Cement is the binding agent in concrete, making it the most consumed man-made material globally. The production of traditional Portland cement is a major contributor to global carbon dioxide (\(\text{CO}_2\)) emissions. This impact stems primarily from the high-temperature process of creating clinker. Calcination of limestone (\(\text{CaCO}_3\)) involves heating it to about 1,450°C, which chemically releases large quantities of \(\text{CO}_2\) and requires substantial energy. The cement industry is responsible for approximately 5% to 8% of the world’s total \(\text{CO}_2\) output, driving the search for lower-carbon alternatives.
Reducing the Carbon Footprint with Supplementary Cementitious Materials
The most widely adopted strategy for reducing concrete’s carbon footprint involves the partial substitution of Portland cement with Supplementary Cementitious Materials (SCMs). SCMs are finely ground materials that react chemically to form binding compounds when mixed with water. Using SCMs allows manufacturers to reduce the amount of high-carbon clinker required in the final cement mixture.
Many SCMs are industrial byproducts, such as fly ash from coal-fired power plants or ground granulated blast-furnace slag (GGBFS) from iron manufacturing. Utilizing these waste streams offers the benefit of reducing clinker demand and diverting materials from landfills. Natural alternatives, including calcined clays and certain pozzolans, are also processed and used as SCMs.
The mechanism by which SCMs contribute to concrete strength is the pozzolanic reaction. When Portland cement hydrates, it produces calcium silicate hydrate (C-S-H), the primary binding compound, and calcium hydroxide (\(\text{CH}\)) as a byproduct. SCMs, which are rich in silicon and aluminum, react with this \(\text{CH}\) to create additional C-S-H gel.
This secondary reaction enhances the long-term strength and durability of the concrete. SCMs are a substitution approach that works within the existing concrete formulation but does not eliminate the need for Portland cement entirely. The degree of replacement varies, but it directly lowers the overall carbon intensity of the concrete mix.
Geopolymers: Replacing Traditional Cement Chemistry
Geopolymers aim to completely replace the traditional calcium-silicate-hydrate (\(\text{C-S-H}\)) chemistry of Portland cement. These materials are formed through geopolymerization, which uses an alkali-activated solution to react with source materials rich in aluminum and silicon. This method entirely bypasses the need for the high-temperature calcination of limestone.
The raw materials for geopolymers are aluminosilicate sources, such as fly ash, metakaolin (calcined clay), or GGBFS. These materials are dissolved and reorganized into a hardened, three-dimensional polymeric structure when mixed with an alkaline activator solution, such as sodium silicate and sodium hydroxide. The resulting binder is often referred to as an alkali-activated material.
Because geopolymer production does not rely on heating limestone to extreme temperatures, \(\text{CO}_2\) emissions are much lower than those from Portland cement. Studies indicate that geopolymers can reduce \(\text{CO}_2\) emissions by 70% to 90% compared to conventional cement. The final product is a non-calcium-based binder with properties like superior acid resistance and fire performance.
Geopolymers are a total replacement for the Portland cement binder. While they offer a large reduction in carbon emissions, widespread adoption is limited by the availability of high-quality aluminosilicate source materials and the specialized handling required for the alkaline activators.
Novel Approaches: Bio-Cementation and Carbon Mineralization
Emerging technologies are exploring new methods for creating cementitious materials, moving beyond industrial byproducts to harness biological and chemical processes. Bio-cementation leverages microorganisms to form a binding agent. This process, known as microbiologically induced calcite precipitation (MICP), uses specific bacteria like Sporosarcina pasteurii.
These ureolytic bacteria hydrolyze urea in the presence of a calcium source, leading to the creation of calcium carbonate (\(\text{CaCO}_3\)) crystals. The resulting crystals precipitate and bind loose particles, such as sand, into a solidified material. This process is energy-efficient since it occurs at ambient temperatures and atmospheric pressure, requiring low energy input compared to traditional clinker production.
Carbon mineralization represents another novel pathway, focusing on the direct capture and permanent sequestration of \(\text{CO}_2\) within the cementitious material. One method involves injecting captured \(\text{CO}_2\) into fresh concrete during the mixing process. The \(\text{CO}_2\) reacts with the calcium oxide components in the mix to form stable calcium carbonate nanoparticles.
The carbon is locked into the concrete, improving the material’s strength and allowing for a reduction in the required amount of traditional cement. Other approaches react \(\text{CO}_2\) directly with industrial waste materials or minerals, forming carbonate products that serve as cement replacements. These technologies reduce the need for conventional cement and actively utilize \(\text{CO}_2\) as a raw material, transforming it into a permanent solid form.