Adsorption of Polycarboxylate Ether Superplasticizer Incorporating Various Functional Groups on the Kaolin (001) Surface: Theoretical Study and Experimental Testing

As an impurity compound in concrete, kaolin could adsorb much polycarboxylate ether (PCE) superplasticizer, depressing the efficiency of PCE significantly. The mechanisms underlying the absorption of carboxylic acid (PCE-C), sulfonic acid (PCE-S), and phosphoric acid (PCE-P) groups on the kaolin (001) surface were investigated by adsorption capacity test and density functional theory (DFT). Adsorption experiments show that the adsorption capacity of PCE-P on kaolin is significantly higher than that of PCE-S, while PCE-C shows the least adsorption on kaolin. Based on DFT calculations, the interaction between PCE-P and the kaolin (001) surface is the strongest, mainly due to the formation of a strong chemical bond, along with hydrogen bonding and electrostatic interactions. However, no stable chemical bonds form between the kaolin (001) surface and either PCE-S or PCE-C. Additionally, the adsorption energy suggests that PCE-S has a higher adsorption capacity on the kaolin (001) surface than PCE-C. Studies have shown that kaolin exhibits a strong adsorption capacity for the phosphoric acid. This adsorption significantly reduces the dispersing ability of the superplasticizer, thereby impairing its effectiveness in reducing water demand and improving workability in concrete. This study aims to provide a theoretical basis for choosing PCEs and improving their efficiency when producing concrete made with kaolin-involved aggregates.