Article updated on 17 August 2021
Image Credit: shutterstock.com/Renata Apanaviciene
Every material undergoes physical and chemical changes when subjected to the extremes of heat, pressure, and other factors. This article will discuss what happens to the mechanical properties of sandstone, an industrially important rock when it is subjected to high temperatures.
The Properties of Rock
In the engineering and construction industries, knowledge of physical and mechanical property parameters is crucial. A misunderstanding of these can lead to industrial disasters, and indeed this has been the cause of many geological disasters in the coal mining industry.
Changes in properties induced by various stressors provide a significant problem for research in areas such as waste storage (especially underground nuclear waste storage) and resource exploitation.
Physical properties include density, permeability, and porosity. Mechanical properties include rock strength, Poisson’s ratio and the rock’s elastic modulus. Subjecting a rock to high temperatures (either by natural or industrial processes) can fundamentally alter its mechanical and physical properties.
Changes which occur are also dependent on other factors, including the rock’s density, porosity and mineral composition.
Properties That are Affected by Temperature
Rocks, including sandstone, can be affected by temperature through many different natural and industrial processes, and heat affects rock in different ways. The properties it influences include thermal expansion, thermal conductivity, radioactive heat generation, electrical properties and magnetic properties.
Original properties of the rock, such as crack density, intrinsic permeability and its mechanical parameters, are modified by factors such as heat.
Industrially important mechanical processes that heat affects include elastic constants. Young’s modulus and the Rigidity modulus of rocks are permanently reduced to values that are determined by the highest heat the rock has been raised to.
This can be explained by the induction of microcracks within the rock’s framework. This can also be influenced by the unequal distribution of minerals in the rock’s structure.
All rocks react in different ways to the application of extremes of temperature due to their physical and chemical structure.
The properties of a material are not only dependent on its structure but also on the scale of observation, and therefore it is of vital importance to understand changes at both the engineering and the micro-scale. This is no different for sandstone.
What is Sandstone?
Sandstone is one of the most widespread sedimentary rocks on earth, and it is clastic in origin. It is made of sand-sized grains consisting of different organic materials along with cementing material.
Clay and salt-sized particles may also occupy the space between the sand particles.
Sandstone can also contain organic particles such as ancient seashell debris from creatures that were alive when the sediments were laid down in ancient times. Layers of sandstone get compacted by overlying sediments.
The sand-sized mineral and rock particles are the result of weathering of other rocks by various elemental forces. They are then transported to the site of deposit by water, ice, or wind. If deposited close to the source rock, the sandstone will resemble it closely.
If the depositional site is far from the source rock, the resulting sandstone will change in composition during transport. Quartz is usually the most found grain in sandstone as it is extremely durable.
Sandstone has been used throughout human history as a material for construction, housewares, and decorative works. It is incredibly easy to work with, which makes it ideal for applications, including asphalt concrete. Some types of sandstone are well-suited to uses, such as grindstones.
Measuring and Identifying the Changes in Mechanical Properties of Sandstone Induced by High Temperature
A paper published online in February 2021 sought to identify how heat affects the mechanical properties of different samples of sandstone from the Upper Silesian Coal Basin in Poland. The team used both meso- and micro-scale experiments to determine the changes at both the engineering and micro-scale.
The methods used for analysis were unconfined compression tests for the meso-scale observation and nanoindentation and X-Ray computed tomography for the micro-scale tests. Results of the morphological and mechanical structure at the microscale were related to the Young’s modulus and uniaxial compressive strength of the sandstone samples.
The two samples were further separated and tested in both an air-dry state and subjected to high temperatures between 100-1000 oC. The heat-treated sample was heated over 8 hours in a resistance furnace at 16 oC/min increments.
By comparing the results, the relationship between mesoscopic properties and changes in the microstructure of the sandstone was determined. A fine-grained and medium-grained sandstone was used in the different samples.
Visually, the samples changed color as the temperature increased. The pink and brown hues induced are associated with changes in mineral composition (in particular, oxidation of iron-containing minerals to hematite.)
In the meso-scale experiments, the Young’s modulus changed similarly to the sandstone’s uniaxial compression strength. In the fine-grained sample, stiffness increased up to 200 oC, whereas the stiffness of the medium-grained sample increased to its maximum at around 400 oC.
At higher temperatures, this was severely reduced. By around 1000oC, the Young’s modulus for both was lower compared to those at ambient temperatures. The ratio between strength and stiffness (E/UCS) was almost constant when the sandstone’s temperature was over 700 oC.
The micro-scale experiments showed why this was happening: high heat caused the formation of cracks on the contacts between grains, leading to a weakening of the microstructure.
One other effect at the microscale was a decrease in the average value of the indentation modulus due to high-temperature treatment. Qualitative trends at the meso-scale could be confirmed by changes in the mechanical properties of the sandstone at the microscale.
As has been shown in the highlighted study, the mechanical properties of sandstone are affected greatly by high heat treatment. The information on these changes can inform the use of the material in critical industrial processes, including the use of waste materials in construction and infrastructure projects.
References and Further Reading