Why Roman concrete is still stronger than RAAC (and other modern concretes)

One of the perplexing aspects of the scandal about ailing concrete panels in British schools and other public buildings is that these components were only ever expected to perform for a few decades before they needed repair or replacement. The porous nature of the reinforced autoclaved aerated concrete (RAAC) at the heart of the problem makes it particularly vulnerable to deterioration, because water can more readily infiltrate and corrode the steel reinforcement. But even the best structures made from Portland cement concrete, the most common variety in use worldwide, are not expected to last longer than a century before needing maintenance.

That might be adequate given the typical rate of construction turnover in many modern cities. But it seems pitiful compared to the longevity of some of the structures made by the Romans. The concrete dome of the Pantheon in Rome remains stable enough for visitors to walk beneath, and some Roman harbours have underwater concrete elements that have not been repaired for two millennia – even though they are in regions often shaken by earthquakes.

Whence this remarkable resilience of Roman concrete architecture? It’s all down to the chemistry. The key component of mortars and cements is lime: calcium oxide, made by heating a calcium carbonate mineral such as limestone to drive off CO₂. Add water and you have slaked lime Ca(OH)₂, which was typically mixed as a paste with sand or rubble (aggregate) to make concrete. The hydroxide gradually absorbs CO₂ from the air and reforms a hard binder of calcium carbonate.