Cenospheres Used for Concrete Countertops

What Is A Cenosphere?

Like fly ash, cenospheres are naturally occurring by-products of the burning process at coal-fired power plants. Unlike fly ash though, cenospheres are lightweight, inert, hollow spheres 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. Following are some of the general properties of cenospheres.

Cenospheres, also sometimes called microspheres, are used in a wide variety of materials, from paints and finishes to plastics and caulking. And although they have been used in concrete for some time, their use is not widely known. Cenospheres have a variety of uses in concrete countertops, including as:

Lightweight Aggregate

As a lightweight aggregate, cenospheres can replace some of the normal-weight sand used in concrete. Cenospheres have a density that is less than water (averaging 0.7 vs water’s 1.0); quartz sand particles typically have a density of about 2.65. This means that 1 pound of cenospheres takes up the same absolute volume as about 3.8 lbs of sand.

Good concrete mix design practices use a variety of aggregate sizes. The broader the particle size range is, the better the particles pack together and the less cement paste is required to coat the particles and provide excess paste “lubrication” necessary for good workability. Cenospheres’ particle sizes vary similarly to screened aggregates. So they can be used just like any other aggregate. However, their extremely small size means that there's a lot more surface area to coat than an equal volume of much larger sand particles. This has implications with regards to cement paste volume and workability.

Most cenospheres are graded so that the largest particles are about the same size as the smallest sand particles typically found in concrete. Some cenospheres come “raw”, so there are a few larger particles that overlap the fine sand particle sizes. Generally a percentage of the sand used in the concrete is replaced with and equal volume of cenospheres. The larger the particle sizes, the greater sand replacement percentage possible. Replacements of up to 33% of the sand have achieved concrete weights of around 124 lbs per cubic foot.

The advantage of cenospheres over other lightweight aggregates is that they are essentially “invisible” in the concrete, even when it is ground and polished. Conventional lightweight aggregate (expanded shale or clay) is normally used as lightweight coarse aggregate. The large particles are very visible when ground, and the porous lightweight gravel does not polish well.

Workability Enhancer

Cenospheres are very small spherical particles. As such they behave just like microscopic ball bearings in a concrete mix. Adding cenospheres to a conventional weight concrete mix will improve workability due to the ball bearing effect, and since the cenospheres are also structural aggregate, they improve concrete density and strength by providing better packing. In addition, the added fines improve trowelability and finishability. Typically dosages of 1% to 5% by weight of aggregate are added to a concrete mix to enhance workability.

Bulk Filler

Since cenospheres are very fine and generally light in color, they are ideal for use in cement grout slurry. 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. After all, cement grout is still a kind of concrete, so the same rules apply. Finally, the spherical shape makes packing pinholes easier and more effective because of the scrubbing action provided by the particles. And the spherical shapes are more likely to “roll” into small holes better than jagged or angular crushed particles. Typical cenosphere dosages are around 10% to 30% by weight of cement.