Shrinkage-reducing measures and mechanisms analysis for alkali-activated coal gangue-slag mortar at room temperature

Abstract

By using slag as a calcium source, high-strength alkali-activated coal gangue-slag (AACGS) cementitious material can be prepared. Given that higher drying shrinkage at room temperature is detrimental to the expanded engineering application of the material, this paper studies several methods for reducing drying shrinkage such as shrinkage-reducing admixture (SRA), polypropylene fiber (PPF), sulfate-enriched materials (gypsum, high-performance concrete expansive admixture (HCSA), U-type expansive admixture (UEA)) and water retaining admixture (WRA). Meanwhile, the mechanisms for drying shrinkage compensation were deeply analyzed by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Experimental results show that the addition of admixtures to reduce the drying shrinkage of AACGS mortar is related to the change of pore size of 2–5 nm, and reduces the volume fraction of the minimum aperture. At room temperature, reducing the surface tension of pore solution, generating expansion products, increasing compressive strength and improving pore structure distribution by adding admixtures are important measures to reduce drying shrinkage. SRA exhibits the strongest capacity to compensate for drying shrinkage, followed by UEA, WRA, HCSA, PPF and gypsum. Despite better compensation for drying shrinkage with the increase in SRA dosage, the reduction of strength value becomes more drastic. The findings of this study provide substantial experimental foundations and in-depth theoretical analyses for the drying shrinkage compensation of AACGS binary cementitious materials.