Engineered stone, reconstituted stone or agglomerate is a term used to describe a man-made stone or tile product that usually consists of stone pieces and/or aggregates held together in a synthetic binder such as a polyester resin or epoxy resin. They offer a cost-effective alternative to natural stone.

Most engineered or agglomerate tile will have similar characteristics to the type of mineral used in the matrix, and are frequently composed of or classified as granite, quartz or marble. For instance, if the constituent mineral is moisture sensitive, the tile may be as well.

In some cases, the type of binder may also have a significant effect on the behaviour and performance of the engineered product. Polyester resins have a high thermal coefficient of expansion and could present problems in differential movement when installed in exterior installations, such as building façades.

Obviously, check with the tile manufacturer to verify the acceptability of each engineered/agglomerated tile for its intended purpose. For instance, certain tiles may not be appropriate on an exterior application due to moisture sensitivity. Some resin and stone pieces are vulnerable to extreme temperature changes, are not resistant to ultraviolet light (UV) or stable in freeze/thaw conditions.
Engineered tile can often be found in larger format sizes, with 900 by 900mm now commonplace.

It is important to note that the facial dimensions and thickness of the engineered tile have a direct bearing on the dimensional stability of the tile.

It is also a good idea to check with the tile manufacturer for their recommendations on setting materials. They may also have requirements on the moisture vapour emission rate (MVER) of the mortar bed or concrete prior to installation of their tile, and may even prohibit the installation of their product using traditional sand and cement methods.

Some engineered tile manufacturers issue a table or chart listing which of their products are suitable for using normal setting materials, which ones call for rapid setting materials, and which tile products need 100% solids epoxy setting material.

Conducting a test in the actual conditions is often prescribed, using the specified installation materials, to determine acceptability of all materials and conditions.
Tiles are manufactured in a broad array of colours and can be engineered to offer technical benefits such as scratch resistance and flexural strength. They are marketed under a wide range of generic names, including conglomerates, resin agglomerated tiles, reconstituted stone, re-composed stone as well as engineered stone.

The manufacturing process combines a mixture of crushed marble or stone chips and powdered stone, together with colours and polyester resin as the binder. The dry materials are combined and the resin and hardener added to make a mass that is placed in an enclosed chamber and put under negative pressure, using a vacuum pump, to remove air bubbles.

After the polyester resin reacts, it binds the entire mass into a solid block that is then cured and sliced into tiles and slabs. These tiles and slabs are then further processed by polishing the front and grinding the back to a uniform thickness. Slabs can also be cast by this process, and will be processed by grinding and polishing to produce finished slabs with the appearance of marble or other stone, but at a more economical cost than natural stone cut from block.

Normally, the back of all the agglomerated stone is passed through a grinder to gauge the material to a uniform thickness and to remove all irregular protrusions. The thickness is normally gauged accurately within +/- 1mm.
This reconstituted stone material is generally used, worked and installed just as natural stone tiles or ceramic tiles. Slabs cut in the larger sizes are used for floors and walls similar to natural granite or marble slabs.

There are two main types of polymer-resin binder used in the manufacture of engineered stone tiles: epoxide or polyester. Typically the ratio of resin binder to agglomerates varies from 4% to 8%, which means, of course, they also contain 96% to 92% of stone/quartz filler. It is a general rule that the higher the percentage of resin present, the lower the abrasion resistance. The proportion of resin also affects the tile’s coefficient of thermal expansion.

This is important due to the increasing use of underfloor or undertile heating systems (UFH), becoming popular as a heating choice. When considering a rigid tile finish over UFH, it is important to understand how the heat change affects the different materials employed within the tiling installation.

The coefficient of thermal expansion for a typical sub-floor – such as concrete or cement – is similar to a cementitious-based tile adhesive. However resin based tiles generally have a higher coefficient of thermal expansion. In simple terms, the tile has a high modulus of rupture, or measure of strength before rupture, and is being “restrained” by the tile adhesive.

However due to heating and cooling cycles, a relatively small amount of heat expansion and contraction can exert a high level of stress on the tile adhesive. And, the larger the tile, the greater the dimensional changes on resin agglomerated stone caused by the thermal expansion or temperature increases.
Resin based stone tiles also have varying degrees of moisture sensitivity. This means that these tiles can also be susceptible to differential moisture expansion. In severe cases, this can lead to “curling” of the tiles.

