Effect of competitive adsorption between polycarboxylate superplasticiser and sodium tripolyphosphate on cement paste fluidity

Sodium tripolyphosphate is a common retarder used to delay the hydration and prolong the setting time of cement paste. It is also sometimes employed to enhance superplasticiser dispersity to improve the fluidity and workability of cement and concrete, but the reason for this effect is unclear. The work discussed in this paper investigated the early hydration process of cement paste and the adsorption of sodium tripolyphosphate and polycarboxylate superplasticiser (PCS). The results show that there is competitive adsorption between PCS and sodium tripolyphosphate, sodium tripolyphosphate inhibits PCS from adsorbing onto the surface of cement particles, and PCS also reduces the adsorption of sodium tripolyphosphate. When both of these are reduced, there is a significant increase in the thickness of the adsorption layer and the zeta potential, which is shown to be the main reason for the improvement in fluidity. This indicates that some salts can be used to adjust the adsorption process of PCS in order to improve the fluidity and workability of cement paste and fresh concrete.