text: According to Rangan, an Australian concrete expert, the growing worldwide demand for concrete is a great opportunity for the development of geopolymer cements of all types, with their much lower tally of carbon dioxide .
  Geopolymer cements do not rely on calcium carbonate as a key ingredient, and generate much less  than Portland cement during manufacture, with claimed reductions in the range of 40% to 80-90% . Joseph Davidovits delivered the first paper on this subject in March 1993 at a symposium organized by the American Portland Cement Association, Chicago, Illinois.
  This section compares the energy needs and  emissions for regular Portland cement, rock-based geopolymer cements and fly ash-based geopolymer cements. The comparison proceeds between Portland cement and geopolymer cements with similar strength, i.e. average 40 MPa at 28 days. There have been several studies published on the subject that may be summarized in the following way:
  Rock-based geopolymer cement manufacture involves: 
  * 70% by weight geological compounds (calcined at 700°C);
  * blast furnace slag;
  * alkali-silicate solution (industrial chemical, user-friendly).
  The presence of blast furnace slag provides room-temperature hardening and increases the mechanical strength.
  Energy needs
  According to the US Portland Cement Association (2006), energy needs for Portland cement is commonly in the range of 4700 MJ/tonne. The calculation for rock-based geopolymer cement is performed considering that the blast furnace slags are available as a by-product from the steel industry (no additional energy needed), or must be manufactured (re-smelting from non granulated slag or from geological resources).
  In the most favorable case — slag availability as a by-product — there is a reduction of 59% of the energy needs in the manufacture of rock-based geopolymer-cement in comparison with Portland cement. In the least favorable case —slag manufacture — the reduction reaches 43%.
   emissions during manufacture
  In the most favorable case — slag availability as by-product — there is a reduction of 80% of the  emission during manufacture of rock-based geopolymer cement in comparison with Portland cement. In the least favorable case —slag manufacture — the reduction reaches 70%.
  Fly ash-based cements Class F fly ashes
  They do not require any further heat treatment. The calculation is therefore easier. One achieves emissions in the range of 0.09 to 0.25 tonnes of  / 1 tonne of fly ash-based cement, i.e.  emissions that are reduced in the range of 75 to 90%.