It adds strength, reduces shrinkage on setting, and may add to the visual appearance of the concrete. Limestone or marble dust are two types of stone dust. Another waste product, this admixture is from metal smelting. It is a very fine dark grey powder. Metakaolin makes the mix creamier, less sticky, and is a plain white in colour.
Particle size is smaller than cement but bigger than silica fume.
Metakaolin is a fairly new additive and may be difficult to find. They disperse into the wet concrete mix and help prevent shrinkage cracks as the concrete sets. Shrinkage cracks can later cause problems. On a very small scale the fibers also add a small amount of tensile strength. They are almost invisible in the final product.