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The correct assessment of industrial by-products and waste materials requires an in-depth knowledge of their characteristics. In thermally activated products, the calcining conditions have an important influence on the properties and later behaviour of cementitious matrices. The present study was carried out to determine the effect of calcining conditions (2-5 h at 700-8008C) on the properties of treated paper sludge. Various chemical, physical, morphological, mineralogical and pozzolanic characteristics were evaluated in order to provide a complete understanding of the properties of the calcined sludge and the possibility of using them in the manufacture of blended cements. The results showed that the thermal conditions have a major influence on the properties of the calcined sludge, primarily the mineralogical and morphological properties, as well as the pozzolanic behaviour.

This paper describes research on clay wastes (CWs) produced in the paper manufacture process. Once activated under controlled thermal conditions, CW is transformed into calcined clay products providing added value as supplementary cementing materials. The obtention of a pozzolanic material (metakaolin (MK)) from valorized CW constitutes an alternative source of pozzolans for the elaboration of blended Portland cements, as well as a priority research line from the environmental point of view. This research work presents the properties of calcined CW (chemical, mineralogical, and pozzolanic) and its influence on Portland cements containing 10% calcined clay product. The results obtained with different characterization techniques (XRF, DTA, XRD, SEM‐EDX) showed that the thermally activated CW exhibits acceptable properties to be used as supplementary cementing materials in the manufacture of commercial Portland cements. The derived MK can react with calcium hydroxide, from cement hydration, producing hydrated phases with hydraulic properties (calcium silicate hydrate gels, tobermorite, C4AH13, zeolite). These novel blended cements comply with the chemical, physical, and mechanical specifications established in the existing standards.