This is the fourth article in a series on making concrete countertops. The previous articles detail concrete countertop supplies, mold making and pouring the concrete. These articles are best read in order.
Curing Concrete Counters
Once the concrete has been placed in its mold and all air bubbles have been released, it is time to let the concrete cure for a couple of days.
- If the environment is particularly dry, tent the mold over with some plastic sheeting to keep the concrete from curing too quickly. Rig something up so that plastic hovers over the mold anywhere from a few inches to a few feet depending on just how dry and hot the air is. Avoid direct sun on the curing concrete.
- After two days, place two pieces of wood on a flat surface and carefully turn the mold over upside down onto the wood.
- Unscrew the mold and pry off the side pieces. Do not pry against the concrete, it is still too soft at this point.
- Lightly mist the concrete then let sit for another day. The concrete will continue lightening in color over the next several days but once the concrete is sealed is will take on a darker color – close to what the cured concrete looks like around day three (this is a rough approximation and varies widely depending on the pigments and concrete mix used).
After the concrete has been out of the mold for two full days, grinding and slurrying in any voids can commence.
Grinding and Polishing the Concrete Counter Top
There are many possibilities for finishing the counter top. The simplest is to do no grinding at all but to simply slurry in any pinholes or voids that there may be on the top surface of the counter top and then to polish the concrete starting with a 200 grit diamond polishing pad.
The other option is to grind the surface to expose the aggregates (rocks and sands) within the mix. By grinding with a 50 grit diamond polishing pad the larger aggregates can be exposed. By starting with 100 grit, the sands can be exposed. It is a matter of taste as to what looks best. With a practice piece it is possible to try different styles in different parts of the same concrete slab.
If the decision is to expose the aggregate, start the wet grinding two days after the slab is flipped out of its mold. If the piece will only be polished wait for day five.
All grinding is done with water streaming onto the counter top. There are a few ways to accomplish this. If a water-fed grinder is not being used one method for keeping the surface water fed is to poke holes in the bottom of a large plastic container (a large vinegar container works well), place it on the counter top and feed a hose into the opening with the water trickling in. A constant stream of water will wash out of the bottom holes onto the counter top.
For exposing the rocks, work slowly with a 50 grit circular diamond pad attached to the grinder across all areas of the surface until the desired aggregate exposure is obtained. Switch to a 100 grit pad and repeat the process until the desired finish is achieved. Hose off the surface well. Slurrying can be done the next day.
Slurrying Concrete Countertop Pinholes and Voids
Usually there will be some pinholes and occasionally a small divot from an imperfection due to a nick on the mold. These will need to be filled with a slurry made from Portland cement, pigment and a bonding agent. The pigment can be dosed in the same ratio as was used with the concrete mix or a lesser or greater concentration of pigment can be used to create a contrasting color. Use one part bonding agent to 10 parts cement. (For details on the bonding agent, see the countertop supplies article listed towards the bottom of the page.)
Mix the cement powder, bonding agent, the pigment and just enough water to get a consistency a bit thicker than sour cream. Work the slurry into the holes using gloved hands and a putty knife. Some people find simply rubbing the slurry in with hands works best then further working it in with the putty knife. Work quickly as the bonding agent will make the slurry set up fairly quickly. Use the putty knife to scrape all excess slurry off the surface, otherwise it will need to be ground off.
Wait one day, then polish with the next grit of polishing pads, most likely 200 grit. Hose the surface off well after polishing. It may be necessary to do another round of slurrying the next day. One day after the last slurrying, wet grind with the 400 grit and 800 grit polishing pads. Then let the piece sit until it has been out of the mold for a total of 10 days before polishing with the higher grit pads.
Final Polishing of Cement Counter Tops
Slowly work through all the grits of diamond polishing pads all the way up to 1500 grit, making sure that all parts of the slab get the same attention. Use the hand pads wherever the electric grinder can't reach. The slab should now be incredibly smooth and ready ready for sealing. Once it is sealed the concrete will take on a coloration half way between its dry unsealed state and what it looked like when wet polishing.
Related Articles:
- Making Concrete Counter Tops: Required Concrete Countertop Supplies and Sources for a DIY Project
References:
- Cheng, Fu-Tung. Concrete Counterops. Taunton Press. 2002.
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