Natural Selection

Natural Selection


landscaping ideasNeed some help choosing the right stone for your next project? Read on…
Story: Jim Mann, Stone Consultant

By learning some simple selection methods and understanding stone’s basic properties, you can make an educated decision when selecting the “right stone” for your next project. The right stone needs to meet requirements based on appearance and performance.
A stone’s physical properties are a useful indicator of performance. Most dimension stone quarried in Australia will have been tested to determine its basic physical properties. Information on imported material may be harder to come by, but a stone merchant should be able to provide you with basic data for the more popular materials.
The Certification of Stone data sheets prepared by Stone Initiatives is a convenient easy-to-read sheet that summarises the stone’s properties and possible uses. Comparison of the basic properties of your short-listed stones can be a great help in finding the right product for you.
There are four basic properties that can be used to determine likely performance.

Stain resistance
Water absorption is a useful indicator of stain resistance. High water absorption suggests the stone will be more susceptible to staining from liquids, as these materials have a greater capacity to hold the staining agents.
Water absorption is usually expressed as “weight per cent”, which is a measure of the weight of water the stone has absorbed compared to the weight of the sample. The water absorption capacity of stone can vary greatly — granite can have water absorption of less than 0.1% while some sandstone can have water absorption of more than 10 per cent. It’s useful to compare the range and standard, as specified in the relevant American Society for Testing Materials (ASTM) standard specifications, as these values can be handy as a benchmark to determine the quality of a material.
Composition also plays a major role in determining the degree of stain resistance. Some stains are caused by a chemical interaction between the stone and the staining agent. For example, most marble types contain calcite and are therefore sensitive to acidic solutions. When a substance such as wine is spilt on these stones, staining occurs not only through the absorption into the pores, but also by the etching of the surface.
Stone composed predominantly of chemically inert (stable) minerals, such as quartz and feldspar (found in granite and sandstone) is more likely to resist etching and is therefore more practical for entertaining areas.
The general appearance of the stone can also affect its “apparent” stain resistance. Highly figured and variegated stone has more potential to hide a stain than a plain white stone with similar properties.

Resistance to abrasion
Low abrasion resistance will result in significant loss of polish, scratching and, in cases of high traffic, significant change in surface profile over time. Abrasion resistance is a property based on the hardness of the minerals that make up the stone and the strength of the bond of the grains within the stone. Let’s compare sandstone with granite. Both usually contain a significant proportion of quartz, considered a hard and durable mineral, but granite is thought to be much more abrasion-resistant than sandstone.
The main reason for this difference is the mineral grains in granite are tightly packed, with the grains literally “welded” to each other. In sandstone, the grains are usually cemented together with a softer and less durable clay mineral. When sandstone is subjected to abrasive wear, the quartz grains can actually pluck out of the stone, while the tight bonding of granite will allow only the surface of the grains to wear away.

In Australia, abrasion resistance is usually evaluated by determining its Resistance to Abrasion Index: the lower the index number, the lower the resistance to abrasion. If resistance to abrasion is important, you should choose a stone that has a high index number. In high traffic areas, consider using a stone that is less likely to show tracking paths, such as stones with honed, sandblasted, antiqued or sawn finishes.

Inherent strength
Strength is an inherent property that relates to the force the material can withstand per unit of cross-sectional area. The standard unit of strength is the Mega Pascal (MPa), which describes the force (newtons) required to cause failure of the material for each mm2 of its cross-section (that is, thickness x width). The term “strength” may relate to either the compressive (crushing) or flexural (bending) strength of the material.
Flexural strength is highly relevant to the adequate performance of a dimension stone. Modern processing facilities now produce stone in longer and thinner slabs, which can be used to span greater distances with relatively lightweight units. Products such as large flagstones and veneer panels must have an adequate flexural strength to prevent failure.

What the end user of the stone product really wants is to be sure the stone will not fail or break in service. Unlike strength, breaking load is determined by the actual dimensions of the finished unit. If you’re about to outlay a considerable sum for new paving, you may want to have the typical breaking load of your flagstone determined by an independent stone expert to ensure the stone is suitable for the intended use. The relatively small additional cost may save you time, money and heartache.

Issue of durability 
Durability has always been an issue for all materials. In Australia, the most common aggressive environments are related to rising damp and salt attack. A stone’s durability in these environments can be assessed by determining its Resistance to Salt Attack. This test is specifically designed for sandstone, but is useful for testing other stone types such as low-density limestone.
The results are presented as a percentage weight loss for the specimen. It’s useful to classify the results into the various grades. Grade A is usually specified for stone to be used as curtain wall cladding and is considered suitable for use as paving around pools. Stone used as coping at the edge of a pool needs to be very durable and therefore should be Grade AA.

Durability is also associated with water absorption. High water absorption can allow aggressive solutions, such as salt-laden or acidic water, into the stone where it can physically or chemically disrupt the structure of the stone. There are exceptions to this rule, as durability is also related to pore size. But if durability (resistance to salt attack) is important to you, choose a stone with a low weight loss after salt-attack testing, a low water-absorption capacity and high strength.

Hopefully, you now have a basic understanding of some of the important properties of stone that will enable you talk to stone merchants in their own language.