Kinetic Investigation of Tobermorite Synthesis for the Recovery of Carcinogenic Respirable Crystalline Silica (RCS)

Respirable crystalline silica (RCS), a hazardous byproduct of quartzite processing, poses severe occupational and environmental health risks. To address both waste valorization and health concerns, this study developed an end-of-waste strategy for converting quartz-rich quarry dust (QD) into substituted tobermorite under mild hydrothermal conditions. A systematic series of syntheses was carried out at 120, 130, and 140 °C under dynamic conditions using controlled mixtures composed of QD together with KRY·AS (a material derived from the thermal inertization of cement asbestos) or CaO as calcium sources and a small amount (2.5 wt %) of phillipsite-rich zeolitic tuff as a catalytic additive. Crystallization pathways and reaction kinetics were analyzed through X-ray diffraction, scanning electron microscopy, and thermogravimetric/thermodifferential methods. Results showed that quartz dissolution is the primary source of silica for tobermorite crystallization, which proceeds according to first-order kinetics. The apparent activation energies derived from Arrhenius plots were 101 ± 33 and 111 ± 34 kJ mol^-1 depending on the Ca source (KRY·AS or CaO, respectively). When using KRY·AS, the coexistence of katoite and amorphous calcium silicate hydrate phases indicated competing reaction pathways, while CaO favored more direct quartz-to-tobermorite conversion. Optimal tobermorite yields were achieved at 140 °C, reaching nearly 48 wt % in CaO-based mixtures. Lower temperatures led to slower growth and persistence of amorphous calcium silicate hydrate phases, whereas elevated temperatures favored rapid and more complete conversion with reduced secondary phase formation. Thermal analyses corroborated these findings, evidencing stable tobermorite and extensive carbonate formation. Scanning electron microscopy further confirmed the complete consumption of quartz, including respirable fractions, validating the process as compliant with end-of-waste criteria. Beyond silica detoxification, carbonate phases consistently formed, suggesting the potential dual benefit of hazardous waste valorization and incidental CO₂ sequestration. Taken together, these results highlight a novel, low-energy valorization route for RCS-containing waste, advancing circular economy goals while offering prospects for both functional material production and carbon capture applications.