Alkalinity input

[Colin Walker <colin.walker@xxxxxxxxxxxxx>]

Dear David Parkhurst,

I am sure you get asked a lot of questions regarding your PHREEQC program,
but having read the manual cover to cover I am still unsure how to
impliment a problem I have. I am using PHREEQC 2.6. I am trying to model
the solubility of Calcium Silica Hydrates (CSH) in a closed system that is
sealed from the atmosphere. Adding CaO and SiO2 as solids to water under a
N2 atmosphere gives the following liquid data.

Ca/Si	pH	[Ca2+]		[Si4+]		Model Solids from Literature
Ratio		millimoles	millimoles
---------------------------------------------------------------------------
----
0.4	10.025	3.04		3.26		Quartz + Tobermorite-14A
0.6	10.183	1.50		1.50			"
0.8	11.420	2.41		0.28			"
0.9	11.912	4.70		0.08			"
1	12.115	6.81		0.08			"
1.1	12.493	23.17		0.07		Tobermorite + Portlandite
1.2	12.547	16.08		0.06			"
1.4	12.586	18.84		0.06			"
1.6	12.652	22.66		0.06		Afwillite + Portlandite
1.7	12.612	20.16		0.06			"
1.8	12.635	21.95		0.06			"
2	12.604	21.22		0.06			"

Above the Ca/Si ratio of 1 then the solution becomes equilibrated with
Portlandite (corresponding pH of ?12.55 ±0.1). Whenever I try to input this
using EQUILIBRIUM_PHASES the percent error on charge balancing is a
staggering 89-90%. I know there are limitations in PHREEQC regarding highly 
alkaline solutions where Alkalinity isn't specified as (CO3)2-, though in
this case I have done everything possible to try and keep CO2 out of the
system. Is there a way of specifying that the Alkalinity is a function of
the dissolution of Tobermorite-14A for Ca/Si < 1.0,  1.0 < Portlandite +
Tobermorite < 1.4 and Afwillite + Portlandite > 1.4. Or maybe using (CO3)2- 
as a buffer, but not including it in any reactions? Thanks in advance for
any comments or suggestions you may have.

Yours sincerely,

Colin Walker


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Colin Walker
Department of Earth Sciences
University of Bristol
WMB
Queen's Road
Bristol, BS8 1RJ


Tel: +44 (0)117 954 5246
Fax: +44 (0)117 925 3385
E-mail: colin.walker@xxxxxxxxxxxxx
Departmental web pages: http://www.gly.bris.ac.uk

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