Porosity and Engineering Parameters of Thermally Degraded Westerly Granite

ABSTRACT We present a textural analysis study based on the combination of ultrasonic testing (UT) and mercury intrusion porosimetry (MIP) of 36 Westerly granite (WG) samples exposed to different heating temperatures in the range between 100°C and 800°C. Specimens subjected to elevated temperature treatment were also subjected to the thin section analysis. Identical WG cylindrical specimens with diameter 11.5 and height 15 mm were used for UT and MIP study. UT of all specimens was made by P and S wave 1 MHz resonant frequency sensors. It was found that P-S wave velocity, amplitude, frequency and elastic modulus decrease more than 60% as a result of thermal rock material disintegration due to the increased temperature treatment. The damage parameters based on measured values were also determined. MIP showed that the pore size distributions vary with different heating temperature in dependence of the thermal WG treatment. The higher rising porosity values were observed for samples with heating temperature > 500°C. The influence of thermal treatment to the 3D – velocity distribution of P, S1, S2 by UT of spherical sample of 50 mm in diameter was also studied, that enables us to calculate full stiffness tensor. Based on the knowledge of stiffness tensor of WG material heated up to 800°C and loaded to hydrostatic pressure up to 120 MPa the 3D distribution of dynamic engineering parameters like Young and shear modulus and Poisson ratio were determined. Fundamental dynamic engineering parameters for material primary directions in temperature range 100 – 800°C and pressure range 0.1 to 120 MPa were determined. INTRODUCTION Granite can be important rock for waste and gas disposal or as water reservoir exhibiting low native porosity/permeability and high integrity (Yang et al., 2017). In the past, high temperature treatment has been demonstrated as having a great effect on the mechanical behaviour of some rocks (Kern et al.; 1978; Heuze, 1983; Zhang et al., 2001; Dwivedi et al., 2008; Lokajicek et al., 2012; Blake et al., 2013; Chen 2017; Lokajíček et al., 2020, etc.). Increasing temperature mostly causes the changes of pore network of the rock by the formation of new microcracks and enlargement of them. Porosity and pore size distribution is the main factor influencing the associated mechanical properties as strength, elasticity, permeability and ultrasonic wave propagation. There was found that poroelastic properties of hard rocks can be affected by microstructural evolution such as porosity change and micro-cracks growth (Sardini et al., 2006, Heap, 2019, Schepp and Renner, 2021, Schild et al., 2021, Kumari and Ranjith, 2022). High temperatures may induce microcrack formation and propagation and result in elastic properties change of rocks and increase of porosity. To investigate the effect of thermal degradation on granite, slow heating thermal treatments were carried out on WG granite samples in this study. The mechanical properties (e.g. elastic properties, elastic modulus, ultrasonic waves propagation, textural properties (pore network, surface area and pore volume etc.), and thermal dependence of all studied parameters were investigated.