Lime and cement both have their place in construction. However, lime is often overlooked as a weak and expensive building material. However, there are many benefits to a weaker mortar. But is lime or cement stronger?
Generally, cement has a far higher compressive strength when compared to even the strongest limes. Cement can be over ten times as strong as lime. This makes cement perfect for high-rise projects where compressive strength is very important. However, weaker lime mortars are both more breathable and flexible. This makes them a great choice for eco-buildings and historic restoration.
The Difference in Strength Between Cement and Lime
Lime mortars are usually weaker than cement mortars. They have a compressive strength of 0.5-5.0 N/mm2, compared to cement which can have more than 50N/mm2 of compressive strength, making them much stronger.
When building, the masonry must be stronger than the mortars used. If a very high-strength mortar is used with soft masonry, the masonry can be severely damaged over time. Usually, the masonry will crack and can pit and blow out. Over time, the masonry will deteriorate and will need to be replaced.
In modern buildings, masonry such as bricks and concrete blocks are engineered to be stronger than the portland cement mortars they are built with, so there are no issues. However, a cement mortar used on an older building with soft masonry can lead to severe damage. This is where lime building materials come in.
Lime mortar, plaster and render are softer, breathable and flexible, which makes them perfectly suited to restoration projects. The soft mortar protects the masonry, unlike when cement is used. It’s far easier to repoint a wall than to rebuild it with fresh masonry because the wrong mortars have been used and caused damage to the masonry of the building.
Related article: What Happens If Mortar Is Too Strong?
The Strengths of the Different Types of Lime
There are three main types of lime used in the construction industry. Quicklime (lime putty or fat lime), hydraulic lime and hydrated lime (not to be confused with hydraulic lime). Each type of lime has its specific use and strength characteristics. Let’s take a closer look at each one.
Quicklime (lime putty or fat lime)
Quicklime is slaked with water to produce lime putty, also known as fat lime. The mortar, plaster and render made from lime putties are the most breathable, flexible and softest. This makes putty-based materials the first choice for restoration projects on historical buildings.
Putty or fat lime-based mortars cure by a different process than other mortars like cement. They undergo a carbonation reaction in which they absorb carbon dioxide (CO2) from the air. This can be a very slow process, and the long set times are the main drawback to using fat lime mortar, plaster and render.
Hydraulic limes are sold as a dry powder like cement. They are mixed with various sands and aggregate to make mortar, plaster and render. Unlike lime putty mixes, hydraulic limes cure via a reaction with water called hydrolysis. This is the same way cement sets and produces a harder and faster set.
There are three strengths of hydraulic lime. NHL 2, NHL 3.5 and NHL 5. The number represents the approximate compressive strength of the mortars and plasters made with the lime. Hydraulic limes are best used in situations where putty-based limes cannot set (constantly wet areas). They are also used in areas more exposed to the weather and elements.
Generally speaking, you want to use the weakest possible lime for historic renovations; this will give the masonry the most protection. If you aren’t sure what strength of hydraulic lime to use, speak to your local lime supplier or expert.
Hydrated Lime (also known as builders lime)
Hydrated lime can often get confused with hydraulic lime and fat limes. They are confused with hydraulic limes because of the similar name. Fat lime (lime putty) is technically (and confusingly) a type of hydrated lime. It’s important to note that proper fat limes are wet and look like very thick cream. They will usually have matured for at least a month.
The hydrated lime I’m talking about here is the dry hydrated lime powder commonly found in builders’ merchants. This type of lime has been slaked like fat limes. The difference is the amount of water added is specially controlled to ensure the lime stays as a powder.
The hydrated lime powder should be used mainly as an additive to cement mortars. In this way, hydrated lime helps to improve the plasticity and workability of cement mortars. Hydrated limes would produce an extremely weak mortar that may not even cure properly. Hydrated lime should never be used to make a lime mortar by itself.
The Benefit of Weaker Mortars
Many people think that a weaker mortar is worse than a stronger one. This isn’t the case. Weaker mortars, such as lime mortar, can have numerous benefits in construction.
Generally, the softer the mortar, the more porous it is. This means that, unlike waterproof cement-based mortars, lime mortars can help control the moisture within a building. They can absorb water and release it into the air without sustaining damage.
Over time, this results in little to no issues with moisture damage, no mould growth and regulation of the internal humidity levels of a building. This also leads to a significant reduction in condensation. Resulting in a very pleasant internal environment.
Lastly, softer lime mortars are far more flexible than cement-based mortars. This results in the mortar, plaster or renders being less likely to crack. Additionally, it means softer mortars can better withstand the movement of buildings. This is especially important when repairing old or historic buildings with soft masonry that are prone to subtle movement.
The Benefit of Stonger Mortars
Whilst weaker mortars have their place in construction, stronger cement mortars are heavily relied on in modern times. There are many benefits to using a stronger mortar.
First, cement mortars have a much higher compressive strength than lime mortars. This allows for the construction of very tall, high-rise buildings. Cement mortars can cope with the enormous loads placed on them. Without cement, constructing these massive buildings may not be as feasible.
Another massive benefit to stronger cement mortars is their speedy set times compared to lime mortars. A fast set time allows for a faster overall build rate; this can massively reduce labour costs. This fast set time can also be beneficial when time is of the essence, such as before some bad weather is expected.
Cement-based mortars have been engineered to work effectively with other modern building materials. This allows them to be used in many projects where perhaps lime mortars would not be as suitable.
Finally, it’s important to note that the choice between using a cement or lime-based mortar depends on the different characteristics and requirements of the project.