Decorative concrete: grinding and professional finishes (polished, satin, raw)

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 (formwork concrete)

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. The architect is looking for traces of the construction process. Grinding must be delicate: pressing too hard exposes the aggregates and destroys the intended effect.

Satin concrete

Satin 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. This is the finish where the difference between a suitable tool and a generic one is most visible.

Polished concrete (mirror)

Mirror polish 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

Tadelakt 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

Fundamental principle

Grit size (abrasive grain size) determines surface roughness. A coarse grit (30) removes significant material and leaves deep scratches. A fine grit (3000) removes very little material and produces a mirror surface.

The golden rule: never skip more than a factor of 2 between successive grits. The recommended progression is:

Roughing: 30, 60 Refinement: 120, 200, 400 Polishing: 800, 1500, 3000

Choosing the first grit

The first grit depends on the initial surface condition:

• New formwork surface with laitance: start at grit 60
• Old surface with defects or excess material: start at grit 30
• Previously ground surface to rework: start at the grit just below the existing finish

Types of abrasives

Diamond: the most durable and precise. Essential for coarse grits (30 to 120) on hard concrete. Sept Tools diamond segments are designed for European concretes (strength 25 to 50 MPa).

Resin: used for fine grits (200 to 3000). Resin discs are more flexible and follow the micro-relief of the surface. They produce the final gloss.

The Mygale: the tool for decorative wall and ceiling concrete

Why the trolley changes everything

Decorative grinding on wall and ceiling 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 2 hours, fatigue alters pressure, which translates into visible finish variations.

The Mygale trolley solves this problem with a counterbalance that compensates for the machine’s weight. The operator guides the movement without bearing the weight. Pressure is calibrated and constant from the first to the last square metre.

Technical specifications for tender documents

For architects and project managers writing specifications, here are the relevant technical specifications of the Mygale system:

• Motor power: 800 W, 1,200 W or 1,600 W depending on surface area and concrete hardness
• Vibrations: 0.35 m/s² at the handle (Directive 2002/44/EC compliant)
• Noise level: 60 dB(A) under load (Directive 2003/10/EC compliant)
• Pad diameter: 165 mm or 225 mm depending on configuration
• Sealing: IP65, compatible with integrated extraction
• Extraction: compatible with class M/H vacuum (IU33 Longopac recommended)

Variants by site type

New build, large volumes: Mygale 1600 W with 225 mm pad and IU33 Longopac vacuum. Maximum grinding throughput, continuous extraction eliminates emptying stops.

Renovation, restricted access: Petit Potam with 165 mm pad. The compact size enables work in corners and recesses inaccessible to the trolley.

High ceilings (over 3 m): Fouine XB165 with telescopic extension. Reach extends to 4.5 m without scaffolding.

Surface preparation before staining or varnishing

Post-grinding cleaning

After the final grinding pass, the surface must be thoroughly dedusted. Concrete microparticles lodged in the pores can prevent surface treatment adhesion. A class M/H vacuum with appropriate extraction flow is essential.

Pre-treatment checks

Three checks before applying stain or varnish:

1. Absorption test: place a few drops of water on the surface. If the water is absorbed within 30 seconds, porosity is sufficient for treatment. If the water beads, the surface is too closed (grit too fine or contamination).

2. Raking light visual check: an LED lamp placed at 15 degrees to the surface reveals defects invisible under diffuse lighting. Residual scratches, overlaps and flatness variations appear clearly.

3. Moisture measurement: the 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

Grinding time estimates

For medium-hardness concrete (30 to 40 MPa), grinding times per square metre are:

These times include disc changes and intermediate checks. They are based on a suitable tool (Mygale or equivalent). With an unsuitable tool or manual grinding, multiply by 2 to 3.

Costing for tender documents

The cost of decorative grinding breaks down as:

• Labour: 60 to 70% of total cost
• Abrasive consumables: 20 to 25%
• Tool depreciation: 10 to 15%

On a 200 m² wall project in satin finish, allow 4 to 5 working days for one operator equipped with a Mygale trolley. The same project with a manual or unsuitable tool takes 10 to 15 days.

Architect approach: integrating decorative concrete into the project

Specifications for tender documents

Architects specifying decorative concrete should include in the technical specifications:

• 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 by glossmeter, in GU)
• The surface treatment (impregnant, varnish, wax)

Samples and approval

Before starting grinding on the full surface, producing a 1 to 2 m² sample is essential. This sample must be approved by the architect before proceeding. It serves as the reference for acceptance inspection.

To size your decorative grinding tooling requirements, use the Sept Tools configurator or view the Mygale product page. For personalised advice, contact our team.

Sept Tools products

Equipment featured in this article

Mygale

Wall and ceiling grinding trolley, constant pressure for decorative finishes

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Fouine XB165

1600 W ceiling grinder, ideal for large concrete surfaces

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Petit Potam

Compact wall sander for restricted access areas

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IU33 Longopac

Class M/H vacuum, concrete dust capture at source

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