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RAR RUS 14301-14301 Irkutsk National Research Technical University Kuz'min Mikhail Mike12008@yandex.ru Irkutsk National Research Technical University Kuz'mina Marina kuzmina.my@yandex.ru Irkutsk National Research Technical University Kuz'mina Alina zhuravlyova-alina@yandex.ru Production of Portland cement clinker from spent chamotte refractory lining Introduction. The cement industry is a major source of CO2 emissions, which highlights the need to implement circular economy principles, particularly the utilization of large-tonnage industrial wastes. Spent chamotte lining (SCL) from aluminum electrolyzers represents a significant environmental challenge – a Class IV hazardous refractory material containing fluorides and alkalis. The aim of this study was a comprehensive experimental assessment of the feasibility of technological integration of two types of waste – leached SCL and amorphous microsilica (MS) – into Portland cement clinker production as a replacement for traditional clayey raw material. Methods. The materials used included limestone, leached SCL, amorphous MS, and an iron-containing corrective sand. The raw mix composition was calculated and optimized using the Pearson's envelope method and solving a system of material balance equations with target values for the lime saturation factor (LSF = 0.90) and the silica ratio (SR = 2.3). Results and Discussion. Laboratory firing of pressed raw mix briquettes was conducted in a muffle furnace following a regime simulating the industrial cycle. The kinetics of decarbonation and phase formation were studied using Thermogravimetric Analysis methods with specifications. The phase composition of the clinker was determined by X-ray Diffraction Analysis, and its microstructure was examined using Scanning Electron Microscopy. The physico-mechanical properties of the obtained cement were tested according to standard methods. Conclusions. It was established that the introduction of a composite SCL+MS component (up to 14.5 % of the raw mix by mass) reduces the decarbonation temperature and the onset of alite (C3S) synthesis by 20–30 °C due to the combined effect of the high dispersity of MS and the residual mineralizing action of fluorides. The experimental clinker-based cement meets the requirements of PC 300 grade, demonstrating a 28-day compressive strength of 29.6 MPa, which is statistically indistinguishable from the control (29.8 MPa). Other critical parameters were evaluated for application which are: the equivalent alkali content (Na2Oₑq.) in SCL should not exceed 2.0–2.5 %, and its proportion in the raw mix can be 8–12 %. The results confirm the technical feasibility of utilizing hazardous waste to produce competitive cement with potential reductions in firing energy consumption. 1.Introduction The modern construction industry faces a global challenge, attempting to reconcile the growing demand for infrastructure with the need for a radical reduction in anthropogenic environmental impact. At the epicenter of this contradiction lies the cement industry – a titanic sector in scale, yet energy- and resource-intensive, responsible for 5–8 % of global CO2 emissions, where the majority of emissions are related to the irreversible process of limestone decarbonation (CaCO3 → CaO + CO2) [1, 2]. In light of the Paris Agreement goals and sustainable development principles, this situation has initiated a widespread search for innovative approaches that go beyond simple optimization of existing processes. The concept of a circular economy comes to the forefront, viewing industrial waste not as an inevitable evil but as potential "secondary resources" or "technogenic raw materials" for related industries, allowing for the simultaneous solution of disposal problems and reduced pressure on natural deposits [3, 4]. One of the most promising and actively studied directions is the partial replacement of primary raw materials (clay, shale, limestone) in the clinker raw mix with wastes containing the necessary oxides (SiO2, Al2O3, Fe2O3, CaO). Successful examples include the use of granulated blast furnace slag, coal fly ash from thermal power plants, nepheline slimes, and metallurgical dusts [5, 6]. However, the effectiveness and safety of such replacement directly depend on the chemical and mineralogical compatibility of the waste with the clinkerization technology, as well as the absence of critical impurities, such as chlorides or excessive alkalis, which can negatively affect the firing process and concrete durability [7, 8]. In the search for such synergistic solutions, the aluminum industry, which generates a range of specific and difficult-to-dispose wastes, is of particular interest. There is already positive experience in using fluorine-carbon-containing gas cleaning sludges as a fluoride mineralizer in cement plants [9]. It has been proven that fluoride ions (F⁻) can lower the temperature of liquid phase formation and reduce the viscosity of the melt in the CaO–SiO2–Al2O3–Fe2O3 system, thereby intensifying the synthesis of the main clinker mineral – alite (C3S) – and reducing specific fuel consumption [10, 11]. This experience was the first step in creating a technological symbiosis between aluminum and cement production. However, alongside these sludges, another large-tonnage and problematic material remains – spent chamotte lining (SCL) from aluminum electrolyzers. Millions of tons of this material are generated annually worldwide, classified as a Class IV hazardous waste due to significant content of mobile fluorides (up to 15 %) and soluble alkalis (Na2O, K2O) [12]. The lining's service life is 3–7 years, after which it is sent for costly landfill disposal or undergoes inefficient processing, posing a serious environmental and economic problem [13]. The key idea of this research is not merely the disposal of SCL but its deep technological integration into the cement production process after preliminary neutralization of the most hazardous components. We hypothesize that after hydrometallurgical leaching, used for fluoride recovery (e.g., in schemes for obtaining synthetic cryolite), SCL – which is essentially a fired high-alumina clay (main components: SiO2 and Al2O3) – can be considered as a source of an aluminosilicate component. However, its original chemical composition is typically characterized by a relative deficiency of silica and a significant excess of alumina compared to traditional clays, as well as the presence of residual alkalis after leaching [14]. This makes the direct use of SCL impossible without correcting the modular characteristics (silica ratio *n* and alumina ratio *p*) of the raw mix. To solve this structural problem, we propose the combined joint use of two types of technogenic raw materials: leached SCL and amorphous microsilica (MS). The latter is a finely dispersed (specific surface area 15–25 m2/g) waste from gas cleaning in the production of crystalline silicon or ferrosilicon, consisting of 85–98 % highly reactive amorphous silicon dioxide [15–28]. Thus, the research is aimed not at simple "disposal" of waste but at the targeted design of a new composite raw material component from them. In this composite, MS compensates for the SiO2 deficiency in SCL, while SCL, in turn, serves as a matrix-carrier for the volatile MS, preventing its losses during transportation and dosing. Furthermore, residual traces of fluorine in SCL may continue to perform the function of a mineralizer. The objective of this work is a comprehensive experimental assessment of the technological feasibility and practical expediency of such a replacement. The following tasks were addressed within the study: Chemical-mineralogical characterization of the initial wastes (leached SCL and MS) and traditional materials. Calculation and design of the raw mix composition using the "SCL+MS" composite to achieve the target clinker modular characteristics. Laboratory modeling of the firing process with a study of the kinetics of decarbonation and phase formation. Comprehensive analysis of the obtained clinker (phase composition, microstructure) and the Portland cement made from it (standard physico-mechanical properties) in comparison with a control produced by traditional technology. 2.Materials The basis for any modeling in cement technology is the accurate chemical composition of the components. We had four key materials at our disposal, each with its own specifics (Table 1). Table 1. Chemical composition of raw materials (wt. %). Component SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 LOI* Limestone 1.4 0.6 0.3 47.4 3.1 – – 0.1 46.5* Spent Chamotte Lining 59.3 32.0 3.0 0.7 0.6 2.0 0.15 0.3 2.5 Microsilica 92.5 0.8 0.6 0.3 0.5 0.40 0.40 0.1 4.6 Iron-containing Sand 28.7 5.4 57.3 3.3 2.8 – – 1.5 1.0   *Note: LOI – Loss on Ignition; for limestone, mainly CO₂.   Limestone served as the main source of calcium oxide (CaO). Leached SCL, after pretreatment, lost a significant portion of fluorine but retained the high alumina (Al2O3) content (32 %) characteristic of refractories and a moderate residual Na2O content (~2 %), which required consideration in the alkali balance. Its main drawback for cement raw mix purposes was a pronounced silica (SiO2) deficiency relative to alumina. To compensate for this deficiency, amorphous MS – an ultrafine powder with a content of reactive SiO2 exceeding 92 % – was introduced. Its high specific surface area predetermined both a potential influence on the mixture's reactivity and a possible increase in the water demand of the future cement. The fourth component was an iron-containing sand, the task of which was to adjust the alumina ratio and ensure the formation of the necessary amount of the aluminoferrite phase (C4AF). 2.1.Design of the Raw Mix Composition The process of designing a raw mix for cement clinker resembles solving a multi-parameter problem. The target values for the lime saturation factor (LSF = 0.90) and silica ratio (SR = 2.3) were set as constants, ensuring the production of clinker with optimal phase composition and hydraulic activity. The main difficulty was working with SCL. Analysis of its composition showed that in its pure form, it cannot serve as a clay analog due to the low SiO2/Al2O3 ratio. To visualize and solve this problem, the classic rule of mixtures – the "Pearson envelope" – was applied. This graphical method clearly showed that to bring the silicate component to the required level (~75 % SiO2), it is necessary to introduce MS in a ratio of 1:0.5 to SCL. Thus, a new, "synthetic" silicate component was created, representing a composite of two technogenic products. Its chemical profile (Table 2), including a reduced content of equivalent alkalis (Na2Oₑq. = 1.3 %), already met the suitability criteria. Table 2. Chemical composition of the modified silicate additive (SCL+MS) (wt. %). Compound SiO2 Al2O3 Fe2O3 CaO MgO Na2Oₑq. Content 75.0 16.0 1.5 0.35 0.27 1.30 Note: Na2Oₑq. = Na2O + 0.658K2O.   Subsequent calculation by solving a system of material balance equations [8] allowed for determining the exact proportions of all components in the raw mix. The final formulation consisted of 81.7 % natural limestone, emphasizing that the proposed technology does not represent a complete replacement of traditional raw materials, but rather a partial, yet significant (14.5 %) replacement of the clayey component. This is a realistic and pragmatic approach for potential industrial implementation. 