I've recently removed some coal dust mortar from the base (lowest six inches) of some walls where it was sounding hollow. (The mortar had been covered with thin lime plaster). Behind the mortar is solid brick wall. See image below.
My question is: what is the best way to replace the mortar? With a replacement layer of mortar? Or would it be best to simply use plaster?
Coal dust mortar is a form of hydraulic lime ( the addition of coal dust allows non-hydraulic lime to become effectively hydraulic lime ) For indoor work the following should work:- Hydraulic lime NHL 3.5 mixed 1 to 3 with sand and use a lime plaster to finish. Do not confuse Hydraulic and hydrated lime they are not the same.
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