Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 31

2.10 SUPERFINE CALCIUM CARBONATE
(PURE LIMESTONE)
Superfine calcium carbonate (passing 75 micrometers) is another relatively
recent introduction. It is primarily used in self-consolidating concrete as a
fine filler instead of cementitious materials. It is usually available in vary-
ing degrees of fineness, with the superfine material being distinctly more
expensive. Calcium carbonate has been used as up to 5% of OPC for many
years, being seen as essentially a diluent and cost-saver. Since the material
is simply calcium carbonate, it is difficult to see any chemical basis for its
beneficial effects, reported to include improved workability and, more sur-
prising, higher very early strength. The assumption is that better particle
packing is at least part of the explanation as well as providing nucleation
sites for hydration.