Compressive strength, modulus of elasticity and hardness of geopolymeric cement synthetized from non-calcined natural kaolin

Abstract

In this work, a natural non-calcined kaolin was used for the synthesis of a geopolymeric paste. The effect of the natural kaolin characteristics and the alkaline activator ratio (Na2O.SiO2/NaOH) on the mechanical behavior of the resulting geopolymeric cement was determined. The chemical and mineral composition of the kaolin were determined by XRF and XRD techniques. For the synthesis, sodium silicate and sodium hydroxide were used in molar ratios of Na2O.SiO2/NaOH = 1.0, 1.6 and 2.2 and the curing of the samples was performed at 40 °C. The compressive strength of the samples was evaluated at 7 and 28 days of age. The modulus of elasticity and hardness were determined by instrumented nanoindentation. Microstructural analysis (SEM) combined with chemical analysis (EDS) was performed to study the morphology of the geopolymeric cement samples. The results showed that the natural kaolin is 95% amorphous (Rietveld) and shows small particle size (<5 μm, by SEM). The most efficient composition, with Na2O.SiO2/NaOH = 2.2 ratio, showed a modulus of elasticity of 10 GPa, hardness of 0.4 GPa and compressive strength of 67 MPa. In the microstructural analysis, the surface of the samples showed some cracks, probably caused by the curing process, and small roughness, but the cured specimens did not show visible flaws. The high strength is due to the chemical composition of the cement, with a SiO2/Al2O3 ratio of 3.5, and due to the degree of amorphism of the natural kaolin, 95%. Therefore, the use of a natural kaolin, without further calcination, reduces costs and environmental impacts in the design of geopolymeric cements.