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Mind the Gap

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Mind the Gap - All about Mortar in Brickwork

Michael Hammett

Mud is man’s oldest building material. The earliest bricks date to about 8,000 years BC and were found in Jericho. They were made of mud, crafted by hand and sun-dried. These were about 12" long, slightly rounded and called adobe bricks. They formed thick walls and were joined with wet mud. About 6,000 yr BC wooden moulds were used to produce rectangular bricks. In hot dry climates the adobe brick is still used today as is seen in the Yemen. The climate must be dry as the bricks would wash away. Chopped straw is added to help dry and add structure. In about 3,000 BC, clay was fired to form a semi-vitrified material for weather-proofing and used to clad the outside. Examples are Chogga Zambil in Iran sand + bitumen was added to the mortar and placed between the bricks. It was also used to face temples and palaces. Bitumen was freely available in this area. Babylon 604-562 BC, Nebuchadnezzar’s palace which occupied 850 hectares was largely constructed of mud bricks but decorated with fine glazed bricks used on the outer facings. The Romans used lime bound mortar for brickwork from about 200BC.

Lime Cycle

Lime or chalk

CO2 from Air Heat about 1,000 ° C

Ca(OH)2 CaO
Calcium hydroxide Calcium Oxide
Slaked Lime Quick Lime
Hydrated Lime


Lime was used in primitive clamps but produced in kilns and also used for plaster and for jointing masonry. It was heated for 3-7 days. The fuel used for the kiln was timber, olive kernels, pine cones, grass or almond shells but later they used coal, oil or gas. Designs in 19th /20th C used vertical lime kilns. Here, lime went in the top, heat was applied and lime came out. Quick lime was drawn into the kiln but had to be slaked so it could be used as mortar. Excess water was added in a tank to slake, hydrate or produce lime putty = mortar. Sometimes it was made in wet sand = Coarse stuff.

Mortar sticks bricks together. It is important that it is solid, stable and robust and seals joints but protects against wind and water damage. It also has to hold the bricks apart and acts as a cushion and spreads the load evenly from brick to brick. It also allows for variations in size and shape of brick to keep it level and ensure plumb alignment. Sand and Binder. Sand is used in bulk to hold bricks and control compression and load. Binder sticks sand particles together to provide tensile strength. Mortar constituents are measured by volume and their proportions are important.
3 parts sand + 1 part binder = 3 parts mortar.

Sand + Lime mortar
The sand is well-graded from good distributors. It is sharp, angular, not rounded equal grain size, no clay, vegetation, soil or salt.

Lime is pure calcium carbonate and impurity in the form of aluminium silicate causes changes in the slaking. It sets by an internal chemical reaction regardless of air because it can set under water = hydraulic.
The amount of clay impurity in chalk varies and therefore alters degrees in hydraulicity.
Non-hydraulic <1% slow to set
Feeble 2-8% clay 15-21 days to set
Moderate 8-18% 5-15 days to set
Eminently hydraulic 18-25% clay 1-4 days to set
Non-hydraulic lime only sets in air (aerial) lime. It is not generally used but is used for joint ribbons, stone masonry, decorative surface washes and internal plaster.
The other three types are used in brickwork jointings and mortars as appropriate.

Lime has many uses. The Romans found pozzolana sandy volcanic ash around Vesuvius. It was added 2:1 by weight and was a strongly hydraulic binder for concrete and mortar.
Trass or Dutch trass is volcanic tuff found in Germany and used in the Netherlands.
Crushed brick and tile, blast furnace slag, fly ash from coal-fired power stations have also been added to improve performance of traditional lime mortars. All these are generally known as pozzolans.
Red rubber bricks are made of fine ground clay. They are cut to precise squareness and size and used for precise brickwork.
In St James’ Street in SW1, the mortar is lime slurry, the brick being dipped in slurry before placed in the brickwork = gauged brickwork.
Tuck Pointing is also used in precision bricklaying. The bricks are irregular and laid in tinted mortar and false white joints added. Regardless of joints, lime putty is added to the joints and cut carefully.
Portland cement
There was an important milestone by Joseph Aspdin 1824 originally used for rendering. This was modified by Johnson. He added calcined limestone, lime and clay into a wet slurry and fired a second time. He also increased the firing temperature to vitrify the limestone. In 1845 the Johnson cement was launched. It was perfected by the introduction of the rotary kiln in 1890s.

Ordinary Portland = normal product for brickwork. Sulphate resisting is not uncommon. White Portland is required for appearance. Masonry cement is premixed to contain a binder 75% and Portland 25% = inert filler.


Types of Portland Cement Mortars:
Sand + Cement 3:1 –general purpose
Sand + cement + lime – non-hydraulic
Sand + cement + air (minute bubbles) - air gives workability and reduces the water requirement
Sand + masonry cement gives a good balance of properties
Sand + cement + lime + air can be made from basic materials.

Combination of cement/lime/air allows variation of cement content >cement = more durable ie frost resistant but most brickwork is not bothered.

The Aesthetics of the Joint


Bricks Mortar
Colour Colour
Texture Width
Shape Profile

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