Properties
Minerals
Sandstone consists of small grains of sand cemented together by a binding material. The sand corns are often of colourless or whitish quartz. The binding agent can contain clay or it can solely be comprised of materials, which have been dissolved in water, which has seeped through the sediment, e.g. silica, calcium carbonate, ferrous carbonate and brownstone.
Silica bound sandstone
The cementing agent in silica bound sandstone is silicon. The sand corns are often packed so tightly, that they seem to touch each other. The stone thus retains moisture from the quarry longer and must be dried well, before it is used in buildings. This is the strongest, hardest and most weather resistant, type of sandstone. The following stone types belong to this category: Nexø, Obernkirschen (Bremen) and Posta sandstones.
Calcium bound sandstone
The cementing agent in calcium bound sandstone is calcium carbonate or calcium-magnesium carbonate (dolomite). This type of stone can be recognized by the fact that it fizzes in contact with acids. It is not as hard or weatherproof as silica based sandstone. In town environments, it is attacked in the same way as limestone and if the stone is very porous, the cementing agent can quickly be dissolved.
Clay bound sandstone
The cementing agent in clay bound sandstone is clay. It can be recognized by the smell it releases, when breathed upon. It is often slated by layers of glimmer, and the stone’s strength and resistance to frost are often poor. Stones of this type are e.g. Cotta sandstone from the Pirna region near Dresden.
The cementing agent, which surrounds the sand corns, is not always homogeneous, and is also divided into contact cement and pore cement. The contact cement, which is the real cementing agent, is only found where the grains almost touch each other. The pore cement, which fills the intermediate spaces, is the dominating material, whereby the stones are classified as described above. Some clay based stones have silicon based contact cement and are thus much stronger and more weatherproof.
Colour
The stone colour is often yellowish or greyish. A large content of ferrous oxides gives a stronger yellow, brown or red colouring and can reduce the stone’s resistance to chemical disintegration. Many strongly coloured stones are though fully equal to lesser coloured stones.
Stratification
Stratification is found in almost all types of sandstone, as they are formed by sedimentation. This enables an easier quarrying, but reduces their resistance to frost. The thickness of the bed differs greatly and the beds are often separated by a layer of clay or glimmer; vertical breaks are also very common in the beds, so the stone is divided into rectangular blocks in the natural environment.
Specific weight
The specific weight of sandstone varies between 1,900 and 2,900 kg/m3 though with an average of about 2,300 kg/m3.
Porosity
The porosity of sandstone is often quite high, 3–27%. Round grains and a high content of grain cement gives a high porosity, whereas angular grains and low cement content gives lower porosity. The higher porosity of sandstone results in a lower heat transmission than in granite or dense limestone and also that lime mortars have good adhesion to sandstone.
Weather resistance
The weather resistance of sandstone differs greatly and is governed mainly by the type of cement found in the stone and by the stone’s stratification. The type of grain present is of lesser influence. Frost is usually more dangerous than chemical attack. Typical signs of a quality stone are thick layers without sharp definition, the absence of continuous layers of glimmer and the absence of pyrites.
The grains should preferably be of quartz rather than feldspar with reference to chemical decomposition. Pyrites are very damaging to sandstone. Sulphuric acid forms gypsum with lime.
Strength
The compressive strength of many good quality stones lies in the region of 70 MPa, but differs greatly.
Hardness
Hardness differs a lot, but moisture saturated stone is always softer than dry stone, for which reason stonemasons in the past, often finished the stones in the quarry. Resistance to wear is relatively low but the stone remains rough. It does not get polished by wear. Fire safety is reasonable.
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