Bluestone is a very common name for stone that is derived from basalt which is formed when lava from a volcano cools. It is commonly available in sawn, flamed, honed or polished form. 

Bluestone is a very popular product but it may be moisture sensitive – a characteristic of the stone (basalt). This means that the natural air holes or vesicles in bluestone tend to absorb water under capillary action.

If they behave like this with water, it is likely to behave the same way when fixed with adhesive – especially one that takes a long time to cure. Many of the adhesives available on the market are in powdered form so they require water to be added to form a slurry that is applied to both the tile and substrate.

Some types of stone are able to “suck up” moisture from the slurry before it sets. Significant pressure may be involved in the process as the stone matrix “loads up” on moisture. This pressure can cause the stone to warp or change shape.

Are Other Types of Stone Water Sensitive?

Whilst some types of bluestone (basalt) are water sensitive not all are. The characteristic may affect other types of stone but it is not a common problem. It is important to ask your stone supplier about the characteristics of the stone you are interested in as it will affect the choice of adhesive that should be used.

Choice of Adhesives for Moisture Sensitive Stone

The manufacturers of adhesives are aware of moisture sensitivity in some types of stone. In such cases they will recommend an adhesive that (1) has greater strength and (2) is quick setting.

Quick setting adhesives that “go off” quickly do not allow the stone enough time to absorb enough moisture to cause distortion in shape.

Some Stone Shapes Are More Vulnerable To Warping Than Others

Long and thin pieces of stone are the most susceptible to warping. Thick pieces of stone in a square shape are the most resistant.

As the thickness of the stone increases, the risk of the stone moving out of shape decreases quite dramatically.A tile that has a length to width ratio of 1: 1 (e.g. 20x200mm) will appear to warp much less than a tile with a ratio of 3: 1 (e.g. 600x200mm) or greater.

Simple Test for Moisture Sensitivity

A simple test is to lay a bluestone tile on a wet towel overnight then place a spirit level on the surface the next day to check whether it is level. If the stone “lips” at the edges, it means that a fast setting adhesive is required as it will set before any distortion in shape will occur.

Further Reading?

If you would like to get a better understanding of how this happens you may wish to read or download a very good paper written by Jim Mann, of Stone Initiatives, called “The Moisture Sensitivity of Bluestone” here.