Colin Walker - Academia.edu (original) (raw)

Papers by Colin Walker

Cement and Concrete Research, 2016

Modeling the solubility behavior of calcium silicate hydrate (C-S-H) gel is important to make qua... more Modeling the solubility behavior of calcium silicate hydrate (C-S-H) gel is important to make quantitative predictions of the degradation of hydrated ordinary Portland cement (OPC) based materials. Experimental C-S-H gel solubility data have been compiled from the literature, critically evaluated and supplemented with new data from the current study for molar Ca/Si ratios = 0.2-0.83. All these data have been used to derive a discrete solid phase (DSP) type C-S-H gel solubility model based on two binary non-ideal solid solutions in aqueous solution (SSAS). Features of the DSP type C-S-H gel solubility model include satisfactory predictions of pH values and Ca and Si concentrations for all molar Ca/Si ratios = 2.7 → 0 in the C-S-H system, portlandite (CH) for Ca/Si ratios > 1.65, congruent dissolution at Ca/Si ratios = 0.85, and amorphous silica (SiO2(am)) for Ca/Si ratios < 0.55 as identified in the current study by IR spectroscopy.

Physics and Chemistry of the Earth, Parts A/B/C, 2007

Data concerning potential solid products of the interaction of cement pore fluids with bentonite ... more Data concerning potential solid products of the interaction of cement pore fluids with bentonite have been reviewed with respect to accurate prediction of bentonite alteration in the long-term. Calcium (aluminium) silicate hydrates (C(A)SH), zeolites, feldspars, hydroxides, carbonates, polymorphs of silica, and some sheet silicates (all of varying degrees of crystallinity) are potential products of cementbentonite interaction. Evidence from natural systems and laboratory studies suggests that most, or all of these phases, may precipitate on timescales of interest to safety assessment of the geological disposal of radioactive wastes. These data indicate that growth kinetics of secondary minerals is equally as important as thermodynamic stability in controlling occurrence. C(A)SH show variable Ca/Si ratio and Al contents. At high pH (>11), the growth of C(A)SH minerals provides a means by which OH À ions from cement pore fluids may be titrated. Although thermodynamic data exist for a number of naturally-occurring crystalline C(A)SH minerals, they are of doubtful quality and should be applied with caution in predictive modelling. Zeolites are likely to form at lower pH than for C(A)SH, with the Si/Al ratio of the zeolite decreasing with increasing pH of the fluid. Zeolite stability is also strongly dependent upon silica activity in the fluid phase. Although silica activity in bentonite pore fluids will be spatially (and temporally) variable as hyperalkaline alteration proceeds, it is likely that minerals which could form would be those stable in quartz-saturated or supersaturated fluids. Currently available thermodynamic data for zeolites tend to overestimate their stability, leading to inaccurate predictions of their occurrence. Notwithstanding this uncertainty, it is considered that the following secondary minerals are the most likely to form in low temperature cement-bentonite systems: calcite, dolomite, chalcedony, C(A)SH of variable Ca/Si ratio, K-feldspar, illite, phillipsite, analcime, clinoptilolite, and heulandite. The relatively more siliceous zeolites (clinoptilolite, phillipsite) are likely to form at lower pH (distal regions of migrating cement pore fluids), whereas C(A)SH, illite, feldspars, and the more aluminous zeolites (analcime, heulandite) are more likely to form at higher pH and hence, the more proximal regions of migrating cement pore fluids. Predominantly Na-, K-bearing solids will be transformed to those dominated by Ca as the composition of cement pore fluids evolves with time.

Environmental Science & Technology, 2005

The weathering and corrosion of depleted uranium (DU) forms a complex series of oxidation reactio... more The weathering and corrosion of depleted uranium (DU) forms a complex series of oxidation reactions, ultimately resulting in metaschoepite, UO3.2H2O. The present work focused on studying the dissolution rate of synthetic UO3. 2H2O using batch and flow-through reactors. Under acidic conditions (pH = 4.4-5.4), atmospheric CO2, room temperature, and 0.1 mionic strength,the log solubility product, log Ksp = 5.26 at equilibrium and a pH-dependent rate law Ro = (0.30 +/- 0.15)[H+]0.83+/-0.1 were established. For consistency, these results were incorporated into the computer program PHREEQC 2.6, and the experimental conditions were simulated. There is generally good agreement between the experimental results and the modeled results. Batch experiments revealed a fast dissolution rate of UO3.2H20 in the first hour, followed by fluctuations in uranium concentration before equilibrium was attained after 3000 h.

