Laitance is the thin, weak, dusty layer that often forms on new or poorly finished concrete surfaces. It looks smooth — but it’s deceptive. Laitance has little strength and very poor bonding properties. If you try to apply microcements, overlays, or coatings on top of it, they’ll quickly fail: peeling, delaminating, or cracking. In this guide, we’ll show you exactly how to properly remove laitance before resurfacing, polishing, or sealing a concrete floor.
Laitance typically appears as a dusty, thin, slightly lighter colored layer on the surface of newly poured slabs. Drag a steel key, coin, or sharp tool lightly across the surface. If it scratches off easily or turns to powder, it's laitance. Water-drop tests also help — if water beads slightly and the surface feels soft compared to underlying concrete, you’re dealing with laitance. Proper identification saves time and ensures you prep the right way.
Laitance must be mechanically removed. Chemical cleaning, acid etching, or pressure washing are not sufficient. Choose between light diamond grinding or shot blasting. For small areas or decorative work, grinding (with 30–80 grit metals) is best. For large industrial floors, shot blasting is faster and roughens the surface better for thick overlays. Never try to coat or resurface over laitance without full mechanical preparation — bond failure is guaranteed.
Grinding or blasting off laitance creates high volumes of concrete dust, which is hazardous to health. Always use dust shrouded grinders connected to HEPA-rated industrial vacuums. If shot blasting, ensure containment barriers are set up to control spent media and dust. Personal protection (P3 respirators, eye protection, gloves) is mandatory. Proper dust control keeps the site clean, protects workers, and meets local silica dust regulations.
Using a medium-weight grinder and 30–80 grit metal bond diamond tools, make slow, overlapping passes across the floor. Aim to remove the soft surface layer fully without digging deep into the strong underlying concrete. If the grinder moves very easily at first, you're still in the laitance zone. As you reach sound concrete, the machine will resist slightly more, and the sound changes from hollow to sharp. Remove laitance fully but avoid unnecessary slab damage.
If you're prepping large commercial slabs, roadways, or warehouse floors, shot blasting can be faster. A shot blaster fires small steel beads at the surface to break up and remove laitance. It leaves an excellent mechanical profile for heavy-duty coatings and industrial resurfacings. Choose a medium blast pattern (CSP 3–5). Avoid over-blasting, which roughens concrete excessively and consumes more overlay materials during resurfacing.
Once the laitance is removed, vacuum the floor meticulously to remove all dust and fine residue. Use industrial vacuums with fine particle filters. A clean, dust-free surface ensures maximum chemical or mechanical bond for overlays, microcements, resins, or sealers. Double vacuuming and even light wet-wiping can be useful before applying primers or resurfacing products — never assume grinding dust alone is safe to coat over.
After removal, double-check the floor. Sprinkle water across different zones — water should darken the surface immediately without beading. Also re-scratch with a coin or key. Sound concrete will resist scratching and won't turn dusty. These simple verification tests confirm that you’ve fully removed the weak laitance layer and exposed solid substrate — ready for priming, resurfacing, or polishing without adhesion risks.
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