Summer 2016, RM writes:
Anyways, Great site!"
This page documents work to refurbish and improve an 1850 brick
homestead. It is a Federalist style house set on a stone
foundation forming basement walls. Above ground walls are solid
12" brick, with beam joists set into wall pockets for the 1st and 2nd
floor. About 50 years after the original house, a "T" addition
back was built to house a kitchen and servant quarters. Electricity was
brought to the house in about 1935 and limited forced air heat was
installed with a coal burning furnace in the basement. All
the chimneys remain operational and 3 of the 5 fireplaces have been
converted to wood stoves and provide heat (with a newer oil forced air
furnace backup) all winter.
The brick repairs were extensive. The newer addition needed the
most work. It was built 2-bricks thick and the outer layer was missing 1/2 to all of the mortar in places. The bricks
were soft orange-clay bricks fired on site in a kiln furnace.
Engineering Mortar - Theory
owning older homes use lime mortars.
If you have a brick building prior to about 1860, it is probably built
with softer--maybe even hand fired--bricks and a lime mortar. Portland Cement
was not invented until 1824 and it was
incorporated into residential homes through the late 1800s. If
you have soft bricks, you need to use a lower
compressive strength mortar. Additionally, older construction
techniques counted on the brick walls being able to breath out the
mortar joints, which cement mortars will not do.
Many masons have high volume constructions skills that are not
suitable for restoration work. Putting Lowe's or Home Depot Type-N
mortar on your house older than
1890 is a mistake. If you want to be convinced, see the
photo of one of the additions on my house - it is an 80 year
study in how cement destroys your older brick walls. Well,
literally it's not the cement but rather the water in the bricks that
wrecks the wall. However, if the mortar properly let
the water pass out (lime not cement), it would not come out through the
bricks and destroy them. Although rain and water washing down the
bricks is most damaging, simple humidity passing through the wall and freezing on the outside has a
similar cumulative effect.
if you're flipping a house or will be out of it in 2-5 years, 98% of
the population will not know the difference, and you could cheaply make
it look good and leave it with an unsuspecting buyer. Somehow
have been used since antiquity, leveraging the chemistry of the lime cycle.
Carbonate (“limestone”) is heated to drive off Carbon Dioxide, leaving
(“quicklime”). This is sold in a
powder form and can be prepared for use by the user by soaking in water
for several days, generating heat and a lime putty. More
conveniently, it can be commercially hydrated adding water to make
Calcium Hydroxide (“slaked lime”), which is conveniently still a
powder. This, in turn, is sold as
hydrated lime powder, which when exposed to carbon dioxide cures back
to the original Calcium Carbonate while giving off water. Notice
an open bag of lime
will slowly absorb water and carbon dioxide over the course of weeks
and months, so seal it up tight in
plastic if you want it to keep.
Here is the cycle of lime, starting with the limestone rock you dig up
from the ground (limestone rock is Calcium Carbonate):
CaCO3 -> CaO + CO2 (crush the
rock and cook it very hot)
CaO + H2O -> Ca(OH)2 (put water back in
because CaO is
not stable; old time done at the job site by "slaking" lime and keeping
it as a putty, but in modern days just buy dry calcium hydroxide in
(use the lime as a mortar at this point)
Ca(OH)2 + CO2 = CaCO3 (the mortar absorbs carbon dioxide and
turns back into rock)
Solid material (for example mortar set into place between bricks)
doesn’t absorb gasses well, so to properly
cure, the calcium hydroxide needs to be kept moist
back to calcium carbonate. That way the carbon dioxide can absorb
into the mortar and get to the lime. Lime mortars require misting or moist burlap bag
coverage for 4-7 days after being used.
Different than hydrated lime, hydraulic
lime has pozzolan additives
that make it set firm in an hour or two instead of many hours.
