Retarding admixtures
These admixtures extend the time taken for the concrete to stiffen and are very important in preventing cold joints between layers or areas of concrete which are placed some time apart. This can be a particular problem in large pours and or in hot weather. Retarders by themselves are not very effective in providing workability retention and need to be used in combination with other admixtures that give cohe sion and increased initial workability It is possible to retard concrete or mortar for extended periods and then bring about setting, usually by the addition of an appropriate accelerator, but for this application advice should always be sought from an admixture supplier.