Almost a century after autoclaved aerated concrete was first developed, it has been rediscovered as the most energy-friendly construction material for both Egypt and the Netherlands. Dr Ahmed Hafez (A+BE) was even surprised about it himself. For application in Egypt, it was its thermal insulation value and low density (0.3-0.6 kg/litre) that made the difference. For the Netherlands, key factors were the minimal energy needed for production, the low CO2 emissions, the potential for reuse and relatively low wall thickness (32 cm). So why is the material, known by its trade name Ytong, still used so rarely? Hafez believes that contractors are primarily driven by low construction costs and the sector tends to stick with the materials it knows.