Fireplace Slabbing

Mixes of cement and sand have been used for years as backing material for tile, brick, stone and marble fireplaces, but such mixes have a number of disadvantages. They add considerable weight to an already heavy unit and transmit large amounts of heat to the surface causing cracking, crazing and loss of adhesion of ceramic tiles or discolouration of light coloured brick and stone. Perlite fireplace mixes on the other hand are light, non-conbustible, stable and unlikely to fail with age during normal service conditions.

Materials required

• Ultra lightweight nature reduces the tendency to sag and run.
• Special grading process ensures absence of oversize particles which could mar the appearance.
• Inert in an industrial atmosphere (chemically like a foam glass) so it is not a contributor to yellowing.
• Forms an excellent insulator in its own right and increases those of the coating to that of wood at the same thickness.

Suggested formulations

It is recommended that the tiles, brick or stone around the fire opening and in the ara of the hearth should be set in a 1:7 by volume high alumina cement:sand mix to a width of 100mm.

The remainder of the fireplace should be filled with a mix by volume of

• One part high alumina cement
• One part sand
• Four parts Perlite

This mix can be made by adding a 100 litre sack of perlite to a premix of 30 litres of high alumina cement, 30 litres of sand with water containing Silvacol and air-entraining admixture (if required).

Factory Mixing

A concrete free-fall (or drum) mixer is satisfactory for producing the mix. Add 40g of Silvacol admixture to every 25 litres of water and allow to stand for 1 ½ hours with occasional stirring. To this solution can be added 150ml of air-entraining admixture if desired (this is only a general recommendation as commercial materials vary considerably and the manufacturer's specific instructions should be followed).

Add the water containing the admixtures to the mixer and then add the high alumina cement and sand, and allow to mix until a slurry has been produced. Then add the perlite and mix for as short a time as possible, adding more water if necessary to obtain the required consistency.

Site placing

Coat the back of the tiles, brick or stone and the fireplace and hearth with a “slab-wash” which is a dilution of polymol adhesive or similar styrene-butadiene co-polymer latex in order to reduce the initial suction rate.

Place around the fire opening to a width of 100mm, and in the hearth area, 1:7 high alumina cement and sand mix.

Fill the rest of the fireplace back with the lightweight 1:1:4 mix, compacting it to ensure that any air pockets are removed. The fireplace should be covered with a sheet of polythene and allowed to cure for at least 18 hours. Power compaction should not be used

Fire protection

Perlite fireplace mixes are non-combustible as defined in BS476 Part 4 1970 and can be designated Class 0 in accordance with the requirement of the Building Regulations. Also BS EN ISO 1182 2002 Reaction to fire tests for Building Products None Combustibility test.

Durabilability

Perlite fireplace mixes, when made and cured in accordance with the recommendations of the data sheet, are completely stable and unlikely to fail with age during normal service conditions.

Packaging

Perlite grade P321 or P35 are supplied in 100 litre plastic bags (3.5 ft3) weighing approximately 10kg.