Hydrothermal reaction between C6S6H and kaolinite for the formation of tobermorite at 180°C

11 Å tobermorites were made from CSH (Ca/Si = 1.14) and kaolinite with Ca/(Si+Al) = 0.8 and Al/(Si+Al)= 0.15 at 180°C. The CSH was prepared from colloidal silica and lime at 130°C and 180°C for 2 h. Reaction gives in succession CSH, poorly crystalline Al-substituted tobermorite, and highly crystalline Al-substituted tobermorite. The addition of kaolinite markedly accelerates the formation of tobermorite within 4 h, more effectively with CSH prepared at 130°C than with that prepared at 180°C. X-ray fluorescence diffractometry shows that the Al coordination number is a mixture of 4 and 6 in the initial products and shifts to 4 with an increase in processing time. This agrees with the results for the degree of reaction of the kaolinite.     

Anomalous tobermorite in autoclaved aerated concrete

Three samples of commercial products were examined by X-ray diffraction, electron diffraction, chemical analysis and d.t.a. - t.g.a. The cementing materials were highly crystalline Al-substituted 11 Å tobermorite with Ca/(Al+Si) = 0.81 and Al/(Al+Si) = 0.10 for specimen A, 0.85 and 0.10 for specimen B, and 0.84 and 0.07 for specimen C. All of these tobermorites showed anomalous thermal behaviour, i.e., the basal spacing did not shrink after heating at 300°C. The content of alkalis in all the tobermorites was insufficient to balance replacement of Si by Al. All the crystals gave SED patterns characteristic of crystals with (001) cleavage, similar to those given by normal specimens.     

Mechanical Property Evolution during Autoclaving Process of Aerated Concrete Using Slag: I, Tobermorite Formation and Reaction Behavior of Slag

The chemical reactions in the formation of autoclaved aerated concrete (AAC) block were investigated. The samples were prepared using blast furnace slag at 180°C under saturated steam pressures for various times from 1 to 128 h. Autoclaving for 1 h yielded 1.1-nm tobermorite with higher Al substitution and lower Ca/(Al + Si) ratio than that made without slag, due to the high solubility of the slag during the initial stage of reaction. This suggests that the utilization of slag has an advantage of reducing processing time. On further reaction, the crystallinity of the tobermorite increases and its Ca/(Al + Si) ratio decreases, becoming constant after 64 h. After a long period of autoclaving. thick reaction rims are formed around unreacted slag particles, interrupting the diffusion of Ca from slag to matrix.     

Hydrothermal Solidification of Kaolinite–Quartz–Lime Mixtures

The strength development of hydrothermally solidified kaolinite–quartz–lime systems with kaolinite as the aluminum source was studied. The starting materials were mixed so that the Ca/(Si + Al) atomic ratio was in the range 0.23 to 0.25, and the Al/(Si + Al) ratio was between 0 to 0.50. Specimens were formed by uniaxial pressing and hydrothermal treatment under saturated steam pressure at 200°C for 2 to 20 h. For quartz-rich systems with Al/(Si + Al) = 0 and 0.05, strength development by the formation of calcium silicate hydrates, such as C–S–H and tobermorite (Ca₅(Si₆O₁₈H₂)·(4H₂O), was observed. On the other hand, in the case of kaolinite-rich systems with Al/(Si + Al) = 0.24 to 0.50, strength development by the formation of hydrogarnet (Ca₃Al₂(SiO₄)(OH)₈) was recognized, resulting in flexural strengths between 15 to 20 MPa. It is proposed that strength development is related to the formation of mesopores (∼0.04 μm) that accompanied formation of the hydrogarnet.     

