Page 88 - Demo

CONCRETE PRODUCTS & CAST STONE86 CPI %u2013 Concrete Plant International %u2013 1 | 2026 www.cpi-worldwide.comThe cement industry is making great efforts to reduce the high CO2 emissions generated during cement production. One important objective %u2013 among many others %u2013 is to reduce the proportion of clinker, which is the main source of CO2 emissions. This report explains the consequences of using CO2-reduced cements on the curing process of concrete products.The production of cement and concrete accounts for a significant proportion of global CO2 emissions. The largest share is caused by the production of cement clinker, the main component of cement. Cement clinker is produced by burning limestone (CaCO2) in rotary kilns at around 1450 %u00b0C. In this process, about two-thirds of the CO2 emissions are produced by the chemical conversion of limestone to calcium oxide (CaO) and CO2. The remaining third can be attributed to the combustion of fossil fuels such as coal, gas or oil, which are essential for achieving the high firing temperatures.The cement industry has drawn up a roadmap with strategies to become climate-neutral by 2045. The strategy has three main thrusts:%u2022 Firstly, the substitution of fossil fuels through the use of alternative fuels or electrification. Already today, over 70% of fossil energy is replaced by biogenic or waste fuels.%u2022 Secondly, the development of CO2-reduced cements. This approach aims to reduce the clinker content in cement by increasingly using ground granulated blast furnace slag, fly ash or calcined clays as grinding additives. This reduces the proportion of CO2-intensive clinker and emissions per tonne of cement by up to 40% today.%u2022 Thirdly, CCS and CCU technologies (capture and utilisation or storage of CO2). This approach is necessary because cement production generates unavoidable process emissions that cannot be completely eliminated, even in the long term.Although the first and third approaches have no direct influence on the manufacturing process of concrete products, the same cannot be said of the second approach. This is because all CO2-reduced cements have one thing in common: They all have a lower hydration heat, i.e. they all release less heat during the curing process, which can have a negative effect on the curing process. This fact will be illustrated using the example of calcined clay.Calcined clays play an important role in the decarbonation of the cement industry, as they can significantly reduce the clinker content in cement without negatively affecting the strength or durability of the concrete. Calcined clays are activated by heating (calcination) at temperatures between approximately 600 %u00b0C and 850 %u00b0C. Activated, in this context, means that the crystal water and other chemical bonds in these clays are removed. This results in a highly reactive material that exhibits pozzolanic properties in cement production. Calcined clays therefore have two significant advantages over clinker fractions. Firstly, they are activated at only approx. 800 %u00b0C, which is significantly lower than the deacidification of limestone at temperatures of 1,450 %u00b0C, and therefore consume significantly less energy. Secondly, calcination produces only metakaolin and water vapour and no additional CO2.As positive as these properties are in terms of avoiding CO2emissions, the question must be asked what consequences this has for the curing process of concrete products.As explained above, CO2-reduced cements have lower reactivity than pure clinker. As a consequence for the curing of concrete products, the hydration process is slower. It should also be noted that the heat release is significantly reduced due to the reduced clinker content. While standard Portland cement of class CEM I has a hydration heat of between 300 and 400 kJ/kg of cement, calcined clays only release about half that amount of heat, at 150 to 250 kJ/kg. Both factors result in slower development of the early strength that is so important for the production of concrete products. Diagram 1 shows the order of magnitude of the loss of hydration heat when using sustainable cements compared to classic Portland cement of class CEM I.The consequences of this development trend in cements are not insignificant for concrete curing. In a modern production plant for concrete products, terms such as \The importance of CO2-reduced cements for the curing processRotho - Robert Thomas Metall- und Elektrowerke GmbH & Co. KG, 57290 Neunkirchen, Germanyn Dipl. Ing. (FH) Erik Leu, Rotho - Robert Thomas Metall- und Elektrowerke GmbH & Co. KG, Germany