Plasterers ‘builders’ have been known to add Gypsum (calcium sulphate hemihydrate) to the cement based mix to speed up the setting time of the mix. This is done in order to get the work complete more quickly. Particularly if the application is thick, with the aim of finishing the job sooner and thereby try to save some money on the labour. This practice is flawed, resulting in the failure of the render coat.
These failures are usually severe with the render coat bursting, swelling and cracking. The material so exposed is friable and powdery. See the photographs below.
The reason for this spectacular failure is, in the main, that the water which would normally be used by the cement component in the process called ‘hydration’ is taken away by the Calcium sulphate hemihydrate (Gypsum) during this curing process which is much quicker than the complicated reactions which occur during hydration (cement and water combination) reaction.