Latex cement, also called rubber cement mortar or polymer-modified cement, is a mortar of Portland cement and sand to which a latex or polymer dispersion has been added. As it cures, the polymer particles form a film between the cement grains, making the mortar tougher, more adhesive and less permeable to water than plain cement mortar.
You will meet it in two forms: as a powder with the polymer already dried into it (mixed with water), and as a two-component system in which a bag of powder is mixed with a can of latex instead of water. Many floor levelling compounds and thin-bed tile adhesives are in effect polymer-modified cement mortars.
Properties compared with plain cement mortar
- Better adhesion to smooth, absorbent or otherwise difficult substrates.
- Tougher and less brittle, so thin layers do not crack or flake straight away.
- Less shrinkage, and therefore less cracking in thin layers.
- Lower water permeability, which suits it to damp-resisting finishes.
- Applied thinner than a traditional sand-cement screed: where screed soon needs centimetres, this can often be worked in millimetres.
Applications
- Levelling floors before laying tiles, vinyl, laminate or LVT — including on awkward substrates such as timber floors, dry screed boards and underfloor heating with a thin covering.
- Concrete repair: filling holes, broken edges and old markings.
- Filling and sealing cracks and joints in concrete and masonry.
- Smoothing and damp-resisting concrete walls and floors, for example in a basement or crawl space.
- As a bonding slurry or bonding bridge under a new cementitious finish.
Application tips
- Substrate first. Dust-free, grease-free and sound. Remove loose material and old adhesive residue; latex bonds well, but not to something that is itself letting go.
- Prime as specified. Absorbent substrates get a primer, smooth and non-absorbent ones a bonding primer. Skipping this is the most common cause of levelling compound lifting.
- Respect the maximum layer thickness. Every levelling compound has both a minimum and a maximum. Spread too thin it cracks; laid too thick it shrinks and curls.
- Work each area in one pass. These mortars are free-flowing with a short open time; otherwise the joins between batches show.
- Carry through the movement joints. Never level over an existing movement joint — reproduce it in the new layer.
- Ventilate, but avoid a draught. Drying the surface too fast leaves a weak, dusty top layer.
Related terms
- Cement
- Ready-mixed concrete
- Polished concrete render
- Cementing
- Concrete injection
- Screed drying accelerator
- Shrinkage joint
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