The compatibility of highly carboxylated polycarboxylate superplasticizer with sodium gluconate retarder in alkali-activated slag system

Abstract

Fluidity control is a critical issue limiting the practical application of alkali-activated slag (AAS). In cement systems, the PCE is usually used combined with retarders for workability adjustment of fresh pastes because the dispersing ability of PCE alone drops quickly over time. However, there is a lack of relevant research in the AAS system. This paper investigates the effect of the combination of PCE and sodium gluconate (SG) retarder on the early workability and hydration of AAS based on the interaction among PCE, SG and slag. PCE existed in the form of aggregates in NaOH solution, and the size of the aggregates in 4M NaOH is larger than that in 3 M NaOH. Results show that SG incorporation significantly prevented the rapid loss of PCE dispersing ability over time due to its strong inhibition on the early hydration of AAS. The initial dispersing ability of PCE was also influenced, which was related to the activator concentration and the dosage of PCE. The initial dispersing ability of PCE was improved at dosages below 0.3 % regardless of the NaOH concentration, as the adsorbed SG occupied adsorption sites, preventing PCE aggregates from bridging multiple slag particles. With increasing PCE dosage, PCE competed with SG for adsorption sites on the slag surface. At 3 M NaOH concentration, the addition of SG reduced the adsorption amount of the smaller PCE aggregates and hence their initial dispersing ability. However, the larger PCE aggregates formed in 4 M NaOH exhibited stronger adsorption capacity than SG and their adsorption was not affected, thus their initial dispersing ability remained unchanged at 4 M NaOH.