Effect of tidal zone and seawater attack on high-volume fly ash pastes enhanced with metakaolin and quartz powder in the marine environment

Abstract The behavior of high-volume fly ash (HVFA) cement pastes modified with metakaolin (MK), quartz powder (Q-P) and their combination in the marine environment was studied. HVFA pastes were manufactured by partially replacing Portland cement (PC) with 70% fly ash (FA). To develop the mechanical performance of this mixture, HVFA was modified by partially replacing FA with 20% MK, 10% MK coupled with 10% Q-P and 20% Q-P. After initial curing, the specimens were transferred to different treatment media, namely air, tap water, seawater and wet/dry cycles for 9 M. In the current procedure, the wet/dry cycle was used to simulate the condition of the tidal zone, of which the specimens were immersed in seawater for 18 h followed by drying for another 6 h per day. The phase composition and microscopic structure of the blended cement composites were studied using X-ray diffraction (XRD), thermogravimetric analysis (TGA/DTG) and scanning electron microscopy (SEM). The results indicated that the deterioration in the specimens exposed to the tidal zone is more severe than their counterparts exposed to seawater attack. Whereas the inclusion of 20% Q-P in the blends resulted in the lowest deterioration, followed by that of 10% MK coupled with 10% Q-P and 20% MK, respectively. The results showed that HVFA specimens blended with 20% Q-P demonstrated superior stability in terms of compressive strength and microstructure among all examined specimens. Accordingly, HVFA blended cement improved with Q-P can be applied as coverings in building structures close to coastal areas to cope with harsh conditions.