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Phosphogypsum (PG) is a solid waste generated during the wet process of phosphoric acid, the utilization of which is hindered by impurities. Herein, natural gypsum and limestone were replaced by PG as the only source of CaO for the production of sulfoaluminate cement (SAC). The effects of fluoride impurities such as NaF, CaF 2 , and Na 2 SiF 6 on the mineral formation and hydration of SAC were investigated. Results show that with increasing fluoride content, the decomposition of CaSO 4 initially decreased and then increased. An increase in the NaF content facilitated mineral formation in the clinkers, whereas an increase of over 1.0 wt% in the contents of CaF 2 and Na 2 SiF 6 each resulted in a positive impact. The fluoride impurities helped convert o-C 4 A 3 S̅ to c-C 4 A 3 S̅ and α'-C 2 S to β-C 2 S as well as influenced the hydration behavior of SAC. They reduced the 1-day compressive strength of SAC while enhancing its 3- and 28- day strengths. Moreover, a quantitative analysis of the distribution of F in the clinkers revealed that F from NaF and Na 2 SiF 6 was uniformly distributed in all clinker minerals, whereas F from CaF 2 was more concentrated in C 4 A 3 S̅ and C 2 S. Na mainly existed as Na 2 SO 4 , which coexisted with CaSO 4. Furthermore, F was immobilized by Ca in the clinker minerals to form CaF 2. This study will promote the use of PG in SAC preparation. • Effect of NaF, CaF 2 , and Na 2 SiF 6 on preparation and hydration of SAC was studied. • Distribution characteristics of F in the clinkers was quantitatively revealed. • SAC was prepared using PG as the only CaO source. [ABSTRACT FROM AUTHOR]

Calcium aluminate clinkers, in which the mass ratio of Al2O3 to SiO2(A/S) is 5, were prepared at 1500℃ for 1h by proportioning pure reagents. The alumina leaching properties and crystal structure of 12CaO·7Al2O3 in the clinkers with different molar ratios of CaO unbonded with SiO2 to Al2O3 (C/A) were studied. The effect mechanism of C/A on the clinkers was also discussed by X-ray diffraction and X-ray fluorescence analysis. When C/A is 1.0 to 1.4, the alumina-leaching ratio improves because the amount of 12CaO·7Al2O3 increases with increasing C/A. When C/A is 1.4, the alumina-leaching ratio reaches the maximum value of 93.30%. In this case, 12CaO·7Al2O3 with defects forms, and the amount of substance of CaO is less than 11.3 mol. With the further increase of C/A, the alumina-leaching ratio decreases, which is because the preferred orientation of 12CaO·7Al2O3 changes from the (211) crystal plane into the (420) crystal plane.

The paper summarizes data on the Pleistocene combustion metamorphic complexes of the Kuznetsk Coal Basin. Paralava and clinker samples are dated by 40Ar/39Ar incremental heating. The 40Ar/39Ar ages of the combustion metamorphic rocks permit reconstructions of the succession of renewed activity of ancient faults in the Salair zone and age estimates for the evolution of the present-day drainage network. Cross sections of burned rocks from the western margin and center of the Kuznetsk Basin are compared. The geologic factors of coal ignition risks are analyzed. On the western margin of the Kuznetsk Basin, paleofires occurred in steeply dipping thick seams with predominant crushed vitrain–clarain coal, which has a high oxygen and methane adsorption capacity. Highly denuded high-temperature combustion metamorphic complexes are most often localized in the arches of slightly broken anticlines. Oxygen was supplied to the coals during the Late Cenozoic renewed fault activity and the subsequent erosion of the sediments. The natural fires in the area were a result of external rather than spontaneous ignition. The depths of the paleofires (up to 200 m) indicate that they occurred in a warm and dry climate. In the center of the Kuznetsk Basin, dispersed fire foci appeared in seams of self-igniting coals with the erosion propagation of the current drainage network. The combustion metamorphic complexes here are partly eroded and consist mostly of clinkers with a low degree of alteration. The 40Ar/39Ar ages and geological data indicate that the earliest large-scale combustion events on the western periphery of the basin occurred in the Eopleistocene (1.3–0.9 Ma). The oldest 40Ar/39Ar age of a coal fire episode (1.7 ± 0.3 Ma) might be the upper age boundary of the altitude differentiation of topography, which corresponds to the renewed activity of the Tyrgan and Afonino–Kiselevsk faults. The later synchronous combustion events on the western margin (0.2 ± 0.1 Ma) and in the center of the basin (0.13–0.02 Ma), most probably, occurred during the Kazantsevian interglacial, which gave rise to the present-day drainage network. [ABSTRACT FROM AUTHOR]

