Decorative concrete transforms a raw material into a finished surface in its own right. Walls, ceilings, floors: the current architectural trend integrates exposed concrete as an aesthetic element in commercial, residential and public projects. For grinding professionals, this high-value market demands tooling capable of producing homogeneous, repeatable finishes that meet architects’ expectations.
Types of Concrete Finishes
Raw concrete retains the marks of its formwork — board imprints, air bubbles, a slight laitance veil. The finish consists of homogenising the appearance without transforming it. A light grind at grit 60 to 120 is sufficient to remove surface laitance and even out the colour. The appeal of raw concrete lies in its authenticity; pressing too hard exposes the aggregates and destroys the intended effect.
Satin concrete offers a smooth feel and a matt to very slightly glossy appearance. It is the most requested finish for interior residential and commercial spaces. The abrasive progression runs from grit 60 to grit 400, with visual inspection between each pass. Satin is less forgiving of irregularities than raw — surface flatness and grinding consistency are critical.
Mirror-polished concrete is the most demanding finish. The surface reflects light like a mirror, revealing the slightest imperfection. The abrasive progression runs from grit 30 to grit 3000, in 8 to 10 successive passes. Each pass removes the scratches from the previous one. Skipping a step — for example jumping from grit 200 to grit 800 — leaves micro-scratches visible in the reflection. Progression discipline is absolute.
Tadelakt concrete is a lime-based plastering technique applied over a concrete substrate, polished with a pebble and treated with black soap. Although it is a plaster rather than concrete grinding per se, the concrete substrate preparation is identical: the surface must be clean, rough (grit 30 to 60) and free of laitance to ensure plaster adhesion.
Grit and Grinding Progression
The golden rule of abrasive progression: never skip more than a factor of 2 between successive grits. The recommended sequence is:
- Roughing: 30, 60
- Refinement: 120, 200, 400
- Polishing: 800, 1500, 3000
The first grit depends on the initial surface condition. A new formwork surface with laitance starts at grit 60; an old surface with defects or excess material starts at grit 30; a previously ground surface to rework starts at the grit just below the existing finish.
For coarse grits (30 to 120) on hard concrete, diamond abrasives are essential — the most durable and precise option. For fine grits (200 to 3000), resin discs are more flexible and follow the micro-relief of the surface to produce the final gloss.
Why the Tool Makes or Breaks Decorative Work
Decorative grinding on walls and ceilings presents a fundamental mechanical problem: the operator must maintain constant pressure while moving a 5 to 8 kg tool in an overhead or horizontal position. After two hours, fatigue alters pressure — and pressure variation translates directly into visible finish irregularities, unacceptable in decorative work.
A counterbalance trolley solves this by compensating for the machine’s weight. The operator guides the movement without bearing it. Pressure is calibrated and constant from the first to the last square metre, eliminating the overlap marks and gloss variations that manual or unsupported grinding inevitably produce.
For high ceilings over 3 m, a telescopic extension can reach up to 4.5 m without scaffolding. For restricted-access areas in renovation work, a compact wall grinder with a 165 mm pad navigates corners and recesses inaccessible to a trolley system.
Surface Preparation Before Staining or Varnishing
After the final grinding pass, the surface must be thoroughly dedusted. Concrete microparticles lodged in the pores prevent surface treatment adhesion. A class M/H vacuum with appropriate extraction flow is essential.
Before applying stain or varnish, three checks are worth running:
- Absorption test: place a few drops of water on the surface. If absorbed within 30 seconds, porosity is sufficient. If the water beads, the surface is too closed — grit too fine or contamination present.
- Raking light visual check: an LED lamp placed at 15 degrees to the surface reveals residual scratches, overlaps and flatness variations invisible under diffuse lighting.
- Moisture measurement: concrete must be dry (residual moisture below 4%) before applying an impregnant or varnish. A pin or microwave moisture meter gives a reliable reading.
Quotation and Site Scheduling
For medium-hardness concrete (30 to 40 MPa), realistic grinding times per square metre are:
| Finish | Number of passes | Time per m² |
|---|---|---|
| Raw (grit 60–120) | 2 | 3 to 5 min |
| Satin (grit 60–400) | 4 | 8 to 12 min |
| Mirror polish (grit 30–3000) | 8–10 | 20 to 30 min |
These times include disc changes and intermediate checks, and assume appropriate tooling. With an unsuitable tool or manual grinding, multiply by 2 to 3.
The cost of decorative grinding typically breaks down as roughly 60–70% labour, 20–25% abrasive consumables, and 10–15% tool depreciation. On a 200 m² wall project in satin finish, allow 4 to 5 working days for one operator with a suitable trolley. The same project done manually takes 10 to 15 days.
Integrating Decorative Concrete into Project Specifications
Architects specifying decorative concrete should include the following in technical documentation: the expected finish type (raw, satin, polished) with visual references, the final grinding grit, flatness tolerances (typically 2 mm under a 2 m straightedge), expected gloss levels measured in GU by glossmeter, and the intended surface treatment.
Before starting on the full surface, producing a 1 to 2 m² sample for architect approval is essential. This sample serves as the reference for acceptance inspection and prevents disputes at project handover.
Frequently Asked Questions
What grit should I use for mirror-polished concrete? The standard progression runs from grit 30 (roughing) to grit 3000 (mirror polishing), through intermediate steps at 60, 120, 200, 400, 800 and 1500. Each step removes the scratches from the previous one — skipping a step is visible to the naked eye on the final finish.
Can you grind concrete less than 28 days old? Technically yes, but the concrete’s strength is not yet optimal. At 7 days, concrete reaches only 65–70% of its final strength — premature grinding risks tearing out aggregates rather than cutting them. Wait 28 days for a finish grind, or 14 days minimum for roughing.
What is the difference between polished concrete and trowelled concrete? Trowelled concrete is achieved by power trowelling fresh concrete. Polished concrete is achieved by abrasive grinding on hardened concrete. Polishing offers superior shine and finish precision, with exposed aggregates if desired. Trowelling is faster but less precise.