You don't need machines, but the machines and plentiful energy make the process cheap.
You can either use labor to crush your aggregate, or use labor to lay your concrete more like brickwork using larger rocks. The latter is what the Romans did, and I'd say that the total labor needed is less that way.
Roman style cement uses calcined limestone, for which 900C or so is good. Going higher actually makes it less reactive, and this was a problem with ancient techniques - they were hard to control so if 100% of your lime was burned, odds were high that parts of it were overburned. The process was also only 20% or so fuel efficient.
You want your cement to be finely powdered to make it reactive. Burnt lime is pretty soft, though, and I don't think the volcanic rock Romans used was all that tough either.
You can either use labor to crush your aggregate, or use labor to lay your concrete more like brickwork using larger rocks. The latter is what the Romans did, and I'd say that the total labor needed is less that way.
Roman style cement uses calcined limestone, for which 900C or so is good. Going higher actually makes it less reactive, and this was a problem with ancient techniques - they were hard to control so if 100% of your lime was burned, odds were high that parts of it were overburned. The process was also only 20% or so fuel efficient.
You want your cement to be finely powdered to make it reactive. Burnt lime is pretty soft, though, and I don't think the volcanic rock Romans used was all that tough either.