Kinetics of the CaO  quartz  H2O reaction at 120° to 180°C in suspensions

Kinetics of hydrothermal reactions have been studied for mixtures of CaO and quartz (<10 μm 10–20 μm) with Ca/Si = 0.8 and 1.0 in stirred suspensions at 120 – 180°C. Reaction proceeds through the sequence: Ca(OH)₂ + SiO₂ → Ca-rich C-S-H + SiO₂ (at 120°C) → poorly crystalline tobermorite (at 140°C)→ highly crystalline tobermorite (at 180°C) → xonotlite at 180°C and Ca/Si = 1.0 and 180°C and Ca/Si = 0.8 if 10–20 μm quartz is used. Reaction is controlled by dissolution of the quartz. For both Ca/Si ratios the radius of the 10–20 μm quartz decreases at a constant rate, viz 0.85 μm/h at 180°C, 0.13 μm/h at 140°C, 0.04 μm/h at 120°C.     

Phase Evolution during Autoclaving Process of Aerated Concrete

The reactions were investigated that occur when lime, cement, and quartz sand are mixed together and molded, then treated at 180°C under saturated steam pressures to produce autoclaved aerated concrete. The hydrothermal treatment of mixtures gives Ca-rich C-S-H with varying Ca/Si ratios as an initial product, which reacts further with silica dissolved from quartz to form 1.1-nm tobermorite with increase of curing time. During autoclaving, the composition of C-S-H and tobermorite as a binder continues to change until after 8 h, when the Ca/(Al + Si) ratio becomes constant at 0.8. As the reaction proceeds, the number of micropores increases, and the strength also increases due to the binder effect of the tobermorite. However, the total pore volume does not change, remaining constant values.     

Structural Degradation of Tobermorite during Vibratory Milling

Mechanochemistry of hydrothermally prepared Al-free and Al-substituted 1.1 nm tobermorite was studied using ²⁹Si NMR, XRD, and TGA-DTA. Tobermorite has a double-chain structure and Al/Si substitution occurs preferentially at bridging tetrahedra. By using the vibration mill, both tobermorites were observed to decompose as a function of milling time to form amorphous C-S-H-like phases. The decomposition rate was higher for the Al-substituted tobermorite. The decomposition process occurs mainly at the bridge portion of the silicate double chain. In the Al-substituted tobermorite, the breakage seems to occur preferentially at the points where Al has replaced Si. On heating the amorphous C-S-H-like phase, wollastonite is formed. However, no decrease was observed at the wollastonite formation temperature.     

Synthesis of normal and anomalous tobermorites

Tobermorites were made from several starting materials at 105 – 180°C and Ca/(Si + A?) 0.8 – 1.0. Reaction gives in succession C-S-H, normal, mixed and anomalous tobermorites, and finally xonotlite. High C/S ratio (1.0), short time, low temperature, stirring, presence of A?, and if quartz is used, small particle size, all tend to stop it at normal tobermorite. In some cases, this effect is due to promotion of crystal growth of normal tobermorite. Low C/S ratio (0.8), long time, high temperature, no stirring, presence of A? plus alkali, and, if quartz is used, large particle size, all tend to give anomalous tobermorite. However, at 180° C this changes easily into xonotlite if C/S > 0.9.     

Influence of aluminium on the conversion of calcium silicate hydrate gels into 11 Å tobermorite at 90°C and 120°C

Mixtures of CaO and colloidal silica with and without γ-A?₂O₃, CaO and alkali-bearing A?₂O₃-SiO₂ gels, or CaO and clinoptilolite were treated hydrothermally at 90°C or 120°C for 4 hr – 4 weeks. Reaction seemed always to proceed through formation of C-S-H gels to 11 Å tobermorite. In the absence of A?, tobermorite crystallized more rapidly at c/S = 1.0 than at C/S = 0.8 but in the presence of A?, it crystallized more rapidly at Ca/(Si + A?) = 0.8 than at Ca/(Si + A? = 1.0. Where the starting materials contained both A? and alkali the tobermorite showed anomalous thermal behaviour similar to that of the natural mineral from Loch Eynort, but where they contained A? but no alkali, the thermal behaviour of the tobermorite was complex.     

