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Managing mortar in extreme conditions



Managing mortar in extreme conditions

11th August 2009:  When modern Portland cement mortars provided an alternative to lime-based mortar almost 100 years ago, it looked as though bricklaying had been freed from the restrictions traditionally imposed by the weather. Mortar made using Portland cement sets at relatively low temperatures and as it sets quickly, it was less likely to be damaged by rain or frost.

Of course, life is never quite that simple and in extreme weather a few simple precautions are needed to ensure a good result.

In very hot weather the main concern is the rate at which water is removed from the mortar either by suction of the masonry or evaporation. Experienced bricklayers know instinctively that on very hot days warm bricks and blocks will generally absorb more water from the mortar. The mortar will also tend to lose its plasticity at a faster rate due to the evaporation of the water from the mix.

Mortar mixed at high temperatures may have a higher water content, a lower air content and a shorter board life. The quality of the bond between the mortar and the brick or block is dependent on having the correct amount of water and so in high temperatures this bond may be affected.

Practical steps to help avoid any problems include storing bricks and blocks in the shade to help control heat gain. Spraying with modest amounts of clean water is another option, although they certainly don’t want to be soaked. In hot summer conditions materials and mixing equipment can be shaded from direct sunlight prior to use. Mortar tubs and mortar boards should be rinsed with cool water before coming into contact with the mortar.

Other suggestions for avoiding problems in hot, dry or windy conditions include finishing the bricks or blocks more quickly after placing the mortar and providing some form of covering or shading.

Where ready-to-use mortar is being stored on site it is obviously important to keep it well covered in the tub. Dry Silo Mortar - where the dry sand and cement mortar is stored on site in a hopper and mixed with water on demand - offers the added advantage of being able to mix small batches which can be used up quickly.

Hydration and strength development - ‘setting’ - in mortar usually occurs at temperatures above 4oC. If mortar is used below this temperature it may not set properly and if water is retained in the joint, frost damage can result.

Fortunately there are relatively few periods in the UK when the daytime air temperature remains below 4oC and if it is below freezing it may be impractical to continue with masonry work in any case; not least because the outdoor water supply will freeze.

However, during the winter months all stocks of bricks and blocks should be covered to provide protection against rain, frost and snow. Bricks or blocks that become saturated should not be used until they have dried out and in cold weather they risk damage if they freeze.

Mortar likewise needs protection during very cold weather. If mortar freezes during storage any frozen material must be discarded. Neither should mortar be laid on frozen surfaces. Anti freeze agents for mortar are not recognised in British or European Standards.

As mortar hardens and develops strength more slowly in cold weather, new masonry, or areas under construction, should be covered and protected from the elements. This is likely to require two layers - thermal protection such as hessian or some form of quilting and a waterproof sheet to stop the under layer getting wet.

Protective covers should not be in contact with the face of the wall to avoid ‘sweating’ and consequent staining. The covers should be secure and kept in place until the mortar dries.

Another undesirable and highly visible feature on new brickwork is efflorescence. This is the white marking which may appear on brickwork a short while after a building has been completed. Its cause is complex and relates to the presence of soluble alkali salts in the building materials and it can be difficult to eliminate completely. However, controlling moisture levels during construction and keeping heavy or prolonged rain off the masonry during construction and before the mortar has set are sensible precautions.

There are cases where mortar must perform under less than idea conditions and there is a range of cements and admixtures, including bonding agents and water repellents from the Admixtures division of CEMEX, the leading building materials provider, that provide mortar with special characteristics.

A series of practical and technical guides and bulletins are available from CEMEX - including the Experts in Mortar Guide

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