No Ash Available--In Need of Aggregates

[JohnWDaileyGLE]

Today's concrete, or "Portland cement", has an aggregate (material used to glue everything together) of sand and gravel.

Ancestral, or Roman, concrete, by contrast, had an aggregate of volcanic ash. Not only does this make the concrete waterproof, it also allows the concrete to harden as it gets older. This may explain why Roman concrete had endured for two millennia.

But Portland cement has no hope of standing for that long. Sand and gravel are more porous than ash, which means water and plants can get through, and in a Life After People, it's estimated that they'd stand for only 100-150 years.

In this alternate scenario, ash is in short supply, and the cities are in need of a different kind of aggregate to keep their concrete strong. Disregarding volcanic ash, sand or gravel, which is the next best material to be used for aggregating the concrete?

[Steerpike]

My somewhat flippant answer is that there's no reason cities have to be built of concrete, or be long-lasting. If we're talking about a scenario in which we've ruled out ash, sand, and gravel as major concrete ingredients, it seems just as likely (if not more likely) to me that your city-builders just construct more ephemeral cities made out of clay, wattle & daub, cob, timber, thatch, stone, turf, etc.