In situ x-ray diffraction at high temperatures: Formation of Ca2_{2}SiO4_{4} and ternesite in recycled autoclaved aerated concrete

This study provides an insight into possible recycling processes for autoclaved aerated concrete (AAC) at low temperatures (<1000 °C). Belite binders were synthesized from wastes of AAC by the addition of CaCO$_{3}$ and adaption of the molar CaO/SiO$_{2}$ (C/S ratio) in the range of 2 to 2.5. An in situ XRD study performed during heating up to 1000 °C and subsequent quenching to room temperature evidenced the formation of ternesite besides C$_{2}$S in sulfate-containing systems. Several factors influencing the reaction kinetics and the evolution of the phase composition were investigated thoroughly. Increased sulfate content and dwelling time during heating increase the ternesite content and promote the formation of highly crystalline α’H-C$_{2}$S. The C/S-ratio of the starting materials has to be adjusted to the sulfate content in order to prevent the formation of ternesite at the expense of C$_{2}$S. Ternesite remains stable during quenching to room temperature or even increases in amount, except in cases of very low cooling rates or high residual quartz contents (C/S-ratio ≤ 2). Temperature and range of α’H-C$_{2}$S to β-C$_{2}$S phase transition on quenching strongly depend on the cooling rate. The onset temperature for β-C$_{2}$S formation varies between 540 °C (slow quench) and 450 °C (fast quench). Thermal expansion coefficients of ternesite are calculated showing similarity with C$_{2}$S. The incorporation of CaSO$_{4}$ modules in the structure switches the direction of maximum compression

Zero emission circular concrete

The project Zero Emission Circular Concrete develops a CO2-neutral, high-quality, and resource-efficient concrete cycle starting with end-of-life concrete. A cement clinker with a reduced CO2 footprint is processed from waste concrete fines at a strongly reduced temperature of approximately 1000°C. The main clinker mineral is belite, Ca2SiO4. The residual CO2 is released in concentrated form and used for the technical carbonation of either waste concrete fines as supplementary cementitious material or of coarse crushed waste concrete. The coarse fraction is treated in a new process based on a pressurized autoclave, where hardening by carbonation improves the properties of the recycled aggregate. Both carbonation options are investigated on a laboratory scale. Recycling cement is developed from belite cement clinker, Portland cement clinker, and other substitutes in a joint project with the industry. A 30% substitution rate of Portland cement clinker compared to European cement standards is targeted. Subsequently, formulations for recycling concrete will be developed from recycled cement and recycled aggregate. The processing of concrete products and precast concrete elements will be tested in plant trials. A pilot plant for belite cement clinker is currently under construction to bring its technology readiness level to four

Toward the Synthesis of New Zeolite Structures in the Presence of Cesium: Zeolite MMU-1

Nanosized small-pore zeolites RHO and MMU-1 (a new Cs-Na modification of an EDI-type zeolite structure) were prepared in the presence of cesium hydroxide by changing the molar composition of the precursor zeolite solution. XRD, Raman, and FTIR results indicated that Cs had a structure-directing effect and controlled the formation of the respective zeolite phases. The presence of water played a crucial role, and when the amount of water was under a certain level, the zeolite cancrinite was formed. Solvent-free syntheses were attempted, and the results confirmed the conclusions drawn based on the water-mediated syntheses, namely that both Cs and water determine the nature of the product formed. The structure of the new zeolite MMU-1 was resolved by Rietveld refinement. MMU-1 was found to exhibit tetragonal symmetry, space group P42$_{1}$c (No. 114), and its framework is composed of four-membered and eight-membered TO$_{4}$-rings. The results reported suggest that there is the potential to synthesize other novel small-pore zeolite structures in the presence of cesium hydroxide by modifications of the chemical composition of the zeolite precursor solution

Thermal stability of C-S-H phases and applicability of Richardson and Groves’ and Richardson C-(A)-S-H(I) models to synthetic C-S-H

Synthetic C-S-H samples prepared with bulk C/S ratios from 0.75 to 1.5 were analyzed by coupled TG/DSC/FTIR and in-situ XRD while heating, in order to correlate observed weight loss curves with the kinetics of evolved gases, and to investigate the transformations C-S-H→β-wollastonite→α-wollastonite. The temperature of the transformation to β-wollastonite increased with increasing C/S. The temperature for the transformation from β- to α-wollastonite meanwhile decreased with increasing C/S; indicating that excess CaO stabilized the α-polymorph. The transformation C-S-H→β-wollastonite was accompanied by the formation of α`LC2S for C/S > 1. In the case of C-S-H with C/S = 1.5, both β-C2S and rankinite were formed and then decomposed before the transformation to β-wollastonite and α`LC2S. C-S-H with low C/S was found to be more stable upon heating. The chemical structural models of Richardson and Groves’ and Richardson C-A-S-H(I) were used to obtain the structural-chemical formulae

Belite cement clinker from autoclaved aerated concrete waste fines with high sulfate content

The processing of belite cement clinker in a rotary kiln at about 1000°C in a CO2 atmosphere is a new recycling option for Autoclaved Aerated Concrete (AAC) waste that otherwise must be landfilled. Waste fine fractions from a sorting facility enriched in sulfate due to intermixing with waste plaster have been processed. During clinkering the cement clinker phase belite (Ca2SiO4) besides technical ellestadite, (Ca10(SiO4)3(SO4)3Cl2), or ternesite, (Ca5(SiO4)2SO4), are formed, depending on the addition of flux minerals. However, not all phases of the novel clinker react hydraulically. Whereas ternesite reacts with water, ellestadite forms complex solid solution series (Ca/Pb, SO4/PO4), which may be used as an insoluble reservoir mineral for undesirable constituents, such as phosphates and chlorides. The produced clinker has been successfully used to partially substitute OPC in AAC production in technical trials. Waste quantities and landfill costs are minimized, while at the same time, CO2 emissions and the primary resource consumption of AAC production are reduced. Joint work with industrial companies is underway to increase technology readiness. Particularly large reduction effects on CO2 emissions can be achieved through electrical heating of the rotary kiln

