Investigation of shrinkage mechanism of alkali-activated slag

Slag is a highly active waste product that can be added to alkali-activated cement to improve their mechanical properties, but also creates significant shrinkage, which limits its usefulness. This study examines the reasons behind the shrinkage of alkali-activated slag (AAS). The research uses a comprehensive approach that includes linear shrinkage testing, comparative chemical analysis, microscopic structural characterization, internal relative humidity evaluations, pore size distribution assessments, and nanoindentation testing. The results show that AAS has more chemical shrinkage, self-drying shrinkage, and drying shrinkage than ordinary Portland cement. AAS’s shrinkage deformation increases with variations in the activator modulus. Chemical reactions that produce aluminosilicate gel with smaller particle sizes and lower bound water are the primary factors contributing to chemical shrinkage in AAS. The high self-drying and drying shrinkage of AAS can be attributed to several factors: AAS experiences a more rapid decrease in internal humidity during the early curing stages, AAS materials typically exhibit smaller most probable pore sizes, and hardened AAS products tend to have a lower creep modulus. The addition of oil shale semi-coke (OSS) to AAS can modify the composition and microstructure of the reaction products, resulting in an increase in porous reaction products and ultimately reducing shrinkage. This study's findings are significant for understanding the causes and potential solutions for alkali-activated cement shrinkage issues.