Impact of MgCl₂ on the mechanical properties of alite pastes at mesoscale and nanoscale

While the MgCl₂ mixing alite paste causes degradation of the modulus and hardness of C–S–H formed, the underlying degradation mechanism needs to be better understood. This study comprehensively analyzed the mechanical properties of alite pastes mixed with MgCl₂ solutions, examining the effects at both mesoscale and nanoscale. The in situ X-ray diffraction analysis revealed that the formation of brucite occurred at a very early age together with the formation of portlandite, which may induce a loosened packing density of C−S−H and served as the primary cause of modulus and hardness degradation. We used Bayesian statistical models to fit the nanoindentation data of C−S−H. By extrapolation at a packing fraction equal to unity, we were able to extract elastic properties (modulus and hardness) of the C−S−H nanograins. The nanoscale C−S−H elastic modulus showed no significant alterations in the mechanical properties of the C−S−H nanostructures. Atomistic simulations also suggested that Mg²⁺ ions preferably substitute interlayer Ca rather than intralayer Ca in C−S−H, and the Mg docking in CSH induced a very modest volume contraction.