Influence of SiO2 modification on hydrogarnets formation during hydrothermal synthesis

Formation and stability of hydrogarnet and Al-substituted tobermorite were examined at 175 °C temperature in saturated steam environment processing CaO-quartz and CaO-amorphous SiO₂ suspensions. A large quantity of Al₂O₃ was added to the starting mixtures [molar ratio A/(S+A)=0.10, duration of hydrothermal synthesis—from 0 to 24 h]. It was determined that hydrogarnets always tend to form more rapidly than 1.13 nm tobermorite. However, later, with extension of synthesis duration, they start to fracture and their quantity reduces almost in half during 24 h. CaO is present in the further reaction with SiO₂ forming hydrated calcium silicates, and released Al³⁺ ions are inserted into Al-substituted tobermorite crystal lattice. Using amorphous SiO₂·nH₂O as SiO₂ component, starting raw materials react considerably quicker—the total Ca(OH)₂ is joined already while increasing the temperature up to 175 °C. Meanwhile, in the mixtures with quartz when their composition is described by the molar ratio C/(S+A)=1.0, traces of Ca(OH)₂ are found even after 24-h isothermal treatment at 175 °C temperature. Moreover, it depends on SiO₂ modification the hydrogarnets of what type are to be formed. Si-free hydrogrossular forms in the mixtures with quartz and katoite in the mixtures with SiO₂·nH₂O. Si⁴⁺ ions are inserted into the crystal lattice of the latter compound while the first one remains undisturbed. This is presumably related to the lower solubility of the quartz. It was also noticed that an isomorphic Si⁴⁺ ions substitution with Al³⁺ ions in the hydrated calcium silicate lattice is considerably quicker when an amorphous SiO₂ is used as SiO₂ component instead of quartz.     

Thermal treatment of C-S-H gel at 1 bar H2O pressure up to 200 °C

Homogeneous CaO-SiO₂-H₂O gels were prepared at Ca/Si molar ratios 0.83, 1.01, 1.21, 1.50, 1.83 and 2.02. These were aged for 12-24 months at 25 °C and subsequently treated in steam, 1 bar total pressure, at 130 or 200 °C; also in water at 55 and 85 °C. Gels with low Ca/Si ratios partially crystallised at 85 °C. At 130 °C in steam, crystalline products included 11 and 14 Å tobermorite, xonotlite, afwillite, portlandite and another incompletely characterised phase. At 200 °C, the gels retained much water but remained amorphous to X-ray powder diffraction (XRD). However, electron microscopy, coupled with diffraction and analysis, disclosed that the “amorphous” product obtained at 85-200 °C had undergone crystallisation with domains typically 10-1000 nm. At higher bulk Ca/Si ratios, 1.83 and 2.02, much nanoscale precipitation of Ca(OH)₂ occurs, probably by exsolution, such that the residual C-S-H product has a Ca/Si ratio in the range 1.4-1.5. The complex thermal history of the products is reflected in their pH conditioning ability, measured at 25 °C. The results are applied to predict the evolution of pH in a cement-conditioned nuclear waste repository which experiences a prolonged thermal excursion.     

Formation of 11 Å tobermorite from mixtures of lime and colloidal silica with quartz

Mixtures of lime, colloidal silica and quartz (<10 μm, 10–20 μm) were treated hydrothermally in stirred suspensions at 180°C to prepare 11 Å tobermorite with Ca/Si = 0.8. The runs made using the colloidal silica and lime quickly formed CSH; but did not convert into crystalline tobermorite even after 20 h. The runs with the mixtures of colloidal silica and quartz gave highly crystalline 11 Å tobermorite after 5 h through the reaction of Ca-rich C-S-H and quartz. The reaction of quartz was controlled by its rate of dissolution. The thermal behaviour of the tobermorites was normal, trending to mixed with increase in processing time.     

