Each year an estimated 17 – 25 Gt of concrete is manufactured worldwide, making it one of the most utilised substances on earth. According to a study published by the US Department of Energy in 2010, the production of Portland cement accounts for more than 2.4 Gt, or ~5%, of global anthropogenic CO2 emissions on an annual basis. This translates to about 810 kg of CO2 emissions for each tonne of Portland cement clinker produced.
Recognising the need to lower both its CO2 and energy footprints, the cement industry actively participates in the World Business Council for Sustainable Development’s Cement Sustainability Initiative. Characterised by a comprehensive CO2 emissions reduction strategy – including the use of alternative fuels, supplementary cementitious materials and sequestration technologies – the initiative has made significant progress. However, there is still work to be done.
In recent years a number of new cement chemistries have been introduced in attempts to reduce the CO2 footprint of Portland cement. These new cements have come from both within and outside of the traditional cement industry. They include products such as:
- Aether cement from Lafarge, which contains primarily belite (C2S) and calcium sulfoaluminate (C4A3S).
- Celitement cement from Celitement GmbH, which is composed of calcium hydrosilicate.
- Novacem cement from Novacem Ltd, which is based on magnesium oxide extracted from naturally occurring magnesium silicates.
- E-Crete from Zeobond Pty Ltd, a concrete product that contains a geopolymer derived from the activation of flyash.
While these products all have unique attributes, their acceptance by the concrete industry and penetration into the concrete marketplace has been slow. The keys to further “greening” cement and concrete industry operations include:
- Modification of cement production practices to reduce the emission of toxic substances and greenhouse gases, including CO2.
- Access to a viable and efficacious CO2 sequestration technology that will further alleviate the industry’s undesirable environmental impact.
- Demonstration of an improved concrete end product that can serve as an alternative to traditional Portland cement concrete.
- Accomplishing the above while remaining compatible with the industry’s existing infrastructure and operations.
Solidia cement offers an innovative approach to reducing CO2 emissions arising from the production and use of a cement product, while addressing the four environmental challenges outlined above.
Solidia cement chemistry and synthesis
Solidia cement is composed primarily of low lime-containing silicate phases such as wollastonite/pseudowollastonite (CaO·SiO2) and rankinite (3CaO·2SiO2). In total, Solidia cement clinker contains between 42 and 48 wt% lime (CaO). This is in contrast to Portland cement, which is composed of lime-rich phases such as alite (3CaO·SiO2), belite (2CaO·SiO2), tricalcium aluminate (3CaO·Al2O3) and tetracalcium aluminoferrite (4CaO·Al2O3·Fe2O3). Portland cement clinker typically contains approximately 65 – 70% lime.
The similarities and differences between the Solidia cement and Portland cement chemistries are significant. The two are similar in the fact that Solidia cement is made from the same raw materials, in the same manufacturing facilities and with the same unit operations as those used to manufacture Portland cement. Compatibility with the existing cement industry infrastructure is mandatory for the quick and efficient implementation of a new product. The chemistry of Solidia cement differs from that of Portland cement as it allows for the reduction of CO2 emissions associated with cement manufacturing and provides the basis for CO2 sequestration during cement curing.
The manufacturing processes for both cements start by creating ground mixtures of limestone as a source of lime (CaO) and sand, clay or shale as a source of silica (SiO2). These materials are present in the quarries located at or near virtually every cement plant worldwide. Lime-rich Portland cement typically requires a mixture that consists of more than 70% limestone. Low-lime Solidia cement requires only around 50% limestone. This difference offers two significant opportunities to reduce the CO2 footprint of the cement clinker. As the raw materials are heated in a cement manufacturing operation, the first significant chemical reaction begins at a temperature of about 800 °C. At this temperature the limestone decomposes, or calcines, to create lime and gaseous carbon dioxide according to this reaction:
- CaCO3 --> CaO + CO2 (g).
With the raw materials mixture for lime-rich Portland cement chemistry, the calcination reaction releases about 540 kg of CO2/t of Portland cement clinker produced. The low-lime raw materials mixture used to make Solidia cement will emit approximately 375 kg of CO2/t of Solidia cement clinker, representing a 30% reduction.
The next significant chemical reaction in cement production occurs at temperatures where the raw materials sinter, react and partially fuse together to form clinker nodules. For the Portland cement chemistry, this reaction occurs at approximately 1450 °C and results in the formation of the requisite alite, belite, tricalcium aluminate and tetracalcium aluminoferrite compounds. These compounds are produced in the following reactions:
- 3CaO + SiO2 --> 3CaO·SiO2 (alite).
- 2CaO + SiO2 --> 2CaO·SiO2 (belite).
- 3CaO + Al2O3 --> 3CaO·Al2O3 (tricalcium aluminate).
- 4CaO + Al2O3 + Fe2O3 --> 4CaO·Al2O3·Fe2O3 (tetracalcium aluminoferrite).
The low-lime chemistry of Solidia cement allows raw materials to sinter, fuse and form clinker at approximately 1200 °C. The resulting wollastonite/pseudowollastonite and rankinite phases, which comprise Solidia cement clinker, occur according to the formulas:
- CaO + SiO2 --> CaO·SiO2 (wollastonite/pseudowollastonite).
- 3CaO + 2SiO2 --> 3CaO·2SiO2 (rankinite).
The ability to produce Solidia cement clinker at a lower peak temperature than that required for Portland cement directly translates into reduced fossil fuel consumption. The fuel combustion required to produce 1 t of Portland cement clinker at a 1450 °C peak temperature will create approximately 270 kg of CO2. One tonne of Solidia cement clinker formed at 1200 °C will emit as little as 190 kg of CO2 from fuel combustion. In total, Solidia cement production can be accomplished with CO2 emissions up to 30% less than that for Portland cement.
Read Part Two of CO2 Reducing Cement here.
Written by Nick DeCristofaro and Sada Sahu, Solidia Technologies®. This is an abridged version of the full article, which appeared in the January 2014 issue of World Cement. Subscribers can view the full article by logging in.
Read the article online at: https://www.worldcement.com/the-americas/09012014/co2_reducing_cement_part_one_solidia_cement_chemistry_and_synthesis_571/