Reactivity of Calcined Clays
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With 3-5 billion tons cement-produced every year, the cement industry is today the third largest emitter of CO2 in the world after fossil-fuel based energy plants serving heating/cooling of buildings and combustion engines in the transport sector. In a short time, the easiest way to reduce CO2 emission attributed to cement making is to replace a large portion of portland clinker in cement making blended cements, or to replace a part of portland cement in the concrete mixing operation with a suitable supplementary cementing material (SCM). However, with increasing trend of making “greener” construction materials, there is a need to look for unexploited pozzolan reserves. Common clays are a large, wide spread resource with great pozzolanic potential when calcined to serve not only the local but also the global cement market. In this study the pozzolanic reactivity of common calcined clayey soils from different regions in Europe with varying mineralogy containing substantial amounts of non clay minerals was investigated by means of calcium hydroxide consumption and mortar strength tests with up to 65 % replacement of OPC. A clear correlation between mineralogy and reactivity was found showing that kaolinite rich clays (AB1080) are most reactive. However, also a smectite rich clay containing large amounts of calcium carbonate (Søvind Marl) showed good pozzolanic reactivity, while the actual origin of calcium carbonate seems to play an important role. The compressive strength of mortars with a replacement of up to 50 % OPC by the calcined marl or the calcined kaolinite rich clay was equal or higher than the reference mortar with 100 % OPC at 28 days. The early strength was sufficient for demoulding concrete in-field practice. For the main investigations the clays were burned in a pilot scale installation at IBU-tec in Germany at temperatures between 700-800 °C. To gain a better knowledge about the reactivity of calcined clays, e.g. due to structural modifications of the clay phases, phase transformations and microstructural changes, the raw and calcined state of the investigated clays was characterized with XRD, TG/DTG, BET, PSD, ICP-MS, SEM, FT-IR, 27Al-MAS-NMR and Mössbauer Spectroscopy. The heat treatment upon complete dehydroxylation of the clay minerals accompanied with oxidation of iron in the clay minerals (if present) induces significant structural distortions leading to metastable reactive phases. In the presence of large amounts of calcium carbonate a reactive vitreous phase forms in the calcined state. The early and long term hydration reactions occurring in calcined clay/lime and calcined clay/cement pastes were studied with XRD, TG/DTG, SEM and isothermal calorimetry. In reactive clays the pozzolanic reaction can start during the first 24 h. AFm phases especially hemi- and mono-carboaluminate hydrates are the favored hydration products formed, but also strätlingite was found in kaolinite rich clays. At long curing times or a higher hydration temperature hydrogarnet (katoite) was detected as well. Substitution of iron for aluminum in calcium aluminate hydrate phases and substitution of aluminum for silicon in C-S-H phases is very common. The two most reactive clays of this study (AB1080 & Søvind Marl) were burned additionally in a lab scale electrical furnace to test the influence of cooling rate, retention time and particle size distribution on the pozzolanic reactivity by means of calcium hydroxide consumption in calcined clay/lime pastes. Retention time and cooling rate do not influence the reactivity while the particle size affects the reactivity considerably. Moreover two full scale industrial trials were performed at existing and operating LECA-plants from Saint-Gobain Weber. The tests indicated that it is possible to produce calcined clay/marl of good homogeneous quality on an economically basis without changing the equipment. The calcination temperature of highest reactivity depends on the mineralogy of the raw material and the atmosphere in the kiln. With good knowledge about the raw material, the measurement of the LOI of the respective calcined clay can serve as a quality test to assess the pozzolanic reactivity of the final material.