The bulk of the cement industry's environmental burden is from the calcareous source. Calcium is mostly available naturally as limestone (CaCO 3), where almost half of the mass is eventually released as CO 2 during clinker manufacture. Iron (Fe) is the fourth most common element in the Earth's crust surpassed only by oxygen, silicon, and aluminium; therefore, potential raw materials for alternative cements can contain significant amounts of iron. This review paper discusses in detail the most abundantly available Fe-rich natural resources and industrial by-products and residues, establishing symbiotic supply chains from various sectors. The discussion then focusses on the impact of high iron content in clinker and on ferrite (thermo)chemistry, as well as the importance of iron speciation on its involvement in the reactions as supplementary cementitious material or alkali-activated materials, and the technical quality that can be achieved from sustainable Fe-rich cements. [Display omitted] • Fe-rich raw materials are available globally at a level appropriate for use in cement • Portland or sulfoaluminate cement clinker rich in iron offer low embodied CO 2 • Fe-rich supplementary cementitious materials (SCM) have minor impact on reaction products • Fe-rich alkali-activated materials (AAM) show important reaction product changes • Reactivity as SCM or in AAM depends on phase composition and speciation [ABSTRACT FROM AUTHOR]

This study investigated the influence of different cooling regimes on the microstructure and consequent reactivity of belite-sulfoaluminate clinkers. The cement clinkers were synthesized by incorporating secondary raw materials, such as titanogypsum and bottom ash, to the natural raw materials. Clinker phases were determined by Rietveld quantitative phase analysis, while the distribution morphology and the incorporation of substitute ions in the phases were characterized by scanning electron microscopy using energy-dispersive X-ray spectroscopy (SEM/EDS). Clinker reactivity was studied using isothermal calorimetry and was additionally investigated through compressive strength, which was determined for the cement prepared from the synthesized clinkers. X-ray diffraction analysis showed that, as well as the three main phases (belite, calcium sulfoaluminate, and ferrite), the clinkers contained additional minor phases (mayenite, gehlenite, arkanite, periclase, and perovskite), the ratios of which varied according to the cooling regime utilized. Microscopic observations indicated that the cooling regime also influenced the crystal size and morphology of the main phases, which consequently affected clinker reactivity. Furthermore, a smaller amount of substitute elements was incorporated in the main phases when cooling was slowed. Results showed that, in comparison to clinkers cooled at slower rates, air quenched clinkers reacted faster and exhibited a higher compressive strength at 7 days. [ABSTRACT FROM AUTHOR]

The mineralogical and geochemical characterizations of the pyrometamorphic rocks caused by coal fires are discussed. The minerals in the combustion metamorphic rocks, as analyzed by X-ray diffraction (XRD), are anorthite, hematite, tridymite and cristobalite, quartz in clinkers; and tridymite, sekaniaite, sanidine, mullite, cristobalite, and quartz in paralavas. Tridymite and sekaniaite account for the largest mineral proportion in paralava. The major elements and rare earth elements (REEs) were determined by X-ray Fluorescence (XRF). Combustion metamorphic rocks are characterized by the major elements and REEs. Three methods to evaluate obtained normalized REE distribution patterns were used to evaluate characteristics of combustion metamorphic rocks. Chondrite-normalized distribution characteristics exhibit intense negative anomalies Eu and lack a Ce anomaly. North American Shale Composite (NASC) normalized patterns show a slight negative anomaly in δEu and lack anomalies in δCe for clinkers, but the paralavas show a positive δEu anomaly. Compared with Upper Continental Crust (UCC) normalized patterns, there is a slight difference (LREE-depleted, but HREE-enriched), and it is similar to NASC-normalized patterns. Primitive mantle-normalized trace elements show significant differences in combustion metamorphic rocks, which the clinkers show larger variations in Pb than paralavas, and paralavas have significant negative Dy anomalies. High content of Fe element may result in enrichment in Ni, Co, and Cu. • We summarized systematically the products in the whole process from ignition to extinguishing. • We examined the main minerals in the combustion metamorphic rocks. • The combustion metamorphic rocks characteristics of major elements and REE distribution are shown in this paper. [ABSTRACT FROM AUTHOR]