Concreting

[journeyman Mick]

Neddy,
a retarder is a chemical additive that slows the concrete without having to add water. Tip: if you find that the concrete is going off faster than you can float or trowel it use a sugar and water mix (as much sugar as you can dissolve into the water) and sprinkle it lightly on top, this acts as a retarder. (as does lemon juice in plaster).

Mick

[silentC]

Some concretors like to add extra water to the concrete as it comes off the truck, or they'll ask for extra water to be added when it is mixed. This makes it easier to work and gives them more time. I was told by an engineer not to let them do this as it makes the concrete weaker.

The message is that you need to have a light hand if you're adding water to it (like Mick says 'sprinkle'). Don't think that adding extra water to the mix to stop it going off is not going to have a downside.

[Dion N]

The water in a concrete mix reacts with the cement and is "used up", ie it undergos a chemical reaction with the lime?? in the cement. The reaction happens quickly at first and then gradually tapers off so that your concrete is never 100% completely dry (although 99.9% is close enough for all practical intents and purposes) At 28 days, the concrete is pretty close the maximum amount of hardness and strength, hence the reason engineers specify a "28 day strength of so many MPa" for buildings and why test cylinders are tested after 28 days.

If you add too much water, the reaction between the water and the cement consumes all the cement, but not all the water and water is not a particularly good building material...