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Old 11-22-2011, 03:07 PM   #16
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Here is a rather long explanation of what's at play with brick walls. Altghough it was aimed at a post where bricks were spalling, parts of it apply to your case. especially the part about mortars.

First, can I suggest we understand the nature of the beast.
Bricks are made from clay. Not all clays are the same, mixed in with the clay are all types of salts and minerals, similarly, all sands used in the mortars are not the same, again sands near a beach for example are full of salt.

When clay is fired in a kiln, it changes properties permanently and becomes hard, the clay technically cannot revert to its former plastic state. The degree of firing and temperature determines how hard the brick becomes. There are poorly fired bricks called "dough boys" which are terra-cotta coloured and extremely hard bricks which have had manganese added called dark manganese bricks. These represent the two extremes. All bricks are porous, the lighter the colour, the more porous and hygroscopic (absorb moisture) they become. Early bricks tended to be under fired.

When mortar is mixed, it too becomes hard. It too is porous and hygroscopic.
Simple science is at work in a brick wall. Sources of dampness like water around foundations, water from rain against the wall, or even damp air, start the process of hygroscopic osmosis. The virgin bricks absorb this moisture from the source, and continue to absorb water till a balance of water content matches the water source externally to the brick or the brick becomes saturated. When the surface dries out, water is pumped (drawn) from the source, through the brick and out through the surface. Osmosis means the salt in high concentrations in the brick and mortar, dissolves in the moisture to equalise the concentration, and is subsequently carried to the brick surface.

As the water evaporates, the salt is left behind, just below the surface and on the surface. (Thatís the white coating you see). Salt crystals form, and clump together and grow in size. This growing force is enough to spall the surface of the brick, hence the brick dust. The process continues forever till there's hardly any brick left while there is a source of water. (See older buildings and how many bricks are hollowed out.)

The key is to stop the water rising into the brick. Hence you need a damp-proof course or moisture barrier near the base of the wall and in two courses just above ground level and weep holes to let excess water out. Rain and dampness are on the outside of the surface and have a limited effect, compared to foundation water, because the pumping cycle is ground, up through brick to air, not the other way round, unless the ground bricks dry out and cycle reverses.

Another source, especially in basements is water bearing soil against the brick wall. This is more severe as the water is drawn from the outside directly through the brick and spalling on the inside surface. You need to tank the outside surface, by a waterproof membrane or bituminized tar layer, and provide drainage pipes at the base of scoria backfill.

The whole plan of attack, is to stop water getting into the brick. It may pay to call in a waterproofing expert, to see what needs to be done. The advice may not cost much, but the prevention measures will. It's an expense you have to make, because nothing else will stop the spalling cycle.

To assist you, here is a good description of different mortars in the following link that you should use to patch up the joints. (Note this is cosmetic and doesn't solve the problem for more than a few years.)
www.abcbricksales.com.au/howtomortar.pdf or www.concrete.net.au/publications/pdf/mortarmixes1.pdf
Cheers, hope you weren't bored to death, from Joe in Oz



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