Study on the strength development, hydration process and carbonation process of NaOH-activated Pisha Sandstone

Pisha Sandstone which caused severe soil erosion is a kind of arenite formed during the Tertiary period in northwest China. Pisha Sandstone when combined with suitable alkali activators and mineral additives can be converted into alkali-activated cement. The compressive strength of alkali-activated Pisha Sandstone at different slag replacement levels, different NaOH dosages and ages were examined. Scanning electron microscopy (SEM)/energy dispersive X-ray (EDS), Fourier transformed infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were used to analyze the hydration process and carbonation process of alkali-activated Pisha Sandstone. TG/DTG was used to quantify the amounts of carbonates which formed from calcium silicate hydrate (C–S–H) at different steel slag additions and ages. The compressive strength of Pisha Sandstone with suitable activators (1% NaOH) and mineral additives (40% steel slag) can reach 46 MPa at 90 days. The results of SEM/EDS, FTIR, XRD and TG/DTG showed the products of NaOH-activated Pisha Sandstone are C–S–H gel and geopolymer gel. However it also showed serious carbonation of C–S–H gel. In particular, the results of TGA/DTG showed that the carbonation rate of tobermorite in C3 reached 37.8% at 28 days. But the carbonation rates of C–S–H gel decreased with the increasing of slag dosages and NaOH concentrations.