Assume you have a large pile of sand, sodaash, limestone and clay. What should we choose to produce with these materials? Cellular glass, a closed cell structure or cellular concrete, an open cell structure. Cellular glass has a higher chemical resistance against water than cellular concrete, is vapour tight and is expected to be stronger in tension and so also in compression than concrete. In the case of cellular concrete, the sand particles are bound with cement while in the cellular glass case, the glass particles are fused into each other, creating a better bond.
Cellular concrete or AAC (Autoclaved Aerated Concrete) can today produced in higher densities (Ytong) and lower densities (Multipor). I found two Environmental Product Declarations, which are describing the process. The Xella Multipor EPD describes the low density (115 kg/m³) process while AKG GAZBETON describes the process for higher densities (385 kg/m³).
For the high density, we read:
The autoclaved aerated concrete products are made of quartzite (40-50%), Portland cement (20-30%), lime (6-12%), gypsum (5-10%), aluminium (0.1-0.2%) and finally recycled waste slurry (closed-loop) (15-20%). In addition, water content of the mix is about 40-50% of the total mixture.
For the low density, we observe:
In all cases, sand, lime (CaO) and gypsum (CaSO4) are bound with cement. The lime is produced by burning limestone at 800°C. Cement is produced from burning a mixture of limestone and clay at 1400°C and fine grinding the residue to a very fine powder. The foaming is done with Al powder and afterwards, autoclaving happens at 180°C and 12 bar to give the foam its strength. In both cases, a lot of water (more than 50%) is heated and evaporated. It is clear that the complete process, including the preparation of the “raw materials” is very energy intensive for the production of AAC.
On the other hand, we could melt a soda lime glass at 1600°C with sand, sodaash, calcium sulphate and limestone. This glass can be ground to a fine powder and foamed above 800°C with carbon black or glycerin to a closed cell structure or with (fine) limestone to an open cell foam. However, the melting and foaming process is more energy intensive than the AAC process and also the investments are a lot larger due to the high temperatures used. At the end, cellular glass based on fresh raw materials is more expensive. It makes only sense to use this type of cellular glass instead of AAC in case closed cells are a must. This is typically the case in flat roofs and industrial insulation.
But if a large pile of waste glass is ready to be recycled, it is clear that this glass has to be foamed to cellular glass with closed or open cells with only one temperature step at 800°C instead of producing cellular glass or cellular concrete with fresh raw materials like sand and limestone with multiple temperature steps. This third option is introduced by GLAPOR in the market and it is easily understood that this option is growing with two digits for ecologic and economic reasons. Especially the current possibility of boards up to 3.2 x 1.5m with compressive strength up to 3000 kPa is responsible for the succes.