INFLUENCE OF ALKALI ACTIVATORS ON THE CARBONATION RESISTANCE OF SUPERSULFATED CEMENT

Supersulfated cement (SSC) is a typical low-carbon cement that has not been widely used due partially to its poor resistance to carbonation. This study investigates the role of cement (the alkaline activator) content (1% 3% 5% 7%) on its carbonation resistance. The evolution of the compressive strength and phase assemblage as a function of the carbonation duration (1 day, 3 days, and 7 days) were analysed using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FT-IR). The results show that a lower cement content results in a higher gain in the compressive strength and carbonation resistance. This was attributed to the higher amount of hydration products (ettringite (AFt) and C-(A)-S-H gels) in the SSC with a lower alkaline activator content. The compressive strength of the SSC samples decreases with the carbonation time (except for the C1 sample), possibly due to the quick decomposition of the main hydration products AFt and the decalcification of the C-(A)-S-H gels. For the C1 sample, the compressive strength increases significantly after 3 days of carbonation and decreases later on. Interestingly, the reduction in the compressive strength of all the samples coincided with the generation of low Ca/Si C-(A)-S-H gels as denoted by the peak at around 825 °C on the TG curve. It is proposed that the carbonation of SSC could be divided into two phases: First, the amorphous silica and aluminate gels produced by the decomposition of AFt and decalcification of C-(A)-S-H gels densifies the pastes, increasing the strength of the SSC; Second, the rapid degradation occurs by the transformation of C-(A)-S-H gels into low Ca/Si C-(A)-S-H gels. The findings indicate that it could be a useful method to increase the amount of C-(A)-S-H gels to improve its carbonation resistance by choosing the proper alkali activator content dosage (1% in this study).