Was the Roman knowledge of how to build aqueducts lost?

The knowledge of how to build aqueducts was not lost. See Vitruvius and Frontinus among other texts. Some few techniques that were more practical knowledge among workers might not have been written down, though.

Musk is totally wrong here. The real reason people after the Romans didn't build aqueducts is not that they couldn't. It's because if you don't build cities with populations in the hundreds of thousands in semi-arid areas, you don't need aqueducts to supply them with water.

If you're an Egyptian and your religion no longer requires you to build gigantic tombs for your dead Pharoahs, you don't build them. Not because you CAN'T build them, but because you have better things to do. For instance, you might construct an impressive lighthouse at your main harbor: Lighthouse of Alexandria. Likewise, if you're a later Roman, you might build an impressive Christian church called Hagia Sophia rather than an aqueduct.

PS: Perhaps it should also be noted that when we build similar things today (or in times past), we usually don't do it the same way the Romans or Egyptians did, because we have invented better ways and have different purposes. For instance, the California Aqueduct is far larger than anything the Romans built. (And is only a small part of the California water system.)

Nor is this just a modern thing. Consider the French Canal du Midi, built in the 17th century, a project the Romans thought of but never did. Likewise the canal systems of Britain & other European countries were much larger in scale than the Roman aqueducts, but constructed for different purposes.

The Romans built the aqueducts -- as well as bridges, piers, and colossal buildings -- out of concrete. Stone and brick were usually just exterior casings for the concrete structural core. And the secret of super-durable Roman concrete was indeed lost for centuries.

Modern concrete uses a paste mixture of water and Portland cement (a fine powder made from limestone & clay) to bind together sand and/or small stones. However, it degrades within decades. Roman concrete used a mix of volcanic ash and lime for their cement - a fact rediscovered only recently. This mixture is stronger than modern cement and lasts much longer. In addition, the source of the volcanic ash matters to the performance of the concrete. So does the environment in which the concrete is used.

In the case of concrete piers, the secrets of Roman concrete durability is all about seawater interacting with the ash. Volcanic ash commonly contains a silicate mineral called phillipsite, which contains aluminum. Concrete containing this mineral, when immersed in seawater, becomes more alkaline, and this in turn appears to cause microscopic aluminium tobermorite crystals - a rare mineral - to grow from the phillipsite. The aluminum crystals flex under stress and wave action instead of cracking. And the longer the concrete sits under seawater, the more of this aluminum there is to resist stress and make the concrete stronger. The same process also self-heals small cracks before they become large.

See this Nature article for more information - https://www.nature.com/news/seawater-is-the-secret-to-long-lasting-roman-concrete-1.22231 .

For freshwater aqueduct construction, the concrete chemistry is less clear. We know from Roman repair records that aqueduct repairs were made with concrete and then water was allowed to return to the pipeline before the concrete cured, which implies it cured under water. Modern concrete made with Portland cement can also cure under water, but if too much water enters the cement, it will be weakened. This was not the case with Roman concrete. Perhaps the use of volcanic ash made the concrete sufficiently more durable than Portland cement for aqueduct use, even in the absence of seawater?

If anyone can point to some research on this point, I'd love to hear more.