Preparation of High-ferrite Portland Cement Clinker Using High-concentration Solar Energy

Cement is a fundamental material for global infrastructure. Global cement production is projected to exceed 4.5 billion tons by 2030. The cement industry contributes about 7.5% of global anthropogenic carbon dioxide (CO₂) emissions. These emissions mainly come from fossil fuel combustion and limestone decomposition during the calcination process. Replacing fossil fuels with concentrated solar energy (CSE) for clinker calcination is a key pathway to achieve carbon neutrality in this sector. The CSE technology can concentrate solar radiation up to 7000 times and generate ultra-high temperatures of above 3000 K. These conditions can meet the thermal requirements for cement clinker production.

The spectral mismatch between cement materials and solar radiation as a critical bottleneck limits the efficiency of solar calcination.. The solar spectrum concentrates its energy in the visible (400–700 nm) and near-infrared (NIR, 700–2500 nm) bands. Most cement raw materials, such as calcium carbonate (CaCO₃), silicon dioxide (SiO₂), and aluminum oxide (Al₂O₃), are wide-bandgap materials. These materials absorb mainly in the ultraviolet region and are nearly transparent to visible and NIR light. This transparency leads to a low solar-to-thermal conversion efficiency. Iron oxide (Fe₂O₃) and iron-bearing ferrite minerals have narrower bandgaps of 2.0–2.2 eV. These minerals offer a stronger light absorption in the solar spectrum. This study was to investigate high-ferrite cement (HFC) clinkers to use the high absorption of the iron phase for better photothermal conversion. The calcination performance in electric furnace and solar furnace was compared and the first-principles calculations were performed. The goal was to reveal the photothermal coupling mechanism that could enable low-temperature and rapid clinker formation.

Three high-ferrite cement raw meals were prepared with analytical-grade reagents. These samples, named HFC-16, HFC-18, and HFC-20, were designed with different mass fractions of tetracalcium aluminoferrite (C₄AF, 16%, 18%, and 20%), respectively. All raw materials were ground and passed through a 200-mesh sieve. The sintering was carried out in a conventional electric elevator furnace and a high-flux solar simulator, respectively. The solar simulator used a xenon lamp array to mimic the solar spectrum (AM1.5). This simulator generated a peak heat flux of > 1800 kW/m² at the focal plane.

For the experiments in the electric furnace, the samples were heated to 800 ℃ and held for 30 min. The samples were then heated at 1000, 1200 ℃, or 1375 ℃ and held for 2 h, respectively. For the experiments in the solar furnace, the samples were rapidly heated to 800 ℃ and held for 1 min. They were then heated to the target temperature and held for 5 min. All the samples were rapidly cooled after firing. The phase composition of each sample was analyzed by X–ray diffraction (XRD). The Rietveld refinement was used for quantitative phase analysis. The optical properties were measured by ultraviolet-visible (UV–Vis) spectrophotometry in the range of 200–2500 nm. The total solar absorptance was calculated by the AM1.5 solar irradiance distribution. The electronic structures were simulated by a software named Vienna Ab initio Simulation Package (VASP). The simulations were based on the Density Functional Theory (DFT) with the GGA-PBE functional and a Hubbard U correction for Fe 3d orbitals.

The UV–Vis spectra of the raw materials show that wide-bandgap oxides (i.e., CaO, SiO₂, Al₂O₃) have a weak absorption in the solar spectrum region. Their absorptance values are all below 20%. Fe₂O₃ is a main absorber among all the raw materials. Its solar absorptance reaches 71.1%. As a result, increasing the Fe₂O₃ content in the raw meal improves the overall light absorption. The solar absorptance of the raw meal increases from 56.6% for HFC-16 to 60.5% for HFC-20. The light absorption of the clinker minerals depends on the calcination temperature. At 1000 ℃, the absorptance decreases slightly. This decrease is due to the decomposition of CaCO₃ into weakly absorbing CaO and the low crystallinity of intermediate phases. At 1200 ℃, the absorptance increases sharply. The iron phase mineral formed at 1200 ℃ shows a solar absorptance of 83.9%. This value is greater than that of the raw Fe₂O₃. For the clinker sample HFC-20, the total light absorptance is 75.6% at 1375 ℃.

The results by the First-principles calculations explain the mechanism behind this enhancement. In the high-temperature ferrite solid solution (C₄AF), Fe³⁺ ions occupy both tetrahedral and octahedral sites. This mixed coordination induces a crystal field splitting and creates diverse intermediate energy levels. The substitution of Al³⁺ for Fe³⁺ also introduces lattice defects. These defects create dense defect states within the bandgap. The density of states (DOS) analysis indicates that Fe 3d orbitals dominate the conduction band minimum. O 2p orbitals dominate the valence band maximum. The strong O 2p → Fe 3d transitions are responsible for the intense broadband absorption in the visible and NIR regions.

The comparison between the two heat sources shows a clear photothermal coupling effect in the solar furnace. The XRD patterns indicate that the solar-calcined samples processed at a thermocouple-measured temperature of only 800 ℃ already contain distinct silicate mineral phases (i.e., tricalcium silicate, C₃S, and dicalcium silicate, C₂S). In the conventional electric furnace, these phases only form at 1200 ℃. This result indicates that the iron phase absorbs a high-intensity photon energy locally and lowers the formation temperature of clinker minerals by approxiamtely 400 ℃. The electric furnace transfers heat slowly through conduction from the surface to the interior. The solar furnace delivers energy directly to the reactive iron-bearing sites. This direct energy delivery greatly accelerates the solid-state reaction kinetics.

At the final sintering temperature of 1375 ℃, the two methods both produce clinkers with the target mineral phases (i.e., C₃S, C₂S, tricalcium aluminate C₃A, and C₄AF). No free CaO appears in either case. This result confirms that the solar furnace can fully sinter cement clinker. The Rietveld refinement of the XRD patterns shows that the solar-calcined clinker exhibits broader diffraction peaks than the electric furnace clinker. The rapid heating rate and the unique photothermal environment in the solar furnace likely caused this broadening. Smaller crystallite sizes or higher lattice defect concentrations are the probable reasons. These findings indicate that solar calcination saves energy and changes the microstructural evolution of the cement minerals.

Based on the UV–Vis absorption testing and XRD analysis of raw materials, raw meals, and samples calcined at different temperatures, the overall light absorption intensity of the raw meal increased significantly as the Fe₂O₃ content in the raw meal increased. Furthermore, the light absorption intensity of the iron phase in the clinker minerals increased with increasing the calcination temperature. At 1200 ℃, its light absorption rate reached 83%. The light absorption intensity of the clinker also increased with the calcination temperature, reaching 75.6% at 1375 ℃. More importantly, the iron phase could trigger a photothermal coupling effect when calcined in a simulated high-concentration solar furnace, significantly lowering the formation temperature of clinker minerals. The results by the XRD Rietveld refinement and hydration calorimetry indicated that, compared with the electric furnace clinker, the simulated concentrated solar calcined clinker (SF-HFC) exhibited diffraction peak broadening characteristics. This could demonstrate that the photothermal effect of the iron phase significantly reduced the calcination temperature of cement clinker and lowered the production energy consumption, providing a material design strategy for high-efficiency and low-carbon cement manufacturing.