It can be interesting to find out how people arrive at your blog. The blog  “And the lightest beam is … cellular glass “, published in January 2018 is getting a lot of attention with an exponential growth.

Michael Ashby has already mentioned that a glass foam would be the most interesting self supporting material but this statement had less value with monolithic boards measuring maximum 60 x 45 cm. However, since GLAPOR introduced the large monolithic cellular glass boards 280 x 120 cm, produced with the modern continuous foaming method, this statement became very important.

Indeed, the following graph shows the number of the specific blog visits since publication in 2018.

It is also clear that besides the large dimensions GLAPOR cellular glass has the most interesting recipe for this application. Indeed, GLAPOR foams directly recycled glass without melting a special composition. This recipe is interesting from the ecologic and economic point of view.

Float glass technology is quite attractive for cellular glass and writing a blog is good method to think about the advantages and disadvantages. I spent already a blog on a patent and on a paper, which showed that the powder method could be history. One major question is why the flat glass world converted to float glass technology?

Before the float glass process became developed, quality glass needed to be ground and polished to get a smooth surface. Like described in this patent of Pilkington, introducing the hot glass on a molten metal bath with temperature treatment delivered a plan parallel glass pane, which could be used without grinding and polishing. The non-interaction between 100% flat molten metal and molten glass is the main reason for this huge development. However, a fire finished glass surface is not needed for cellular glass.

Like shown in the previous blogs, the tin bath is suggested only to be used as transport carrier during the foaming of the glass. This is a much more expensive solution than the a belt which turns around within the furnace without leaving and so reheating. A glass fleece can be used to avoid sticking of the foam on the steel belt.

The more I think about using a tin bath, I see a lot of  advantages evaporating if compared with a steel belt, turning around strictly inside the furnace. Especially the previous paper, which gives an an alternative to the powder method, could be much easier executed by putting the ribbon on a belt,  instead on a tin bath. But on the other hand, the float process could become a 100% non-waste process because the ribbon should be perfectly flat and plan parallel. If the non-powder method on a belt would be the second generation, on a tin bath would it become the third generation process. I have the feeling that after almost 90 years cellular glass, we are just at the beginning.

2018 was indeed a fruitful year for the development of cellular glass.  In a previous post, I already declared the Aalborg – Ljubljana team as the one who made the largest progress during the last year with consequences on the market the next years.

But yesterday, I found a new paper, published in 2018 with an extremely important experimental fact. This paper proves that it is possible to produce a glass foam with an homogeneous and CLOSED cell structure with only a melting step at 1100°C without grinding and sintering process. I give here under the abstract:

Glass foams are being widely used as constructional materials due to their unique properties in thermal insulation, fire retardation, and shockwave absorption. However, the cost of energy consumption and processes in a conventional glass foam production limited the use of glass foams as sustainable materials. In this study,
for the very first time, thermally tunable CaO−SnO2−P2O5−SiO2 glass foams with controllable pore size were presented as a novel category of melt-casting and float-manufacturable glasses. It was found that the pore size and thermal properties become tunable by manipulating the glass network, i.e., connecting linear chained Sn−P
network with [SiO4] units. In addition, the unique combination of thermal properties and porous structure of CaO−SnO2−P2O5−SiO2 glasses shows potential in float glass foam production, which can produce glass foams sheet-by-sheet with less complexity in
manufacturing processes.

This means that this glass foam can be formed with the floating method on a tin bath, generally know as the float glass process, invented by Sir Alistair Pilkington.

This foam should have NOX gases in the cells but density, thermal conductivity and compressive strength are not given. The authors claims that this will be the route to low cost cellular glass. I see the following positive and negative points.

• By working with top rollers like for float glass, it could be possible to flatten the cells and to tune the couple thermal conductivity / compressive strength. 
• There are no belts or molds which have to be replaced, molten tin is doing this. 
• There is one heating step to 1100°C to be compared with a heating to 800°C after energy intensive grinding. 
• The glass composition with phosphates and tin oxide is probably costly.
• It is not clear that replacing tin is less costly than replacing a steel belt or molds.
• A lot of (expensive) heat has to be extracted at the bottom of the tin bath to avoid that molten tin hits the steel casing. 
• It is not clear how we keep the same temperature under and above the foam because heating the molten tin is not needed for float glass. 
• A typical float glass line costs typically 130 000 000€ while a low cost cellular glass production line, based on direct foaming of waste glass costs less than 20 000 000€.