2.2.Laboratory Modeling of the Firing Process The transition from calculation formulas to real material began with the preparation of a homogeneous raw mix and pressing it into wet briquettes (Fig. 1). Figure 1. Briquettes of pressed raw mix. Firing was carried out in a muffle furnace under a strictly controlled temperature-time regime, which served as a laboratory model of an industrial rotary kiln operation: from 100 °C to 800 °C – 45 min; from 800 °C to 1100 °C – 50 min; from 1100 °C to 1300 °C – 10 min; from 1300 °C to 1450 °C – 20 min. Key stages were: slow heating to remove free and chemically bound water, intensive decarbonation in the range of 700–900 °C, formation of intermediate oxide compounds, and finally, the culmination of the process – the synthesis of clinker minerals at 1450 °C. Rapid cooling of the obtained clinker nodules (Fig. 2) was necessary to fix the metastable high-temperature phases, primarily alite (C3S), which is standard practice in cement technology. Figure. 2. Obtained clinker samples. For a comprehensive characterization of intermediate and final products, an arsenal of modern physicochemical methods was used. Thermogravimetric Analysis (TGA/DTA) acted as a "real-time observer" of the process, allowing for the recording of mass loss during decarbonation and thermal effects accompanying phase transformations. X-ray Diffraction Analysis (XRD) allowed not only qualitative identification of crystalline phases in the clinker but also their quantitative assessment using the Rietveld method. Scanning Electron Microscopy (SEM) provided a unique opportunity to see the material's microstructure: the shape, size, and mutual arrangement of crystals, which directly correlates with the strength properties of the future cement. Standard physico-mechanical tests of cement paste and mortar became the final and most important criterion for success, translating scientific data into the language of practical construction standards (GOST). 3.Results and Discussion Comparative analysis of thermograms of the experimental and control mixtures revealed important differences (Fig. 3). Figure 3. Comparative thermograms (TGA/DTA) of the experimental and control raw mixes. The main endothermic peak, corresponding to the decomposition of calcium carbonate (CaCO3 → CaO + CO2), in the mixture containing SCL and MS, was shifted to a lower temperature region by 20–30 °C. This can be interpreted as a result of two factors. Firstly, finely dispersed MS, possessing an enormous specific surface area, creates an extensive contact zone with limestone particles, facilitating diffusion processes. Secondly, residual fluoride ions, preserved in SCL after leaching, may act as a catalytic mineralizing additive. It is known that even small amounts of fluorine contribute to the destruction of the crystal lattices of initial oxides and reduce the viscosity of the forming melt, thereby intensifying the reactions of solid-state and liquid-phase synthesis of calcium silicates [9]. This effect is also confirmed by the shift of the onset of the exothermic effect associated with alite (C3S) formation towards lower temperatures. Thus, the first important conclusion is that the introduction of the technogenic composite not only does not impair but potentially improves the kinetics of the firing process, promising direct energy benefits on an industrial scale. The obtained diffractograms, presented in Fig. 4, clearly demonstrate that the main qualitative composition of the clinkers is identical. Figure 4. X-ray diffractograms of the experimental and control clinker. In both cases, peaks characteristic of alite (C3S), belite (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF) are clearly identified. However, quantitative calculation (Table 3) revealed nuances. Table 3. Phase composition of clinkers (wt. %), determined by the Rietveld method. Sample C3S (Alite) C2S (Belite) C3A C4AF Glass phase / Other Experimental 52 ± 2 25 ± 2 8 ± 1 12 ± 1 3 Control 56 ± 2 22 ± 2 7 ± 1 13 ± 1 2     In the experimental clinker, the proportion of alite – the most valuable and fast-hardening phase – was 4 % lower (52 % vs. 56 %), while the proportion of belite, which gains strength more slowly but more stably, was correspondingly higher. The explanation for this phenomenon can be found when examining the microstructure (Fig. 5). a b Figure 5. SEM micrographs (BSE mode) and EDS mapping of clinker samples. (a) Control clinker: prismatic alite (C₃S, light grey, 20–35 µm) with rounded belite (C₂S, darker grey) and interstitial phase (bright, Fe-rich). (b) Experimental clinker with SCL+MS additive: alite crystals (15–40 µm) show slightly irregular morphology; belite content visually higher. SEM micrographs show that alite crystals in the experimental sample have a slightly less regular shape and a larger size distribution (15–40 µm vs. 20–35 µm in the control). This may be related to changes in the rheological properties of the melt. Highly dispersed MS, actively interacting with the melt, may locally increase its viscosity. This creates kinetic barriers for the growth of large, ideal alite crystals and complicates the final stage of its formation from belite and free lime. Interestingly, energy-dispersive X-ray analysis (EDX) confirmed the hypothesis about alkali distribution: sodium primarily concentrated in the inter-crystalline glassy phase rather than entering the lattice of the main silicate minerals. This is a favorable scenario, as such immobilization reduces the risk of harmful alkali-silica reaction in concrete [6]. The compressive strength was determined at 28 days of age on a hydraulic press in accordance with GOST 310.4. Standard specimens made from the experimental and control cements were tested (Fig. 6). a b Figure 6. Process of testing cement specimens for compressive strength: (a) specimen installed in the grips of the testing machine; (b) general view of the hydraulic press during testing. The key confirmation of the new cement's quality is the compliance of its properties with standard requirements. The data presented in Table 4 and Fig. 7 allow for a definitive conclusion: the cement based on the experimental clinker fully meets the requirements of PC 300 grade.         Table 4. Properties of Portland cements based on experimental and control clinker. Characteristic Experimental Cement Control Cement Norm for PC 300 (GOST 31108) Specific surface, m2/kg 355 350 Not less than 300 Normal consistency, % 28.50 25.75 – Initial setting time, min 220 200 Not less than 45 Final setting time, min 430 470 Not later than 600 Compressive strength, MPa:       3 days 12.5 ± 0.8 14.1 ± 0.9 ≥ 12.0 7 days 21.3 ± 1.2 22.8 ± 1.3 ≥ 19.0 28 days 29.6 ± 1.5 29.8 ± 1.5 ≥ 28.0 56 days 34.2 ± 1.6 33.1 ± 1.5 ≥ 32.0   Some differences from the control sample are characteristic and explainable. The increased normal consistency (28.5 %) is a direct consequence of the presence of ultrafine MS, which requires more water for wetting. This is a technological nuance that must be considered when selecting water-reducing admixtures in concrete mix design. The kinetics of strength gain (Fig. 7) demonstrate a classic pattern for clinker with a slightly increased belite content. Figure 7. Variation of compressive strength with curing time (3, 7, 28, and 56 days). In the first three days, the strength of the experimental cement slightly lags, which correlates with the slightly lower amount of fast-reacting alite. However, by 7 days, the gap narrows, and by 28 days, the strengths become statistically indistinguishable. This vividly illustrates the compensatory role of belite: although its hydration proceeds more slowly, it continues for a long time, ensuring confident strength growth in the medium- and long-term perspective. Thus, a minor shift in the clinker's phase composition did not lead to a deterioration of its key strength characteristic. Comparison with previous studies. The observed reduction in decarbonation temperature (20–30 °C) due to the combined effect of fluoride residues and amorphous silica is consistent with the findings of 9. K. Vance et al. [9], who reported that fluoride ions lower the liquid phase formation temperature by 15–40 °C in cement raw mixes. Similarly, Katey O’Quinn et al. [11] demonstrated that fluoride-containing mineralizers enhance alite formation at lower temperatures. However, unlike previous studies that used pure fluoride salts, the present work achieved a comparable effect using residual fluorides in a waste material (SCL), which is more economically and environmentally attractive. The 28-day compressive strength of 29.6 MPa obtained in this study is slightly lower than the 32–35 MPa reported by Abdul,  W. et al. [27] for SCL-fly ash blends, but this difference is attributed to the higher alkali content in our SCL (2.0 % Na2O vs. 0.8 % in their study). Vuthaluru [19] also noted that alkalis above 1.5 % Na2Oₑq. can suppress alite formation, which agrees with our observed 4 % reduction in C3S content compared to the control. 4.Conclusion The conducted study, consistently from the calculation model to the testing of the finished product, proved the fundamental possibility of deep technological integration of wastes from aluminum and silicon production into the Portland cement manufacturing process. We did not merely add an inert filler but purposefully designed a new composite component (SCL+MS), which in quantities up to 14.5 % of the raw mix mass can fully replace traditional clay. The main conclusions of the work are of both scientific and practical application value: The combined use of leached SCL and MS allows for the mutual compensation of their shortcomings: MS compensates for the silica deficiency in SCL, while SCL serves as a matrix-carrier for finely dispersed MS. Residual fluorides in SCL and the high reactivity of MS have a positive effect on the firing kinetics, lowering the temperature of key reactions, which opens the way to fuel savings. Despite a slight modification of the phase composition (shift in the C3S/C2S balance) and the clinker's microstructure, the obtained Portland cement satisfies all regulatory requirements for PC 300 grade, demonstrating full competitiveness. For successful implementation, it is necessary to control the alkali level in the incoming SCL (Na2Oₑq. ≤ 2.0 %) and its proportion in the raw mix (8–12 %), as well as to account for the increased water demand of the cement when designing concrete mixes. Prospects for further research logically follow from the obtained results. The next necessary step should be pilot industrial trials on an operating rotary kiln, which will allow assessing the real impact on energy consumption, process stability, and emissions of volatile compounds. In parallel, an in-depth study of the durability characteristics of concrete based on the new cement is required, especially its resistance to freeze-thaw cycles and corrosive aggression. Future work should also include a comprehensive evaluation of concrete durability, including freeze-thaw resistance, sulfate attack, and alkali-silica reaction, as the presence of residual alkalis may pose long-term risks. In a broader context, this work is a clear example of implementing circular economy principles in heavy industry. It offers a specific, technically justified solution for two environmental problems simultaneously, transforming the costs of hazardous waste disposal into investments in the production of a demanded construction material. 10.34910/MCE.143.1 691.3 spent chamotte lining amorphous microsilica Portland cement clinker circular economy waste utilization fluorides alkalis mineralizer raw mix firing microstructure compressive strength https://engstroy.spbstu.ru/article/2026.143.1/

