Sealing polished concrete

Some think that because concrete is polished it will not stain, but a polished floor or countertop is still porous. Gum tends not to stick to a highly polished surface but coffee and oils will penetrate.

There are several densifier and sealer options for polished surfaces. They include new lithium silicates, traditional sodium and potassium silicates, and various sealers. Silicates react with free calcium in concrete to form calcium silica hydrate, which hardens the surface. "Sodium and potassium silicates form a soluble bond that can absorb moisture and expand, then contract again with drying," says Kent Barrus technical services director for Convergent Concrete Technologies, Orem, Utah. "This cycle can cause cosmetic micro cracking in concrete (among other potential causes like poor curing). Lithium silicates form an insoluble bond that doesn't tend to contribute to surface crazing. A micro-coating based on lithium is available to help preserve the shine and protect polished concrete from staining."

Many excellent solvent-based sealers are also available for stain protection, though the VOC content of solvent-based sealers is a concern in some states. But "what coatings and sealers adhere to bare polished concrete is another question to consider," says Girard.

Floors

"Polished concrete is a material that says 'modern' and people respond to it for that reason," says Tom Marble, Marble Architecture, Los Angeles. "They like it because it is low maintenance, a sustainable material, and looks great. It acts as a thermal mass and is good for radiant floors."

Mark Foreman, owner of AK4 Concrete Solutions, Auburn, Calif. says that "most applications are indoors. People have tired of carpet and are mixing up the décor combining high tech and old world." Foreman creates samples for people "to get their creative juices flowing." He emphasizes the importance of knowing each client's preferences. Do they see a crack in the concrete as an aesthetic benefit or do they consider the crack an imperfection?

Foreman speaks of creating the artwork and locking it into the slab with staining and polishing, and notes that it is unnecessary to have high reflectivity to have an attractive floor. "On the first grind I visualize the floor and run the machine to deliberately leave some fines and expose them in areas to create a natural look. Know how your machine affects the concrete and plug those variables into your artistry," says Foreman. "Once the grit level is achieved, add stain and then densify. After the densification process, clean the floor. What remains is a hard dense surface that won't dust like untreated concrete and the art is trapped in the concrete." While it may be unnecessary to polish to the higher levels, people find it fascinating. "Residential clients become entranced and want to take the polish to the 3000 grit," says Foreman. "Most houses have low light, so a light reflective shiny floor is very rewarding."

"Every floor is different," says Greg Demmert, Demmert & Associates, Glendale, Calif. "They are inconsistent with various faults and degrees of hardness that affect grinding. Polishing contractors must be patient and willing to experiment." Demmert mostly polishes dry-a process yielding a burnished darker look than wet polishing. His floors often use earth-tone water-based stains applied after the 150-grit pass and finished with a high-solids wet-look lacquer and topped with a high-grade acrylic floor finish.

"If you want a subtle shine and color," says Demmert, "apply an acid stain and then polish to an 800-grit finish being careful not to remove color with additional grinds." He seals the 800 grind with an impregnating sealer that seals the pores without changing the color of the concrete or adding shine. Many of Demmert's clients prefer the natural stone effect of stained and polished concrete without the true diamond polished 3000 grit reflectivity. "There is a place in the market for floors with only a few grinds that remove a credit card size thickness of concrete," says Demmert. He recommends trying out different floor polishers. The different weights of machines using the same number diamond will polish differently and diamond pads vary between manufacturers. Look at smaller equipment for corners and next to walls. "There is still a lot of hands and knees work when polishing concrete," says Demmert.