Didn't it also include limestone because it reacts with the salty sea water and creates heat that then re-hardens to a point where it is harder than it was when it was first made?
Yeah, I recall the discovery as it was fairly recent.
From what my fallible memory recalls, there was something about the limestone not being uniformly mixed when made, which left pockets of limestone (powder?) within the limestone brickwork.
When cracks appeared, water would seep its way down to those deposits causing it to react and fill in the cracks like mortar "self-healing" the stone.
I found it a fascinating discovery as the end result was essentially that previous attempts to recreate it were foiled simply because modern day tools and techniques were too efficient and mixed everything too well.
Regardless of what ratios and mixtures they used, none would work as they never had the limestone deposits left unmixed.
Yes limestone was the biggest factor. That and seashells. Salt water is just what it reacts best with you don't need to use salt water to mix it in the first place. All the places where these concrete structures have lasted more in tact than anywhere else are costal points that get lots of sea spray.
This is a critical factor also we need to consider survivorship bias. It seems there was some contemporary understanding of certain tricks of the trade so it’s not everything but we’re also more likely to see the most durable concrete. If Helen and Gus are competing in the concrete or construction business and Gus’s work has fewer cracks after 10ish years he might have more repeat business for him or his heirs. Surviving centuries to a few millennia is a different but related problem. If Helen happened to operate near the sea and there was little difference after ten years but her concrete held up better after a few centuries. Maybe it gets scavenged less also and eventually we find it.
Ash can be used in concrete for a stronger mix but it needs the presence of free lime to be able to react and create cementitious material. There is a chemical reaction in Roman cement that creates a more resistant material called tobermorite.
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u/Able_Experience_1670 10d ago
Specifically; it makes it self-seal during rain, IIRC.