RAR RUS 14302-14302 University of Anbar Khalaf Khalaf kha22e1007@uoanbar.edu.iq

Ramadi, Anbar, Iraq

University of Anbar Aljanab Khalid kr_aljanabi@uoanbar.edu.iq

Ramadi, Anbar, Iraq

Shear strength and durability behavior of organic soils treated with recycled glass powder-based geopolymer Organic soil has many construction problems due to its being weak in engineering properties, such as low shear strength and high deformation or compressibility. This study's primary goal was to determine whether employing recycled glass powder-based geopolymer can enhance the engineering properties of organic soil through tests of durability and unconfined compressive strength (UCS). The impact of an alkaline activator solution on UCS and durability, as well as the effects of recycled glass powder (RGP) contents on maximum dry unit (MDU) and optimum moisture content (OMC), were also examined in this study. RGP-based geopolymer was used with organic soil to improve durability and compensate for reduced shear strength. The RGP was added with different percentages (5, 10, 15, 20, and 25 %) by dry weight of soil. Sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were used to prepare the geopolymer. First, sodium hydroxide with constant molarity (4M) was used with RGP to find the optimum mixture, then sodium hydroxide (SH) with sodium silicate (SS) (SH:SS = 72:28 and SH:SS = 50:50) with the same molarity was used with the optimum mix only. According to the findings, MDU weight increases as RGP content is added, but OMC decreases. Additionally, as the RGP content increases up to 20 %, the results demonstrate that the UCS increases. For the optimum mix (20 % RGP), the UCS increases by 29, 38, and 46 times, and the failure strain decreases by 31.4, 31, and 39.3 % with curing times of 7, 14, and 28 days at 65 °C, respectively, compared to the untreated soil. A durability test was conducted on compacted natural soil and stabilized organic soil with three different alkaline activator solutions 100:0, 72:28, and 50:50 for a curing time of 28 days at a curing temperature of 65 °C according to ASTM D559. Results showed that after 12 cycles of the wetting and drying cycles, UCS decreased by 20, 28.5, and 35.1 %, respectively. 1.Introduction Organic soil has several engineering problems due to its low shear strength, high compressibility, swelling, and shrinkage characteristics. The construction process presents a challenge for geotechnical engineers to address these issues. Today, Portland cement is a widely utilized material for soil stabilization. Still, cement manufacturing generates environmental impacts because it involves the significant release of carbon dioxide (CO2) emissions. Organic matter in the soil can reduce the reaction between cement and soil particles [1]. Replacing the natural soil layer with suitable soil from another site represents another method of organic soil stabilization [2]. As a result of this process, sometimes the project's cost may rise, particularly if the proposed location for the replacement soil is relatively far away. This has prompted researchers to explore alternative solutions. One of these methods involves soil stabilization with geopolymers. According to ASTM D2974, soil is classified as organic soil if the organic content exceeds 20 % [3]. Previous studies have investigated the engineering properties of organic soil, showing its high compressibility and low shear strength. Khairina et al. demonstrated that adding 10 % gypsum to organic soil with concrete waste increases the unconfined compressive strength (UCS) and reduces compressibility [4]. Adel et al. demonstrated that adding an amount of lime to organic soil, including both decomposed and non-decomposed samples, significantly affects their shear strength. The results showed that the cohesiveness values (c) increased by 5 % in non-decomposed samples and 7 % in decomposed samples with higher organic content [5]. Nath et al. evaluated the behavior of organic soil stabilized with fly ash as a function of strength. They found an increase in UCS with increasing fly ash amount due to the pozzolanic reaction of fly ash [6]. Ibrahim et al. demonstrated that increasing crushed waste concrete (CWC) to 50 % can increase organic clay's UCS values by 25 % [7]. Mohd et al. showed that adding lime and salt to organic soil increased the cohesion value from 7.1 to 23.6 kPa [8]. Some studies found that adding fly ash to organic soil increased the UCS and resilient modulus [9]. Moayedi et al. illustrated that adding 5 mol/L of sodium silicate (SS) to stabilized organic soil can increase the UCS values [10]. Habbi studied the organic soil's compressibility. The results demonstrated that an increase in organic content for all samples increases the compressibility characteristics (compression index, rebound index, coefficient of consolidation, and coefficient of secondary compression). The use of cement dust and fly ash has improved organic soil's compressibility [11]. Ali and Tatt studied that adding additives such as cement, lime, and fly ash to a mixture of organic soil improved its strength, particularly after the 7-day curing period [12]. Abbas et al. studied that additive of cement dust and fly ash with percentages (3, 6, and 9 %) by weight of organic soil improved compressibility characteristics [13]. Boobathiraja et al. studied that adding cement and lime in the range of 10–30 % as a percentage of dry soil mass caused an increase in the UCS of organic soil [14]. Rafizul et al. investigated how organic soils behave when mixed with cement, bentonite, and lime with different ratios of (5, 10, 15, 20, and 25 %) of the dry weight of organic soil. They found that MDU increased while OMC decreased as the admixture percentage increased [15]. Talib and Noriyuki developed another alternative binder using sugarcane bagasse ash (SCBA) for organic soil stabilization. Research demonstrated that combining SCBA with Portland cement enhanced the amount of UCS-stabilized organic soil [16]. As for improving the consolidation of organic soil. Thiyyakkandi and Annex showed that the consolidation coefficient decreased with increased organic matter [17]. Wardwell and Nelson showed the increase of organic content accompanied by a rise in the coefficient of secondary compression (Cα), causing an increase in the void ratio of soil [18]. Mustapa et al. showed that adding gypsum content with known percentages led to an improvement in the strength of organic soil. They found that any increased addition in gypsum content led to increased UCS values while the compressibility was reduced simultaneously [19]. The above literature review shows no application for organic soil stabilization using RGP-based geopolymers. The main aim of this study is to assess the effectiveness of using recycled glass powder-based geopolymer in improving organic soil engineering properties by conducting UCS and durability tests. Also, this study investigated the effect of RGP contents on MDU and optimum moisture content (OMC), as well as the effect of an alkaline activator solution on UCS and durability. 2.Methods and Materials 2.1.Soil It was collected from the agricultural area in Anbar Governorate, west of Iraq. Soil samples were collected from a depth (0.3) m. According to the Unified Soil Classification System (USCS), the soil was classified as low-plasticity organic silty clay (OL). Tables 1 and 2 show the physical properties and chemical composition properties of soil. Table 1. The physical and classification properties of soil. Property Soil Specification Liquid Limit (LL) (%) 44 According to ASTM D4318 Plastic Limit (PL) (%) 27 According to ASTM D4318 Plasticity index (PI) (%) 17 According to ASTM D4318 Specific gravity (Gs) 2.45 According to ASTM D854 Organic content (OC) (%) 20.8 According to ASTM D2974 Passing sieve #200 (%) 86 According to ASTM D422 Unified Soil Classification System (USCzS) OL According to ASTM D2487 Optimum moisture content) (%) 20 According to ASTM D698 Maximum dry unit) (KN/m3) 15 According to ASTM D698   Table 2. The chemical composition of soil by XRF. Chemical composition (%) SiO2 Al2O3 CaO Fe2O3 MgO Na2O K2O SO3 Mn Organic soil 39 20.91 20.4 5.74 9.28 1.24 1.31 1.10 0.08   2.2.Recycled Glass Powder The waste glass powder was collected from a waste landfill in Ramadi city. The glass was crushed using the traditional (metal mortar and pestle) tools. Then, it was sieved through sieve #200, and the particles finer than 75 μm were used as the base of the geopolymer. The glass powder specific gravity was determined to be 2.52, according to ASTM D854. The XRF test of RGP sample shows that its generally composed of SiO2, Al2O3, CaO, Fe2O3, MgO, etc. and it is generally composed of a high percentage of silica (SiO2), which is about 72.1 %. The XRF is conducted at the laboratories of Ministry of Industry and Minerals of Iraq. Table 3. Shows the chemical composition of RGP. Table 3. The chemical composition of RGP by XRF. Chemical composition (%) SiO2 Al2O3 CaO Fe2O3 MgO Na2O K2O SO3 Mn Glass powder 72.1 1.30 6.06 0.30 4.60 15.20 0 0.30 0   2.3.Alkaline solution This study used sodium hydroxide (SH) (NaOH) and sodium silicate (Na2SiO3) as alkaline activators with a constant concentration of 4M and their properties are listed in Tables 4 and 5, respectively. Table 4. The properties of (NaOH). Chemical formula Mass (g/mol) Purity (%) Density (g/cm3) pH NaOH 40 98.4 2.13 @ 20 °C 14     Table 5. The composition and properties of Na2SiO3. Na2O (%) SiO2 (%) Viscosity (N.s/m2) Specific gravity pH 13.7 33 800 1.534 12   2.4.Geopolymer It is an alternative binder instead of cement, made from reacted silica and alumina with an alkaline solution. It is used in many engineering applications, such as soil stabilization for instance. Geopolymer was developed as an eco-friendly alternative to Portland cement, as it is produced with less consumption of energy and reduces the emission of CO2. Geopolymers have many properties, such as flexibility, corrosion resistance, and being more resistant to environmental and climatic conditions, making them important materials in a variety of engineering applications. Many laboratory tests were achieved on untreated organic soil. On the other hand, the testing was conducted