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Tuesday, 19 October 2010
Advice on re-pointing of old brickwork
Answer: Possibly the worst thing you could do would be to re-point using a cementatious mortar. Even one with lime added.
Older, softer bricks and stones were always laid with a lime mortar. This allowed any moisture within the wall to evaporate through the mortar rather than the walling medium. Inevitably, over time, the action of the weather would mean that the mortar would recede and therefore need re-pointing. But this was by design.
If you replace the lime mortar with a cement mortar, then it will be stronger than the bricks. It will also block the egress of any moisture and, instead of the mortar dying back it will be the bricks or stones that flake off and spall.
Traditional lime mortar is made by mixing one part lime putty, which is available in 55lb (25kg) tubs, with 3.5 parts well-graded sharp washed sand. Adding a pozzolan of burnt dry clay, one part to eight of mortar, can aid the setting in an exterior situation. The tubs will cost about £8.70, and a semi-skilled person could expect to re-point about two square metres a day.
David Snell Daily Telegraph 14th October 2010