A. Kindness

Stability and solubility relationships in AFm phases: Part I. Chloride, sulfate and hydroxide

Abstract Portland cements contain an AFm phase whose anion content is initially dominated by OH − (hydroxy) and SO 4 2− (sulfate); variants such as those based on C 2 ASH 8 may occur in blended cements. In service conditions AF m phases may exhibit anion exchange, principally with carbonate, chloride, and additional sulfate. The chemistry, stability, and crystal chemistry of AF m phases are reviewed briefly. New data are presented on the role of 3CaOAl 2 O 3 0.5CaCl 2 0.5CaSO 4 10H 2 O, which, it is proposed, should be named Kuzel’s salt after its discoverer.

Immobilization of chromium in cement matrices

Portland cement and blended cements containing blast furnace slag afford both physical and chemical immobilization of chromium. To separate physical and chemical effects, the pore fluid contained in set, hydrated cements has been expressed and analyzed. In Portland cement spiked with 5,000 ppm Cr(III), pore fluid levels are 0.1--1 ppm, whereas in well-cured slag blends, they decrease to <0.01 ppm. Both cement types give chemical immobilization, but slag cements give the better performance. Slag-containing cements are the most effective at removing Cr(VI) from the pore fluid, probably by reducing Cr(VI) to Cr(III). Electron microscopy coupled with energy dispersive X-ray analysis shows that Cr(III) can be substituted for Al in most of the calcium aluminated hydrate phases. In synthetic preparations, substitution is complete resulting in Ca-Cr phases that are isostructural to calcium aluminate phases. Three Cr analogues of calcium aluminates were synthesized: Ca[sub 2]Cr(OH)[sub 7] [center dot] 3H[sub 2]O, Ca[sub 2]Cr[sub 2]O[sub 5] [center dot] 6H[sub 2]O and Ca[sub 2]Cr[sub 2]O[sub 5] [center dot] 8H[sub 2]O, as well as solid solutions, e.g., Cr substituted hydrogarnet 3CaO [center dot] (Al[sub 2]O[sub 3]/Cr[sub 2]O[sub 3]) [center dot] 6H[sub 2]O. There is no real evidence that Cr is taken up by C-S-H gel.

Impact of carbon dioxide on the immobilization potential of cemented wastes: Chromium

Portland cement and blended cements containing blast furnace slag afford both physical and chemical immobilization of chromium. Chromium occurs in aqueous solutions in two oxidation states, Cr(III) and Cr(VI). Slag-containing cements are very effective at removing Cr(VI) from the internal pore fluid, probably by reducing Cr(VI) to less soluble Cr(III). Carbon dioxide attack, or carbonation, is probably the most common form of concrete environmental attack and it promotes changes to the cement chemical composition and physical properties that can affect the long-term retention of heavy metals. In the present paper the effect of carbonation on the immobilization of Cr(III) and Cr(VI) has been studied in both Portland cements and blended cements containing blast furnace slag. The results show that although Portland cements matrices are more resistant to carbonation than slag-containing cement matrices, the increase of chromium content in pore solution is more marked for Portland matrices. After 60 days the pore fluid of carbonated Portland cement spiked with 50000 ppm Cr(III) or Cr(VI) contains 20 or 40000 ppm respectively, whereas after carbonation of slag blends for the same time, the corresponding pore fluid Cr contents are 1 and 16000 ppm respectively.

Thermodynamic investigation of the CaO-Al[sub 2]O[sub 3]-CaCO[sub 3]-H[sub 2]O closed system at 25 C and the influence of Na[sub 2]O

The solubilities of calcium hemicarboaluminate, calcium monocarboaluminate and calcium tricarboaluminate have been determined and the equilibrium phase diagram for the CaO-Al[sub 2]O[sub 3]-CaCO[sub 3]-H[sub 2]O closed system at 25 C has been calculated. Six isothermally invariant points have been located involving six stable hydrates: CH, C[sub 3]AH[sub 6], AH[sub 3], calcium hemicarboaluminate, calcium monocarboaluminate and calcite. Calcium tricarboaluminate, the carbonate analogue of ettringite, does not appear to be stable at 25 C. This study was part of a larger study on radioactive waste solidification.

