How Long Does Lime Mortar And Plaster Remain Workable?


There are a few different types of lime, and they all set via different processes. This can make it difficult to understand how long a lime mortar will remain plastic and usable. In addition, certain lime building materials have shelf lives while others don’t. So, how long does lime mortar remain workable?

Putty-based (fat lime) mortars will remain workable indefinitely as long as they are kept in airtight packaging and frost free. Pre-mixed lime putty mortars are often sold in plastic bags, which can be stored for years before use. Pre-mixed putty mortars gain workability as they mature, effectively improving with age. Hydraulic lime mortars react with water. Usually, as soon as they are mixed, they begin to set. Depending on the strength, they can remain workable for up to 24 hours after water is added.

Many factors can influence how long a mortar or lime will remain workable. Let’s investigate how long specific mortars and plasters should remain workable and how long they can be stored.

Lime Putty (fat lime) Mortars And Plasters

Lime putty (also known as fat lime) starts its life as quicklime. Quicklime is burnt rock such as limestone and is highly reactive with water. If exposed, quicklime reacts with water and moisture in the air. This process is called slaking. Quicklime will keep well in airtight and waterproof packaging. In theory, it should keep indefinitely as long as no air or moisture can get to the quicklime. In practice, this can be tricky; it’s usually recommended that bagged quicklime is used within 12 months to ensure it’s still in good condition.

Once quicklime has been slaked (mixed with water), it is known as lime putty or fat lime. Lime putty sets via a reaction with carbon dioxide in the air. This means it can last for years as long as it’s kept airtight and free from harsh sunlight and frosts. Putty is usually matured for at least one month for building mortars and three-plus months for plasters. Usually, the longer lime putty is matured, the better its workability and plasticity (the ease of shaping and moulding a material).

Mature Lime putty.

Because lime putty reacts with carbon in the air and not water like cement or hydraulic lime, it can be mixed with various aggregates and stored as a wet ready-to-use mortar or plaster. These are called pre-mixed mortars and plasters because they require the tradesperson to give it a quick mix before application (known as ‘knocking up’). Over time, pre-mixed mortars may stiffen up in the bag. This is normal and doesn’t mean they aren’t usable. Once knocked back up, it will begin to become more plastic. During this process, try not to add any water.

Some pre-mixed mortars will contain pozzolans. Adding a pozzolan to a putty-based mortar gives the mortar very similar characteristics to hydraulic limes. Pozzolanic materials such as ash will react with water to help the mortar form a set. Pre-mixed pozzolanic mortars and plasters will usually have a recommended use-by date.

Related article: What Are The 3 Main Types of Lime Used in Construction?

Hydraulic Lime Mortars And Plasters

Hydraulic limes differ from putty-based limes because they are set via a chemical reaction with water called hydrolysis. This means that they cannot be stored as pre-mixed mortars, as any water or moisture will cause the chemical reaction to begin. Hydraulic limes are sold in paper bags that sometimes have a plastic liner. However, most do not so bagged hydraulic limes must be stored in a dry, sheltered, and frost-free environment.

There are three different strengths of hydraulic lime. NHL 2, NHL 3.5 and NHL 5. The bigger the number, the stronger the lime. As the strength of the lime goes up, the less time it will stay workable once mixed. Generally, hydraulic lime mortars and plasters should remain reasonably workable for 24 hours. It’s best to use bagged hydraulic lime powder six months after purchase when stored well; the official use-by date is 6-12 months.

hydraulic lime in paper bags
Hydraulic lime bags

Of course, using a strong hydraulic lime and putting additives such as ash in the mortar or plaster will reduce the time it will stay workable. If you intend to leave a hydraulic lime mortar overnight, you must ensure it’s stored well. It must be kept free from frost, drying winds and rain. Your best bet is to ensure it’s stored in a cool, dry area and is covered with a damp cloth, such as hessian. This should prolong the mix until the following day. Remember, with lime mortars, always thoroughly remix the mortar before adding water.

Hydrated Lime And Cement Mortars

Hydrated lime, also known as ‘builders lime’, is quicklime with a controlled amount of moisture added so that it has slaked but remains a powder. This type of lime is often confused with hydraulic lime because the names are similar. It should never be used as a replacement for hydraulic limes, quicklime or lime putty. Mortars and plasters made from hydrated limes will almost certainly fail. This is why hydrated lime is often considered an inferior lime.

Most commonly, hydrated lime is added to cement mortars. This helps improve characteristics such as workability and plasticity. Lime mortars and plasters are naturally much more fat, plastic, and sticky than cement mortars and plasters. Because limes improve the workability of mortar and plaster, adding even inferior lime (hydrated lime) to cement mortars will increase their workability. Hydrated lime should only ever be used as an additive and not to make mortars in its own right.

Related article: Why is Cement Used Instead of Lime in Modern Buildings?

Don’t get me wrong; hydrated lime is a perfectly good choice if you’re only using it to help improve the workability of cement or gypsum. Because it is already slaked, it is much safer and easier to use. It’s also cheaper than the other limes.

Because cement sets so quickly, it will also lose its workability quickly. Some cement mortars can lose their workability in as little as an hour. Even when mixed cement mortar is stored well, it will still not last overnight like some lime mortars. As a general rule, cement mortars lose their workability within 90 minutes of leaving the mixer, so they must be used relatively quickly.