Understanding synergistic effects between MgO expansive agents and hydrophilic nano-silica in UHPC

Abstract

Ultra-high performance concrete (UHPC) incorporating MgO expansive agents (MEAs) demonstrates reduced early shrinkage but can potentially compromise mechanical strength, which may lead to serious quality incidents during early construction. Nanomaterials, with small particles, large specific surface area, and high hydration activity, have proven effective in enhancing the mechanical properties of cementitious materials. Nonetheless, limited research exists on the impact of nanomaterials on the performance of MEA-containing UHPC. This study aims to enhance the mechanical strength of the hybrid system by incorporating nanomaterials, specifically hydrophilic nano-SiO2 (HNS). The effect of HNS and/or MEA on UHPC performance was systematically investigated through various tests, including heat of hydration, MIP, XRD, TG-DTG, FTIR, and SEM-EDS. The results show that HNS improves the hydration process of cement, and effectively increases the compressive strength of MEA-containing UHPC by up to 30.87 % at 3 days. The improvement of the mechanical properties of MEA-UHPC was attributed to the filling effect of the unreacted HNS, which effectively enhanced the densification of the matrix. Additionally, the crystallization and nucleation effect of hydrated HNS produced more C–S–H gels with high stiffness, which resisted the stresses generated by the micro-expansion of MEA. Importantly, in terms of less shrinkage, HNS exhibits a synergistic effect with MEA, wherein MEA provides the necessary weakly alkaline environment for the dissolution of reactive silica, while HNS appropriately increases the initial hydration degree of MEA. These combined effects further reduce the early shrinkage of UHPC, consequently lowering the risk of cracking. The collective improvement of all the aforementioned results suggests that HNS holds significant potential for application in MEA-containing UHPC.