Changhui Yang

Sodium Sulphate Activated GGBS/PFA and Its Potential as a Nuclear Waste Immobilisation Matrix

The UK currently uses Portland cement composite blends to immobilise/encapsulate intermediate and low level radioactive wastes (ILW and LLW). However, among other things, the high pH of these systems causes the corrosion of some metals, which can lead to expansion and excess generation of hydrogen. Therefore, in order to immobilise nuclear waste where corrosion is an issue, a near neutral cementing system is desirable. Among the activators which can be used in the alkali-activated slag (AAS) systems, a solution of Na 2 SO 4 is near neutral but the ground granulated blast-furnace slag (GGBS) itself has a pH about 11, increasing the pH within the Na 2 SO 4 activated AAS. As low calcium Pulverized Fuel Ash (PFA) only has a pH about 9, using a GGBS/PFA blend activated by Na 2 SO 4 offers the potential to develop a near neutral cementing system for nuclear waste immobilisation purposes. In this paper, the replacement of GGBS in the Na 2 SO 4 activated AAS system with PFA at 0%, 10%, 20% and 30% by mass was examined. The pH and corrosion of Al were determined and used as primary criteria for judging the feasibility for further development of a Na 2 SO 4 activated GGBS/PFA matrix for immobilising nuclear wastes. The microstructure of the matrices was studied by SEM. Leaching studies were carried out to examine the possibility of immobilising Cs + within these Na 2 SO 4 activated GGBS/PFA matrices. The potential of using Na 2 SO 4 activated GGBS/PFA for immobilising nuclear wastes is discussed.

Immobilization of Cr(VI) by hydrated Portland cement pastes with and without calcium sulfate

This work aims to illustrate the impact of high concentrations of Cr(VI) (based on Na2CrO4) on the hydration assembly and microstructural development of hydrated Portland cement, and the results also present the role of calcium sulfate on the immobilization of Cr(VI) in Portland cement. The results showed that the immobilization of Cr(VI) in hydrated Portland cement was attributed to the formation of CrO4-U phase, an analogue of SO4-U phase (3CaO·Al2O3·CaSO4·0.5Na2SO4·15H2O). The growth of CrO4-U phase on the surface of clinker particles formed a diffusion barrier and hence increased the setting time. Increasing the calcium sulfate dosage impaired the Cr(VI) immobilization due to the competition between CrO42- and SO42- integrated into the U phase. The generalized acid neutralization capacity (GANC) test indicated that the Cr(VI) leaching behavior was a function of the leachate pH value. As the pH decreased to 11.8, the CrO4-U phase was converted quickly to CrO4-ettringite, which generated a slight increase in Cr(VI) concentration. The most leaching sector, approximately 89.3% of added Cr (1wt% of cement), was found in the pH range 11.8-10.5 due to the dissolution of secondary CrO4-ettringite. It could also be shown that the C-S-H had little chemical binding for Cr(VI).

Use of Two-Pressure-Head Method to Assess Water Permeability of Structural Concrete

Determining the water permeability of concrete in structures remains a conundrum because of difficulties in removing the influences of moisture. This study describes the extended flow-net theory developed on the basis of the two-pressure-head concept, which provides a means of measuring permeability under the partially saturated condition. Surface-mounted tests and standard laboratory water penetration tests were carried out to verify this approach. Before determining the water permeability, steady-state flow rates at two different pressure levels were evaluated and the effects of initial moisture conditions on flow behavior were investigated. The results indicate that the proposed approach does offer a useful means of determining the water permeability of structural concrete, although it cannot be claimed to be universally applicable for all moisture conditions likely to be encountered in practice.

Effectiveness of Vacuum Saturation Preconditioning Regime for Assessing Water Permeability of High Performance Concrete

Saturating high performance concrete (HPC) for assessing water permeability is a challenge. This paper reports a testing program established to determine the reliability and efficiency of an in situ preconditioning regime, the vacuum saturation, for the water permeability test. The vacuum saturation regime was examined through changing the parameters used and to justify if the field conditioning regime is able to yield similar results as in the laboratory, the results after vacuum saturation were statistically analysed and compared with that after incremental immersion. Its accuracy was further examined by additional experiments, whereas factors considered include different concrete mixes and different initial moisture conditions. The results suggest that the proposed method can eliminate the influence of moisture for different HPCs under the high initial moisture content. It is not effective, when the initial moisture content is low.