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.