Which material can be added to concrete to reduce shrinkage and improve flow?

Bentonite clay is a type of clay that can be added to concrete to help reduce shrinkage and improve flow characteristics. Its unique properties allow it to absorb water, resulting in a more workable mix that minimizes the likelihood of cracking as the concrete dries and cures. By incorporating bentonite clay, the concrete retains moisture better and can enhance overall flexibility.

The other materials have different roles in concrete mixtures. Sand is typically used as an aggregate that can improve the density and strength of the concrete but does not have the same benefits for shrinkage control and flow improvement. Gravel serves a similar function by providing bulk and strength when mixed with cement and water, but again, it doesn't address shrinkage effectively. Adding more cement would generally increase the strength and initial set of the concrete but could lead to increased shrinkage because of the greater curing heat and moisture loss.