Additives could be done artificially at a job site, but because of
production controls, this is not favored. Plus, who has large
piles of powdered clay, fly ash, or crushed volcanic rocks to use as
additives? At some places in the world “just the right
amount of the right stuff” comes out of the ground with the lime …hence
“Natural Hydraulic Limes.” Because it is imported to the United
States, it’s pretty expensive – running $45-$65 per 55 pound bag.
The composition is well controlled and comes in three strengths NHL 2,
NHL3.5, or NHL5 (the numbers are compressive strength in N/mm2).
This is good stuff if the careful labor of keeping it moist while
drying is done.
The problem is that NHL are not used in huge new building quantities so
chances are nobody around you carries the stuff. In south central
Ohio, big mortar cement suppliers mostly haven't heard of it. Two
had heard of it before, but were not interested in ordering more than a
skid pallet full. The big distributor is Trans Mineral USA. One of their
very active distributors is LimeWorks.
LimeWorks lists a few retailers
in the central and eastern US.
As an alternative, many folks are mixing in small amounts of Portland
cement to an otherwise pure hydrated lime mix to make lime mortar set
firm quicker yet still show many nice
character qualities of lime mortar. Hydrated lime can be obtained
for $4-$8 per 50 pound bag. There are a few established
recipes, which come in different types, delineated in decreasing
compressive strength by the phrase MaSoN wOrKs. The National Park
Service (scroll down near the bottom) and the University of Vermont agree on this recipe data, which
are all relative volumes of dry material. Lime in the table is
either Type-S lime
(not Type-S mortar) or NHL
(“Natural Hydraulic Lime”).
|slaps, mortar for hard stones
|base coats, stones
|Quickrete 60lb "Type-N" mortar from Lowe's or Home Depot
|290, 757d, 42012m|
|mortar for weak stones, finishes
|"L" (lime only)
In addition to compressive strength, it’s important to understand that
concrete blocks water vapor flow through the brick constructed
project. It forces the vapor out through the bricks rather than
allowing it to pass through the mortar. The result can be
spalling and degradation of soft bricks. Only the NHL and L
mortar do not have cement. NHL cures up a lot quicker like a cement
mortar. It's only technical downside is that it must be babied
when curing - keeping it actively moist for 4 days or more. It is
also approximately 9x more expensive! Assuming a 1:2.5 rate, NHL mortar (lime after diluted with
sand) will be about 2.5x the cost of hydrated lime mortar.
- Type-L (lime only) at a ratio of 1:3 has been tested
to show compressive strengths of 261 psi after a day to 725 psi after
- The compressive strengths of 1:2.5 NHL mortars are after 12
months, taken from and Trans
Mineral's test results and NHL product data sheets. Sand ratios of 1:2 or 1:3 pull the
strength higher or lower by about 25%-30%. See LimeWork's Tech
- The compressive strengths of MSNOK mortars are taken from the NPS
restoration documents and the University of Vermont.
- The NPS document recommends for "Minimally durable soft hand-made
brick" a Type L, K, or O based on sheltered, moderate, or severe
exposure to weather.
- Notice Type-K is the same ratio
as Type L (1:2.5) with an additional small 1/3 component of cement/sand
(1:3.5). I cannot explain why it's compressive strength is not
the same as Type-L plus a
little bit since it should be at least as good as Type-N.
When combining cement and lime, there have been some studies that the
cement crystalline structure competes adversely with the calcium
carbonate crystalline structure, and so purists would choose the higher
cost pure NHL, while a pragmatic person might use an NPS recipe.
Instead of buying cement, and mixing with lime and sand, according to the table above, it may be
buy bags of Quickrete Type-N mortar pre-mix (750 psi compression strength) from Lowe’s or Home Depot,
and use that as
base. It already has cement and a nice aggregate size
distribution. The downside is you don't know the pedigree or
quality of the cement or if the "weaking agent" is lime. Some Home
Depots are selling only Type-N "Masonary Cement" which makes the same
thing after you add 3 parts equal volume of sand, but way the aggregate
distribution is your own choice.