Belite cement clinker from autoclaved aerated concrete waste – A contribution towards CO₂-reduced circular building materials

The processing of belite cement clinker in a rotary kiln at about 1000oC is a new recycling option for autoclaved aerated concrete (AAC) waste that otherwise must be landfilled. The clinker produced can partially substitute ordinary portland cement (OPC) in AAC production. Waste quantities and landfill costs are minimized, while at the same time CO2 emissions and the primary resource consumption of AAC production are reduced. The technology is currently under development. New analytical possibilities and modeling have made it possible to optimize the process conditions to such an extent that the use of belite cement clinker in aerated concrete production has already been technically tested. Particularly large effects on CO2 emissions can be achieved through the electrical heating of the rotary kiln and the coupled sequestration of the released CO2 in other secondary products such as recycled aggregate for concrete production from waste concrete. Comparable concepts for the AAC cycle are currently being worked on together with the industry partner Xella. Although decentralized plant concepts would be useful in order to minimize transportation, small plants are currently not economical according to initial estimates. In the long term, emission-free product cycles are aimed at

Catalytic Tar Conversion in Two Different Hot Syngas Cleaning Systems

Tar in the product gas of biomass gasifiers reduces the efficiency of gasification processes and causes fouling of system components and pipework. Therefore, an efficient tar conversion in the product gas is a key step of effective and reliable syngas production. One of the most promising approaches is the catalytic decomposition of the tar species combined with hot syngas cleaning. The catalyst must be able to convert tar components in the synthesis gas at temperatures of around 700 C downstream of the gasifier without preheating. A Ni-based doped catalyst with high activity in tar conversion was developed and characterized in detail. An appropriate composition of transition metals was applied to minimize catalyst coking. Precious metals (Pt, Pd, Rh, or a combination of two of them) were added to the catalyst in small quantities. Depending on the hot gas cleaning system used, both transition metals and precious metals were co-impregnated on pellets or on a ceramic filter material. In the case of a pelletized-type catalyst, the hot gas cleaning system revealed a conversion above 80% for 70 and 110 h. The catalyst composed of Ni, Fe, and Cr oxides, promoted with Pt and impregnated on a ceramic fiber filter composed of Al2O3(44%)/SiO2(56%), was the most active catalyst for a compact cleaning system. This catalyst was catalytically active with a naphthalene conversion of around 93% over 95 h without catalyst deactivation

Total oxidation of carbon monoxide, VOC and reduction of NO₂ with catalytic ceramic filter media

This paper deals with the effect of catalyst loading on the activity and selectivity of CuOx MnOy catalysts on alumina-silicate supports (fiber material-Al2O3(44)/SiO2(56)). A special focus lies on the oxidation of CO, on mixtures of VOC from 1-butene, isobutane, n-butane, propane, ethene, and ethane, as well as on CO oxidation in the presence of NO2. The catalysts are prepared through wet impregnation of the filter section with an aqueous solution of copper and manganese nitrate. The rate of CO oxidation for small carbon monoxide concentrations of up to 1 vol.% is independent of catalyst loading in the filter material. In contrast, at a carbon monoxide concentration of around 3 vol.%, it is found that the rate of CO oxidation increased rapidly with increasing catalyst loading of the filter material. The highest catalytic activity of over 93% CO elimination is achieved at 290 ◦C for 1 vol.% CO and smaller catalyst loading and for 3 vol.% CO with higher catalyst loading. In long-term stability tests, complete CO conversion is measured without deactivating the catalyst at 390 ◦C for at least 100 h. The highest catalytic activity for VOC elimination of 90% is achieved in the temperature range of 350–420 ◦C. During the CO–NO2 reaction with and without O2, a constant decrease in the CO oxidation rate is observed, while the NO2 reduction rate remained constant at a temperature below 300 ◦C

Chlorellestadite (Synth): Formation, Structure, and Carbonate Substitution during Synthesis of Belite Clinker from Wastes in the Presence of CaCl2_2 and CO2_2

The synthesis of low-temperature belite (C$_2$S) clinker from wastes of autoclaved aerated concrete and limestone was studied in the presence of CaCl$_2$ as a mineralizing agent. Synthetic chlorellestadite (SCE; Ca$_{10}$(SiO$_4$)$_3$(SO$_4$)$_3$Cl$_2$) forms in experiments at temperatures between 700 and 1200 °C. Samples were investigated by X-ray diffraction and Raman spectroscopy. In general, the amount of SCE depends mainly on the sulfate content and to a lesser extent on the synthesis temperature. At lower temperatures of formation, a non-stoichiometric SCE seems to crystallize in a monoclinic symmetry similar to hydroxylellestadite. Rietveld refinements revealed the presence of chlorine and calcium vacancies. Raman spectroscopy proved the partial substitution of sulfate by CO$_3$$^{2−}$ groups in ellestadites formed at 800 °C and 900 °C in air. Incorporation of CO$_3$ results in a shorter unit cell parameters and smaller cell volume similar to CO$_{3−}$apatite. At low temperatures, SCE coexists with spurrite intermixed on a very fine nm scale. At temperatures above 900 °C in air, ellestadite is carbonate-free and above 1000 °C chlorine loss starts in all samples