The world’s Portland cement production is the third largest emitter of anthropogenic CO2 after heating/cooling of houses and transport. If no measures are taken, 1 tonne of CO2 is emitted per tonne of clinker produced, where 60% comes from the raw meal (i.e., decomposition of limestone) and 40% from the fuel (most commonly coal). Limestone is the dominating calcium oxide source of Portland cement clinker consisting of about 60% CaO, and hence the cement plants are located near a limestone deposit as limestone constitutes about 80% of the raw meal fed to the rotating kiln for clinker production.
In many areas, good limestone sources are close to depletion for several reasons (e.g., MgO should be < 5% in cement clinker), and the objective of this MSA short course contribution was to see if there are other Ca-bearing natural minerals available that can replace limestone, in particular non-carbonaceous minerals that also will lead to a reduction in CO2 emissions.
The conclusion is that there is no material as widespread and abundant as limestone that can replace it on a global basis. However, there are some minerals in large deposits locally that may be utilized when new cement plants are to be localized. The most promising being wollastonite (CaSiO3) that also often occur together with limestone. The second being larnite (Ca2SiO4), but it only occurs in few places. Another mineral, anorthite (CaAl2Si2O8), is too high in alumina to play a significant role in Portland cement clinkering, and is also not of interest in calcium aluminate and calcium sulfoaluminate cements due to too high silica content.
As a curiosity, gypsum may replace limestone in Portland cement clinkering, but then sulfur dioxide (SO2) is released rather than CO2, which in turn may be used for production of sulfuric acid. However, the demand of sulfuric acid is much less than the demand of Portland cement on a world basis, so this route is not viable and sulfuric acid can be obtained much cheaper from other routes (e.g., roasting sulfide ore).