Cement and Concrete Research, 2007

New data relevant to calcium silicate hydrate (C-S-H) gels prepared at room temperature have been... more New data relevant to calcium silicate hydrate (C-S-H) gels prepared at room temperature have been obtained over a time period of up to 112 weeks. X-ray diffraction (XRD) indicates equilibrium was attained after 64 weeks. Coupled with fourier transform infrared (FT-IR) spectroscopy, a phase change in C-S-H gel at Ca/Si ≈ 1.0 was identified and the occurrence of portlandite as a distinct phase for Ca/Si N 1.64. The incongruent dissolution of C-S-H gel was modeled as a non-ideal solid solution aqueous solution (SSAS) between the end-member components CaH 2 SiO 4 (CSH) and Ca(OH) 2 (CH) using equations defining the solidus and solutus curves on a Lippmann phase diagram. Despite being semiempirical, the model provides a reasonable and consistent fit to the solubility data and can therefore be used to describe the incongruent dissolution of C-S-H gels with compositions Ca/Si ≥ 1.0.

Applied Geochemistry, 2011

The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal f... more The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal facility for radioactive wastes creates potential issues concerning chemical reactivity. From a long-term safety perspective, it is desirable to be able model these interactions and changes quantitatively. The 'Long-term Cement Studies' (LCS) project was formulated with an emphasis on in situ field experiments with more realistic boundary conditions and longer time scales compared with former experiments. As part of the project programme, a modelling inter-comparison has been conducted, involving the modelling of two experiments describing cement hydration on one hand and cement-rock reaction on the other, with teams representing the NDA (UK), Posiva (Finland), and JAEA (Japan). This modelling exercise showed that the dominant reaction pathways in the two experiments are fairly well understood and are consistent between the different modelling teams, although significant differences existed amongst the precise parameterisation (e.g. reactive surface areas, dependences of rate upon pH, types of secondary minerals), and in some instances, processes (e.g. partition of alkali elements between solids and liquid during cement hydration; kinetic models of cement hydration). It was not conclusive if certain processes such as surface complexation (preferred by some modellers, but not by others) played a role in the cement-rock experiment or not. These processes appear to be more relevant at early times in the experiment and the evolution at longer timescales was not affected. The observed permeability profile with time could not be matched. The fact that no secondary minerals could be observed and that the precipitated mass calculated during the simulations is minor might suggest that the permeability reduction does not have a chemical origin, although a small amount of precipitates at pore throats could have a large impact on permeability. The modelling exercises showed that there is an interest in keeping the numerical models as simple as possible and trying to obtain a reasonable fit with a minimum of processes, minerals and parameters. However, up-scaling processes and model parameterisation to the timescales appropriate to repository safety assessment are of considerable concern. Future modelling exercises of this type should focus on a suitable natural or industrial analogue that might aid assessing mineral-fluid reactions at these longer timescales.

Cement and Concrete Research, 2016

Modeling the solubility behavior of calcium silicate hydrate (C-S-H) gel is important to make qua... more Modeling the solubility behavior of calcium silicate hydrate (C-S-H) gel is important to make quantitative predictions of the degradation of hydrated ordinary Portland cement (OPC) based materials. Experimental C-S-H gel solubility data have been compiled from the literature, critically evaluated and supplemented with new data from the current study for molar Ca/Si ratios = 0.2-0.83. All these data have been used to derive a discrete solid phase (DSP) type C-S-H gel solubility model based on two binary non-ideal solid solutions in aqueous solution (SSAS). Features of the DSP type C-S-H gel solubility model include satisfactory predictions of pH values and Ca and Si concentrations for all molar Ca/Si ratios = 2.7 → 0 in the C-S-H system, portlandite (CH) for Ca/Si ratios > 1.65, congruent dissolution at Ca/Si ratios = 0.85, and amorphous silica (SiO2(am)) for Ca/Si ratios < 0.55 as identified in the current study by IR spectroscopy.

Physics and Chemistry of the Earth, Parts A/B/C, 2007

Data concerning potential solid products of the interaction of cement pore fluids with bentonite ... more Data concerning potential solid products of the interaction of cement pore fluids with bentonite have been reviewed with respect to accurate prediction of bentonite alteration in the long-term. Calcium (aluminium) silicate hydrates (C(A)SH), zeolites, feldspars, hydroxides, carbonates, polymorphs of silica, and some sheet silicates (all of varying degrees of crystallinity) are potential products of cementbentonite interaction. Evidence from natural systems and laboratory studies suggests that most, or all of these phases, may precipitate on timescales of interest to safety assessment of the geological disposal of radioactive wastes. These data indicate that growth kinetics of secondary minerals is equally as important as thermodynamic stability in controlling occurrence. C(A)SH show variable Ca/Si ratio and Al contents. At high pH (>11), the growth of C(A)SH minerals provides a means by which OH À ions from cement pore fluids may be titrated. Although thermodynamic data exist for a number of naturally-occurring crystalline C(A)SH minerals, they are of doubtful quality and should be applied with caution in predictive modelling. Zeolites are likely to form at lower pH than for C(A)SH, with the Si/Al ratio of the zeolite decreasing with increasing pH of the fluid. Zeolite stability is also strongly dependent upon silica activity in the fluid phase. Although silica activity in bentonite pore fluids will be spatially (and temporally) variable as hyperalkaline alteration proceeds, it is likely that minerals which could form would be those stable in quartz-saturated or supersaturated fluids. Currently available thermodynamic data for zeolites tend to overestimate their stability, leading to inaccurate predictions of their occurrence. Notwithstanding this uncertainty, it is considered that the following secondary minerals are the most likely to form in low temperature cement-bentonite systems: calcite, dolomite, chalcedony, C(A)SH of variable Ca/Si ratio, K-feldspar, illite, phillipsite, analcime, clinoptilolite, and heulandite. The relatively more siliceous zeolites (clinoptilolite, phillipsite) are likely to form at lower pH (distal regions of migrating cement pore fluids), whereas C(A)SH, illite, feldspars, and the more aluminous zeolites (analcime, heulandite) are more likely to form at higher pH and hence, the more proximal regions of migrating cement pore fluids. Predominantly Na-, K-bearing solids will be transformed to those dominated by Ca as the composition of cement pore fluids evolves with time.

