Chemical process involved in setting plaster/resin

[mummytait]

Please can someone explain in simple terms the chemical process involved when water is added to dry plaster. How does this differ from the reaction involved when catalyst is added to resin?

[Clanad]

Our friends at Wikipedia have this to say about one of the more common types of plaster:
Lime plaster is a mixture of calcium hydroxide and sand (or other inert fillers). Carbon dioxide in the atmosphere causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone). Whitewash is based on the same chemistry.

To make lime plaster, Limestone (calcium carbonate) is heated to produce quicklime (calcium oxide). Water is then added to produce slaked lime (calcium hydroxide), which is sold as a white powder. Additional water is added to form a paste prior to use. The paste may be stored in air tight containers. Once exposed to the atmosphere, the calcium hydroxide turns back into limestone, causing the plaster to set.

Lime plaster is used for true frescoes. Pigments, diluted in water, are applied to the still wet plaster. The pigments bind with the plaster as it sets.

There are several other types of plaster.

So, although there is a chemical cahnge when either dries, it is not equivalent to the true chemical process that happens when using catalysts of any kind, including resins. Called polymerisation, the process begins when catalysts are added to the resin system shortly before use to initiate the polymerisation reaction. The catalyst does not take part in the chemical reaction but simply activates the process. An accelerator is added to the catalysed resin to enable the reaction to proceed at workshop temperature and/or at a greater rate. Since accelerators have little influence on the resin in the absence of a catalyst they are sometimes added to the resin by the polyester manufacturer to create a �pre-accelerated� resin. (Source SP Systems)

[Gef]

Clanad, sorry for being pedantic, but if a catalyst does not take part in a reaction (as your source suggests) then how can it affect the reaction?

[Clanad]

Not being a chemist, Gef. forgive me if my explanation is simplistic, but catalysts used in resin curing is extremely acidic and works by lowering the PH of the resin. Most resins I've worked with can be cured without the catalyst by simply increasing the temperature in the room. In fact the amount of catalyst required is substantailly less the higher the temperature. Catalysts and accelerators only affect the hardening or curing time without chemically bonding with the resins. A lot of effort is actually made to recover some of the catalyst after curing in some high-tech applications I've read of...

[Gef]

No problem Clanad you are forgiven. Unfortunately I am a chemistry teacher and I would not forgive my pupils if they said that, lol.

Catalysts take part in the reaction but can be recovered unchanged at the end of the reaction. They are used for two reasons - to speed up the reaction and/or to allow it to take place at a lower temperature.

Anyway, I was being pedantic and I apologise for that.