Thermodynamic Evaluation of Pozzolanic Reactions between Activated Pozzolan Mix of Clay Waste/Fly Ash and Calcium Hydroxide

Abstract

The mixture of activated paper sludge and fly ash can successfully be used as pozzolans for the manufacture of a more ecological portland cement. The saturation indexes and the mineral stability fields of a pozzolanic reaction are studied at 1, 3, 7, 28, 90, and 360 days into the reaction. The system is formed of a pozzolanic mix (activated clay waste and fly ash) and a saturated solution of Ca(OH)2 at 40°C. The behavior of the reactions is predicted in this study by means of a thermodynamic model running on a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. The concentration of soluble species in the aqueous solution is determined by inductively coupled plasma mass spectrometry. The evolution of the hydrated phases formed in this study and the mineral stability fields are evaluated with the geochemical programs PHREEQC and SUPCRT92. The hydrated phases produced during the pozzolanic reactions were C─ S─ H gels, C4AH13 (calcium aluminate hydrate), C4AcH12 (calcium monosulfoaluminate), and layered double hydroxide (LDH)–type structures or hydrotalcite are anionic clays. The saturation index values indicate that the LDH-type structures (phyllosilicate/carbonate) are the most thermodynamically stable phase in the activated clay/fly ash–Ca(OH)2 system after 7 days of reaction. Mineral stability fields situate all the samples among the LDH-type structures (phyllosilicate/carbonate).