Plastering is the oldest form of decorating walls. Today, there are many types of plasters on the marketplace, with very different properties designed for different applications. Here is a quick overview of the main types of plasters:
Lime Plasters
Lime plasters are the oldest form of plasters. Their history goes back about 7,000 years predating the Romans and Egyptians. They are manufactured from lime, sand and water. Different aggregates can also be added to obtain certain desirable properties (e.g. to make it softer, harder, water repellent, able to set under water etc.). Lime has been extensively used throughout the centuries and has a very rich heritage. It is the right material for old buildings; practically it is the best choice for any building built before the 1900s.
Portland Cement Plasters
The modern alternative, developed in the second part of the 19th century at the dawn of the Industrial Revolution. By adding clay to lime and burning it at much higher temperatures results in a much harder material. Cement is unsuitable for the renovation of old buildings – the next chapter explains why.
Gypsum Plasters
Mainly used as interior decorative finishes on top of other plasters, being very soft they have excellent workability resulting in silky smooth finishes. On the downside: being so soft, in the presence of humidity and salts they easily break down.
Which One is Better?
If you have an old building, chances are that you have heard about the lime-cement debate, namely that “lime is good” and “cement is bad”. This is not quite true. Both lime and cement plasters have their advantages and disadvantages. Both are suitable for some applications and unsuitable for others. The point is: they are very different. Cement plasters are not recommended for the renovation of old buildings (e.g. the ones built before the 1900s); instead lime is the recommended choice. To understand why, we need to look at the fundamental differences between older and newer houses, on how they have been built.
Older vs. Newer Houses
Older houses have been designed and built to be water permeable. They got wet – no big deal – because they dry out. Being built from breathable materials, they naturally dry out. Newer houses on the other hand are built watertight using modern materials (including many plastics) designed to keep moisture out.
Mixing old and new technologies has a detrimental effect on old buildings, usually leading to their rapid decay. Here are some of the reasons why:
Hardness: new, modern building materials are harder. A new modern brick for example is expected to be stronger than a 400-year-old aged hand-made brick. Thus when old and new materials are mixed together, the older and softer materials will give way and suffer, leading to the gradual loss of the original building fabric.
Breathability: newer materials being less breathable, they do not allow for the evaporation of moisture. The accumulated moisture will again damage the weaker and older materials resulting in the decay of the historic building fabric.
Now, this doesn’t mean that you shouldn’t use any modern building material in an old building. Life is an evolution. Buildings are also constantly adapted to new uses to suit a purpose, and this often involves significant changes. However, without extensive knowledge of materials and/or building physics one can seriously damage old buildings – most of the time, unwillingly.
Lime vs. Cement – Why Lime?
Lime is the recommended choice for the repair and renovation of old buildings because lime is:
Lime is the recommended choice for the repair and renovation of old buildings because lime is:
Soft: lime mortars are softer than portland cement plasters. In fact, their hardness can be controlled during the manufacturing process. However if chosen correctly, lime must be softer than the bricks they bind together. Imagine lime mortar as an air cushion on which bricks rest. Buildings are subject to constant ground movement and vibrations. A soft cushioning can accommodate this minute movement without cracking. Over the years the soft lime will erode but it will protect the surrounding bricks, keeping the historic building fabric intact for centuries.
Cement render on the other hand is rigid. It cracks badly, letting rainwater in, resulting in increased dampness and potential frost damage. Cement mortar being harder than surrounding bricks, will erode or crack them, damaging the softer historic building fabric.
Breathable: on molecular level lime allows water vapors to evaporate freely, keeping its surface dry. Cement, on the other hand, prevents evaporation and traps humidity. Moisture under the cement plaster gradually builds up, “amplifying” dampness problems, making them worse. Cement around timber can lead to rot.
Warmer: lime having large air-filled pores, is a good thermal insulator, making it a cozy and warm material. Cement, being significantly denser with few pores, it is much colder which often leads to condensation and mold problems.
Antibacterial: lime being alkaline, it has natural antibacterial properties. It is a natural mold killer. A natural way of disinfecting cellars in the past was to paint them with lime wash.
Eco-Friendly: lime is eco-friendly and carbon-neutral. After reaching its end-of-life it can be recycled: crushed and re-used for the next lot of lime mortar. Cement is not reusable but ends up in the landfills. It also has a heavy environmental impact: the cement industry produces 10% of global man-made CO2 emissions. Heavy metals are also commonly found in cement in non-negligible concentrations.
Why Cement?
Despite many of its advantages, lime plasters are still used a lot less than cement plasters. This happens for several reasons:
History: before the turn of the century (1900s) lime was the “golden standard” for thousands of years. Then cement has been invented. Being waterproof, fast curing, harder, capable of supporting larger loads etc. it quickly became the new “wonder material” allowing properties to be built on industrial scale: more, faster, taller – something which was not possible with lime. Thus lime has fallen out of fashion being largely “forgotten” for several decades. During this time the shortcomings of aging cement came to light – problems of durability, cracking. Poor aging etc. – and after the 1980s the building industry started returning to lime. Today lime is living its second Renaissance, becoming increasingly popular again,
Lack of knowledge / education: due to lime being largely forgotten in the 1970-80-90s, many builders and small traders just don’t know or have never been taught about lime as an alternative. So they just use cement. According to customer feedback about 70% of the tradesman / builders out there do not work with lime, so we know at times it can be challenging to find the right tradesman for your old building project.
Time considerations: lime cures slower than cement, it needs more time to dry. There is a 1-2 days wait period after each coat applied. Cement on the other hand sets within a few hours, making possible a faster turnaround between jobs. As “time is money”, many builders choose to use cement because it’s more cost effective to them; they can get more jobs done in less time.
Things are slowly changing though. More and more building owners as well as professionals are becoming aware of the real benefits of lime. Many heritage organizations and groups are making strides in popularizing lime, a key ingredient in better preserving our old buildings for the future.
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