Synergistic Effects of Multiscale MgO Expansion Agent and SAP on the Mechanical and Shrinkage Properties of UHPC

Abstract

Because of its low water-to-binder ratio and high cementitious material content, ultrahigh performance concrete (UHPC) exhibits significant autogenous shrinkage at an early age, which makes shrinkage cracking a common occurrence. This study addresses this issue by combining a multiscale MgO expansion agent and a super absorbent polymer (SAP) internal curing material to significantly enhance or completely compensate for the early age autogenous shrinkage of UHPC. The effect of multiscale MgO expansion agent and SAP on the mechanical, working, and shrinkage properties of UHPC was investigated, and the synergistic mechanism of mixing multiscale expansion agent and SAP was elucidated using scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), mercury intrusion porosimetry (MIP), and thermal gravimetric (TG) analyses. The results indicate that the multiscale MgO expansion agent is more conducive to the development of the mechanical properties of UHPC than the single-scale expansion agent, and provides a constant expansion source throughout the entire hydration process of the UHPC system. Because of the additional water introduced by SAP, which supports the hydration of the cementitious material and the multiscale MgO expansion agent, the introduction of SAP can enhance the workability of UHPC while further reducing the autogenous shrinkage of UHPC at an early age. This study can provide statistical support and theoretical guidance for resolving the issue of significant autogenous shrinkage in the early stages of UHPC.