TELLING ARCHITECTURAL - Building Materials and Architectural Building Products
Using a modern day manufacturing process, the Unilit range of natural hydraulic lime products replicate
the strength, porosity, adhesion and flexibility of historic mortars. Telling are credited with the reintroduction
of hydraulic lime to the UK in the early 1990's and lead the field in quality, range and performance in the use of
natural hydraulic lime. Our knowledge and experience is also without equal.
Developments in cement technology for volume construction in the 19th and early 20th century, together with the
perceived advantage of increased mechanical strength, served to eliminate the need for the traditional skills of
craftsmen to create a lime mortar.
This has led to a distortion of the concept of mortar to such an extent that today a "traditional mortar" is
defined as a blend of cement/sand/lime whereas it is little more than a modern process adopted to satisfy the
demands of mass building.
Most home extensions requiring planning permission for historic buildings will probably benefit from using their
products during the build. Many Architects or House Extension Designers can incorporate their
products within the scheme design drawings and specifications.
The following article may be of interest for homeowners researching this type of building product for inclusion
within their own house extension scheme.
Natural Hydraulic Lime Mortar
By Jonathan Owens
Mortar and Longevity: Will Our Buildings Stand the Test of Time?
Did you ever look closely at the exterior of brick or stone home, and see hairline cracks running up the wall?
Or sometimes the mortar joints have lots of cracks running across them, so that the mortar looks like it could fall
out in 3-inch sections. Even an old (or not so old) concrete block building, like a garage, can have joints
cracking out all over the place.
Natural Hydraulic Lime Mortar
Whether your a homeowner, or someone in the building trades, you may have noticed mortar joint failure, and
maybe you had a thought like, "What is going on here? I thought masonry was the best exterior-supposed to last
Isn't a stone home supposed to last forever? What about the castles in Scotland and the palaces in Prussia?
We are stonemason's, and these are questions we have started to ask also. For us, our livelihood depends on the
answers because we can already see cracked mortar joints in work we did 5 years ago! Could the mortar that we were
trained to mix and use be flawed?
Why do patios and sidewalks crack so quickly-often in less than 10 years? Why does almost every stone, brick, or
block building show cracks in just a few years after it's built? That's not the longevity we expect from using such
historic and time-tested materials!
Materials scientists have been asking these same questions. After studying those castles in Scotland and masonry
buildings all over the pre-modern world, the answers are starting to come out. Turns out the mortar we use today is
not at all historic. Time has tested it, and it is failing the test.
A little construction history might help at this point. For at least 7,500 years, man had been using (roughly)
the same process to make mortar: burn high-calcium limestone by layering wood and stones inside of a really fat
chimney (kiln) and then lighting it on fire. The resulting burnt stones are then crushed and mixed with sand and
water to make mortar. The burnt lime reacts with the water, causing it to get sticky and then harden, lasting for
centuries or longer in between the stones in a wall.
This lime is called hydraulic lime because it hardens without the presence of air. Getting hard is a chemical
reaction that is different from just drying out. It will get hard under water.
Now don't confuse hydraulic lime with hydrated lime. Hydrated lime is a different process, a different material
altogether. Hydrated lime can't be used as the binder in mortar because it never gets hard. Hydraulic lime
Now, jump forward in the history of mortar to the late 1800's when various inventors began experimenting with
new processes and materials for making cement. Portland cement, the almost exclusive binder and hardener in today's
mortar, concrete and stucco. It got its name from the Isle of Portland in the English Channel where limestone had
been quarried for centuries and admired for its building qualities. By 1878, the British government had issued a
standard for Portland cement, and in 1907, production began in the United States. It came to be the main ingredient
in mortar and concrete throughout the country by the end of World War II.
Now, Portland cement has proven its superiority to hydraulic lime in many departments.
In the speed-of-getting-hard department: Portland's the champ.
In the waterproof department: no contest. Portland wins.
In the hardness department: Portland wins again.
Game over? Not yet.
As it turns out and according to research on the old hydraulic lime mortar, using Portland may be a strategic
error. At least, as far as longevity is concerned.
It all boils down to the way we think about buildings and how they weather. Everyone knows that the point of a
building is to keep out water, right? In recent decades research on building materials and techniques has gone
farther and farther down the road of keeping out 100% of all moisture and all air. Now we are combating mold, air
quality, and condensation problems.
But back to the Portland vs. hydraulic lime debate.
In the longevity department: no contest. Hydraulic lime wins. Hands down. Why?
Yes, Portland cement seals out water. Hydraulic lime allows water to penetrate. The problem is that most masonry
units (like brick, stone, and block) absorb small amounts of moisture from the air and rain. Hydraulic lime acts
like a wick to get that water back out - FAST!! Portland won't let the water pass, trapping it in the wall where it
does damage-cracking the joints and even the faces of the bricks or stones. That's why you see the faces of old
brick buildings popping off. Repointing with Portland destroys the building-FAST! Repointing an older building
using Portland cement starts the countdown to it's demise.
Yes, Portland cement is harder. But harder is also more brittle. Portland is fired at about 2,600 degrees
Fahrenheit, as compared to hydraulic lime's 1,800 degrees. When you look at the two under a microscope, hydraulic
lime particles are like plates that interlock; Portland's are like needles. Any movement in the building is going
to make Portland crack all over the place, while the more flexible lime mortar can move with the building without
It gets better.
When you get down to the microscopic level, Portland cement has salts in it that actually degrade the mortar
from the inside out. This stuff starts decomposing as soon as it gets hard!
You guessed it: hydraulic lime has a little secret of its own, and its not the kind that brings the wall down.
Hydraulic lime has small amounts of free lime-lime that never reacted with the water in the beginning, after it was
burned. This free lime actually dissolves in the water that is escaping out of the wall, and in the process it
fills any cracks that may have formed. The experts call it "autogenous healing." Like a lobster growing back its
claw, I guess.
No wonder they used it for 7,000 years, Plus. No wonder the historic restoration movement is switching from
Portland-and-lime mortars to historic, hydraulic-lime mortars.
Hydraulic lime is still a bit hard to find in this country. To my knowledge it's not produced in the U.S. yet.
What we use is imported from France. Different grades can be used to make plaster, stucco, lime paint, mortar, and
So next time you're wanting you're chimney repointed, a stone or brick historic building restored, find a
historic restorations contractor who knows about hydraulic lime. As a mason, the choice is clear to me. What's the
point of building new or restoring the old, if our work is not going to stand the test of time?