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Dyoung Sun Kum, Yi Hyung Woo, Boo Byung choi,
김경선, 우이형, 최부병

The properties of a investment material can be described by the consistency at the slurry state,
the setting time, the compressive strength and the thermal expansion during the casting. In this
study the effect of the production parameters which are included the ratio of quartz and
cristobalite, the content of binder, the water powder ratio and the content and concentration of
additives on the gypsum-bonded investments has been investigated with help of the consistency
test, the vicat needle test, the compressive strength test, the thermal expansion test, x-ray
diffraction and DTA thermal differential test. The experimental results showed that the
constitution of a investment with W/P ratio of 0.34, 30% of gypsum, 0.8% aluminium sulfate, 2%
magnesium sulfate, 0.6% sodium phosphate was adapted for the properties of the KDA Spec. No.
13 type Ⅰinvestment. The important experimental results are summarized as follows.
1.  The consistency of the investment decreased with increasing amount of aluminium sulfate
and decreasing amount of sodium phosphate. An addition of magnesium sulfate up to 2% an
increase of the consistency was shown. But 3% magnesium sulfate in investment showed a
decrease of the consistency. The consistency did not vary significantly with a variation of the
content of gypsum and cristobalite and the W/P ratio.
2.  Aluminium sulfate and the magnesium sulfate promoted the hardening and the aluminium
phosphate delayed the hardening. The setting time increased with amount of gypsum. The
effect of the matrix on the setting time was insignificant. With the W/P ratio.
3.  The compressive strength decreased with the amount of alumnium sulfate up to 0.25% and
increased with the amount of aluminium sulfate greater than 3%. The compressive strength
decreased as decreasing the amount of magnesum sulfate and gypsum and as increasing the
W/P ratio. The effect of the refractory on the compressive strength was also not significant.
With the W/P ratio of 0.34 the compressive strength was 34Kg/㎟.
4.  The 1st thermal expansion was found at the temperature near 250℃ and the steady state or
the contraction stage was found at the temperature between 250℃ and 500℃. The amount
of thermal expansion increased with decreasing the content of magnesium sulfate,
aluminium sulfate and gypsum and the W/P ratio. And the amount of thermal expansion
increased as the content of sodium phosphate and cristobalite. With the W/P ratio of 0.34 the
amount of total expansion was 1.2%.

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