Influence of the CaO/SiO2 molar ratio on the technological properties of alkali-activated fly ash mortar with quicklime

Abstract

This study aimed to investigate the influence of the CaO/SiO2 molar ratio on the physical, chemical and mechanical properties of alkali-activated fly ash with quicklime and assess the technical viability of the mortar with the highest compressive strength. Eight mortars were prepared, with a fixed sand to combined activator/precursor ratio of 3:1, varying the CaO/SiO2 ratio from 0.25 to 2.00 every 0.25. The doses that resulted in the highest and lowest compressive strength were CaO/SiO2 = 0.75 and 2.00, respectively, which reached 17.9 and 3.8 MPa at 28 days. Analytical techniques were performed to identify the products formed and the morphological differences in order to justify the variation in mechanical strength between mortars. EDS/SEM showed the formation of C-S-H, C-A-S-H and CH as well as other compounds in the mortars. XRD demonstrated greater CH formation in the mortars with the highest CaO/SiO2 molar ratio and CaCO3 in all the mortars, indicating that all the samples underwent carbonation. The TG/DTG results confirmed the presence of C-S-H, CH and CaCO3 phases in the mortars. Mercury intrusion porosimetry showed a significant difference in porosity between mortars with the highest (15.4%) and lowest (4.6%) compressive strength, confirming that this parameter also justifies mechanical strength differences.