Fireplace Surround Progress

Here are the finishing steps and the current progress;

1. Strip the forms

2. Thick slurry coat to fill the large bug holes

3. Quick grind 50 grit through 200 grit (about 30 minutes)

4. Light slurry coat

5. Quick grind quick grind 200 grit through 400 grit (about 30 minutes)

6. Spot slurry as places that are still low

7. Finish grind at 400 grit (about 30 minutes)

8. Seal and wax

9. Lightly rub with steel wool to remove the shine

Really this project hasn't taken much time or money. The mold only took a few hours to build and get in place. The concrete was 6 bags of quikcrete at $4.50 each, 3 gallons of latex at $10 each, $10 worth of pigment, and about $10 for reinforcing steel. The concrete placement only took about 4 hours from buying the materials to setting up, placing, and clean up. The real cost was the dry grinding setup since most people are against wet grinding in their living rooms. Although if you are planning on two or more project, the speed and cleanliness of the dry setup is well worth the money.

Finishing the Concrete Fireplace

After curing under plastic and wet towels for four days the first slurry coat was applied to fill in the largest bug holes and honeycombing. A light grind will knock down any high places. A second slurry will fill in any places that still need it.

Since there isn't a good way to wet polish concrete inside, the fireplace will be dry polished. The dry polishing setup uses a low-speed grinder with hot-pressed pads and a dust shield. This method is much faster and cleaner than the wet method.

Starting on the front corner.

Concrete Fireplace Surround Reveal

After two days of curing in the mold, under plastic, and under wet towels the forms were carefully removed. As expected there were a few places that had honeycombing and need some slurry. The larger proportions make the fireplace stand out better.

Some honeycombing under the braces used to hold the mold together.

The concrete after a batch of colored slurry. The slurry was made using the same proportions of cement and pigment held together with just enough latex.

It is a little splotchy as the slurry dries. Another coat or two of slurry will be required to completely fill in all of the bug holes and honeycombing. A light grind will smooth the surface back out.

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.

Concrete Fireplace Surround

The next concrete at home project is replacing this beige tile fireplace surround with a concrete one. The surround will be flat black concrete and be a little thicker and taller than the current version. Afterward the gas log will be removed and replaced with a low profile burner and crushed glass. The tile was held on with a combination of mortar and construction adhesive. Either way the underlying material is very rough and will create a great bond with the new concrete.
Pretty standard melamine construction. This mold used (2) 24"x48" pieces which cost about $20.
The new concrete will be about 1 1/2" out from the original. Because of the small clearance the concrete will have a maximum aggregate size of 3/8" to allow placement.
Since the floor is concrete and I did not want to drill into the slab, the mold has feet out from the front and back. Silicone adhesive was used to seal the mold to the concrete and glue it into place. It will be fine as long as the concrete is not vibrated too much during placement. In order to accomplish this, the concrete for this placement will be self consolidating.
Due to the upcoming volume of pieces that will be created this summer I have started modifying a prebagged quikcrete mixture for various applications. By adding additional supplementary cementitious materials, admixtures, and fibers I have created mixtures both for inside use and outside. Once the testing matrix is complete I will post the results of my research.

Finishing Up the Sink

Other project have been keeping me busy so I better finish posting the pictures of the concrete sink before showing the next concrete items. After the sink was polished a semi-penetrating polymer sealer was applied to prevent unwanted stains or rings. A few coats of bees wax was applied over the sealer. This step is not really necessary and a holdover from colored concrete floors of the early 1900's that were only sealed with wax.

This picture shows the copper piece in the bottom of the sink. There was no functional reason for the copper, I just liked the look of copper patina. It is a textured copper. Since the inset and cabinet fronts will be walnut I stained the inside of the sink with walnut acid stain.

The copper patina was chemically accelerated. Although I made backsplashes to match the sink, they seemed a little heavy (visually) for the space. The backsplash ended up being tiled.

A mock-up of the removable walnut inserts.

The cabinet was notched to provide a small reveal to highlight the concrete sink.
Looking up the sink at the new backsplash.

To date the walnut inserts and cabinet fronts have not been completed. Pictures will be posted when everything is finished.

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.