Abstract
Thin slices of white Portland cement-low calcium pulverized fuel ash (pfa) blended cement pastes containing 30 or 50% pfa were leached progressively in de-ionized water. The paste with 50% pfa was aged 13years prior to leaching and those with 30% pfa were aged 1 and 13years. Pastes were leached for up to 75days and were characterized using thermal analysis, X-ray diffraction, analytical scanning and transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. Leaching affected the pastes in the following sequence: (i) crystals of Ca(OH)2 large enough to be resolved by backscattered electron imaging were removed completely prior to any effect on C-A-S-H; (ii) the Ca/Si ratio of C-A-S-H reduced from ≈1.4 to ≈1.0 whilst the aluminosilicate structure was unaffected; (iii) further reduction in the Ca/Si ratio of C-A-S-H was accompanied by lengthening of the aluminosilicate chains; (iv) the Ca/Si ratio of C-A-S-H reduced ultimately to ≈0.6.
Highlights
Cementitious materials will be used in a wide range of applications in the geological disposal facility (GDF) for radioactive waste in the UK
Whilst some residual belite would be expected to be present in a neat white cement paste after 1 year hydration at ordinary temperatures (e.g. [24]), the hydration of belite has been reported to be retarded by the presence of fly ash particles (e.g. [25])
An AFm phase was not observed by X-ray diffraction (XRD) but a peak assigned to it is present on the 27Al nuclear magnetic resonance (NMR) spectrum, which indicates that any AFm was at best poorly crystalline, perhaps due to the opposite charges on the main layers in the structures of AFm and calcium aluminosilicate hydrate (C-A-S-H) producing strong mutual attraction that results in interstratification and loss of long-range order [26]
Summary
Cementitious materials will be used in a wide range of applications in the geological disposal facility (GDF) for radioactive waste in the UK. These include: waste encapsulation grouts, concrete waste containers, vault/tunnel backfills, lining and plugs; fracture grouts; and floors and roads [1,2,3,4]. The long-term ability of a cementitious backfill to maintain an alkaline environment and take up radionuclides from solution are important contributors to the containment of radionuclides in ILW after the closure of a GDF [7]. Richardson et al compared the data from [10,11,12] and concluded that the morphology of the outer product (Op) calcium aluminosilicate hydrate (C-A-S-H) in the hydrated cement is associated with a change of pH of the pore solution that occurs as the pfa reacts, the degree of saturation with respect to CH, and the Si concentration [13]
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