The Chemistry
Concrete contains calcium silicate hydrate (the binding compound) and calcium hydroxide (lime), which is water-soluble. When water moves through a concrete slab — whether from rain, ground moisture, or condensation — it dissolves the calcium hydroxide and carries it to the surface. As the water evaporates, it deposits the dissolved calcium as calcium carbonate — the white, chalky efflorescence you see.
Why Houston Sees So Much of It
High annual rainfall, high humidity, and expansive clay soil that drives water upward create constant moisture movement through concrete slabs in the Greater Houston area. New concrete produces more efflorescence as it cures and its high lime content is gradually depleted. Older concrete can continue to effloresce if there's a persistent moisture source driving water through the slab.
Primary vs. Secondary Efflorescence
Primary efflorescence occurs in new concrete as excess mix water evaporates during curing. It's usually light, dusts off easily, and decreases over time as the concrete matures. Secondary efflorescence occurs in older concrete from external water sources — ground moisture, rain penetration, or condensation. Secondary efflorescence is a symptom of ongoing moisture movement and won't resolve without addressing the moisture source.
Why It Matters for Epoxy
Efflorescence is an adhesion barrier. The calcium carbonate crystals sit on and in the concrete pores, preventing epoxy from making direct contact with the concrete substrate. Even after brushing away visible deposits, soluble salts remain in the pore structure and will continue to migrate — creating a built-in release layer beneath the epoxy. Acid washing neutralizes the salts; diamond grinding removes the affected surface layer.
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