1860 Homestead Mortar and Bricks

Rev 1.1

 Brian Mork - 2016
© 2016 Increa Technology

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Summer 2016, RM writes:
Anyways, Great site!"

Introduction


This page documents work to refurbish and improve an 1850 brick homestead.

Engineering Mortar - Theory


People 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 big mistake.  If you want to be convinced, see the photos below of one of the additions on my house - it is an 80 year study in how cement destroys your older brick walls.  Of course, 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 that seems wrong.

Lime mortars have been used since antiquity, leveraging the chemistry of the lime cycle.   Crushed Calcium Carbonate (“limestone”) is heated to drive off Carbon Dioxide, leaving Calcium Oxide (“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 bags.)

(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 while converting 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 minimal 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 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”).

Mortar  Type
Cement
Lime
Sand
PSI
M
1
0.25
3-3.75
2500
S
1
0.5
4-4.5
1800
N
1
1
5-6
750
NHL5
0
2.5
2.5
725, 122412m
NLH3
0
2.5
2.5
507, 85512m
O
1
1
8-9
350
NHL2
0
1
2.5
290, 757d, 42012m
L
0
1
2.25-3
261-725
K
1
3
10-12
75-?

Table Notes:
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.

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, it may be convenient to buy Type-N mortar from Home Depot or Lowe’s, etc 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 the "weakening agent" (lime?) used to make the commercial Type-N.  If one assumes commercial Type-N mortar is 1:1:5, per the recipe table above, it can be “converted” to Type-K by adding only lime and sand.

Type-N mortar    1:1:5 (7 parts)
Add        0:2:5 (7 parts)
Result        1:3:10 (14 parts)

Summarizing, 7 scoops of Type-N, plus 2 parts of hydrated lime, plus 5 parts of sand roughly make up Type-K mortar.

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 : 1/7 : 5/7
1 bag Hydrated Lime    0 : 1 : 0
~1 bag sand        0 : 0 : 1
Result            1/7 : 1-1/7 : 1-5/7
Multiply by 7 simplicity    1:8:12

So, this is 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.

Another archtect recommended old houses with clay bricks be mortared with a mixture of 1 bag of cement and 1 bag of lime, plus about 1 bag of sand.  Based on the NPS table above, this is similar to Type-N mortar


Testing Some Mortars


From 30-40 years of experience, the mason really wanted to use less sand than the NPS recipes indicated.  All his mental recipes were done with bags and scoops (shovel fulls).  He was comfortable with a bag of cement, a bag or two of lime, and 10-12 scoops of sand, equivalent to a 1:2:2 recipe. Our unit of measure was 1/2-bag, which turned out to be a 5-gallon bucket full up to where the handles connect.  With half-bag measures, he really did not want to go over 7 scoops of sand, which was already double what he was used to.  For him, 5 "scoops" was a bucket.

I mixed up four types of mortar for comparison.  See the photos below. 

On the left uses 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 1:2:5 for the "new" part of the old house.

Middle-left is NHL2:sand 1:2.5 (no cement).  By the end of the project, we had adjusted to 1:2.

Middle-right is a lime:sand 1:2.5 mortar made with "ag lime" picked up from a local farm store. Don't use ag lime!  It's crush up calcium carbonate which has already cured.  You want calcium hydroxide that cures between your bricks into calcium carbonate.

On the right is 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.  The blob on the brick was doing adhesion tests, and the blobs on the bottom were used for strength tests (punction, cracking, chipping)


The NHL mixture is mathematically the same as the 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 to the existing mortar on the oldest part of the house.

The ag lime 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.

After 4 days of "keeping moist to cure", the Type-L is a little bit softer when scraping with a screwdriver tip.  The Type-K and NHL2 are about the same.  In order to compare each to the soft bricks I'm using, I used a 2" steel ball (the type inserted and locked into a trailer hitch for security).  The testing idea was this.  I can apply force to each test piece according to the formula 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; 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 want to collected after {pending} days of curing time.  This shows what height is 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.

Test Article
Ball  Height
Estimated PSI

Type-N
20"
750

Soft Brick
15"
487

Type-L



NHL2



Type-K





What We Actually Used


On the southern addition, probably early 1900s, we accepted some Type-N cement.  On the original 1860 rectangular 2-story section, we used no cement. For the southern chimney veneer stones (manufactured cement), the mason used Type-S 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 (1890s?) section of the house. kdk The first ratio shows 5-gallon buckets for the first two numbers and shovel scoops of sand for the third number.  This should be the same as the second ratio, which shows direct volume ratios (cups, pints, buckets, whatever you want).
Numbers below are (NHL 2.0: Sand) by volume used on the original 1860 section of the house.

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 between can 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 water 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. 

Saver Systems
Applied Technology

These are ~not~ sealants, like "Thompson's water seal".  For a healthy brick wall, film forming sealants are bad.  Penetrating repellants are good.  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 ft2 per gallon.  A 40% silane mix may be twice that cost.




Additional Resources

  1. Homeowner NHL-ing his stone house foundation
  2. Dominant buyer-friendly NHL distributor
  3. Lime Mortar: Stone Foundation Friend

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The 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.