Abstract
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.
Highlights
Chemical recycling of cementitious wastes, i.e., their use as secondary raw materials for the production of C2 S-based (C2 S = Ca2 SiO4 according to the Cement nomenclature, i.e., C = CaO, S = SiO2, S = SO3, H = H2 O) binders, is a promising approach to reduce the environmental impact of the cement industry in several aspects
Comparison of the quantitative results from Raman and X-ray diffraction studies with different heating rates and different C/S-ratios proves that a maximum amount of ternesite forms in samples with high C/S-ratio or after very long dwell times
The experiments up to the maximum temperature of 1000 ◦ C evidenced the formation of ternesite besides C2 S in sulfate-containing systems
Summary
Chemical recycling of cementitious wastes, i.e., their use as secondary raw materials for the production of C2 S-based (C2 S = Ca2 SiO4 according to the Cement nomenclature, i.e., C = CaO, S = SiO2 , S = SO3 , H = H2 O) binders, is a promising approach to reduce the environmental impact of the cement industry in several aspects. The CO2 emissions from the cement industry amount to 8% of the global CO2 emissions [2]. These emissions include the “process emissions”, due to the calcination of CaCO3 , and the “energy emissions” from the firing process at 1450 ◦ C. Even though the wastes from mineralogical building materials have a high recycling rate, the recycling possibilities are limited essentially to the production of aggregates that are only suitable for addition, e.g., in asphalt production
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