ALKALI-METAL SALTS AS SET ACCELERATORS FOR HIGH ALUMINA CEMENT

The effect of alkali metal salts on the setting time of high aluminia cement (HAC) has been studied. The influence of concentration of the salts, chemical nature of anion and the type of alkali metal cation have been investigated. The results of the research indicate that alkali metal salts are set accelerators of HAC. The lithium cation has more effect on the setting time than other alkali cations because of its ability to form tetrahedral symmetry while the others will form the octahedral type. Lithium salts remove the nucleation barrier, caused by an initially fast precipitation. The influence of pH in mixing water is not important. The effect of hydroxyl group is greater than effect of other investigated anions due to the replacement leading to a further centre for oxobridge formation. The lithium salts cause decreasing of flexural and compressive strength of HAC mortars but also cause the strength development at early ages.