More recently, a project sponsored by WRAP in the UK came to the same conclusions. The WRAP report can be downloaded at
http://www.wrap.org.uk/applications/glass/documents/details.rm?doc_id=489
In a test, that report says that making bricks without glass used 2825 btu/pound, while with 5 percent glass, 2249 btu/pound was needed. Energy consumption per pound is a function of scale as well as temperature, so the WRAP report may not be a reasonable comparison.
All of the studies so far do to things:
they focus on very fine (200 mesh and finer) glass, and
they treat glass as a flux.
A flux is defined in McGraw Hill Dictionary of Scientific and Technical Terms as “A substance used to promote the fusing of minerals or metals.” The definition implies that the minerals are doing the fusing, and the flux is the promoter. The promotion can be by creating better thermal conductivity between grains or acting as a catalyst of some sort.
There are several problems associated with using only 200 mesh glass. Among them are
there are no large-scale producers of 200 mesh glass
compared with mined clay, 200 mesh is expensive to make
at higher percentages, 200 mesh glass decreases the workability of the clay mixture.
Instead of treating the glass as a fine flux, what if we think of it as an aggregate and a glue? Brick manufacturers already add up to 50 percent of a combination of sand and coarse temperature-stable pieces called “grog” to reduce shrinkage, reduce water requirement, and accelerate drying.
So add glass in a gradation similar to grog and sand, at 50 percent of dry weight. The mixture forms and dries like a normal brick blend. In fact it needs somewhat less water because the glass particles absorb no water, compared with grog particles, which tend to be porous.
At room temperature, the brick clay forms and dries normally. When heated, however, at temperatures as low as 1250 F, the glass particles begin to fuse together, giving the brick body strength. To get good strength, the brick needs to be heated considerably higher, because the viscosity of the glass must be reduced enough to surround the clay particles.
But the clay particles never reach their reaction temperature. The function of the clay has been to give the clay workability and green strength at room temperatures, then to hold the brick in shape as the glass particles fuse to final strength. That’s the principle behind this project. I call it the “principle of reactive aggregate,” because the glass acts as an aggregate at low temperatures, then becomes the reactive material at higher temperatures, reversing the normal state of affairs, where the aggregate is the inert material, as it is in concrete or normal brick manufacturing.
Early in this project we sent a cover letter, sample tiles, and a summary of this principle to ceramic manufacturers in California. With two exceptions, we got no traction. In fact, on trying to visit three manufacturers to speak with them in person, we basically got the “bum’s rush” out the door. This is understandable, I suppose. If your entire career was predicated on certain chemical reactions happening at certain temperatures, and someone came along with an idea that threatened the importance of those reactions, you might respond pretty defensively as well.
One brick manufacturer gave a serious trial, but decided to focus on the finer glass only and insisted on trying to use old technology furnaces. One thing about using glass is that old-fashioned brick firing on rail cars stacked four feet high with multiple day firing schedules won’t work. Glass is more sensitive to temperature and time variation than clay, so furnaces made with lightweight insulation and good instrumentation are necessary. If the upside is saving half the energy and more than half the time, then welcome to the 21st century.
One tile manufacturer in California gave us a fair hearing. As a result of that contact, we are currently developing a monolithic glazed countertop that will be five-feet long, made from a single slab of recycled glass and ceramic, and fired in a few hours start to finish.
There will be a lot more about energy issues as the project proceeds. Now on with the testing.