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Alkali-activated materials (AAMs) are regarded as promising alternatives to Portland cement. However, the significant high shrinkage of AAMs, particularly in alkali-activated slag system, is a critical issue that hinders their wide application. This study investigated the effect of expansive admixture, i.e. CaO on the autogenous shrinkage and drying shrinkage of alkali-activated slag/ fly ash microsphere (AASFAM). The effect of CaO content on heat evolution, reaction products, microstructure development and compressive strength was systematically investigated. The results revealed that the addition of CaO can effectively reduce both the autogenous and drying shrinkage of AASFAM. Mixture with 3% and 5% CaO exhibited expansion within the first 24 hours after casting, while the drying shrinkage was reduced by 72% and 82%, respectively. The addition of CaO promoted the formation of crystalline phases including Ca(OH) 2 , hydrotalcite and calcite, leading to a lower free water content and coarser pore structure, which in turn reduced the capillary stress during the drying process. The results demonstrated the promising potential of CaO in effectively mitigating the high autogenous shrinkage and drying shrinkage of AAMs without compromising its mechanical properties. • The addition of CaO can effectively reduce both the autogenous and drying shrinkage of AASFAM. • The addition of CaO facilitates the crystalline phases in AASFAM including Ca(OH) 2 , hydrotalcite and calcite. • The formation of Ca (OH) 2 led to a larger pore size distribution. [ABSTRACT FROM AUTHOR]