Almost 37 years ago, a report about the production of foam glass from waste glass was published. This report describes three foaming systems:
- After water absorption by the waste glass powder
- After mixing the waste glass powder with carbon (black)
- After mixing the waste glass with milled limestone
The report learns that up to 6% water can be absorbed in waste glass by using an autoclave and the addition of NaOH in the water. This glass can be foamed but only small pellets are possible. The glass with 6% water has a low viscosity and during reheating, we have steam as the driving gas for the foaming. When the water has left the glass, the viscosity increases and the foam freezes.
But also carbon black can be mixed to the waste glass. In this case, also bentonite is added to able to form green pieces. This clay also seems to reduce the water absorption of the foam. Between the carbon blacks we find exotic ones and the ASTM defined ones, produced mainly in China. The paper assumes that the absorbed gas in the carbon is inducing the foaming, while we know that the sulfate in the glass is also important (Demidovich p12). Last but not least, the importance of a reducing atmosphere is mentioned.
CaCO3 was also mixed with the glass together with again bentonite. It was found that milled limestone gives the best results and larger dimensions are possible. A thermal conductivity as low as 0.052 W/mK is reported with this recipe at 160 kg/m³. Controlling temperature and foaming agent allows to foam a closed cell structure.
The report is in favor for the carbon black method but gives only thermal conductivity measurements for the CaCO3 system. Personally, I think that this last system deserves more attention.