Lime mortar or cement mortar? While both materials are used in construction, some might remain unclear on how they differ and unsure of what to look for when trying to identify a mortar. This guide will introduce various types of mortars, how they differ, and a few identifying markers for each.
It’s worth noting that while visual characteristics can be a useful starting point, identifying the difference between mortars without laboratory analysis can be difficult due to the similarities between some binders. As such, this guide only introduces each and is not a comprehensive methodology for identifying mortar types.
The use of historic lime mortars and how they differ
For thousands of years, lime mortars have been used worldwide and are still used today. Historically, in the UK it’s not uncommon to find that buildings constructed with stone, brick or cob were often built with lime mortars or renders. However, historic mortars will vary considerably depending on the period and location the mortar was used.
Pre-1800s and heading into the early 1800s, many of the mortars used were quicklime mortars. These mortars were created by mixing quicklime and aggregates, such as coarse sand. The quicklime was then slaked by adding water during the mixing process. A database interrogation of the mortar analysis performed at the Scottish Lime Centre found that around 4/5ths of mortars of this age were made this way. See our helpful guide for more information on quicklime and hot mix mortars.
Quicklime mortars often have tell-tale lime inclusions. They will appear as soft, white, powdery lumps in the mortar. These inclusions are created where the quicklime was slow slaking and only converted to building lime over time. Lime inclusions are pretty easy to spot in most cases and very recognisable once you’ve looked for them. In addition, fragments of coal or charcoal can usually, but not always, be found coupled with these lime inclusions, as they are often left over from the original cooking of the limestone.
In the UK, in areas along the early railway lines or cities with an industrial coal mining heritage, like Bristol or Cardiff, you will find mortars with excessive quantities of fuels or fuel remains present. These mortars are known as ash mortars, where ash gave the mortar a charcoal grey colour. Occasionally, ash mortars may contain no aggregate, with only lime and ash used to make the mortar.
As the 1800s progressed and kiln designs, commerce, and industrial transport improved, natural hydraulic lime mortars became more commonly used. Naturally hydraulic limes (NHLs) are a product of their source rock, so early on were limited to regional areas only. Examples of NHLs being shipped worldwide from the UK, such as the famous Charlestown lime in Scotland, became more of a familiar picture. The most common NHLs in England come from the Blue Lias belt, which stretches from Devon to the Humber River.
Identifying natural hydraulic limes by eye can be tricky, as often, but not always, lime inclusions can be present. The presence of these inclusions depends on how the lime was slaked originally and how strong the lime was. Many very early limes will be feebly hydraulic in nature as the localities of truly pure limestone in the UK are very limited. To identify specific hydraulicity, we recommend performing a mortar analysis. When in doubt about a mortar or before undertaking a project, seek advice from a professional or get in touch to learn more about our mortar analysis service.
The use of natural cement and earth mortars
Natural Cements were initially patented in 1796 by James Parker. Over the next one hundred years, various versions would appear around the United Kingdom as new source rocks which met the mineralogical requirements were identified.
Natural cement mortars are often binder rich, strong and typically characterised by their distinct colouring, often an iron-like orange or red. Lime mortars on the other hand often develop their colour from the sand used.
Throughout London, you will see a plethora of natural cements used as both mortars and renders. Old natural cement renders often exhibit hydraulic shrinkage cracking, which looks like the pattern of dried clay on the bottom of an empty river bed.
Grey natural cements also exist and are found around Dorset and Devon, with some more limited examples elsewhere in the UK.
Earth mortars are considered somewhat common and are often created using a sub-soil material, typically sourced locally. However, earth mortars may be rare in locations where access to lime was readily available for those who could afford it. Those working with this type of mortar would bed the stone into the earth and often point the joints with lime mortar afterwards.
It’s not uncommon to also find that quicklime has been added to these mortars, which can be identified by the presence of soft, white and powdery lime inclusions. Lime addition rates in earth mortars are generally lower than when used as a sand and lime mortar in our experience.
The difference between historic and modern cement
Modern Portland cement mortars differ entirely from the original historic Portland cement mortars. The original Portland cement, named after the colour of Portland stone, is believed to be no stronger than a modern NHL5 or grey natural cement in nature. It can be easy to confuse stronger hydraulic limes with early Portland cements, as they can be very similar in chemistry. Modern Portland cements, however, are usually quite noticeable as they are predominantly a strong grey colour and finished with a smooth surfaced texture. In addition, cement mortars are typically very hard and brittle in nature; breaking a piece by hand is often difficult.
It is also possible to find that white cement has been blended with a hydrated lime which can give the appearance of a lime mortar. However, the strength and water absorption characteristics are the easiest to identify outside of a lab as they are usually quite strong and impervious. When looking under a microscope or magnifying lens Portland cement has an almost glasslike aesthetic quality to the binder, where lime tends to look more crystalline and fluffy.
After an initial visual inspection, we recommend completing a mortar analysis if you’re ever unsure what the mortar is. It is also often required to have a mortar analysis report to get listed building consent to perform repairs or renovations to a property. It’s worth noting that typical mortar analysis reports for listed building consent are often performed as “wet bench” type experiments. This method may not provide a detailed analysis of the specific features of a mortar. A more thorough report using methods such as thin section, soluble silica testing and XRD will give substantially more information and better inform what the mortar comprises of in greater detail. While not often necessary for listed building consent, it may be worth considering depending on the nature of your project.
The information provided by Cornish Lime is for informational purposes only and does not amount to a specification. Every project is unique, so please consult a professional before undertaking a project. Use of this site and reliance on any information on the site is solely at your own risk.
