Shrinkage mechanisms and shrinkage-mitigating strategies of alkali-activated slag composites: A critical review

Alkali-activated binders are considered as promising alternatives for ordinary Portland cement (OPC) binders due to their positive environmental effects and excellent mechanical properties and durability. However, significant shrinkage of alkali-activated composite systems, especially alkali-activated slag (AAS) systems, induces nonuniform deformation across the material to form tensile stresses and then causes the formation of harmful cracking. These harmful cracks can create pathways for various corrosive substances to penetrate the composites, seriously decreasing the load-bearing capacity and affecting the durability of concrete structures. Therefore, this paper presents a literature review on the shrinkage of AAS composites and systematically summarizes the shrinkage characteristics, shrinkage mechanisms, and shrinkage-mitigating strategies of AAS systems. The effects of various shrinkage-mitigating methods, i.e., reducing alkali activator modulus and alkaline content, incorporating mineral admixtures, adding chemical additives, toughening with fibres, introducing porous aggregates, and optimizing curing conditions, on the mechanical behavior and shrinkage of AAS systems were compared and analyzed. It is concluded that the shrinkage-reducing mechanisms obtained by applying these methods are mainly attributed to densifying the microstructures of the pastes, coarsening the pore structures, lowering the surface tension of the pore solution, and increasing the formation of crystalline phases (e.g., calcium hydroxide and ettringite).