There are many different concrete finishes that may require sanding to improve their appearance and usability. These include concrete floors, work tops, counter tops, walls, ceilings and decorative items. Sanding concrete to produce a smooth shiny finish is referred to as “Polished Concrete“. Heavy duty polishing machines are used to gradually grind down the surface of the concrete to eventually produce a smooth shiny finish. The technique is comparable to sanding down wood, except to sand wood requires only sandpaper but sanding concrete requires tools that are diamond-tipped as it is much harder.
2. Hand Held Grinders
Hand held cement grinders typically have a rotating disk, typically 7 inches across. They have a motor built into the handle and also have a 2nd handle that is used to give more control and steady the grinder during use. Their price ranges are from around $250 up to $1,000 for a top of the range model. Good Hire shops and hardware suppliers also rent these out which may be a more cost effective way of completing the task.
3. Types of Grinding Disk
There are several types of concrete cutting disk currently available on the market. Each has its particular use and careful selection is recommended to reduce unnecessary time taken to complete the job and to end with a suitably high quality finish.
These types of disk are best used when a large amount of material is to be removed quickly such as removing items embedded into the concrete such as glass or stone. Turbo cup disks are rigid metal disks with a metal segment around the edge that is impregnated with diamonds. They are available in a variety of sizes and styles, typically 5 or 6 inches. The larger disks spin faster and so they can cut quicker. Because they spin faster, they tend to be more stable and are less likely to dig into the concrete. Smaller diameter Turbo Cup disks are used for edges and smaller sections of concrete. Because the outer edge of a larger disk has to travel further, it will be moving faster. Faster travelling disks tend to be more productive as they will lose diamond pieces quicker so will stay sharp. Smaller diameter disks will be the opposite and as their outer edge is slower moving, less diamond pieces will become dislodged so the diamonds will not be as sharp. The disk will last longer but will be less productive.
Turbo cup disks can be purchased for either wet or dry use.
There are three parts to a Turbo Cup disk. 1. The plate which is the backing part of the disk. This varies in design from manufacturer to manufacturer but typically has some series of holes that are designed for cooling. 2. Segments are the bits of the disk that actually do the cutting. They are manufactured using a compound mix impregnated with diamonds. Once again they vary from manufacturer to manufacturer. Once the segments are worn away, the disk will no longer be able to cut and will have to be discarded. 3. High temperature solder is then used to secure the segments to the plate.
Turbo Cup Disks are designed based on four factors:
This can be calculated by measuring the surface area of one segment, then multiplying by the number of segments on the disk. The smaller this surface area is, the more aggression or speed the disk will have. Disks with larger surface area will therefore have lower aggression.
This differs from manufacturer to manufacturer and can affect the disks aggression as well. In general, disks with segments that have sharper leading edges will have the highest aggression.
This is the type of bond that is used to make the segments in the factory. Its purpose is to hold the “grits” of diamond. When the disk is cutting, the bond wears away exposing the diamond grits that are then used in the grinding process. As the diamond grits slowly break off or break, the bond is eroded thus exposing more diamond grit so the disk can continue to cut. Once all of the bond has been worn away and all of the diamond grit has been used, the disk is then useless and would need to be replaced.
This is the reference number used to determine the finishing qualities of the cutting diamonds embedded into the segments. The lower the number, the coarser the pieces of diamond are, and the higher the number, the finer the embedded diamond pieces become. Typically in concrete sanding, the coarsest grit required is around 30 or 40. Coarser than this, and the disks will struggle to maintain sharpness, and finer than this will slow production as the disks will not be able to cut sufficiently.
These disks are for general concrete smoothing where a Turbo Cup is not required. They are thin plastic disks that are impregnated with cutting diamonds. They typically have a backing of Velcro so they can be quickly fixed and unfixed to a Velcro faced backer pad that is attached to the concrete sander.
Diamond pads come in a variety of different grits, or coarseness. The grit numbers available range from 50, being coarse and used for honing up to 3,000 grit which is very fine and used for polishing.