The (UK) Tile Association technical document on ‘Tiling with Resin Agglomerated Tiles’ suggests that: “Cementitious floor screeds to receive resin based tiles should be completely cured and tested to ensure that they have a moisture content of not more than 2% by weight or 75% relative humidity using the appropriate equipment and that an appropriate adhesive is chosen”.

This means that the use of a cementitious-based tile adhesive or screed will introduce moisture directly beneath the resin agglomerated stone tile which may be taken up slowly by the tile. Some loss of moisture will occur when the tile joints are unfilled; although this process will be slower.

The larger the format size, the more likely it is that moisture will become trapped at the tile/adhesive interface. In the case of some resin based tiles, this will increase the risk of curling of the tiles away from the tile adhesive bed.
It is common practice for the manufacturer of large format tiles to fix a reinforcing mesh to the underside. Consideration must be made to the type of adhesive used to fix the mesh so it will not have an adverse effect on the adhesive bond.

British Standards Code of Practice BS 5385-3 proposes that the mesh on the back of the tile should not obscure more than 25% of the total surface area. If it does, at least 75% of the mesh must be removed or the tiles be mechanically fixed. BS 5385-3 recommends further that resin based agglomerated stone tiles should not be used externally.

Therefore, the correct selection of tile adhesive is essential when fixing reconstituted resin based tiles, and advice must be sought from the tile manufacturer or supplier. It is always good practice to carry out a trial test area first.

Adhesive selection guidelines

Engineered tiles are neither classed as ceramic nor, strictly speaking, as natural stone. BS 5385, states: “To avoid moisture from the adhesive bed distorting resin-based agglomerated stone, reaction resin adhesives, or quick drying low alkalinity cement-based adhesives should be used”.

When installing a ceramic tile or natural stone tile using a tile adhesive, the accuracy of a sub-floor should be such that it facilitates the accurate fixing to the desired planarity. If this is not the case, this may be rectified using a smoothing or levelling compound prior to the tile installation.

In the case of fixing large format tiles – in particular, resin agglomerated stone tiles – the surface regularity should be of a much higher tolerance for both the wall and floor substrates.

The tile adhesive should be applied using a notched trowel to the wall or floor and additional back buttering of the tile with the tile adhesive to ensure a full solid bed and adequate adhesive coverage.

When fixing resin agglomerate tiles to both walls and floors, solid bed fixing is essential to:
•     Ensure full contact between the tile adhesive and the tiles
•     Eliminate voids beneath the tiles

Engineered tiles are generally not recommended for use in frequently wet areas such as wet rooms, walk-in showers, or swimming pools. For domestic showers and wet rooms, specifiers should seek advice from the tile supplier or manufacturer with respect to their suitability in these applications. It is also important to establish what, if any, additional precautions may be necessary to take both during and following completion of the tiling installation.
Fundamentally the wall and floor background substrates should be protected from moisture ingress or leaks by using a waterproofing tanking system. (Laticrete has its Hydro Ban product.)

Additional protection from potential water ingress may be afforded by using an impervious reaction resin grout. (Laticrete makes a Spectralock Pro Premium Grout.) But the use of impervious grouts and adhesives is no substitute for a tanked installation.

The need for inclusion of movement control joints within any tiled installation is explained in the BS 5385. Any movement, such as drying shrinkage, thermal expansion/contraction and moisture movement will generate stresses within the tiling system which can lead to de-bonding or cracking of the grouting and/or tiles.

Where the resin based tile is deemed to be suitable for floors subjected to direct heat or heat from an environmental source, the (UK) Tile Association advises the frequency of movement joints need to be increased and fit for purpose. For instance, the ‘Tiling with Resin Agglomerated Tiles’ technical document says the floor area should be divided into bays not exceeding 25 to allow for the anticipated increase in thermal expansion of the resin agglomerated tiles with UFH. Under normal dry internal conditions, the advice is that bay sizes should be reduced to bay sizes not exceeding 36sqm.

It is always advisable to use white adhesives for installing white or light-coloured engineered/agglomerate tile. In all cases, proper substrate preparation and attention to detail is paramount to a long lasting and problem free installation.
Attention to detail will minimise lippage, reduce moisture-related concerns and eliminate problems caused by poor choice of setting materials. Once again, it is recommended to verify the suitability of the installation system by conducting a test area. This can help provide peace of mind for the architect, builder, tile contractor, and building owners.

By Michelle Costigan, technical product manager, Laticrete UK