How late in the year can cement be poured?

50–60 degrees F

Below 40, the curing reaction doesn’t complete

Below freezing and the water freezes. Not good

If you are are making a cement overcoat, it probably doesn’t matter.

@josie -Thank you for your answer
How do you explain Roman cement?

Where do you live? The Romans discovered then perfected cement. The key was volcanic ash. They also gave us the arch and dome, and thus engineered the practical marvels we worship to the present day

@stanleybmanly -I live in USDA zone 6. (0 to -10 but have experienced much colder)
Roman conrete is amazing in that it cured underwater.
I’ve used volcanic ash in ceramic glaze formulation and interesting effects can be had.
(Come to think of it, these glazes hard pan easily in the cups I use to mix them)

The Romans also perfected different formulas for different uses when it came to cement and concrete. Vitruvius actually discusses different aggregates and their ideal applications.

@stanleybmanly – I wonder how these formulations were ever lost? I understand wanting to guard one’s livelihood but also wanting to pass down your secrets to future craftsman.

@lucillelucillelucille

I had to look it up, and I still don’t get it.

@josie – Made ya look! XD
It was able to cure underwater and people couldn’t figure how how they concocted it.
I went off on a tangent. :)

The “art” of concrete was lost with the collapse of Rome and cessation of civil engineering mastery in the West. The fulcrum of great architecture shifted to the Eastern realm of the empire, where the traditions were maintained and actually advanced in the glory days of Justinian, but by the time the cannons of Sultan Mehmed pulverized the walls of Constantinople, the arts of Roman building had withered to obscurity.

You do not want fresh poured concrete to freeze.
Roman concrete was great because they used pumice which is like lava rock. Very porous so the cement and the pumice would bind very well to make a very strong concrete. Most people these days use crushed or river run gravel, which is round and/or not porous, so the concrete can break much easier than the Roman stuff.

@stanleybmanly -Thank you for answering :)
@kritiper -Was it a capillary action of the pumice that made it stronger?

No, it was just the irregular, uneven surface of the pumice. Compare lava rock to round river gravel. When it is broken, the break will occur AROUND river gravel, but the Roman concrete, when broken, has to break THROUGH the rock.

@kritiper – Makes sense. Some volcanic ash glazes can be very hard.