When sanding and polishing cement, it is important not to skip grit sizes. Typically pads with grit 50 will be used first, followed by 100, 200, 500, then 3,000. If you try to jump from 50 grit to 3,000, you will end up becoming frustrated as a grit 3,000 is not coarse enough to be able to remove the scratches left by the 50 or even 200 grit.
If you are not looking to get a highly polished finish, then when working your way through the grit sizes, you should be able to achieve a sufficiently good finish by using 200 grit as your last set of disks. Grit 500 up to 3,000 are typically used for polishing and glass like finishing.
Use a Mohs Hardness Pick Set to find out how hard the concrete is. Using this set you can discover how hard or soft your concrete is compared to other minerals. This will enable you to fine tune your concrete polishing methods. Softer concrete will take less time to polish and generally use less abrasive disks.
The concrete surface should then be inspected. If there are any cracks, these should be repaired first using specialised concrete crack filler which should be squeezed directly into the crack then given enough time to dry. If the cracks are small, an epoxy type filler can be used to bond the concrete together.
The surface of the concrete then needs to be cleaned using household detergent mixed with water. To remove mould or stubborn stains, Hydrogen Peroxide, TSP or Ammonia may be used, but select only one chemical and only use that. Do not mix chemicals together as it may be dangerous. When handling chemicals or detergents, always wear goggles and protective gloves for safety and ventilate the area well.
5. Protective Clothing
When grinding concrete, protective clothing should always be used. Depending on the type of concrete grinder you have depends on the amount of dust produced. Some grinders have a built in dust collection system and these tend to work very well, but there is always a small amount of dust released into the air. Obviously if you are using a grinder without a dust collection system, the dust generated will be far more.
You should wear an approved dust mask and goggles as a minimum requirement. Ear protectors can be worn if required and also a paper dust suit can be worn to protect your clothing. Make sure your footwear has sufficient grip and you are not at risk of slipping.
6. Sanding and Polishing
You will need to select the coarseness of grinding disks to be used. Only select metal bonded diamond disks as non-diamond disks will not be hard enough to grind down your concrete.
When you first start grinding, if there are marks or stains that remain after the initial cleaning stage, use a 40 grit grinding disk to remove these. Read the concrete grinder instructions how to attach the disks safely to the grinder. Once any stains are removed, move on to an 80 grit grinding disk. Cover the whole concrete area working from corner to corner.
Note the grinding disk numbers indicate the disks coarseness. Lower numbers are more coarse, than higher numbers which are finer and should be used later. You will need to progress gradually changing disks to higher and finer disks. Depending on the hardness of your concrete, this may take some time but as a general guideline, 40 grit disks can then be followed by 80 grit, 150 grit, 200 grit and then finally 400 grit disks for a polished glass like finish.
When you change grinding disks, ensure you have passed over the entire working area first. With each change of disk, the idea is to polish away all the scratches left from the previous disk. Do this by crossing over previous scratch lines in a perpendicular direction.
7. Applying Liquid Chemical Hardener
This chemical is often referred to as a “Densifier” and when applied to concrete, it helps to solidify it and produce a strong and durable finish.
Depending on the hardness of your concrete, this should be applied to the concrete’s surface at different stages of the process. For soft to medium concrete, apply the Densifier after using 80-grit grinding disks. For medium to hard concrete apply the Densifier after using 200-grit grinding disks.
Read and follow the instructions on the bottle how to apply the Densifier. Typically you will need to use a small sprayer to apply the product evenly over the concrete surface and then let it dry before continuing.
8. Completing the Polishing.
After the Densifier has dried, continue polishing using 3000-grit disks. These final disks are so fine, they will actually begin polishing the surface to give a high gloss like finish. Once again, work over the entire area from corner to corner until the surface is polished to the standard you require.
After the 3,000-grit disks have been used, vacuum the entire area so it is free of any remaining dust or debris.
Finally the polished concrete can be buffed using a buffing machine. You may then want to protect the surface by applying a sealant or stain guard product. This will help protect the surface if it is in an area that may become exposed to oil, grease or chemicals. Usually two coats of stain guard are sufficient to protect the floor for several years.