Potentialities of magnesium phosphate cement-based ultra-high-performance concrete in extremely cold weather construction

Abstract

The aim of this study was to design serviceable ultra-high-performance concrete (UHPC) based on magnesium phosphate cement (MPC) in severely cold weather and investigate the effects of different temperature conditions on UHPC performance, including constant temperatures, natural temperatures, and variations from sub-zero to normal-temperature conditions. The MPC composition was first modified for higher early-age performance, and then the mix proportion of MPC-UHPC was designed based on the modified MPC. Compared with the water content calculated based on the complete consumption of phosphates, optimal mechanical properties were achieved with the lowest water content necessary for acceptable moulding. The designed UHPC was prepared and cured under various low-temperature conditions to investigate its strength development and microstructural maturation. It developed 120 MPa above compressive strength and 24 MPa above flexural strength after 28 d under constant −10 ℃ or the daily fluctuating temperatures of the freezing Harbin winter (−27 to 3 ℃). At low temperatures, the strength was attributed to a combination of MPC hydration, pore solution freezing, and steel fibre reinforcement. However, this study was the first to discover degradation in flexural strength and impact ductility when frozen specimens were moved to warm environments, even though the microstructure was continuously maturing, presenting an increase in hydration products and a decrease in capillary porosity. This paradox was attributed to freezing regression. Finally, the shrinkage and durability of UHPC were tested and verified to meet the requirements of the relevant standards. These findings convincingly demonstrate that MPC-UHPC with an appropriate mix proportion is ideal for winter construction in severely cold regions.