[Strawbale] Rlimestone screenings

Fri Apr 20 17:00:47 CDT 2007

The following edited extract from my book "Practical Straw Bale Building"
might help to enlighten you on the chemistry of lime. Sorry I cannot put it
more simply.You will see, for example, that limestone does not "suck up" CO2
from the atmosphere. For a start, "limestone" is calcium carbonate, so it
has already "sucked up" all CO2 that it can suck! 

Murray Hollis, NSW, Australia

What is lime?
In the context of plastering straw bale walls, lime is hydrated calcium
hydroxide (Ca(OH)¬2) plus impurities and, sometimes, additives. However,
lime can refer to ‘agricultural lime’, which is calcium carbonate (CaCO-3),
which is not used in plastering, and to ‘quicklime’, which is calcium oxide
(CaO). 
The use of the word ‘hydrated’ for Ca(OH)¬2 can be misleading, because
people with some knowledge of chemistry will recall that a hydrated molecule
usually refers to a one to which a certain number of water molecules are
attached, the material still existing in a dry form. Use of the term
‘hydrated’ here refers to the fact that hydrated lime is formed by the
reaction of quicklime with water to form molecules (Ca(OH)¬2) that do not
have attached water molecules.
Lime exists naturally as calcium carbonate in limestone and chalk, which are
common minerals.
Quicklime (also called burnt lime, unslaked lime or caustic lime) can be
manufactured from limestone and typically contains 80% to 90% calcium oxide,
the rest being impurities carried over from the mined limestone. These
impurities commonly comprise oxides of silicon, magnesium, aluminium and
iron. Heating limestone to about 500°C, which causes it to decompose into
calcium oxide and carbon dioxide, makes quicklime.
Quicklime can be converted into calcium hydroxide by reacting it with water,
which produces a violent reaction that is accompanied by the generation of a
large amount of heat and resultant steam; that process is referred to as
‘slaking’ lime. When CaO (quicklime) reacts with excess water, 1 kg of CaO
will combine with 0.32 kg of the water to produce 1.32 kg of Ca(OH)2
(hydrated lime), which will be suspended in any excess water. The amount of
excess water will determine the thickness of the lime putty so produced. For
example, one commercial lime putty is made from a mix of 38% CaO and 62%
water (by weight), which produces a lime putty that is 50.2% solids
(Ca(OH)2) and 49.8% excess water. Quicklime is a hazardous material and
should be handled only by appropriately trained and experienced people.
Hydrated lime can be in the form of a white powder (builders’ lime,
brickies’ lime, industrial hydrated lime, slaked lime) or as thick white
slurry (lime putty, which typically contain about 50% solids). The resultant
lime putty can be dried to produce hydrated lime, which may have additives
incorporated, such as air-entraining agents, which are included in some
builders’ lime products, but not all of them. The usual process uses an
air-swept mill to directly lift the hydrated lime from the precipitate
formed by slaking lime, thereby directly producing a fine powder without the
need for any grinding process. The hydrated lime can have some colouration
due to impurities such as iron oxide, but such colouring usually is not very
significant.
Mixing hydrated lime and water is not accompanied by chemical reaction or by
significant dissolution of the hydrated lime, which has very low solubility
in water (about 20 grams/litre), but rather results in a suspension of the
lime in water with the same chemical composition as lime putty, but not the
same flow properties, or rheology.
How does lime plaster set or cure?
Lime-based plaster (lime/sand/silt) sets by drying and undergoes some
conversion to calcium carbonate very close to the surface during its drying.
In the literature, particularly on the Internet, widely divergent statements
can be found about the way that lime in lime-based plaster changes over time
from calcium hydroxide to calcium carbonate due to its interaction with
atmospheric carbon dioxide, a process called carbonation. These range from
claims that lime plaster sets by carbonation to the scientific observation
that lime in mortar from Roman times (about 2000 years old) still has not
fully converted to calcium carbonate [R. Malinowski. Concretes and Mortars
in Ancient Aqueducts, Concrete International, v. 1, pp. 66–76 (1979);  B
Marchese, Non-Crystalline Ca(OH)¬¬2 in Ancient Non-Hydraulic Lime Mortars,
Cement and Concrete Research, v. 10, pp. 861–64 (1980)].
The fact is [O Cazalla, C Rodríguez-Navarro, E Sebastián, G Cultrone & M J
de la Torre, Aging of lime putty: effects on traditional lime mortars
carbonation, Journal of the American Ceramic Society, v. 83, pp. 1070-1076
(2000)] that the setting of lime plaster occurs almost entirely by
drying—simply the evaporation of water. A very thin surface layer will
carbonise quite quickly (typically in a matter of days) but that layer then
acts to inhibit deeper carbonisation and also the evaporation of free water
retards the carbonisation, because the reaction with the atmospheric carbon
dioxide depends on the presence of water. O Cazalla et al have examined the
ageing and carbonation of various lime putties and provided a detailed
analysis of their properties and changes in their crystalline structure, and
measured the rates of carbonation of a range of samples.
The conversion of calcium hydroxide to calcium carbonate occurs through two
stages. First the carbon dioxide in the air is dissolved in water to form
carbonic acid (H2CO3), a very weak acid, which in turn reacts with the
calcium hydroxide to form calcium carbonate. The rate that this reaction
occurs in plaster after its initial drying is slow, because the
concentration of both water and carbon dioxide is low. The rate depends on
the humidity (it is faster when there is high relative humidity), the
exposure of the plaster to liquid water (e.g. via rain), the atmospheric
concentration of carbon dioxide (it might, for example, be higher in some
industrial areas) and the microstructure of the plaster. Carbonation can be
accelerated greatly by putting concentrated carbon dioxide gas in direct
contact with wet plaster, but this is only of academic interest since it is
not likely to be practicable to do this during normal building construction
processes.
As a result it can take many decades for the lime binder in lime plaster to
convert substantially into calcium carbonate.

> -----Original Message-----
> From: strawbale-bounces at listserv.repp.org [mailto:strawbale-
> bounces at listserv.repp.org] On Behalf Of Derek Roff
> Sent: Saturday, April 21, 2007 4:14 AM
> To: strawbale at listserv.repp.org
> Subject: [Strawbale] Rlimestone screenings
> 
> > I don't know what kind of stone you have in Nova Scotia but if it's
> > limestone (as it is here in Ottawa) compacted limestone screenings
> > does   result in a concrete-like material as it sucks up CO2 from the
> > atmosphere   and turns back into a monolithic-like mass.
> 
> Tell me more about limestone screenings.  Is this fragments of
> limestone, crushed and/or graded?  In any case, what is the chemical
> reaction that allows it to suck up CO2 from the atmosphere?
> 
> Derelict
> 
> (Allegedly from a chemistry exam:  H2O is the chemical formula for hot
> water.  CO2 is the formula for cold water.)
> 
> Derek Roff
> Language Learning Center
> Ortega Hall 129, MSC03-2100
> University of New Mexico
> Albuquerque, NM 87131-0001
> 505/277-7368, fax 505/277-3885
> Internet: derek at unm.edu
> 
> 
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