Effect of shrinkage reducing admixture on early expansion and strength evolution of calcium sulfoaluminate blended cement

Abstract

This research examines the influence of presence of shrinkage reducing admixture (SRA) on expansion, hydration, and strength evolution of calcium sulfoaluminate-based expansive agent (CSA-based EX) blended with ordinary portland cement (OPC) in the absence of initial moist curing. The performance of inclusion of SRA in OPC-CSA system and without moist curing was compared to the moist-cured OPC-CSA system made without any SRA. The investigation uses the measurements of cement hydration from isothermal calorimetry, autogenous and drying volume changes, internal relative humidity, X-ray diffraction (XRD), and thermogravimetry, as well as compressive strength to characterize the expansion and hydration of OPC-CSA mortar systems. The results indicate that under unsealed conditions, the expansion and strength evolution of OPC-CSA system is significantly sensitive to the presence of moist curing. Mortar containing 15% CSA-based EX resulted in no expansion and had comparable magnitude of total deformation to that of the plain OPC system when no moist curing was adopted. This is linked to the: (i) reduced ettringite formation; and (ii) decreased stiffness which translates to the lower resistance to deformation. Analysis of calorimetry, XRD pattern, and thermogravimetry indicates the synergistic effect of using SRA in OPC-CSA system, which contributes towards higher early-age expansion and increased later-age strength compared to the similar system made without any SRA. In the absence of moist curing, the OPC-CSA system containing SRA exhibited 20% higher non-evaporable water content than OPC-CSA system alone, confirming the effect of SRA addition on hydration reaction progress of OPC-CSA system. The outcomes of this research provide insights into formulation of CSA-based suspensions with the aim of reduced shrinkage, when insufficient initial moist curing is applied.