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

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Abstract

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. Formation of gibbsite, Al(OH)3, was also increased as a hydration product in the CAP and a 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.

Introduction

Chemically-bonded phosphate cements and ceramics can have a wide range application, from the technical cements such as dental cements [1] and shielding material for neutrons or immobilizing matrix for highly radioactive fission products [2], to the products as common as the construction cements and the corrosion and fire protection coatings [3].

Calcium aluminate cement (CAC) modified with phosphates are a type of chemically-bonded phosphate cements, and can be found in various studies. CAC modified with ammonium polyphosphate was investigated by Sugama and Callciero as an alternative for the magnesium phosphate cement system that experiences a hydrolytic deterioration [4]. They later improved their system by replacing the ammonium phosphates to sodium phosphates as the environmentally safe reactant [5] to avoid the potential emission of NH3 gas. Ma and Brown used sodium phosphates to modify CAC hydration, as a means to suppress the formation of metastable CAH10 and C2AH8 and avoid the consequent conversion reaction into stable C3AH6 [6,7].

The well-known characteristics of the CAC modified with phosphates (CAP) are; rapid setting [8], high mechanical strength [9] and relatively low internal pH [10]. Taking these characteristics into account, the CAP was investigated for solidification/stabilization of toxic metal solutions as an alternative of ordinary Portland cement [11]. CAP systems have also been investigated for cementation of radioactive wastes [12] including the aqueous secondary wastes arising from the water treatment of Fukushima Daiichi Nuclear Power Stations [13].

The CAP usually consists of anhydrous CAC powder, phosphate powder(s) and mixing water. It is considered that the CAC hardens together with phosphate component through the acid-base reaction [9]. Compositional effects on the characteristics of the CAP systems have been investigated, but the focus of the investigations have been on the phosphate components, either the types of phosphates [12] or the amount of the added phosphates against a fixed composition of CAC system [8]. The effects of the mixing solution with varied phosphate and water amounts on the characteristics of the CAP have not been openly reported, despite the significance of water contents in the properties of cementitious systems. Numbers of studies on the water contents can be found for the other cement systems [[14], [15], [16], [17]].

The present study focused on the amount of mixing solution in the CAP system. Based on the formulation in the literature [9], CAC was modified with a sodium polyphosphate, but the amount of mixing solution was altered with a constant amount of sodium polyphosphate (a constant phosphate to CAC ratio) or a constant concentration of sodium polyphosphate (a constant phosphate to water ratio), and their effects on the reaction of CAC clinker phases, resulting product phases and basic microstructure of the hardened products were examined.

Section snippets

Materials

Secar® 51 supplied by Kerneos Ltd. was used as the basis of the binders studied in the present study, combined with reagent grade of sodium polyphosphate, (NaPO3)n (65–70%, Acros Organics Ltd.). The oxide composition of Secar® 51 is shown in Table 1 [13]. Distilled water was used for dissolution of sodium polyphosphate and then mixed with CAC.

Sample synthesis and testing procedures

Sodium polyphosphate powders were premixed with distilled water on a roller mixer for 24 h at room temperature. The mixed solution was then added to the

Effect of water on reaction of CAC clinkers

Fig. 1 shows the X-ray diffractograms for the samples. Well defined and intense reflections of the crystalline calcium aluminate clinker phases were observed in the diffractogram of the CAPref, indicating monocalcium aluminate, CA (CaAl2O4, pattern diffraction file PDF #01-070-0134), gehlenite, C2AS (Ca2Al2SiO7, PDF #00-035-0755), and calcium titanate, CT (CaTiO3, PDF #01-075-2100). In good agreement with the other reports [8,9,12,13], the CAPref shows no obvious reflection for either the

Conclusions

The present study investigated the effects of the amount of mixing solution on the characteristics of the CAP system. When the mixing solution was increased with a constant amount of sodium polyphosphate, the consumption of monocalcium aluminate in the CAC was enhanced, resulting in the increased formation of gibbsite and the reduced free water and water bonded with the amorphous gel phase. No crystalline hydration products with calcium was identified, suggesting that it is forming a part of

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by the Engineering and Physical Sciences Research Council, UK (EP/N017684/1) and the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (research grant no. 273604).

References (27)

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