on organic soil mixed with 5, 10, 15, 20, and 25 % RGP to study the impact of RGP-based geopolymer on the UCS of the stabilized soil. Many specimens of natural soil mixed with 5, 10, 15, 20, and 25 % RGP were performed according to ASTM D698 by using the standard proctor test to obtain the MDU and OMC. To find the value of UCS for each mixture with different ratios of RGP, specimens were prepared at similar MDU and OMC values found from the standard proctor test. Also, a small plastic spilt mold with a hammer was used to achieve the processing of the specimen's compaction. As shown in Fig. 1, the plastic mold dimensions are 50 mm in inner diameter and 100 mm in height, while the hammer's weight is 2.5 kg, falling from 30.5 cm in height. Compaction processing was conducted in 3 layers, and every layer was compressed with 8 blows. Where layers and blow numbers were obtained to get a similar compaction effort (596 kN.m/m3) for the proctor test. At first, the different ratios of RGP+ soil (5, 10, 15, 20, and 25 %) by dry mass of soil were mixed to prepare testing specimens. The solution of the alkaline activator was prepared with a constant concentration of NaOH (4M), and after this, it was added to the mixture of soil and RGP. The specimens of untreated soil were tested directly, and other treated specimens were cured at 65 °C for 48 hours and then kept at room temperature of 25 °C for 7, 14, and 28 days. For each mixture and curing period, three specimens were prepared [20, 21]. For the durability test, alkaline activator solution in three different ratios of solution SH:SS represented by 100:0, 72:28, and 50:50 were used to prepare soil specimens. All these specimens were exposed to wetting and drying cycles; similarly, treated soil specimens were exposed to wetting, drying, and brushing cycles. Each cycle started with submerging the specimens for 5 hours in the distilled water, 42 hours in the dried oven (71±2 °C), and brushing with a wire scratch brush. At the end of each cycle, the specimens are weighed and measured in volume. As expected, all specimens lost weight with time. After completing 12 cycles, the specimen was put in the oven (110 °C) to dry it until the mass became constant, and the UCS was conducted on all specimens. Figure 1. Plastic spilt mold with dimension (50×100 mm). 3.Results and Discussion Compaction Tests The compaction test showed a relationship between dry density and moisture content for unstabilized (natural soil) and stabilized soil with different percentages (5, 10, 15, 20, and 25 %) of RGP, as shown in Fig. 2. The results show that as the RGP content ratio increases, MDU values increase and OMC values decrease. This may be attributed to the fact that finer particles of RGP fill the voids existing between soil particles [22]. This is similar to what was observed by the previous studies [23–25]. (a)        (b)                                                                     (c) Figure 2. a) The compaction curves of RGP-soil mixture, b) MDU versus RGP content, c) OMC versus RGP content.   Unconfined Compressive Strength 4.2.1. The impact of RGP content and curing period The UCS values of soil with geopolymer of many percentages of RGP content (5, 10, 15, 20, and, 25 %) and curing time 7, 14, and 28 days with fixed alkaline solution SH:SS = 100:0, molarity 4M at a curing temperature 65 °C are shown in Fig. 3. As expected, the values of UCS for all soil specimens stabilized with RGP geopolymer were more than that of the untreated soil for all three curing times. Also, this figure shows that the value of UCS increases with increasing RGP content up to 20 %, where this increase represented by 29, 38, and 46 times compared to untreated soil for 7, 14, and 28 days respectively. Figure 3. The values of UCS versus RGP content for different curing times 7,14 and 28 days at curing temperature 65 °C. The maximum value of UCS was obtained at 20 % RGP content due to the stronger bond between the soil particles and RGP geopolymer gel [26]. This is similar to what was observed in previous studies [27–29]. A slight decrease in UCS values above 20 % RGP content was observed. The additional amount of RGP may have adverse effects on the UCS. These effects might be caused by the substantial amount of RGP particles contacting the soil structure, which would reduce soil cohesiveness. RGP serves as a filler material rather than contributing to the geopolymer binder [29]. 4.2.2. Stress-strain behavior of RGP geopolymer-treated soil Fig. 4 illustrates the impact of RGP Geopolymer on the stress-strain behavior of treated soil for three curing times 7, 14, and 28 days. All treated specimens had a lower failure strain compared to untreated soil. Where the axial strain values of specimens treated with 20 % RGP decreased by 31.4, 31.5, and 39.37 % of curing times 7, 14, and 28 days. As seen in the behavior of treated soil closer to ductility with increasing RGP content, this may be returned to a delay in developing geopolymer bonds due to the low curing temperature. Similar studies demonstrated that treated soil with metakalin-based geopolymer developed UCS and ductility [27], and another explained that RGP-based geopolymer can improve soil ductility [29]. Figure 4. Axial strain (%) values versus RGP content for different curing times 7,14 and 28 days at curing temperature 65 °C. Durability According to ASTM D559, the durability test was conducted on all treated soil specimens prepared for this purpose. For the durability test, three different ratios of alkaline activator solution SH:SS represented by 100:0, 72:28, and 50:50 were used separately with 20 % RGP+80 % soil)to prepare two geopolymer specimens for each mixture. The durability test for all prepared specimens was tested by wetting and drying (w/d) cycles. After the end of 12 cycles, the loss in mass and change in volume were recorded and discussed, as shown below. 4.3.1. Volume change All specimens stabilized with RGP geopolymer have approximately changes (±1 %) in volume and survive all 12 w/d cycles. Whereas untreated soil specimens are destroyed within the first w/d cycle. Fig. 5 illustrates the natural soil and treated soil specimens before and after the w/d cycles. 4.3.2. Mass loss (%) The mass loss (%) for all treated soil specimens with the RGP geopolymer tested to the durability cycles is shown in Fig. 5. As shown in Fig. 6, it can be observed that a gradual increase in mass loss was observed until the fifth cycle for mixes treated with 20 % RGP+ (100:0) and 20 % RGP+ (50:50). After this, the mass loss approximately remained constant up to the twelfth cycle. Whereas a slight gradual increase after the fifth cycle for mix treated with 20 % RGP+ (72:28) up to the ninth cycle remained constant up to the twelfth cycle. The losses in mass for treated soil specimens for SH:SS = 100:0, 72:28, and 50:50 for 65 °C were 4.9, 6.65, and 8 %, respectively. In any case, if they were compared to untreated soil, the treated soil specimens behaved better durability in terms of mass loss. All the mass loss (%) for all treated soil specimens was limited according to American standards [25, 26]. (a)                                                                                      (b) (c)                                                                                      (d) Figure 5. Durability test (w/d cycles): a) natural soil during cycle No. 1, (b) natural soil after (10min) during cycle No. 1, (c) treated soil specimens during cycle No. 1, (d) treated soil specimens after cycle No. 12. Figure 6. Mass loss (%) versus No. of w/d cycles at 65 °C. 4.3.3. Strength loss The UCS of the stabilized soil specimens exposed to w/d cycles was tested after 12 cycles. Table 4 displays the UCS values before and after the durability test. This result indicates that a sudden reduction in strength values for all specimens stabilized with 20 % was 20, 28.5, and 35.1 %, respectively, for curing temperature 65 °C recorded at the end of 12 cycles. A similar result has been observed with [29, 30]. Additionally, several studies showed that the UCS values of the treated soil were negatively impacted by the durability test [31, 32]. In comparison to soil that had not been treated, the results were deemed to be highly significant. Table 4. Shows the results of UCS before and after durability (w/d cycles) for different soil mixtures. Mixture UCS before durability (MPa) UCS at the end of durability (MPa) Soil+20% RGP+(SH:SS=100:0) 9.41 7.5 Soil+20% RGP+(SH:SS=72:28) 8.12 5.8 Soil+20% RGP+(SH:SS=50:50) 8.01 5.2   4.Conclusion This paper aimed to study the behavior, shear strength, and durability of organic stabilized with RGP-based geopolymer. According to the results from the testing, the conclusions below can be found. When the RGP was added to the natural soil (organic soil), the MDU values increased and the values of OMC decreased. The results obtained from UCS tests for all specimens treated with RGP geopolymer demonstrated a significant increase in the shear strength of stabilized organic soil when the RGP content increased by more than 20 %. The strain values at failure decreased by 31.4, 31.5, and 39.37 % of the curing times 7,14 and 28 days, respectively, less than that of untreated soil, indicating a substantial effect of the RGP geopolymer additive on this parameter. The durability of all treated soil specimens was tested with wetting and drying cycles; all specimens survived 12 cycles. The volume change was approximately equal (±1), and the mass loss was 4.9 %, 6.65 %, and 8 % for treated soil specimens for SH:SS = 100:0, 72:28,and 50:50 at 65 °C, respectively, and it was found to be within the required limits. There was a reduction in strength values for all specimens stabilized with 20 % RGP represented by 20, 28.5, and 35.1 %, respectively, for SH:SS = 100:0, 72:28, and 50:50 at 65 °C. The strength values obtained after the wetting and drying cycles prove that the RGP geopolymer is a promising alternative solution of chemical stabilizers and a potential stabilizer and can be accepted for organic soil stabilizing. 10.34910/MCE.143.2 624 shear strength durability organic soil RGP geopolymer recycled glass powder unconfined compressive strength https://engstroy.spbstu.ru/article/2026.143.2/

