Frequently Asked Questions

How does Fosroc Cemseal crystalline waterproofing work inside concrete and what chemical reaction forms the waterproofing crystals?

The crystalline waterproofing mechanism of Fosroc Cemseal relies on a chemical reaction triggered by the combination of water and the active chemical compounds in the Cemseal formulation with the un-hydrated and partially hydrated cement compounds within the concrete pore solution. Concrete contains significant quantities of un-reacted and partially reacted cement minerals — particularly unhydrated calcium silicates (C3S and C2S) and calcium aluminate (C3A) — that remain in the concrete pore structure even after the bulk hydration is complete. These reactive cement minerals are present as fine particles distributed throughout the concrete matrix. When Cemseal is applied to a moist concrete surface as a slurry, the active chemical components (typically sodium silicate derivatives, aluminium compounds, and proprietary organic activators) dissolve into the water present in the surface pores and diffuse into the concrete — the diffusion process is driven by the concentration gradient of the active chemicals, and the active compounds migrate typically 50 to 150 mm into the concrete from the surface of application (much deeper than any surface coating system). Within the pore structure, the active Cemseal chemicals react with the un-hydrated cement minerals and with calcium hydroxide (a by-product of cement hydration present in the pore solution) in the presence of water to form insoluble crystalline calcium silicate hydrate compounds — similar to but different from the C-S-H gel that is the primary strength-giving product of cement hydration. These insoluble crystals precipitate within the pore spaces and gradually fill the capillary pores, blocking the water pathways. The crystal growth is progressive — it continues as long as water and un-reacted cement minerals are present — and the crystals are permanently stable under subsequent dry conditions and reactivate (re-crystal growth) when the concrete becomes wet again in service, which is the mechanism behind the self-sealing property for subsequent hairline cracking.

Can Fosroc Cemseal be applied to the inside face of a basement wall that currently has active water seepage and what treatment is required before crystalline application?

Yes — the ability to be applied to the negative side (interior face, against the direction of water pressure) is one of the most important practical advantages of crystalline waterproofing systems like Cemseal, and the treatment of active seepage and damp areas before crystalline application is a critical part of the repair procedure. For a basement wall with active water seepage from the exterior groundwater, the treatment before applying Cemseal depends on the severity of the seepage. For damp walls with moisture but no active dripping or flowing water (moisture reaching the surface by capillary absorption and evaporation rather than pressure-driven flow), the concrete surface must be cleaned (remove all paint, coatings, efflorescence, and contamination by wire brushing, grinding, or hydro-jetting), but the dampness itself is actually beneficial for Cemseal application — crystalline waterproofing systems require a moist concrete substrate to initiate the crystal growth reaction, and applying Cemseal to perfectly dry concrete produces inferior crystal development and penetration. For areas with active dripping or flowing seepage (pressure-driven water flow through a crack or joint), the active water flow must first be arrested before Cemseal can be applied — because flowing water would wash the freshly applied Cemseal slurry off the surface before the active chemicals can penetrate and the crystals begin to grow. Active seepage is stopped using Fosroc Lokset or Renderoc Plug (rapid-setting hydraulic cement — hand-packed into the crack and held in place for 2 to 4 minutes until initial set arrests the water flow), then allowed to cure for 1 to 2 hours before the Cemseal surface application. After arresting all active seepage, apply Cemseal as a two-coat system (two brush-applied slurry coats at the specified coverage rate, applied wet-on-wet for the second coat) and maintain the applied surface in damp condition for 72 hours by misting with water to allow the crystal growth to continue in adequate moisture.

What is the difference between using Fosroc Cemseal as a surface-applied slurry and using it as an integral concrete mix addition and which method provides better waterproofing performance?

Fosroc Cemseal can be used in two application modes — as a surface-applied brush-coat slurry on existing or new concrete, and as an integral admixture added to the concrete at the batch plant during mixing — and the two methods provide different coverage and performance characteristics that make each better suited to different project scenarios. As a surface-applied slurry: Cemseal is mixed with water to a thick slurry and applied to the prepared concrete surface in two coats by stiff brush; the active chemicals penetrate from the surface into the concrete by diffusion. The penetration depth (typically 50 to 150 mm from the surface) means that the crystalline waterproofing is most concentrated near the treated surface and decreases in effectiveness with depth. This method is used on existing concrete structures (basement walls, water tanks, and retaining structures that have already been built and cannot receive an integral admixture) and provides effective waterproofing for the surface zone of the existing concrete. It is the only applicable method when the concrete already exists. As an integral concrete mix addition: Cemseal powder is added directly to the concrete batch at the RMC plant (typically 0.8 to 1.5 per cent by weight of cement — added with the dry materials or to the mixing water) and is distributed uniformly throughout the fresh concrete as it is mixed. The active chemicals are therefore present throughout the full cross-section of the concrete element (in the interior of the slab, wall, or raft as well as at the surface), and the crystalline growth is initiated throughout the concrete simultaneously as water is present. This uniform distribution provides waterproofing protection through the full thickness of the concrete — not just in the surface zone. The integral method is used in new construction — water tanks, basements, retaining walls, and infrastructure — where the waterproofing can be specified as part of the mix design rather than applied as an afterthought. In terms of performance, the integral method provides more complete, through-thickness waterproofing and is generally considered the superior application when the choice is available (new construction); the surface-applied slurry is the appropriate solution for existing concrete where integral addition is not possible.