Cementitious Coatings

A cementitious coating is a protective or waterproofing layer based on portland cement or a related hydraulic binder, typically blended with fine aggregate, admixtures and a polymer modifier. Cementitious coatings bond well to concrete and masonry because they are chemically similar to them. They are used for waterproofing, resurfacing, fire protection and wear protection, and as a base for other coatings.

Composition

A typical product contains:

• Hydraulic binder. Usually portland cement, sometimes with calcium aluminate or other specialty cements for fast setting or chemical resistance.
• Fine aggregate and fillers. Graded silica sand or limestone.
• Polymer modifier. Acrylic, styrene-butadiene or vinyl acetate polymers, supplied as a liquid latex or as a redispersible powder. The polymer improves adhesion, flexibility and water resistance.
• Admixtures. Water reducers, set controllers, and in crystalline products, reactive chemicals that form crystals in concrete pores.

Products are sold as one-component powders mixed with water, or two-component kits of powder and liquid polymer.

Main types

Polymer-modified cementitious waterproofing

These coatings are brushed, troweled or sprayed onto concrete to form a dense, waterproof layer. Flexible grades with high polymer content can bridge fine shrinkage cracks. They are used on foundation walls, water tanks, basins and below-grade structures, often as part of a broader waterproofing membrane strategy.

Crystalline waterproofing

Crystalline products contain chemicals that react with moisture and by-products of cement hydration to form insoluble crystals inside the concrete’s capillary pores. They are applied as a slurry or included as an admixture. They are often used on the positive or negative side of water-retaining structures.

Resurfacers and repair mortars

Thin-section resurfacers restore worn or scaled concrete. Repair mortars rebuild spalled areas before coating (see concrete crack repair before coating).

Cementitious fireproofing

Spray-applied cementitious fireproofing on structural steel provides fire resistance ratings in buildings. It is covered in fireproofing structural steel.

Cementitious floor toppings

Urethane cement flooring combines cement with polyurethane chemistry. It is classed with resinous floors rather than ordinary cementitious coatings.

Properties

Strengths:
– Compatible with concrete and masonry; similar thermal expansion.
– Vapor permeable, so moisture in the substrate can escape.
– Tolerant of damp surfaces; many products require a saturated surface-dry substrate.
– Non-combustible and generally low in VOC.
– Suitable for negative-side waterproofing in many cases.

Limitations:
– Limited flexibility compared with elastomeric membranes, so moving cracks may reflect through.
– Moderate chemical resistance; acids attack cement.
– Lower abrasion resistance than many resinous systems.
– Cure is sensitive to temperature and drying; early drying causes cracking and dusting.

Surface preparation

Cementitious coatings need a sound, clean, open-textured surface:

• Remove laitance, form oil, curing compounds and contaminants by concrete surface preparation methods such as water jetting or abrasive blasting.
• Aim for the profile the data sheet calls for (see concrete surface profile (CSP)).
• Pre-wet the substrate to saturated surface-dry so the concrete does not draw water out of the coating.
• Repair voids and route cracks as required.

Application

Methods include brush, roller, trowel and spray. Key practices:

• Mix with the exact water or liquid ratio stated. Extra water weakens the coating.
• Apply within the stated working time.
• Build thickness in the number of coats required.
• Cure as instructed, often by keeping the surface damp or protecting it from sun and wind for a set period.

Use with resinous coatings

Cementitious products are sometimes used below resinous coatings as leveling layers or moisture-tolerant bases. In that case the cementitious layer must be fully cured and prepared to the resin manufacturer’s requirements, and its moisture condition checked (see concrete moisture testing). Some systems are designed specifically for this, so use the products the manufacturer approves.

Applications

• Foundation and basement walls
• Water and wastewater basins (see water and wastewater coatings)
• Potable water tanks, where the product is certified for drinking water contact
• Swimming pool shells
• Tunnels and below-grade structures
• Masonry block walls (see coating masonry and block)

In practice

On a typical project, the specifier first decides whether the cementitious product is the finished surface or a base for another system. A resurfacer that will receive an epoxy or polyurea topcoat needs to be cured and dry enough for that topcoat, so the schedule must allow for it. A crystalline or polymer-modified waterproofing coat on a tank or basement wall may be the final layer. In both cases the substrate is usually dampened before application, unlike most resin coatings, and the product data sheet sets the limits for temperature, thickness per coat and curing.

Key terms

• Portland cement. The hydraulic binder in most cementitious coatings, which hardens by reacting with water.
• Polymer modification. Adding acrylic, styrene-butadiene or other latexes to cement mixes to improve adhesion, flexibility and water resistance.
• Crystalline waterproofing. Cement-based products with active chemicals that form crystals inside concrete pores.
• Hydration. The chemical reaction between cement and water that builds strength.
• Curing (of cement). Keeping a cementitious material moist and within temperature limits so hydration can proceed.
• Parge coat. A thin layer of mortar or cementitious coating trowelled over masonry to fill and level the surface.

Frequently asked questions

Are cementitious coatings waterproof?

Many are designed as waterproofing layers. Performance depends on the product, thickness and whether cracks move.

Can they be applied to damp concrete?

Usually yes, and many require a damp but not wet surface. Standing water must be removed.

Do they resist sewer gases?

Ordinary cementitious coatings are vulnerable to acids formed from hydrogen sulfide. Sewer rehabilitation usually uses specialty mortars or resinous linings (see sewer and manhole rehabilitation).

Related entries

• Coating concrete
• Waterproofing membranes
• Efflorescence
• Concrete surface preparation
• Concrete surface profile (CSP) (new)
• Concrete crack repair before coating (new)