RAR RUS 14303-14303 55875561800 0000-0002-8745-7059 Babol Noshirvani University of Technology Rezaei Sadegh S_Rezaei1366@yahoo.com

Babol, Mazandaran, Iran

0000-0002-8470-6160 Department of Civil Engineering, University of Science and Technology of Mazandaran Moradi Majid m.moradi@mazust.ac.ir

Behshahr, Iran

0000-0002-5918-8920 Department of Civil Engineering, Ayatollah Amoli Branch, Islamic Azad University Soleimani Kutanael Saman samansoleimani1616@yahoo.com

Amol, Iran

Effects of near-fault and far-fault earthquakes on the site response Investigation of previous earthquakes shows that the distance to the fault, from which the earthquake is sourced, is a paramount factor contributing to the site response and magnitude of the induced damages. In this research, equivalent linear and nonlinear methods were used to assess the effect of the earthquake field on the site response. Assessments included investigations of time histories of acceleration, velocity, displacement, as well as their peak values and response spectra. Records of the Bam earthquake acquired at two different stations were used for this purpose. Results showed that the peak values of the response spectrum induced by a near-field earthquake record occurred at higher frequencies than those seen in a far-field quake, with the nonlinear effects being more pronounced in the near-field earthquake record than in the far-field one. Additionally, the difference of the obtained responses from the nonlinear and equivalent linear methods was greater for the near-field earthquake record than for the far-field earthquake record. Finally, it is worth noting that the damage potential of an earthquake event cannot be adequately assessed unless the frequency content of the earthquake, soil effects, and structure stiffness are considered simultaneously.                                                                                                    1.     Introduction Earthquakes are among the most significant natural hazards humans have faced since their emergence on Earth. More than 5 million earthquake events occur each year, making it crucial to study every aspect of this phenomenon [1, 2]. A major characteristic of an earthquake that affects the induced damage and soil behavior is the distance to the quake-inducing fault. Based on the data on previous earthquakes, researchers have identified three types of quake events as near-field (within 20 km of the quake-inducing fault), mid-field (within 20–60 km of the quake-inducing fault), and far-field earthquake records (beyond 60 km of the quake-inducing fault) [3]. Near-field earthquake records are usually larger in magnitude than the far-field earthquake records. The near-field records exhibit intense pulse-like ground motions, which can be explained by the release of large amounts of energy in a short time upon faulting. This pulse-like ground motion imposes devastating amounts of energy on overlying structures at the start of the motions [4–6]. A near-field earthquake record generates higher frequencies than a far-field earthquake record because the closer distance to the source of the earthquake waves prevents the absorption of higher-frequency events, so that the resultant records contain higher frequencies than the records of far-fault areas [7–10]. Another characteristic of a near-field earthquake record is what is referred to as directivity. Pulse-like motions are particularly more pronounced in forward-directivity areas where the fault failure approaches at a velocity close to the shear-wave velocity. Another difference between the far- and near-field earthquake records is the peak vertical acceleration [11–15]. Typically, peak vertical acceleration is equal to two-thirds of the peak horizontal acceleration. However, studies on previous earthquakes have shown that the actual ratio is higher for near-fault areas and lower for the far-fault zones [16]. Very near-fault earthquake records (within 500 m of the fault) indicate substantial permanent ground displacement, which is a result of tectonic deformation of the ground in a process called the fling step. This deformation occurs during slippage along the fault plane and is therefore usually observed in the parallel component to the fault plane [17]. Considering the importance of the abovementioned topics, the present research was focused on evaluating the effect of the earthquake field on the site response with the help of equivalent linear and nonlinear methods. This evaluation included investigations of time histories of acceleration, velocity, displacement, as well as their peak values and response spectra. Records from the Bam earthquake, acquired at two different stations, were used for this purpose (Bam Station as near-field earthquake record and Ravar Station as far-field earthquake record). Despite extensive research on seismic site response and earthquake ground-motion characteristics, most previous studies have primarily focused on either near-fault ground motions, nonlinear soil behavior, equivalent linear analysis, or structural response independently. Limited attention has been given to a comprehensive comparative evaluation of near-fault and far-fault earthquakes using both nonlinear and equivalent linear approaches under identical soil conditions and real earthquake records. In particular, the combined investigation of acceleration, velocity, displacement, amplification behavior, and response spectra throughout soil depth has not been sufficiently addressed in earlier studies. Therefore, the present research aims to investigate the effects of near-fault and far-fault earthquakes on site response characteristics through nonlinear and equivalent linear analyses using real earthquake records. The study further evaluates how different seismic field conditions influence amplification behavior, spectral response, and depth-dependent ground-motion parameters, while also examining the role of soil nonlinearity in modifying seismic demand. Unlike previous investigations, this study provides an integrated assessment of seismic response characteristics together with their engineering implications for resonance behavior, structural vulnerability, and site-specific seismic design.                                                                                                        2.     Methods 2.1.               Input Ground Motion (Bam Earthquake) In the early morning (5:26:56 a.m. IRST) of Friday, December 26th, 2003, the city of Bam, Iran, was hit by a powerful earthquake. Fig. 1 shows the horizontal component of the studied earthquake for the Bam and Ravar Stations. As seen in the figure, the record at Bam Station (near-field) exhibits a peak horizontal acceleration of PHA = 0.777 while the corresponding figure to the Ravar Station (far-field) is PHA = 0.0126.   Figure 1. Horizontal acceleration records for the Bam earthquake at: a) Ravar Station; b) Bam Station. Fig. 2 exhibits velocity records for the Bam and Ravar Stations. As observed, the record for the Bam Station is clearly pulse-like at the start of the ground motion, while the record for Ravar Station does not exhibit such pulses.   Figure 2. Horizontal velocity records for the Bam earthquake at: a) Ravar Station; b) Bam Station. As explained in the Introduction, near-field earthquake records exhibit directivity. Fig. 3 indicates this fact. As seen in this figure, the component  of the horizontal record at Bam Station exhibits some pulses that are almost attenuated in the component   Figure 3. Horizontal acceleration records for the Bam earthquake: a) component L; b) component T. Fig. 4 shows the vertical acceleration of the studied earthquake for the Bam and Ravar Stations. As seen in the figure, the record at Bam Station exhibits a peak vertical acceleration of PVA = 0.994, while the corresponding figure to the Ravar Station is PVA = 0.006088 Moreover, the ratio of the peak vertical-to-horizontal acceleration was seen to be 1.28 (> 2/3 = 0.666) at Bam Station and 0.48 (< 2/3 = 0.666) at Ravar Station.   Figure 4. Vertical acceleration records for the Bam earthquake at: a) Ravar Station; b) Bam Station. Fig. 5 indicates the displacement records at Bam and Ravar Stations for the component  According to this figure, permanent displacements recorded at Bam and Ravar Stations were 2.8 and 0.0002 (~ 0) cm, respectively.   Figure 5. Horizontal displacement records for the Bam earthquake at: a) Ravar Station; b) Bam Station. Fig. 6 depicts the response spectrum for the records at Bam and Ravar Stations for a damping ratio of 5 %. For shorter periods (i.e., higher frequencies), the response spectra of the Bam Station exhibits higher values than those for the record at Ravar Station. The opposite was observed for longer periods. In general, far-field and near-field earthquake records are the controlling factors for the structures of long and short periods, respectively.   Figure 6. Response spectrum for a damping ratio of 5 %. 2.2.               Modeling In order to evaluate the site effect, Fig. 7 was considered as the base model for the soil layers. Four reference points (A, B, C, and D) were considered at the interfaces of successive layers, and the results were studied at these reference points.   Figure 7. Characteristics of soil layers. In this research, the effects of the earthquake and the soil were evaluated via two approaches, namely equivalent linear and nonlinear methods. The equivalent linear method uses linear properties for the elements. Obtained as average values over dynamic motion, these properties then remain constant during the ground motions. Accordingly, the phenomena that are driven by interactions among multi-frequency components encountered in a nonlinear material are actually omitted in an equivalent linear analysis. The equivalent linear method provides no information on irreversible deformation and permanent changes, as it barely models the vibrational motions. The so-called plastic flow is inappropriately modeled in the equivalent linear method [18–21]. In contrast, the nonlinear method can follow any predetermined nonlinear behavior of the material. Applying nonlinear rules for the material, mixing of different multi-frequency components can be naturally captured, and irreversible deformations and other permanent changes can be automatically modeled. Although this methodology can follow any stress-strain relation, its results are highly sensitive to the details of the constitutive models used [18–22]. Different steps of modeling by the nonlinear method in the PLAXIS software include general setting, geometry design, application of boundary conditions, definition of material properties, generation of element grids, establishment of initial conditions, and processing the computations. In this research, the Mohr–Coulomb constitutive model was implemented, with material damping approximated by the Rayleigh damping model, which assumes proportionality between damping and material mass and stiffness. Given that calculating the Rayleigh damping ratio requires the angular frequency in two vibration modes, the angular frequencies were calculated by modal analysis in the Abaqus software. Modeling by the equivalent linear method in the Deepsoil software goes through 5 steps: (1) configuring the analysis, (2) applying the properties of the soil layers and bedrock, (3) determining the input ground motion, (4) analyzing the model, and (5) producing the results. The most important part of an equivalent linear analysis is the proper choice of the diagrams of shear modulus and damping ratio, which shall be based on the studied soil layering.                                                                                    3.     Results and Discussion 3.1.               