Should I add retarding agents to the concrete mix?

Do NOT add any retarding agents to a concrete mix that is going to be ground and sealed or polished. Retarding agents WILL affect the seal and may react badly with the polishing process. Simply apply a light sprinkle of water to the slab as required.

What happens if someone spills petrol or turps on a Grind and Seal or a Hyperfloor?

Wipe as soon as possible with a damp cloth.

What sorts of questions should people ask before choosing a polished concrete floor?

The home owner should consulate with a professional concrete polisher to decide on how much aggregate/stone should be visible.

More info

We have a tiled floor but they are old and don't look very good. We are thinking of going polished concrete but how do we get all the glue off and do we have to pour new concrete on top of the mess that will be underneath the tiles?

Usually this is not required, once the tiles have been removed the remaining glue and mess can be ground to a brand new surface. However there can be exceptions.

Does polished concrete save on lighting costs?

Mitre 10 estimates that their highly polished floors save 80% of their lighting bill because of their light reflection.

Do I need to advise the concreter and or supplier that the concrete will be polished?

Yes, this will avoid the concrete supplier from sending truck deliveries from different depots and using different batches of concrete. The concreter can then also take special care to avoid screed marks and aggregate inconsistency.

How long does it take to polish a concrete slab?

This depends on a number of factors, firstly how big the job is, how accessible the site is, and what finish you have selected. i.e. CCC Polished  Concrete or Grind and Seal.

At what stage should we polish?

About a week to 10 days after the slab is poured.