[Strawbale] Chemistry and physics of initial lime curing

[CALXA at aol.com]

Hi Derek,
 
I think the folks are correct..the starting of carbonation...
 
Ca(OH)2 + H2O = Ca++ plus OH--.  Calcium Plus charged ions, and  negative 
charged Hydroxide ions 
 
What happens chemically, I think, is first at the high pH from the lime  
causes the silica to become soluble, forming a silica gel at the surface of the  
silica sand particle. This gel attains a negative ionic charge. This negative  
charge attracts the Calcium ions which are of a positive charge. (Opposites  
attract....... :)  ... forming calcium silicate.
 
Ca++ plus Silica-- forms Calcium Silicate. I understand there are three  
forms of calcium silicate CaS, CaS2, CaS3:
Calcium Silicate, Calcium Di Silicate, and Calcium TriSilicate.. 
 
(I understand the final natural state is Calcium Silicate, which  
theoretically, all forms will eventually migrate to this state.)
 
Then old mother nature works her miracles, and begins to attract carbon  
dioxide from the surrounding air........this re-carbonates the calcium silicate  
molecule, bringing the pH down below the soluble point of the silica ..about pH 
 11. no longer being soluble, the material cures/hardens back to a 
solid....All  of these reactions require a layer of water to properly transport the CO2 
to the  Calcium silicate molecule in a timely manner. If left to dry out 
before these  chemical reactions go to completion (satisfying the opposite ionic 
charges  attractions ), then there is no cross linking of the formed  
crystals....leaving the particles standing alone, with no, of little strength.  This 
inter linking is also occurring as the excess lime is also re-carbonating,  
forming interlinking crystals between the calcium silicate.
 
The re-carbonation of the excess lime particles, now take the place of  
excess water, as the water evaporates, filling the voids and locking all  particles 
together......no shrinkage cracks, and great strength.
 
  This, basically is the same chemistry that occurs in all  
cements..including Roman Cement, Portland cement and natural cements. High pH  makes the silica 
( and alumina,too) soluble, so it can mix with the calcium. 
 
Impurities ( which all limes contain) help or hinders the process.  Iron, 
sulphur and aluminia compounds form an interesting compound with  lime...Calcium 
alumino (or Iron) sulphate..called ettringite.
The Chinese have been making this stuff for ever... and is used as a  
Blastless Demolition Agent. Mix it dry, drill a hole in a rock, Place it in the  
hole, add a little water, and plug the hole with a wooden peg. Now, in a few  
minutes, hours or days, the Ettringite absorbs the water, the dry chemicals  react 
and expand at about 20,000 psi, and swell about 30% in volume. the rock  
slowly forms cracks, and falls apart.
 
Once the Errtingite is formed, it no longer expands, and is incorporated in  
the mixture... 
 
This is the reaction which causes "Pop Outs " in plaster, cement, stc., and  
is one good reason to let lime putty age...to insure all of these impurities  
fully react before re-carbonation/curing occurs.
 
The reactions are:
 
(Ca(OH)2 hydrated lime, is formed by calcining/burning (heating)  limestone -
 
                    CaCO3  + Heat = CaO Quicklime + CO2 gas.
 
 now, add water:
 
                    CaO  + H2O = heat + Ca(OH)2. 
 
Now add Carbon Dioxide (CO2):
 
                    Ca(OH)2  + CO2 = CaCO3    Back where we started.
 
 
There are suggestions that the ancients added other "soils" to the lime  
plasters and mortars....such as Natron - Sodium Carbonate. If one adds Sodium  
Carbonate to Calcium hydroxide (lime putty), the resulting reaction forms Sodium  
hydroxide.  Ca(OH) 2 + Na2CO3 yields  NaOH + CaCO3.  
 

The pH of hydrated lime is 12.4 @25 deg C. The pH of Sodium Hydroxide is  
about 13.
 
At the higher pH the silica is even more solubilized and formed into Sodium  
Silicate gel. Now add excess lime, and the Sodium is exchanged/replaced with 
the  Calcium. With the more completely solubilized Silica Gels (not just a 
surface  reaction), the Calcium forms a more complete cement. The washng out of 
the  sodium is important, as Sodium salts are water soluble, while  Calcium 
reactions such as Calcium silicates are not ( Calcium Chloride  is a soluble 
Calcium Salt). 
 
Hope this further explains the chemistry and physics involved in the curing  
of lime and cement mixtures.... There are probably many more intervening  
reactions going on, but I think this is a good overview.
 
Attached is a paper discussing these reactions and possible uses by the  
ancients. Also, you will find references to research on lime and various  
additives that may be of interest. This is where I found support about the use  of 
Natron.
 
Enjoy.
 
Regards,
 
Harry Francis



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