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Autoclaved aerated concrete
Published in Ash Ahmed, John Sturges, Materials Science in Construction: An Introduction, 2014
Aircrete consists of 60–85 per cent air by volume, depending on density. The solid material part is a crystalline binder, which is called tobermorite. Besides the binding phase, tobermorite, grains of quartz and small quantities of other minerals also exist. The chemical composition of tobermorite comprises of silicium dioxide, calcium oxide and H2O (water). It is tobermorite that provides the relatively high compressive strength and stability of aircrete in spite of the high proportion of pores and lack of course aggregate in this material.
Potassium-bearing species in fertiliser obtained by hydrothermal modification of glauconitic siltstones with calcium hydroxide
Published in Mineral Processing and Extractive Metallurgy, 2022
Rodrigo A. O. Peixoto, Kátia D. Oliveira, Cícero N. Ávila-Neto
The hydrothermal chemical route in the presence of calcium carbonate or calcium hydroxide, the subject of this study, has been studied for decades and is still the subject of recent researches. The term hydrothermal refers to reactions conducted in pressure vessels (autoclaves) where aqueous solutions at high temperature and high vapour pressure crystallise into specific crystal structures. The hydrothermal methodology has been considered an approach with lower consumption of disposable mineral resources and energy, better use of K-bearing ores, with cleaner production (Hongwen et al. 2015). The paper published by Assarsson (1960) is probably one of the oldest reports on the solubilisation of muscovite and K-feldspar by Ca(OH)2 in hydrothermal environment. In that study, both minerals (muscovite and K-feldspar) could be solubilised by Ca(OH)2, K+ was leached in the form of KOH, and the nature of hydrothermal products were found to depend essentially on the Al/Si ratio of the parent mineral surface. In addition to easily identifiable insoluble crystalline products such as tobermorite, α-dicalcium silicate and hydrogrossular, the author reported the formation of materials with little or no crystallinity (amorphous). Although tobermorite is a calcium silicate hydrate mineral, there are reports of Al- and K-substituted tobermorites obtained after hydrothermal treatments of K-feldspar with Ca(OH)2 at temperatures higher than 463 K (Liu et al. 2015). This indicates that even after extraction in alkaline solution, the extracted K will not necessarily be fully available in a water-soluble form.
Technical and environmental assessment of hydrothermally synthesized foshagite and tobermorite-like crystals as fibrillar C-S-H seeds in cementitious materials
Published in Journal of Sustainable Cement-Based Materials, 2023
Somayeh Nassiri, Ananya Markandeya, Md Mostofa Haider, Antonio Valencia, Milena Rangelov, Hui Li, Aaron Halsted, David Bollinger, John McCloy
Minerals within the tobermorite supergroup include tobermorite 14 Å (also known as the mineral plombièrite) with a structure similar to tobermorite 11 Å (known as tobermorite if orthorhombic or clinotobermorite if monoclinic, C5S6H5 in cement chemist notation) just with a larger interlayer spacer due to more water molecules [39, 43,44]. Upon further dehydration (250–450 °C), the interlayer water is removed, producing tobermorite 9 Å (usually called riversideite). However, riversideite has rare or possibly no reported natural occurrence, thus making it a questionable mineral, despite its synthetic existence [39].
Effects of ultrasonically dispersed nano-slurries on solid waste-based autoclaved concrete (SWAC) and its leaching of heavy metals
Published in Journal of Sustainable Cement-Based Materials, 2022
As shown in Figure 10, heavy metals are mainly solidified in two forms in tobermorite: (1) Ca2+ in the Ca-O layer of tobermorite is replaced by heavy metals such as Pb and Cr [55]. (2) The heavy metals are adsorbed or complexed between the layers or on the surface of tobermorite [56]. The layered tobermorite is composed of the silicon-oxygen tetrahedron single chain and CaO7 polyhedron in the form of ‘dreierkette’ structure with repeating unit p = 3 [57]. The Ca existing on the crystal boundary and the silicon-oxygen tetrahedron are bonded together by ionic bonding, and it is difficult to be replaced by heavy metals. With the leaching of water molecules, nevertheless, the external heavy metals can enter the tobermorite crystal lattice and replace the free Ca2+ between the tobermorite layers [58], thus stabilizing/solidifying heavy metals. Pardal et al. [59] deduced that charge imbalance existed in the tobermorite basing on the increasing substitution of Al for Si in tobermorite, which has to be compensated through a coupled substitution of type: □+OH−→Ca2++O2- where □ represents an ‘empty’ position or a defect site in tobermorite structure [60]. The most common form of ‘□’ is Na+, in addition, other metal ions such as Cd2+, Pb2+, Co2+, Ni2+ and Cs+ can also act as charge compensator. This characteristic gives tobermorite surfaces superior adsorption performance [61]. Zhao et al. [62] have recently shown that hydrothermally synthetic tobermorite can exhibit adsorption capacities for Cr6+ ions as high as 30 mg/g and they indicated the optimal hydrothermal condition of 180 °C and 8 h and ideal pH of 4. Tsutsumi et al. [63] reported that the hydrothermally prepared tobermorite showed the highest removal amount of Cs+ and Sr2+ when a hydrothermal condition of 180 °C and 3 h was adopted, whereas the Cs+ and Sr2+ selectivity experiments showed the highest performance when a hydrothermal condition of 180 °C and 48 h was adopted. In addition, Coleman et al. [64] also reported that the hydrothermally synthetic tobermorite exhibited good surface immobilization and was effective in the removal of Cd2+, Pb2+, and Zn2+ from acidified aqueous wastes.