Abstract
The mechanism of setting expansion of phosphate-bonded investment was studied by SEM and X-ray diffraction analysis. Six kinds of investments were prepared by mixing commercial glass beads (as a base), magnesia clinker and NH4H2PO4 (as binders). The content of glass beads in the investment powders was kept constant at 80%. The ratio of magnesia clinker to NH4H2PO4 varied from 12/8 to 4/16. Three kinds of 20% colloidal silica solution were mixed with the investment powders at the L/P ratio of 0.24. The findings were as follows.
(1) The time on setting expansion decreased with increasing ratio of magnesia clinker to NH4H2PO4.
(2) The setting expansion after 2 hr showed the maximum at the ratio of magnesia clinker to NH4H2PO4 of 7/13 or 6/14. However, the X-ray diffraction data of MgNH4PO4・6H2O did not agree with this result.
(3) In spite of the same concentration of colloidal silica solution, the values of setting expansion of mixtures differ from each other. The solution at maximum setting expansion had uniform sized and spherical silica particles, and had high ability to hold water.
(4) As an initial product of setting reaction, large needle-like crystals (probably Mg(NH4)2H2(PO4)2・4H2O) were found. The shape of final product MgNH4PO4・6H2O was plate or block.
