Incorporation of AlCl 3 and As 2 O 3 in Composite Cement Systems
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,QFRUSRUDWLRQRI$O&ODQG$V2LQ&RPSRVLWH&HPHQW6\VWHPV J Hill and J H Sharp Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK $%675$&7 As part of an investigation into the consequences of adding inorganic metal salts to composite cements, based on blastfurnace slag (BFS) and pulverised fuel ash (PFA), the effect on the hydration behaviour of adding AlCl3 or As2O3 to the mix water of a number of cementitious systems was investigated using isothermal conduction calorimetry (ICC) and x-ray diffraction (XRD). Four cement systems were investigated; ordinary Portland cement (OPC), 3:1 BFS: OPC, 9:1 BFS:OPC and 3:1 PFA:OPC. AlCl3.7H2O and As2O3 were added to the mix water at 0.1% and 1% concentrations. Results from ICC indicated that the addition of As2O3 had very little effect on the setting and early hydration of any of the cements. AlCl3, however, inhibited the setting of the composite cements at the 1% addition level but accelerated the OPC at 0.1%. The expected hydration products were observed, with the additional observation of Friedel’s salt in the presence of aluminium chloride additions and vaterite in both the aluminium and arsenic-containing systems after 180 days hydration. ,1752'8&7,21 Understanding the science associated with incorporation of intermediate level toxic and radioactive waste into cement systems is important for their safe and cost effective storage and disposal. Aluminium and arsenic are significant wastes and are included in the Environmental Protection Agency list in the USA. It is, therefore, important to understand the effect of these elements on the hydration of cementitious systems, especially as there is little published literature for either of the elements. However, a review by Mattus and Mattus [1] highlighted that, in alkaline solutions, aluminium is usually in the form of the aluminate anion (AlO2-) which will accelerate the set of cement especially when there is blast furnace slag or fly ash present [1]. Studies using simulated Hanford waste solutions have shown that problems with excessive heat generation arose from the presence of aluminate although this can be overcome by adjusting the grout formulation, such as by addition of another material to lower the amount of active slag or fly ash. In addition to soluble aluminium salts, metallic aluminium causes hydrogen gas generation problems [1,2]. A few studies have been carried out to ascertain immobilisation techniques for As(III) and a number of these [3-6] report the formation of CaAsO2(OH) in the presence of calcium hydroxide. Due to its insolubility in water, formation of this compound limits the extent of arsenic leaching from the cement. (;3(5,0(17$/ Four cement systems were chosen as being of interest to British Nuclear Fuels plc (BNFL): 100% OPC (included as a reference material), 3:1 BFS:OPC, 9:1 BFS:OPC and 3:1 PFA:OPC. The OPC and BFS were supplied by Castle Cement and the PFA by Pozzolanic Lytag. All the powders were prepared
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