Residual flexural strength of usual set plaster (CaSO₄·2H₂O) after the immersion in water for a day is lower than 50% of initial strength. In present study, the relationship between composition of polymer-gypsum composite and water-resistivity of resulting composite material was investigated.
After several attempts, we succeeded to prepare highly water-resistant polymer-gypsum composite starting from the mixture of gypsum, resorcinol, formalin, methanol and catalytic amount of acid.; residual flexural strength of present composite after immersion in water at room temperature for 84 days kept about 95% of initial strength and increased to approximately 140% of the initial strength when it was air-dried at 30°C for 24h.

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In order to lower the basicity of soil quality stabilizer on hydration, substitution of Ca²⁺ ion with Mg²⁺ ion in C₃A and Hauyne which form Ettringite, a major hydration phase of cement type soil stabilizer, on hydration and hydration characteristics of substituted products were examined. The substitution of Ca²⁺ ion with Mg²⁺ ion in the framework of C₃A and Hauyne was confirmed by change of XRD patterns of products and the substituted amounts were estimated to be within 0.4 and 0.5 mol, respectively. The Mg-substituted C₃A and Hauyne were found to be effective to lower the pH value upon hydration. Ettringite which formed from both solid solutions was considered to contain also Mg²⁺ ion in the framework up to about 0.3 mol. In the course of hydration in the system of [(3-x) CaO·xMgO] ·Al₂O₃+3CaSO₄, considerable expansion of body was observed and products showed low compressibility. On the other hand, products did not expand much on hydration and showed high compressibility in the case of [(3-y) CaO·yMgO] ·3Al₂O₃·CaSO₄+2CaSO₄.

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A commercial hydroxyapatite, which has been regarded as a powder with low sinterability, was ground by four kinds of wet ball-milling in order to improve the sinterability. The effect of the grinding on the sinterability was investigated by comparing the four methods. When the powder was ground under water as a dispersion media, fine particles generated by the grinding cohere to reconstruct aggregates, then milling effects disappear. The fine particles without reconstructed aggregates can be obtained by ball-milling under ethyl alcohol as a dispersion media. The grinding efficiency can be improved by using balls with a high true density. The sizes of aggregates were diminished to 0.1-0.2μm by ball-milling using nylon coated iron balls. The powder ground by the nylon coated iron ball was formed into disks and then heated at 1200°C for 1 h. The bulk density of the sintered body was >98% of the theoretical density.

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Dissolution of phosphate ion from the incineration ash with dilute sulfuric acid was investigated in order to establish the recovery of useful ingredients from the incineration ash of sewage sludge. As a result, the concentration of the phosphorus was increased with the increase of the pH of the dilute sulfuric acid and reached the maximum at pH = 2. The concentration of the phosphorus, however, was decreased with the further increase of the pH of the dilute sulfuric acid.
Dissolution of phosphate ion from the incineration ash in supersonic flow field was investigated. Consequently, the concentration of the phosphorus showed the maximum with the ultrasonic wave for two minutes. However, the concentration of the phosphorus decreased with increasing time above two minutes. As compared with the effect of agitation on the average dissolution rate, the rate with homogenizer was about 40-70 times faster than the rate with magnetic stirrer.

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The effect of oxygen enriched air on the firing rate was investigated in a ceramic-sintering shuttle kiln equipped with a 4 m3 combustion chamber. The kiln was heated to 1700°C according to a prescribed firing schedule. When oxygen enriched air containing 23% oxygen was supplied with a gaseous fuel during a part of the firing period, the whole firing time was shortened by 14 h (from 62 to 48 h). The addition of oxygen to the combustion air was conducted under two different conditions. The one was the addition of oxygen from 800 to 1500°C (enriched air (I)) and the other was from 1100 to 1600°C (enriched air (II)). The amount of fuel consumption and CO₂ emission was reduced by 25% for enriched air (I) and reduced by 32% for enriched air (II). The results are explained on the basis of reduction of exhaust gas for oxygen enriched combustion.

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