The competitive hydration of [formula omitted] and Cl− in alkali-activated slag cementitious materials

Abstract

This article presents a study on alkali-activated slag cementitious materials that were doped with varying levels of calcium chloride and calcium sulfate. The article delves into the hydration products and mechanisms of the cementitious material, utilizing pH values, X-ray diffraction (XRD), thermogravimetric (TG), and scanning electron microscopy (SEM). The results show that under the condition of Ca2+ dosage less than 2.5%, CaCl2 is more advantageous than CaSO4 in improving compressive strength. Under the condition of Ca2+ dosage of 2%, adding CaCl2 to the cement paste increased the compressive strength of the test block by 230% at 3 curing days and by 190% at 28 curing days compared to the sample with calcium oxide. After calcium chloride replaced calcium sulfate, the compressive strength was improved. At a Ca2+(CaCl2)-Ca2+(CaSO4) ratio of 1:1, the strength was increased by 44% and 18% at 3 days and 28 days, respectively, compared with the addition of calcium sulfate alone. SO42- can promote the formation of ettringite (AFt), while Cl− can promote the formation of Friedel's salt (3CaO·Al2O3·CaCl2·10H2O, F’s). The contribution of hydrated Friedel's salt to compressive strength is greater than that of AFt. As Cl− is added to the alkali-activated slag cementitious materials stimulated by SO42-, Cl− is dominant in the hydration process, and form Friedel's salt with hydrated calcium aluminate and calcium hydroxide, while promoting the increase of C-S-H gel quantity, thus improving the compressive strength of the material.