Suketoshi Ito

Decomposition of synthesized ettringite by carbonation

The stability of ettringite (3CaO·Al2O3·3CaSO4·32H2O) in contact with CO2 gas has been studied using synthesized ettringite. Carbonation were carried out at water/solid ratios(W/S) of 0.6∼3.5 in a moist CO2-incubator. According to the SEM-EDAX examination, the carbonation without water makes compositional changes slightly in ettringite, maintaining fibrous form. By the carbonation with excess water, ettringite clearly decomposed to gypsum, calcium carbonate, and alumina gel. The apparent carbonation rates were calculated from the amount of calcium carbonate. Jander equation was taken to model the carbonation kinetics.

Formation and carbonation of C-S-H in water

Abstract The formation of calcium silicate hydrate (C-S-H) and its carbonation were investigated under two different atmospheric conditions. The C-S-H was prepared with calcium hydroxide solution and tetraethyl orthosilicate to have different Ca/Si mole ratios. With rising initial Ca/Si mole ratio, the amount of C-S-H increased and that of co-existing silica gel decreased under the isolated condition. The Ca/Si mole ratios of formed C-S-H have the approximately constant value of 1.2 and its mean solubility product is 5.5×10−49 at room temperature. Under exposure to CO2, the C-S-H formed in the liquid decomposes by carbonation and then forms calcite and silica gel. This carbonation process of C-S-H could be divided into four stages: (I) Formation of C-S-H, calcite and the Ca(II)-modified silica gel, (II) Formation of calcite from the residual calcium ion, (III) Decomposition of C-S-H and release of silicate ion, (IV) Change from the modified silica gel to the pure silica gel.

Phase Transition in SrAl2O4

Strontium-monoaluminat SrAlaCU lagert sich, wie wir fanden, bei 650 °C reversibel in die unverzerrte BaAUCM-Struktur um. Die Hochtemperaturform hat die Raumgruppe P6322 und die auf Zimmertemperatur extrapolierten Gitterkonstanten a = 5,13 Â und c = 8,44 Â. Das Pulverdiagramm der Tieftemperatur-Modifikation wurde mit einer monoklinen Zelle von = 10,20 Â, b = 20,26 Â und c = 8,42 Â sowie = 60,53° indiziert. Die Umwandlungsenergie von 2.3 cal/g ist mit der von oc—ß—S1O2 vergleichbar.

High-pressure modifications of CaAl2O4 and CaGa2O4

Abstract The modifications of CaAl 2 O 4 and CaGa 2 O 4 with the stuffed tridymite structure were examined under high temperatures (600 ∼ 1500 °C) and high pressures (10 ∼ 40 kb). Calcium monoaluminate CaAl 2 O 4 was found to transform to three kinds of high-pressure modifications. The original CaAl 2 O 4 (CA-I) changed to the phase CA-II which had m-CaGa 2 O 4 type structure with a different array of tetrahedra in the six-membered rings of tetrahedron. The phase CA-II transformed either to the phase CA-IV with CaFe 2 O 4 type structure or to an unknown phase (CA-III) under high pressures. The phase CA-IV was obtained under the pressures above 30 kb and at the temperatures above 1000 °C. Calcium monogallate CaGa 2 O 4 transformed to the CaFe 2 O 4 type structure above 30kb and 700 °C. No phases such as CA-II and CA-III were found. The structural relations among these modifications were discussed.

Approach by zeta-potential measurement on the surface change of hydrating C3S

Abstract The adsorption of Ca 2+ on C 3 S surface and the formation of C-S-H in the induction period of C 3 S hydration were investigated by measuring zeta-potential, heat evolution change and by analyzing dissolved ions. Within 20 minutes, the dissolution of Ca 2+ and silicate ions and the adsorption of Ca 2+ on the surface of C 3 S were alternately repeated. In the induction period (20 ∼ 50 minutes), Ca 2+ was adsorbed fairly strongly on C 3 S surface giving positive zeta-potentials and the dissolution stopped for a time. In the hydration between 60 and 140 minutes, the C-S-H gel proceeded to crystallize and Ca 2+ began to dissolve again, giving negative zeta-potentials by Si-O − . After 140 minutes, the second evolution was caused by the crystallization of C-S-H and Ca(OH) 2 .

Effect of NaCl or NaOH on the formation of CSH

Abstract Formation field and composition of CSH in the presence of NaCl and/or NaOH were investigated to clarify their effects on the durability of concrete. The CSH and other hydrates were prepared from the mixtures of various solutions. The silica gel, CSH and calcium hydroxide are precipitated in turn with increasing initial Ca/Si mole ratio of mixture. On the formation of hydrate, the liquid composition moves to and then situates on each solubility product curve regardless of ionic species or concentration. The CSH formed in this experiment has lower Ca/Si mole ratio than that of normal CSH formed in water and contains sodium ion. However, this CSH easily returns to normal CSH after dispersing in water. These characteristic of CSH are confirmed thermodynamically.

