This review examines the detailed chemical insights that have been generated through 150 years of work worldwide on magnesium-based inorganic cements, with a focus on both scientific and patent literature. Magnesium carbonate, phosphate, silicate-hydrate, and oxysalt (both chloride and sulfate) cements are all assessed. Many such cements are ideally suited to specialist applications in precast construction, road repair, and other fields including nuclear waste immobilization. The majority of MgO-based cements are more costly to produce than Portland cement because of the relatively high cost of reactive sources of MgO and do not have a sufficiently high internal pH to passivate mild steel reinforcing bars. This precludes MgO-based cements from providing a large-scale replacement for Portland cement in the production of steel-reinforced concretes for civil engineering applications, despite the potential for CO2 emissions reductions offered by some such systems. Nonetheless, in uses that do not require steel reinforcement, and in locations where the MgO can be sourced at a competitive price, a detailed understanding of these systems enables their specification, design, and selection as advanced engineering materials with a strongly defined chemical basis.

A cementitious system for the immobilisation of magnesium rich Magnox sludge was produced by blending an Mg(OH)2 slurry with silica fume and an inorganic phosphate dispersant. The Mg(OH)2 was fully consumed after 28 days of curing, producing a disordered magnesium silicate hydrate (M-S-H) with cementitious properties. The structural characterisation of this M-S-H phase by (29)Si and (25)Mg MAS NMR showed clearly that it has strong nanostructural similarities to a disordered form of lizardite, and does not take on the talc-like structure as has been proposed in the past for M-S-H gels. The addition of sodium hexametaphosphate (NaPO3)6 as a dispersant enabled the material to be produced at a much lower water/solids ratio, while still maintaining the fluidity which is essential in practical applications, and producing a solid monolith. Significant retardation of M-S-H formation was observed with larger additions of phosphate, however the use of 1 wt% (NaPO3)6 was beneficial in increasing fluidity without a deleterious effect on M-S-H formation. This work has demonstrated the feasibility of using M-S-H as binder to structurally immobilise Magnox sludge, enabling the conversion of a waste into a cementitious binder with potentially very high waste loadings, and providing the first detailed nanostructural description of the material thus formed.

Magnesium potassium phosphate cements (MKPCs), blended with 50 wt.% fly ash (FA) or ground granulated blast furnace slag (GBFS) to reduce heat evolution, water demand and cost, were assessed using compressive strength, X-ray diffraction (XRD), scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) spectroscopy on 25 Mg, 27 Al, 29 Si, 31 P and 39 K nuclei. We present the first definitive evidence that dissolution of the glassy aluminosilicate phases of both FA and GBFS occurred under the pH conditions of MKPC. In addition to the main binder phase, struvite-K, an amorphous orthophosphate phase was detected in FA/MKPC and GBFS/MKPC systems. It was postulated that an aluminium phosphate phase was formed, however, no significant Al-O-P interactions were identified. High-field NMR analysis of the GBFS/MKPC system indicated the potential formation of a potassium-aluminosilicate phase. This study demonstrates the need for further research on these binders, as both FA and GBFS are generally regarded as inert fillers within MKPC.

Standard methods to assess the durability of vitrified radioactive waste were first developed in the 1980’s and, over the last 40 years, have evolved to yield a range of responses depending on experimental conditions and glass composition. Mechanistic understanding of glass dissolution has progressed in parallel, enhancing our interpretation of the data acquired. With the implementation of subsurface disposal for vitrified radioactive waste drawing closer, it is timely to review the available standard methodologies and reflect upon their relative advantages, limitations, and how the data obtained can be interpreted to support the post-closure safety case for radioactive waste disposal.

