All proprietory Post Mix in bags have rapid hardening cement - its Ordinary Portland + aluminium. A mix of 1 of Ordinary Portland to 6 of ballast is quite sufficient - 1:2:4 is far to strong and unnecessary. However, with Post Mix all you have to do is drag the bag to the hole, half fill the hole with water, tip sufficient of the bag in until you have a paste, stand there for 30 seconds, then move on to the next one. Much easier than knocking up concrete, whether by hand or mixer.