mortar

  • Mortar JOINTING AND POINTING

    Jointing is the word used to describe the finish of the mortar joints between bricks, to provide a neat joint in brickwork that is finished fairface. Fairface describes the finished face of brickwork that will not be subsequently covered with plaster, rendering or other finish.

    Most fairface brickwork joints are finished, as the brickwork is raised, in the form of a flush or bucket handle joint. When the mortar has gone off, that is hardened sufficiently, the joint is made. Flush joints are generally made as a ‘bagged’ or a ‘bagged in’joint. The joint is made by rubbing coarse sacking or a brush across the face of the brickwork to rub away all protruding mortar and leaving a flush joint. This type of joint, illustrated in Fig. 65, can most effectively be used on brickwork where the bricks are uniform in shape and com¬paratively smooth faced, where the mortar will not spread over the face of the brickwork.

    A bucket handle joint is made by running the top face of a metal bucket handle or the handle of a spoon along the joint to form a concave, slightly recessed joint, illustrated in Fig. 65. The advantage of the bucket handle joint is that the operation compacts the mortar into the joint and improves weather resistance to some extent. A bucket handle joint may be formed by a jointing tool with or without a wheel attachment to facilitate running the tool along uniformly deep joints.

    Flush and bucket handle joints are mainly used for jointing as the brickwork is raised.
    The struck and recessed joints shown in Fig. 65 are more laborious to make and therefore considerably more expensive. The struck joint is made with a pointing trowel that is run along the joint either along the edges of uniformly shaped bricks or along a wood straight edge, where the bricks are irregular in shape or coarse textured, to form the splayed back joint. The recessed joint is similarly formed with a tool shaped for the purpose, with such filling of the joint as may be necessary to complete the joint.
    Of the joints described the struck joint is mainly used for pointing the joints in old brickwork and the recessed joint to emphasise the profile, colour and textures of bricks for appearance sake to both new and old brickwork.

    Pointing


    The words jointing and pointing are commonly loosely used. Jointing is the operation of finishing off a mortar joint as the brickwork is raised, whereas pointing is the operation of filling the joint with a specially selected material for the sake of appearance or as weather protection to old lime mortar.
    Pointing is the operation of filling mortar joints with a mortar selected for colour and texture to either new brickwork or to old brickwork. The mortar for pointing is a special mix of lime, cement and sand or stone dust chosen to produce a particular effect of colour and texture. The overall appearance of a fairface brick wall can be dramatically altered by the selection of mortar for pointing. The finished colour of the mortar can be affected through the selection of a particular sand or stone dust, the use of pigmented cement, the addition of a pigment and the proportion of the mix of materials.

    The joints in new brickwork are raked out about 20 mm deep when the mortar has gone off sufficiently and before it has set hard and the joints are pointed as scaffolding is struck, that is taken down.
    The mortar joints in old brickwork that was laid in lime mortar may in time crumble and be worn away by the action of wind and rain. To protect the lime mortar behind the face of the joints it is good practice to rake out the perished jointing or pointing and point or repoint all joints. The joints are raked out to a depth of about 20 mm and pointed with a mortar mix of cement, lime and sand that has roughly the same density as the brickwork. The operation of raking out joints is laborious and messy and the job of filling the joints with mortar for pointing is time consuming so that the cost of pointing old work is expensive.
    Pointing or repointing old brickwork is carried out both as protection for the old lime mortar to improve weather resistance and also for appearance sake to improve the look of a wall.

  • Lime mortar

    Lime is manufactured by burning limestone or chalk and the result of this burning is a dirty white, lumpy material known as quicklime. When this quicklime is mixed with water a chemical change occurs during which heat is generated in the lime and water, and the lime expands to about three times its former bulk. This change is gradual and takes some days to complete, and the quicklime afterwards is said to be slaked, that is it has no more thirst for water. More precisely the lime is said to be hydrated, which means much the same thing. Obviously the quicklime must be slaked before it is used in mortar otherwise the mortar would increase in bulk and squeeze out of the joints. Lime for building is delivered to site ready slaked and is termed ‘hydrated lime’.

    When mixed with water, lime combines chemically with carbon dioxide in the air and in undergoing this change it gradually hardens into a solid mass which firmly binds the sand.

    A lime mortar is usually mixed with 1 part of lime to 3 parts of sand by volume. The mortar is plastic and easy to spread and hardens into a dense mass of good compressive strength. A lime mortar readily absorbs water and in time the effect is to reduce the adhesion of the lime to the sand and the mortar crumbles and falls out of the joints in the brickwork.

    Mortar for general brickwork may be made from a mixture of cement, lime and sand in the proportions set out in Table 2. These mixtures combine the strength of cement with the plasticity of lime, have much the same porosity as most bricks and do not cause efflorescence on the face of the brickwork.

