Effect of nano MgO/SiO2 additive on hydration and strength of self-compacting concrete fabricated by different processing methods

The effects of the addition of 0–4 wt% nano MgO and 0–2 wt% nano SiO₂ (with respect to cement content) on hydration, microstructural, and compressive/flexural strength of Self-Compacting Concrete (SCC) were investigated. Two different post-treatment conditions with water and CO₂ gas were used to study the processing method on the samples. The results of rheological tests showed that the addition of nanoparticles decreased the flowability of SCC. The results of the density showed that the sample containing 4 wt% of MgO processed in the water environment had the lowest density and the sample processed in the CO₂ environment had the highest density. In the samples containing SiO₂ nanoparticles, an increase in density was observed with the increase of SiO₂ nanoparticles in both cases. It was also determined the amount of heat released for the samples with nano MgO was higher than the samples with nano SiO₂, which can be attributed to the heat of the hydration reaction of MgO or the formation of calcium carbonate. The mechanical properties of the samples were investigated. The compressive strength significantly improved after the addition of MgO/SiO₂ nanoparticles. However, this improvement was more remarkable in the case of post-treatment with CO₂ compared to the samples fabricated with water. SEM results showed that the samples treated under CO₂ gas had irregular and needle-like morphology. The samples prepared by normal processing had CaCO₃ and SiO₂ phases, whereas the ones fabricated under CO₂ gas contained CaCO₃, SiO₂, and Ca(OH)₂. With the addition of nano MgO, the density of concrete decreases in the samples post-treated with water, whereas it increases for the samples post-treatment with CO₂ gas. Adding nano MgO-SiO₂ to concrete and further post-treatment with CO₂ for 45 days could increase the mechanical properties.