Effect of Sodium Silicate to Hydroxide Ratio and Sodium Hydroxide Concentration on the Physico-Mechanical Properties of Geopolymer Binders

Abstract

Geopolymers are emerging materials in the construction industry that are yet to achieve uniform standards in their mode of synthesis. Several factors over the years are said to affect the performance of the binders which vary widely. This paper investigates the effect of silicate-to-hydroxide ratio and sodium hydroxide concentration on the mechanical properties of metakaolin-based geopolymer binders. Four SS/SH ratios of 0.5, 1.0, 1.5, and 2.0 and four SH concentrations of 8M, 10M, 12M, and 14M were studied with the kaolin calcined at 800oC. Physical and mechanical tests were conducted on sixteen (16) samples at testing ages of 7, 14, and 28 days. The mean density of the geopolymer binders was 1.86 g/cm3, and the samples increased with increasing SS/SH ratio and SH molarity with the 2MK14 having the maximum density of 1.95 g/cm3. The compressive strength and flexural strength of the samples generally increased with testing age (7-28 days). The compressive strength of the samples further showed a direct increase with SS/SH ratio, though, above SS/SH = 1, the compressive strength began to decrease gradually. The maximum compressive strength for these binders 14.6MPa which was recorded by the 1MK10 sample. The Pareto chart for the compressive strength of the samples showed that both the SS/SH ratio and SH concentration contributes significantly (95% significant level) to the compressive test results. However, the SS/SH ratio had a more standardised effect on the compressive strength of the binders. The maximum flexural strength (3.45MPa) was obtained from the 1MK8 geopolymer binder, and similar to the compressive test results, strength values were obtained at the SS/SH ratio of 1