Cob Re: lime-hardening

[CALXA]

The term "lime " refers to both quicklime (CaO - Calcium Oxide) and hydrated
lime ( slaked lime - calcium hydroxide - Ca(OH)2 )

Lime is made from heating limestone to drive off the carbon dioxide - called
calcining.
Our forefathers made lime by gathering the limestone rocks, and placing them
in a pit in the ground filled with fuel - wood 0r coal. After lighting the
fuel, they kept the fire burning about a week. This was a community affair,
held about twice a year - providing sufficient lime for mortar, plaster,
whitewash, pickling, egg preservation, outhouse sanitation, agricultural use,
poison oak and bug bite medicine, colic, and bone building food additive,
amoung others.

Limestone       + Heat (about 1850 F)+    = Calcium Oxide     + Carbon Dioxide
CaCO3                    heat                              CaO
CO2

CaO        +   Water  H2O             + Ca(OH)2     +  heat

Ca(OH)2              +  CO2                   +   CaCO3
slaked lime      Carbon dioxide                          Calcium Carbonate
Hydrated lime                                                  Limestone
lime putty

When lime is mixed with fine soil and sand, and kept damp, the alkalinity
rises to a pH of 12.4, which is somewhat above the soluability of the silica
and alumina  componants of the soil, which forms a gel coating. The silica and
alumina then react with calcium from the lime forming calcium alumino
silicates (same thing as Portland cement) Here you have made a cement out of
the soil. The mass then absorbs carbpon dioxide from the air - catalized by
the water - bringing the pH down below the soluable point of the silica and
alumina, forming a hardened cement and calcium carbonate. Seems like Mother
Nature knew what she was doing ???

Calcium hydroxide will revert to calcium carbonate,but it is difficult to make
a structural mass without the addition of silica or alumina. Under
considerable pressure, temperature, and CO2, you can make artificial marble.

Hope this helps,

Harry