If one assumes commercial Type-N mortar is
per the recipe table above, it can be “converted” to Type-K by adding
only lime and sand.
||1:1:5 (7 parts)||1:1:5 (7 parts)
|0:2:5 (7 parts)||0:0:3 (3 parts)
||1:3:10 (14 parts) essentially Type K|
|1:2:8 (10 parts) esentially Type O
Summarizing, 7 scoops of Type-N commerical mortar, plus 2 scoops of
hydrated lime, plus 5 scoops of sand roughly makes up Type-K mortar. 7
scoops of Type-N commercial mortar, plus 3 scoops of sand approximates
One architect recommended old houses with clay bricks be mortared
with a mixture of 1 bag Type-N and 1 bag of lime, plus about 12 scoops
(1 bag) of sand. Because lime comes in 50 pound bags that are
physically larger than 55 pound bags of Type-N, there is a maybe a 10%
error in the math, but this is often swamped by field conditions and
small changes to handle the environment at the job site.
1 bag Type-N mortar
||1/7 bag: 1/7 bag: 5/7 bag
1 bag Hydrated Lime
||0 : 1 : 0
~1 bag sand
|0 : 0 : 1
1+1/7 : 1+5/7
Multiply by 7 to keep ratio but simpler numbers. Approximiately Type-K mortar.
So, this architect would end with with mortar similar to Type-K mortar except it has 2.5x as much
lime. It’s a lot purer lime, and has relatively less
concrete. This is probably good for older, softer bricks if the
worker is willing to soak the bricks before applying the mortar and
keep them moist for about 4-7 days afterwards -- lime cures a lot slower than cement.
Testing Some Mortars
From 30-40 years of experience, the mason working with me really wanted to
sand than the NPS recipes indicated.
All his mental recipes were
done with bags and scoops (shovel fulls). For him, 5 "scoops" was equal to a 5-gallon bucket full up to where the
handles connect. Turns out 1/2 bag conveniently also filled up a 5
gallon bucket. So.. 5 scoops is about 1/2 bag.
He was comfortable with
cement, a bag or two of lime, and 10-12 scoops of sand, equivalent to a
1: 1.5: 1.1 recipe. Mixing half-bag recipes, he really did
not want to go over 7 scoops of sand,
and preferred 5 scoops of sand. This means his max-sand was a
1:1.5:1.4 recipe and he wanted to use a 1:1.5:1 recipe.
Basically, he was okay with a high lime content like Type-N mortar but
wanted higher strength by cutting back on the sand aggregates.
I mixed up four types of mortar for comparison. See the photos
||The left two blobs use Type-N cement and Type-S Hydrated
Lime from Menards, 1:3:1.4. This is the K recipe with a lot less sand
since 1:3:11 was way too
much sand for the mason to be comfortable. By the end of the
project we had adjusted to 1:2:1 for the "new" part of the old
The three blobs middle-left is
NHL2:sand 1:2.5 (no cement). By
the end of the project, we had adjusted to 1:2 because the sand was too much for color and texture.
Middle-right is a lime:sand 1:2.5 mortar made with "ag lime" picked up
from a local farm store. Don't use
It's crush up calcium carbonate which has already cured. You want
calcium hydroxide that cures between your bricks into calcium
The right two blobs are a lime:sand 1:2.5 mortar using Type-S Hydrated Lime
(not Type-S mortar) from the local Menard's store and sand aggregate
that was a clean mix
of several sizes of sand. It works, but it looks weak. This is what the
ancients used with ratios of 1.5 up to the over-assumed 3.0.
||Months later, with the ag lime mix gone. Equivalent Type K - NLH2 - Type L shown.
