Glass and concrete-without-cement as foundation?!

The great thing about building our own house is that we will get to know all the materials that we are working with. But there are so many options and things you can take into consideration for choosing all materials. We are always trying to get an optimum between price, quality and sustainability. Often we choose the most natural and renewable materials, which is maybe more expensive but has less impact on the environment. Preferable the wood we buy is grown locally, or at least in Europe for less transport pollution. We also like to pioneer in using materials which are not widely known in the building sector yet. Sometimes we choose for second-hand products or leftover stocks which will not be used otherwise. And sometimes we will just go for the cheapest options, if there are not so many other differences.

Little stools made from leftover materials

In this blog we like to highlight two of our special material choices, which is that we have used recycled glass as insulation material and concrete without cement as foundation. I hope it inspires you as well to not always follow the conventional ways but dare to pioneer as well.

Glass as foundation?

The first special thing is that we are using glass as insulation; foam glass gravel to be precise (or foam glass aggregate or granulate). Foam glass gravel is 99% made from recycled glass. Glass bottles that are no longer suitable to reuse are broken into small pieces and ground into a powder. This powder is ‘foamed’ with CO2 in a special oven at 900 °C [1]. During the baking process bubbles of gas are released, creating a network of porous cells in the material. The foam glass cake that is coming out of the oven is cracked into smaller chunks which can be used as insulation material.

We were using the brand Geocell schaumglass [2], which has manufacturing sites in Germany and Austria. Twenty-five bigbags of Geocell foam glass were delivered to our door. It looks a bit like volcanic rock, but luckily it is very lightweight (150 kg/m3). It took a lot of wheelbarrows to get all those 37,5m3 of glass into our foundation pit.

The closed cell-structure gives a thermal conductivity value of λ=0,084 W/m·K [3]. Into our pit we are putting a layer of 60 cm giving a thermal resistance of Rc=7,14 m²·K/W. As a reference the construction decrees in the Netherlands (het bouwbesluit) formulates a minimum floor insulation value of Rc=3,7 m²·K/W [4]. With our Rc-value of 7 we are reaching the passive housing design norms.

Glass foam can be used as a shallow foundation (slab on grade or ‘fundering op staal’) as it is very strong and can bear a heavy load (compressive strength is 275 kN/m2). This means that after compacting the glass foam (it has a compression factor of 1:1,3 [1]) you can use it as your bottom layer for your concrete slab. With the high insulation value you have a thermal bridge-free floor. In our case we couldn’t directly put our concrete slab on the glass foam gravel, as we are living on a very weak layer of top soil of clay and peat. Therefore we had to drive in piles to connect the load of our house to the stronger layer of sand at 7 meters below the ground. The reinforced slab is in this case connected to the piles, instead of resting on the foam glass gravel.

At the bottom of our pit we have put a drainage tube. As glass granulate has a very open structure it also has a drainage function. We extended the glass granulate floor 50 cm out of the house (so the concrete slab is smaller than the glass granulate floor). We have put PVC pipes for waste water discharge into the granulate. Also the electrical connection to the grid is there. As we don’t have any crawlspace under our house we can never change this again!

Geopolymer concrete

One thing we have fought hard for is the use of geopolymer concrete. This was quite complicated as this type of concrete didn’t have the same certifications and is not widely used in the construction industry as conventional concrete yet. The only project we heard of in the Netherlands was that of Ecodorp Boekel where they also have used geopolymer concrete as foundation. Luckily our constructor was very open to do the calculations, and together with SQAPE technology and Jansen Beton at the table we determined it was possible to use this for our house!

In conventional concrete, cement is used as binding agent for the concrete. The cement industry is one of the main producers of carbon dioxide, as it takes a lot of energy to manufacture the cement. Cement is the source of about 8% of the world’s CO2 emissions, which is huge [5].

In our case we are using RAMAC geopolymers to replace the cement, and in this way concrete is made without cement. RAMAC is an innovation from SQAPE geopolymer technology which is a company in the Netherlands. Compared to conventional concrete the production of RAMAC results in 85% CO2 reduction and a very low ECI value. Even the mechanical properties are an advantage; it has a higher density, shrinks less and has a high acid resistance. The ingredients of RAMAC concrete are produced from 100% recycled mineral waste flows; sand, gravel and alkalis. A globally patented additive makes the process controllable. Amongst other things blast-furnace-slag is used from the steel industry which is a waste product. RAMAC is a fully circular product, and much less water is used during its production process. At the end of the life cycle of this concrete everything can be used again, so a circular product. [6]

Jansen Beton is working closely with SQAPE as they are choosing to prioritise sustainable production and a circular economy [7]. The certified raw materials are thermally cleaned at their concrete plant in Son after which they are re-entering the value chain. In our case the plant had to be completely emptied so our concrete could be mixed.

The processing of the geopolymer concrete is similar as normal concrete but there are two things which should be taken into account. Firstly, because of the reaction process the RAMAC concrete has a higher acidity degree when it is being processed, it should be avoided that it gets on your skin whilst pouring the concrete. Secondly, the hardening time is very short and therefore everything needs to be processed quickly, but with good preparation and helping hands we managed to get a very nice floor.

From our left over concrete and rebars we made some very ecological little stools :). We hope you will be inspired to also search for innovative and sustainable materials to build with!


[1] Geocell Structural and Civil engineering Brochure

[2] Geocell Schaumglas GmbH

[3] Geocell Technical Data sheet

[4] Insulation values Dutch ‘Bouwbesluit’

[5] Environmental impact of concrete

[6] RAMAC concrete

[7] Jansen Beton

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