Effect of Phosphoric Acid and Phosphate Additives on the Strength and Other Properties of Heat-Insu lators in the Vermiculite-Bond Clay System

Abstract

The reactivity of phosphoric acid and dihydrogen phosphate additives with expanded vermiculite aggregates and with bond clays at firing temperatures of 800° and 1000°C was investigated from the measurements of X-ray diffraction, water absorption and pH value. The aggregates were prepared by the calcination at 1050°C of the vermiculites from Fukushima Prefecture, Japan and from Palaboroa area, N. E. Transvaal, S. Africa. The bond clays were Mitsuishi-pyrophyllite from Okayama Prefecture and kibushi-clay from Aichi Prefecture. The effect of the additives on the compressive strength of fired insulators of the expanded vermiculite-bond clay system is discussed in connection with the above reactivity investigation. A remarkable increase in strength was observed when magnesium dihydrogen phosphate was added as an additive: The strength was about 22kg/cm2 for the test-specimens obtained by the firing at 1000°C of the mixture of 15 expanded vermiculite, 10 kibushiclay and 75 pyrophyllite by wt%, whereas the strength of about 39kg/cm2 was achieved when 1.5wt% magnesium dihydrogen phosphate was added into the mixture. This is attributed to the formation of bridge bonds due to both reactions of magnesium dihydrogen phosphate with magnesium oxide component in vermiculites and with aluminum oxide component in bond clays. Little or no change in strength was observed when aluminum dihydrogen phosphate was added. This is connected with the X-ray diffraction evidence that aluminum dihydrogen phosphate does not appreciably react with vermiculites though reacts with bond clays. The thermal conductivity shows a small decrease with increasing the content of additives. This corresponds to the porosity increase due to the dehydration condensation of the additives on firing.