Study on the improvement of anti-carbonation performance of (modified) high volume fly ash cement mortars

Carbonation of high volume fly ash (HVFA) cement cementitious materials has been reported to cause corrosion and cracking of reinforcing bars in structures. In addition, although the early strength of the modified high volume fly ash (MHVFA) cement mortars with the addition of nano-silica (NS) and silica fume (SF) is significantly enhanced, its strong pozzolanic reaction consumes a large amount of calcium hydroxide (CH), which may further decrease carbonation resistance. In this paper, the effects of hydrated lime (HL), calcined hydrotalcite (C-HT), magnesium oxide expansive agent (MEA) and polypropylene fibers (PPF) on the carbonation resistance of HVFA cement mortars and MHVFA cement mortars were investigated, the possible ways of carbonation resistance enhancement were explored both chemically and physically. In addition to fluidity and compressive strength, the anti-carbonation effect of the four additives was evaluated using accelerated carbonation test and pH test, and the anti-carbonation behavior was further analyzed by x-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen (N2) adsorption/desorption test. The test results showed that the additives with the best effect in enhancing the mechanical properties and carbonation resistance of (M)HVFA cement mortars were HL for HVFA cement mortars and MEA for MHVFA cement mortars, respectively.