Phosphate-modified calcium aluminate cements

The effect of phosphate modification on CAC hydration is poorly understood, and the investigation in this thesis focuses on the sodium phosphate modification of a commercially available calcium aluminate cement, examining the following : (i) the effect of modification upon the fresh state properties, (ii) long-term phase evolution, (iii) binding phase characterisation, and (iv) trials of aluminium encapsulation. Formulations of CAC modified by sodium polyphosphate, sodium monophosphate and mixtures of these, in varying proportions, are investigated. ATR-FTIR and solution NMR are used to examine the chain length of phosphate ions in solution prior to mixing. Immediately after mixing the fresh state properties are investigated using isothermal calorimetry, to assess the effect of average phosphate chain length on the heat of hydration and thermal behaviour of pastes during the initial curing period. The phase assemblage over the long term is examined by XRD and TGA up to a period of 180 days, elucidating trends in hydration behaviour with phosphate modification. This focuses upon assessment of the degree of conversion, identifying phosphate modifications which prolong the presence of metastable CAC hydrates for up to 180 days and also formulations that prevent any conventional phases forming at all and hence avoid conversion. Promising formulations with no conventional CAC hydrate formations are studied up to a period of 1050 days to confirm the longer-term stability of the alternate hydrates being formed. Characterisation of these samples after hydrothermal treatment showed the formation of hydroxyapatite, boehmite and a zeolite type phase. The disordered binding phase of this system is further investigated using solid-state nuclear magnetic resonance (NMR) to probe the environments of the 31P and 27Al nuclei. Results from advanced REAPDOR NMR experiments, used to assess the interactions between these nuclei, are reported and confirm the presence of a disordered C-A-P-H type binding phase. i Results from trials of aluminium encapsulation are also reported, where corrosion is assessed by hydrogen evolution evaluation using mass spectroscopy and water displacement, differential scanning calorimetry, scanning electron microscopy, and mass loss measurements. Promising formulations with phosphate modification outperformed the neat CAC encapsulants in all experiments performed, considering both powder and plate aluminium. The series of formulations with polyphosphate to cement mass ratio of 0.4 are recommended for further investigation as waste encapsulants. It was determined from the results of this study that altering the average phosphate ion chain length in solution prior to mixing can be used as a tool to tune the fresh state properties, including the heat of hydration and setting time. The kinetics of long term hydration and phase assemblage development maybe affected with the addition of sodium monophosphate, and radically altered away from conventional CAC hydration and instead the formation of an x-ray amorphous binding phase. This binding phase, optimal formulations, is shown to be stable in to a minimum of 1050 days with elevated compressive strength. NMR spectroscopy is used to positively verify the binding phase to be a calcium aluminium phosphate hydrate phase. Optimised phosphate modified CAC formulations are shown to outperform other conventionally used cementitious encapsulants, including OPC, OPC/BFS and neat CAC.