Influence of Curing Humidity on the Strength and Microstructure of Cement Paste Incorporating Metakaolin and Limestone

In this study, the effect of curing humidity on mechanical performance, hydration as well as microstructure for cement paste containing metakaolin (MK) and limestone powder (LP) with various water to binder (w/b) ratios, such as 0.24, 0.36, and 0.48, were investigated. A total of 18 types of slurries were prepared and cured under three levels of humidity [55% relative humidity (RH), 95% RH, and water curing at 20 degrees C] for 28 days. The compressive and flexural strengths were measured to evaluate the influence of humidity on the strength performances of pastes. The MK reaction degree of pastes was tested using the method of selective dissolution in hydrochloric acid. X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) tests were employed to evaluate the hydration and microstructure of paste. Research results demonstrated that the strengths of pastes generally decreased with curing humidity. Curing at 55% RH can obviously degrade the flexural strength of pastes compared with curing at 95% RH and water curing. The influence of humidity on the compressive strength of MK-containing pastes decreased with the decrease of w/b. The compressive strength of 0.24 w/b pastes containing MK at various humidities were similar. Increasing humidity can enhance the MK reaction degree of pastes with different w/b ratios, which is responsible for the enhanced strength tendency of pastes with humidity. Similar to the compressive strength tendency, humidity had insignificant effects on the MK pozzolanic reaction of pastes with a low w/b ratio. XRD results demonstrated that raising humidity can further increase the synergistic effect of MK and LP. The quantities of monocarboaluminate (Mc) and hemicarboaluminate (Hc) in paste with a high w/b under water curing were greater than those in pastes cured at 55% RH. The microstructures of pastes investigated using MIP and SEM tests also corresponded well to the results of mechanical performances. The findings of this study can provide some guidance for manufacturing normal concrete (NC), high-performance concrete (HPC) and ultrahigh-performance concrete (UHPC) incorporating MK and LP under different levels of humidity.