Everything involves luck, but until very recently (past 100 years) mortar maker was an expert occupation with thousands of years of accumulated art and science.

Everything involves luck, but until very recently (past 100 years) mortar maker was an expert occupation with thousands of years of accumulated art and science. These days most professional concrete and foundation contractors don't have a clue about mixing or pouring anything other than portland cement and whatever is most popular at Home Depot. Who knows how much knowledge has been lost, but my guess is that the Romans developed their processes much more deliberately than haphazardly. You don't have to understand the 21st century chemistry of why something works in order to build a systematic and even scientific methodology. It was part and parcel of a mortar makers job to understand how to make use of regional materials and handle local environmental conditions.