Failure mechanisms and nanoindentation response of metakaolin geopolymer: an atomistic investigation

: Geopolymer is attracting increasing attention due to their excellent mechanical properties and environmental friendliness, as well as the high energy consumption and substantial pollutant emissions associated with cement production. In this work, molecular dynamics (MD) simulations were employed to systematically investigate the failure mechanisms and nanoindentation process in geopolymers with varying Si/Al ratios and water contents. MD results showed that the ultimate tensile, compressive strengths and hardness increase with increasing Si/Al ratio, whereas the elastic modulus is marginally affected. As the water content increased, the tensile elastic modulus, compressive elastic modulus, reduced elastic modulus, and ultimate compressive strength all increased, whereas the ultimate tensile strength decreased. The marked reduction in Al–O coordination confirmed that the failure was primarily driven by Al–O bonds breakage, indicating that Al–O bonds were the weakest link governing structural failure. Water molecules formed a H-bond network play a crucial role in resisting deformation. H-bond breakage promoted tensile failure, whereas additional H-bonds formed under compression contributed to the high residual compressive stress. The H-bond network also enhanced the hardness of geopolymer under nanoindentation.