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.     

Hydrothermal reaction between CSH 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.     

Synthesis of Calcium Silicate Hydrate with Ca/Si = 2 by Mechanochemical Treatment

An amorphous, C-S-H-like phase with Ca/Si = 2 was synthesized from amorphous precipitated silica, calcium oxide, and water by mechanochemical treatment using a vibration mill at room temperature. The product was studied by XRD, ²⁹Si MASNMR, TEM, analytical TEM, and TGA-DTA. After 14 h of treatment, the starting materials react to form an amorphous phase (C-S-H-like phase). The XRD pattern of C-S-H-like phase resembles the C-S-H formed hydrothermally or by cement hydration except it has no reflection at 0.182 nm. The ²⁹Si NMR results revealed a silicate anion structure of C-S-H-like phase consisting mainly of a mixture of a monomer and a dimer. On heating, the C-S-H-like phase decomposed into β-dicalcium silicate below 1000°C.     

亚熔盐法粉煤灰脱铝渣水热处理后碱含量的影响因素

以亚熔盐法处理粉煤灰的脱铝渣为原料,采用动态水热法分解脱碱,研究了不同A/S(Al2O3/SiO2质量比)、C/S(CaO/SiO2质量比)和不同脱铝溶出工艺对硅渣碱含量的影响. 结果表明,随脱铝渣A/S增加,碱含量先降低后升高,脱铝渣A/S为0.11,硅渣Na2O含量降至1.18%,适当的A/S有利于提高硅渣中含铝托贝莫来石的晶化程度;脱铝渣C/S为0.98,硅渣Na2O含量仅有1.31%,随脱铝渣C/S增加,硅渣碱含量增加,C/S过高会降低硅酸钠钙(NaCaHSiO4)的分解率,不利于生成含铝托贝莫来石相;溶出时间和停留时间较长的脱铝渣在脱碱过程中不易生成含铝托贝莫来石.     

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.     

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.     

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.     

Al-substituted tobermorite: shows cation exchange

It has been found that the tobermorite family--fundamental to cement chemistry can exhibit base exchange behavior. Several Al-substituted tobermorites were synthesized and these 11Å type tobermorites were discovered to have cation exchange as well as selective cation adsorption properties. For example, Al-substituted tobermorite (synthesized from a mixture of clinoptilolite and mortar) has a cation exchange capacity of 56 meq/100g and a selective Cs adsorption K_d of 3400 (ml/g). The cation exchange and selective properties of Al-substituted tobermorite are analogous to those of zeolites because of its rigid structure but unlike those of smectites or vermiculites where the basal spacing varies with the nature of the interlayer cation.     

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.     

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.     

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.     

Synthesis and characterisation of wollastonite with aluminium and fluoride ions

This work aims to investigate the influence of aluminium and fluoride ions on the mechanism of wollastonite formation during a two-step synthesis. For the synthesis, three mixtures with different chemical compositions were prepared. The first step of the synthesis was carried out under hydrothermal conditions (stirring – 50 rpm; 200 °C, 4–48 h); the second step was carried out in a furnace with a temperature range of 900–1100 °C for 1 h. The chemical composition of the primary mixture has been determined to exert a significant influence on the mineralogical composition of the hydrothermal synthesis products, since tobermorite and xonotlite dominated in the pure system, while tobermorite and cuspidine dominated in the mixture with aluminium and fluoride ions. After calcination of the precursors, pure wollastonite was obtained using a mixture of amorphous silica and calcium oxide, while in the other two mixtures, it was identified together with wollastonite cuspidine and fluormayenite or gehlenite. Dilatometric and BET analyses of the products showed that Al³⁺ and F^? ions caused densification and loss of porosity during the formation of wollastonite. The synthesised products were characterised by STA, XRD, DIL and BET analyses.     

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⁺.     