X-ray photoelectron spectroscopy of aluminium-substituted tobermorite

We have synthesised 11-Å tobermorite hydrothermally, both pure and with increasing isomorphic substitution of aluminium for silicon. The samples were analysed by X-ray photoelectron spectroscopy (XPS). Aluminium was found, on the basis of its Al 2p binding energies, to be tetrahedrally coordinated. We observed no changes in Ca/(Si+Al) ratio upon aluminium substitution, implying that charge balancing does not occur via the incorporation of additional calcium into the tobermorite structure. Aluminium substitution into the silicate structure led to a decrease in Si 2p binding energies. This implies one of two alternatives. Firstly, that charge balancing occurs via substitution of OH⁻ for O²⁻ in the tobermorite structure. Secondly, the presence of aluminium in the tobermorite structure may negatively influence the degree of silicate polymerisation. Further work is required to determine which of these possibilities is the case.     

Synthesis of 11 Å Al-substituted tobermorite from trachyte rock by hydrothermal treatment

The alkaline hydrothermal activation of trachyte rock led to synthesis of technologically important 11 Å tobermorite. Tobermorite synthesis was studied by X-ray diffraction, scanning electron microscopy and ²⁹Si and ²⁷Al high resolution magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The influence of the reaction conditions such as different temperatures (150–170 °C), times (5–20 h) as well as different Ca/Si ratios of 0.6, 0.9 and 1.3 on tobermorite formation were investigated. The results showed that the main rock constituents were completely converted into a well crystallized Al-substituted 11 Å tobermorite when hydrothermally activated with 3.0 M NaOH under the optimum hydrothermal conditions of 170 °C for 20 h and using Ca/Si and Al/Al + Si ratios of 0.9 and 0.17, respectively. The local structure of the synthesized tobermorites as determined by MAS-NMR spectroscopy implied an alumino-silicate mean chain length of 5.9 units with 79% of the interlayer cross-links which are of Si–O–Al configuration. The present results show that trachyte rock could be considered as a new economic resource for synthesizing Al-substituted 11 Å tobermorites.     

Synthesis and characterization of calcium–alumino-silicate hydrates from oil shale ash – Towards industrial applications

The influence of the alkaline medium on the hydrothermal activation of the oil shale fly ash with NaOH and KOH was studied using SEM/EDX, XRD, ²⁹Si and ²⁷Al high-resolution MAS-NMR spectra. In the presence of NaOH the silicon in the original fly ash was completely converted into calcium–aluminum–silicate–hydrates, mainly into 1.1 nm tobermorite structure during 24-h treatment at 160 °C. At similar reaction conditions, the activation with KOH resulted only to the formation of amorphous calcium–silica-hydrate gel on the surface of ash particles at temperature. The results obtained in this study indicate that the oil shale fly ash can be used for production of Al-substituted tobermorites when strongly alkaline media (NaOH) is applied. The synthesized product was used in a catalytic d-lactose isomerization reaction.     

29Si NMR Spectroscopy of Silicate Anions in Hydrothermally Formed C-S-H

Hydrothermal treatment of lime–silica mixtures under saturated steam pressures below 200°C usually gives C-S-H as an initial product, which reacts further to give crystalline calcium silicate hydrates. In this paper, C-S-H was hydro–thermally prepared using CaO and silicic acid at Ca/Si ratios of 0.3 to 2.0 and 120° to 180°C for 2 h. The C-S-H was examined mainly using ²⁹Si NMR by the magic angle spinning gate proton decoupling and cross polarization magic angle spinning methods. XRD for all of the C-S-H showed bands at 0.304, 0.280, 0.183, and 0.166 nm. NMR results showed that all of the C-S-H contained single chains of silicate anion, which became progressively longer as the Ca/Si ratio decreased, i.e., as the system became richer in silica. This was independent of the preparation temperature. The 0.8 ratio preparations at 180°C contained small amounts of double-chain structure of 1.1-nm tobermorite. The reaction processing in the lime- silicic acid mixtures is also discussed.     

