THE IMPORTANCE AND BENEFITS OF USING SLAKED LIME IN MORTAR AND PLASTER MIXES
WHAT IS LIME?
Lime is made by burning Calcium Carbonate to form Calcium Oxide and then water is added to the oxide which rapidly converts to Calcium Hydroxide (slaked lime), a very fine white powder.
HOW DOES IT HELP US?
Lime has been used in conjunction with cement in mortar mixes for many years. Although it has nowadays often been replaced by chemical admixtures, it has many benefits, which if properly understood, would result in it being used. We cannot emphasise its importance enough.
- PLASTICISER. The extremely fine lime particles fill the voids between the larger sand particles of the mix and retain the moisture between the grains. This makes the mortar mix workable and buttery. The mortar clings to the trowel and edges of the bricks and doesn’t fall off. This results in less mortar waste.
- BOND STRENGTH. Lime mortar has a 60% greater bonding strength to sand cement only mortars. This helps to reduce moisture penetration through the mortar as well as prevent mortar waste occurring when mortar falls off masonry, causing waste. Typically builders use up to 3 times the amount of mortar originally estimated as a result of waste.
- EVEN CURING. The fine particles of the lime retain moisture in the mix resulting in the continued curing of the cement, which requires water to cure. Mortars often dry out too quickly resulting in low strength mortars and increased mortar waste.
- SLOW CURING. The slight alkalinity of the lime retards the setting of the cement allowing the mortar to remain malleable and take up slight movements of the brickwork without cracking. This means that less dried out mortar is wasted from each batch produced.
- SELF SEALING. All brick work (clay and concrete) moves continually as a result of changes in temperature and moisture as well as ground conditions. Eventually the mortar bond between the bricks may fail leaving hairline cracks. Over time the lime component of the mortar will convert to Calcium Carbonate and fill the hair line fracture.
- MOISTURE RESISTANCE. The denser mix created by the presence of fine lime particles helps resist the entry of moisture through the mortar. Moisture almost always penetrates through mortar joints and not through the bricks themselves.
In summary, almost all of the problems experienced with cracking can be avoided by using a lime mortar and plaster. Whilst upfront it may appear to add to the cost of the mortar and plaster mix, the overall savings as a result of less waste, better quality finish and elimination of cracking and moisture penetration more than outweighs the upfront cost.