Sandstone is a sedimentary stone. This means that it is formed under pressure without heat. It is caustic meaning it is composed of fragments of other rocks mainly sand-sized minerals or small grains of other rocks
Sandstone, so often sealed using penetrating systems, is not always best protected with this technology. In many cases a topical or coating sealer is the better choice.
The reason for this lies in the fact that there are many different types of sandstone. To accurately access the sealer required you must firstly understand the way and stone is formed and the different resulting types.
Sandstone is a sedimentary stone. This means that it is formed under pressure without heat. It is caustic meaning it is composed of fragments of other rocks mainly sand-sized minerals or small grains of other rocks. The main minerals are quartz (silica) and or feldspar (a silicate of mainly soda or potash).
The grains are cemented together with compounds such as calcite (calcium carbonate) iron oxides (typically from clays) and silica.Sandstone forms during two principal stages –
- A layer of sand forms as a result of sedimentation from water(lakes, rivers) or from air as in a desert.
- Once the sand has accumulated the particles are cemented together by the precipitation of minerals (calcium carbonate being the most common) within the pore spaces between the grains.
There are 3 main groups of sandstone. Arkosic , characterised by having a high amount of feldspar usually greater than 25%; Quartzose, which have a high content of quartz greater than 90% and Argillaceous which contain significant amounts of clay or silt.
It is the type of mineral and cementing agent that most influences the particular sandstones performance as well influencing how it should be sealed. Sandstone with a high feldspar or silica content is going to be more chemical resistant,harder and hence more durable. However it is the type of cementing mineral that most influences performance. For example the hardest most chemical resistant sandstone is one where a high content of silica is cemented with silica. A softer stone will have a significant quantity of clay cemented with the more common cement calcium carbonate. The density and texture of the stone is also related to its chemistry.
Sandstone with high silica in both grains and cement is going to be older and hence exposed to greater pressure over a longer period of time. This creates a denser stone with finer grain size whereas a stone with high clay and calcium content is younger, more porous with a larger more uneven grain size.
All of these factors that contribute to the formation of sandstone directly effect how a sealer technology will perform. For example dense sandstone made and cemented with silica is going to be less porous making the job of a sealer easier. The grain sizes are finer, more even and tightly cemented together making it easier for film forming sealers to bond. However the same sealer may struggle to perform on coarser grained stone where the greater surface texture and grain size variation makes it difficult for the sealer to bond. Factors such as more extreme variation of expansion and contraction between the varying grain sizes and types of minerals (silica and calcium have completely different rates of expansion and contraction) also come into play in these sandstones adversely affecting sealer performance.
So which sealers are most effective on sandstone?
In most cases penetrating type sealers work best. However I am not a fan of the lower cost siliconate sealers commonly used by the industry. Siliconates work by creating a salt crystal (either potassium or sodium) that plugs the pores of the stone. They work best where there is a good quantity of calcium to act as a reactive catalyst. In sandstone although calcium is present as a cement in many sandstones it is not present in all therefore making siliconate sealers less effective in a percentage of the sandstone family.
Siliconate sealers due not have a high threshold for temperature or expansion making them less effective on the more textured unevenly grained sandstones especially those with high calcium content.
It is the industries premium siloxane, silanes and Fluoro based sealers that best meet the characteristics of sandstone. These all have good resistance to expansion variations as well as a good ability to form within a reasonable variation in particle size. These can be carried in either water or solvent and for the main it is the water based premium sealers that work best on sandstone.Solvent carriers have very low surface tension which enhances their ability to penetrate into dense high energy surfaces. However when applied to a sandstone which has a high porosity the combination of high porosity and low surface tension means the sealer travels too deeply into the stone. To get the sealer to sit just below the surface where it performs best you need to apply a very liberal amount of a solvent sealer as compared to the higher energy water based sealer.
However there is another sealer family that is also recommended for some sandstone, these are topical or coating sealers. In many cases the more textured larger grained sandstone don’t just suffer from below surface staining due to porosity, they also suffer from surface soiling caused by the texture.
For example graffiti can be very difficult to completely remove from textured sandstone. A penetrating sealer gives no surface protection hence the need in these cases for a surface sealer. In most cases these are premium acrylic based sealers that completely coat the surface of the stone. By doing so they keep contaminants on the surface and out of the grains where they are easily removed.
The down side to this technology is that the sealer becomes the walking or wearing surface and being softer than the stone will need more frequent reapplication than a penetrating sealer.
Water based coatings are best as they have higher rates of vapour transmission (extremely important on a high porosity stone such as sandstone) than those carried by solvents and they tend not to alter the natural colour of the stone where solvents invariably do.
The type of sandstone can also have a large affect on the way it should be maintained after and before any sealing. For example in many cases a light acid wash is used after installation to remove any grout or pointing residue. However acid can damage many types of sandstone contrary to what many people believe. The most common mineral cementing the grains is calcium carbonate and acid will damage this almost immediately. Many types of sandstone have a red vein or hue which is created by iron oxide. In some cases this is soluble and easily attacked by acid the result being a bleaching of the original colour. The use of ph neutral cleaners and high alkaline cleaners is the safest way to maintain and or prepare sandstone for sealing.
In conclusion not all sandstone is composed of the same minerals.
The different combinations of base minerals and cements create different sandstones with completely different characteristics. The high silica and or feldspar stones cemented with silica will be harder, finer grained in the main and will even take a polish whereas the stones composed of clay, or lower quantities of silica or feldspar cemented with calcium carbonate will be more textured, softer and more porous. To offer the best sealer performance across such a wide range of characteristics you need to embrace and consider both penetrating and coating technology.