Alternative Low-CO2 “Green” Clinkering Processes

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).