    The mixes set out in Table 2 are tabulated from rich mixes (1) to weak mixes (2). A rich mix of mortar is one in which there is a high proportion of matrix, that is lime or cement or both, to sand as in the 1:3 mix and a weak mix is one in which there is a low proportion of lime or cement to sand as in the mix 1:3:12. The richer the mix of mortar the greater its compressive strength and the weaker the mix the greater the ability of the mortar to accommodate moisture or temperature movements.

  • Cement mortar

    Cement is made by heating a finely ground mixture of clay and limestone, and water, to a temperature at which the clay and lime¬stone fuse into a clinker. The clinker is ground to a fine powder called cement. The cement most commonly used is ordinary Portland cement which is delivered to site in 50 kg sacks. When the fine cement powder is mixed with water a chemical action between water and cement takes place and at the completion of this reaction the nature of the cement has so changed that it binds itself very firmly to most materials.

    The cement is thoroughly mixed with sand and water, the reaction takes place and the excess water evaporates leaving the cement and sand to gradually harden into a solid mass. The hardening of the mortar becomes noticeable some few hours after mixing and is complete in a few days. The usual mix of cement and sand for mortar is from 1 part cement to 3 or 4 parts sand to 1 part of cement to 8 parts of sand by volume, mixed with just sufficient water to render the mixture plastic.

    A mortar of cement and sand is very durable and is often used for brickwork below ground level and brickwork exposed to weather above roof level such as parapet walls and chimney stacks.

    Cement mortar made with washed sand is not as plastic however as bricklayers would like it to be. Also when used with some types of bricks it can cause an unsightly effect known as efflorescence.

    This word describes the appearance of an irregular white coating on the face of bricks, caused by minute crystals of water soluble salts in the brick. The salts go into solution in water inside the bricks and when the water evaporates in dry weather they are left on the face of bricks or plaster. Because cement mortar has greater compressive strength than required for most ordinary brickwork and because it is not very plastic by itself it is sometimes mixed with lime and sand.

  • Ready mixed mortar

    Of recent years ready mixed mortars have come into use particularly on sites where extensive areas of brickwork are laid. The wet material is delivered to site, ready mixed, to save the waste, labour and cost of mixing on site.

    A wide range of lime and sand, lime cement and sand and cement and sand mixes is available. The sand may be selected to provide a chosen colour and texture for appearance sake or the mix may be pigmented for the same reason.

    Lime mortar is delivered to site ready to use within the day of delivery. Cement mix and cement lime mortar is delivered to site ready mixed with a retarding admixture.
    The retarding admixture is added to cement mix mortars to delay the initial set of cement. The initial set of ordinary Portland cement occurs some 30 minutes after the cement is mixed with water, so that an initial hardening occurs to assist in stiffening the material for use as rendering on vertical surfaces for example.

    The advantages of ready mixed mortar are consistency of the mix, the wide range of mixes available and considerable saving in site labour costs and the inevitable waste of material common with site mixing.

  • Mortar plasticiser

    As an alternative to the use of lime it has become practice to use a mortar plasticiser with cement in the mix of cement mortars. A plasticiser is a liquid which, when combined with water, effervesces to produce minute bubbles of air that surround the coarse grains of sand and so render the mortar plastic, hence the name ‘mortar plasticiser’ mixes.

  • ‘Compo’ mortar

    During the last 50 years it has been considered good practice to use a mortar in which the advantages of lime and cement are combined. This combination or ‘compo’ mortar is somewhat messy to mix.

  • Matrix for mortar

    The material that was used for many centuries before the advent of

    Portland cement as the matrix (binding agent) for mortar was lime. Lime, which mixes freely with water and sand, produces a material that is smooth, buttery and easily spread as mortar, into which the largely misshapen bricks in use at the time could be bedded.

    The particular advantage of lime is that it is a cheap, readily available material that produces a plastic material ideal for bedding bricks. Its disadvantages are that it is a messy, laborious material to mix and as it is to an extent soluble in water it will lose its adhesive property in persistently damp situations. Protected from damp, a lime mortar will serve as an effective mortar for the life of most buildings.

    Portland cement, which was first manufactured on a large scale in the latter part of the nineteenth century, as a matrix for mortar, produces a hard dense material that has more than adequate strength for use as mortar and is largely unaffected by damp conditions. A mixture of cement, sharp sand and water produces a coarse material that is not plastic and is difficult to spread. In use, cement has com­monly been used with ‘builders’ sand’ which is a natural mix of sand and clay. The clay content combines with water to make a reasonably plastic mortar at the expense of loss of strength and considerable drying shrinkage as the clay dries.

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