Influence of microfractures and porosity on the physico-mechanical properties and weathering of ornamental granites

The purpose of this paper is to demonstrate the influence porosity and fissuration exert on the physico-mechanical properties and durability of nine Portuguese ornamental granites with different petrographical and physical characteristics. Scanning electron microscopy (SEM) allows the type of cracks (intergranular, intragranular and transgranular) to be identified. A microfracture index was calculated based on the collected data. This index is called linear crack density (LCD) and is defined by the number of cracks per length unit. The results reveal that intragranular cracks are the most frequent and represent between 62.9% and 82.3% of all the cracks observed. A physical weathering classification system based on linear crack density values of the studied granites is presented. The voids included in these types of rocks are predominantly microfractures and correlate closely with open porosity and linear crack density. Uniaxial compressive strength and P-wave velocity (VP) appear to decrease as linear crack density increases, albeit with low correlation coefficients. This may be due to the fact that linear crack density does not take into account possible preferential crack orientation and both properties are strongly dependent on this preferential orientation. Other textural characteristics, such as grain size and preferential orientation of certain minerals, may contribute to these low correlation coefficients. In ageing tests, the salt crystallization test demonstrated greater material loss in granites with higher linear crack densities and thermal shock testing proved that thermal cracks have a greater influence on VP in rocks with lower linear crack densities. In light of the ageing tests results, precaution is recommended when using granites with effective porosities greater than 3% as dimension stone out-of-doors in polluted continental or marine areas.