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There are many additives available that can greatly improve cement based mixes such as mortar or concrete, some will be described below.
A powdered dye can be added to a mortar mix to dye the mortar a different colour, Black is a popular colour. Cement dye's should be added to the dry mix before the gauging water is added. Colour matching mortar is difficult so if you are making several mixes be sure to add the same amount of dye on each batch that you do! Here you can see examples of how different coloured mortar can effect the overall look of brick walls.
Light Grey Mortar
Plasticiser comes in many different guises but basically it all does the same thing. It makes the mortar easier to work with by air entraining. Washing up liquid was once used for this purpose but this is no longer an accepted practice as it significantly weakens the mixture. Special plasticisers are available for when the mixture is to be used for rendering, these make it far easier to handle than mortar mixed without the plasticiser.
PVA ( polyvinyl acetate )
PVA adhesive can be mixed with the gauging water to increase adhesion and improve workability. For more information on the many uses of PVA please click here
Mortar and concrete can be made waterproof by adding a waterproofing chemical to the mix, this is ideal for rendering internal walls that have had a replacement damp proof course installed etc. Mortar and concrete absorb a certain amount of water but this is reduced considerably by adding this chemical. This room was rendered with mortar and a integral waterproofing chemical before it was dry lined.
Mortar and concrete do not like frost as it can seriously damage the mix, adding a frost proofing chemical means that mortar and concrete can be used in temperatures of -4 degrees C. Some of these chemicals can be used as rapid hardeners which make the cement set a lot faster than normal.