By Brad Sleeper and Steven H. Miller, CDT
The sheen of the floor in an Edmonton Value Village store may look like polished concrete, but it is, in fact, neither polished nor concrete. The surface material is actually a bonded topping—a relatively soft cementitious material used as underlayment for carpet or vinyl tile, which would not normally be considered ‘shinable.’ Instead of the labour-intensive finish made by polishing it with fine diamond abrasives, it was honed with only medium-grit abrasives, hardened with an advanced-chemistry densifier, protected with a breathable sealer, and buffed to a near-polished shine—a much more affordable treatment.
Since the introduction of diamond-polished concrete floor systems in the 90s, demand for glossy concrete floors has grown enormously. However, the desire for a reflective concrete floor often outstrips the available budget. Diamond grinding and polishing can be costly, both because of labour and the consumable diamond-impregnated tools.
In response to the need for more affordable reflective concrete floors, necessity has given birth to invention. Advancements in chemistry and abrasives have been the focus of experimentation by resourceful contractors. They have developed several fast, economical methods of putting a reflective surface on concrete without diamond polishing by taking advantage of certain unique properties of a chemical compound called reactive colloidal silica. (Silica is more properly known as ‘silicon dioxide’—the hard mineral compound familiar in its crystalline form as quartz or beach sand.) These floors provide the performance and decorative options of polished concrete at a price point well below diamond polishing—sometimes 65 per cent less. In many cases, their gloss is indistinguishable from a true polish to the untrained eye.
Decorative concrete: a grassroots revolution
Development of new decorative concrete methods and products has been driven by practitioners—applicators, finishing contractors, and artisans. When one talks to the people who founded numerous companies manufacturing decorative concrete products, the same story is repeated with startling fidelity:
- They started as contractors, working in the field making things out of concrete, and constantly struggling to get the right result using the accepted practices of the day. They thought, “There has to be a better way.”
- They took a risk, modified materials, improved methods, and got results that nobody had ever seen before.
- Other practitioners began asking how they did it, so they gave lessons.
- They began selling their own concoctions to their students and, suddenly, they were manufacturers.
The next step, of course, was their students began innovating, inventing, and creating the next generation of new looks and applications. Demand for exposed concrete floors and other architectural surfaces and objects—countertops, sinks, fireplaces, and furnishings—continues to bring talented contractors into decorative concrete, even in a down economy, because the concrete option is often more affordable and more sustainable than traditional alternatives. Therefore, innovation has continued to grow, even as budgets have slumped.
The (re)active ingredient
The glossy exposed concrete floor is becoming more common, with diamond polishing being the traditional means to achieve it. In an era when budgetary pressure is even greater than usual, however, innovation has been driven in the direction of finding ways to offer glossy floors at lower cost. In this context, the peculiar properties of reactive colloidal silica, a water-based suspension of almost pure silica, have proved very exploitable. (Colloidal silica is amorphous silica, and should not be confused with crystalline silica and its associated health risks. Crystalline silica is present in concrete, however, and appropriate protection against inhalation of silica dust should always be used when grinding or polishing any concrete). It is used as a densifier, and is also included in several other products connected with exposed concrete floor finishing.
Densifiers are chemicals that react with lime in the concrete to produce new cementitious material. It partially fills, or ‘tightens,’ the concrete pore structure as shown in Figure 1. To improve wear-resistance and minimize dusting, densifiers have been used for decades (often under the term, ‘hardener’) on exposed slabs that were not being given decorative treatment. They were introduced for a new purpose in the mid-1990s—concrete polishing. First came sodium silicate and potassium silicate, then lithium silicate in the late 90s, and reactive colloidal silica five years ago.
Reactive colloidal silica was not invented for use in concrete; rather, it was discovered by two polishing contractors who found it superior to the silicate densifiers they had been using. Unlike sodium silicate and potassium silicate, it does not have to be scrubbed in or off. Spray-applied and dry within an hour, it requires no overnight dwell time, and does not produce a caustic residue subject to hazardous waste disposal restrictions.
Unlike a mishandled silicate, reactive colloidal silica does not produce whiting—a hard residue that would have to be ground off, essentially starting the polishing process over. It is also lower in pH, making it safer to handle. Still, the most versatile advantage turns out to be the ability of reactive colloidal silica to bond to other silica, including itself. It not only reacts with the lime in concrete, but also bonds to the cement’s silica. Silica in suspension will also bond to silica that has already bonded to the concrete, allowing it to accumulate. This bonding and buildup property does not occur in any of the silicates.
Applicators using reactive colloidal silica as a densifier noticed a floor that had been ground smooth could be buffed to a nice sheen even before it was fully polished. They realized they were getting the gloss from the densifier buildup. This discovery led to numerous new methods of finishing floors using reactive colloidal silica in two different types of product: densifiers and stain protectors.
The latter is a category of breathable sealers that should always be used to complete polished concrete applications. Even after densification, concrete surfaces remain somewhat porous, and can still be infiltrated by liquids. Conventional stain protectors (their brand names often include the words, ‘protector,’ ‘guard,’ or ‘shield’) are one-part polymer solutions that deposit a thin, breathable protective layer on the concrete surface, usually darkening the colour slightly and imparting a slightly wet look. This application eventually wears off and needs to be re-applied. Wear time depends on both product quality and traffic conditions, and may be as little as six months or more than two years.