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.

Factory made standard cellular glass for thermal insulation is assumed to have 100% closed cells like described in the standard  EN 13167. Indeed, the mu-value is not allowed to be lower than 40000. Measurements on materials with such a high mu-value easily take 3 months, which means that this method cannot be used for daily quality control.

On the other hand, the gas – pycnometer is a nice and simple instrument to measure the volume of the closed cells. If this volume is in line with the geometric volume, we know that the cells are closed and that the mu-value will be above 40000. Because this method takes a few minutes, it is the ultimate instrument for QC in factories where open (acoustic ) and closed cell material (thermal insulation) is produced.

The following paper describes nicely the principle of the measurement.

An over pressure in the sample chamber is released into the (calibrated) reference chamber. The pressure before and after release, together with the known volume of the reference chamber allows to calculate the volume of the sample chamber. If this volume is also known, we can calculate the volume of the sample. From this sample volume, we can calculate the amount of closed / open cells.

The instrument can be quite simple like shown here under:

For about 2000€, this instrument can be built and connected to a PC. It is not clear why the above commercial instrument costs about 16000€. For larger samples, a pycnometer with compressor in situ is also available on the market like shown in this leaflet.

In Europe, we are going from huge piles of waste glass to a higher demand than available. A long time ago, manufacturers were paid absorbing waste glass but today they have to pay.

But there is still one type of glass for which the demand is in fact not existing. Indeed, since the Cathode Ray Tube (CRT) televisions are replaced by LCD (Liquid Crystal Display), the CRT´s are not remelted anymore. And as a consequence, a huge pile of glass is available for other applications.

One function of CRT glass is to absorb the X-ray radiation, generated by the high velocity electrons bombarding the front panel with phosphor. For this important function, PbO and BaO are introduced in the glass. PbO is introducing a brown color into the screen after long use. For that reason, the PbO is replaced by SrO in the front panel of the color television CRT. This and more is well explained in the following paper about CRT´s.

The source of CRT-panel glass waste seems to be very large like found in the following reports:

• The following  paper describes the worldwide situation and even mention cellular glass as a way to recycle CRT glass.

Foam glass is a lightweight and handy product that is especially used where heat and sound insulation is necessary [122]. Its use has increased in recent years due to it being nonflammable, waterproof, a good insulator and having a long
life. Foam glass is produced at temperatures of 700 °C and 900 °C [123]. In the production of these products, CRT glass components with a lower melting temperature can reduce the product’s melting temperature in some cases [12].
Although first applications were made entirely of pure glass, the rate of these industrial waste products being used has increased to 98% today [56]. In order to improve the mechanical properties of these products, different materials have
been used besides CRT [124]. Investigations have shown that the amounts of lead leaked through the glass foams are acceptable [125]. Foam glass is usually produced by adding a gas producing material to powdered glass and then baking it to trap the gas bubbles in the glass. These products are generally used as foaming agents, carbon-containing materials, organic compounds, and carbonates [126], [127]. Depending on the area to be used and the desired properties, other minerals can be added to the mixture and the mixture is then sintered. During sintering, the foaming agent content reacts to increase the volume and obtain the glass foam [128]. The foaming agent added to alter the properties of the produced glass foam varies depending on the grain size of the glass used and the cooking temperature [129].

• Another paper gives the situation in the UK for CRT glass recycling and also mentions cellular glass:

Foam glass is an insulating material which can be made from post-consumer waste glass. Experience in Norway indicates that it is feasible to incorporate at least 20% CRT panel glass in foam glass. There are no known technical  barriers to using CRT glass and no adverse environmental impacts compared with using other types of waste glass. Demand for foam glass in the UK, however, is limited and there is currently no production capability. Production facilities
are being considered, but the projected demand for CRT glass in this application is low, starting at 3,000 tonnes per annum and rising to a maximum of 9,000 tonnes per annum.

• Another  paper  mentions the recycling problems of Pb-glass from CRT and the different laws about this in Europe.
• Further, we also found equipment to separate the panel and other parts of the CRT in the following leaflet.
• A research paper  warns up for the eventual leach out of CRT-glass and so cellular glass based on this.
• Last but not least, we found a another paper about the foaming of CRT-glass with SiC and TiN.

Vacuum cellular glass is still fascinating some people, like can be found on this link of the Technical University in Freiberg, Germany. I give a citation here under:

VIsus3D

Das Ziel des Projektes liegt in der Entwicklung eines nachhaltigen, brandfesten und tragfähigen vakuum-isolierten Bauelementes (VIP), mit erheblich verbesserten Eigenschaften im Vergleich zum Stand der Technik. Dazu sollen die derzeitig für den Stützkern verwendeten Werkstoffe durch Schaumglasstrukturen substituiert werden.

Zwei Wege werden untersucht, um die Dämmeigenschaften konventioneller VIPs mit Schaumglas zu erreichen. Einerseits soll ein offenzelliges Schaumglas entwickelt werden, welches im Nachgang evakuiert wird. Andererseits ist ein geschlossenzelliges Schaumglas Ziel der Entwicklungen, welches bereits im Schäumungsprozess mit einem sehr geringen Poreninnendruck versehen wird.

Also in this case, it is mentioned to develop a recipe for open cell cellular glass, which will be evacuated later on. The evacuation has to be done to a very low pressure because the mean free path of the gas molecules must be larger than the cell size. A very nice introduction to vacuum technology, written by Leybold, can be found here.

Breathing walls are requested due to the “sick building syndrome“. This illness is attributed (without real proof) to air conditioning, out-gassing of some materials (VOC), too small intake of fresh air, mold and ozone. We may assume that depending on the person some of these products are indeed not positive for the health. Generally, there are two solutions:

• The passive housing system with air tight walls and forced ventilation
• The breathing wall system

The passive housing system is the well known. Air tight walls with a very large thermal resistance (U=0.1 W/m2K) where all the fresh air comes from forced ventilation. The intake air is heated by the air leaving the building with a heat exchanger between both flows. Due to the low heat flow, the wall may be a source of interstitial condensation, generating mold while also the heat exchanger needs regular cleaning to avoid mold. On top of that, the limited diameter of the ventilation channels is the reason of high air speed regions in the house, which is not comfortable. It is clear that passive housing is not the favorite for people suffering from the sick building syndrome.

On the other hand, the breathing wall system is solving all the above problems. In a previous post, we used another definition: Dynamic insulation. In this case, cold outside air is sucked through the wall and heated with (low value) heat, which tries to leave the building. This sucking happens with a fan and the heat is stripped of the (warm) air by a heat exchanger and returned into the building. In this way, we have a low energy building with a very low air velocity ventilation and no possibilities for interstitial condensation in the wall, while the cleaning of the heat exchanger remains. A serious disadvantage is that wind on the building creates pressure differences in the building and also outside odor comes into the building.

An Hungarian paper lists the advantages of breathing walls (or dynamic insulation) while a more technical paper treats the dynamic insulation as a heat exchanger. Theory and experiments are given to know the real efficiency of dynamic insulation.

It is clear that open cell cellular glass can play a role in this dynamic insulation but I do not believe in breathing (dynamic insulation) walls due to wind problems. However, a breathing ceiling would be a nice application. Fresh air is pumped through a ceiling, constructed with open cell cellular glass, generating a ventilation over a large surface with extremely low air velocity. Besides ventilation, the open cell cellular glass serves also as acoustic absorber. I can imagine that this a perfect system for a student restaurant at the university. Ventilation and acoustic absorption, besides food are there the main issues.