Effect of Na2O concentration and water/binder ratio on carbonation of alkali-activated slag/fly ash cements

Abstract

In this study, the effects of Na2O concentration and water/binder (w/b) ratio on the phase evolution and carbonation of alkali-activated slag/fly ash cements are studied. After sealed curing for 28 days, the alkali-activated cement powders were performed accelerated carbonation for 7, 14, 35, 60, and 120 days, at CO2 concentrations of 1.0 ± 0.2%, 20 ± 2 °C, and RH = 65 ± 2%. In alkali-activated slag at w/b ratio = 0.5, when the Na2O concentration increases from 4% to 8%, the amount of calcium carbonates after carbonation increases due to a higher activation degree of slag and the decalcification of more C-A-S-H gels, but the crystallinity of calcium carbonates decreases. At a fixed Na2O concentration of 6%, when the w/b ratio decreases from 0.5 to 0.3, the formation rate of calcium carbonates and the crystallinity of calcium carbonates decrease. When the content of slag is 60%, the formation rate of calcium carbonates decreases with the increased Na2O concentration from 6% to 8%, due to the higher reaction degree of fly ash and the formation of more N-A-S-H gels; and the amount of calcium carbonates increases with the decreased w/b ratio. When the content of slag is 20%, the increased Na2O concentration could decrease the formation rate of calcium carbonates significantly, but the effect of the w/b ratio is limited.