Lythic Solutions: Concrete Polishing 101

Concrete 101: Learn about how concrete is polished and what to expect.
Find out about polishing New Concrete
Find out about polishing Existing Concrete

CONCRETE POLISHING 101

Polished concrete is in fashion for some very practical reasons. It's an aesthetic and economic solution both for new construction and remodels. It offers a range of finish options to meet varied designs. Its first cost is highly competitive, and its life-cycle cost is even better. Polished concrete treated with Lythic™ Solutions technology has low maintenance requirements and high performance characteristics. Finally, it is one of the most sustainable options available.
          Popularity has produced wide availability of professional expertise in the field, and encouraged creativity and craftsmanship. Because of these creative professionals, new appearance innovations are constantly being created.
          Concrete can be left in its natural tone, or it can be colored – even multi-colored – if desired. Highly artistic color schemes have been created.
          Concrete itself can present different surfaces, depending on how it is handled. The finely-grained, uniform surface of troweled concrete can be polished to any level from a slight matte appearance to a high gloss. The sand and gravel in the concrete – the aggregate – can be revealed by grinding off the fine surface. Polished concrete with revealed aggregate can resemble terrazzo, or even natural stone. In new slabs, aggregate can be specially selected for color and size to create a desired look when polished. Some practitioners seed the surface of new slabs that are intended for polishing with "creative aggregate"– stainless steel screws or seashells, for example; when ground flat, they become eye-catching details that enliven the pattern and make a floor absolutely unique.

The polishing process has three major stages: Grinding, Densifying, Polishing: Color addition may occur at different stages, depending on the color method used. Stains or dyes can color the surface of newly cast or existing slabs. New concrete can be colored at the time it is placed by including integral coloring pigments in the concrete mix.
Stains and dyes can be employed on new or existing concrete. They are useful on new concrete if a more mottled or variegated appearance is desired. Several colors can be used in combination, and they can be combined with integral color, multiplying the possible visual effects.
Concrete must be hard before grinding or polishing can begin. A new slab must cure 28 days before polishing.

GRINDING
Once the concrete has cured, it is ground flat using diamond abrasives mounted on large grinding machines. Abrasives are graded by their coarseness, like sandpaper. A new slab that has been finished well can be ground very lightly, just to improve flatness and evenness, leaving the fine cement "paste" surface intact. It can also be ground more deeply to reveal aggregate, if desired. An existing slab in good condition with a fairly smooth surface may only need a light grind with a reasonably fine grit. A slab that has been abused or damaged may need several passes, starting with a coarse grit and getting finer until a smooth matte surface has been achieved; in such instances, revealing aggregate may be necessary simply because the damage to the slab requires deeper grinding to achieve smoothness.

DENSIFYING
After grinding, the densifier is applied. Densifiers react with excess lime that is present in the concrete to create increased cementitious materials, filling in the pore structure of the concrete surface. Concrete normally has pores because some of the water used to mix it evaporates out during curing, leaving behind tiny empty spaces. Filling in this pore structure makes the surface more dense, and less permeable to liquids (and therefore more resistant to accidental staining).
The additional cementitious materials also improve the concrete's ability to take a polish, in two ways. The mechanical process of polishing concrete, or any other material, is actually a process of scratching it. A series of ever-finer abrasives are used until the scratch patterns become too fine to see. As the peaks between the scratch-valleys become more finely spaced, there is less and less room for them to cast shadows, creating the glossy, reflective effect we associate with polished surfaces.
To be polished effectively, a material must be hard enough not to give up large chunks under the stress of abrasion. The harder a material is, the better it will take a polish.

Concrete, in its native state, presents two problems to polishing:

• The softest part of the slab is generally the top layer of fine, smooth-grained "paste," because it contains soft materials that are brought to the surface during concrete finishing.

• The pore structure introduces random spaces into the scratch pattern.

Both these problems are solved by applying Lythic™ Densifier before polishing. It fills in the pores by adding new cementitious material, which also increases the hardness of the surface. Lythic Densifier begins reacting within 1-2 minutes. After one hour, polishing can begin.

POLISHING
Polishing is similar to grinding, except that the grit of the abrasives is much finer. Again, diamond-tooled machines are used. Both wet- and dry-polishing are possible. Each has advantages and disadvantages that should be discussed with your applicator.
Most polishers progress from 400-grit up to 800-grit diamonds. Extremely high-gloss polish, sometimes referred to as "mirror polish," is possible. Such a glossy surface may actually look slippery, but testing has shown that a polished concrete floor is safe in terms of slip-resistance.