Abstract
In this study, a multi-technique approach was utilised to determine the high temperature performance of magnesium potassium phosphate cement (MKPC) blended with fly ash (FA) or ground granulated blast furnace slag (GBFS) with respect to nuclear waste immobilisation applications. Conceptual fire conditions were employed (up to 1200 °C, 30 min) to simulate scenarios that could occur during interim storage, transportation or within a final geological disposal facility. After exposure up to 400 °C, the main crystalline phase, struvite-K (MgKPO4·6H2O), was dehydrated to poorly crystalline MgKPO4 (with corresponding volumetric and mass changes), with MgKPO4 recrystallisation achieved by 800 °C. XRD and SEM/EDX analysis revealed reaction occurred between the MgKPO4 and FA/GBFS components after exposure to 1000–1200 °C, with the formation of potassium aluminosilicate phases, leucite and kalsilite (KAlSi2O6 and KAlSiO4), commensurate with a reduced relative intensity (or complete elimination) of the dehydrated struvite-K phase, MgKPO4. This was further supported by solid-state NMR (27Al and 29Si MAS), where only residual features associated with the raw FA/GBFS components were observable at 1200 °C. The high temperature phase transformation of blended MKPC binders resulted in the development of a glass/ceramic matrix with all existing porosity infilled via sintering and the formation of a vitreous phase, whilst the physical integrity was retained (no cracking or spalling). This study demonstrates that, based on small-scaled specimens, blended MKPC binders should perform satisfactorily under fire performance parameters relevant to the operation of a geological disposal facility, up to at least 1200 °C.
Highlights
Cementitious binders based on magnesium potassium phosphate (MKPC) bonding are, in general, near-neutral pH systems formed from the acid-base reaction of KH2PO4 and dead burnt magnesium oxide (MgO) [1,2]
In the FA/MKPC binder (Fig. 2A), a progressive colour change was observed between 20 ◦C and >800 ◦C, which was thought to be associated with iron oxidation in the fly ash; this is examined in more detail below using Mossbauer spectroscopy (Section 3.5)
In the granulated blast furnace slag (GBFS)/MKPC binder (Fig. 2B), a more subtle colour change was observed, and the green tinge observed after low temperature exposure (200 ◦C) was attributed to polysulfide species released by the GBFS [40]
Summary
Cementitious binders based on magnesium potassium phosphate (MKPC) bonding are, in general, near-neutral pH systems formed from the acid-base reaction of KH2PO4 and dead burnt MgO [1,2]. MKPCs benefit from a near-neutral pH, low drying shrinkage, low demand for water during mixing and high compressive strengths from early ages [2,3,4], compared to other cementitious systems. These properties have led to consideration of MKPCs as encapsulants/grouts for the stabilisation of nuclear wastes, including (but not limited to): the physical encapsulation of plutonium contaminated ashes [5], liquid wastes [2,6] and the immobilisation of cesium (accommodated within struvite-K by partial K replacement) [7]. The corrosion reaction products both have potential to cause pressurisation and distortion of the waste container [15], which is undesirable from a long-term safety aspect
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