Environmental Science & Technology, 2005

The weathering and corrosion of depleted uranium (DU) forms a complex series of oxidation reactio... more The weathering and corrosion of depleted uranium (DU) forms a complex series of oxidation reactions, ultimately resulting in metaschoepite, UO3.2H2O. The present work focused on studying the dissolution rate of synthetic UO3. 2H2O using batch and flow-through reactors. Under acidic conditions (pH = 4.4-5.4), atmospheric CO2, room temperature, and 0.1 mionic strength,the log solubility product, log Ksp = 5.26 at equilibrium and a pH-dependent rate law Ro = (0.30 +/- 0.15)[H+]0.83+/-0.1 were established. For consistency, these results were incorporated into the computer program PHREEQC 2.6, and the experimental conditions were simulated. There is generally good agreement between the experimental results and the modeled results. Batch experiments revealed a fast dissolution rate of UO3.2H20 in the first hour, followed by fluctuations in uranium concentration before equilibrium was attained after 3000 h.

Cement and Concrete Research, 2007

New data relevant to calcium silicate hydrate (C-S-H) gels prepared at room temperature have been... more New data relevant to calcium silicate hydrate (C-S-H) gels prepared at room temperature have been obtained over a time period of up to 112 weeks. X-ray diffraction (XRD) indicates equilibrium was attained after 64 weeks. Coupled with fourier transform infrared (FT-IR) spectroscopy, a phase change in C-S-H gel at Ca/Si ≈ 1.0 was identified and the occurrence of portlandite as a distinct phase for Ca/Si N 1.64. The incongruent dissolution of C-S-H gel was modeled as a non-ideal solid solution aqueous solution (SSAS) between the end-member components CaH 2 SiO 4 (CSH) and Ca(OH) 2 (CH) using equations defining the solidus and solutus curves on a Lippmann phase diagram. Despite being semiempirical, the model provides a reasonable and consistent fit to the solubility data and can therefore be used to describe the incongruent dissolution of C-S-H gels with compositions Ca/Si ≥ 1.0.

Applied Geochemistry, 2011

The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal f... more The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal facility for radioactive wastes creates potential issues concerning chemical reactivity. From a long-term safety perspective, it is desirable to be able model these interactions and changes quantitatively. The 'Long-term Cement Studies' (LCS) project was formulated with an emphasis on in situ field experiments with more realistic boundary conditions and longer time scales compared with former experiments. As part of the project programme, a modelling inter-comparison has been conducted, involving the modelling of two experiments describing cement hydration on one hand and cement-rock reaction on the other, with teams representing the NDA (UK), Posiva (Finland), and JAEA (Japan). This modelling exercise showed that the dominant reaction pathways in the two experiments are fairly well understood and are consistent between the different modelling teams, although significant differences existed amongst the precise parameterisation (e.g. reactive surface areas, dependences of rate upon pH, types of secondary minerals), and in some instances, processes (e.g. partition of alkali elements between solids and liquid during cement hydration; kinetic models of cement hydration). It was not conclusive if certain processes such as surface complexation (preferred by some modellers, but not by others) played a role in the cement-rock experiment or not. These processes appear to be more relevant at early times in the experiment and the evolution at longer timescales was not affected. The observed permeability profile with time could not be matched. The fact that no secondary minerals could be observed and that the precipitated mass calculated during the simulations is minor might suggest that the permeability reduction does not have a chemical origin, although a small amount of precipitates at pore throats could have a large impact on permeability. The modelling exercises showed that there is an interest in keeping the numerical models as simple as possible and trying to obtain a reasonable fit with a minimum of processes, minerals and parameters. However, up-scaling processes and model parameterisation to the timescales appropriate to repository safety assessment are of considerable concern. Future modelling exercises of this type should focus on a suitable natural or industrial analogue that might aid assessing mineral-fluid reactions at these longer timescales.