Influence of the smectites in the microstructure and the mechanical properties of an altered basalt

Here, the microstructure and mechanical properties of a basalt with three alteration degrees (low, medium, and high) are studied, and these properties are compared to the alteration degree of the basalt. The representative samples of each alteration degree have been studied both in raw state as after a short ageing treatment with ethylene glycol. The properties studied are density, P-wave velocity (V P ) and uniaxial compression strength (UCS). The microstructure has been studied via optical and scanning electron microscopy and X-ray computed microtomography (microCT), which also provides quantitative data on connected and non-connected porosity. The representative samples of the three alteration degrees show clear differences and V p and UCS values are progressively lower as higher the alteration. The loss of the good properties of low altered basalt was due to smectites filling a dense network of micro- and nano-fractures that crossed the full rock. Smectites as alteration products of the basalt explained the behaviour of these rocks. With only three swelling–shrinking cycles of smectites, the porosity grew ∼500% in the most altered sample, which explains the cohesion loss of the rock and its lower UCS. Statistical analysis showed the negative correlation among the connected porosity and the mechanical properties of these basalts.