Hydrothermal alkaline treatment of oil shale ash for synthesis of tobermorites

The hydrothermal alkaline activation of the oil shale fly ash was studied using SEM/EDX, XRD and ²⁹Si and ²⁷Al high-resolution MAS-NMR spectra. The silicon in the original fly ashes was completely converted into calcium-alumino-silicate hydrates, mainly into 1.1 nm tobermorite structure during 24 h treatment under hydrothermal conditions at 160 °C in the presence of NaOH. The local structure of synthesized tobermorite samples implies long silicate chains with small number of bridging sites. The results obtained in the study prove that the oil shale fly ash can be used for production of Al-substituted tobermorites.     

Incorporation of Al and Na in Hydrothermally Synthesized Tobermorite

Calcium silicate hydrate and its Al‐substituted form synthesized by a hydrothermal process were investigated by X‐ray diffraction, compositional analysis, and magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, in order to determine the mechanism of Al and Na incorporation in the tobermorite structure with varying molar ratios of Ca/Si and Al/Si. At a high molar ratio of Ca/Si, the silicate chains of tobermorite are ruptured, the degree of polymerization of the silicate chains is lowered, and the high calcium concentration lowers the content of Na₂O in the structure. Solid‐state ²⁹Si and ²⁷Al MAS NMR spectroscopy confirm that all Al atoms were incorporated in the silicate chains of tobermorite. The tetrahedrally coordinated Al (Al(IV)) could either act as the bridging tetrahedron () for the dreierketten chain of tobermorite, or be present in Q³ sites that link two dreierketten chains together. Therefore, the degree of polymerization of the silicate chains of tobermorite is increased at high molar ratio of Al/Si. Furthermore, the greater charge deficit due to the replacement of Si⁴⁺ by Al³⁺ ions is compensated by increased adsorption or binding of Na⁺.     

Geopolymers from Algerian metakaolin. Influence of secondary minerals

The influence of secondary phases (illite, quartz) on the geopolymerization reaction of metakaolin has been investigated by comparing two metakaolins, one prepared from a pure kaolinite and the other from illite- and quartz-containing Algerian kaolin from the Tamazert region, respectively. Geopolymerization was achieved by mixing the metakaolins with an alkaline sodium silicate solution at room temperature and curing at 50 °C. The products were characterized by X-ray diffraction and ²⁹Si and ²⁷Al MAS-NMR. The results show that the secondary phases, at the concentration used in this work, do not prevent the geopolymerization reaction.     

A study of fly ash-lime granule unfired brick

In this paper, the properties of fly ash-lime granule unfired bricks are studied. Granules were prepared from mixtures of fly ash and lime at fly ash to hydrated lime ratios of 100:0 (Ca/Si = 0.2), 95:5 (Ca/Si = 0.35) and 90:10 (Ca/Si = 0.5). After a period of moist curing, the microstructure and mineralogy of the granules were studied. Microstructure examination reveals that new phases in the form of needle-like particles are formed at the surface of granule. The granules were used to make unfired bricks using hydrothermal treatment at temperature of 130 ± 5 °C and pressure of 0.14 MPa. The microstructures, mineralogical compositions, mechanical properties and environmental impact of bricks were determined.The results reveal that the strengths of unfired bricks are dependent on the fineness of fly ash. The strength is higher with an increase in fly ash fineness. The strengths of the fly ash-lime granule unfired brick are excellent at 47.0-62.5 MPa. The high strength is due to the formation of new products consisting mainly of hibschite and Al-substituted 11 Å tobermorite. The main advantage of utilization of granule is the ability to increase the pozzolanic reaction of fly ash through moisture retained in the granule. In addition, the heavy elements, in particular Cd, Ni, Pb and Zn are efficiently retained in the fly ash-lime granule unfired brick.