New cement saves energy and CO2

A lot of energy is consumed in the built environment. Not only for heating, but also – perhaps not so well known – for producing building materials. The cement industry alone uses between two and three per cent of the worldwide energy demand for its energy intensive production processes and emits five to seven per cent of the anthropogenic greenhouse gas CO₂ produced by its raw materials and fuels into the atmosphere. The potential for making savings here is huge, and scientists from the Karlsruhe Institute of Technology now want to tap into this. They have developed a new process for producing cement that only consumes half as much energy and emits even less carbon dioxide. “This development builds solely on completely new insights gained into the processes taking place as the material sets,” explains Dr. Peter Stemmermann, who heads the project.”

Conventional cement is made by burning limestone and clay in rotary kilns at around 1,450° Celsius. Subsequently, the clinker brick is finely ground together with other additives to produce the finished cement. This produces huge amounts of carbon dioxide, not only through the high kiln temperatures that swallow large amounts of fuel, but also because lime, the main component, releases 480 kilograms of CO₂ per tonne of cement. “The high lime share is the key factor, which is why we started there,” says Stemmermann. So, Stemmermann and his team studied the processes in detail before and while the conventional cement set. Two results were decisive. On the one hand, structural studies using the Karlsruhe Synchrotron Radiation Source ANKA show that only around half the lime is needed for the cement to bond with water and set, the so-called hydration stage. “That wasa surprise,” explains Stemmermann. “The hydration process involves two steps. First, the surrounding water leaches out the calcium ions from a layer that is just a few nanometres thick and replaces these with protons. This layer then has a low calcium level. However, it is only this leached product that, in a second stage, actually forms the calcium-silicatehydrate phases that are decisive for the hardness of the cement by reacting with water.” These CSH phases grow in the form of thin, strutted films so that the cement can set. By contrast, the other half of the lime is bound in the form of calcium hydroxide and other calcium hydrates and even reduces the quality of the cement, because these hardly set and can easily be affected by acids.

And so the idea was that much less lime could be used and the burning could be dispensed with if the basic ingredients could be left to react with water in autoclaves, a kind of pressure cooker, with temperatures up to 300° Celsius. This produces polycrystalline phases that grow in water and are stabilised by hydrogen bonds. However, to ensure that the cement can still store water and set, these hydrogen bonds have to be destroyed, for example, by grinding. “That’s the crux, we need less lime and manage with lower temperatures,” emphasises Stemmermann. But the scientists also used another observation to their advantage. Conventional cement often continues to react many years later, even though the required hardness is already seen after a month. The nucleus of the cement minerals only has a mechanical function, as a support particle. “Then we can also simply add grains of sand around which the cement material can then settle as a thin layer, like a skin,” says Stemmermann, describing the line of thought. Together, these two approaches save at least half the energy and even more greenhouse gas emissions as well. “And that’s a modest estimate. At lab scale, we can achieve a lot more, but we now want to transfer the method to the industrial scale.”

Meanwhile, the composition of the new cement and the process steps have been patented under the name of “Celitement”. After two years of support funding by the KIT Innovation Department and the Helmholtz Enterprise Fund, the day finally came in spring 2009. The Karlsruhe Institute of Technology (KIT), the inventors and the industrial partners from the SCHWENK Group founded the Celitement GmbH company. This will now build a pilot plant that aims to deliver some 100 kilograms of the new binder per day as from 2011. As from 2014, the industrial partner aims to establish a plant capable of producing 30,000 tonnes per year. “For the cement industry, these are still test-tube dimensions,” says Stemmermann, “but it is the first step into a mass market.”