Calorimetric evolution of the early pozzolanic reaction of natural zeolites |
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Authors: | R Snellings G Mertens J Elsen |
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Institution: | (1) Department of Earth and Environmental Sciences, KULeuven, Celestijnenlaan 200E, Box 2410, 3001 Heverlee, Belgium |
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Abstract: | The pozzolanic reaction between natural zeolite tuffs, portlandite and water was investigated over the course of the early
reaction period up to 3 days. Isothermal conduction calorimetry experiments supplemented by TG/DTG and XRD analyses assisted
in the elucidation of the sequence of reaction processes taking place. The calorimetry experiments clearly showed the dependence
of the pozzolanic reaction rate and associated heat release on the fineness of the zeolite tuff. Higher external surface areas
of pozzolans yield higher total heat releases. Also the exchangeable cation content of the zeolites influences the reaction
rate. Release of exchangeable alkalis into solution promotes the pozzolanic reaction by raising the pH and zeolite solubility.
The appearance of an exotherm after approximately 3 h of reaction is more conspicuous when alkali-rich zeolites are reacted.
This exotherm is conceived to be related to a transformation or rupture of initially formed reaction products covering the
zeolite grains. The formation of substantial amounts of ‘stable’ calcium silicate hydrate (C–S–H) and calcium aluminate hydrate
(C–A–H) reaction took place after an induction period of more than 6 h. The openness of the zeolite framework affects the
proneness of the zeolite to dissolution and thus its reactivity. Open framework zeolites such as chabazite were observed to
react much more rapidly than closed framework zeolites such as analcime. |
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