Nevertheless, this invention is of major importance especially when the waste glass becomes less available and in that way more expensive. In fact, we could speak about the continuous foaming second generation and it will be a major challenge to develop this new process. Alistair is dead, long live Alistair.

In a very short time, I see that phosphates are introduced in the cellular glass world. The first time as a crystallization inhibitor, the second time now for a “grinding-sintering free” cellular glass process.

In this blog, we have written about boards, gravel and boards made of foamed glass granulate. But up to now, we never discussed the foamed glass granulate itself. Foamed glass granulate is produced in a rotary furnace. I guess that white granulates  are foamed by using white sodalime glass with Calciumcarbonate as foaming agent. The following picture shows a rotary furnace from Liaver.

The Genius group, an organization which focuses on innovation, uses foamed glass granulate for fire protection and to absorb liquids. I give hereunder a citation from their website

• PyroBubbles^® have been positively tested by the MPA Dresden (in compliance with DIN EN 3-7) as an extinguishing agent for solid and liquid flammable materials (Fire Classes A, B, D and F).
• PyroBubbles^® are the ideal filler for storing and transporting lithium-ion batteries (UN3480, UN3090) and are lighter than sand (approximately 235 kg/m³).
  PyroBubbles^® consist mainly of silicon dioxide with an average grain size of 0.5 to 5 mm.
• PyroBubbles^® absorb electrolytes (BAM tested).
• PyroBubbles^® have a low thermal conductivity and electrical conductivity and are electrically insulating.
• PyroBubbles^® are heat resistant to approx. 1050°C. At higher temperatures they begin to melt and form a closed and thermally insulating layer around the source of the fire.
• PyroBubbles^® float on the surface of the liquid and are particularly well suited for fighting flammable liquid fires, independent of polarity.
• PyroBubbles^® can be collected and to a large extent be reused after every application.
• PyroBubbles^® are hydrophobic and resistant to aging.
• PyroBubbles^® comprise porous hollow glass granules and are classified as Class A1 building materials (DIN 4102 and EN13501); they feature excellent processing capabilities.
• PyroBubbles^® require no maintenance and thus generate low maintenance costs.

This is clearly a new approach to clean up (industrial) disasters with waste glass and it can be recovered to be used again. In the datasheet, they mention a thermal conductivity and acoustic absorption. These properties are important for boards produced with these granulates like already mentioned in a previous post.

In a previous post, we have already written about the mechanical stability of cellular glass. We even have discussed the static fatigue limit of glass and so cellular glass. These posts, based on acoustic emission experiments and self organized criticality (1988) take into account the interaction between the different cells. In case a cell breaks, the load is redistributed over the neighbor cells.

In a 1981 paper of the NASA, the mechanical stability of cellular glass is experimentally and theoretically studied. I give the abstract in the following:

Cellular glasses are prime candidate materials for the structural substrate of mirrored glass for solar concentrator reflecting panels. These materials possess properties desirable for this application such as high stiffness to weight ratio, dimensional stability, projected low cost in mass production and, importantly, a close match in thermal expansion coefficient with that of the mirror glass. These materials are brittle, however, and susceptible to mechanical failure from slow crack growth caused by a stress corrosion mechanism.
This report details the results of one part of a program established to develop improved cellular glasses and to characterize the behavior of these and commercially available materials. Commercial and developmental cellular glasses were tested and analyzed using standard testing techniques and models developed from linear fracture mechanics. Two models describing the fracture behavior of these materials are developed. Slow crack growth behavior in cellular glass was found to be more complex than that encountered in dense glasses or ceramics. The crack velocity was found to be strongly dependent upon water vapor transport to the tin of the moving crack. The existence of a
static fatigue limit was not conclusively established, however, it is speculated that slow crack growth behavior in Region I may be slower, by orders of magnitude, than that found in dense glasses.

