Influence of reactive MgO and silicate modulus on alkali-activated GGBS-MK: Analysis of macro-micro properties and hydration mechanisms

In view of the critical involvement of MgO in the hydration of alkali-activated materials (AAMs), a rigorous understanding of its functional role and regulatory influence is essential. This study, therefore, focuses on the effects of reactive MgO (R-MgO) and it's sodium silicate modulus (Ms) on the workability, mechanical performance, and chemical shrinkage of GGBS-MK based AAMs, with the hydration mechanisms further interpreted through microstructural analysis and thermodynamic modeling using the GEM-Selektor 3 software (GEMS). The results indicate that both excessively high and low Ms (Ms = 2.0 and 1.0) lead to a reduction in paste strength after curing 7 days, while chemical shrinkage at 72 h increases as Ms decreases. The incorporation of R-MgO into AAMs does not exert a single effect; an appropriate amount of R-MgO (10% MgO content) increased the reactivity of the paste, leading to the generation of more hydration products. Additionally, the hydration of R-MgO produced expansive products that effectively reduced the chemical shrinkage of the paste and refined the pore structure. The paste containing 10% R-MgO exhibited a 31% improvement in compressive strength after curing 7 days compared to the control with 1.6% R-MgO, with chemical shrinkage reduced by 39%. Nevertheless, excessive R-MgO (18.4% MgO content) leads to the deterioration of the paste pore structure, resulting in a reduction in strength. Moreover, the hydration products identified in the paste exhibit substantial agreement with those predicted by GEMS simulations. These findings provide profound insights into the roles of R-MgO and Ms in alkali-activated materials and offer guidance for the development of alkali-activated materials utilizing composite activators.