Design, structure, microstructure and gamma radiation shielding properties of refractory concrete materials containing Ba- and Sr-doped cements
Introduction
Concrete, being a construction material composed of cement, both fine and coarse heavy weight aggregates mixed with water which hardens with time, has been used in the construction of nuclear facilities because of two primary advantages, its structural strength and its ability to shield radiation. Several attempts have been made to investigate e.g. the effect of cement and aggregate type and water to cement weight ratio (w/c) on the physical properties, gamma and neutron shielding effectiveness of concretes [[1], [2], [3], [4], [5], [6]]. It has been reported in the recent literature that, many efforts have been undertaken to find new concrete having a high resistance against elevated temperature [[7], [8], [9]].
As is well known to those skilled in the concrete technology, the binder choice drives the optimization of overall properties of concrete including engineering, mechanical, structural, and radiation shielding properties. Three well-known strategies for solving these optimization problems are the binder design, the choice of aggregates and the concrete mix design. Portland cements are concerned as the representative binder of commonly used binder for shielding concretes together with other coarse aggregates. Heavy-weight and nuclear-shielding coarse aggregates types are natural mineral and synthetic aggregates. The commonly used types of natural mineral aggregates are boron minerals (colemanite), barium minerals (barite) and iron minerals and ores (hematite, magnetite, limonite). The commonly used types of manufactured synthetic aggregates are boron frit glasses, ferroboron, boron carbide, ferrophosphorus, steel punchings, heavy slags and ferrosilicon [[10], [11]].
With regard to the systematic literature review on hydraulic binders for shielding concrete, the divalent metal aluminate phases and aluminates-containing composite cements seem to have an increasing interest, especially due to their excellent refractory properties. Nowadays, calcium aluminate cements are the only used aluminous cements designed for high performance unshaped refractory materials (monolithic refractories) [[12], [13], [14]]. However, the doping of the calcium aluminate (Ca–Al–O) series with different CaO to Al2O3 proportions with Sr2+ or Ba2+ ions can predispose these cements to be used as components of radiation shielding ceramic materials [[15], [16], [17], [18]]. It is found that the single-phase solid solutions of Ca1-xSrxAl2O4 exist only over a very limited range, x < 0.35 and x > 0.85 [19]; x = 1 to x = 0.5 [20] or x < 0.25 and x > 0.75 [21]. It was also found that the miscibility between two endmembers CaAl2O4 and BaAl2O4 is incomplete [22]. Two binary solid solution systems with the general formula (Ca,Ba)Al2O4 exist – one on the Ca side yielding Ca rich solid solutions, the other on the Ba side giving rise to Ba rich solid solutions. Many interesting applications of the Ba-bearing compounds in the ceramic technology can be found in [[23], [24], [25], [26]]. These applications include refractory castables based on barium aluminate cements [[23], [24]], engineering ceramics or refractory components containing barium aluminates [25] or refractory concrete containing barium aluminate-barium zirconate cements [26], most of which exhibit gamma ray shielding properties. Recent studies from our laboratory have shown that the Ba2+-, Cu2+- or Bi3+-doped CaO–Al2O3–ZrO2-based cements [27], Sr-doped Ca7ZrAl6O18 cement [[28], [29], [30]] and SrAl2O4 [31] cement play a key role in the radiation shielding concretes technology.
With this systematic literature review, we aim to fill gap in providing an implementation of new high-alumina refractory cements in the technology of radiation shielding concretes. With going into the details of this issue, the aim of this work is to assess the gamma radiation shielding effectiveness of calcium aluminate cement-containing refractory concretes, and concretes which were additionally doped with strontium or barium. For this purpose, the high purity corundum aggregate was used as a filler together with both commercially available calcium aluminate cements and as-synthesized cements both belonging to the CaO–Al2O3–ZrO2 system doped with divalent cations Sr or Ba and strontium monoaluminate (SrO·Al2O3) cement.
Section snippets
Synthesis of Ba-/Sr-doped Ca7ZrAl6O18 and SrAl2O4 cements and their hydration characteristics
The preparation of samples with chemical formulas Ca7-xMex2+ZrAl6O18 (where Me2+ = Sr2+ or Ba2+ and x = 1.0) and SrAl2O4 was performed by standard ceramic method. The molecular concentration x is substituted for Sr2+ in the chemical formula, as it was presented before [[28], [29], [30]], but x is not substituted for Ba2+ in this chemical formula, as it was also presented in Ref. [27]. The oxides Al2O3 and ZrO2 from Acros Organics (with 99.0% purity), and carbonates SrCO3 and BaCO3 from Avantor
AS-SYNTHESIZED cement clinkers
Three aluminate-based cement clinkers Ba-doped Ca7ZrAl6O18, Sr-doped Ca7ZrAl6O18 and SrAl2O4 clinkers were successfully synthesized via a two-step ceramic route through calcination and sintering. The Ba-doped C7A3Z clinker contains one major (Ca7ZrAl6O18) and two minor constituents (Ba0.8Ca0.2ZrO3, BaAl2O4) as presented in Ref. [27], whereas the (Sr,Ca)7ZrAl6O18 solid solution exists as the main constituent of the Sr-doped C7A3Z clinker with some admixture of (Ca,Sr)ZrO3 phase [[28], [29], [30]
Conclusions
Results of the described research can be summarized with the following conclusions:
- 1.
New hydraulic cements belonging to CaO–Al2O3, SrO–Al2O3, SrO–CaO–Al2O3–ZrO2 and BaO–CaO–Al2O3–ZrO2 systems designed for shielding concretes were developed as an alternative binders to ordinary Portland cement (OPC) heavy concretes.
- 2.
The high purity corundum was used as an aggregate to assess the gamma radiation shielding performance of these cements.
- 3.
The doping alkaline earth elements Sr and Ba were chemically
CRediT authorship contribution statement
Dominika Madej: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Michał Silarski: Formal analysis, Funding acquisition, Investigation, Methodology, Resources, Software, Validation, Visualization, Writing - review & editing. Szymon Parzych: Investigation.
Declaration of competing interest
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 project was financed by the National Science Centre, Poland, project number 2017/26/D/ST8/00012 (Recipient: DM). We acknowledge support from the National Center for Research and Development through grant No. LIDER/17/0046/L-7/15/NCBR/2016 (Recipient: MS). The authors (MS and SP) acknowledge technical support of Artur Michałek and Adam Mucha from the Physics Laboratory II of the Faculty of Physics, Astronomy and Computer Science, Jagiellonian University.
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