HISTORIC MORTARS: THE CHEMISTRY BEHIND WHAT BINDS OUR BUILT ENVIRONMENT

What is mortar?

A mixture of various compounds including Lime and or Cements mixed with water and sand to bond or cover Masonry i.e. brick, stone, terracotta, stucco, and plaster.

Mortar ≠ concrete or cement

Historic mortars

Historic mortars Gypsum

Great pyramids in Egypt Interior plaster Stone mortar

Lime Primary building binder

for 3000 years Natural Cement Portland Cement

Recent applications 1824 England and 1870 in U.S.

Types of historic lime mortars

• Air Lime Cycle Three basic types

Hi-calcium Lime Dolomitic Lime Magnesium Lime

Hexagonal plate crystal structure

Sets Via Carbonation Impurities can

produce hydraulic behavior

Burned @ 950 F

Types of historic lime mortars

Natural Hydraulic lime Cycle Two basic types

Argillaceous Clay impurity

siliceous Pure Silica

Hexagonal plate crystal structure

Sets Via Carbonation Set Via Hydraulic reaction

with Silica Belite (di-calcium Silicate)

Types of historic lime mortars

Pozzolanic Hydraulic lime Cycle Needs a reactive silica source

Calcinated material Brick, tile, fly-ash, Blast furnace slag,

glass, silica fume…. Must be the right fineness

At least 180 microns

Hexagonal plate crystal structure Sets Via Carbonation Set Via Hydraulic reaction with

Silica Belite (di-calcium Silicate) Varying minerals formed

Manufacture and mixing techniques of historic mortars

• 3000 B.C. – Roman Empire Limestone burned in

Ricks and stone kilns Most often mixed and

transported by hand in baskets

Primarily hot lime mixing Quicklime mixed on site

with sand Laid fresh

Lime slaking for plaster and washes Fresco

Manufacture and mixing techniques of historic mortars

• Fall of Rome – 1700’s Vitruvious

1 lime : 3 sand introduced

Quicklime not hydrated lime

Pozzolanic mix designs for “Roman concrete” Mixed very stiff and hand

compacted Used vitrified clay (tiles)

NHL used in Europe “Roman Cement” lost Basic lime mortar used

and sourced locally

Manufacture and mixing techniques of historic mortars

• 1700’s – 1870’s 1759-John Smeaton- Builds Eddy

stone Lighthouse with NHL + pozzolan

1796-James Parker- Patent for artificial hydraulic lime, called “Roman Cement”

1818-Natural Cement found in Madison county, NY

1824-Joshph Aspdin- Patents Portland Cement in England First cement to contain “Alite” Tri-

calcium silicate 1868-First Shipment of Portland

Cement to U.S 1871-First Portland Cement Plant

opens in Coplay, PA

Manufacture and mixing techniques of historic mortars

• 1870’s – 1913• 1880’s-experimentation

with rotary kiln • 1896-first year U.S.

Portland cement production exceeds importation

• 1900-First year Portland cement Production exceeds Natural Cement

• 1904-1913 – Rotary kiln’s become primary kiln design

• Burned @ 2400 F

The debate has not changed

1914 article in ”Engineering News”

Manufacture and mixing techniques of historic mortars

1913-Present Modern Portland Cement

reigns in engineering projects

Slag Cement produced Lime fades as primary

binder around after WWII 1:1:6 mix becomes

common Beginning of restoration

with high lime content mortars in 1960’s

Mid 1990’s – Beginning of lime renaissance

Compare and contrast Compressive strength

The more hydraulic a mortar the greater its strength Lime mortars are slowest setting (carbonation &

belite) Portland cement is fastest setting (Alite and Belite) Pozzolans consume free lime to gain strength

Compare and contrast Flexural strength

The more free lime the greater a mortars flexibility

High calcium and Natural Hydraulic limes have the most free lime thus greatest flexibility

Pozzolans consume free lime to gain strength thus reduce healing ability

Compare and contrast Vapor permeability

Question & Answers