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

Our plain-English summary of Kinetic Investigation of Tobermorite Synthesis for the Recovery of Carcinogenic Respirable Crystalline Silica (RCS), by D. Malferrari, G. Galamini, M. Bernini, et al (ACS Omega, 2025).

The study

• Authors: D. Malferrari, G. Galamini, M. Bernini, R. Fantini, G. Malvolti, A.F. Gualtieri
• Journal: ACS Omega
• Published: 2025;10(43):51284-51296
• Read it: PubMed · Publisher · Free full text

Abstract

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.

© 2025 The Authors. Published by American Chemical Society.

In plain English

This was a laboratory experiment on materials. It did not study people, asbestos fibers or health effects. The authors tested whether quartz-rich dust from quartzite quarry processing in northwest Italy, which contains respirable crystalline silica, could be turned into a mineral called tobermorite. They mixed the dust with a calcium source and 2.5 wt % zeolitic tuff. The calcium source was either KRY·AS (a material made by heat-treating cement asbestos) or pure calcium oxide. The mixtures were heated in sodium hydroxide solution at 120, 130 and 140 °C for 4 to 144 hours. Tobermorite formation followed first-order kinetics. Tobermorite content reached 47.8 wt % in the calcium oxide mixture (140 °C, 144 hours) and peaked at 26.1 wt % in the KRY·AS mixture (140 °C, 48 hours). Electron microscopy of these two samples showed no quartz. The authors noted that X-ray diffraction cannot reliably detect quartz below about 0.5 wt %. The authors concluded that the respirable silica was transformed into safe phases and that the process met End-of-Waste criteria.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory hydrothermal synthesis experiment with kinetic analysis (no human or animal subjects)
• Who was studied: Materials, not people: quartz-rich quarry dust (QD) from quarry processing sites in northwest Italy, mixed with KRY·AS or calcium oxide and a phillipsite-rich zeolitized tuff from a quarry near Grosseto, central Italy
• Exposure: No health exposure was studied. The materials tested were quartz (respirable crystalline silica) in quarry dust and KRY·AS, a calcium source made by thermal inertization of cement asbestos. Mixture M1 was 75 wt % KRY·AS, 22.5 wt % QD and 2.5 wt % ZT. Mixture M2 was 42.0 wt % CaO, 55.5 wt % QD and 2.5 wt % ZT. Mixtures were heated at 120 to 140 °C in 1 M NaOH.
• Main results:
  • The quarry dust had a high quartz content and a median particle size, Dv(50), of 10.6 μm.
  • M1 (KRY·AS): at 120 °C, tobermorite rose from 7.2 wt % at 12 h to 21.9 wt % at 144 h.
  • M1 (KRY·AS): at 140 °C, tobermorite peaked at 26.1 wt % at 48 h and then decreased slightly.
  • M2 (CaO): at 140 °C, tobermorite rose from 27.8 wt % at 12 h to 47.8 wt % at 144 h.
  • Apparent activation energies were 101 kJ/mol (error 33) for M1 and 111 kJ/mol (error 34) for M2.
  • In both mixtures, at every temperature, the silicon for tobermorite came from quartz.
  • Quartz was completely consumed at 130 and 140 °C and persisted longer at 120 °C.
  • Tests run for 144 h at 110 °C produced no tobermorite and left considerable residual quartz.
  • Electron microscopy of the highest-yield samples (M1-48H-140 and M2-144H-140) indicated absence of quartz.
  • Katoite formed with M1 but was not observed with M2.
  • Calcium carbonates formed. In sample M1-48H-140 thermal analysis gave 29.2 wt % calcium carbonates.
• The authors' conclusion: The authors concluded that the combined X-ray diffraction and electron microscopy data showed the respirable crystalline silica in the quarry dust was transformed into safe phases. They stated that this demonstrated the process fulfils the criteria for an End-of-Waste designation.
• Limitations the authors state:
  • X-ray diffraction may not distinguish small amounts of quartz (usually below about 0.5 wt %) from background.
  • The microscopy maps could only indicate that quartz was absent or negligible.
  • The authors could not rule out a second reaction pathway contributing to tobermorite formation.

Topics

Medical terms in this study, explained in our Lexicon: Carcinogenic, Crystalline, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Silicate, Respirable Fraction.

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