The motivation of NASA was already described in a previous post and it even already applied to replace concrete. On top of that, cellular glass and glass have the same thermal expansion coefficient if the foam is made from the same glass. This makes cellular glass as the preferred support for mirrors.

The mechanical stability was measured by measuring the velocity of a single crack under a load. In a previous post, we described how the acoustic emisson detects the interaction of many micro cracks. In fact, these microcracks are a complex system inducing small and large events. We prefer this method and not the study of a single macroscopic crack. The acoustic emission technique allows to measure the static fatigue limit of cellular glass.

Cellular glass according to EN 13167 “Thermal insulation products for buildings – Factory made cellular glass (CG) products – Specification” needs to have a water vapor diffusion resistance factor µ of at least 40000. This property has to be measured according to EN 12086:2013 “Determination of water vapour transmission properties”. This is a difficult way to state that cellular glass according to EN13167 has to have 100% closed cells.

The µ-value is expressed as the ratio to the water vapor transport properties in air. µ=40000 for a material means that the water vapour is diffusing 40000 times slower in the material than in air. Like shown in the following, this is hard to measure for two reasons:

• The seal between the cup has to “vapor tight” but not one flexibe material has a µ-value like cellular glass. This induces always an error.
• The weight changes of the samples are extremely low, which means that the weight balance has to be stable and reproducible.
• The EN standard also request that the system is in a stationary state.

Indeed, for a sample of 40 mm thickness and a diameter of 100 mm we get a transmission of 0.004 mg/h if the dry cup is placed in 50% humidity. To have a certain accuracy (10%), the total weight difference needs to be about 10mg (we measure with 1mg error) which means we need to wait 10/0.004 = 2500 hours or about 100 days. It is clear that this test takes a long time and cannot be used for daily quality control.

Therefore, we suggest to measure daily the amount of closed cells with a pycnometer, like described in a previous post.

GLAVEL as a name for cellular glass gravel is already genial, it says everything in one word. The mission of this company is surprising, contrary to what I am used in the USA.

Foam glass gravel is a highly stable and versatile product that has been successfully used in building and infrastructure construction in Europe for over twenty-five years. Our mission is to bring this product to the construction industry in North America.

Although a lot of American companies import European technology, most of them never state this explicit. But the Glavel management is different and also clearly ecologic minded about the climate change.

Clean Tech Block is a project from  Gråsten Teglværk, the University of Aalborg and the University of Ljubljana. A few posts were already written about Aalborg and in fact, they collaborate with Ljubljana. I had the occasion to met very interesting people: Y. Yue and Martin B. Østergaard in Aalborg and Jakob König in Ljubljana.

Prof. Y. Yue

Martin B. Ostergaard

Jakob König

The complete team proud about their invention

This team did what I believed for a long time was impossible. They managed to foam directly waste glass (without melting) with closed cells, filled with CO2 with a minor content of H2. In my opinion, this is the best progress in cellular glass land in the last five years. In this way, cellular glass with 0.040 W/mK and 600 kPa compressive strength based on waste glass without remelting becomes possible at prices of about 150€/m³. They want to use this cellular glass between bricks to able to build houses like explained here under. It is a Google translation from Danish to English.

In a previous post, we mentioned already PhD work on cellular glass and also one  on foaming of CRT-glass.  However, the thesis in this post has a lot of attention for the methodology of searching for a new glass foaming system. Hereunder the abstract of the thesis is given.

Exponential growth of papers on cellular glass

Foam glass has been used for over 70 years in construction and industry for thermal insulation. Foam glass is mainly made of recycle glass. Strict energy policy motivates foam glass manufactures to improve the thermal insulation of foam glass. The effort to understand the making of foam glass with good insulation ability is scarcely reported. The goal of this Ph.D. thesis is to reveal the underlying mechanism of foaming reaction, foam growth and the heat transport of solid foam glass. In this thesis, the panel glass from cathode ray tubes (CRT) will serve as a key material to reveal the mechanisms.