Struvite-K (MgKPO4·6H2O) is a magnesium potassium phosphate mineral with naturally cementitious properties, which is finding increasing usage as an inorganic cement for niche applications including nuclear waste management and rapid road repair. Struvite-K is also of interest in sustainable phosphate recovery from wastewater and, as such, a detailed knowledge of the crystal chemistry and high-temperature behavior is required to support further laboratory investigations and industrial applications. In this study, the local chemical environments of synthetic struvite-K were investigated using high-field solid-state 25Mg and 39K MAS NMR techniques, alongside 31P MAS NMR and thermal analysis. A single resonance was present in each of the 25Mg and 39K MAS NMR spectra, reported here for the first time alongside the experimental and calculated isotropic chemical shifts, which were comparable to the available data for isostructural struvite (MgNH4PO4·6H2O). An in situ high-temperature XRD analysis of struvite-K revealed the presence of a crystalline–amorphous–crystalline transition that occurred between 30 and 350 °C, following the single dehydration step of struvite-K. Between 50 and 300 °C, struvite-K dehydration yielded a transient disordered (amorphous) phase identified here for the first time, denoted δ-MgKPO4. At 350 °C, recrystallization was observed, yielding β-MgKPO4, commensurate with an endothermic DTA event. A subsequent phase transition to γ-MgKPO4 was observed on further heating, which reversed on cooling, resulting in the α-MgKPO4 structure stabilized at room temperature. This behavior was dissimilar from that of struvite exposed to high temperature, where NH4 liberation occurs at temperatures >50 °C, indicating that struvite-K could potentially withstand high temperatures via a transition to MgKPO4.

Fenton or Fenton-like oxidation for treatment of organic radioactive wastes is a promising technology with applications to a range of organic wastes. This review details this process; exploring potential challenges, pitfalls and opportunities for industrial usage with radioactive wastes. The application of this process to real radioactive wastes within pilot-plant settings has been documented, with key findings critically assessed in the context of future waste production. Although this oxidation process has not found mainstream success in treatment of radioactive wastes, a lower temperature oxidation system bring certain benefits, specifically for higher volume or problematic organic wastestreams.

Novel cements can contain up to 50 wt% Mg(OH)2, offering a new route to immobilisation of this nuclear waste constituent.

8This paper is a discussion of two recent papers by Unluer & Al-Tabbaa [1, 2] which analysed accelerated 9 carbonation of reactive MgO blocks. We suggest that the authors have incorrectly analysed key data, leading 10 to overstated claims of MgO carbonation. Based on reassignment of their X-ray diffraction data, it is proposed 11 that little MgO carbonation occurred in the samples discussed in those papers, with CaCO3 instead forming 12 during accelerated carbonation. We also draw attention to the thermodynamic instability of nesquehonite 13 under ambient conditions, which calls into question the long-term stability of these binders. 14 Discussion 15Cements containing reactive magnesia are of great interest as alternative binders, as they have been 16 proclaimed to embody potentially lower CO2 emissions during manufacture and service. Two recently 17 published papers by Unluer and Al-Tabbaa [1, 2] have added to the body of literature on these cements, 18 studying the effect of hydrated magnesium carbonate (HMC) addition and curing conditions, respectively, on 19 the properties and structure of porous reactive MgO cement blocks exposed to accelerated carbonation 20 conditions. We will focus the discussion here on the first of these two papers, as the results presented in the 21 second are largely an extension of the first, and contain similar points requiring re-analysis. 22In these papers the authors claim to carbonate MgO to form a range of magnesium carbonates which 23 constitute their binding phases; this is a key aspect of the 'green' credentials proposed for these alternative 24 cements. Unfortunately, we are unable to reach the same conclusions made by the authors, based on our 25 own analysis of the data presented in their papers. In our opinion, the scientific discussion in these two 26 papers is based upon poorly-assigned X-ray diffraction patterns, which have led to incorrect interpretations 27 of thermal analysis data, and consequently erroneous claims of high levels of carbonation. 28In the paper "Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO 29 cements" [1], the authors produce blocks containing natural aggregates, pulverised fuel ash (PFA) as filler, 30and MgO as the key anhydrous precursor, with hydrated magnesium carbonates (HMCs) added to some of 31 the mixes. The combination of hydration and carbonation is proposed to lead to the formation of additional 32 hydrated magnesium carbonates as binding phases, when cured under either natural or accelerated 33 carbonation conditions. The authors achieved some interesting strength data, exceeding 20 MPa in 34 compression in some instances, which shows that their methodology is of some interest. 35However, there are several apparent discrepancies in the peak assignments in the two XRD patterns used by 36 the authors to identify hydration products after accelerated carbonation (Fig. 7 in [1]). This graphic is 37 reproduced here as Figure 1, with our suggested peak assignments for their first set of XRD patt...