Thermally induced degradation of Westerly granite microstructure documented by dynamic elastic properties and pore space and damage characteristics

Abstract

Ultrasonic sounding, petrographic image analysis, and mercury intrusion porosimetry was performed on 52 Westerly granite (Rhode Island, USA) specimens exposed to controlled heating from 100 °C to 800 °C. Elastic wave velocities, dynamic elastic moduli, and amplitudes decreased quasilinearly by more than 65% (P-wave), 75% (S-wave), and over 90% (elastic moduli). Damage evaluation by using two crack density parameters proved similar behavior. Direct and indirect evaluation of thermal treatment related microcracks by means of petrographic image analysis and mercury intrusion porosimetry revealed exponential character of porosity evolution, being accelerated above the \(\alpha -\beta\) quartz phase transition. Discrepancy between thermal treatment and profoundly non-linear increase in porosity and microcrack density can be satisfactorily explained from direct microscopic observation which revealed formation of crushed/powdered minerals within newly formed microcracks in 650‒800 °C range being related to tension-shearing along grain boundaries of phases with contrasting linear thermal expansion coefficients. This damage phenomenon, resulting in the so-called “clogged” porosity evidently overvalued measured elastic wave velocities. Current results thus underline importance of application of various methods / techniques during examination of changes in rock microfabric from decay processes because none of the methods is capable to cover all important factors alone.