Effect of reactive magnesium oxide in alkali-activated fly ash mortars exposed to accelerated CO2 curing

Abstract

The effect of reactive magnesium oxide (MgO) as partial fly ash replacement in alkali-activated fly ash mortars (0, 10 and 20 %) was studied under two different environments: conventional climatic chamber and climatic carbonation chamber with 0.04 and 5 % level of CO2 respectively. A complete characterisation of all raw materials was performed. The effect of replacing fly ash with MgO resulted in a decline of compressive (up to 52.3 % at 28 days of age and 20% of MgO replacement) and flexural strength, ultrasonic pulse velocity and dynamic Young’s modulus mainly due to the formation of weak and expansive brucite, detected by X-ray diffraction (XRD). Carbonation depth and shrinkage also were obtained. CO2 curing improved the physicomechanical properties, due to the formation of nesquehonite in samples with MgO. An improvement of CO2 absorption of 2 g CO2/kg mixes with the CO2 curing and the substitution of fly ash with MgO was obtained (determined by thermogravimetric and differential thermal analysis). The increase in mass due to CO2 capture was of 6.91%, 9.39% and 12.26% at the age of 7, 14 and 28 days, respectively, when compared to mixes cured with 0.04 (reference) and 5 % (CO2 curing) level of CO2.