Takeshi Mitsuda

Synthesis of normal and anomalous tobermorites

Abstract Tobermorites were made from several starting materials at 105 – 180°C and Ca/(Si + Al) 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 Al, 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 Al 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.

Hydrothermal reaction between C‖S‖H and kaolinite for the formation of tobermorite at 180°C

Abstract 11 A 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.

Influence of quartz particle size on the chemical and mechanical properties of autoclaved aerated concrete (I) tobermorite formation

Abstract The chemical reactions in the hardening of autoclaved aerated concrete (AAC) block were investigated using ground quartz of different particle sizes. The samples were prepared at 180 °C under saturated steam pressure for various times from 0.5 to 64 h Autoclaving for 0.5 h yielded 1.1-nm tobermorite, suggesting that the utilization of finer quartz has the advantage of reducing the processing time. The tobermorite formed had higher crystallinity with coarser quartz. The degree of reaction of finer quartz was higher than that of coarser quartz after shorter periods of autoclaving, but lower after longer periods of autoclaving. Gyrolite was formed after 64h when finer quartz was used.

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

Abstract Mixtures of lime, colloidal silica and quartz ( μ m, 10–20 μm) were treated hydrothermally in stirred suspensions at 180°C to prepare 11 A 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 A 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.

Formation of magnesium silicate hydrate and its crystallization to talc

Abstract Mixtures of Mg(OH)2 and colloidal silica with Mg/Si = 0.75 were treated hydrothermally at 180° to 600°C for 4 h − 8 weeks. Reaction seemed always to proceed through the formation of magnesium silicate hydrate to talc, which had random displacements of layers parallel to b of nb/3, and gave a decrease of the basal spacing with increase of crystallinity. The magnesium silicate hydrate gave X-ray powder patterns indicative of two-dimensional crystals without basal spacing, had a specific area of about 300 m2/g, and gave an exotherm at 830°C. Dehydration and infrared spectra for products are also described.

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.

Influence of quartz particle size on the chemical and mechanical properties of autoclaved aerated concrete (II) fracture toughness, strength and micropore

Abstract The development of mechanical properties of autoclaved aerated concrete (AAC) block was investigated using ground quartz of different particle sizes. The samples were prepared at 180 °C under saturated steam pressure for various times from 0.5 to 64 h. With increasing tobermorite formation, the compressive strength, Youngs modulus and fracture energy increased for the samples made using the coarser quartz, and decreased for the samples made using the finer quartz. After the completion of tobermorite formation, the coarser quartz yielded the higher compressive strength, Youngs modulus, fracture energy and crack growth resistance. Using the finer quartz, gyrolite was formed by the decomposition of tobermorite after 64 h autoclaving, reducing the compressive strength and Youngs modulus with changes in the micropore size distribution.

Paragenesis of 11 Å tobermorite and poorly crystalline hydrated magnesium silicate

Abstract Poorly crystalline talc-like material and highly crystalline 11 A tobermorite occurred as small veins in a basic tuff. These are the first assemblages of this type to be recorded in nature. The 11 A tobermorite was rich in Al and very poor in Mg, and it showed anomalous thermal behaviour similar to that of the Loch Eynort specimen. These results suggest that Mg cannot enter into the tobermorite structure but forms poorly crystallized hydrated magnesium silicates, when the hydrothermal treatment is carried out at low temperature from Mg-bearing raw materials such as Portland cement, high-magnesia cement, slag, etc.

Hydrothermal formation of γ-dicalcium silicate from lime-silica mixtures using a rapid-heating method and its reaction to give kilchoanite or calciochondrodite

Abstract γ-Dicalcium silicate was formed as an initial product from mixtures of lime with silicic acid or finely divided quartz at Ca/Si = 1.5 – 3.0 above 220°C under saturated steam pressures using a rapid heating method, in which water is not admitted into the reaction vessel until the working temperature has been reached. The γ-dicalcium silicate forms poorly crystallized prismatic aggregates and reacts further to give kilchoanite at Ca/Si = 1.5, or to give poorly crystallized calciochondrodite at Ca/Si = 2.0 – 3.0.

Anomalous tobermorite in autoclaved aerated concrete

Abstract 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 A 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.