Arms Race

I have enough work coming up in the next few months that I could justify adding one more piece of equipment. I picked up a planetary grinder setup for dry grinding. The conventional grinder works but takes a long time and requires enough pressure that the thoughts of doing more than one kitchen make my arms hurt. The planetary grinder gets the job done about 5 times faster and has enough weight that all I have to do is hold on. This should pay off in time very quickly.

For a test I quickly ran up through 3000 grit on spare piece in the garage. Although you wouldn't go up to 3000 for a kitchen or bathroom, I just wanted to see how it works. This is a 30 inch by 20 inch piece and it took less than 20 minutes to work from 50 to 3000. The general surface texture is much better/more consistent than the single head unit. 

It will be a busy fall.

On-going and Up-coming Projects:

1. Concrete planters

2. Fireplace surround, bathroom vanities, bathroom sink, and bar top for a new remodel

3. Island countertop

4. Kitchen remodel

Finishing the White Desk

The white concrete desk was placed two weeks ago. Form were stripped after 4 or so days. Initial grouting was done at 4 days and moist cured. Initial grinding 50 and 100 grit was done at 7-days. A second thinner grout was applied with a rubber trowel. The countertop was moist cured for another 7-days until polishing. At 14 days after placing the top was polished through 1500 grit. So the full polishing scheme went 50, 100, 200, 400, 800, and 1500. I skipped 3000 grit since the sealer doesn't really stick to it. Four coats of sealer went on today. The sealer will cure for another day (it is really hot and humid in the garage right now) and will get waxed before installation.

Placing the Concrete Fireplace Surround

The fireplace surround had reinforcing steel mesh embedded 1-inch into the surface. Since it is such a long piece (about 7 feet) which is restrained by the existing fireplace, the steel helps prevent shrinkage cracking. Even though the carpet here will get replaced it was covered with cardboard. Integrally mixed concrete pigment will stain pretty much everything.

The setup in the garage. The fireplace took 3.3 cubic feet of concrete so two different batches were required. The mixture was a modified quikcrete mixture containing lower w/c, latex, high range water reducer, viscosity modifier, fibers, and pigment.
Same mixer as last time, still too small. Each batch was 1.65 cubic feet.

The mixture was self-consolidating to minimize honeycombing against the small gap between the mold and the fireplace.

The surface was hard-troweled and covered with plastic.

And covered with wet towels for a couple days.

Polishing the Concrete Sink

Alright, our concrete has stayed nice and moist for a few days and has gained some strength. Now comes the finishing. Finishing takes that boring looking piece and really makes it stand out. We must first distinguish finishing techniques. Concrete can be ground or polished using either wet or dry techniques.

Grinding - We consider the grinding phase when finishing young concrete (5-7 days old) with sanding grits less than 400 grit. Typically grinding is very aggressive to expose decorative aggregate or glass. At about 7-days old the concrete becomes too hard to grind and tends to go through grinding pads quickly.

Polishing - Polishing is finishing older/harder concrete with pads 400 to 1,500 or 3,000 grit. This produces the shiny glass-like surface.

Depending on the project you can have various grinding/polishing levels.

- Unground/unpolished is the concrete as is

- Ground/unpolished exposes aggregate but leaves a dull surface

- Ground/polished exposes aggregate with a shiny surface

- Unground/polished leaves the uniformly colored cream on a shiny surface without exposing any aggregate.

The sink contains no aggregate so only a light grind was applied to the surface at 5-days after casting. These are backsplash pieces with inlaid tiles. The top piece had a light grinding while the bottom shows the piece straight from the mold. Notice the ground piece has a slightly darker color. The concrete surface has high levels of calcium hydroxide (free lime) which is white. Grinding removes this layer.

After the concrete had cured an additional 5 days (10 days after casting), the polishing begins. For this project I used a wet grinding technique with a pneumatic grinder, since I have a large two cylinder air compressor. Wet-feed electric grinders are available. Wet grinding poses a few interesting logistical items. First, a constant supply of water is required. Mine was from a hose and milk jug with a few holes. Second, the water needs somewhere to go. My rolling cart allow the grinding to be done outside so the water just ran into the yard. Polishing inside requires capturing the water. These items mean that wet grinding requires some forethought on the setup. Notice the plastic lip on the table to prevent me from getting wet. Plan on wet grinding in a swimsuit, it makes a mess.The first round of grinding/polishing will open up some bug holes. These are small pockets of air which stuck to the mold when the concrete was cast. Most project for the kitchen or bathroom require a smooth surface for cleaning purposes, so the holes need to be filled. There are several colored-matched slurry kits available. These contain colored, polymer-modified cement paste (no aggregate or sand) which is used to coat the surface and fill the holes. Here is a picture of the sink after a slurry coat. The chips in the front lip of the sink were filled with this slurry. Small holes are generally filled with one application. Larger holes may require two or three slurry coats.
After the slurry coats were finished, stains can be applied. Stains consist of water or acid borne pigments which penetrate the concrete surface leaving behind a color. Since the vanity for this sink will have walnut drawers, the sides of the sink got a walnut color.

The sink was wet polished through 1,500 grit. As you see it is glass smooth and very reflective. The next project will use a dry-grinding technique.

After polishing we can consider sealing/surface protection options.