N H L is a three letter abbreviation  which stands for 'Natural Hydraulic Lime'. There are basically 3 options available: NHL 2, NHL 3.5, and NHL 5 They are easily  available in 25 kg bags and are pre-mixed in dry, powder form to be added into a mix in a very similar way to cement being added to a mix, so from the point of view of a newcomer to limes they are quite attractive. But it's not quite as simple,e as that! 

Firstly people really do need to know about lime, and as we have touched on in some of our other pages, there's really only two types of lime that people need to know about for pretty much any type of general repairs,building works and internal plastering / external rendering works.  (We like to keep things simple) :- Non Hydrauli lime and Hydraulic lime. BUT as you will see We are going to provide av few areas, becsuse we think it's  a maze that does need roper  mapping to get to the other side and really find out where this old chestnut began. 

1. Non Hydraulic Lime

NHL's really appeared in the 1990's, but Hydraulic Lime mixes have been around for a very long time, in fact a few thousand years before the 1990's!

Hydraulic lime mixes were more of the ingenious works of the Romans. the Romans were great experimeters with lime, and amongst many other experiments they carried out  they  discovered that by adding Calcium Silicates  and Calcium Aluminates to quicklime (Calcium Oxde), they could produce a lime which was faster setting and also had a greater density meaning it had higher compressive strengths to non hydraulic lime mixes, which was  because the water interacts with the added Calcium Silicates and Calcium Aluminate and causes this faster setting, hence the term "Hydrauiic Lime" was born.

Now, just to start your confusion, in today's world of Lime we have hydraulic Lime, Hydrated Lime,and we also have Calcium Hydroxide! And just to throw in another chunk of confusion we also have Calcium Hydroxide which are all common terms in Lime, but all need to be completely separated so there are no naughty mix-ups happening, We'll try our best here  to make some everyday English language out of it all, to make life easier:  

1. Hydraulic Lime: you're reading about it now
2. HYydrated Lime: this is powdered Quicklime which has been left out to absorb some atmospheric moisture, resulting in a partially slaked lime (or as the old boysc who taugh us, used to call it: "Girls Lime"), now don't go crying, the world actually had a sense of humour before pronouns and green hair took over! 🤣 👍 - now to level things up, if you see your big, burley builder arriving at your property and  unloading bags labelled "hydrated lime" to carry out some lime repointing works, then call him a girl and send Her away quickly because this is an old trick where they will make out that they know about lime repointing, but that Hydrated lime is simply going to get mixed with common cement once your back's turned to create a very fast setting mortar which simply means a fast increasing bank balance for him/Her/,   whilst leaving you with a mortar that is too hard for your lovely soft period bricks and will eventually just trap water which will lead to internal damp issues in your home.
3. Calcium Hydroxide: this is Quicklime that has been hydrated - it's commonly known today as 'Lime Putty' which is fully slaked Quicklime.

But again, it's important to remember that these three are all individual and must remain as seperates, at least until you've got a few years of experience and knowledge behind  you. 

Calcium silicates and calcium aliminate are compounds that  harden in contact with water. This contrasts with calcium hydroxide;  also called slaked lime or air lime that is used to make lime mortar, the other common type of lime mortar )non hydrauli lme), which sets by carbonisation (re-absorbing carbon dioxide (CO₂) from the air). Whereas NHL's set by hydration and due to their added calcium silicates and calcium aluminate  compounds which harden in contact with water. This contrasts with calcium hydroxide which is also called slaked lime, lime putty  or air lime (colldctinely n

known as non hydraulic lime) that is used to make lime water which also sets by carbonisation (re-absorbing carbon dioxide (CO₂) from the air). Hydraulic lime provides a faster initial set and higher compressive strength than air lime, and hydraulic lime will set in more extreme conditions, including under water.

2. Non Hydraulic Lime

Now here's some more potential confusion because this could also very easily be abbreviated to HNL, but it isn't! It is very often called "air Lime"  because it works by the water content first evaporating out into the atmosphere and then hardens by absorbing CO² from the atmosphere, so why isn't Mr 'Net Zero' Milliband calling for the use of lime in new buildings?

The production of modern day Cement requires a lot of heat in order for the limestone content, which accounts for 40 - 60% of today's cement to drive out the moisture and the CO² from the stone itself, which then obviously increases the quantity of CO² in the earth's atmosphere.

