Performance and mechanism of amino acids (AAs) on the gypsum setting-time control

Abstract

Protein retarders can not only prolong the setting time of gypsum in application, but also lower the reduction in strength. The chemical activity of proteins is determined by the basic unit of amino acids (AAs). To enhance the understanding of protein retarding action mechanism, this paper focuses on the AAs-adsorbed gypsum structure, and investigated the retarding effects, crystal morphologies, and nanoscale interactions. Results indicate that the AAs had varying retarding effects on gypsum, and the effects from greatest to smallest were L-arginine, L-glutamic, and L-leucine acid. The rate of initial hydration heat release was reduced, and the peak of hydration release was delayed. Retardation of AAs on gypsum involves chemical adsorption on the surface of dihydrate gypsum crystals, which mainly occurs on the (1 2 0) face and affect gypsum morphology. The mechanism of AAs adsorption involves electrostatic interactions between charged functional groups of AAs and gypsum particles, as well as hydrogen bonding at additional carboxyl and amino groups. The adsorption strength depends on longer chained structure and number of functional groups of the AAs. And intrinsic strength of AAs-adsorbed gypsum nanocrystals contributes to the strength reduction on the macroscale. This paper provides basic data for designing and tailoring enhanced protein retarders for gypsum.