For years, most designers considered concrete a dull and drab building material. However, these days, it is very common to see contractors, designers and even homeowners create their own custom-coloured concrete floors, countertops and walkways from acid-stained concrete. Acid stains contain hydrochloric acid, which etches the concrete, preparing it for colour, and acid-soluble metallic salts, which react chemically with the lime content within the concrete. The salts create a permanent colour with the concrete, resistant to fading and chipping.
Colour charts for acid concrete stains vary between manufacturers, but typically, acid stains come in three main colour groups. The brown group is a result of iron salts and produces shades of brown, tan, gold and umber. Rust salts produce deep shades of earthy red, dark brown and nearly black colours, while copper salts produce shades of blue, aqua and green.
Acid-based concrete stains interact with the free lime within the concrete and colour variations will occur depending on the amount of lime in the concrete. Since lime dissolves over time, newer concrete provides the best surface for accepting acid stains. Concrete, even parts of the same slab, will accept stains in different variations and intensities. Unlike water-based stains and paints, acid stains give a mottled appearance. Contractors can adjust colour variations by applying acid stains over tinted concrete, sealing acid-stained concrete with water-based stains, or by mixing different acid stains.
Colour intensity from acid concrete stains varies because the strength on the colour depends greatly on the mixing process of the concrete, the type and quantity of aggregates used in the concrete, the amount of free lime and the level of alkalinity in the concrete. Contractors often use spray bottles to apply a generous level of stain to the concrete for better coverage and intensity and allow it to react for at least four hours, but the reaction process can continue for months after application. Acid stains are translucent, so surface defects in the concrete will still show through the stain.
Contractors apply two or more coats of acrylic concrete sealant to the newly stained floor to not only intensify the colour and sheen, but also to protect the floor from scratches and other stains. Acrylic sealants allow water to evaporate from the concrete, which could leave hazy white marks in the protective layer. Other sealants, like epoxies and urethanes, are available for use on countertops and food-prep surfaces. Afterwards, the surface is waxed with a special concrete wax to yield further protection, shine and colour depth.