The acceleration and retardation of set high alumina cement by additives

The set of high alumina cement (HAC) pastes may be considerably influenced by the addition of very small quantities of chemical compounds. from the measurement of setting time and temperature rise, with a conduction calorimeter. It is shown that both cations and antons have a profound effect on the chemical reactions which cause the hardening of HAC. The effects on the time of setting may be summarised as follows:- for cations Ll ⪡ Na < none < K. < Ca < Mg < Sr < NH4 for anions OH ⪡ none < CL < NO3 < Br < acetate The effect of anions, except for the hydroxyl ion, causes a retardation in the setting as a result of replacement of OH groups linked to Al, referred to as ion penstration, preventing the formation of oxobridges. The retardation of sot. will be governed by the rate of replacement of OH by other anions and the stability of the new species formed. The dramatic effect of OH is due to the replacement of H2O by OH in the Al environment leading to a further centre for oxobridge formation.