Solidification and stabilization of strontium and chloride ions in thermally treated calcium aluminate cement modified with or without sodium polyphosphate

Irisawa, Keita; Namiki, Masahiro*; Taniguchi, Takumi; Garcia-Lodeiro, I.*; Kinoshita, Hajime*

Cement and Concrete Research, 156, p.106758_1 - 106758_8, 2022/06

https://doi.org/10.1016/j.cemconres.2022.106758

Cementation of aqueous radioactive waste contaminated with a significant Sr is challenging, and utilization of calcium aluminate cement (CAC) modified with sodium polyphosphate (CAP) is interesting. The present study investigated solidification and stabilization of Sr and Cl ions in CAC and CAP cured in open system at 90C and in closed system at 20C. A leaching test showed that Sr ion could be stabilized more effectively in the CAP than in the CAC. On the other hand, the CAC cured at 20C had the best stabilization for Cl ion among the samples. Friedel's salt formed in the CAC may have contributed to the immobilization of Cl ion. Although the stabilization of Cl ion by CAP was less effective than CAC, it was significantly improved by the thermal treatment. The results may suggest that Cl ion in the CAP was incorporated in the poorly crystalline apatite structure.

Influence of mixing solution on characteristics of calcium aluminate cement modified with sodium polyphosphate

Irisawa, Keita; Garcia-Lodeiro, I.*; Kinoshita, Hajime*

Cement and Concrete Research, 128, p.105951_1 - 105951_7, 2020/02

https://doi.org/10.1016/j.cemconres.2019.105951

This study investigated characteristics of a calcium aluminate cement modified with a phosphate (CAP) by changing an amount and concentration of mixing solution with sodium polyphosphate. When the amount of mixing solution was increased with a constant amount of sodium polyphosphate, an enhanced consumption of monocalcium aluminate was observed compared with gehlenite in calcium aluminate cement (CAC). Formation of gibbsite, Al(OH), was also increased as a hydration product in the CAP and the possible reduction of water in the amorphous gel phase. When the amount of mixing solution was increased with a constant concentration of sodium polyphosphate, the enhanced consumption of monocalcium aluminate was not observed. Neither gibbsite nor any other crystalline hydration products were identified in this series. In addition, unreacted sodium polyphosphate remained in the system. The increased formation of gibbsite and the possible reduction of water from the amorphous gel phase appears to contribute to the improvement of the microstructure in the products.

Application of phosphate modified CAC for incorporation of simulated secondary aqueous wastes in Fukushima Daiichi NPP, 1; Characterization of solidified cementitious systems with reduced water content

Garcia-Lodeiro, I.*; Lebon, R.*; Machoney, D.*; Zhang, B.*; Irisawa, Keita; Taniguchi, Takumi; Namiki, Masahiro*; Osugi, Takeshi; Meguro, Yoshihiro; Kinoshita, Hajime*

Proceedings of 3rd International Symposium on Cement-based Materials for Nuclear Wastes (NUWCEM 2018) (USB Flash Drive), 4 Pages, 2018/11

Heat treatment of phosphate-modified cementitious matrices for safe storage of secondary radioactive aqueous wastes in Fukushima Daiichi Nuclear Power Plant

Irisawa, Keita; Taniguchi, Takumi; Namiki, Masahiro; Garca-Lodeiro, I.*; Osugi, Takeshi; Sakakibara, Tetsuro; Nakazawa, Osamu; Meguro, Yoshihiro; Kinoshita, Hajime*

Proceedings of 2017 International Congress on Advances in Nuclear Power Plants (ICAPP 2017) (CD-ROM), 6 Pages, 2017/04

