Investigation on the Immobilisation of Carbon in OPC-BFS and OPC-PFA Systems
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Investigation on the Immobilisation of Carbon in OPC-BFS and OPC-PFA Systems Hajime Kinoshita1, Paulo H. R. Borges1, Claire A. Utton1, Neil B. Milestone1 and Cyril Lynsdale2 1 Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, U.K. 2 Department of Civil and Structural Engineering, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, U.K. ABSTRACT The reaction of CO2 gas with OPC, OPC-BFS and OPC-PFA composite cement systems were studied using XRD, SEM and TG to investigate the applicability of these materials to immobilise carbon arising from graphite waste. XRD results suggested that calcite formed in OPC system after the carbonation reaction, whereas calcite and vaterite were observed in OPCBFS and OPC-PFA systems. In OPC system, nearly half of Ca(OH)2 was consumed to form CaCO3. In OPC-BFS and OPC-PFA systems, the amount of CaCO3 formed, corresponded to the consumption of greater than 100% of Ca(OH)2 initially present, suggesting that other hydration products e.g. C-S-H were also consumed, either directly or indirectly during the carbonation process. The OPC-BFS system became more porous after carbonation. OPC-PFA system indicated a high efficiency on the conversion of Ca in the system into CaCO3. INTRODUCTION A large amount of graphite waste exists and will increase in UK from the decommissioning of Magnox stations. It has been reported that such graphite can go through self-sustaining oxidation at temperatures of about 550°C [1], which would eventually produce CO2 gas. It is known that the reaction of cementitious systems with CO2 occurs even under ambient condition due to the high thermodynamic stability of the carbonates of alkali-earth elements e.g. CaCO3 and MgCO3. It may be possible to apply this reaction to immobilise the carbon originating from the Magnox graphite waste by deliberately oxidise graphite wastes into CO2. The reaction between cementitious materials and CO2 has been of interest, especially in recent years, due to its potential as a raw material for the sequestration of CO2 [2-4]. Present work is a preliminary study focused on the reaction of CO2 gas with neat ordinary Portland cement (OPC), OPC-blast furnace slag (BFS) and OPC- pulverised fuel ash (PFA) composite cement systems. These cementitious systems were selected as they are often used to immobilise low and intermediate level nuclear wastes [5]. Experimental Experimental conditions are detailed in Table I. A neat OPC grout, a mixture of BFSOPC (9:1 BFS:OPC) and that of PFA-OPC (3:1 PFA:OPC) were prepared, with a water to solids ratio (w/s) of 0.33. The grouts were moulded in prisms of 25 x 25 x 285 mm and cured at 40 oC
Table I. Experimental details. W/S Curing Composition (wt%) Size ratio condition OPC=100 Temp: 40oC Particles: 3.36-5.00mm BFS:OPC=90:10 0.33 R.H.: 100% Powder:
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