Every professional concrete countertop manufacturer has his or her very own "specially formulated" concrete mix and they guard the formula as closely as Coca Cola guards their recipe for their soft drink. Most have experimented for many years to create a concrete formula that works perfectly for them.
If you are planning on going into the concrete countertop business or if you are planning on designing and molding concrete architectural products for resale, then we highly recommend that you spend time, a lot of time, experimenting with concrete mixes. But for those of you who are planning on designing and building one or two concrete countertops, a lot of experimentation is probably not worth your time, effort and the expense.
We do however suggest that you do mix your own concrete although you can purchase premixed concrete or have the concrete delivered to your home, mixing your own concrete will ensure that you have complete control over the quality of the finished countertop.
Understanding the concrete mix
We will not waste your time by delving into the history of concrete nor shall we discuss the chemical interactions that take place. However, that being said there is one important point to discuss and that is "water"!
Water is the catalyst that makes the chemicals in the concrete react with one another. Yet, water is the one item that most home handymen use far too much of when mixing a batch of concrete. You want to use as little water as possible as the more water you use the more shrinkage that will occur and the more water you use, the more likely you are too have water stains and efflorescence on the concrete countertop. A minimum amount of water also produces a stronger and more durable cured concrete.
The strength of the concrete is also dependent upon the type of aggregate used, how well the concrete is mixed and how thoroughly the concrete is vibrated after it is poured into the mold.
You should also be aware that the temperature and humidity in the room where the concrete is curing will also have a great deal of effect on the finished quality of the concrete countertop.
Note: It is of the utmost importance the mold is not disturbed in any manner while the concrete is in its initial curing process, at least 72 hours.
The main ingredient in any concrete is the portland cement. There are a number of different types of portland cement, they are listed in Roman numerals Type I through to Type V, white cement and there are some other portland cement mixtures designed for very special applications.
The most common Type of portland cement is Type I. Type I is probably used more than all the other Types combined. It is the cement that is used for most home handyman projects such as driveways, patios and sidewalks.
However, for architectural products such as concrete countertops, Type I portland cement is not really the best choice. For architectural products you should use either Type II, Type III or white cement.
The primary difference between Type II and Type III portland cement is the curing time. Type II portland cement should be left in the mold at least 1 week before it is removed. Type III, which we recommend for the mix we detail further on, can be removed from the mold after 72 hours. Remember that you must control the temperature and humidity for the curing time. So if you are in a cold dry climate and using Type II portland cement you will have to maintain control over the environment for a week, where with Type III portland cement you only have to maintain a proper curing environment for 72 hours.
White cement is similar to portland cement Type II, without iron. White cement is generally used when bright or light color pigments are added to the concrete. Using bright or light colored pigments complicates the mix in other manners and if one is a novice at building concrete countertops should probably be avoided.
Note: Never mix portland or white cement from different manufacturers in the same pour. There are slight differences in the color grey and white depending on the manufacturer of the cement. If you do multiple batches of concrete mix using cement from different manufacturers the difference in color may be apparent in the finished product. Some manufacturers will also put a lot or batch number on the bag. Try to use the same lot or batch number for each pour.
Note: Sand is very small sized aggregate.
Purchasing sand is more complex than you might consider and the biggest problem is that sand is not transported around the country, it is quarried locally and because of that the sand that is available in one geographic area may not be available in another.
The sand quarries all have their own name for the sands that they sell. Because of this we will not state specific sand by name. Natural river sand, as shown in Figure B, is generally the best product to use when casting architectural pieces from concrete. It is graded sand with a mix of small and large particles.
Figure B - River sand
Note: You do not want to use "beach" sand because of the salt that is contained within the sand as it will destroy your concrete mix.
The best way to purchase the sand you require for your concrete countertop project is to visit a sand supplier or local quarry and discuss your requirement with the company.
How to Make Concrete Countertops - Index