"Imagine a baker opening the door of the oven and hoping that the bread, cake or   soufflé according to the new recipe won't collapse". This is how the Belgian  describes the moment of truth. Unfortunately, in spite of all the efforts made, difficult baked goods have a tendency to collapse. Sometimes the oven temperature is wrong; sometimes the humidity in the room is too low or too high; sometimes it is too cold or too hot outside; sometimes an ingredient in the recipe needs a tiny correction.
And then comes the moment when everything is just right: the soufflé is perfect! Civil engineer Elly Van Overmeire knows this roller coaster ride. That's what it was like for her during the many years she spent together with the Xella Technologie- und Forschungsgesellschaft mbh team testing the new Ytong 07 autoclaved aerated concrete. The demands are high. The new product should make it possible to comply with the thermal insulation regulations for external walls (U < 0.24 W/m²K) in Belgium with a monolithic wall with a thickness of 300 mm - such a lightweight autoclaved aerated concrete (AAC) has never existed before.

High technical requirements

But all good things take time. The success story started more than ten years ago. The European Union set new, more demanding specification for the thermal performance of external walls. The key factor for a good thermal product is the thermal conductivity or lambda value. The lower the thermal conductivity, the better the insulation. AAC with a lambda value of 0.10 W/(m∙K) conducts around 25 percent more energy than one with a lambda value of 0.080 W/(m∙K). What sounds like a minor detail to an architect has a massive impact on a family's heating budget.
That was not the only challenge. "What is required is a difficult combination of minimum compressive strength and maximum insulation value," explains Elly Van Overmeire. "A wall made of this can be monolithic and finished with an external plaster or a wall with facing bricks in a double-skin construction".
The target for the new AAC was lambda 0.07 W/(m∙K). "But our starting point was lambda 0.075 W/(m∙K). On the way to 0.07 W/(m∙K) we had to overcome many challenges. Technically, we're at the extreme end of what's possible for autoclaved aerated concrete," she explains. Like a good baker who guards her recipe, she does not reveal any details.

 
Trial and error

"We went through a phase where we thought, 'Let’s concentrate on other products for now,’" recalls Elly Van Overmeire. It was not until sometime later that a new manager brought fresh impetus to the development, this time at a Xella plant in France. "With a lot of trial and error, we further optimized a very interesting passive block variant, because the original product was not yet of the same quality as Ytong 07," recalls Elly Van Overmeire. "But by the end of the year 2020, we introduced it as an innovation project and  gradually worked our way up to the final stage of testing. The recipe was optimized right up to the last stage before it was released internally in October this year.
"We" means almost 40 international colleagues from the Xella Group. Different qualifications, united by a great deal of patience. They are production experts, but also sales colleagues who are in touch with wishes of the market. They were joined by colleagues from the Xella Technologie- und Forschungsgesellschaft mbh (T&F) and the innovation team in Germany. They all contributed their knowledge - and experience. "For example, if we are testing a new recipe at the Xella plant in Malsch, Germany, a mobile team of production experts will come and spend a week with us, constantly trying out different versions. Finding the optimal recipe for the environmental parameters then requires a gut feeling on the part of the production staff. "It is like a baker who knows that every oven is different. The engineer calls it: "managing the process". Only the dimensions are larger. In order to produce the new product in an existing AAC plant, extensive investments are required, for example in silos.

 
Final fine-tuning

In the meantime, the colleagues from Xella T&F have increased their internal requirements for the new material. National specifications vary from country to country. Even the seemingly harmless rounding of numbers becomes a new challenge. "In Germany we can round down the lambda value, but in Belgium we always have to round up, i.e., towards the value that is more difficult to achieve. To achieve the Belgian value, we had to adjust the very fine material components again."
Elly Van Overmeire and the team at the plant have worked through one task after another. Now the market introduction is being prepared. The final recipe is ready to be produced at the Ytong plant in St. Saulve, France.
A key ingredient in this recipe is persistence. "But we have plenty of that," says Elly van Overmeire, describing the motivation of the international team. "We saw this as an opportunity to offer an entirely new solution that didn't exist at Xella or any of our competitors."
Mission accomplished. AAC Ytong 07, the Belgian-French-German joint venture, is about to set a new standard in European construction.