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Use of recycled paper sludge to produce a binder for concrete

The goal of the project was to develop an innovative re-use of paper sludges produced either by de-inking plants or wastewater paper mills. The option considered in the present research was to develop a controlled calcination process using the organic content of the sludge as fuel and allowing the re-use of the resulting ash as a pozzolanic material usable in cement and concrete. The transformation of paper sludge into a high-value added material represents a good development both for the paper industry, which is looking for creative uses for sludge, and the building industry which needs mineral additives to improve the durability of concrete.

In the European Community, about 5.8 million tonnes of sludge are produced every year. This amount represents an important source of secondary raw materials for the production of pozzolans. Processing a waste, which has a negative value (that of landfilling), should be an opportunity to introduce a cheaper pozzolan into the market and increase the use of such product.

Laboratory, pilot and industrial tests of calcination were carried out in the temperature range 600-800 C. The best product was obtained using the multi-hearth furnace heated at 630 C. The resulting ash was a mixture of metakaolin and calcite, able to quickly consume the calcium hydroxide produced by cement hydration. The main parameters influencing the pozzolanic activity of calcined sludge are: the kaolinite content of the inorganic fraction, the quantity of decarbonated calcite and the amount of particles lower then 10 µm. Superficial defects occur during the calcination and enhance the reactivity of the product at an early age. Dioxins were detected in the emitted gases and have to be treated in an industrial plant.

The investment cost, however, is very high due to the need of emissions treatment and it should be interesting to investigate more deeply the ashes produced by the incineration of paper sludges in paper mills, to verify if some of them fulfill the requirements of a good pozzolan and if the calcination process could be modified to get suitable products.

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