About Fosroc Nitocote CM660 Masilla

Fosroc Nitocote CM660 Masilla is the crack-and-joint sealing component of the Nitocote CM660 waterproofing system. (“Masilla” is the Spanish term for mastic or putty — the product originates in Fosroc’s European/Iberian range, where the CM660 system is widely specified.) Where the main Nitocote CM660 product is a flexible, polymer-modified two-component cementitious coating that waterproofs the broad surface, the Masilla handles what a brush-applied coating cannot reliably do on its own: locally seal and reinforce discrete cracks, fissures and construction joints so they do not become leak paths under hydrostatic pressure.

In practice, waterproofing failures rarely begin across a sound, intact slab — they begin at cracks, cold joints, honeycombing and re-entrant corners. A thin cementitious coating laid straight over an open or moving crack is liable to split along that line. The CM660 Masilla is a deliberate detailing material: it is a two-part elastic mastic, supplied as a base and a reactor/hardener, that you mix and press into routed-out cracks and joints to create an elastic, watertight plug before the membrane goes on. Because it is damp-tolerant, it can be used on substrates that are still green or cannot be fully dried — a major practical advantage on Indian sites where below-grade tanking, water tanks and monsoon-affected roofs frequently must be sealed against the clock.

The Masilla is not a stand-alone waterproofing membrane and is not intended to be used in isolation. It is a system accessory: the documented sequence is to prepare and seal the cracks with the Masilla, then apply the Nitocote CM660 flexible cementitious coating over the whole area. The CM660 system as a whole is supplied grey, bonds without a primer to sound concrete and masonry, resists both positive and negative hydrostatic pressure, and — in its European certification — is approved for contact with public/potable water supply (per Spanish RD 1262/2005 and RD 140/2003). For Space Arc’s contractor and consultant base, the Masilla is best understood as the joint-and-crack detailing material that makes the CM660 coating perform as a continuous, crack-tolerant tanking system.

The technical insight

The engineering logic of CM660 Masilla is local stress relief. A rigid cementitious coating has limited strain capacity; if a substrate crack opens beyond that capacity, the coating fails at the crack line regardless of how sound it is elsewhere. By plugging the crack with an elastic mastic first, the system separates two functions — the Masilla accommodates movement and seals the discontinuity, while the overlying Nitocote CM660 coating provides the continuous, flexible waterproof skin. Fosroc’s own guidance on the system notes that, with a 2 mm coating applied, the CM660 system can accommodate fine cracking up to around 0.8 mm developing after cure; the Masilla extends this by pre-treating wider or pre-existing cracks and joints that exceed what the coating alone can bridge. The two-part chemistry (base plus reactor) means cure is by reaction rather than purely by drying, which is what allows application onto damp substrates — a behaviour conventional drying-type fillers cannot match. The correct workflow is therefore non-negotiable: open the crack to a sound channel, clean it, press in the mixed Masilla, allow it to cure, then overcoat with CM660. Skipping the crack-sealing step and relying on the coating alone over open cracks is the single most common cause of recurrent leakage in cementitious tanking.

Where it is used

• Sealing cracks and fissures in concrete water tanks and reservoirs prior to internal cementitious waterproofing
• Treating construction and cold joints on flat roofs and terraces before applying the Nitocote CM660 coating
• Crack repair and joint sealing in basements, lift pits and below-grade retaining walls subject to ground water
• Detailing of cracks in wells, sumps and underground water-retaining structures
• Sealing surface and shrinkage cracks on balconies and parapets ahead of waterproof topcoating
• Localised crack treatment in concrete refurbishment and rehabilitation works on damp substrates

Key advantages

• Damp/wet substrate tolerant — cracks can be sealed without fully drying the concrete, saving programme time on monsoon and below-grade work
• Elastic after cure, so it accommodates limited movement and seals cracks that would split a rigid filler or coating
• Two-part reactive chemistry gives a durable, watertight seal rather than a drying-only paste
• Purpose-matched to the Nitocote CM660 flexible cementitious coating for a single, compatible waterproofing system
• Suitable for water-retaining structures, including potable-water tanks where the certified CM660 system is used
• Trowel/knife applied into routed cracks — straightforward for trained applicators with no specialist equipment

Limitations & considerations

• Not a stand-alone waterproofing membrane — it seals cracks and joints and must be overcoated with the Nitocote CM660 coating to complete the system
• A detailing/accessory product, not a structural crack-injection grout; live structural cracks needing resin injection require a Nitomortar/Conbextra injection system instead
• Mixed material has a finite working life (pot life); only mix what can be applied before it stiffens
• Damp tolerance is not the same as suitability for running water — active leaks under pressure must first be plugged with a quick-set plug mortar
• Substrate must be properly prepared: cracks opened to a sound channel, cleaned of dust, laitance and loose material before sealing
• Exact published India pack sizes, mixing ratio and coverage should be confirmed against the current regional TDS, as these can differ between Fosroc territories

Selection & best practice

Specifications

Industry use cases

How we position Fosroc Nitocote CM660 Masilla

Within Fosroc’s range, Nitocote CM660 Masilla is best understood as an accessory to the Nitocote CM660 flexible cementitious waterproofing coating — it is the crack-and-joint detailing material that lets the CM660 membrane perform as a continuous, crack-tolerant tanking system. It sits alongside other Fosroc detailing options: for moving expansion joints and sealant work, Fosroc’s Nitoseal and Thioflex 600 (polysulphide) sealants are the right tools, and for structural crack injection the Conbextra and Nitomortar resins apply. In the wider Fosroc cementitious-waterproofing context, related products include Brushbond RFX and the Nitocote/Nitoproof families. Because the CM660 system is more commonly specified in Fosroc’s European market, Indian projects more often see Fosroc’s Brushbond range for cementitious tanking, with the appropriate Fosroc crack-sealing accessory used for detailing. Credible alternatives a consultant may evaluate include Sika’s two-component cementitious systems (e.g. SikaTop Seal) with Sika crack-sealing mastics, Dr. Fixit’s Pidicrete/Roofseal cementitious coatings, MBCC/Master Builders Solutions (formerly MBS) cementitious waterproofing, and STP’s range. The decision usually turns on the specified waterproofing membrane: choose the crack-sealing mastic that is part of, and warranted with, that membrane’s system — mixing a coating from one manufacturer with a crack sealer from another can void compatibility and warranty. Space Arc Engineering, as an authorised Fosroc distributor and applicator across Delhi NCR and Pan-India, can confirm current availability, the matching CM660 coating, India-specific data, and the right detailing product for your structure.

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