The importance of Portland cement in the present world can certainly not be overstressed. As the primary binder in concrete, of which around 10 km3 is produced annually, it has significant social and economic importance. However, its production contributes greenhouse gases (0.8-1.0 tons of carbon dioxide are emitted per ton of cement, contributing 5-8% of global anthropogenic CO2 emissions) and requires high energy consumption. Reduction in gas emissions and lower energy use are thus highly desirable for economic as well as for environmental reasons.
The research team in Aberdeen has found that including partially decarbonated (calcined) limestone as a lime source in the composition of a pozzolanic cement, can provide pastes and mortars with properties meeting international building standards. Production of such cements normally involves crushing a lime source with high calcium carbonate (CaCO3) content, such as limestone or chalk, and calcining it (or “decarbonating” it, ie eliminating the carbon dioxide, CO2) to generate a reactive calcium oxide (CaO). Researchers have found that a lesser degree of decarbonation achieved by controlling calcination conditions can retain the cementitious properties of the clinker but reduces both carbon dioxide emissions and energy consumed.
Partially decarbonated particles are made of substantially unchanged, fully carbonated and unreactive inner cores, and a decarbonated reactive outer shell. After decarbonation the cement properties are determined by the materials mixed with the partially burnt limestone. For this cement the researchers have identified rice husk ash or RHA, which due to its high silica content and pozzolanic properties reacts with the de-carbonated shell and with water to form a cementitious product. In a hydrated pozzolanic cement system, the unreacted inner cores constitute a microaggregate, offering further mechanical advantages. Alternatively, these components can also be used with Ordinary Portland Cement (OPC).