This is Part Two of our series on choosing stones. In this article, we explore essential physical properties of stones, including density, water absorption rate, hardness, compressive strength, and flexural strength.
The physical properties of stone are determined by its chemical composition, mineral constituents, and crystallization methods. With a vast array of stone types available, understanding these properties is essential for making informed decisions in choosing stones for applications such as construction and landscaping. This article explores key properties of stones, including density, water absorption rate, hardness, compressive strength, and flexural strength.
Density
It is important to distinguish between density and relative density. The density of a stone is defined as the ratio of its mass to its volume, indicating how much mass of that specific type of stone is contained in a unit volume. Relative density, also known as specific gravity, is a dimensionless value that refers to the ratio of the weight of a mineral to the weight of an equal volume of water at 4°C. It provides a way to understand how heavy the mineral is relative to water. The relative density of a stone is influenced by the types and proportions of minerals it contains, as well as their respective relative densities, which in turn influences the overall density of the stone.
Stones with a higher content of dark minerals generally exhibit a greater density. For stones with similar structures and compositions, variations in mineral content can lead to differences in relative density. For example, the density of gabbro with an equigranular structure and massive texture, such as Shanxi Black, is approximately 3.07 g/cm3. In contrast, the density of Fujian Sesame Black (G654), a neutral diorite, is about 2.80 g/cm3. Shanxi Black contains a high proportion of ferromagnesian minerals, such as pyroxene, amphibole, and biotite, which can constitute up to 90% of its composition and have a relative density exceeding 3.1. In contrast, the primary minerals in diorite are plagioclase, which accounts for 55%, and quartz, which makes up 10%, with dark ferromagnesian minerals like amphibole and biotite only comprising 30%.
The density of stone is closely related to the compactness of the rock from which it is formed, which is influenced by the size of the mineral crystallization particles and the structural characteristics of the rocks. Generally, fine-grained rocks tend to exhibit greater density, while coarse-grained rocks often have relatively lower density. This difference in density can be attributed to the natural packing of mineral particles within the rock. In fine-grained rocks, smaller particles allow for tighter packing, resulting in less void space and a denser structure. Conversely, in coarse-grained rocks, larger particles create looser packing, leading to greater voids between the particles and, consequently, a lower overall density.
Water absorption rate
The water absorption rate refers to the ability of stone to absorb water under standard atmospheric pressure, expressed as a percentage of the moisture absorbed by the stone. There is a mutual relationship between the water absorption rate, porosity, and density of the stone. Generally, coarse-grained granite exhibits a higher water absorption rate compared to fine-grained granite, and coarse-grained sandstone has a greater absorption rate than fine sandstone and siltstone. This indicates that stones with larger pore spaces have a higher volume proportion of voids. Consequently, stones with a significant proportion of pore space tend to have lower relative density.
The water absorption rate significantly impacts the usability of stone. During installation, cement is used as an adhesive. The alkaline components and salts in the cement paste can migrate through the stone's pores to the surface, leading to efflorescence or water stains. Additionally, the porosity and water absorption characteristics of stones should be considered based on the environment where they will be used. In cold regions, using stones with high water absorption rates as exterior cladding materials can result in moisture within the pores freezing and expanding. This can lead to freeze-thaw cycles that compromise the stone's integrity.
Shore hardness
Shore hardness is a standard measure of material hardness, first proposed by the British inventor Albert F. Shore. The Shore hardness value, denoted as HSD, accurately reflects the hardness of stone. This method employs an elastic rebound technique, where a small conical indenter, often tipped with a diamond, is dropped from a specified height onto the surface of the material being tested, causing it to rebound. The height of the rebound indicates the hardness of the material.
To test the Shore hardness of stone, samples should measure 100 mm × 100 mm in length and width, with a thickness greater than 10 mm. A minimum of three samples should be tested per group, with nine points selected for testing on each sample. The arithmetic average of these measurements will represent the Shore hardness value for that group of samples.
It is important to note that there is no mathematical correspondence between the Shore hardness of stones and the Mohs hardness scale. While Mohs hardness is a comparative measure, Shore hardness is an absolute measure. Generally, stones with a higher content of minerals that possess greater Mohs hardness will also exhibit higher Shore hardness values; conversely, stones with lower Shore hardness values will typically contain minerals with lower Mohs hardness.
Compressive strength
The compressive strength test for stone involves placing a sample at the center of the loading plate of a testing machine. A load is then applied until the sample fails, and the load value at the moment of failure is recorded. Compressive strength is calculated by dividing the measured load at failure by the surface area of the sample that was subjected to the force. The unit of compressive strength is expressed in megapascals (MPa).
Compressive strength is a crucial mechanical property, as it indicates the ability of stone to withstand compressive loads without failure. This metric is particularly significant for load-bearing applications of stone, making it essential to consider the level of compressive strength when using structural stone in construction.
Flexural strength
Stone is a rigid material that exhibits elasticity; when subjected to external forces, it undergoes some bending deformation and returns to its original shape once the external force is removed. However, if the applied force exceeds the stone's capacity to withstand it, the material will reach its limit of deformation and may fracture.
Flexural strength refers to the maximum stress that a material can withstand under bending loads before breaking or reaching a specified deflection, expressed in megapascals (MPa). It is a critical indicator of the physical properties of stone, representing its ability to resist bending under surface loads without fracturing.
This metric is particularly important for applications involving floor slabs and exterior wall cladding. Selecting stone varieties with higher flexural strength for these architectural elements can help resist dynamic pressures from above and significant wind forces, thereby enhancing the durability and integrity of the stone.
