Lime is a very old material, being used by the Greeks and Romans to
construct large buildings and civil engineering projects that are still here
Lime in mortar form is being used to repair the Sphinx today proving a more suitable material to resist ground salt attack than cement mortar. Lime is produced by burning limestone (calcining) in its quarried form (rock lime or nut lime.). The calcining breaks the rock lime into a powder which is then prepared for mortar by slaking (hydrating).
Slaking is the immersing in water of the calcined lime. When first placed in the water the chemical reaction causes the water to boil.
Keeping the lime wet maintains its use for mortar. Lime sets by taking carbon dioxide from the atmosphere, this is the gas that is driven off when the limestone is calcined. The setting process can be fairly slow with the lime mortar gaining strength very slowly.
Two types of lime are in use, hydraulic lime and non-hydraulic lime.
Hydraulic lime will set under water and today is used for tuckpointing and
where lime mortar is specified.
Non-hydraulic limes tend to be used with cement in mortar, they produce
'fatty ' consistency to the mortar.
Lime, when added to mortar, produces the following benefits;
Makes the mortar more workable;
Acts as a water retentivity agent;
Gives the mortar 'autogenous' self-healing properties. Lime,
when added to mortar, improves it when both plastic and set.
Lime was (and should still be) the principal material for mortar. Lime adds a workability quality to mortar that no other plasticiser can produce. It also
gives a degree of flexibility to the hardened mortar which can allow slight
movement in the wall fabric without serious permanent cracking occurring.
The lime will cause self-healing to occur across any cracks which may form.
Recommended: Lime be used in bricklaying mortar.
Sand is used as a filler for mortar. Mortar will normally comprise three parts of sand to one part of binder (cement or lime).
Mortar for bricklaying should comprise one part general purpose cement, one part hydrated lime and six parts clean sand.
Sands are available in many forms and grain sizes, from filling sands, coarse river sands for paving, or concretes, to Sydney sands and bush sands for bricklaying
The bush sands consist of very fine grains or clay. The reason the bush sand or clay is mixed with Sydney sand is to improve its water carrying capacity. The smaller the sand grain size the larger the surface area available to carry water. The more water contained in the mortar makes it more workable, more able to resist high initial rates of absorption, retaining enough water in the mortar to set the cement.
The amount of clay or bush sand added to the Sydney sand is often not clearly known but where a percentage can be ascertained 3% to 5% is considered ideal to act as a plasticiser.
Obviously, the more fine particles contained in a mortar also affects the way the mortar shrinks as it dries. As the water leaves the mortar, the particles move back to touch so the benefits of increasing the fine particles must be balanced by the increased shrinkage.
The shrinkage may also affect the final appearance of the mortar joint finish.
Although no actual library on the proportions or type of ingredients to achieve any particular colour exists, many experienced bricklayers will be able to suggest a suitable sand, cement and/or lime for a particular mortar colour.
The use of oxides to colour mortar is necessary where specific colours are
required. Their use will enhance the mortar colour but certain conditions
must be observed.
- The mortar proportions must be accurately batched.
- The oxide must be measured and added during mixing.
- The mixing time must be a minimun of 7 minutes.
- The mortar must be used fresh and not re-tempered.
The intensity of the colour depends on the percentage of oxide added to the mix but no improvement will be gained where the oxide content exceeds 10% of the cement volume.
When using oxide, the saturation percentage should be obtained from the manufacturers. It should also be remembered that oxides are 'fines' added to the mix and can increase the shrinkage during drying. The addition of oxides may also suggest a reduction in the clay content in the sand.
The oxides work by combining with the cement paste and coating all the sand grains producing the desired colour.
Air Entrainers/Bycol Ect
The addition of air entrainers to a mortar mix is to improve workability and
flow. The mortar made from cement and brickies sand will often be sticky
and although workable will not flow or slide freely off the trowel. The
addition of an air-entrainer will correct this producing a fully workable mortar.
The amount of air entrainer allowed per mix is not well known even though the manufacturers instructions are on the can. 25ml er bag of cement is specified for normal mortar with 50ml being the highest amount for very coarse or 'hungry' mortars.
The Air Entrainer works by forming tiny air bubbles right through the mortar mix. These tiny air bubbles work like a lubricant 'aerating' the mortar and making it lighter to lift and spread. The bubbles are also formed from water ensuring more water is held in the mix.
The amount of air 'entrained' into the mortar depends on two things, the
quantity of air entrainer added to the mix and the time the mortar is mixed.
About 10% air in the mortar is acceptable, above that and the mortar is
weakened. The air bubbles remaining in the mortar after it has set reducing the contact between mortar and brick and creating a mortar like an Aero bar.
The longer the mortar is left in the mixer, the higher the air content becomes.
Several air entrainers are available on the market, all work in much the same way. If instructions are observed and the product is good, the mortar
strength will not be too seriously affected.
Note: Washing up liquids and detergents should not be added to mortar to improve workability. Although the mortar will be more workable the amount of air 'entrained' into the mortar cannot be controlled and will probably result in weakened mortar.
Plastamasta and Fireclay
Plastamasta or fireclay is added to mortar to increase the amount of 'fines'. The clay acts as a plasticiser but can also increase the shrinkage rate of the mortar if over used. 5% fines in sand is considered suitable.
Fireclay should not be used in bricklaying mortar. Fireclay is manufactured for refractory work and has its own setting protocol, either heat set or air set. Using fireclay with Portland cement will probably affect the strength of the mortar. Fireclay is not recommended for use in bricklaying mortar.
Accelerators are used to promote earlier setting times for mortar. Often used in cold weather where the setting time can be slowed.
Note. Can attack steel in ties and fitments.
Used to slow down the setting actions of mortar, CSR Readymix use a product called R.P.R
Bricklayers on-site use sugar. Sugar will slow down the initial set of mortar from about 40 mins to 6 to 8 hours depending on how much sugar has been added to the mix. The use of sugar is not recommended and is certainly not listed in the Masonry Code as an allowed additive to bricklaying mortars.
A.S.3700 states that retarded mortar must be factory batched and delivered without any further addition to them.
Mortar which contains sugar is proving difficult to clean as it forms a hard shell on the mortar surface preventing the acid from penetrating the mortar to soften the cement.
The ingredients of mortar will affect the productivity of the bricklayers, which allow or prevent good joint finishes being formed. In fact, making some joint finishes difficult to produce.
Mortar is the most important single factor in the production of quality brickwork, equivalent to good workmanship.
The quality of mortar relies solely on the knowledge, technical expertise and competence of the operative on-site charged with proportioning and mixing it. Its importance to the integrity of the construction cannot be overemphasised and should never be underestimated.