Multistep crystallization pathways of a new carbonation-hardened Ternesite binder

Abstract

Developing low-carbon clinkers and binding materials is a promising way to achieve net zero in cement industry. Ternesite is a low-carbon clinker mineral in terms of reduced CaO content and sintering temperature. More importantly, its hydration inertness can be compensated by carbonation curing to develop high strength rapidly. This investigation hence aims at disclosing the mechanisms and kinetics associated with carbonation hardening of ternesite. The X-ray diffraction, thermogravimetry, Raman spectroscopy and ²⁹Si nuclear magnetic resonance were used to determine the phase assemblage evolution of carbonated ternesite at different periods, while back-scattered electron microscopy and scanning electron microscopy with energy dispersive spectroscopy were employed to illustrate the microstructure characteristics. The calcium carbonates formed in carbonated ternesite were calcite, aragonite and more interestingly amorphous phase and monohydrocalcite, with the crystallization degree increased with carbonation time. Si transferred into highly polymerized Q³ and Q⁴ units, with content increased with time. This unique carbonation behavior of ternesite should be governed by the competition on “capturing” Ca between C, Si and S. The Ca²⁺ is partially combined with SO₄²⁻ to form gypsum, which leads to decreased Ca/C ratio of calcium carbonate and thus weak crystallization, also lower Ca/Si ratio of silica gel hence higher polymerization degree.