Effect of Predrying Temperature on Carbonation of Alkali-Activated Slag Pastes

According to standard testing methods, predrying of portland cement (PC) usually is required as a preconditioning step before the determination of carbonation. However, the hydration process, the hydration products, and the microstructure of alkali-activated slag (AAS) differ from those of PC, and little research has focused on the effects of conventional predrying methods (applying to PC) on the microstructure change and carbonation process of AAS. To study the effect of predrying temperature on the carbonation performance of alkali-activated slag, AAS pastes were prepared with the activator of sodium hydroxide (NaOH). The carbonation depth of AAS was characterized by phenolphthalein spraying, differential thermogravimetry (DTG), Fourier-transform infrared spectroscopy (FTIR), and X-ray computed tomography (CT) at predrying temperatures of 40 degrees C and 60 degrees C. The results showed that predrying temperature had a remarkable effect on the degree of drying in the matrix. The higher the drying temperature, the greater was the water loss rate due to the inward movement of the actual evaporation surface. Higher predrying temperature also resulted in changed hydration products, and more hydrotalcite was detected in the AAS specimens conditioned at a predrying temperature of 60 degrees C. Compared with the lower predrying temperature of 40 degrees C, the predrying temperature of 60 degrees C led to a coarse microstructure and cracks in AAS, resulting in greater carbonation.