However, if the grout surrounding the heat-exchange pipes cracks or shrinks, the boreholes housing the pipes can channel surface runoff contaminants directly into groundwater. This concern and the poor performance of conventional grouts led the New Jersey Department of Environmental Protection (NJDEP) to ban their use in 1998.

Search for a better grout

Brookhaven was already working on grouts with increased thermal conductivity. These grouts also had advantages in terms of sealing capability, reduced shrinkage, and improved crack resistance. The laboratory performed additional tests to ensure that the formulation would meet NJDEP requirements.

“This is a great example of scientists working together with industry and environmental regulators to solve a real-world problem,” Berndt said.

The new grout, called Mix 111, is comprised of cement, water, silica sand, and small amounts of superplasticizer and bentonite. Brookhaven does not manufacture the grout, but has made the formula available to the industry.

“The whole objective was to come up with something people in the geothermal heat pump industry could use. They can buy the ingredients themselves and mix it themselves to keep the cost down,” remarked Berndt.

Tests have shown that Mix 111 is less likely to be infiltrated by water, bonds more firmly to pipes, and is much more resistant to shrinkage and failure than conventional grouts. When tested in two different climates and geological areas, the new grout was 29% to 35% more efficient at heat transfer than traditional grout.

Mix 111 is now approved for use in New Jersey. It has already been used in several residential and commercial projects throughout the United States.