The best of both worlds
The introduction of Portland cement in the early 19th century must have seemed a Godsend for Victorian builders. Here was a material that was seemingly very tough and relatively cheap which could be successfully used for a host of construction applications. However, its application to mortar was destined to have unfortunate consequences.
For centuries, mortar used for pointing masonry was normally a mixture of lime and local sand. In such constructions, the natural elasticity of lime proved the perfect partner for the soft, low crushing strength bricks, resulting in a construction which was relatively flexible and could accommodate movement without the need for movement joints.
However, the introduction of cement enriched mixes tended to result in very strong, yet inflexible mortars which would stick to the bricks and dry very hard, thereby producing a relatively inflexible construction. The mortar would inevitably crack, thereby promoting water ingress. Worse, if the mortar was harder than the brick itself - as was often the case - cracking of the bricks might occur.
But traditional lime mortar, which for a variety of reasons is currently enjoying a revival of interest, also has its drawbacks. Making lime putty on site can be a dangerous process, as the mixing of quicklime with water causes a violent reaction as the lime hydrates. Clearly, with ever more stringent health and safety standards and the alarming increase in construction industry fatalities, such antiquated practices must seem anachronistic in a modern world.
But there are also problems associated with the site mixing of ordinary mortars. Weak mixes, colour variations and mistakes in the quantities of air entraining agents used can all have catastrophic aesthetic and cost implications; nobody wants a situation where raking out all the joints and repointing is necessary, or worse, where an entire outer leaf has to be taken down and rebuilt.
An alternative mortar, used by some builders, is to make a lime mortar variant by using a bagged hydrated lime and mixing it on site with Portland cement, sand, water and, for workability, an air entraining agent (normally washing-up liquid). This, however, does not give consistent results. Ready mixed lime putty is an alternative, but would it not be ideal to combine the strength benefits of cement with the benefits of lime?
The answer is yes, thanks to a product called Limebond from Buxton Lime, Britain's largest producer of lime-based products for the building industry. A pre-mixed lime based masonry cement, Limebond is a unique, safe, all-in-one product which combines the strength, durability, frost and sulphate resistance of Portland cement with the flexibility and workability of lime. As only sand and water are required, there is no need for expensive cement additives, so Limebond is quick to make up, reduces wastage and can lower costs by as much as 20%. Now used widely by volume housebuilders, bricklayers have found it hangs well on the trowel, spreads easily and clings well to perpend joints, thereby encouraging good workmanship and durability.
Limebond can be used in accordance with the manufacturer's recommendations for a variety of applications including bedding above and below DPCs, brick laying, blockwork, internal plastering and external rendering according. Because it is factory blended, the ICI formulation of Portland cement, hydrated lime and chemical admixtures eliminates the dangers which can occur when proportioning ordinary cement, lime, sand and air entrainment agents. This means Limebond produces a consistent result every time.
Indeed, consistency in site mortars is a virtue extolled by the NHBC which recognises that getting a mortar mix right has traditionally been left to the labourer on site. It therefore urges specifiers and contractors to 'take appropriate action to ensure that the right mortar mix is used. NHBC inspection staff will be looking at mortar more closely and may take samples for analysis where they believe the mortar is not up to strength.'
Given the importance placed by the NHBC on correct mortar mix, it must surely make sense to use factory-blended products such as Limebond. Given it combines the benefits of both Portland cement and lime, it must surely be regarded as much of a Godsend as was Portland cement all those years ago.
