Calcined clays as pozzolanic man constituent for composite cementsPart 2: Concrete investigations with special consideration of durability

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Due to the climate debate, work is being carried out on the development of sustainable cements. One important strategy here is to replace the clinker content in cement, which is associated with high CO2 emissions, with suitable composite materials. Calcined clays, which are available worldwide in large quantities offer an alternative. The aim of the first project phase was to evaluate the basic suitability of calcined clays based on very different main clay minerals as composite materials in cement. The investigations in the first part of the project were carried out on pure clay minerals. Optimum firing temperatures were determined for the various clays with respect to maximum pozzolanity, which are about 700 °C for kaolinite and about 900 °C for illite and montmorillonite. The reactivity of the calcined clays decreases in the order metakaolin < metamontmorillonite < metaillite. In the second part of the first phase of the project, adequately composed commercial clay raw materials were then investigated for their activatability when different calcination techniques were applied. Reactive metatones can be produced in all the firing units investigated, requiring one hour in the muffle furnace, 10 minutes in the rotary kiln and only 1 - 2 seconds in the flash calciner. The formation of amorphous phases prior to glass formation leads to a significant increase in pozzolanic reactivity in all clays. In blended cements (with 30 wt% metaclay), all clays show only low reactivity at the beginning of hydration. After 28 days, the blended cements with metakaolin and metabentonite show higher strengths than the underlying CEM I cement. In terms of strength contributions, the metaillite is in the range of good hard coal fly ashes. Based on the results of the first part of the project, a parameter model was developed in nomogram form. In the proposed second project phase, the durability properties of concretes with calcined clays will be investigated. For the investigations, the commercial raw clays of the first project phase will be used in order to be able to utilize the already available investigation results for the second phase. Furthermore, for the tests, these raw clays will be further diluted with quartz powder until the normative limit regarding reactive silica is reached (≥ 25.0 wt.%). After optimal calcination and grinding of the clays, mixed cements CEM II/B-S, CEM II/C-M (Q-LL) and CEM VI (Q-LL) are produced, which are then used to carry out the durability tests on the concrete. Both the clay, the clinker base and the sulphate carrier are varied. The impact of cement will be investigated on concretes for the exposures carbonation, chloride penetration and freeze/thaw attack. In fundamental investigations carried out in parallel, an interpretation basis for the determined concrete durability is to be established. Both physical and chemical-mineralogical influencing factors will be investigated.