The present paper concerns with a method of the improvement of dolomite plaster by the addition of waste sludge from aluminium product factory.
The sludge was air-dried, heated at 100-110°C and milled (under 149 micron) to give “sludge powder”.
The sludge powder, alone or with portland cement, was mixed with the dolomite plaster on the market, and the mixtures were blended with water, poured into forms, and stored 7 or 14 days in a humid chamber.
These specimens were tested on the properties by the conventional methods.
The following results were obtained :
(1) In all runs the setting time was decreased with the increasing contents of the sludge powder.
(2) The maximum values of bending strength and compressive one, were observed both at the 5% addition of the sludge powder alone; each of the values is 2-3 times larger than that of dolomite plaster only.
(3) The optimum amount of the sludge powder adding for the adhesive force was at a range of 1-3% when gypsum board were used as the base.
The series of the mixtures (dolomite plaster-portland cement-sludge powder) give similar results on adhesive force with gypsum plaster (3.05 kg/cm²).
It is 3-5 time larger than dolomite plaster.
(4) The optimum mixing ratio was as follows; dolomite plaster 73.5-79.0%, portland cement 20-25%, sludge powder 1-1.5%.
(5) The chemical composition of sludge powder was found to consist mainly of Al₂O₃ with small amounts of others, such as BaSO₄, P₂O₅, etc.
The X-ray diffractions of the air-dried sludge were same with that of BaSO₄ (Barite), and by heating at 1300°C they were changed to that of α-Al₂O₃, and the others were BaO·Al₂O₃·2SiO₂, α-SiO₂ (Quartz).

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The changes in weight of the samples due to isobaric water adsorption have been measured with an apparatus employing a weighing method subject to the different vapor pressures of 4. 6 and 17.5 mmHg and an evacuating state of 10^-2-10^-3 mmHg. The adsorption ρ is expressed by the following equations,
ρ= βe^E/^RT : for the evacuation
ρ= β (p/A) ^0.333e^E/^RT : for the admission of water vapor, where β and E are constants determined from the measured values of weight and depend only on the kinds of samples, and A is a constant of the same dimension as that of pressure p and of an amount equal to one expressed in mmHg. The increase in temperature of the samples after the admission of water vapor to the evacuated state has also been measured. The heat of adsorption evaluated from the increase, and the calculated value of E seem to be comparable to each other. The values of adsorbed water computed from the equation are dicussed with relation to the phenomena which have been noticed heretofore.

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