Approaches to control the quality of cementitious PFA grouts for nuclear waste encapsulation

Pulverised Fuel Ash (PFA) is combined with Ordinary Portland Cement (OPC) powder and water to form cementitious grouts for use in various aspects of nuclear waste encapsulation. Whilst specific PFA supplies in the United Kingdom currently deliver adequate grout performance it is also clear that some alternative supplies result in inferior performance, leading to concern over the long term availability of suitable raw material. This paper presents the results of an investigation into the characteristics of PFA that affect critical aspects of grout performance and identifies strategies that could be used to ensure high quality PFA supplies in the future. A key grout performance characteristic is the requirement to produce a high level of fluidity without generating a significant volume of bleed water after setting. This must be achieved within a set operating envelope which precludes the use of Superplasticisers and limits water content. Specific sources of PFA (individual power stations) have previously been identified as having an adequate performance and therefore form the current supply. Some other sources are generally known to produce materials that result in very poor performance or exhibit large performance fluctuations over time. The identification of physical and/or chemical characteristics of PFA that influence grout performance is therefore essential in ensuring a consistent long term, high quality, supply for encapsulation plants. A wide range of PFAs have been characterised encompassing the current supply, known poor quality supplies and potential future supplies. After preparation into grouts, using standard industry acceptance criteria, PFA characteristics such as particle size distribution and unburned carbon content have been used to create simple performance models. Whilst unburned carbon and aspects of the particle size distribution each appear to influence grout performance, the relatively weak models developed from each parameter alone indicate that a combination of these factors control final performance. Suitable processing techniques, which could be employed to upgrade previously unsuitable supplies, were then identified and practically tested at laboratory scale. One such technology is based on the established mineral processing technique of froth flotation. This technique is demonstrated as being able to reduce unburned carbon present in PFA whilst retaining a desirable particle size distribution in the reduced carbon product. Using this approach the performance of a number of PFAs, which would normally be rejected under acceptance criteria, were upgraded to produce grout fluidity levels in excess of the best known current supplies. The mechanisms which lead to the performance enhancement are discussed and used to identify other processing techniques that might be used to upgrade PFA in the future. Froth flotation offers a route to upgrade previously unsuitable PFA supplies for use in waste encapsulation grouts. Fluctuations in grout performance can be reduced whilst the significant performance advantages over standard materials leads to the potential for reducing grout water content, which itself has a number of desirable process implications in waste encapsulation.