Layer Formation and Apparent Activation Energies of Formation of Calcium Aluminates

The layer formations of various calcium aluminates were examined by using three kinds of reaction couples C3A/CA2, C12A7/CA2 and C12A7/AI2O3 at 1000, 1100, 1200 and 1300°C. The layer thicknesses of C12A7-, CAand CA2-phases formed were measured with polarizing microscopy and electron microprobe analysis on thin sections of each couple and the apparent actuation energies of the formation for each phase were calculated. The apparent activation energies of CA formation are found to depend strongly on the original phases; 120 kcal/mole from Ci2A7-phase and 85 kcal/mole from CA2-phase. No appreciable difference in activation energies was found among three couples. The ratio in observed activation energies of the CA-phase was exactly the same as the ratio of CaO molecules which diffuse in and out in order to form the CAphase in the original phases.

Clinker grindability and textures of alite and belite

Abstract Superior in fracture toughness to the other constituents of portland cement clinker, belite, when clustered, deteriorates the grindability of clinker to a considerable degree and appears in the ground product mostly as angular fragments of clusters. When belite exists separately throughout clinker without forming clusters, spherical single grains of belite result from grinding and the grindability of clinker is much improved. Belite clusters thus play a much more important role in grinding than pores, especially in relation to fine grinding. Microcracks within alite crystals occur mainly through the volume shrinkage during crystallization of the interstitial liquid and hence depend on the rate and extent of crystallization during cooling. Quenching in air from just above the crystallization temperature of the interstitial liquid produces microcracks in abundance.

Formation of belite clusters from quartz grains in portland cement clinker

Abstract The conversion process of quartz grains into belite clusters in portland cement clinker has been followed under isothermal heating conditions at 1400 °C. On heating, quartz grains are surrounded by layers consisting of belite and liquid, which continue to grow thick with the progress of conversion (outer layers of belite). The conversion process has been divided into three stages considering the chemical composition of the liquid phase formed in each stage. In the first stage are formed two kinds of liquids the compositions of which lie across the primary phase field of wollastonite from each other. The belite crystals precipitated constitute thin, dense layers (inner layers of belite). At the beginning of the second stage, the inside of the belite layers is completely converted into a liquid without quartz and wollastonite. The inner layers extend inward and grow thick; the impurity concentration is lowest for this part of the belite layers. With further progress of diffusion, belite crystals nucleate and grow independently inside the layers. The composition of the coexisting liquid, rich in alkalies, moves on the primary phase field of C 2 S toward that of C 3 A. In the third stage the conversion is completed and the liquid phase, enriched with Al 2 O 3 and Fe 2 O 3 , is similar in composition to the interstitial liquid in portland cement clinker. The belite crystals, especially those of the inner layers, undergo grain growth with an increase in impurity content with duration of heating. Three kinds of belite crystals different in origin, texture and composition are thus distinguishable in the belite clusters.

Effect of Na, K and Fe on the formation of α- and β-Ca2SiO4

Abstract The contribution of Fe to C 2 S polymorphs is effectively revealed for β-C 2 S formation and is not for α′- or α-C 2 S. However in co-existing of Na or K with Fe, the α-C 2 S is easily stabilized, though accompanied with small amount of crystalline Ca 2 Fe 2 O 5 as the interstitial material. This effect of Fe on α- and β-C 2 S synthesis was investigated by XRD, chemical analysis and Mossbauer spectra observation. Comparing calculated Mossbauer parameters of α- and β-C 2 S with those of other minerals, it was confirmed that they included only Fe 3+ at octahedral and tetrahedral sites with the ratio 30:70 in NaFe substituted α-form and with 63:37 in KFe substituted α-form. In β-C 2 S, Fe 3+ was mostly situated at tetrahedral site. The formulas of α- and β-C 2 S were decided and shown as, (Ca 1.88 Fe 0.05 Na 0.24 )(Si 0.88 Fe 0.11 )O 4 , (Ca 1.94 Fe 0.09 K 0.18 ) (Si 0.88 Fe 0.05 )O 4 for α-form and (Ca 1.93 Fe 0.003 Na 0.04 ) (Si 0.99 Fe 0.05 )O 4 , (Ca 1.94 Fe 0.01 K 0.04 ) (Si 0.92 Fe 0.13 )O 4 for ɜ-form.