Sulfur Concrete – Que?

We’ve introduced Dr. Marwa Al-Ansary to you as a civil and environmental engineer, working for Shell on (amongst others) sulfur utilization in materials such as Thiocrete (a type of sulfur concrete).

But what is sulfur concrete really, and why do we care about this? As with many questions we might have, Wikipedia has an answer to that:

Sulfur concrete is a composite construction material, composed of sulfur, aggregate (generally a coarse aggregate made of gravel or crushed rocks and a fine aggregate such as sand). Cement (commonly Portland cement) and water important compounds in normal concrete are not part of sulfur concrete. The concrete is heated above the melting point of sulfur ca. 140°C. After cooling the concrete reaches a high strength, not needing a prolonged curing like normal concrete. Sulfur concrete is resistant to some compounds like acids which attack normal concrete. Sulfur concrete was developed and promoted as building material to get rid of large amounts of stored sulfur produced by hydrodesulfurization of gas and oil. Sulfur concrete is also a possible building material for a lunar base. Up to 2011 Sulfur concrete is only used in small quantities when fast curing or acid resistance is necessary.

Sadly enough, Wikipedia is using a lot of jargon in this explanation. The explanation focuses on the following topics:

What ingredients do we need for sulfur concrete?

Regular concrete is a combination of sand, stones, cement and water. For those of you that like to DIY around in the backyard, you might be familiar with the 1-2-3 and 1-2-4 recipes of throwing it all together and making fresh concrete. Sulfur concrete on the other hand is made without water, and without cement: it’s simply sulfur, sand and stones.

How do we make sulfur concrete?

Unlike regular concrete, in which you mix everything together, get a liquid product and then wait, sulfur concrete is made by heating the sulfur product with the stones and sand such that they get glued together. This process is similar to making asphalt. Upon cooling, the product has its final strength, and you don’t need to wait like with regular concrete, which reaches its strength over time.

What are the benefits of sulfur concrete as compared to normal concrete?

Sulfur concrete is made without water and cement. As a result, the costs and energy for water supply becomes obsolete. Moreover, the carbon footprint of the product is reduced. Regular concrete leaves a large carbon footprint because the production of cement requires very high (1450^oC) temperatures and the process itself is responsible for 5% of all CO₂ emission worldwide.
In regular concrete, the cement forms “tubes” inside the material which can take in water. Upon freezing, this water expands and the inner pressure can become so high that the concrete cracks. Sulfur concrete does not have these tubes, and therefore performs better in freeze-thaw cycles.
As sulfur concrete is made by heating the components, it can also be recycled by crushing, reheating and remolding, reducing the waste associated with regular concrete construction.
If you’ve made regular concrete before, you’ve seen that at first it is almost liquid, and then it starts building up its strength over time (we typically test it after 28 days to see how strong it is). Sulfur concrete gains it strength right when it is made – upon cooling down the product has its final strength.

Why is Shell making concrete?

Sulfur is a byproduct from the process of refining gas and oil. To look for ways to implement this product, Shell stimulated research into sulfur concrete, using a sulfur-product to replace part of the bitumen in asphalt and innovative fertilizers for agriculture.

Is it a new invention?

Sulfur concrete has been around since the 1970s. However, because the cost of the modification of sulfur for use in concrete used to be very high, the range of applications was rather small. Now, with the development of the competitively-priced sulfur-concrete binder, the product can finally compete with regular concrete in price, and a wide range of possibilities opens up.

What’s the caveat?

As you can recycle sulfur concrete by heating it up to 135^oC, it is not suitable for applications which require temperatures of above 100^oC. Therefore, building with this product will require the right provisions such that in the case of a fire, the structure is safe (and doesn’t “melt down”).