Acceleration Figs. 8 and 9 illustrate the time histories of acceleration produced by the nonlinear and equivalent linear methods, respectively, at the reference points under the influence of near-field and far-field earthquake records. As can be seen, the magnitude of the acceleration time history increases as one moves toward the surface. In general, it can be inferred that, given the lower shear-wave velocity and density of the material on the surface compared to the underlying materials and ignoring the damping and propagation decay effects, the principle of the conservation of elastic energy implies that the flow of energy shall remain unchanged, and this explains that the reduction in the shear-wave velocity and density of the soil tends to increase the magnitude of the associated ground motions. Moreover, when the earthquake waves reach the surface, a significant portion of their energy is reflected back to the crust, so that parts of the ground surface are simultaneously affected by the upward- and downward-propagating waves [23]. These two reasons can explain the increase in the ground motion from the bedrock to the ground surface. Of course, one should further consider the fundamental frequencies of the site and the ground motion with a focus on possible resonance. As can be seen from the results, the predicted increase by the equivalent linear method exceeds that by the nonlinear analysis. Indeed, the linear nature of the equivalent linear method leads to unreal resonance. It is further clear that the acceleration at the ground surface is higher in a near-field earthquake record than in a far-field earthquake record. This can be attributed to the attenuation of ground motion waves as one moves farther from the earthquake source. Another observation is that the time history of acceleration for a near-field earthquake record contains higher frequencies than a far-field earthquake record because the distance from the source of earthquake energy is insufficient to damp high-frequency events.   Figure 8. Time history of acceleration at reference points under the influence of a near-field earthquake record (Bam Station): a) nonlinear analysis; b) equivalent linear analysis. Figure 9. Time history of acceleration at reference points under the influence of a far-field earthquake record (Ravar Station): a) nonlinear analysis; b) equivalent linear analysis. Fig. 10 shows the Peak Horizontal Acceleration (PHA) as a function of depth for the near-field and far-field earthquake records using the equivalent linear and nonlinear analyses. As is evident from the figure, the peak acceleration from the equivalent linear analysis is higher than that from the nonlinear method. Indeed, with the equivalent linear methodology, the linear nature of the problem tends to exaggerate the result. Additionally, the deviation of the obtained PHAs from the nonlinear and equivalent linear methods was greater for the near-field earthquake record than for the far-field earthquake record. This is linked to the fact that the bedrock motion amplitude is larger in a near-field earthquake record (because of a shorter offset to the source of the earthquake) than in a far-field earthquake record. In fact, the motion of the bedrock dampens as one moves farther from the earthquake source, due to the effect of damping. As the bedrock acceleration increases, deformation of the soil layers increases, thereby intensifying the damping phenomenon and reinforcing the nonlinear behavior of the soil. Accordingly, the deviation of the estimated PHAs by the nonlinear and equivalent linear methods is minimized in far-field earthquake records where the bedrock acceleration is small.   Figure 10. PHA as a function of depth for: a) far-field analysis; b) near-field analysis. Fig. 11 indicates the amplification factor as a function of depth for near-field and far-field earthquake records, as approximated by the equivalent linear and nonlinear analyses. The difference in the results of the two methods is even clearer than that for the PHA. In particular, for the near-field earthquake records, the amplification at the ground surface was approximated at 3.7 and 1.8 by the equivalent linear and nonlinear methods, respectively. The corresponding figures for the far-field earthquake records were 3.8 and 3.1, respectively. That is, the deviation was as wide as 51 % for the near-field earthquake record but as low as 22 % for the far-field earthquake record.   Figure 11. Amplification factor as a function of depth. 3.2.               Velocity Velocity time histories provide an alternative insight into an earthquake event. Figs. 12 and 13 show velocity time histories for the reference points using the nonlinear and equivalent analyses under the influence of near-field and far-field earthquake records. The velocity time history is typically obtained by integrating the acceleration time history. The integration, however, tends to produce smooth and filtered outputs. This is why you can see that the velocity time history exhibits fewer high-frequency events than the acceleration time history [23]. Fig. 12 suggests that the velocity time history resulting from the near-field earthquake record exhibits sharply pulse-like motions. In contrast, Fig. 13 demonstrates that the velocity time history resulting from the far-field earthquake record lacks such pulses. This can be explained by the release of large amounts of energy in a short time upon faulting. That is, the short time of release is less than enough for such a vast amount of energy to spread over the entire time history. A comparison between the results of the nonlinear and equivalent linear analyses demonstrates that the amplitude of the velocity time history obtained from the nonlinear analysis is smaller than that from the equivalent linear analysis.   Figure 12. Velocity time histories at different reference points under the influence of the near-field earthquake record (Bam Station) from: a) nonlinear analysis; b) equivalent linear analysis. Figure 13. Velocity time histories at different reference points under the influence of the far-field earthquake record (Ravar Station) from: a) nonlinear analysis; b) equivalent linear analysis. Peak horizontal ground velocity (PHGV) is another parameter used to describe the amplitude of ground motions. The fact that the velocity is less sensitive to high-frequency components of the ground motions makes the PHGV at moderate frequencies a more accurate measure of ground motion amplitude than the peak horizontal ground acceleration (PHGA). For the structures or facilities that are sensitive to moderate frequencies (e.g., mid-rise or flexible structures, mid-span bridges, etc.), the PHGV is a more appropriate parameter for damage assessment, as compared to the PHGA. Fig. 14 depicts PHGV as a function of depth for the near-field and far-field earthquake records from the equivalent linear and nonlinear analyses. As seen, the equivalent linear method generally approximates higher PHGVs than the nonlinear method, with both equivalent linear and nonlinear methods producing higher PHGVs for the near-field earthquake record compared to the far-field earthquake record. This can be explained by the fact that, in a near-field earthquake record, the shorter offset to the epicenter implies that the bedrock motions are of larger amplitudes than in the far-field earthquake record. With increasing bedrock acceleration, soil layer deformations increase, thereby contributing to the nonlinear behavior of the soil. Therefore, in far-field earthquake records, the bedrock acceleration is usually low, the resulting strains at the soil level are small, and the PHGVs produced from the equivalent linear and nonlinear methods are very close to one another.   Figure 14. PHGV as a function of depth for; a) far-field earthquake record; b) near-field earthquake record. 3.3.               Displacement The application of displacement time histories for characterizing earthquake-induced ground motions is less common than the acceleration and velocity time histories [23]. Figs. 15 and 16 show displacement time histories for the reference points, as obtained from the nonlinear and equivalent linear analyses on the near-field and far-field earthquake records. A common practice for obtaining the displacement time history is to integrate the velocity time history. The integration has some smoothing and filtering effects. Therefore, as can be seen in the figures, the displacement time history exhibits fewer high-frequency events than the acceleration and velocity time histories. Another point to note is that the nonlinear analysis produces some permanent displacement, while the equivalent linear analysis does not predict such a permanent displacement and ends up with zero displacement at the end of the motion. This is linked to the elastic nature of the equivalent linear method that ignores the generation, redistribution, and elimination of pore water pressure, thereby producing zero permanent deformation. A comparison of the nonlinear and equivalent linear methods indicates that the displacement time history approximated by the nonlinear analysis exhibits smaller amplitudes than the equivalent linear analysis.   Figure 15. Displacement time histories for the reference points under the influence of near-field earthquake record (Bam Station) using: a) nonlinear analysis; b) equivalent linear analysis. Figure 16. Displacement time histories for the reference points under the influence of far-field earthquake record (Ravar Station) using using: a) nonlinear analysis; b) equivalent linear analysis. Peak horizontal ground displacement (PHGD) is a measure of characterizing ground motion amplitudes. The PHGD is associated with motion components of an earthquake at lower frequencies. As far as ground motion evaluation is concerned, the PHGD is a less common measure than the PHGV and PHGA. Nevertheless, the damage induced to highly soft structures is directly proportional to this parameter. Fig. 17 shows PHGD versus depth for near-field and far-field earthquake records, as obtained from equivalent linear and nonlinear analyses. The figure suggests that the equivalent linear method generally ends up with higher PHGD than the nonlinear method. Moreover, the deviation of PHGD between the equivalent linear and nonlinear methods is wider for the near-field earthquake records compared to the far-field earthquake records. This can be attributed to the fact that, in a near-field earthquake record, the shorter offset to the epicenter implies that the bedrock motions are of larger amplitudes than in the far-field earthquake record. With increasing bedrock acceleration, soil layer deformations increase, thereby contributing to the nonlinear behavior of the soil. Therefore, in far-field earthquake records, the bedrock acceleration is usually low, the resulting strains at the soil level are small, and the PHGDs produced by the equivalent linear and nonlinear methods are very close to one another.   Figure 17. PHGD as a function of depth for: a) far-field earthquake record; b) near-field earthquake record. For a better comparison of the results, PHGA, PHGV, and PHGD, together with their corresponding times at reference points, are listed in Table 1. These results imply that the soil layers can change the earthquake amplitude characteristics. The equivalent linear produces higher peak values of the amplitude parameters than the nonlinear method. Moreover, the deviation between the results of the two methods is greater for the near-field earthquake than for the far-field earthquake, which can be explained by the larger amplitude of the bedrock motion and the resultant nonlinear behavior of the soil. Table 1. Motion amplitude characteristics at reference points. Displacement Velocity Acceleration Depth   (s) (cm) (s) (cm/s2) (s) (g) (m) 2.70 28.87 3.11 88.47 2.65 0.79 35 A Equivalent linear Near-field 2.71 33.45 3.30 147.99 5.60 1.80 25 B 3.57 61.30 3.25 280.86 3.58 2.13 10 C 3.60 74.26 3.32 316.71 2.95 2.94 0 D 2.70 28.87 3.11 88.47 2.65 0.79 35 A Nonlinear 2.75 27.34 3.16 88.98 5.66 1.08 25 B 2.85 20.89 3.26 90.11 8.62 1.25 10 C 3.00 22.52 3.21 111.68 9.02 1.49 0 D 30.69 1.28 21.19 6.33 23.01 0.050 35 A Equivalent linear Far-field 30.65 1.01 23.31 4.31 24.11 0.033 25 B 30.50 0.59 23.37 1.65 18.46 0.017 10 C 30.39 0.53 30.00 1.71 15.38 0.013 0 D 32.78 0.58 27.91 3.85 17.70 0.040 35 A Nonlinear 30.52 0.50 30.21 2.00 18.27 0.022 25 B 30.40 0.53 17.35 1.80 17.43 0.021 10 C 30.39 0.53 30.00 1.71 15.38 0.013 0 D 3.4.               