ATOMIC-LEVEL INSIGHT INTO ADSORPTION OF POLYPHOSPHATE SUPERPLASTICIZERS ON CALCIUM SILICATE HYDRATE SURFACES

Abstract

This study is dedicated to atomic-level observations of the adsorption of polyphosphate superplasticizers on calcium silicate hydrate (C-S-H) surface. Phosphate-based comb-shaped polymers represent a new type of concrete admixtures which provide exceptional calcium binding effect and adsorption behavior near interface. In this work, the conformational properties and interactions of polyphosphate superplasticizers with C-S-H surface were analyzed by the application of the molecular dynamics simulations. The effect of calcium ions concentration in the solution on the conformational behavior of phosphate-based polymer was initially analyzed by radius of gyration of the polymer. The value of the radius of gyration was greater in solution with lower concentration of calcium ions in the solution. The results obtained from the radial distribution function (RDF) between the phosphate groups and calcium ions in the solution showed that the high concentration of calcium ions can weaken the effect of steric hindrance and in the case of low calcium ions concentration the peaks on the RDF obtain higher values. Consequently, the structural properties of polymer in mineral-polymer-water-ions system were analyzed and compared with the results from simulations of polymer-water-ions system. It was observed from the results of the radius of gyration evolution that polymer obtains more collapsed conformations near the calcium silicate hydrate surface and binding capacity to calcium ions is higher compared to results from polymer-water-ions system. In all dynamics simulations polymer backbone changes its conformation with time. Negatively charged phosphate containing groups move close to the surface of mineral. Whereas polyethylene glycol (PEG) containing groups of the copolymer chain mostly do not interact with the C-S-H surface.