Monday, 15 July 2013

Cement mortar vs. lime mortar

Why use cement mortar? Cement vs. lime mortar.

Before we go down the route where this question will be answered I'd like to point out that there is a difference between lime putty and  natural hydraulic lime. Putty is non-hydraulic and cures during exposure to carbon dioxide. It comes in a tub and is wet. In direct contrast, natural hydraulic lime is dry, comes in a bag, needs to be mixed with sand and is reliant on the addition of water to cure. It is a more recent invention and is graded as a NHL 2, NHL 3.5 or NHL 5 with 5 being the hardest and only really for use on chimneys, canal sides and seafronts. Most people would struggle to find a use for NHL 2 therefore when chosing to use natural hydraulic lime a 3.5 is the choice for almost every job.

In the old days the most important building in a village was the church. When the foundations were sunk for one of these leviathans the rockiest ground was chosen because it created an instant bed on which stonemasons and builders could lay their base stones. If non-rocky ground was chosen then massive foundation stones would have to be laid. This would mean added expense and lots of extra work such as deep digging because if shallow foundations were dug then this would affect the structural integrity of the build due to future ground movement.
Therefore, the ideal ground on which old churches were built was bedrock. Once the site was chosen, the immense structure would wend its way skyward without fear of collapse as it was built on solid rock. The village would then spread out concentrically from the church. In the surrounding fields, farm hands would then begin systematically clearing rubble and use it to build boundary walls. This was a cheap way for landowners to create demarcation lines as farm labourers were paid a pittance and the stone was free. Some of this stone was used to build houses and is referred to as rubble stone.
The choice of mortar in those days was lime putty. The fact that foundations were minimal means that these old structures were – and still are - prone to movement but putty is a good load bearer, is soft and will move with the building. What this translates to is the fact that old houses - although prone to movement - don't end up with spalled masonry and big cracks running from top to toe in the same way as houses built with cement mortar. 

The use of lime putty can be traced back to Egyptian and Roman times. Natural hydraulic lime (NHL) came in during the 18th century and is a response to the need to have a mortar which would set under water. However, it was English inventor Joseph Aspdin who changed the course of building history by inventing Portland Cement. This is referred to as OPC and, any lime based mortar relies on a chemical set. In direct contrast, putty works on carbonation. What this means is that the carbon (carbon dioxide) is drawn out of the atmosphere allowing for a slow reaction where strength is built up over a period of years. Add to this the fact that manufacturing of lime putty mortar involves the production of less carbon dioxide and you begin to realise that using it has definite and measurable benefits. 
Both lime putty and NHL are easily obtainable and the information below provides a case to discontinue the use of cement. 

Advantages of lime over cement

  • Lime putty absorbs CO2 in the curing process. In fact, it absorbs nearly 75% of its own weight in CO2 whilst it cures.
  • By being produced at lower temperatures than cement, both lime putty and NHL require less energy - this results in up to 20% less CO2 output.
  • When cement goes off it needs to be thrown away. In direct contrast, neither NHL or lime putty needs to be disgarded because it has a slow curing time. A barrow full of it can be mixed on Monday morning and if it isn’t used by the close of business it’s still usable the next day, and the next. In fact, if it's kept wet and small amounts of lime and aggregate are added you can keep it going for as long as you need. 
  • Bricks laid using lime based mortar can be recycled unlike those bonded with cement. Once glued together with the grey stuff they can only be used for hardcore or thrown away. In direct contrast, lime based mortars can be easily stripped away. 
  • Although people might think otherwise, lime is strong, flexible and permeable. Contrary to popular belief, it will outlive and outperform cement.
  • Traditional buildings laid on shallow foundations and built using lime mortar disperse moisture which means there is no need for any kind of damp course
  • Lime based mortars flex with the structure and so prevent joints and masonry from cracking. 
  • By using lime based mortar expansion joints can be avoided. In direct contrast, the imperviousness of cement mortar prevents the absorbtion of water from the structure whereas lime based mortars act as a kind of 'wick', absorbing the moisture and allowing it to evaporate. By absorbing moisture, lime based mortars keep the masonry dryer and lessen the risk of spalling. 
  • And now the technical bit: thermal movement. All buildings absorb the sun’s radiation and get hot it summer. When this happens the stone expands - hence the need for expansion joints in modern builds. Buildings which have been built with Cotswold stone (limestone) need a natural bonding product which moves thermally at the same rate as they do. Lime based mortars, which originate from limestone, will do this. What this translates to is the fact that neither masonry or the mortar which it binds will crack. However, as cement mortar moves at a different rate to lime based mortars then it is not compatible with natural stone. If cement is ever used to bond natural stone then it should be mixed at a ratio of 13:2:1. That’s 13 spades/containers/bags of sand to 2 portions of hydrated lime to 1 portion of cement. Many builders today do not know this and some will mix cement mortar at a ratio of 5:1 which is far too hard. If cement is ever  used then white should be the colour of choice. Although much more expensive than the grey it will not set as rigid and it will look much better.

As a point of information: The reason we know about the 13:2:1 mix is that builders in the 1940s often used it and 70 years later we find no evidence of cracking. What this means is that the mix is weak enough to accommodate thermal movement in combination with the movement of the buildings. However, there is no evidence to date, to my knowledge, which suggests this kind of weak mix will allow for the evaporation of water.

And lastly, many builders will combine cement with natural hydraulic lime mortar to obtain a chemical set. This is bad practise as it means you get two substances holding a building together which respond at different rates to thermal changes.

Believe me, as a person whose routine revolves around the time it takes for natural hydraulic lime mortar and lime putty to cure, it is very tempting to add cement to make mortar go off more quickly. If I were to do this then my lime mortar would go off the same day it was applied and I wouldn't have to return the next day to brush it down. Just a little white cement would mean a Saturday morning off. But if I went down the cement route then this would mean I’d be selling out to the money which would ultimately result in my contributiion to the demise of Oxfordshire’s beautiful heritage.   And I can't do that.

Having said that, if one wants to plasticise a cement mortar then feel free to go ahead without compunction and add some natural hydraulic lime in the same way you would add hydrated lime. It'll work in the same way. But - in direct contrast to what most people think - what you won't have is lime mortar - all you'll have is plasticised cement mortar.

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