The blob on
the brick was doing adhesion tests, and the blobs on the bottom were
used for strength tests (punction, cracking, chipping)
The middle NHL mixture is mathematically the same as the Type-S lime (Type L mortar) far right in the
pictures, but the
NHL has various prozzolithic impurities that make it set quicker than
the hydrated lime formula. It cures up about the same hardness
but behaves more familiar to masons that are used to hard-setting
concrete formulas. Important
for my project, it has a softer
tan color - not grey like the concrete and not white like the hydrated
lime. The mixed particle size sand made a very close match
existing mortar on the oldest part of the house.
After 4 days of "keeping moist to cure", Type-K and NHL2
are about the same when scratched with a screwdriver; the Type-L is a little bit
softer. In order to compare compression strength of each to the soft bricks
I'm using, I will use a 2" steel ball (the type inserted and locked into a
trailer hitch for security) and drop it on the mortar.
The testing idea was this. I
can apply a controlled force to the test pieces by dropping the ball from a controlled height.
F=m*a. Mass of the steel ball stays the same. The distance
of de-accelerating is assumed to be the same when impacting the test
piece, so what I can change is the speed the ball hits the brick or
mortar test piece by dropping it from higher or lower: F=m*d*s. I controlled the steel ball speed by dropping it from different
heights. speed = sqrt( 2 * g * height). So the applied
force is F=m*d*sqrt(2*g*d). Comparing two different drop heights,
F2 = F1 * (d2/d1)^1.5.
Here are the data I plan to collected after a week of curing
shows the lowest height required to drop the 2" steel ball to crack the
brick or mortar. The PSI for Type-N is not calculated. It
is looked up in a table and assumed to be perfectly true as F1 in the
above formula. The other numbers are based on this. So, the
relative values are good. The absolute values are good if the N
number starts good. The goal is to find a mortar that is just
slightly softer than the brick, so relative numbers are fine.
The ag lime will not be tested because it is a total failure. Don't make this mistake! Ag
lime is crushed rock (calcium carbonate) and adds zero strength to a
mortar. If you mistakenly use this to make a cement/lime mortar,
it will appear to harden up due to the cement but will be horribly weak
because the lime is acting like an aggregate not a binder. This is a
lesson in failure because the lime starts
as Calcium Carbonate lime, instead of Calcium Hydroxide lime and hardening into Calcium Carbonate
once layed and cured with moisture absorbing atmospheric carbon dioxide.
you're sure you have the right lime, check again. A prominant DIY help
website showed the picture duplicated here on the right side. The
picture represents what you'll find out there. Look at the red-arrow annotation. It
says "hydrated." Then see "for Agricultural Uses". Then look at the
first sentence of Wikipedia for Agricultural Lime: "Agricultural lime,
also called aglime, agricultural limestone, garden lime or liming, is a
soil additive made from pulverized limestone or chalk."
limestone and chalk is NOT hydrated lime and is NOT the right lime to
use in masonry work. Pulverized limestone and chalk is not the same as
hydrated lime. The product pictured could be hydrated lime or could be
ag lime, but it cannot be both! I know this is confusing because a
mason I hired showed up with bags of ag lime and I insisted he not use
it. I ran some test and showed him that Hydrated Lime makes a decent
but weaker than Cement mortar. The ag lime test piece crumbled when
squeezed in our hands. If you build a house out of ag lime, the wall
will wash away with the rain and your wall will fall down!
Ask for "Type-S Hydrated Lime" (which is NOT Type-S mortar that
everybody else is asking for). See the black-banded picture to the
left above. I could not find it at Home Depot or Lowes, but Menards
What We Actually Used
On the southern addition, probably added early 1900s, we
Type-N cement. On the original 1850 rectangular 2-story section,
used no cement. For the southern chimney on the addition, the bricks
proved to be too fragile to properly re-tuck. The mason used
veneer stones (manufactured
cement), and modern Type-S mortar (not lime) and the recipe directly
off the bag but he
doubled the amount of cement; he intended this to be a very hard shell
around the old decaying bring chimney.
