Effect of carboxylic and hydroxycarboxylic acids on cement hydration: experimental and molecular modeling study

Abstract

Most concrete produced includes chemical admixtures such as air entrainers, set modifiers, water reducers, etc., many of which include organic molecules. Hydroxycarboxylic acids, in particular, retard portland cement hydration. The interaction of such acids with hydrating cement phases is a complex, multi-parameter problem. To elucidate the interaction of hydroxycarboxylic and carboxylic acid retarders on hydration of cement, a combined experimental and molecular-computational approach was used. Glycolic acid, acetic acid, calcium glycolate and calcium acetate were used as model compounds. Molecular dynamics simulations were performed to simulate the interactions of select test compounds with the (001) surface of the portlandite crystal (calcium hydroxide) and the (040) surface of the tricalcium silicate crystal. Hydrogen bond density profiles and binding energies were evaluated. The adsorption isotherm for chelate complexes was determined experimentally by equilibrating aqueous solutions of the agents in the presence of various amounts of solid-phase calcium hydroxide. Finally, isothermal calorimetry experiments were used to quantify effects on hydration rate. The glycolic acid shows significant cement retardation, whereas acetic acid does not retard. Glycolic acid was found to retard hydration via calcium chelation and surface adsorption that involves the adsorption of the calcium chelate complex preferentially on tricalcium silicate. Simulation results reveal that calcium glycolate forms a strong hydrogen bonding network near to calcium hydroxide and hydrated tricalcium silicate surfaces and are responsible for its strong adsorption on these surfaces. While acetic acid forms a strong calcium chelate, it does not associate with calcium hydroxide or unhydrated or hydrated tricalcium silicate surfaces.