Frequently Asked Questions

How many coats of Cementite are needed to waterproof an underground sump and what is the minimum curing time before filling with water?

For waterproofing an underground concrete sump or collection tank, a minimum of three coats of Cementite are recommended to achieve a continuous, pinhole-free waterproof coating. The first coat fills the surface porosity of the concrete or masonry and provides the adhesion base for subsequent coats; the second coat applied after the first coat has dried builds additional film thickness and seals any pinholes in the first coat; and the third coat provides the final waterproof film surface. Each coat should be applied in a cross direction to the previous coat (the brush or roller strokes running perpendicular to the previous coat direction) to ensure uniform coverage. The drying time between coats at 25 degrees Celsius is typically 4 to 6 hours — the previous coat should be firm and dry to the touch before the next coat is applied. In Indian summer conditions (35 degrees Celsius), drying between coats may be as short as 2 to 3 hours. After the final coat is applied, the minimum curing period before filling the sump with water is 7 days at normal ambient temperature — this allows the cement in the Cementite coats to hydrate and develop adequate waterproof film strength before water pressure is applied. If the sump is filled too early (before 7 days), the immature Cementite film may soften, peel, or allow water ingress before full strength is achieved.

Can Cementite be used on a masonry overhead water tank that is currently leaking and must be stopped as an emergency repair?

For a masonry overhead water tank that is actively leaking, Cementite alone is not suitable as an emergency stopping material. Cementite is a cement-based putty that requires proper surface preparation, multiple coat application, and adequate curing time before water contact — none of which are compatible with an active leak situation where water is running from the tank. For emergency stopping of an active leak in a masonry tank, the first action should be to temporarily stop the water flow using a hydraulic setting mortar (such as Fosroc Renderoc Plug or a similar rapid-setting waterproof mortar) that can be packed into the crack or joint and held under pressure until it sets within minutes against actively flowing water. Once the active leaks have been stopped by Renderoc Plug or equivalent, the tank should be fully drained and allowed to dry to the extent possible. The interior surfaces of the tank are then inspected, all loose mortar joints are raked out and repointed with Fosroc cementitious mortar, and the full interior surface is then coated with Cementite in the recommended multiple coat system with adequate curing before refilling. For severe leakage or for tanks with structurally deficient masonry joints, a complete cementitious slurry waterproofing using Nitoproof 600 or Brushbond RF over the repointed and primed masonry is a more robust repair system than Cementite alone.

Does Cementite have potable water contact approval for use in domestic drinking water overhead tanks in India?

For domestic drinking water overhead tanks and underground sumps that store drinking water for household use, the waterproof coating used on the internal surfaces must be confirmed as safe for potable water contact — the coating should not leach harmful chemicals, flavours, or odours into the stored water. Fosroc Cementite, being a cement-based product, has inherent chemical safety advantages over synthetic polymer coatings in that the cured cement matrix does not leach organic chemicals into water. However, the specific formulation of Cementite (including any additives or polymer modifiers) and its compliance with Indian potable water contact standards (BIS standards and guidelines from state water and sanitation boards) should be confirmed with Fosroc India before specifying Cementite for drinking water tank lining. In general, cement-based waterproof coatings without polymer additives or chemical admixtures are considered safe for potable water contact after adequate curing, as the cement is chemically stable and inert to water after full hydration. Any residual alkalinity from fresh cement leaching into the first tank fill (which is common with new cement-based coatings) can be managed by discarding the first tank fill after the tank is put into service, which allows the surface alkalinity to reduce before the water is used for drinking. This practice is standard for new concrete water tanks and cement-lined tanks throughout India.