Fracture toughness of cement paste constituents assessed by micro-scratching correlated with acoustic emission

The fracture toughness of cement paste is difficult to quantify both by standard nanoindentation tests and by time-consuming and expensive measurements on micro-specimens milled with a focused ion beam. Here, a well-calibrated scratch test with simultaneous recording of acoustic emission signals was used to quickly and easily provide statistically relevant quantitative results for a wide range of scales (1–100 µm). The microscale fracture toughness for the main hydration products reached 0.54 ± 0.03 MPa$\cdot$m${}^{1/2}$ for the outer product, 0.64 ± 0.05 MPa$\cdot$m${}^{1/2}$ for the inner product, 0.66 ± 0.06 MPa$\cdot$m${}^{1/2}$ for Portlandite, and 1.24 ± 0.20 MPa$\cdot$m${}^{1/2}$ for clinker on well-hydrated sample. Two primary deformation mechanisms inherent in the micro-scratch process, material compaction and “ripping off”, were identified and their impact on fracture toughness and friction coefficient was quantified. Simulation of cracking and damage mechanisms, plus estimation of the otherwise unavailable tensile strength of compacted cement paste constituents, were successfully modeled using a Griffith-type fracture model.