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 CaCO3 and adaption of the molar CaO/SiO2 (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 C2S 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-C2S. 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 C2S. 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-C2S to β-C2S phase transition on quenching strongly depend on the cooling rate. The onset temperature for β-C2S formation varies between 540 °C (slow quench) and 450 °C (fast quench). Thermal expansion coefficients of ternesite are calculated showing similarity with C2S. The incorporation of CaSO4 modules in the structure switches the direction of maximum compression.
The synthesis of low-temperature belite (C2S) clinker from wastes of autoclaved aerated concrete and limestone was studied in the presence of CaCl2 as a mineralizing agent. Synthetic chlorellestadite (SCE; Ca10(SiO4)3(SO4)3Cl2) 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 CO32− groups in ellestadites formed at 800 °C and 900 °C in air. Incorporation of CO3 results in a shorter unit cell parameters and smaller cell volume similar to CO3−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.
Phosphorus (P) is an essential macronutrient, playing a role in developmental and metabolic processes in plants. To understand the local and systemic responses of sorghum to inorganic phosphorus (Pi) starvation and the potential of straw and ash for reutilisation in agriculture, we compared two grain (Razinieh) and sweet (Della) sorghum varieties with respect to their morpho-physiological and molecular responses. We found that Pi starvation increased the elongation of primary roots, the formation of lateral roots, and the accumulation of anthocyanin. In Razinieh, lateral roots were promoted to a higher extent, correlated with a higher expression of SbPht1 phosphate transporters. Infrared spectra of straw from mature plants raised to maturity showed two prominent bands at 1371 and 2337 cm−1, which could be assigned to P-H(H2) stretching vibration in phosphine acid and phosphinothious acid, and their derivates, whose abundance correlated with phosphate uptake of the source plant and genotype (with a higher intensity in Razinieh). The ash generated from these straws stimulated the shoot elongation and root development of the rice seedlings, especially for the material derived from Razinieh raised under Pi starvation. In conclusion, sorghum growing on marginal lands has potential as a bio-economy alternative for mineral phosphorus recycling.
The suitability of CaCl2 as a mineralizing agent in the synthesis of a low-temperature C2S-cement clinker from wastes of autoclaved aerated concrete was investigated. As chlorellestadite is a potential host mineral for the immobilization of chlorine, the formation conditions for the highest joint content of chlorellestadite and C2S were studied in samples with different sulfate contents. Oven experiments were conducted at temperatures between 700 and 1200 °C. The samples were analyzed by X-ray diffraction in combination with chemical and thermal analysis and Raman spectroscopy. Calculation of the yield of C2S and ellestadite for all samples proves the optimum temperature range for the C2S-ellestadite clinker from 950 to 1000 °C. At lower temperatures, the formation of a carbonate-rich halogenide melt promotes the crystallization of a significant amount of spurrite at the expense of C2S. Ellestadite formation mainly depends on the sulfate content and to a lesser extent on the synthesis temperature. However, at higher temperatures, with ternesite another sulfate coexists in sulfate-rich samples at the expense of ellestadite. In addition, distinct evidence for non-stoichiometry and carbonate substitution in the structure of low-temperature ellestadite was found. Low sulfate content leads to the crystallization of Ca10[Si2O7]3Cl2 at higher temperatures. In all samples treated at temperatures above 1000 °C chlorine loss starts. Its extent decreases with increasing sulfate content.
The processing of belite cement clinker in a rotary kiln at about 1000 o C 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 CO 2 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 CO 2 emissions can be achieved through the electrical heating of the rotary kiln and the coupled sequestration of the released CO 2 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.
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