Part Three: Choice of mortar and application.
The usual constituents of mortar are the binder, i.e. lime, and an aggregate in the proportions of one part binder to two or three parts of aggregate. The choice of mixture and aggregate depends on the material to be pointed up. In wider joints, coarser aggregates and galleting (see below) can be used. This helps reduce shrinkage and prevent the mortar cracking. Fine joints should be filled with a stronger mix, up to 1:1, using stone dust as an aggregate.
The type of lime used depends on the situation in which it is to be applied. As well as considering the material to be repaired, the environment in which it needs to perform must also be taken into account. As noted in Part One stone needs to be repaired with a compatible mortar. Oolitic limestone, which incorporates small pores, needs a porous lime mortar made using lime putty. The more open textured weatherstones from the upper limestone beds, known locally as Minchinhampton stone, will be tolerant of a hydraulic lime mortar. Weatherstone is used in the more exposed positions on a building, e.g. parapets, cills and plinths, where a hydraulic lime will perform better than a ‘fat’ lime mortar.
Accelerated setting of lime putty mortars can be achieved by the use of ‘pozzolanic’ materials; this term derives from volcanic ash found in Pozzuoli near Naples in Italy, which was used by the Romans to hasten the setting of lime mortar. Pozzolans include substances such as crushed brick or tile and PFA (Pulverised Fly Ash) which contain silica and alumina. These induce a hydraulic set and produce a stronger mortar with greater resistance to water. When used they typically make up a tenth of the volume of the mortar. The addition of small amounts of cement to lime mortars has been common practice but is not recommended as research has shown that these mortars are prone to failure.
‘Fat’ lime mortars (made with lime putty) should be used externally when there is no risk of frost and ideally in temperatures over 5 degrees centigrade. Low temperatures will inhibit carbonation and in extreme conditions cause the water content to freeze. High temperatures and strong winds can dry out the mortar too quickly. In all conditions, the carbonation process will only take place when the mortar is damp. Before repointing the open joints should be thoroughly wetted. In wide joints wetted stones can be used to pack out the joint (galleting) and reduce the amount of mortar employed. A round ended gauging trowel or similar tool should be used rather than a ‘pointing’ or bricklayers trowel. When filling the joints the mortar needs to pressed firmly in and left slightly proud of the surface. After about 24 hours and as the mortar stiffens it should be pressed back to prevent cracks developing. Following this excess mortar and trowel marks should be removed with a stiff bristle brush; a churn brush is very effective. This also gives the mortar an open texture and consistent appearance. During the carbonation process the mortar should be kept covered for at least two days with dampened sacking or plastic sheeting to reduce evaporation and the work regularly damped down with a water spray. The mortar should be finished flush with the surface of the stone to promote water run off. Ribbon pointing should be avoided as the ledges encourage damp penetration into the stone, see photo 2. The colour of the mortar is also important if only for aesthetic reasons; a light mortar highlights the stone whilst a dark mortar emphasises the pointing, see photo.