Effect of sodium citrate on compressive strength and microstructure of NaOH-activated slag pastes/mortars exposed to high temperatures

Abstract

Fire resistance is a critical consideration in the performance of concrete. This study investigated the impact of sodium citrate on the compressive strength and microstructure of NaOH-activated slag cement at temperatures ranging from 200 to 1000 °C. The findings indicated that the addition of sodium citrate led to an increase in matrix density and a decrease in porosity, ultimately resulting in a notable enhancement compressive strength for both AAS paste and mortar samples. Nevertheless, this condition was not conducive to the high temperature resistance of AAS paste due to the elevated internal pressure resulting from the vaporized water within the matrix pores. The incorporation of fine aggregates could notably enhance this capability, particularly in sodium citrate-activated specimens. This enhancement can be attributed to the optimization of the pore structure of the sand, facilitating the elimination of free water without compromising the integrity of the structure and minimizing the occurrence of micro-cracks. Furthermore, the elevated temperature of 200 °C facilitated the AAS reaction, resulting in an increased formation of gel phases.