Sandstone, the signature stone of Sydney, could be considered Australia’s premier building material. It was a source of shelter and protection well before European settlement, and early settlers quickly identified its workability. The survival of some of Sydney’s earliest buildings is a testament to the durability of this versatile stone.
Modern manufacturing processes have freed sandstone from its early use as large hewn blocks and it is now used in many different formats on the inside and outside of commercial and public buildings as well as homes throughout Australia. The modern use of sandstone as thin tiles and large format cladding presents new challenges to the performance of the stone and it is now more important than ever to have an understanding of what sandstone is and how to choose a good quality stone.
Sandstone is a sedimentary rock composed of sand-sized particles and weathered rock fragments which are deposited at locations such as beaches, flood plains and deltas over millions of years. After deposition the sediment undergoes a process called diagenesis which involves de-watering, compaction and cementation due to pressure applied by overlaying sediment which results in the formation of sandstone rock. During diagenesis the individual grains are bonded or cemented together by other minerals such as clay (such as in Wondabyne or Helidon sandstone), silica (e.g. Donnybrook) or iron oxides (e.g. Northern Territory Red).
Within Australia, sandstone can be designated into two major groups based on their bonding materials. The largest group is classified as argillaceous sandstone where fine clay particles that are interspersed with the sand grains are compacted by pressure forming a cementitious matrix. The presence of the clay within the voids between the sand grains reduces the porosity of the stone and increases its strength.
The type of clay present is a determining factor in the stability and durability of the sandstone. Clay types such as kaolinite are relatively stable as they only experience minor swelling when wet and return to their original dimensions upon drying. Clay types such as smectite and vermiculite are unstable as they undergo significant swelling and can experience a change in dimensions upon drying. Excessive swelling of clay particles can tear the stone apart resulting in cracking or delamination. All argillaceous sandstones will undergo loss of strength when wet.
Siliceous sandstone is composed predominantly of quartz grains which are chemically bonded or ‘welded’ together by the deposition of silica. Siliceous sandstone is usually quartz rich and referred to as quartzite. The absence of densely packed clay within the voids can result in a porosity that is higher than an argillaceous sandstone. However, chemical welding of the sand grains in this type of sandstone usually produces a strong stone that maintains more of its strength when wet as it does not rely on clay for bonding.
The colour of sandstone is greatly influenced by the minor minerals and trace oxides present. Siliceous sandstone which is often relatively pure and quartz-rich is usually white or cream. Argillaceous sandstone is often grey to tan or brown due to the clay component and the presence of iron bearing minerals. As sandstone is formed from sediment, stone types can vary in appearance depending on whether they are viewed perpendicular or parallel to the bedding plane.
Soluble minerals present within the groundwater can lead to the production of banding within the stone known as Liesegang Rings (e.g. Mount White). The rings are formed by the deposition of dissolved minerals at ‘tide lines’ as the level of the groundwater rises and falls throughout the seasons. As Liesegang Rings are produced in an ‘onion ring’ fashion, the appearance of the bands will vary depending on the slabbing orientation.
Some stone types change colour after quarrying such as the ‘yellow block’ oxidising sandstones from the Gosford region (e.g. Piles Creek Guinea Gold). These sandstones develop their colour change as soluble minerals such as iron and manganese oxides which are present within the natural moisture in the stone are deposited at the surface. The colour change can commence within days of quarrying and continue for months with minor changes in hue occurring over years as the minerals are exposed to the environment.
As sandstone is formed from sediment, stone types that have variability in the bedding sediment are more likely to display changes in appearance and strength when slabbed in different orientations.
Sandstone is classified as a ‘free stone’ and is able to be easily worked in all orientations making it a very versatile stone. There are literally hundreds of available sandstone types suitable for many different uses. Fine, even-grained sandstone is suitable for carving into intricate statuary, and ornate figuring adorns many of our historic buildings. Other sandstone types with adequate strength and durability are suitable for use as paving and external cladding. High-strength siliceous sandstone can be processed into thin tiles for floor tiles and wall cladding.
Large cubic-form sandstone ashlar is used extensively in new buildings as well as maintaining our heritage buildings, while even larger sandstone blocks are commonly used for retaining walls and breakwaters.
Sandstone is a sedimentary rock which can produce significant variations in strength in different orientations. The multitude of bedding layers present in a sandstone can be compared to the pages of a telephone book where it is easy to turn the pages but extremely difficult to tear the book in half. In a similar way, a sandstone will always be stronger when loaded perpendicular to the bedding compared to a load applied parallel to the bed. The difference in strength between orientations will vary depending on the stone composition but it is always good practice to ensure any load is applied perpendicular to the bedding. As an example, paving units should always be laid with the bedding horizontal.
Argillaceous sandstone can lose significant strength when wet. It is important that this strength loss is taken into account when determining load bearing requirements.
The use of sandstone as pool surround paving is increasing in popularity as it is naturally slip resistant and cool under foot. Careful selection is required for sandstone that is to be used in an environment where it will be exposed to salt or frequent wet-dry cycles. To assist in determining the suitability of the stone, ask your supplier for Resistance to Salt Attack test results as this will give you guidance on the durability of the stone. A maximum weight loss of 1.0 per cent is recommended for sandstone used in the vicinity of pools and preferably less than 0.1 per cent for stone used at the pool tide line. It is also important to note that the improper use of coating or sealers can actually reduce the durability of the stone.
Sandstone has a relatively low abrasion resistance compared to granite and although this is not usually an issue it should be taken account in high traffic areas especially where smooth finishes are used.
Standard specification ASTM C616-10 provides a guide to the selection of sandstone dimension stone suitable for general building and structural purposes. The standard provides separate requirements for sandstone (usually argillaceous), quartzitic sandstone and quartzite (usually siliceous). These classifications are based on the free silica constant of the stone. The specification of any dimension stone should be based on location, design and engineering considerations specific to the intended use. The specification states a minimum strength requirement which requires the determination of both dried and soaked strength. It is also important to note that the strength results are likely to vary significantly with bedding orientation.
Water absorption and modulus of rupture are the key performance indicators for this stone and should be evaluated closely throughout the project supply phase to ensure adequate performance in service.
Sandstone is a versatile stone type available in a wide range of colours and textures. It is a stone type that is equally appealing when used as a large edifice or as backyard paving. Taking note of the following points will assist you in the selection of the right stone for the job and its maintenance well into the future:
- Ensure soaked strength results are used to determine load bearing capacity of paving units in wet environments.
- Ask for resistance to salt attack test results when selecting stone for use as pool surround to ensure the durability of the stone is adequate.
- To maximise the load bearing capacity of paving the bedding should be laid horizontally.
- If the stone has inadequate durability for use in an aggressive environment – don’t use it.
If you are considering sealing your sandstone:
- Think twice about why you are sealing – most sealers are designed to improve stain resistance not the durability of the stone.
- NEVER use a topical sealer if the stone may become wet as it will trap in moisture.
- ALWAYS use an impregnating sealer which will allow the stone to breathe.
- Check pavement design to ensure salt will not become trapped behind the sealer.
Article by Jim Mann, stone consultant and principal of Stone Initiatives.