What's problematic with cement mortar? Modern cement pointing differs significantly from lime mortar. It is rigid, less porous, and at times entirely waterproof. Its application on traditional masonry causes various types of harm.
Cement pointing, being harder than soft brick or stone, lacks the flexibility to accommodate wall movement or settlement. Consequently, when movement occurs, the stone or brick edges press against the unyielding mortar, resulting in masonry chipping and mortar cracking.
Rainwater penetration into the pointing's cracks and around stone edges exacerbates damage. As the mortar is impermeable, trapped moisture cannot evaporate from the joints post-rain. Instead, it forces evaporation through the brick or stone surface, causing crystallization of soluble salts, leading to decay and crumbling of surface layers in the masonry. In severe cases, the entire stone face might deteriorate, leaving the rigid cement pointing intact. This trapped moisture enhances vulnerability to frost damage in winter.
In contrast, flexible lime mortar enables moisture movement, allowing for evaporation and salt deposition in the mortar joints, shielding the stone or brick from decay. Repairing or replacing damaged brick or stone is more complex and expensive than repointing a wall, and it involves less loss of historically significant material.
Concerning cement render, it presents different issues. Fine cracks typically form due to shrinkage in the render, leading to capillary-driven rainwater penetration, which diffuses into the wall. Once inside, the impermeable render traps moisture, contributing to rising damp, and results in internal dampness, damaging plaster and decorations. Strangely, waterproof render can elevate internal dampness levels. Conversely, a porous lime render aids in moisture evaporation, thereby mitigating the effects of penetrating and rising damp."
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