Numbers below are "Type-N
cement: Hydrated Lime: Sand" used on the newer addition of the
One each line, the first ratio shows 5-gallon buckets for the first
two numbers and shovel scoops of sand for the third number. This
is what the mason liked to use. This is mathematically
identical to the second ratio, which shows direct volume
ratios (cups, pints, buckets, whatever you want).
Numbers below are (NHL 2.0:
Sand) ratios by volume used on the original 1860 section of the house.
- 1:3:12 or 1:3:1.4 on south side (a bit too lathery and maybe weak)
- 1:2:10 or 1:2:2 on east utility room and west dormer (a little bit grainy)
- 1:2:5 or 1:2:1 on east dormer and chimney gables and south side old house,
east (hopefully the best).
- 1:2.5 above dining window, but the color came out a little bit
too tannish sandy colored.
below dining window and south side old house, west. This proved
to be the best recipe with the sand (including small sized mixed
aggregates) that we found locally.
Weight Bearing with Weak Mortar
An interesting side note is that even with low compressive strength
Type-K mortar, a standard 4x8 brick has 32 square inches and the mortar
therefore support 2400 pounds (we assume the brick is stronger, which
my experience shows is not true with some soft orange clay locally
fired bricks). With 48 courses of brick on a
2-story house, and about 6 pounds per brick, the bottom bricks support
about 6x48=>288 pounds of static brick (roof weight is
ignored). This load is only 1/10th of what the brick can support.
This is true for any thickness of wall because weight and load
capability both go up proportionately.
A ½-brick joist pocket in the bottom course has an
extra 1200-288=>912 pounds it can support. This allows a joist
to carry double that (two ends) of about 1824 pounds. Subtracting
off its own weight, maybe 1524 pounds. If joints are spaced 1.5’
across a 20’ wide room, each needs to support 30 board feet of
floorboards or maybe 200 pounds. That leaves a usable load of
about 1324 pounds per joist.
If 10 people stand on 1 joist,
that’s about 2000 pounds, but remember the floor boards will transfer
some of the load to the next two adjacent joists. I can't imagine
10 people huddling close enought to weight down one joint. It's
possible that maybe 10 people would be in one room. I
would pay attention if you start housing 800-1000 pound gun safes.
Back to the roof. Now it matters how many bricks thick the wall
is. We calculated the joints above assuming the joist pocket uses
just the internal thickness of brick. The outer two layers give
up to 4800 pounds per brick of support that
is not tapped for internal structures – only the roof. I
can't imagine any roof weighing anything close to 4800 per top surface
area of brick, so the roof essentially weighs nothing.
You may also want to consider lateral loads of roof, wind, and catenary
power lines, etc.
Seal the Wall
After mortar work is done, many contractors will seal the wall with a
repelling silane (smallest molecules, 10 years or more lifespan,
requires catalyst while applying to slightly damp surface), siloxanes
(slightly larger, react with glass, at least 10 year lifespan),
silicates, methyl siliconates.
These are ~not~ sealants, like "Thompson's water seal". For a
healthy brick wall, film forming sealants are bad. Penetrating
It's of like Gortex for bricks - keep water out while water vapor
passes through. The bricks have to breath, not receive a
waterproof coat or else water vapor will be trapped IN the brick due to
temperature gradient condensation. You'll know if you're buying
the correct stuff because good repellants are expensive
– maybe $200 for 5 gallons of 7% siloxane, with a coverage of 90-150
gallon. A 40% silane mix may be twice that cost.
- Homeowner NHL-ing
his stone house foundation
buyer-friendly NHL distributor
Mortar: Stone Foundation Friend
skeleton of this document was originally created using AbiWord
under a Gnome desktop. It was subsequently edited by Konquerer to
become the web page you are reading. Last
updated May 2013. Suggestions for changes, comments, and questions are
always welcome. The easiest way is to contact me via e-mail.