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Calcium silicate hydrate (C-S-H) gel dissolution and pH buffering in a cementitious near field

Published online by Cambridge University Press:  05 July 2018

G. M. N. Baston ,
A. P. Clacher ,
T. G. Heath ,
F. M. I. Hunter ,
V. Smith  and
S. W. Swanton
G. M. N. Baston
Affiliation:
AMEC, Building 150, Thomson Avenue, Harwell, Didcot, Oxfordshire, OX11 0QB
A. P. Clacher
Affiliation:
AMEC, Building 150, Thomson Avenue, Harwell, Didcot, Oxfordshire, OX11 0QB
T. G. Heath*
Affiliation:
AMEC, Building 150, Thomson Avenue, Harwell, Didcot, Oxfordshire, OX11 0QB
F. M. I. Hunter
Affiliation:
AMEC, Building 150, Thomson Avenue, Harwell, Didcot, Oxfordshire, OX11 0QB
V. Smith
Affiliation:
AMEC, Building 150, Thomson Avenue, Harwell, Didcot, Oxfordshire, OX11 0QB
S. W. Swanton
Affiliation:
AMEC, Building 150, Thomson Avenue, Harwell, Didcot, Oxfordshire, OX11 0QB
*
*E-mail: tim.heath@amec.com
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Abstract

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A cementitious backfill has been proposed in many geological disposal concepts for intermediate-level waste and low-level waste in the UK and elsewhere. In this paper, the main features of the chemical evolution of backfill and the associated changes in the near-field pH are illustrated with results from recent work. For example, interaction of the groundwater with calcium silicate hydrate (C-S-H) phases in a backfill is expected to play an important role in the long-term pH-buffering behaviour. Existing experimental data for the dissolution of C-S-H gels are compared with recent experimental results from leach tests on gels of a lower calcium to silicon ratio (C/S) to provide a consistent set of data across the full C/S range. The results confirm that a congruent dissolution point around C/S = 0.8 is approached by leaching from below (i.e. for gels with 0.29 < C/S < 0.8), as well as from above, as reported elsewhere. In addition, a spreadsheet model has been developed to calculate the volume of backfill required at the vault scale to meet specified pH performance criteria. This model includes the major reactions of the backfill with the groundwater, waste encapsulants and waste components. It can also consider the effects of specific waste packages on local pH performance to allow comparison with the vault-scale calculations.

Keywords

evolutionnear-field performancebackfillengineered barriercalcium silicate hydrateC-S-H gelsdisposal concept
Type
Research Article
Information
Mineralogical Magazine , Volume 76 , Issue 8 , December 2012 , pp. 3045 - 3053
Creative Commons
Creative Common License - CCCreative Common License - BY
© [2012] The Mineralogical Society of Great Britain and Ireland. This is an open access article distributed under the terms of the Creative Commons Attribution (CC BY) licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2012

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