Turning waste glass into expanded glass
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How turning post-consumer recycled glass into Poraver® contributes to complete the recycle circle

Recycling is efficient and sustainable, saves energy and natural resources. Glass is one of the very few examples where the same material can be recycled over and over again without any loss in quality. The clean color separated stuff can be turned into new bottles and jars. But what to do with the broken multi-colored bits that can not be used for recycling? There’s a company in Innisfil, Canada, with a state-of-the-art plant that can take this tiny bits and turn it into an innovative product and contributes to complete the reycle circle.

Traves Ogilvie, LEED AP and head of sales for the U.S. and Mexico for Poraver North America, is explaining the process of manufacturing Poraver® expanded glass, our functional raw material with distinctive advantages made of post-consumer recycled glass.

Crushing and grinding alone doesn’t bring waste glass processing far enough to make a functional raw material for building products manufactures or other industries. Rather than a re-engineered raw material you just end up with a resized material. Manufacturing Poraver® expanded glass is a process where crushed glass cullet is upcycled on the particle size level and re-formed into spherical-aerated-granules. The product is stable and inert with a chemical composition containing around 71% silicon dioxide (SiO2) and is used as versatile raw material and lightweight filler in different industries.

The Poraver's process of expanding glass

After learning how to expand clay for his family’s precast concrete business in the Bavarian region of Germany in the ’60s and ’70s, Hans Veit Dennert upped his game in the early ’80s when he discovered how to expand glass. Subsequently, in 1983 he founded Poraver GmbH and today the largest producer of expanded glass aggregates worldwide. He still presides over the company today.

The inhouse developed process begins with a very fine gradation of post-consumer recycled glass. The cullet is dried and sent to the ball mill where it is ground down to very fine flour (< 36 microns). The next step is to mix the glass flour with a silicate binder and an expanding agent and blended into a viscous slurry. For larger grain sizes above 1 mm the semidry material is applied to an inverted rotating granulating dish. The slurry then is formed into spherical-shaped grains. The size of the grains can be altered by changing the inversion angle or the rotational speed. To make grain sizes below 1 mm a spray dryer is used.

After the granulating process the spheres go into a rotary kiln that’s heated to more than 1,400 degrees F. In the kiln the expand to 200 % volume. Then the granules are cooled, sieved and packaged into seven different grain sizes ranging from 0.04 mm to 4 mm.

How to use Poraver® expanded glass in concrete

First match the particle size of your existing fine sand with the corresponding expanded glass grain size. For face mixes, self-consolidating concrete and sprayable mixes, the Poraver® grain size is typically 0.25 - 0.5 mm. For gravity-cast mixes, architectural and structural precast, the grain size is typically 0.25 - 1 mm. If there are larger aggregates in the mix, you can replace some (or all of them) with grain size 1 - 2.5 mm. Finally, you must decide how much you’re going to replace. This ratio is driven by the concrete’s mechanical properties and performance requirements.

A key component of reformulating with expanded glass is to replace the sand in the mix by volume. This is because expanded glass has a specific gravity value less than 0.550. Most sands and aggregates have specific gravity values north of 2.5. Thus replacement by volume is key as expanded glass is nearly five times lighter than silica sand.

Once you’ve decided on a replacement ratio, you’re ready to mix and cast. Poraver® expanded glass uptakes water at a rate of 20 - 30 % mass. One hundred pounds of dry expanded glass will absorb 20 to 30 pounds of water. This is actually minimal absorption when you consider that 100 pounds of Poraver® is 7 cubic feet, or a quarter yard, of material. The water that weeps into the expanded grains begrudgingly weeps back out of them. This water remains available, within the matrix, for cement hydration. Expanded glass works as an internal curing agent extending the dehydration time of concrete and dry mortar. This is due to the initial water absorption followed by a slow return of the mix water to the matrix from within the expanded glass granules.

Mixing methods

The first method is to take 20 % of the calculated mix water and pre-hydrate the expanded glass with it. Absorption takes only a few seconds. The Poraver® grains can then be blended with other aggregates and the mixing process can proceed as usual. The second method is to run through the mixing process as usual and introduce the quenched EG after the cement, sand, aggregates, water and additives are blended. This method is most commonly used in ready-mix and structural pre-cast concrete operations.

Glass aggregates in concrete are sometimes associated with the problem of alkali-silica reaction (ASR). Alkalis, present in portland cement, that encounter “reactive aggregates” initiate a chemical reaction that forms a gel that expands when hydrated. Signs of ASR are only seen in compact concrete. With expanded glass as an admixture, you create porous concrete. In these enhanced concretes the swelling pressure caused by the ASR gels is dissipated through the pores. No hairline cracks, edge spalling, drops of gel or other peculiarities appear. The positive effect of Poraver against ASR is officially tested confirmed.

By using Poraver® as a lightweight replacement for sand and aggregate, distinct benefits can be obtained

  • Performance benefits – Including extreme density reduction, improved thermal and acoustic absorption values, protection from ASR, and lowering the e-modulus which makes the concrete less brittle.
  • Health benefits – Including reducing crystalline silica content of the overall matrix since Poraver® contains no crystalline silica. Using Poraver® can help concrete product manufacturers conform to the new OSHA regulations regarding airborne crystalline silica.
  • Environmental benefits – Poraver® expanded glass is a sustainable aggregate and is considered 100% recycled content. It helps products and projects earn LEED points.

Poraver® expanded glass is widely available throughout the United States. It’s packaged in various sizes and is also available in bulk for silo storage. If you would like to test Poraver® expanded glass for your applications, just request a free sample here.

Contact the author:
Director of Sales USA, Mexico
E-mail: [email protected]

Traves W. Ogilvie, LEED AP, is Director of Sales for the U.S. and Mexico for Poraver North America. With nearly 30 years of experience in the commercial construction and building products industry, Traves has a broad knowledge of all types of composite materials from concretes and mortars to thermoset FRP and polymer concrete. As a LEED accredited professional, he has worked extensively with the USGBC to quantify how recycled content contributes toward LEED points. Today, Traves travels the country as an expanded glass formulation expert in GFRC, concrete, FRP and polymer concrete. He can be reached at [email protected]

This article was also published in the magazine concreteDECOR.