Temperature-Induced Explosive Behaviour and Thermo-Chemical Damage on Pyrite-Bearing Limestones: Causes and Mechanisms

Abstract

In this investigation, two different varieties of 'Prada' limestones were studied: a dark grey texture, bearing quartz, clay minerals, organic matter and pyrites, and a light grey texture with little or no presence of such components. We have observed two effects of different intensity when heating the dark texture from 400º C: i) the explosion of certain samples and ii) greater thermal damage than in the light grey texture. Chemical and mineralogical composition, texture, microstructure, and physical properties (i.e. colour, open porosity, P and S-wave velocity) have been evaluated at temperatures of 105, 300, 400 and 500º C in order to identify differences between textures. The violence of the explosive events was clear and cannot be confounded with ordinary splitting and cracking on thermally-treated rocks: exploded samples underwent a total loss of integrity, displacing and overturning the surrounding samples, and embedding fragments in the walls of the furnace, whose impacts were clearly heard in the laboratory. Thermogravimetric results allowed the identification of a process of oxidation of pyrites releasing SO 2 from 400º C. This process jointly with the presence of micro-fissures in the 2 dark texture, would cause a dramatic increase in pore-pressure, leading to a rapid growth and coalescence of microcracks that leads to a process of catastrophic decay in rock integrity. In addition to the explosive events, average ultrasound velocities and open porosity showed a greater variation in the dark grey texture from 400º C. That results also points towards a significant contribution of oxidation of pyrites on the thermochemical damage of the rock, among other factors such as the pre-existence of microfissuresand the thermal expansion coefficient mismatch between minerals. Implications in underground infrastructure and mining engineering works are critical, as the explosive potential of pyrite-bearing limestones bear risk for mass fracturing and dramatic strength decay from 400º C. Moreover, SO 2 released has harmful effects on health of people and the potential to form acid compounds that corrode materials, shortening their durability and increasing maintenance costs.

Thermal Effects on the Drilling Performance of a Limestone: Relationships with Physical and Mechanical Properties

“…The effects of high temperatures in textural, physical, and mechanical features from the Prada formation were described in previous research [27]. A dark grey fraction from Prada limestone exhibited an increased thermal damage and explosive behaviour when heated to above 400 • C, and this is related to an increase in the pore pressure caused by SO 2 released during the thermal oxidation of pyrites [23]. Due to such different effects produced by the thermal treatment, the dark grey fraction was separated from the rest of samples and was not considered in this research.…”

“…Quartz-bearing limestones experiment a dramatic microcracking and volume increase at the phase transition between 550 and 600 • C, with a strong peak at 573 • C [30,31]. Local thermal stress concentrations and microcracking occur due to mismatches in thermal expansion coefficients of different mineral particles [25,[55][56][57], especially in the range of temperatures between 400 and 500 • C [54]; and thermal oxidation of pyrites leads to a dramatic increase in pore-pressure on pyrite-bearing limestones, resulting in increased thermal damage and explosive behaviour [23]. All these processes affect rock integrity and cause a continuous and gradual decay in physical and mechanical properties of rock with temperature-with some threshold temperatures marking changes in the general trend [54].…”

“…Andriani and Germinario [19] observed a clear reduction in uniaxial compression strength (UCS) from 500 • C on calcareous and dolomitic rocks from Apulia in Italy, with temperatures above 600 • C usually marking a dramatic decline in UCS [20,21] Beck et al [22] explored colourimetry to determine thermal damage in buildings and described a trend of limestone to redden (later confirmed in Prada limestone and related to oxidation of iron compounds) [23]. Natural limestone becomes lighter in appearance with increasing temperature [23,24]. Zhang et al [25] determined that from 200 to 500 • C porosity and pore size rapidly increase, and from 500 to 600 • C UCS, elastic modulus, Poisson's ratio, and hardness decreased.…”

“…Later, Zhang and Lv [26] described a strong relationship between mineral content and thermal damage in limestones (China). Martínez-Ibáñez et al [23] described a significant contribution of the thermal oxidation of pyrites in the explosive behaviour and thermo-chemical damage of Prada limestone from 400 • C. Martínez-Ibáñez et al [27] identified micro-structural changes produced by high temperatures and cooling methods in Prada limestone, and related them with severe variations in the physical and mechanical features of this type of rock. Such dramatic changes are mainly explained by the anisotropic expansion of calcite [15,28], and by other physicochemical processes such as the mismatch in thermal expansion coefficients between mineral particles [29] and the quartz phase transition [30,31].…”

Effects of Wildfire on Rockfall Occurrence: A Review through Actual Cases in Spain

“…Other processes, such as dehydration or decarbonization, can lead to significant physical changes and mechanical properties of rocks [40,42]. Thermo-minero-chemical changes also cause polymorphic transformations, melting, and even the disappearance of certain minerals [40,[43][44][45]. These processes are behind the development of new microcracks and the coalescence and enlargement of existing ones that are responsible for the important changes in the physical and mechanical properties of rocks after a fire [39,41,42,[46][47][48][49] (Figure 2).…”