Fracture surfaces and compressive strength of hydrated cement pastes

The properties of the fracture surfaces of hydrated Portland cement pastes were investigated. Analyses of fracture surfaces proved to be capable to provide information about the actual values of compressive strength. Since the compressive strength of cement pastes, σc, is dependent mainly on capillary porosity and the porosity is a controlling factor of height irregularities of fracture surfaces, the compressive strength, as a consequence, shows a functional dependence on the height irregularities. As a measure of height irregularities the 3D profile and roughness parameters were employed. The 3D surface profiles of the fractured specimens were registered by the confocal microscope. The parameters SPa and SPq, i.e. the arithmetic average and the quadratic mean (rms) heights, respectively, were found to be statistically reliable indicators of surface irregularity and thus the dependences σc(SPa) and σc(SPq) were also shown to be relevant indicators of compressive strength. It is hypothesized that the height irregularities of fracture surfaces might become a basis for the microscopic assessment of the compressive strength of cement pastes.

► The profile parameters of fracture surfaces bear information on compressive strength.
► Confocal microscopy is convenient to create digital replicas of fracture surfaces.
► The three-dimensional profile parameters can be evaluated from the confocal replicas.
► The profiles of fracture surfaces depend on porosity which influences compressive strength.