Contractors are finding more opportunities in concrete countertops and are seeking mixes that set and gain strength faster than portland cement.
CTS Cement Contractors are finding more opportunities in concrete countertops and are seeking mixes that set and gain strength faster than portland cement.

In recent years, concrete countertops have seen a rise in popularity among homeowners for their beauty and durability. Contractors are finding more opportunities in this decorative concrete niche market and are seeking mixes that set and gain strength faster than portland cement — two important concerns for concrete countertop construction.

Portland cement versus Rapid Set cement

Portland cement comes in five different types. Type I and III are most often used by concrete countertop fabricators. Type I is ordinary portland cement and is widely available. Type III is a high early-strength cement. It is ground finer and reacts faster than Type I, so the early strength gains are greater. Concrete made with Type III will have higher shrinkage. Type III can be difficult to find in less than pallet quantities. Both Type I and III are available in white or gray.

Type I cement-based concrete generally reaches about 60% of its 28-day strength in the first three days; Type III cement achieves about 70% of its 28-day strength after three days. Some contractors choose Rapid Set Cement, which achieves 80% of its 28-day strength in the first day, and can reach greater than 90% of its strength within the first three to seven days.

Due to its high strength, concrete made with Rapid Set cement reduces the curing time from days to hours and the hydration process reduces shrinkage and cracking compared to most portland cement-based systems. For concrete countertop construction, these benefits offer faster turnaround and more stable concrete. This fast strength gain means that large pieces don’t need to stay in the forms for several days while they cure and gain sufficient strength to be stripped, handled, and processed. Even in the winter, large pieces can be stripped the next day where portland-cement based concrete can take three to five days.

In many cases concrete countertops are ground and polished. The quality of the polish depends on the hardness of the concrete and faster strength gain means diamond polishing can begin much sooner.

Rapid Set Cement achieves 80% of its 28-day strength in the first day, and can reach greater than 90% of its strength within the first three to seven days.
CTS Cement Rapid Set Cement achieves 80% of its 28-day strength in the first day, and can reach greater than 90% of its strength within the first three to seven days.


Adjusting the working time

One of the most important considerations for Rapid Set Cement-based systems is working or set time. The set time can be controlled through the use of commercially available set-retarding admixtures or common citric acid. The amount of working time can be adjusted by the admixture dose. Using set-retarding admixtures will have no negative effects on long-term performance. Another way to increase working time is to reduce mix temperature using chilled water. This is an excellent alternative when set retarders are not readily available. 

Countertops for company headquarters

Polished concrete countertops were needed for the Thompson Shipping Company headquarters in Grand Cayman. The main contractor was Terry Wilson, a former Concrete Countertop Institute (CCI) student. Jeff Girard, Wilson’s instructor at CCI and CCI president, helped with the project. They had only one week to complete the job because the countertops had to be installed before New Year’s Eve.

The countertops were 50 square feet by 2 inches thick polished concrete made with stone imported from Jamaica because it matched the aggregate used in the polished concrete floors. Because of the exposed aggregate in the concrete, the counters were ground and polished. As such, the concrete had to reach its maximum strength for contractors to grind and polish it in such a short timeframe. If grinding were attempted on concrete that was too soft, especially with hard aggregate and soft cement, the diamond grinding would tear up the surface.

The concrete was pigmented to complement the Jatoba woodwork and the polished floor in the lobby where the countertops were installed. Custom stainless steel, brass and copper logos were embedded in the floor and countertops.

Girard and Wilson had to develop a color and mix design on the first day on site, which was one day before casting. The very short timeframe and the requirement to produce beautiful polished concrete countertops on the first try could only be achieved using a rapid hardening cement system.

In making the polished concrete countertop, the bags of Rapid Set Cement were mixed in a towable mixer and then vibration cast, ground, and polished.

For more, visit CTS Cement at www.ctscement.com.