Experimental investigation of the effect of quenching cycles on the physico-chemical properties of granites

Abstract

In this study, the physicochemical properties of granitic rocks subjected to quenching cycles were studied experimentally. Four granites of similar mineralogy but with different degrees of initial weathering (porosity between 1 and 6%) were slowly preheated at two peak temperatures (200 and 400 °C) and then quenched 35 times. To study the effect of thermal cycling on the physical properties, non-destructive tests were used such as water porosity, capillary water absorption tests, P- and S- wave propagation velocities, nuclear magnetic resonance relaxometry, and X-ray micro-tomography. Chemical analysis of the granites was performed using X-ray fluorescence, which provided information on the major and trace elements. Water-granite interactions were followed using inductive plasma mass spectrometry (ICP-MS). The variation of all the measured parameters indicates the creation of cracks with thermal fatigue. The porosity, water uptake, size, and volume of cracks increased while P- and S- wave velocity and Young's modulus decreased. At 200 °C, the changes were progressive up to ten cycles, from which the stress threshold was reached and only small readjustments took place. At 400 °C, the greatest damage was observed during the first five cycles. These changes were a direct consequence of the propagation of the microcracks induced by the strong gradient during the quenching tests. For both temperatures, the changes depended on the initial weathering conditions of the granites. Initially, weathered granites showed crack development or crack closure during quenching, meanwhile the damage on the sound rocks was characterized by the creation of intragranular microcracks. The analysis of the experimental fluids showed enrichment in K, Na and Ca in the solution as consequence of the dissolution of K-feldspar, plagioclase and the degradation of mica and clays, independently of the physical and mechanical modifications.