Immobilisation and fixation of molybdenum (VI) by Portland cement

The immobilization of molybdenum (6) in Portland cement has been studied. Analytical methods are reviewed. As a precursor to studies using commercial cements, the cement constituent phases Ca[sub 3]Al[sub 2]O[sub 6] and Ca[sub 3]SiO[sub 5] and mixtures of Ca[sub 3]Al[sub 2]O[sub 6] and gypsum (CaSO[sub 4][center dot]2H[sub 2]O) have been synthesized and subsequently hydrated with an aqueous solution containing initially 2,000 ppm Mo. The aqueous Mo contents decrease slowly over the first 12 days before attaining a steady-state concentration, ca. 40--50 ppm. Its main speciation is as molybdate, MoO[sub 4][sup 2[minus]]. Commercial Portland cement was also studied; chemically soluble Mo was determined following pore fluid extraction. The 28-day Mo concentration, in the pore fluid, was in the range 50--80 ppm. the insoluble Mo in Portland cement is distributed mainly between powellite, CaMoO[sub 4], and a calcium aluminate sulfate hydrate with a very small amount in cement gel, a hydrated calcium silicate. The synthesis and characterization data are reported for Ca[sub 3]Al[sub 2]O[sub 6][center dot]CaMoO[sub 4][center dot]10--14H[sub 2]O (Mo-AFm). Solubility data are reported for synthetic powellite and Mo-AFm at 25 C.

Solubility properties of ternary and quaternary compounds in the CaO Al2O3 SO3H2O system

Solubility measurements are reported for four hydrated cement phases. A technique employing repeated dispersion and filtration is described which enabled the solubility (activity) products of ettringite (C6AS3H32) and hydrogarnet (C3AH6) to be calculated with confidence. Free energies of formation are also given. These phases dissolve congruently, whereas monosulphate (C4ASH2) and tetra-calcium aluminate hydrate (C4AH13) exhibit incongruent dissolution, and can therefore not be described by a conventional solubility product. The results represent additional data for inclusion in thermodynamic databases currently being applied to the modelling of cement systems.

Sulphate attack on concrete: limits of the AFt stability domain

Abstract The compositions of the solutions at the four invariant points that define the AFt equilibrium surface, have been calculated at 25°C with and without sodium ions. The results indicate that AFt is stable over a large range of sulphate concentration depending on both the calcium and aluminium concentrations. The presence of sodium modifies the AFt stability domain but it remains stable even at high sodium concentrations.

Corrosion behaviour of steel in high alumina cement mortar cured at 5,25 and 55 °C: chemical and physical factors

The corrosion behaviour of embedded steel was related to the composition of the pore phase in equilibrium with the hydrated phases and the porosity of the high alumina cement mortars subsequent to curing at 5,25 and 55 °C. The corrosion of reinforcements was evaluated by electrochemical techniques. The effect on corrosion of 3% by weight of cement of NaCl, added during the mixing process, and of the accelerated carbonation of mortars in CO2 atmosphere were also determined. The pH value and the chemical composition of pore fluid of plain high alumina cement (HAC) mortar cured at all three temperatures suggested that the embedded steel was in a passivated state. The resistance of HAC to carbonation and its greater potential for chloride binding by chloroaluminate formation are believed to make HAC inherently more protective to steel, relative to normal Portland cement, during ingress of chloride from external sources. High corrosion rates reported in literature for steel embedded in HAC may be attributable to bad practice, not to lack of passivity.

The use of silver as a selective precipitant for 129I in radioactive waste management

Abstract 129 I is one of the more hazardous nuclides occurring in radioactive waste. In the form of I − , its most likely speciation, it is poorly sorbed on most geologic media. Several workers have suggested the use of silver to precipitate I − as the insoluble AgI, in a cemented waste form, or as a “getter”. The efficacy of this procedure is examined by experiment, in conjunction with thermodynamic predictions. The addition of AgNO 3 to Portland cement leads to coprecipitation with C-S-H, with low Ag solubilities (∼ 10 μ mg/L); 2–;3 orders of magnitude lower than predicted (from Ag 2 O). AgI is stable in these matrices, with low aqueous I concentrations ( − passing into solution; concentrations up to 900 mg/L were observed. It is shown that repository conditions, on closure, are also likely to induce solubilisation of I − from AgI. It is concluded that the use of Ag is unlikely to significantly improve the immobilisation properties of the near field for radioiodine.

Determination of sulphonated naphthalene formaldehyde superplasticizer in cement: A new spectrofluorimetric method and assessment of the UV method

Abstract The quantitative determination of SNF in the aqueous phase of fresh cement-water systems have been investigated by UV spectrophotometry and spectrofluorimetry. The conventional UV method gives a spectrum with three λ max values at ca. 227, 292 and 328nm, which may be used for analysis. However, different λ max values produce rather different results for the same species of SNF in the aqueous phase of fresh cement mixes although all follow the same trends. The SNF concentrations in solutions measured at ca. 227nm are significantly higher than those obtained at 292nm and 328nm. Alternatively, a method based on spectrofluorimetry may be used for SNF analysis. The results obtained using this method are in very good agreement with those produced by the UV method at 227nm, suggesting that true SNF concentrations in fresh or hardened cement pastes can be measured either by spectrofluorimetry or by UV spectrophotometry at 227nm.