Dolomite calcined clay composite cement

The cement industry is putting huge efforts in developing cements with reduced CO2 emission. This is done in various ways. During the production of cement CO2 is emitted due to the calcination of limestone (i.e. remove CO2 from limestone), which is one of the main raw materials for Portland cement, and due to the high temperatures needed in the kiln to form Portland clinker from the raw materials. Portland clinker is the main constituent for Portland cement. Fossil fuels have traditionally been used as the energy source in clinker production, but waste based fuels are increasingly applied to reduce the CO2 emission. In addition, so-called supplementary cementitious materials (SCMs) are used to replace parts of the Portland clinker in cement, to reduce the clinker content. Since these supplementary materials often are by-products, the CO2 emission associated with these materials is generally much lower than for clinker. Thus, cements containing supplementary materials are generally manufactured with considerable reduced CO2 emission than traditional Portland cements. The use of supplementary materials is an effective way to reduce CO2 emission from the cement industry. However, because of the increasing environmental awareness, the demand for traditional supplementary materials may exceed their supply in the future. There is therefore a need to consider and investigate alternative supplementary materials for cement production. These materials should preferably be available locally and in sufficient quality and quantity. The PhD project investigated the possibility of using dolomite in combination with calcined clays as SCMs in Portland composite cements. In this project, a pure metakaolin was used as a model material for industrially available calcined clays. The use of dolomite as an SCM provides new options for Norcem AS and HeidelbergCement to exploit local resources. This will contribute to reduce the CO2 emissions of future cements, due to the reduced transport distance. The use of calcined clays as SCMs is especially interesting for cement producers in areas, where the demand for pozzolanic materials exceeds the supply of traditionally used ones. When Portland cement is mixed with water, it sets and gains its strength due to a hydration reaction. The first objective of the PhD project was to understand the hydration of such a new Portland composite cement containing dolomite and metakaolin. The focus was put of the hydration phases that are forming, and how dolomite is affecting the compressive strength of these new binder compared to the commonly used limestone. It was shown that similar amounts of a Portland metakaolin cement can be replaced with either dolomite or limestone without impairing the compressive strength. When dolomite is added to Portland cement, hydrotalcite was shown to form in samples where little or no metakaolin was present. During the service life of concrete structures, they might be exposed to various aggressive environments. This may lead to the deterioration of the concrete structures and can cause high costs for maintenance or repair. Therefore, concrete made with new composite cements has to provide a similar or even improved durability compared to today’s commercial cements. The second goal of this project was to investigate how stable the phases that form in such new composite cements are when they are exposed to various aggressive environments. The hydrotalcite formed by the reaction of dolomite in the cementitious systems was shown to be a very stable hydration product. It withstands high degrees of leaching and carbonation without changes in its composition. The hydrotalcite formed was also shown to increase the chloride binding of the cement pastes.

Norcem, and other companies in HeidelbergCement Group, are putting large efforts into developing cements with reduced CO2 emissions. One of the most effective ways of reducing CO2 emmision from manufaturing of cement is to replace parts of the Portland cl inker by so-called supplementary cementitious materials (SCMs). Commonly used SCMs are limestone, fly ash and GGBFS. In this project we are investigating SCMs not commonly used in the cement industry, but which potentially can be used successfully in the cement industry to reduce CO2 emission. The SCMs in question are dolomite and calcined clay. Using these two materials in combination is of particular interest since we expect interaction between these two SCMs. We anticipate that the use of these SCMs will provide us with an increased number of possibilities for manufacturing composite cements with reduced CO2 emission in the future. This is important since the supply of commonly used SCMs today can be restricted and too expensive. Therefore, it is imp ortant to develop alternative composite cements with reduced CO2 emission. Both clays and dolomite are present in large quantities in Norway and many other areas. We consider the main critaical R&D challenges to be: - Selection of materials and combina tions for the main study will be based on theoretical considerations and relatively limited experimental tests, there is thus a risk that potentially interesting combinations become overlooked. -There is also a certain risk that the durability study do n ot fully reveal the risk of unsoundness, in particular due to too short testing time. We rely on accelerated exposure testing which may not always be representative for natural conditions. We will consider to make additional specimens to be stored under n atural conditions and to test these over a period of years after the termination of this project.