Microstructural Aspects of Zeolite Formation in Alkali Activated Cements Containing High Levels of Fly Ash
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ABSTRACT Wasteforms made by reaction at elevated temperature of a highly alkaline simulated lowlevel nuclear waste solution, having high sodium ion concentration, with a cementitious blend high in fly ash have been studied. Significant formation of Na-P1 zeolite (gismondine framework) and of a sodalite occurred. The time evolution of the crystalline phases over the first 28 days is reported for both adiabatic and isothermal curing, and the role of these phases in microstructure development is discussed. The level of carbonate ions in solution was found to have a substantial effect on strength evolution and chemistry.
INTRODUCTION Cement stabilization has considerable potential for the immobilization of low-level nuclear waste solutions and other hazardous wastes; reactions that occur due to the high pH of the pore solution reduce the mobility of many species
of
concern
[1].
Zeolites
have
also
found
considerable application for immobilization of radioactive species; for example, clinoptilolite can selectively absorb species such as cesium ions [2] by ion exchange. In situ zeolite formation has been observed previously in alkali activated cement systems with high levels of pozzolanic admixtures [3,4]. Hoyle and Grutzeck have demonstrated the potential for removal of cesium from solution by alkali activated hydration of calcium aluminosilicate glasses [5]. In this paper we consider the phase evolution and microstructure of a cement-stabilized wasteform in which zeolite formation occurs spontaneously. The wasteform contains Na-Pt (gismondine framework) and sodalite zeolites, which may enable immobilization of both cationic and anionic species. The wasteform was generated by reaction at elevated temperature of a simulated alkaline waste solution with a solid blend containing a high proportion of fly ash. The composition of the full solution is given in Table I; sometimes a simplified solution was used (bold type in Table I.) The cementitious solids (dry
Table I: Composition of the full simulated low level waste solution. Components used to make up a simplified solution are shown in bold type Compound NaOH AI(NO 3)3 .9H20 Na 3PO4 .12H 2 0 NaNO 2 Na 2 CO 3 NaNO 3 KC1 NaCl Na3Citrate.2H 20 Na2B407 Na2SO4 Ni(N0 3)2.6H2 0 Ca(N0 3)2 .4H 20 Na4(EDTA).2H 2 0 Na3(HEDTA) Glycollic Acid M6(NO•),.6HO
199 Mat. Res. Soc. Symp. Proc. Vol. 370 01995 Materials Research Society
9/1 74.80 128.00 74.40 36.90 36.20 8.58 1.83 2.50 2.50 0.13 3.89 0.30 0.44 1.42 5.29 0.65 0.03
Table II: Composition of the dry blend used for solidification of the simulated low level waste solution. A mix ratio of lkg preblended solids to 11 solution was used. Material Cement Fly Ash
Amount 20.7% 68.3%
Attapulgite
11.0%
Details of Supplier Type II OPC ex Ash Grove Cement Co., Durkee, OR Class F (but high Ca) ex Centralia, Ross Sand and Gravel Co., Portland, OR Attapulgite Clay ex Engelhard Co., Iselin, NJ
blend) was added with a ratio of lkg / 1 liter of waste solution, and had the composition given in Table II. This blend has been confirmed t
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