This paper presents the effect of compaction pressure level and wheat straw addition on the durability, mechanical behavior, and thermal conductivity of geopolymer stabilized rammed earth. Rammed earth specimens were prepared by static compaction to 5, 10, and 25 MPa of mixtures containing predefined amounts of sand, silt, clay, and wheat straw, stabilized with fly ash geopolymer. A number of unstabilized specimens made of the raw materials were also prepared for comparison. These specimens were cured inside plastic bags at 35 °C so that the least energy consumption is achieved. The durability was investigated by performing a dip test and spray test. The results of the geopolymer stabilized specimens demonstrated an excellent resistance to erosion by water, unlike the unstabilized specimens, which almost failed completely. The mechanical behavior was evaluated by performing unconfined compression test. The results indicated that material’s stiffness and strength increased considerably with increasing compaction pressure and curing age, with the majority of the increase occurring during the first month of curing. Compressive strength values of 4.2 and 10 MPa were recorded from tests on stabilized specimens compacted to 5 and 25 MPa, respectively, then cured for 60 days. These figures are promising, keeping that, a threshold unconfined compressive strength of 1–2 MPa is typically acceptable by many building codes. A relatively low thermal conductivity of about 0.35–0.5 W/(K.m) was recorded from the hot wire method on specimens prepared from various mixtures, suggesting that the stabilized rammed earth outperforms most of the traditional building materials such as concrete. The inclusion of wheat straw improved material’s ductility by increasing strains, at which shear failure occurs. However, this inclusion led to undesirable reduction in strength and stiffness over the first two months of curing and almost no change in thermal conductivity, with respect to those obtained on the stabilized specimens. This response was attributed to material wise incompatibilities.
1.Introduction
Rammed earth is a technique that uses sand, silt, clay, and sometimes gravel as raw materials by compacting them into forms to construct walls. Since ancient times, humans have used earthen materials in construction. They are available at a low cost and have good mechanical and insulation properties so they have been used as an excellent solution for construction throughout history [1–3]. Recently, rammed earth construction has become more popular in the sustainability arena since the materials are still inexpensive and locally available [4], and well suitable for mixing with stabilizers such as lime [5, 6], waste materials, and factory side products such as fly ash [7, 8], or similar mixtures [9–15].
Although rammed earth has limited compressive strength, it has often been used for structural purposes [16]. For this reason, rammed earth is often stabilized using Portland cement. The use of Portland cement has a negative environmental impact, as cement production consumes a lot of energy and water, which represents 9 % of the global consumption of industrial water [17, 18], and it is responsible for producing 5 % of harmful gases every year [19]. Hence, it is indispensable to search for alternative stabilizers such as fly ash, crushed bricks, or natural additives (e.g., natural fibers) [18, 20]. The use of stabilizing materials has a positive effect on improving hygroscopic properties and reducing thermal conductivity [21]. According to [17, 22], thermal conductivity decreases when the wheat straw is added for rammed stabilization. On the other hand, the use of wheat straw reduces compressive strength compared to samples that do not contain wheat straw.
The current study explores the effect of compaction pressure and the addition of wheat straw on several properties of rammed earth, i.e., durability, strength, and thermal conductivity. In practice, reducing the compaction pressure to a level as low as 5 MPa has many advantages, including lower production costs and improved thermal insulation.
2.Methods and Materials
2.1.Materials
Soils. The soils were obtained from quarries near Ramadi, where sufficient quantities of coarse and fine soils were collected and transported inside plastic bags to the soil laboratory at the University of Anbar (Fig. 1a and 1b). The soils were dried at 110 °C, crushed, then samples were taken for classification. Table 1 shows index properties of these soils. Soil 1 was classified as poorly graded sand, while Soil 2 was classified as fat clay.
Figure 1. Materials used in this work: a) Soil 1; b) Soil 2; c) crushed wheat straw; d) fly ash and NaOH.
Table 1. Index properties of Soil 1 and Soil 2.
Property
Soil 1
Soil 2
Specification
Gravel (>4.75 mm, %)
0
0
ASTM D422-2007 [23]
Sand (4.75–0.075 mm, %)
100
0
Silt (0.075–0.005 mm, %)
0
35
Clay (Compaction Characteristics
(a)
(b)
Figure 5. Compaction characteristics of: a) raw material; b) stabilized material.
< >Durability
Time (min) of test
Depth of erosion, D (mm)
Criteria
Erodibility index
Unstabilized material
Stabilized material
15
10
≈ 0.0
0 ≤ D < 20
20 ≤ D < 50
50 ≤ D < 90
90 ≤ D < 120
D ≤ 120
1
2
3
4
5 (fail)
30
18
45
24
60
30
< >Unconfined Compression
(a)
(b)
Figure 7. Typical stress-strain curves after 30 days of curing for: a) stabilized material without wheat straw; b) stabilized material with wheat straw.
Figure 8. Variation of the average compressive strength with curing age for: a) stabilized material without wheat straw; b) stabilized material with wheat straw.
Figs. 9 and 10 analyze the effect of compaction pressure as well as curing age on the elastic modulus and compressive strength, respectively. The geopolymer acted as a cementitious material, which provided additional bonding to sand particles and, by other words, restricting the relative movement at the interparticle contacts. As a result, the geopolymer-stabilized specimens exhibited very much stiffer response than raw material compacted to the same pressure. For example, the specimen of raw material, which was compacted to 25 MPa, displayed an elastic modulus of only about 16 MPa, compared to 1306 and 560 MPa displayed by the corresponding specimens without and with wheat straw, respectively. This also indicates that wheat straw inclusion led to a large decrease in the elastic modulus. The elastic modulus increased substantially over the first month with curing then showed lesser increase by the end of the second month.
Inspection of Fig. 10 indicates that compressive strength values of the stabilized specimens were remarkably higher than those made of the raw material, owing to the cementation function of the geopolymer. Quantitatively, an increase of about one to two orders of magnitude can always be observed in the compressive strength.
(a)
(b)
Figure 9. Variation of the elastic modulus with compaction pressure: a) without wheat straw b) with wheat straw.
(a)
(b)
Figure 10. Variation of the compressive strength with compaction pressure: a) without wheat straw b) with wheat straw.
< >Thermal Conductivity SEM ImagingOverall, the stabilized rammed earth examined in this work demonstrated promising durability, mechanical, and thermal properties. For instance, a threshold unconfined compressive strength of 1–2 MPa is recommended by many building codes. In addition, the average thermal conductivity of the geopolymer stabilized rammed earth is much lower than that of many building materials such as ordinary concrete, which has a thermal conductivity of about 1.5–3.5 W/(m.K).