This must be done only just prior to use as the mortar will start to set once the pozzolan is added. You would usually need approximately 1 part of pozzolan for every 9 parts of mortar (by volume). To ensure maximum effect the pozzolan must be very fine and thoroughly mixed into the mortar. Some examples of pozzolans are stone/ brick dust or ash. Pozzolans are naturally or artificially fired clays which are reactive when mixed with lime putty. POZZOLANS are materials that enable lime mortars to set more rapidly and add strength. Stone repair mortars and sheltercoats usually have a higher proportion of stonedusts whereas construction, pointing mortar and plasters and renders have a high proportion of sand. Stonedusts are usually crushed and sieved limestone. It is important that the sand is well graded and angular and preferably washed. Sands are usually mixed and graded at the quarry to satisfy British Standards. SANDS and stonedusts are mixed with LIME PUTTY to form mortars, plasters and renders. Casein (protein from milk) can be mixed with limewash to improve binding and can be mixed with lime putty to make a strong adhesive for both stone and wood. Tallow (animal fat) was traditionally added to limewash for the same purpose. LINSEED OIL can be added in small quantities to improve the weathering of LIMEWASH. However, they have the advantage of being cheaper and much easier to add to the mortar / plaster. Polypropylene fibres work in the same way as the addition of horse hair in adding extra strength to the mix. We now have the option of a ready mixed plaster made with the addition of polypropylene fibres. *Human hair is unsuitable as it is regarded as too oily. The cut edge on the trowel should show an even distribution of hair with a "beard of bristles" clearly showing. To test for consistency, place some mortar on a HAWK with a trowel and, with a sweeping action, cut and remove a sample off the hawk. Although sprinkling the hair evenly into the mortar whilst it is in a mixer can work effectively. The easiest way to mix it is by sprinkling it evenly over mortar which has been spread out flat. It is best mixed in just before the plaster is to be used because in the presence of wet lime it will quickly degrade in strength. Thorough mixing into the plaster is essential to ensure consistency and prevent tangled hair. The most commonly used is horse hair or goat hair which is usually about (2-3 inches) long and tough. When added to lime plaster helps binds it together and gives it additional tensile strength.
Laths, Woodwool Boards, Bricks, Hessian & Mesh.
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