Harrow and Hillingdon Geological Society
Mind the Gap
Mind the Gap - All about Mortar in Brickwork
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 or chalk
CO2 from Air Heat about 1,000 ° C
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.
Sand + Lime mortar
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.
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.
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:
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