Influence of calcium lignosulfonate on the setting time and compressive strength of cement paste

Owing to the retarding and air-entraining properties, lignosulfonates (LS) can improve the workability of concrete. But these two properties also bring bad effect, which makes the compressive strength of concrete drop sharply, when the dosage of LS exceeds 0.25%. The influences of calcium lignosulfonate (CLS) on the setting time and compressive strength of cement paste were studied systemically involving the molecular weight, hydrophilic groups, kinds of cations and sugar content. Results show that among the four factors, the types and contents of hydrophilic groups were chiefly responsible for the retardation of LS. By turning the hydroxyl group in CLS molecule into carboxyl group through oxidation or increasing the sulfonic degree through sulfomethylation can weaken the retarding effect of CLS and improve the compressive strength of hydrated cement markedly. Study of ultrafiltrated fractions indicates that LS with a molecular weight of 10000 similar to 30000 can relatively accelerate the set of cement paste and endow it with higher strength. The compressive strength of hydrated cement blended with the aluminium, ferrum or chromium salt of LS raises 20%similar to 60% comparing with that was blended with CLS.