Abstract
Phosphate geopolymers are part of chemically bonded phosphate cements obtained from an aluminosilicate and phosphate solution. Their structure consisting of phosphate bonds makes them suitable for use as refractory material. This study deals with the influence of phosphoric acid concentration (6, 8 and 10 mol/L) on the stability of volcanic ash-based phosphate geopolymers exposed to 100, 600 and 1000 °C. The results reveal that the onset of crystallization is about 600 °C with the formation of aluminum phosphate (V) and tridymite, then crystallization of iron (III) phosphate (V) and hematite at 1000 °C. The degree of crystallization of these phases increases with phosphoric acid concentration. The geopolymers obtained with 8 mol/L of phosphoric acid showed the best thermal stability at 1000 °C in terms of compressive strength change. The maximum thermal linear shrinkage recorded was 3%. The major phases of all geopolymers remain stable up to 1000 °C, after which the melting of phases happens.
Highlights
Geopolymers are known as 3D network inorganic polymer obtained by chemical reaction of a solid precursor and an alkaline or acidic medium [1]
It was shown that volcanic ash based-geopolymers obtained with potassium silicate as alkaline solution develops superior stability at elevated temperatures with thermal shrinkage less than 3% compared to the ones with sodium silicate [11]
Few studies deal with the phase stability or physical properties of geopolymers obtained in an acidic medium after exposure at elevated temperatures
Summary
Geopolymers are known as 3D network inorganic polymer obtained by chemical reaction of a solid precursor (aluminosilicate) and an alkaline or acidic medium [1]. When exposed to elevated temperatures the physical properties of geopolymers obtained in alkaline solution are significantly deteriorated in the range of 750–900 oC [12, 13] This is due to cations present in the alkaline solution which lower the temperature of glass transition, induce sintering with the decrease of the mechanical strength along with severe thermal shrinkage [8, 11]. Few studies deal with the phase stability or physical properties of geopolymers obtained in an acidic medium after exposure at elevated temperatures
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