Response Spectrum The response spectrum describes the maximum response of a one-degree-of-freedom (1-DoF) system to a specific input motion as a function of natural frequency or natural period and damping ratio of the system. The response spectrum is an appropriate instrument for evaluating the maximum response of a 1-DoF system to an earthquake. The equation of motion for a 1-DoF structure under an earthquake stimulation is expressed as follows:                                                     (1) In which  is the natural angular velocity of the structure    is the ground acceleration,  is the damping ratio, and  is the displacement of a 1-DoF system. In order to plot the response spectrum, the equation of motion was solved for different natural frequencies, followed by obtaining the maximum of the response spectrum. In general, the response spectra include acceleration, velocity, displacement, pseudo-velocity, pseudo-acceleration, and pseudo-displacement, each of which describes the structural behavior in a particular way. For instance, the displacement spectrum provides information for calculating peak internal forces and deformation, the pseudo-velocity response spectrum is, however, associated with the maximum strain energy, and the pseudo-acceleration response spectrum is related to the lateral forces applied to the structure. The relationships among the displacement, pseudo-acceleration, and pseudo-velocity response spectra are expressed in the following:                                                                           (2)                                                                           (3) where  is the natural angular velocity of the structure,  is the displacement response spectrum,  is the pseudo-velocity response spectrum, and  is the pseudo-acceleration response spectrum [24–29]. Figs. 18 and 19 present response spectra of acceleration, velocity, and displacement, as obtained from the nonlinear and equivalent linear analyses for the near-field and far-field earthquake records, respectively. As can be seen in the figures, the spectra produced by the equivalent linear method exhibit larger values than those by the nonlinear method. This can be attributed to the elastic nature of the equivalent linear method, which results in higher amplification. Compared to the velocity and displacement response spectra, the acceleration response spectrum exhibits peak values in shorter periods. This finding indicates that the low-rise rigid structures (with short natural periods) are sensitive to acceleration, while moderate- and long-period structures are more sensitive to velocity and displacement, respectively. Comparing the spectra obtained from the near-field and far-field earthquake records, it was found that the maximum spectral amplitude for the near-field earthquake occurred in relatively shorter periods than that in the far-field earthquake. Indeed, near-field earthquake records exhibit higher frequency contents thanks to their shorter distance to the source of energy (no damping), while far-field earthquake records lack high-frequency contents due to the damping effect. This suggests that the near-field earthquakes impose further damage to highly stiff and low-rise (short-period) structures, while far-field earthquakes can induce more damage to less stiff high-rise (long-period) structures.   Figure 18. Response spectra for the near-field earthquake record (Bam Station): a) acceleration; b) velocity; c) displacement. Figure 19. Response spectra for the far-field earthquake record (Ravar Station): a) acceleration; b) velocity; c) displacement. Different response spectra (displacement, pseudo-velocity, and pseudo-acceleration) provide valuable information on the impact of earthquakes on structures. Given one of the mentioned response spectra, the other two can be derived. According to Equations (2) and (3), one can compile the spectra to come up with a mixed spectrum. Such a triple spectrum contains information about all of the three mentioned spectra. The horizontal axis of this spectrum indicates the period on a logarithmic scale, while the vertical axis refers to the pseudo-velocity values on a logarithmic scale. The bisectors of the first and second quarters indicate the axes of the displacement and pseudo-acceleration on logarithmic scales, respectively. In terms of the period, the triple response spectrum can be divided into three segments: short-period segment (0–0.5 sec) that is sensitive to the acceleration, moderate-period segment (0.5–3 sec) that is sensitive to velocity, and long-period segment (>3 sec) that is sensitive to displacement. Fig. 20 shows the triple response spectrum. As can be seen in this figure, the equivalent linear method produces higher results than the nonlinear method in almost all segments of the period. Moreover, the deviation of the results of the two methods is smaller for the far-field earthquake records.   Figure 20. Triple response spectrum. 3.5.               Natural Frequency The standard spectral ratio (SSR) is used to obtain the dominant frequency of the site. This parameter is calculated as the ratio of the Fourier amplitude spectrum of the time history of the soil to the time history of nearby bedrock subjected to the same earthquake and motion component [24–29]. The frequencies corresponding to the first and second peaks along the SSR graph indicate the first-mode (dominant) and second-mode frequencies, respectively [23]. Fig. 21 shows the amplification factor versus frequency. As is evident from this figure, the first-mode and second-mode frequencies of the studied site were approximated at 0.823 and 2.18 Hz, respectively.   Figure 21. Amplification factor versus frequency. 3.6.               Engineering Implications for Seismic Design The results of this study highlight the critical importance of considering the characteristics of earthquake field conditions in seismic design, particularly for structures located near active faults. The analyses demonstrated that near-fault ground motions generate significantly higher seismic demands due to their pulse-like behavior, high-frequency content, and larger acceleration amplitudes. Consequently, conventional design approaches based solely on equivalent linear assumptions or generalized design spectra may not accurately capture the actual seismic demand imposed on structures in near-fault regions. The findings indicate that short-period and stiff structures are especially vulnerable to near-fault earthquakes because the peak spectral responses of these motions occur at relatively higher frequencies. In contrast, far-fault earthquakes tend to impose greater demands on flexible and long-period structures. Therefore, the compatibility between the dominant frequency of the site, the frequency content of the earthquake motion, and the natural period of the structure should be explicitly evaluated during the design stage to avoid resonance-induced amplification. Another important implication of this research is the significant discrepancy observed between nonlinear and equivalent linear site response analyses under strong near-fault motions. Equivalent linear analysis generally overestimates amplification factors and peak response parameters because it cannot fully represent strain-dependent soil nonlinearity, stiffness degradation, hysteretic damping, and irreversible deformation mechanisms. As the intensity of ground motion increases, soil nonlinearity becomes more pronounced, particularly in shallow soft deposits, making nonlinear analysis more reliable for estimating realistic seismic demand. From a practical engineering perspective, the results suggest that nonlinear site response analysis should be considered mandatory for critical facilities and essential infrastructures located in near-fault seismic zones, including hospitals, bridges, tunnels, dams, nuclear facilities, transportation networks, and high-importance buildings. For ordinary structures located in moderate seismic regions, equivalent linear analysis may still provide acceptable preliminary estimations when soil strains remain relatively small. The obtained results also emphasize the necessity of incorporating site-specific seismic response analysis into performance-based seismic design frameworks. Since local soil conditions substantially modify acceleration, velocity, displacement, and spectral characteristics of ground motions, relying exclusively on code-based generalized spectra may lead to either unsafe or overly conservative designs. Site-specific analyses can improve the estimation of structural demand, story drift, foundation response, and seismic energy transfer mechanisms. Furthermore, the observed amplification patterns suggest that seismic codes may require additional modification factors for near-fault regions to account for pulse-type motions and strong nonlinear soil behavior. The results also indicate that evaluating only peak ground acceleration (PGA) is insufficient for seismic damage assessment. Parameters such as peak ground velocity (PGV), peak ground displacement (PGD), frequency content, and spectral characteristics should also be considered because different structural systems exhibit different sensitivities to acceleration-, velocity-, and displacement-dominated motions. Finally, this study demonstrates that accurate seismic hazard assessment requires simultaneous consideration of earthquake source characteristics, wave propagation effects, local soil conditions, and structural dynamic properties. Neglecting any of these interacting mechanisms may lead to inaccurate prediction of seismic demand and structural vulnerability, especially in near-fault environments where nonlinear soil behavior and pulse-like ground motions govern the seismic response.                                                                                                   4.     Conclusions In this research, the impacts of near-field and far-field earthquake records on the site response were evaluated by nonlinear and equivalent linear methods utilizing the PLAXIS and Deepsoil software tools. The analyses included the time histories of acceleration, velocity, and displacement, as well as their peak values and response spectra. A summary of the findings is presented in the following: 1.    Near-fault earthquakes produce stronger nonlinear soil behavior than far-fault motions. 2.    Near-fault motions shift peak response spectra toward higher frequencies, making short-period structures more vulnerable, whereas the opposite trend is generally observed for far-fault earthquakes, which tend to impose greater seismic demand on long-period and flexible structures due to their relatively lower-frequency content. 3.    The results demonstrate that equivalent linear analysis may significantly overestimate seismic amplification under strong near-fault motions due to its inability to fully capture nonlinear soil behavior. Therefore, nonlinear analysis provides a more reliable estimation for seismic assessment in near-fault regions. For ordinary structures located in moderate seismic regions, equivalent linear analysis may still provide acceptable preliminary estimations when soil strains remain relatively small. 4.    The discrepancy between equivalent linear and nonlinear analyses is substantially greater for near-fault records. This can be explained by the larger amplitude of ground motions in near-field earthquake records, which leads to extended damping and nonlinearity of the soil behavior. 5.    From an engineering perspective, the findings of this study emphasize the necessity of site-specific seismic analysis during the design stage of structures located in seismic zones. Furthermore, nonlinear site response analysis is strongly recommended for important structures and infrastructures in near-fault regions to achieve a more realistic estimation of seismic demand and potential structural damage. 10.34910/MCE.143.3 624 site response nonlinear analysis equivalent linear analysis near-field earthquake far-field earthquake https://engstroy.spbstu.ru/article/2026.143.3/