A solidification technique with minimized water content is being developed using phosphate cements for the safe storage of secondary radioactive wastes in the Fukushima Daiichi Nuclear Power Plant. Conventional cement systems become solidified via hydration reactions, and need a certain water content. Phosphate cement systems, however, become solidified via an acid-base reaction, and so they only require water mainly for reasons of workability. A reduced water content of phosphate cement systems is beneficial for the immobilization of the radioactive wastes from mitigating the potential to generate hydrogen gas by the radiolysis of water by radioactive wastes. The current study investigated the water content and mineralogy of calcium aluminate cement (CAC) and phosphate-modified CAC (CAP) cured in open systems at 60, 90 and 120 C and in a closed system at 20 C as a reference case. Water contents in both the CAC and the CAP were seen to decrease as curing progressed. For 90 C, the CAP contained less water than CAC. Free water in CAC converted to structural water by heat treatment, but this was not the case for CAP. An orthophosphate hydrate salt, a precursor phase of hydroxyapatite, was found in CAP when cured at 20 and 60 C, and a mixture of the orthophosphate hydrate salt and hydroxyapatite, Ca(PO)(OH), were formed in the CAP when cured at 90 C. Phosphate products in CAP cured at 120 C appears to consist of a different phosphate phase compared with the CAP cured at 20, 60 and 90 C.

Development of solidification techniques with minimised water content for secondary radioactive aqueous wastes in Fukushima, 6; Incorporation of simulated secondary aqueous wastes in CAC and CAP cements

Garca-Lodeiro, I.*; Lebon, R.*; Mahoney, D.*; Zhang, B.*; Irisawa, Keita; Osugi, Takeshi; Nakazawa, Osamu; Kinoshita, Hajime*

no journal, , 

Use of NaHPO and (NaPO) as a phosphate source for development of magnesium phosphate cements

Garca-Lodeiro, I.*; Kai Wuen, O.*; Irisawa, Keita; Nakazawa, Osamu; Kinoshita, Hajime*

no journal, , 

Development of solidification techniques with minimised water content for secondary radioactive aqueous wastes in Fukushima, 7; Leaching test of CAC and CAP containing simulated secondary wastes

Garca-Lodeiro, I.*; Lebon, R.*; Mahoney, D.*; Zhang, B.*; Irisawa, Keita; Osugi, Takeshi; Nakazawa, Osamu; Kinoshita, Hajime*

no journal, , 

Leaching tests have been conducted on CAC and CAP systems prepared with minimised water content, containing simulated secondary aqueous wastes to investigate their feasibility as wasteforms. In CAC, the effect of reduction in water content on leaching was small whereas CAP system indicated a significant benefit of reduction in water content. CAP appears to be beneficial to retain alkaline earth elements such as Sr, when cured at elevated temperatures.

Effects of composition on calcium aluminate phosphate cement for caesium nitrate incorporation

Irisawa, Keita; Yeung, S. W.*; Suriwong, P.*; Kinoshita, Hajime*

no journal, , 

In order to evaluate the applicability of calcium aluminate phosphate (CAP) cement as a matrix for nuclear wastes, we investigated the compositional effects on the CAP cement focusing on the caesium incorporation. The CAP samples were prepared by mixing calcium aluminate cement (CAC) with the aqueous solution of (NaPO) and CsNO. CsNO re-precipitated in the conventional CAC sample, suggesting a limited interaction of CAC with CsNO. The increase in the polyphosphate content limited the conventional hydration of CAC and the relative amount of CsNO re-precipitation. This is interesting as it may show a potential interaction between CAP matrix and CsNO.

Development of solidification techniques with minimised water content for secondary radioactive aqueous wastes in Fukushima, 2; Roles of phosphate on mineral formation of calcium aluminate cement

Irisawa, Keita; Garca-Lodeiro, I.*; Osugi, Takeshi; Nakazawa, Osamu; Meguro, Yoshihiro; Kinoshita, Hajime*

no journal, , 

In order to reduce hydrogen gas explosion and leakage risks for these wastes, we are aiming to develop phosphate cements with minimised water content. The present study investigates roles of phosphate modification on mineral formation in calcium aluminate cement.