Using the correct mortar is essential when building and repointing.
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If the mortar is too strong for the brick then the moisture in the wall is retained in the brick rather than joint, this means the brick is overworked during evaporation and freezing of the moisture. This will eventually result in the spalling of the face of the brick . Once spalling occurs the erosion of the face is increased. So remember, the mortar must never be stronger than the brick it is used with, dense bricks, dense mortar, soft bricks, softer mortar. Mortars are differentiated primarily by their strength: M is the highest strength, S is next, and N is a moderate strength mortar. (O and K are lower strengths yet, which is important in repair jobd so as not to create a mortar that is stronger than the wall/units where it is being placed.) Polymer cement mortars (PCM) are the materials which are made by partially replacing the cement hydrate binders of conventional cement mortar with polymers. The polymeric admixtures include latexes or emulsions, redispersible polymer powders, water-soluble polymers, liquid resins and monomers. It has low permeability, and it reduces the incidence of drying shrinkage cracking, mainly designed for repairing concrete structures