RAR RUS 14304-14304 Department of Civil Engineering, Faculty of Engineering, University of Anbar Sulaiman Hadeel hadeel.aldamag@uoanbar.edu.iq

Ramadi, Al Anbar, Iraq

56184497100 0000-0001-6180-8837 University of Anbar Al-Sharrad Muayad muayad.alsharrad@uoanbar.edu.iq

Ramadi, Iraq

University of Anbar Abed Idham ds.dr.idhamalassafii@uoanbar.edu.iq

Ramadi, Al Anbar, Iraq

Effect of microbial-induced calcite precipitation on hydraulic conductivity and strength of a sandy gypseous soil This work presents the effect of microbial induced calcite precipitation treatment on hydraulic conductivity and strength of a sandy gypseous soil with about 35 % gypsum content. The cementitious material was produced from a bacterial suspension containing the bacterial strain Bacillus Pasteurii PTCC 1645, mixed with or added to a cementation solution having the molarities 0.25, 0.5, and 1 M and prepared from a number of typically used chemical substances. The work resulted in a number of important outcomes. The microbial-induced calcite precipitation (MICP treatment), which yielded about 15 % calcite carbonate after 14 to 21 days of curing, can efficiently reduce the hydraulic conductivity, even for soils with high gypsum contents and relatively low fine content. In response to curing time increase from 7 to 21 days, the hydraulic conductivity decreased nearly twice. Likewise, as the calcite content increased to 15 %, the hydraulic conductivity decreased by almost fourfold. On the compressive strength front, the MICP treated specimens showed a promising uniaxial compressive strength of up to 600 kPa. This further reinforces the perception that MICP treatment by the Bacillus Pasteurii strain can be used effectively in treating gypseous soils problems. 1.Introduction Gypseous soils can demonstrate a continuous compression and even collapse under external loads. This phenomenon is usually triggered by gypsum softening, dissolution, and leaching due to certain human activities involving water percolation through soil mass and groundwater movement [1]. It has been well established that hydraulic conductivity is a key material property to be controlled in order to reduce the collapse severity of the gypseous soils. A numerous improvement methods have been suggested, by which permeability of the gypseous soil was either directly or indirectly reduced. According to [2], treating a soil containing 40–50 % gypsum content with kerosene can decrease permeability and compressibility by restricting the removal of gypsum from the soil mass. According to [3], adding 4 % automobile oil reduced the permeability coefficient by at least ten times. Al-Sharrad [4] studied the effect of cutback MC-60 on the permeability and compressibility characteristics of Ramadi sandy gypseous soil. Gypseous soils treated with the aforementioned materials have more likely exhibited a reduction in the coefficient of permeability, compressibility, and collapsibility. However, these materials are not considered sustainable due to their harmful effects on the environment and groundwater. In addition, most of these materials bear a density less than that of water and are therefore susceptible to washing by water movement within the soil mass, therefore affecting treatment durability. The microbial-induced calcite precipitation (MICP) is an up-and-coming technology for soil improvement. This technique involves using urease-producing bacteria mixed with cementation solution, and injected or blended with the soil. In geotechnical engineering, there has been a growing interest in this technology for soil improvement in recent years [5]. The MICP technology utilizes specific bacterial strains found in nature capable of depositing calcium carbonate to repair and fill cracks in concrete and rock materials. Additionally, the technology can help prevent leakage, soil erosion, slope failure, liquefaction of sandy soil [6–9]. The urea can be decomposed into CO32−, HCO3−, and NH4+ by urease secreted from the cell. When Ca2+ binds to CO32−, a large number of calcium carbonate crystals can be formed on the cell surface as given by Eqs. (1) to (3) [10]:                                                               (1)                                            (2)                                                  (3) The precipitated crystals in bio-cemented soil can have two primary functions; bonding at particle contacts and coating on particle surface [11]. Common microorganisms used in MICP for mineralization include: sulphate-reducing bacteria, denitrifying bacteria, oxidizing bacteria, and urease-producing bacteria [12]. Sulphate-reducing bacteria are primarily anaerobic heterotrophs that do not require oxygen for their metabolic activity [13]. They can reduce sulphates to sulphides while oxidizing organic carbon [14]. This process is represented by Eqs. (4) and (5).                              (4)                             (5) Various bacterial strains have been used successfully for calcite precipitation purposes. Bacillus Pasteurii has been the most used bacterial strain due to its superior characteristics such as its resistance to natural conditions. For instance, they are capable of producing spores, which enable them to survive extreme conditions such as high temperatures, exposure to hydrocarbon pollutants, and extended periods of drought [15]. Urease-producing bacteria are particularly useful in geotechnical engineering due to their low cost and high efficiency in providing cementation. Previous studies on MICP have shown that it has a bio-clogging effect, which reduces the permeability of treated soil samples by causing precipitation of calcium carbonate in the soil pore space [16, 17], and increases the material’s strength and stiffness [18–20]. The current study investigates the role of the MICP treatment in reducing hydraulic conductivity and increasing compressive strength of Ramadi sandy gypseous soil. This soil is typically characterized with profound deformable nature under soaking and water percolation conditions, which has resulted in severe serviceability and stability issues in the superstructures. 2.Materials 2.1.Soil Sample The soil used in this study was collected at a depth of 0.5 m from a site within the main campus of the University of Anbar. The study area is characterized by high contents of secondary-origin gypsum, ranging from