Effect of Na sub( 2)SiO sub( 3)/NaOH Ratios and NaOH Molarities on Compressive Strength of Fly-Ash-Based Geopolymer

Carbon dioxide (CO sub( 2)) emissions from the production of 1 ton (2204.62 lb) of cement vary between 0.05 and 0.13 tons (110.23 and 286.60 lb). It is important to reduce CO sub( 2) emissions by the greater use of substitutes for portland cement (PC), such as fly ash (FA), clay, and other geo-based materials. This paper studies the processing of geopolymers using FA and alkaline activators. The factors that influence the early-age compressive strength, such as the sodium hydroxide (NaOH) molarity and Na sub( 2)SiO sub( 3)/NaOH ratios, were studied. Sodium hydroxide and sodium silicate solutions were used as alkaline activators. The geopolymer paste samples were cured at 70 degree C (158 degree F) for 1 day and kept at room temperature until testing (the seventh day). The compressive strength was measured after 7 days. The results show that the geopolymer paste with a combination of an Na sub( 2)SiO sub( 3)/NaOH ratio of 2.5 and a 12 M NaOH concentration produces the highest compressive strength. The density obtained for geopolymer for PC is in the range of 1760 to 1855 kg/m super( 3) (0.064 to 0.067 lb/in. super( 3)). The porosity of the geopolymer was in the range of 12.16 to 26.19%, and the water absorption was in the range of 5.03 to 8.13%. The results of scanning electron microscopy (SEM) indicated that the samples with a denser matrix and less unreacted FA contributed to the maximum compressive strength. In the X-ray diffraction (XRD) patterns, the intensity of quartz content at 12 M was highly detected compared to the 6 and 10 M solutions.