About MC-Bauchemie Oxal TKM-HS

Oxal TKM-HS sits inside MC-Bauchemie’s Oxal range, the German manufacturer’s dedicated line for natural-stone and heritage masonry restoration. The “HS” stands for highly sulphate resistant, and “TKM” traces back to Trasskalkmörtel — trass-lime mortar — rather than an ordinary Portland-cement (OPC) mix. Instead of a cement binder that relies on tricalcium aluminate (C3A) for early strength, Oxal TKM-HS uses a one-component, tricalcium-aluminate-free binder built around trass, a natural volcanic pozzolan that has bonded lime mortars in European masonry for centuries. That distinction is not academic: C3A is the reactive phase that combines with sulphates from gypsum plaster, brick salts or sulphate-bearing ground water to form expansive ettringite — the mechanism behind the cracking, staining and spalling so common where old masonry has been repaired in ordinary cement mortar. Taking C3A out of the binder removes that failure path at source. For India’s stock of colonial-era brick and lime masonry, forts, havelis, temples and civic buildings — most of it originally built and plastered in lime, and too often patched since in cement — this is a chemistry-first answer to a problem restoration contractors run into constantly.

In use, Oxal TKM-HS is a genuinely universal mortar within its scope: it works as a masonry-bedding mortar, a render/plaster base coat, and a thin-bed mortar for natural stone, across interior and exterior, new and old construction alike. It arrives factory-proportioned as a single powder component, mixed only with water in a forced-action or slowly rotating double mixer — MC-Bauchemie’s own application instructions explicitly rule out hand-mixing and part-batching, because uneven mixing compromises the lump-free consistency the binder needs. Once mixed and applied by trowel or float, it develops a deliberately moderate compressive strength — around 5 N/mm² at 28 days, mortar class M5 to EN 998-2 — and that moderation is the point. In conservation work, a mortar harder than the masonry unit or stone it beds is a defect, not a feature: it concentrates stress and cracks the original fabric instead of the joint. Oxal TKM-HS is calibrated to stay weaker than the substrate while still doing its structural job, and to stay open to water vapour diffusion so damp historic walls can dry outward instead of trapping moisture behind an impermeable patch.

The technical insight

The two figures worth reading carefully on this datasheet are the M5 strength class and the tricalcium-aluminate-free binder. M5 — approximately 5 N/mm² compressive strength at 28 days per EN 998-2 — is intentionally modest. It keeps the mortar “sacrificial” relative to soft historic brick or stone, a core conservation principle, rather than chasing the higher strength classes used in modern load-bearing masonry, where a stronger-than-substrate mortar would eventually damage the very fabric it is meant to protect. The tricalcium-aluminate-free, trass-lime binder is what actually delivers the “HS” claim: trass is a natural pozzolan that reacts with lime to build strength hydraulically, without the C3A phase that ordinary Portland cement needs for early strength and that reacts destructively with sulphates to form expansive ettringite. Vapour permeability follows the same lime-based chemistry, avoiding the dense, low-porosity matrix typical of polymer-modified cement renders, which is why this system is specified wherever breathability is a conservation requirement rather than an afterthought.

Where it is used

• Repointing and rebedding masonry in heritage buildings, forts, havelis and religious structures
• Render and plaster repair on old brick or stone walls where gypsum plaster or high-sulphate substrates are present
• Thin-bed bedding and jointing of natural stone masonry — sandstone, granite ashlar, stone cladding
• Base-coat render for restoration render systems (compatible with Oxal’s WPw/NHL restoration finishing coats)
• Masonry repair in salt-laden or damp-affected walls, including rising-damp-prone ground-floor masonry
• New masonry construction in a heritage idiom within conservation zones and precincts
• Masonry repair in industrial or coastal settings with sulphate-rich ground water or soil
• Dressing and filling of natural stone joints during facade conservation work

Key advantages

• Tricalcium-aluminate-free trass-lime binder resists sulphate attack, avoiding the ettringite-driven cracking common with cement mortars on gypsum-contaminated masonry
• High resistance to efflorescence and surface staining, keeping restored facades visually clean over time
• Open to water vapour diffusion, letting damp historic walls dry outward instead of trapping moisture behind the repair
• One-component, factory-proportioned mix gives consistent quality bag to bag versus site-batched lime mortars
• M5 strength class is matched to soft, old masonry units, avoiding stress concentration that damages fragile historic fabric
• Works across interior and exterior, new and old construction — one product for most of a restoration scope
• Simple trowel-and-float hand application; no specialised spray equipment required
• Backed by a formal EN 998-2 Declaration of Performance and by Space Arc’s site support across Delhi NCR

Limitations & considerations

• Must be machine-mixed in a forced-action or double mixer for the full 3 minutes — hand-mixing and part-batching are explicitly disallowed, even for small heritage patch repairs
• Moderate M5 strength (~5 N/mm²) is not intended for structural or load-bearing repair work — it is a masonry/render mortar, not a structural patch material
• Needs curing protection from rapid drying, direct sun and wind; careless curing in hot, dry or windy site conditions risks shrinkage cracking
• Working time is only around 2 hours at 20°C, shorter in Indian summer heat, which demands disciplined batch sizing on site
• Maximum layer thickness is limited to about 25 mm per pass, so deeper repairs need multiple built-up layers
• Supplied only in grey — colour-matching to a heritage lime finish depends on pairing it with the correct top render or limewash system

Selection & best practice

Specifications

Industry use cases

How we position MC-Bauchemie Oxal TKM-HS

Within MC-Bauchemie’s own Oxal range, TKM-HS is the workhorse masonry-bedding mortar in a broader heritage restoration system: pair it with Oxal NHL 3 historic for cement-free lime render, Oxal WPw or WPw WTA as the finishing render coat, Oxal SPM where a sulphate-resistant waterproof repair mortar is needed, and the Oxal VP I T / VP III HS trass-based grouts where voids or cracks in old masonry need injection rather than surface repair — Space Arc can specify the full system rather than a single product for a restoration scope. Outside the Oxal family, there is no genuinely equivalent product in Fosroc’s India catalogue: Fosroc’s well-known repair mortars (the Renderoc range and similar polymer-modified cementitious systems) are OPC-based products engineered for concrete rehabilitation, not lime-based, sulphate-resistant masonry mortars for heritage stone and brick — different substrate, different failure mode, different chemistry. Where the brief is genuinely a general concrete patch repair rather than lime-masonry conservation, a standard OPC-based repair mortar is the more appropriate and economical choice; where the brief is heritage masonry, gypsum-affected walls, or any project where breathability and sulphate resistance are non-negotiable, Oxal TKM-HS is the more specific and defensible specification.

Related Concrete Repair products

View all Concrete Repair products →