Shrinkage of ultra-high performance concrete with superabsorbent polymers

During the period of hydration and hardening in ultra-high performance concrete (UHPC), a number of destructive processes occur that affect the formation of the structure of the material, which ultimately seriously affects its technical properties and performances. In this regard, the article explores the possibility of increasing the effectiveness of UHPC through the use of polymineral binders (PB) and superabsorbent polymers (SAP). For this, such characteristics as the effect of SAP on the plastic shrinkage of cement paste were investigated; hydration and hardening of cement in the presence of SAP; rheological and physical-mechanical properties of the cement system with the addition of SAP; shrinkage deformations of UHPC. It has been proven that SAP is an effective means of reducing shrinkage deformations of the cement system at the initial stage of structure formation, which helps to reduce the number of destructive phenomena of cement paste. The additive with a dispersion of more than 200 µm reduced vertical deformations by approximately 30%, and with a dispersion of less than 200 µm, by 50%. Despite the fact that SAP does not have a chemical effect on the process of cement hydration. However, the introduction of an additive in an amount of more than 0.1% by weight of PB leads to a significant increase in the setting time (period of active hydration) through the absorption and desorption of water. After 17 and 24 hours after watering, the crystals of neoplasms in the control samples without additives developed more rapidly and had a larger size. However, 41 hours after mixing with water, the larger size of hydration products is typical for samples containing SAP. The introduction of SAP into the composition of cement paste leads to the formation of closed pores filled, depending on storage conditions, with water or air, and a decrease in  mechanical properties (by 10-20%). A technological line for the production of UHPC based on polymineral binders using SAP has been developed, it consists of three main stages - the preparation of the polymineral binder, UHPC with desired properties and the manufacture of products from it.