Adhesive mortar — also called bonding mortar — is a general term for a thin mortar used wherever a better bond with the background is wanted. Where an ordinary masonry mortar mainly fills and bears, an adhesive mortar is meant to stick: applied thin, combed with a notched trowel, and the material pressed into it. The Dutch term is kleefmortel.
Adhesive mortars are usually cement-based and contain polymer additives that improve adhesion, open time and flexibility.
Where it is used
- Tiling — fixing wall and floor tiles; on walls the mortar’s non-slump property keeps tiles from sliding down
- Bonding insulation — in an external wall insulation system the boards are bedded on adhesive mortar (and normally mechanically fixed as well)
- Copings and units — fixing concrete coping stones, natural stone elements and profiles
- Bonding coat under render — a thin bonding coat on a smooth or poorly absorbent background
Practical tips
- Pick the right class — tile adhesives and adhesive mortars are classified; for large formats, external work and underfloor heating you need a flexible, high-grade mortar rather than the cheapest standard product
- A flat background is a precondition — in thin-bed work the background has to be flat and the tiles of even thickness (for natural stone: calibrated slabs). Thin-bed adhesive is not a levelling compound
- Solid bedding outside — for external paving and loaded floors, full bedding (buttering and floating) is required: voids under a tile fill with water and burst in frost
- Watch the open time — once the combed ridges have skinned over, the tile will not bond; comb the bed through again
- Temperature — do not use cement-based adhesives in frost or full sun; follow the manufacturer’s limits
Related terms
- Mortar
- Drywall adhesive
- Adhesive
- External wall insulation
- Floor tile
- Bastard mortar
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