Type smeaton project into Google and it brings up a series of interesting articles about the mixing of lime and cement in mortars. What is or was the Smeaton Project? John Smeaton (1724-92) was an eminent Georgian who is sometimes called the father of civil engineering. He built canals and bridges and, perhaps most famously, the Eddystone Lighthouse off the Devon coast. In order to do this he began tinkering around with lime mortars, trying to get them to be waterproof and this work led onto the invention of cement, as we now know it, in 1824.
In its early years, Ordinary Portland Cement (OPC) was expensive and it was used quite sparingly, mostly as an additive to lime mortars to help them set. The typical Victorian building site used lime mixed with sand for its mortar and whilst this was fine for internal work, it proved very slow to set in cold and wet weather and any additive which speeded up this process was welcome. Thus started the practice of mixing lime and cement.
In the 20th century, the use of lime declined and OPC started to be used exclusively for mortars and concrete. These days, lime kilns have become a relic of a bygone industrial age and OPC is king throughout the world.
But 15 years ago, a lime revival got underway and slowly but surely it has gathered pace and momentum. Conservationists got interested and bodies like the Society for Protection of Ancient Buildings and English Heritage began beating the lime drum. However, the lime revivalists were split into two camps: on the one hand were/are the purists who believed that lime simply made better mortars than cement; on the other, the traditionalists who saw no harm in adding a bit of cement, or even mixing lime and cement half and half, so as to get the benefits of both.
And so a bun fight ensued. People began arguing about all manner of weird and wonderful properties that different mortars might or might not have. Advocates of lime mortars said that they were better because they could “breathe” and “self-heal” and that they were “simply beautiful.” Conservators became terribly enthusiastic about the use of lime to repair old buildings and terribly negative about the use of over-strong cement mortars. And arguments raged about sustainability: cement production was said to be responsible for 3% of the world greenhouse gas emissions. Or was it 6%. Or even 8%. In reality nobody has a clue, but it’s a big number, because cement production is a) very common and b) uses a lot of energy.
Not that replacing cement with lime would really help matters, because lime also uses a lot of energy to produce, but when a bun fight has started, who cares about boring little things like logic.
But whilst open season was declared on cement-heads by the limeys, the lime revival was still split down the middle by the disagreement between the purists and the mixers. This is where the Smeaton Project came in. Because this was a piece of research, funded by English Heritage, which looked at how the different mortars performed and it came to a significant conclusion. Just chucking a bit of cement in a mainly lime mix was not a good idea because there was a danger of something called segregation.
Here’s what Graham O’Hare, Conservation manager of Wells Cathedral Stonemasons, had to say about this topic
Segregation is a major hazard of gauging lime mortars with cement. As the mortar sets, the cement colloid tends to migrate into the pores of the lime mortar as they form, clogging them and leading to a greatly reduced porosity. If the proportion of cement is high enough, segregation is much less likely to occur, but the resulting mortar will be hard. If the cement proportion is low, the mortar will be less hard, but segregation is more likely to occur. The resulting mortar will be seriously weakened, with a poorly formed pore structure leaving it very susceptible to frost damage and deterioration, even after carbonation of the non hydraulic lime present has taken place.
It sounds just dreadful, doesn’t it. “Major hazard”, “clogging”, “greatly reduced porosity,” “seriously weakened”, “poorly formed pore structure”, “susceptible to frost damage and deterioration.” What a catalogue of woe! What we have here is an attempt to drive a wedge between the purists, who are delighted with the results from the Smeaton Project, and the mixers. But there is something just a little bit too alarming about these findings for my liking. Whilst they may have found segregation taking place on certain mixes, how can they be so sure that all these horrors will result? After all, the test of a good mortar is how it behaves after 50 years in place. The choice of language used by Graham in this piece is, to my mind, a dead giveaway that we are not seeing pure science at work here, with its understated conclusions and room for doubt, but an attempt to rubbish a practice that has been in widespread use since the 1830s.
What’s ironic is that Smeaton himself was pioneering the way for cement to be used in place of lime, whilst the Smeaton project seems to be doing just the opposite.