Effects of sodium citrate on compressive strength and microstructure of NaOH-activated fly ash/slag cement exposed to high temperature

Abstract

This paper reports the effects of sodium citrate on compressive strength and microstructure of NaOH-activated fly ash/slag cement at elevated temperatures from 200 to 800 °C. The binders were prepared by blends of fly ash/ground granulated blast furnace slag at a ratio of 50/50 wt%, and a NaOH/sodium citrate solution. Specimens free of loading were heated at a constantheating rate of 5 °C/min to a target temperature in a furnace until the specimens reached a steady state (1 h). Then the compressive strength and volume changes of heated samples were tested. The characterization of the fly ash/slag cement after thermal treatment were investigated by XRD, DTA, FTIR, SEM-EDX, MIP and Micro-CT. Our results suggest that: First, the use of sodium citrate in NaOH-activated fly ash/slag cement can result in a more densified and less porous matrix with much higher compressive strength; Second, the sodium citrate-affected specimens had higher residual strength of up to 600 °C and a better volume stability at below 700 °C; Third, similar chemical process occurred in both the sodium citrate-affected pastes and the NaOH-activated samples during thermal treatment, such as the decomposition of gel products, and the recrystallization of new phases. However, different volume changes were observed in the former samples (expansion) and in the latter reference pastes (shrinkage), indicating that the significant effect of sodium citrate on the microstructure changes of NaOH-activated fly ash/slag cement during the high-temperature treatment. It is perhaps due to its initial denser microstructure with lesser and smaller pores, which can influence the transportation of water in the matrix at high temperature.