Foaming is commonly achieved by adding metal oxides or metal carbonates (foaming agents) to glass powder. At elevated temperature, the glass melt becomes viscous and the foaming agents decompose or react to form gas, resulting in foamy glass melt. Subsequent cooling to room temperature, lead to solid foam glass. Metal carbonates decompose due to surface reaction. Based on Na2CO3, we show the reaction is fast and the glass transition is changed considerably. We propose the reaction rate is dependent on contact area between glass melt and Na2CO3, melt viscosity and Na+ diffusion.

A method is developed for optimising process parameters. Characteristic temperatures are derived from a deformation curve and the deformation rate curve. Maximum expansion rate was linked to closed porosity. Using this knowledge the method is applied to literature data to analyse for optimal conditions. The resulting conditions were in agreement with industrial conditions. Since no foam glass properties are necessary to measure, the method allows fast investigation of process parameters.

The melt viscosity is an important parameter for foam growth. We compared bubble- and crystal free melt viscosity with foam density and show in order to minimise the foam density, the heat-treatment should be performed in the viscosity regime of 103.7-106 Pa s.

The thermal conductivity of foam glass made is often reported to be linear dependent on porosity or foam density. Foam glasses made from CRT panel glass and different foaming agents confirm this trend at high porosity level (85-97%). The experimental data suggests the solid conductivity is dependent on the foaming agent applied.

The research is part of a project to build “passive houses” where the cellular glass is a structural element and not only thermal insulation. In fact, the cellular glass takes all the load while thin bricks are just creating a standard house look. Houses built from cellular glass was already mentioned in a previous post.

Werner Sobek is a German famous architect and structural engineer. I cite a part of the Wikipedia link.

Werner Sobek^[1] was born 1953 in Aalen, Germany. From 1974 to 1980, he studied structural engineering and architecture at the University of Stuttgart. From 1980 to 1986, he was post-graduate fellow in the research project ‘Wide-Span Lightweight Structures’ at the University of Stuttgart and finished his PhD 1987 in structural engineering. In 1983, Sobek won the Fazlur Khan International Fellowship from the SOM Foundation.

In 1991, he became a professor at the University of Hanover (successor to Bernd Tokarz) and director of the Institute for Structural Design and Building Methods. In 1992 he founded his own company Werner Sobek which now has offices  in Stuttgart, Frankfurt, London, Moscow, New York, and Dubai. The company has over 200 employees and works with all types of structures and materials. Its core areas of expertise are lightweight construction, high-rise construction, façade design, special constructions made from steel, glass, titanium, fabric and wood, as well as the design of sustainable buildings.^[2]

Since 1994, he has been a professor at the University of Stuttgart (successor to Frei Otto) and director of the Institute for Lightweight Structures and of the Central Laboratory for Structural Engineering. In 2000, he took over the chair of Jörg Schlaich, and fused the Institute for Lightweight Structures and the Institute for Construction and Design into the Institute for Lightweight Structures and Conceptual Design (ILEK). Both in its research and teaching, the ILEK at the University of Stuttgart unites the aspect of design that is dominant in architecture with the focus on analysis and construction from structural engineering as well as materials science. On the basis of a goal-oriented and interdisciplinary approach, the institute is concerned with the conceptual development of all types of construction and load-bearing structures, using all types of materials. The areas of focus span construction with textiles and glass all the way to new structures in reinforced and prestressed concrete. From the individual details to the whole structure, the approach focuses on the optimisation of form and construction with respect to material and energy use, durability and reliability, recyclability and environmental sustainability. The results of this work are published in the bilingual (German/English) serial from the institute (IL) or published individually in special research reports on particular topics.^[3]   

I got a presentation  of his view on the future building world and was surprised by his preference for cellular materials where possible. These materials have to be by preference recycled materials. It is clear that cellular recycled glass, foamed with a minumum of energy in large boards is exactly what he is looking for. Indeed, a simple structure where this material is a structural element AND thermal insulation allows easy recycling later on. The figure on the right is exactly how we should NOT work.