利用脱硅滤液水热合成硅灰石

节能环保是现代化工业生产的新指标,这不仅可以达到环境保护的目的,同时还能带来可观的经济效益。工业中碱法生产氧化铝产生了大量的滤液,可通过水热合成法除去滤液中的硅杂质并合成雪硅钙石,经煅烧后,得到结晶度较好的硅灰石,从而变废为宝,应用于各种领域。本试验将脱硅滤液做为原料,通过水热合成法制备硅灰石。根据正交试验的条件设定,确定水热合成硅灰石的最适宜n(Ca)/n(Si)为0.6,最适宜合成温度为180℃,在此条件下合成出的硅灰石产品,纯净度和结晶度最好。     

Quantum Chemical Determination of the Al-Substituted Site in Tobermorite

Ab initio and semiempirical molecular orbital (MO) calculations have been performed to determine the position of the substitution of Al for Si in tobermorite. Replacement energies were calculated for the cluster models of tobermorite framework. Al was found to be substituted mainly in chain middle groups (Q² sites). Correlation has been recognized between the site occupancy order in the unit cell and structural parameters such as bond lengths and bond angles. It was also found that the variation among energies required for double replacements at the different sites can be explained in terms of the Löewenstein rule.     

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.     

Drastic enhancement of mechanical strength and thermostability of bright yellowish-red hematite/alumina composites having a unique disk-like structure via silica coating

Surface coating is a key technique used to enhance various properties of functional inorganic materials. We adopted a silica-coating treatment for preparing bright yellowish-red pigments using hematite/alumina composites, which have a unique porous disk-like structure. This treatment enhanced color, mechanical strength, and thermostability. The porous disk-like structure comprising Al-substituted hematite particles and alumina nanoparticles, which is fragile to mechanical impact and/or high-temperature heating, was a key factor to obtain high chroma bright-red color in the present system. Silica-coated samples having various Si molar ratios [x=Si/(Fe + Al + Si)] were prepared, and mechanical-strength and thermostability tests were conducted. The mesopores generated by an assembly of hematite particles and alumina nanoparticles were gradually filled with amorphous silica, thus achieving significant enhancement in mechanical strength and thermostability in color. The composite pigment can be used as a colorant for red overglaze enamels for porcelain, and shows significant potential for various other industrial coloring applications.     

Substituted Tobermorites: 27Al and 29Si MASNMR, Cation Exchange, and Water Sorption Studies

²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy revealed that the maximum amount of Al that can be substituted for Si in the tobermorite structure is 15 to 20 mol%. Powder X-ray diffraction and cation exchange studies corroborate the above finding. Anomalous tobermorite structures resulted in all cases, and hydrogarnet appeared beyond the 15 to 20 mol% Al substitution limit for Si. The sorption of water molecules by synthetic [Al³⁺+ Na⁺]-substituted tobermorites and their cation-exchanged forms heated at 200°C under vacuum (≥10⁻³ torr) was measured at 25°C in order to probe the nature of the ion-exchange cavity. Samples with ≤30 mol% of Al substitution for Si showed isotherms of Brunauer, Demming, Demming, Teller (BDDT) type I and gave better linearity with the Langmuir plot than with the BET plot. Samples with ≥35 mol% of Al substitution for Si showed BDDT type II isotherms and gave better linearity with the BET than with the Langmuir equation. The Langmuir monolayer capacity was found to depend on the Al content and on the cationic form. The Li⁺- and Na⁺-exchanged tobermorites with about 20 mol% of Al substitution for Si showed the highest Langmuir monolayer water sorption capacity. The Cs⁺-exchanged tobermorites showed a smaller capacity than the Li⁺- or Na⁺-exchanged samples, which can be ascribed to clogging of the ion-exchange cavity by the large Cs⁺ ions.     

Effect of temperature and time in the hydrothermal treatment of HY zeolite

Zeolite HY was steamed at 1 bar water pressure in steps of 100 K from 473 K to 1073 K in dependence of the time (0.5–20 h). Three routes of hydrothermal reactions can be outlined: At low temperature (473 K), a partly framework dissolution occurs accompanied by the formation of X-ray amorphous material. At intermediate temperatures (773–873 K), framework dealumination takes place with the occurrence of different non-framework species. Thereby, with increasing steaming time, saturation of the Si/Al ratio occurs on low levels (Si/Al ratios of about 4 and 5, respectively). At 973 K, a crossover from a low level saturation in Si/Al (6) towards higher values (11) is observed, which is accompanied by a partial amorphization of the zeolite with formation of aluminium oxide and silica gel and a significant decrease in crystal size as well. At high temperatures (1073 K), a fast thermal decomposition occurs. The IR spectra directly reveal an aluminosilicate phase as decomposition product of the framework. At 573 K, the zeolite framework remains unchanged.     

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.