It was a hot day, and the time between loads was longer than we would have liked, so it started going off fast, but we changed the spec to make it wetter & in the end it was all poured. The driver was somewhat traumatised by our lane (narrow with lots of overhanging vegetation) and by our turning area (tight), but by the third load he had mastered it all.
See earlier post on why we used concrete - resilience and robustness. We specified a 40%mix of PFA (Pulverised Fly Ash - waste from power stations) to reduce the cement content, but there is still a heavy carbon footprint associated with its use. We consider we made the best choice in the circumstances - it will also allow the use of further heavyweight materials at low level, increasing the thermal mass/ comfort of the final building & reducing heating/ cooling loads.