Elastic Response of Cementitious Gels to Polycation Addition

The high compressive strength of cementitious materials stems from the creation of a percolated network of calcium silicate hydrate (C-S-H) nanoparticles glued together by strong Ca²⁺-Ca²⁺ correlation forces. Although strong, the ion correlation force is short range and yields poor elastic properties (elastic limit and resilience). Here, the use of polycations to partially replace Ca²⁺ counterions and enhance the resilience of cementitious materials is reported. Adsorption isotherms, electrophoretic mobility, as well as small angle X-ray scattering and dynamic rheometry measurements, are performed on C-S-H gels, used as nonreactive models of cementitious systems, in the presence of different linear and branched polycations for various electrostatic coupling, that is, surface charge densities (pH) and Ca²⁺ concentrations. The critical strain of the C-S-H gels was found to be improved by up to 1 order of magnitude as a result of bridging forces. At high electrostatic coupling (real cement conditions), only branched polycations are found to improve the deformation at the elastic limit. The results were corroborated by Monte Carlo simulations.