Tadahiro Nishikawa

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.

Powder characteristics, sintering behavior and microstructure of sol-gel derived ZTA composites

Abstract A series of alumina/zirconia composites of varying compositions of zirconia were prepared through the sol–gel technique. Precursors were calcined at different temperatures ranging from 300 to 1400°C and sintered at 1530°C for 3 h. Compacts made from the powder calcined at 950°C yielded density up to >99% of theoretical density by pressureless sintering. Pore size distribution and the densification behavior were explained with respect to calcination temperature. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia grains in the alumina matrix. It is also observed that the faceted intergranular zirconia grains are at the grain junctions and the corners of the alumina matrix.

Pressureless sintering of sol-gel derived alumina–zirconia composites

Abstract Alumina–zirconia (pure zirconia, 12 mol % ceria stabilized zirconia and 3 mol% yttria stabilized zirconia) composites containing 5, 10, 15, 20 and 25 vol.% zirconia were prepared by sol-gel technique. The procedure involved the following steps: preparation of stable (hydrous) alumina and zirconia sols, mixing of sols in proper ratio, to obtain the final precursor with the desired composition and finally stabilizing the mixed sols. Thermal analysis was carried out for the dried precursor. The sol-gel derived precursors were calcined at different temperatures and their crystallization behavior was studied using X-ray diffraction (XRD) technique. Samples calcined at 950°C sintered well than those calcined at other temperatures. All the samples were sintered at 1530°C for 3 h by pressureless sintering, which yield upto 99.8% of the theoretical density of the composite.

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.

Influence of MgO on microstructure and properties of mullite–Mo composites fabricated by pulse electric current sintering

Abstract Mullite–Mo (10 vol.%) composites and monolithic mullite were fabricated using a pulse electric current sintering technique. Both monolith and composites of mullite were sintered up to theoretical density at 1500°C within few minutes. MgO of 0.25 wt.% was added as a sintering aid to both the mullite and composites. Addition of MgO significantly increased the bending strength of the monolithic mullite and mullite/10 vol.% Mo composites to 441 and 634 MPa respectively. The apparent increase in the bending strength of the composites was attributed to the combinational effect of Mo and MgO present in the composites. The fracture toughness of the composites also increased from 2 to 3.9 MPa.m 0.5 for the mullite/10 vol.% Mo composites, which was nearly twice that of the mullite. Crack-bridging and frontal process-zone elongation were expected to be the toughening mechanisms operated in these composites. The addition of Mo having high thermal diffusivity slightly increased the thermal diffusivity of the composites, because the 10 vol.% Mo particles were well dispersed and discontinuous in the matrix. Elongated mullite grains were observed for the composites without MgO, whereas the composites with MgO have a controlled microstructure.

Fracture behavior of hardened cement paste incorporating mineral additions

Abstract Various powder materials have been added on the hydration of cement to improve its mechanical and chemical properties. Industrial waste or by-product powder is consumed in large quantities as aggregates or hydraulic reactants. In the present study, hardened cement pastes (HCP) were prepared with ordinary Portland cement containing fine powder consisting of isotropic graphite or glass waste to investigate their microstructure and mechanical properties. Fracture behavior was investigated by compact tension, by which the fracture energy and fracture toughness of a specimen were obtained. The amount of calcium hydroxide and Ca Si molar ratio of CSH were constant in spite of being mixed with graphite. Graphite powder did not take part in the hydration of cement. An improvement in fracture toughness and a remarkable plastic behavior were observed because of the dispersion of graphite particles. On the other hand, glass waste powder reacting with alkaline solution in the pores resulted in decreasing the Ca Si molar ratio of CSH. Similar behavior was observed in a mixture of cement and fine blast furnace slag. Bending and compressive strength increased, but fracture toughness decreased. The lower elastic fracture energy in HCP containing reactive fine powder facilitated crack propagation and enlarged the fracture surface.

Heat transmission during thermal shock testing of ceramics

The thermal shock resistance of ceramics is generally evaluated by the water-quench test, in which it is important and necessary to understand the heat-transmission behaviour. A novel and simple method for measuring the transitional changes of temperatures in ceramics has been proposed. Changes in temperature at two different positions in zirconia ceramics were measured to estimate the temperature distribution. From the analytical results, it was clear that the heat-transmission behaviour changed with the quenching temperature or water temperature. The Biot number also changed remarkably with time or with the surface temperature in this experiment. These results are useful in practice for examining the cooling conditions in the thermal shock test.

Chemical conversion of CSH in concrete

The C-S-H and other hydrates in cement paste or concrete can be analyzed quantitatively by a novel analytical method using heavy water. The hardened cement paste was prepared by the change of water/cement ratio, by mixing with a salt solution or through exposure to CO2 gas. The amount of hydrates and Ca/Si molar ratio of C-S-H were measured and estimated. Several cement pastes of concrete were also analyzed. Deterioration resulted in a lower Ca/Si molar ratio of C-S-H and progressive carbonation. (A)

Sintering and microstructure of mullite-Mo composites

Abstract Mullite–Mo composites of different compositions (0–100 vol.% Mo) were sintered to near theoretical density by pulse electric current sintering (PECS). The densification behaviour and the microstructure of mullite–Mo composites as a function of Mo content were studied. The addition of 10 vol.% Mo significantly enhanced the strength and toughness of monolithic mullite to 556 MPa and 2.9 MPa m 1/2 , respectively. SEM observations revealed the modification of discrete isolated Mo particles to continuosly interconnected network with the increase in the Mo content. Mo grains were located at the grain boundaries as well as inside the mullite grains. The addition of Mo to monolithic mullite led to a change in the fracture mode.