Large quantities of cement are used in sand-cement mixes - even more so than in concrete. Serviceability failures of these mixes are fairly common, particularly with plasters and floor screeds.
Typical problems include:
- Poor abrasion resistance
- Unacceptable surface finish
As far as mortar mix is concerned, a common problem is the leaching of lime from the mortar in face brick masonry.
In all cases the failures can be traced to one or more of the following:
- Inadequate specifications
- Poor materials selection
- Incorrect mix proportions
- Poor site practice and bad habits
This tip covers plaster mix and mortar mix. Part two (tip 9) will cover sand-cement floor screeds.
Traditionally sand-cement mixes have always been specified in terms of mix proportions by volume and not by performance. This is still almost universal practice in South Africa. The exception is SABS 0164, The Structural Use of Masonry. This standard specifies mortar mix by compressive strength in Table 1 and then goes on to suggest suitable mix proportions which should satisfy those strength requirements in Appendix C-2.
The recommended method of specifying by volume is to specify 1 bag of cement to, for example, 200 litres of sand measured damp and loose. The reasons are that specifying a mix as 1:6 by volume is open to serious misinterpretation (e.g. 1 bag of cement to 6 wheelbarrows of sand) and the fact that that sand’s bulk changes when damp - often by as much as 30% by volume.
To put some numbers on bulking, 200 litres of dry sand would typically weigh about 320 kg whereas the sand component of the same sand in a damp condition could weigh as little as 205 kg if the sand bulks 30%.
In terms of mass, the mortar mix proportions could therefore vary from 1:6,4 to 1:4,1 - depending on the moisture content of the sand.
The following cements are suitable for use in a mortar mix or plaster mix :
- CEM I
- CEM IIA
- CEM IIB
- CEM IIIA
- MC 12,5
(Fulton’s Concrete Technology, 8th edition)
Builders Lime (complying with SABS 523) is used to improve the workability, plasticity and water retention of a mortar mix or a plaster mix . Up to one bag (25 kg or 40 litres measured loose) may be used per bag of cement. The quantity added depends on the fine content of the sand.
Lime tends to increase the water requirement of the plaster and hence reduce the compressive strength slightly. The improved workability and water retention results in better bonding and increased impermeability. Lime must not be used with masonry cements.
Chemical Admixtures And Additives:
These include plasticisers, retardants, accelerators, bonding aids, water proofing agents and pigments. There is a large range of proprietary admixtures and additives available.
Generally speaking the manufacturers’ instructions should be followed closely and expert advice should be sought on admixtures, if in doubt.
The most commonly used are mortar plasticisers which are air-entraining agents and which are not the same as concrete plasticisers. Their use in concrete will seriously affect the compressive strength of the concrete. Mortar plasticisers must also not be used with masonry cements.
The quality of the sand is the main determinant of the quality of the mortar mix or plaster mix . Sand for use in mortar and plaster is specified in SABS 1090.
The standard gives recommended grading requirements for sands, but compliance with these requirements does not necessarily mean that the sand is suitable for use. The converse also applies. Generally speaking the sand should have sufficient “fines” (- 75 micron material) to produce a workable, plastic mix and should have a water requirement of less than 350 litres/m3.
The best indicator of sand quality, apart from experience, is a simple field test:
- Dry approximately 50 kg of the sand (air dry is sufficient)
- Weigh out 5 kg of cement and 25 kg of the sand and measure 5 litres, 1 litre and 1,5 litres of water in separate containers
- Mix the sand and cement to a uniform colour
- Mix in the three quantities of water, one after another, until the mix is workable enough for use as plaster
If 5 litres of water is enough, the quality of the sand is good. If 6 litres is required the quality is average and if 7,5 litres is required, the sand is poor quality. If more than 7,5 litres is required the sand is very poor.
Good quality sand is suitable for all grades of mortar and plaster. Average quality sand is suitable for mortar and interior plaster. Poor and very poor quality sands should not be used if at all possible.
Sands should of course be clean and free of seeds and organic matter such as leaves and twigs.
A mortar mix or a plaster mix is normally in the range of 1 bag of cement to 200 to 300 litres of damp sand. Mixes richer than this are typically used where masonry is highly stressed or where plaster is subject to impact - for example squash court walls. Leaner mixes are used on soft, friable surfaces such as poorly baked and sun-dried bricks.
A common problem with plasters is the use of too rich a mix. Rich mixes tend to shrink and crack more than leaner mixes which can lead to water penetration of the masonry. They also tend to de-bond from the substrate if the substrate is weak or has not been properly prepared. A less common problem is the use of too lean a mix which results in soft, friable plaster.
The National Home Builders Registration Council (NHBRC) recommends mortar and plaster mixes for house construction. These recommendations are, however, currently under review.
Poor site practice is the cause of many plaster problems and should be avoided to ensure the durability of a project. Some of these practices include:
- Floating neat cement into the surface of the plaster to “improve” the finish
- Mixing too much mortar or plaster at one time – this results in re-tempering of the mix, sometimes even on the following day. Plaster and mortar should be used within two hours of mixing
- Adding gypsum based plaster (e.g. Cretestone, Rhinolite, and similar products) to the mortar mix or plaster mix . This is done to get the mix to stiffen quickly when plastering door and window reveals. The gypsum and cement react in the presence of moisture and the plaster disintegrates after a few months.
- Adding extra cement to the mix to improve workability
- Inadequate preparation of the substrate, particularly concrete surfaces
- Inaccurate and inconsistent batching
- Over-reliance on the abilities of bonding aids