Abrasives 101

Abrasives are the most important tool for mechanically processing concrete to a polished finish. They come in many forms, shapes and sizes and are made out of various materials whose bonding varies in hardness. Many people think a densifier is required to create a polished finish — a densifier will increase the durability of a polished finish and aesthetically improve it, but it is not required to achieve a polish.

There are multiple variables and conditions that need to be considered when choosing your abrasives, including physical make up and the conditions under which they will be used. The manufacturer of the abrasive should be able to give you guidelines on how their abrasives best work under certain conditions. Always keep in mind the material you are abrading (concrete and aggregate, glues, mastics, coatings) has to work together with your abrasive. Not using abrasives properly will affect both their performance and life.

Abrasive particulate can be coated or uncoated. Since the concrete polishing industry primarily uses uncoated abrasives, coated abrasives will not be discussed in this article.

Grit

Abrasives are made of a bonding material and particles called grit. Grit is measured in micrometers (also called microns) and assigned a numeric value based on its size. The smaller the grit designation, the larger the grit particle is. Exact sizing of particulate is expensive so an abrasive will have a range of particulate whose sizes overlap. For example, an abrasive classified as 120 grit will not only contain 120 grit but also have a percentage of 100 grit and 150 grit.

A typical grit sequence when polishing concrete does not contain all of the grit sizes available to an abrasive manufacturer but from whatever the starting grit is each consecutive grit used is approximately doubled in size. For example, staring with 50 grit, the next grit is 100 or 120, followed by 200 or 220, 400, 800, etc. There is 600 grit particulate available for purchase, but rarely do you see a 600 grit abrasive for sale in the concrete polishing industry.

Particulate grit can consist of natural or man-made material. Man-made material is most often used due to expense and the ability to create consistency. Man-made particulate comes in various forms of alumina, silicon carbide and most widely used are superabrasives made from cubic boron nitride and synthetic diamond.

Bonding

Grit particulate is bonded together by various bonding material to form an abrasive. Abrasives may be made through an injection process, hot and cold pressing, electroplating, and vacuum brazing. The two primary bonding materials used are metal and resin.

Metal abrasives are made from a blend of micro-graded granulated metal oxides and/or powders. Harder metal bond abrasives are made from cobalt, brass and titanium. Softer bonded metal abrasives may be made from copper. Metal bond abrasives are used in coating removal and medium to heavy grinding. Use high-grit, metal-bonded tools on softer concrete.

Resin abrasives are made from poly-phenolic and ester-phenolic for wet and dry use and thermoplastic-phenolic resins for dry use only. They are ideal for light grinding, honing and polishing. Also on the market are hybrids that are made from a combination of bonding materials and ceramics. These are used for light to medium grinding, removal of heavy metal bond scratch patterns and offer heavier grinding ability with the finesse of a resin-bond abrasive.

Abrasives come in several hardnesses of bonding ensuring that diamonds are exposed most efficiently for maximum cutting. The concrete and the aggregate in the concrete will determine the hardness of the abrasive bonding for the most efficient cutting. Hard concrete will require a softer bonding so that new abrasive grit is exposed for maximum cutting. Soft concrete will require a harder bonding. Soft concrete produces very abrasive particulate that rapidly wears the bonding. The harder bond slows down the deterioration of the abrasive. Abrasives can be used wet or dry. The bonding of the abrasive is made for wet use only, dry use only or wet/dry use.