Phase evolution of slag-rich cementitious grouts for immobilisation of nuclear wastes

Abstract

An updated calcium silicate hydrate (C–S–H) model incorporating aluminium-containing end-members was used for thermodynamic modelling of blended cements using blast-furnace slag and Portland cement (BFS:PC) with ratios of 1:1, 3:1 and 9:1, using GEMSelektor. Selective dissolution and magic angle spinning nuclear magnetic resonance (MAS NMR) studies were performed to determine the degree of hydration (DoH) of the anhydrous material as an input parameter for the modelling work. Both techniques showed similar results for determining the DoH of the BFS within each sample. Characterisation of the hardened cement pastes over 360 days, using X-ray diffraction analysis and MAS NMR, demonstrated that the use of the updated C–S–H model can highlight the effect of different blend ratios and curing ages on the phase assemblages in these cements. Validation using this modelling approach was performed on 20 year old specimens from the literature to highlight its applicability for modelling later-age blended cements.

Effects of plutonium dioxide encapsulation on the physico-chemical development of Portland cement blended grouts

“…The sulfate content of the BFS:PC systems will only increase if both the degree of hydration (DoH) of BFS increases and there is a sufficiently oxidising environment to convert the released sulfide to sulfate. Prentice et al [33] found the DoH in Sellafield Ltd specification 3:1 BFS:PC system to be between 50-55% for BFS after one year, with alite and belite hydration at 85% and 53% respectively. There was no intermixing of Calumite® with the GGBS fraction in the Prentice study, and the larger particle size of the Calumite® component will reduce the degree of reaction in the current study; Sanderson et al found the behaviour of Calumite® to be similar to an inert filler [34].…”

“…Given the estimated range of DoH, the concentrations are not anticipated to vary significantly without oxidation of the sulfide (Table 5). Thermodynamic modelling of similar systems (unirradiated) [33] predicts ettringite to be replaced by monosulfoaluminate in the phase assemblage by 28 days; monosulfoaluminate and hemicarboaluminate were predicted to be the major AFm components. The persistence of ettringite together with AFm phases can be seen in the XRD data for the BFS:PC control samples (Figure 8), indicating the sulfate had not decreased as predicted.…”

“…The persistence of ettringite together with AFm phases can be seen in the XRD data for the BFS:PC control samples (Figure 8), indicating the sulfate had not decreased as predicted. The replacement of ettringite requires a reduction in the SO3/Al2O3 ratio of the system, which occurs as depletion of gypsum and hydration of BFS progresses [11,31,33]. The stability of the AFm phases that are formed upon replacement of the ettringite is also dependent upon the bulk CO2 content of the system, as the stability of monosulfoaluminate decreases significantly with increasing carbonate contents; the phase assemblage identified by XRD indicates molar bulk ratios of CO2/Al2O3 and SO3/Al2O3 of below 0.5 and between 1-3 respectively [31].…”

Development of technology of industrial wastes treatment products disposal by ferritization in the matrix of alkali-activated cements

“…[11,12]. Also, alkaline cements are characterized by the ability to fix in their structure the elements of radioactive and heavy metals not only on the physical, but also on the chemical level [13].…”

Hydration kinetics and products of MgO-activated blast furnace slag

“…The chemistry of slag-derived binders is strongly influenced by the selection of the 9 activator [8][9][10]. The main reaction product of slag-blended PC is C-A-S-H gel, while the formation of secondary phases depends on the slag:PC ratio; increasing the slag content in slag-PC blends leads to a decrease in portlandite and ettringite content, and an increase in hydrotalcite [11,12]. Alkali-containing C-A-S-H (often represented as C-(N-)A-S-H using Na as the archetypal alkali metal) is the main reaction product in alkali-activated slag binders [13].…”