The immobilization of Cs in cement based matrices
Ordinary Portland Cement (OPC) is a material of complex chemical and phase composition. Therefore its sorption characteristics were determined by examining the properties of its individual phases, as well as bulk OPC. The principal constituents of OPC along with its hydration products were tested for their potential to sorb Cs using radiocaesium tracer studies. Results showed that OPC had no significant sorption potential for Cs. It was therefore suggested that the cement composition required to be tailored by including materials which were likely to sorb Cs and moreover, improve the physical properties of the cement matrix. Two categories of materials were chosen for study:- (i) those which are normally permitted in civil engineering practice, including amorphous silica, pulverised fuel ashes (PFA) , natural pozzolari and blast furnace slag (BFS); (ii) those which were known or suspected to be good Cs sorbers but whose stability in cement were as yet unknown. These included hydrous titanium dioxide, calcined kaolin, acid zirconium phosphate, tobermorite and xonotlite. Amorphous silica, several PFA's, natural pozzolan and BFS were all examined for their sorption potential. All the materials showed, to a greater or lesser degree, a potential for immobilising Cs. The amorphous silica showed the greatest uptake of Cs and was chosen for further study. The sorption of Cs by amorphous silica was determined over a- wide range of Cs concentrations. daData were treated by the Freundlich isotherm and indicated that different sorptive mechanisms may be operating at different concentration aranges. The effect of water: solid ratio and initial pH were examined. Results showed that pH had 2 opposite effects on the silica:- (a) the high pH (-12.5) increases its sorption efficiency for Cs, compared to neutral conditions, presumably due to an enhanced negative charge on the silica surface; (b) the presence of Ca(0H)2, which was used to adjust the pH arid would be present in hydrated OPC, decreases the available silica content due to pozzolanic reactions consuming silica to form Calcium Silicate Hydrate gels (C-S-H). Hence if a silica containing material was added to cement and was the only sorbing material present, it might have to be proportioned carefully to maintain a permanent excess of silica. Leach tests were carried out on mixes of cement with amorphous silica, PFA and BFS. All gave significant improvements in leach characteristics compared to OPC alone. The effects-of curing, conditions were studied. Results showed that increasing the curing time from 28 days to 84- days or 168 days or increasing the curing temperature from 20 G to 4-0 C or both improved the leach resistance. Mercury intrusion porosimetry gave little indication that this was totally due to physical encapsulation and studies of the pore fluids on the samples showed beyond doubt that a strong chemical mechanism was involved. It was hypothesised that C-S-H with a high silica content was responsible for the chemical retention of Cs. In OPC the C-S-H has a C/S ratio of -1.7 but when amorphous silica or PFA is added this ratio is reduced and can be as low as 0.8. BFS hydrates independently from OPC but it also produces a C-S-H with a low C/S value. This hypothesis was confirmed by studies on synthetic C-S-H with a range of C/S ratios and by analysing OPC/PFA and OPC/silica pastes in the analytical electron microscope. Sorption experiments were also carried out on the second category of materials. All showed good sorption potential for Cs in neutral solutions. Sorption remained significant in the presence of Na and K as competitive species. However, the effect of Ca (OH)2, differed. Tobermorite and Xonotlite appeared to show the most significant sorption of Cs in the presence of Ca(OH,)2 while the others either reacted with Ca(OH)z or discharged Cs - presumably by exchange for Ca. Tobermorite and Xonotlite were further studied. Desorption experiments and the effect of varying the Cs concentration as well as pH effects were investigated. Both materials gave straight line plots on a Freundlich isotherm indicating that one sorptive mechanism operated over a wide concentration range. Sorption increased with pH but unfortunately desorption studies showed that the Cs was easily removed. Leach studies were carried out on composites with OPC containing 5-10% tobermorite. Such additions decreased the leach rate of Cs compared to OPC.