Glass is transparent and strong, but the melting temperature is high and the material is brittle. PhD research by Clarissa Justino de Lima shows that it is possible to improve its properties. She developed a type of glass from phosphate and fly ash with a low melting temperature and an unusual strength.

Standard glass consists largely of silica or silicon oxide and has an incredible theoretical strength of up to 7000 MPa. But unfortunately, due to its brittleness, it achieves a practical strength of only 35 to 70 MPa. Fragility remains the Achilles heel, even in architectural applications such as glass bricks. Are there no possibilities to improve that, De Lima wondered? She experimented in the laboratory with new glass compositions, starting with phosphorus pentoxide (P2O5) and a little fly ash and blast furnace slag - waste products from incinerators and blast furnaces.

Phosphate glass is not entirely new. Advanced types of glass have been developed with phosphate since the 1950s, which reduces melting temperature and viscosity. "However, an important problem is that phosphate absorbs a lot of water," says De Lima about her experiment. "The glass produced turned out to be extremely hygroscopic." To test the quality, she submerged the produced workpiece in water for a month. The result: the glassware lost 100 percent of its weight; all phosphate had disappeared.

When she tested glass with an 85 percent potassium phosphate content, half of the mass had disappeared after a month. De Lima: "It wasn't glass anymore; it became a kind of gel." This changed when she reduced the potassium phosphate content to a percentage of 50 to 75 percent and added at least 15 percent aluminium oxide (Al2O3) to increase water resistance. She also increased the content of fly ash and blast furnace slag considerably. In this case the loss of mass turned out to be negligible.

Fly ash and blast furnace slag are widely used as additives in cement, but also prove to be very suitable for use in glass in this experiment. Their transparency only appears to be affected at a content of over 35%. At a higher percentage, the fly ash causes crystallization during cooling - which is not favourable for its strength and transparency. When the composition of the aggregates changed, De Lima also saw the colours of the glass change. She produced colourless, but also brown, blue, yellow, and green glass.

The various mixtures of phosphate glass that rolled out of the tests already had one pleasant property: a low melting temperature. It melted around 1200°C instead of 1500 to 1600°C, such as borosilicate and soda lime glass. This makes a big difference in energy costs and environmental impact. Moreover, the price per kilo is roughly three times lower than the above-mentioned types of glass. This makes it potentially an interesting product, provided the mechanical properties are sufficient.

De Lima carried out laboratory tests to determine those properties. The tests showed that the modulus of elasticity and hardness of the phosphate glass produced is lower than that of standard silicate glass, but the breakage resistance of some mixtures is higher. This may come in handy when phosphate glass is used as a construction material. Thermal stability is also high, while expansion remains limited. Some mixtures could therefore be suitable for use in 3D printers.

The lab research yielded another interesting discovery. For certain compositions, the developed phosphate glass – different from any other type of glass – can be anisotropic. "This is exceptional, because it means that it could have a large deformation in one direction, while there is only a small deformation in the other direction," explains De Lima. "This could mean that it's much stronger in one direction than in the other." Remarkably, the anisotropic glass retained its transparency. In theory, this glass allows for extraordinary properties. A construction made of this type of phosphate glass could withstand specific loads through clever orientation. Is it therefore suitable for applications in construction, for example as glass bricks or elements for building bridges? De Lima: "It could very well be. But that's a subject for further research."

Published: March 2020