Brief introduction of limestone cement
Limestone has been utilized as a mineral addition in cement industry for a long time. The latest European Standard (EN 197-1-2000) allows up to 5% limestone as a minor additional constituent, and also identifies four types of Portland limestone cement containing 6-20% limestone (types II/A-L and II/A-LL) and 21-35% limestone (types II/B-L and II/B-LL), respectively. The ASTM Standard (C150-04) allows up to 5% of limestone filler. Since the early 1980s, the Canadian cement standard has allowed the inclusion of up to 5% limestone addition to Portland limestone cement.
Similar trend could also be found in Latin-American countries such as Argentina, Brazil, and Mexico (Menendez). According the review Cement Standards of the World (CEMBUR-EAU 1997), more than 25 countries allow the use of between 1% and 5% limestone in their Portland cement. Many countries even allow up to 35% replacement in Portland composite.
The main composition of limestone is calcite (CaCO3), with smaller proportion of quartz or amorphous silica and sometimes of dolomite. Table above is composition analysis of several typical limestones. Though the LOI content of limestone is up to 42.5% or even more, they must be low in clay minerals and organic matter to guarantee the water demand and setting time. As for the quality requirement, according to ASTM Standard (ASTM C-150-4), the calcium carbonate (CaCO3) content calculated from the calcium oxide content should be at least 70% by mass.
Limestone is softer than cement clinkers, thus could be grind into finer particles. When it interground with clinkers, for an overall specific surface area (Blaine) of 420 m2/kg, 50% of the limestone can be below 700nm, compared with 3 µm for the corresponding clinker. Addition of finer limestone particles to clinker completes the fine fraction in the granulometric curve of cement without an increment on water demand, and improves the cement packing and blocks the capillary pores.