What Are Cenospheres and Why Use Them in Countertops?

Cenospheres are tiny, lightweight spherical particles
that can replace some of the normal-weight sand
used in concrete countertops.

Question:
I have been hearing about lightweight aggregates called cenospheres. What are they, and how are they used in concrete countertops?

Answer:
Cenospheres (also called microspheres) are used in a wide variety of materials, from paints and finishes to plastics and caulking. In concrete countertops, they have a variety of practical applications, including as a lightweight extra-fine aggregate and a workability enhancer. They also are used as a bulk filler and shrinkage reducer in cement grouts.

Cenospheres are very small spherical particles, and like fly ash, are naturally occurring by-products of the burning process at coal-fired power plants. Unlike fly ash, cenospheres are lightweight, inert and hollow, comprised largely of silica and alumina and filled with air and/or gases. Since they are inert, they are not considered a pozzolan. And because they are very small and have high compressive strengths, cenospheres can be used as a structural lightweight filler.

As a lightweight aggregate, cenospheres can replace some of the normal-weight sand used in concrete. About 1 pound of cenospheres takes up the same absolute volume as about 3.8 pounds of sand. Replacements of up to 33% of the sand with large-particle-size cenospheres can result in concrete weights as low as 124 pounds per cubic foot.

Adding cenospheres to a conventional-weight concrete mix will improve workability because they behave just like microscopic ball-bearings in a concrete mix. Since cenospheres are also structural aggregate, they improve concrete density and strength by providing better packing. In addition, the added fines improve the trowelability and finishability of concrete. Typically dosages of 1% to 5% by weight of aggregate are added to a concrete mix to enhance workability.

Since cenospheres are very fine and generally light in color, they are ideal for use in cement grouts, or slurry mixes. Not only does the added bulk from the cenospheres increase the volume of grout without adding more cement, the fine-aggregate gradation of the particles helps to reduce shrinkage, and the spherical shape makes packing pinholes easier because the spheres “roll” into small holes better than jagged or angular crushed particles. Typical cenosphere dosages in a cement grout formula are around 10% to 30% by weight of cement.

For more detailed technical information and resources for purchasing cenospheres, please read this article on the Concrete Connections website.