Determining physico-chemical parameters for high strength ambient cured fly ash-based alkali-activated cements

Abstract High strength ambient cured alkali-activated cement was developed from original Class-F (ASTM) fly ash using 1:2.5 mass ratio of 16 molar sodium hydroxide and sodium silicate liquid activator with solution to solid binder ratio of 0.36 for in-situ cast application. Graded strength of high, medium and low in the range of 60–103, 30–56 and ≤ 20 MPa were obtained using mono-component original fine, medium and coarse sized fly ashes respectively. The inclusion of 35 wt% GGBS in the medium sized Class-F fly ash improved the strength from 45 MPa to 60 MPa while 100% GGBS showed higher strength of 81 MPa. Chemically, CaO/SiO2 ratio in the composition influenced the fresh and hardened properties of alkali-activated fly ash/GGBS cements. For instance, in the Class-F fly ash (Si/Al=1.55-1.99) based compacts with CaO/SiO2 ratio ≤ 0.03, the strength improved with increasing atomic Al/(Na + K) ratio. Conversely strength decreased with increasing Al/(Na + K) ratio in bi-component fly ash and GGBS based alkali-activated cements where CaO/SiO2 ratio is higher in between 0.17-0.75. Interestingly Si-dissolution from subcrystalline silica or fine quartz from mechanically activated Class-C fly ash did not improve strength. Lignite ash with high specific surface area as obtained in low temperature combustion did not react with high pH alkaline solution indicating need for vitreous aluminosilicate glass. X-ray diffraction and attenuated total reflectance Fourier transform infrared spectroscopic analyses show that plagioclase and C-S-H were the reaction products responsible for strength gain in ambient cured alkali-activated fly ash/GGBS cements.