Microstructural characteristics of lime-pozzolan pastes made from kaolin production wastes: Fid-Bau Portal

Microstructural characteristics of lime-pozzolan pastes made from kaolin production wastes

de Azeredo, Aline F. Nóbrega

Lime-pozzolan mortars are often used in restoration of historical buildings. In this paper pastes prepared with hydrated lime and metakaolin from kaolin production wastes were examined in order to verify the performance of this waste as a pozzolanic material in mixes containing hydrated lime. Some specimens were cured in moist air and some in dry air. The compositions and microstructures at curing times up to 180 d were assessed using thermal gravimetric analysis, X-ray diffraction and scanning electron microscopy. The major phases found in the lime-metakaolin pastes were strätlingite (C2ASH8), monocarboaluminate (C4A $$\overline{\text{C}}$$ C ¯ H11), portlandite (CH) and calcite (C $$\overline{\text{C}}$$ C ¯ ). The phases formed in these pastes were affected by curing conditions. In the moist cured samples the main phase was strätlingite and in the dry cured samples the main phase was monocarboaluminate. The portlandite was consumed by 28 d in moist curing and by 180 d in dry curing. SEM images showed that the morphology of lime-metakaolin pastes was influenced by curing condition and metakaolin content. In the moistured samples the morphologies of the main phases appeared very well formed with sharp edges, whereas in the dry cured samples the morphologies did not appear very well formed. This work shows that pastes using metakaolin made from kaolin production wastes had similar microstructural characteristics as have been reported for pastes made using commercial metakaolin. So this metakaolin is a suitable and environmentally friendly pozzolanic material for use in lime mortars.