Optimizing the concentration of Na2O in alkaline activators to improve mechanical properties and reduce costs and CO2 emissions in alkali-activated mixtures

Abstract

This article evaluates the influence of Na2O concentration from sodium silicate and metasilicate activators on the physical, mechanical, and chemical/mineralogical properties of blast furnace slag alkali-activated mixtures. The mixtures were prepared with Na2O dosages of 3, 4.5, 6, and 7.5%. Compressive strength results showed that the maximum values were found for the dosages of 6 and 7.5% at 28 days, while in the flexural strength, the optimal dosages were between 4.5 and 6%. Through thermogravimetry, X-ray diffraction, and infrared spectroscopy tests, it was possible to evaluate the C-S-H production, which occurred more intensely at Na2O concentrations of 6 and 7.5%. However, at the dosage of 7.5%, it was observed utilizing scanning electron microscopy that excessive cracking exerts a preponderant influence on the mechanical properties, especially on the tensile strength. Finally, an assessment of the sustainability of the mixtures was carried out by calculating the CO2 emission efficiency and costs. In this analysis, it was concluded that the mixtures with 4.5% and 6% Na2O were more efficient in these aspects. Therefore, in addition to increasing the amount of microcracks and decreasing the production of C-S-H, excess alkali increases the costs and CO2 emissions of alkali-activated materials.