Technology Roadmap Low-Carbon Transition in the Cement Industry

Table 3: Process CO
2
 emissions released upon calcination of raw materials
by clinker compounds
Note: Limestone is considered to provide the main source of calcium in clinker.
Source: UNEP (2016), 
Eco-efficient Cements: Potential, Economically Viable Solutions for a Low-CO
2
, Cement-based Materials Industry
.
Process CO
2
 emissions 
(kilogramme of CO
2
 per tonne of material)
Alite (Ca
3
SiO
5
)
579
Belite (Ca
2
SiO
4
)
512
Tricalcium aluminate (Ca
3
Al
2
O
6
)
489
Tetracalcium alumino-ferrite (C
4
Al
2
Fe
2
O
10
)
362
Lime from limestone (CaO)
786
Wollastonite (CaSiO
3
)
379
Ye’elimite (Ca
4
Al
6
SO
16
) from calcium sulphate
216
Periclase from magnesium carbonate (MgO)
1 100
Periclase from magnesium silicate rocks (MgO)
0

---

43
4. Carbon emissions reduction levers
compared to PC (Figure 16). These CSA clinkers 
have rapid strength development and reduced 
drying-shrinkage cracking of cement (Gartner 
and Sui, 2017). They have been commercially 
produced for more than 30 years, primarily in 
China where current annual production is around 
2 Mt/yr (ECRA and CSI, 2017). 
z
Alkali‑activated binders
(sometimes called 
geo-polymers) are produced by the reaction 
of an alumino-silicate (the precursor) with an 
alkali activator. They can reduce CO
2
emissions 
depending on the carbon emissions associated 
with the production of alkali activators. They 
rely on materials similar to those used in blended 
PCs to reduce the clinker to cement ratio, 
either from natural (e.g. natural pozzolana) or 
industrial (e.g. GGBFS) origin. Therefore, their 
availability has a high regional dependence, 
and is expected to decline in the future for 
GGBFS or fly ash. Given the wide range of mix 
designs, sources and doses of the activator and 
the energy mix to produce the precursors, it is 
impossible to provide a single value or even a 
well-defined range to describe the CO
2
footprint 
of producing alkali-activated binders compared to 
PC clinker. Two extreme cases could be 97% CO
2
and energy savings compared to PC related to 
mortar based on blast furnace slag with sodium 
carbonate as the activator and containing high 
amounts of granular limestone, and less than 
10% savings for an inefficient mix design and 
an energy mix highly reliant on coal (Provis, 
2017). Global commercial use of alkali-activated 
binders remains limited, and they have been 
primarily used in non-structural applications 
(ECRA and CSI, 2017; UNEP, 2016). Australia, 
Brazil, Canada, China, Czech Republic, India, 
Netherlands, Russian Federation (hereafter 
“Russia”), South Africa, Ukraine, United Kingdom 
and United States have reported commercial-scale 
production and use (Provis, 2017).
KEY MESSAGE: Alternative binding materials offer possibilities for reductions in the generation
of process CO
2
 emissions in cement manufacturing.