Admixtures for low-carbon cements

Low-carbon cements with reduced clinker factors increasingly rely on high-specific-surface-area supplementary cementitious materials (SCMs), notably calcined clays and emerging recycled fines. These binders often exhibit increased water demand, faster workability loss, and reduced early strength relative to ordinary Portland cement (OPC) and blended cements with fly ash or slag, thereby challenging the robustness of conventional admixture formulations. This review synthesizes current mechanistic understanding of how superplasticizers and hydration-controlling admixtures govern hydration kinetics and fresh-state performance in composite cements, with an emphasis on calcined-clay systems. For polycarboxylate ether (PCE) superplasticizers, we discuss structure–activity concepts in terms of adsorption, evolving surface area during early ettringite formation, and the effects on C₃S dissolution and C-S-H/CH nucleation. For accelerators and strength-enhancing admixtures, we evaluate calcium salts, alkanolamines, and C-S-H seeding, highlighting their impact on the hydration of SCM-rich binders. For retarders, we compare dissolution-, nucleation-, and growth-controlled mechanisms and explain how the presence of SCMs alters them. Finally, we connect the hydration processes of low-carbon cements to rheology through surface coverage, time-dependent specific surface area, particle packing, and the percolation threshold, and identify key knowledge gaps for predictive admixture design in low-carbon concretes.