R. G. Forrester - Trading as
Omega Consulting Services
Concrete Technology, Structural Repair, and Rehabilitation Specification Services
P O Box 782Magaliesburg1791
Tel: +27 14 577 2165Fax: 088014 577 2165
Cell: +2782 905 2818E-mail: omegaconsult@telkomsa.net
20 March 2006
Using Natural Sandstone Products.
Sandstone has been in use as a building material for thousands of years. It has an extremely acceptable appearance, which
enhances any structure where it is used. Its natural beauty is added to its exceptional weathering ability, and it requires no
further surface treatment to provide a long lasting finish.
Types.
There are a number of different products available from Lesotho Stone Enterprises (Pty) Ltd. These include, loose and
interlocking rivens, dry wall packing, block wave, random, regular, flat, raised, and crazy cladding, blocks, window sills, tiles,
bullnoses, pillar caps and fireplaces.
Description.
Lesotho Stone Enterprises (Pty) Ltd sandstone is a natural product, which has a reasonably high compressive strength and
relatively low water absorption qualities. The sandstone is cut from natural resources, using advanced stone mining
machinery and technology. Large quantities of the material are mined, dressed and erected annually.
In order to enjoy the benefits of the sandstone facades and finishes, it is essential that the materials be correctly erected.
Following the simple rules will provide long-lasting results from the naturally warm and beautiful appearance of the material
Application.
Natural sandstone has a reasonably low water absorption. In addition, it has a compressive strength much higher than
manufactured bricks and pavers. Therefore, it is important that the material is used correctly to harness its qualities correctly.
The average water absorption of sandstone products is of the order of 8%, which is about half the water absorption exhibited
by baked common clay bricks. Concrete bricks, on the other hand, have a lower water absorption than sandstone.
Suction
It is a fact of nature that when a dry material is placed against a wet material, there is a transfer of the fluid material to restore
equilibrium. Since there is some water absorption by sandstone, it is important that the products be soaked in water before
erection. This is to prevent the suction of moisture from the mortar used to bond the material to the substrate.
Whilst the water content of mortar is higher than is used in concrete, because mortar is applied in a relatively thin layer, the
suction of the bricks, and sandstone, can rob the mortar of the moisture required for hydration. This results in the mortar layer
losing its ability to adhere, or bond, to the back of the sandstone as well as the substrate.
If the sandstone products are to be placed against common clay bricks, the clay bricks must ALSO be well wetted to prevent
the same phenomenon from occurring. A saturated, surface dry condition is required. Since common clay bricks can absorb
500 ml of moisture into one clay brick in only 30 minutes, it is useless to merely sprinkle water onto the surface. The surface
must be totally saturated before applying the mortar. If concrete bricks are used, the wetting of the wall, prior to applying the
mortar, may be excluded.
Cement.
Cement can be CEM I, CEM II-A, or masonry cement. All cement should comply with the requirements of EN 197.If masonry
cement is used, builders lime must NOT be added to the mix. Masonry cement mortar needs to be richer in cement quantity
than common cement mortar because of the lower strength achieved when using masonry cement.
Sand.
The use of a good quality mortar is recommended. It is important that good quality mortar sand be used. High clay content
materials should be avoided at all costs as the clay component has a very high water demand, which will negatively affect the
strength of the mortar.
To test a sand, mix 5 kg cement with 25 kg of the sand. Then measure the amount of mixing water that is required to give a
good consistency to the mix. An excellent sand will require 5 litres of water, and an average sand will require 6 litres. If the
mix requires more than 6 litres of water, look for a better sand source.
Mortar.
A 1:6 by volume mix could be used. This means 1 bag cement to 3 builder’s wheelbarrows of sand. 1 bag of cement has a
volume of only 33 litres, which is equal to only 0.5 of a wheelbarrow. In addition, when filling the wheelbarrow with sand, it
MUST be leveled off (not heaped).
If the sand has a shortage of fine material, building lime may be added to the mix. 40 litres of building lime may be added to a
50 kg bag of cement. Do not use quick-lime or agricultural lime, as these materials are not suitable for mortar.
Mix only in sufficient quantities to ensure that the full mix can be used in 2 hours. It is more sensible to make 3 – 4 mixes per
day of mortar than one big mix. Mixing the big mix takes longer, and requires frequent re-mixing, with the unsuitable addition
of further water. Smaller mixes can be done quicker, and the mortar is always fresh.
Re-tempering.
It is extremely bad practice to re-temper mortar mixes. Re-tempering means the addition of more water to previously mixed
mortar. It is a fact of life that on most building sites, a large volume of mortar is mixed at the beginning of the day, and then
water added, throughout the day, to maintain the workability. This practice is guaranteed to produce failures, and must not be
done.
In tests, it has been proved that mixing smaller mixes, throughout the day, produces better workmanship, saves time and
wastage, and improves production. Remember that each time that water is added, the mix is weakened, and extra labour is
required.
Bond coats.
Bond enhancement can be carried out if required. A bonding slurry can be applied to the wall and the back of the sandstone
materials. This does not mean that the pre-wetting of clay brick or sandstone can be eliminated.
Bonding slurry normally comprises of a 1:1 cement:mortar-sand mix. A bonding liquid, such as styrene butadiene rubber
(SBR), acrylic, or polyvinyl acetate (PVA) emulsion, may be added to the mixing water. Follow the emulsion manufacturer’s
instructions, but never use un-mixed emulsions as they tend to skin over. Always apply the mortar to the WET bond coat,
which should be mixed to a thick fluid consistency.
Omega Consulting Services
Gordon Forrester
A.C.T.Dip. C&G Concrete Technology Dip