Non hydraulic  lime products re-absorb atmospheric CO² in order to harden,  modern day cement doesn't re-absorb any - we are going to copyright our thinking (in fact we already  own the copyright to everything we have written on our website) so Heritage Lane today, saving the world tomorrow, move aside Mr Net Zero and learn!

Our tests

We don't have high quality laboratorys like the big manufacturers have, so we simply carried out some tests in our own workshop that We reconfigured a bit to make life easier for us. We managed to find an old, but fully functioning 'Kenwood chef' food mixer which came with a stainless steel mixing bowl (perfect for when the lime goes into 'exothermic' mode, as todays plastic bowls tend to melt a bit fast. We made 3 non hydraulic lime  mixes in September 2024 and we also made 3 Hydraulic lime mixes. They were all formed in cubes measuring 6 inches x 6 inches x 6 inches ( 150mm x 150mm x 150mm ) because these are the standard sizes used in conventional concrete testing (plus they're the sizes that we used to make many years ago when we carried out concrete testing in our old college lab when we were apprentices.

We asked an Engineering friend to build us a hydraulic ram operated crushing machine that would create up to 10 tons of pressure, and we started testing our cubes to destruction. But that didn't last long because we were concerned that each time we made new cubes, and Despite us meticulously weighing each gauge each time,  we felt that we would be better by carrying out Non Destructive Testing because that mean't the exact same cubes were periodically tested which is more relevant to lime mortar pointing for example. So after some searching through the memory banks we found a rebound hammer and started over again  with measuring the compressive strengths of the Hydraulic lime mixes made by our "chef": Mr Ken Wood!

Now. We could simply put our monthly measurements on here, but that seems like a lot of effort to state that we found no increase in the compressive strengths of either of the 3 common NHL's: 2.0, 3.5 and 5.0

And just for a bit more context, for each grade we created a builders sand mortar mix for repointing, a lime / sharp sand mix for ecreeding and a lime-crete  mix.

Each mix was meticulously labelled with it's contents, it's claimed compressive strength an it's mix date, then tested once per month, over 12 months, and to reiterate we did not see any measured increases nor decreases in the claimed compressive strengths of each mix.

Now for our  big conclusion!

We think that someone somewhere maybe once read about Roman Hydraulic Cement which  was used in Roman  concrete and they got their wires very crossed.

Being completely honest we have have simply copied and  pasted the last few paragraphs below from a wikipedia page for  two reasons : 1) it's far quicker than hours of typing when  2). Dinner and bed are calling.

Roman  Concrete, also called opus caementicium, was used in construction in ancient Rome. Like its modern eqivalent , Roman concrete was based on a hydraulic setting cement added to an aggregate. 

The Pantheon in Rome is an example of Roman Concrete construction. Cesarean harbour is an example of underwater Roman concrete technology on a large scale. Many buildings and structures still standing today, such as bridges, reservoirs and aqueducts, were built with this material, which attests to both its versatility and its durability. Its strength was sometimes enhanced by the incorporation of pozzolanic ash where available (particularly in the Bay of Naples). The addition of ash prevented cracks from spreading. Recent research has shown that the incorporation of mixtures of different types of lime, forming conglomerate "clasts" allowed the concrete to self-repair cracks. Roman concrete was in widespread use from about 150 BC; some scholars believe it was developed a century before that.

It was often used in combination with facings and other supports, and interiors were further decorated by stucco, Fresco paintings, or colored marble. Further innovative developments in the material, part of the so-called 'concrete revolution' contributed to structurally complicated forms. The most prominent example of these is the Pantheon dome, the world's largest and oldest unreinforced concrete dome.

Roman concrete differs from modern concrete in that the aggregates often included larger components; hence, it was laid rather than poured. Roman concretes, like any hydraulic concrete, were usually able to set underwater, which was useful for bridge stantions and other waterside construction like Betty's for tying ships to.

And that's our take on why there is far too much incorrect online information regarding Hydraulic time's today:someone read something and completely got it wrong, but of course they just started some Chinese whispers instead of carrying out some of their own simple tests, just as we did. But as a guess, armchairs must also be comfortable for sitting and reading in before making yourself out to be an expert in lime 🤔

So could this be where the false claim that NHL's continue to harden over time originated? We think that's a pretty strong  possibility, for the reasons we've shown above.