Effects of sodium chloride on the mechanical strength of alkali activated volcanic ash and slag pastes under room and elevated temperatures

This study investigated effects of the sodium chloride on the microstructural and mechanical properties of alkali activated volcanic ash (VA) and ground granulated blast furnace slag as well as Portland cement pastes. To this end, unconfined compressive strength (UCS), scanning electron microscopy (SEM)-EDS-Mapping, FTIR, and XRD tests were conducted. Furthermore, effects of curing temperature on the binding capacity of chloride in alkali activated cements were examined in both elevated (HT) and room (RT) temperature conditions. The VA was replaced by slag at 0, 50, and 100 wt%. Based on the results, samples containing 100 wt% slag showed the highest mechanical strength in both curing conditions. Besides, addition of sodium chloride from 0 to 10 wt% did not significantly affect the strength of samples containing 100 wt% volcanic ash in both curing conditions. On the other hand, in HT condition, mechanical strength of samples containing 50 and 100 wt% slag, as well as Portland cement pastes increased with increasing sodium chloride from 0 to 2.5 wt%, and further addition of sodium chloride by up to 10 wt% led to a reduction in their strength. However, compressive strength of samples containing 50 and 100 wt% slag, as well as Portland cement samples, decreased with the addition of sodium chloride from 0 to 10 wt% in RT condition. Microstructural investigations were conducted, aiming to find the mechanism controlling the reactions. It was found that (N,C)-A-S-H and C-S-H gels were the dominant factor in the solidification and encapsulation of chloride ions in slag-based samples.