Effect of calcium sulfoaluminate in fly ash–metakaolin blended binders on hydration and mechanical properties

Abstract

Advancements in sustainable cement technology rely on the use of supplementary cementitious materials to enhance performance and reduce environmental impact. Mineral additives such as fly ash (FA), blast-furnace slag, silica fume and limestone are commonly used to lower clinker content, refine microstructure and improve durability. Among these, calcium sulfoaluminate (CSA) expansive agents are widely applied in shrinkage-compensating concretes, grouts and repair mortars, as they induce controlled early expansion that mitigates shrinkage and cracking. This study investigates the effect of CSA on the hydration and mechanical behaviour of cementitious systems containing FA and metakaolin (MK). The binders consist of CEM II cement with 12% limestone filler, combined with FA and MK, and are analysed using X-ray diffraction, thermogravimetric analysis, isothermal calorimetry and cement Gibbs energy minimisation software modelling. Results show that CSA promotes early ettringite formation and accelerates hydration, increasing overall product formation but slightly reducing C–S–H content and compressive strength while enhancing the flexural strength. The combined use of CSA, FA and MK yields balanced reactivity, with early acceleration from CSA and later pozzolanic activity, achieving strengths comparable to the reference system.