The mortar joints in masonry fasten the stones or bricks together into a structural whole and make a binding, watertight seal. When mortar deteriorates and decays, it must be replaced to restore the masonry’s structural integrity and appearance. This replacement process is called repointing. If you have decided to take on the repointing job yourself, your first task should be to decide what type of mortar mix to use.
Mortars are all made out of the same basic ingredients, which are water, an aggregate, and a binder. Sometimes additives are blended in, for altering the mortar color, increasing frost resistance and making setting time faster.
Other additives include bonding agents, which are not recommended due to their tendency to make mortar less breathable and more brittle, and air entraining agents, which are intended to improve mortar plasticity and cold resistance in colder climates.
What mortar mix needed will all depend on the type of masonry, it’s function and what condition it is in. There are four types to choose from; Very Soft, Soft (type O), Medium (type N), and Hard (type S) mortar. The letter types refer to the corresponding ASTM standard specs. The less the lime content, the harder the mortar and the quicker it will set.
Very Soft Mortar
This is composed of 1 part Portland cement, 4 parts lime, and 11-15 parts sand. It is mostly used for interior walls of low strength (marble, weak limestone or sandstone and brick). It can also be used in exterior walls that are sheltered and made of low strength masonry materials.
Type O Soft Mortar
This type made of 1 part Portland cement, 2 ½ parts lime, and 8-10 parts sand. It is used when repointing sheltered or exposed exterior walls of low strength stone and average strength stone (hard limestone or sandstone and facing brick). Also on interior walls of high strength- granite and paving brick).
Type N Medium Mortar
Composed of 1 part Portland cement, 1 /14 part lime and 7-9 parts sand. Used for repointing of highly exposed exterior walls made of low and average strength masonry, and paving of low strength masonry.
Type S Hard Mortar
This type mortar is made of 1 type cement, 1 ½ part lime and 4-5 parts sand, and used on paving of average and high strength stone.
Sand and Binders
Sand in repointing mortars should be clean and match the color of the original mortar as close as possible. Unfortunately, on older buildings, sand colors can range from gray to white to dark yellow all on one wall. It may be necessary to blend sand of several colors and grain size to match some older mortar, as modern mortar sands are mass manufactured, with sharper edges, although it does make stronger mortar.
The binders are materials that cement together the sand, and are a variable blend of lime and Portland cement. Lime adds softness and porosity to mortar, as well as makes it less sensitive to temperature changes.
Portland cement is a very hard type of cement which is impermeable to water. Mortar with too much cement content becomes overly rigid and can shrink when setting to the point it leaves air gaps between the mortar and the masonry.
Careful mixing is a must in order to prevent uneven coloring and lumps and to give consistent strength and texture to the repointing. Add dry ingredients first, then add the water. Only half the required water should be added at first, then the mix should be mixed for around five minute. The remainder of the water is to be added in small amounts until the consistency you desire is achieved.
You want the consistency of the mortar to be just enough water added so that the mortar will stick to a trowel held upside down. The mortar should be applied before it starts hardening, which is usually within 1 to 2 hours of mixing, so do not mix up too large a batch. Mortar should not be mixed or applied at temperatures below freezing, as the water component will solidify prematurely.