INTRODUCTION: The improvement of the output efficiency in CSP plants is a challenge of major interest. To achieve this objective, an increase of temperature in the heat transfer and the storage media is required to increase power block efficiency. Among the suitable material options, concrete was determined to have attractive properties. Concrete is a heterogeneous material, made of several components that allows to choose the best dosage to obtain the desired properties in the final blender. Worldwide availability of concrete components and their low cost, contribute to implement eco-friendly measures. The environmental impact of the materials is a substantial parameter to take into consideration when selecting concrete components. There are by-products from industries that can be revalorized by including them into concrete mixtures, replacing cement percentage or using them as aggregates. A by-product from the metallurgy industry, known as steel slag or electric arc furnace slag, was previously studied as aggregates in concrete for thermal energy storage. In this paper, the thermal stability under high-temperature cycles, of two cement types with steel slag as the only aggregate in the concrete mixture is compared. As further novelty, the thermal cycles were adapted to heating rates currently used in the operation of a CSP plant. MATERIALS AND METHODS: In the present study two concrete dosages were designed. One using ordinary Portland cement and the other with calcium aluminate cement. In both binders, steel slag was used as aggregate, considering granulometries of 0-12 mm. Physical, thermal and mechanical properties were studied before and after thermal cycles. To study the thermal stability, 10 continuous thermal cycles were carried out. The thermal cycle profile consisted on a first cycle with a low heating rate 1 ºC/min up to 290 ºC followed by an isothermal step of 4 hours. Continuously, temperature increased up to 700 ºC with the same heating rate and maintaining during 4 hours. After, the samples were cooled down to 290 ºC, at a rate below 1 ºC/min. For the next cycles the heating rate was increased to 15 ºC/min, considering the same time for the isothermal steps and the cooling part. RESULTS: The external appearance showed a geometry preservation in all samples. There were no important cracks in the surface, only micro-fissures in the samples after some thermal cycles. There is a strong connection between the aggregates and both types of cement paste, conferring a bonding in the concrete mixture. No spalling signs nor thermal expansion of aggregates was observed. The mass loss was around 4 % in both mixtures after the thermal cycles. The thermal conductivity was reduced 50 % after the 10 thermal cycles. Despite the high compressive strength obtained before any thermal treatment, the compressive strength drastically decreased 80 % after the 10 thermal cycles. CONCLUSIONS: Despite the low thermal conductivity achieved after thermal cycles, a high integrity preservation of the concrete mixtures was observed. The compatibility of steel slag highlight with both cement types conferring a hydraulic bonding to the mixture, appreciating small fissures.

CSPplus is supported under the umbrella of CSP-ERA-Net 1st Cofund Joint Call by AEI - Spanish Ministry of Science, Innovation and Universities, TÜBITAK - Scientific and Technological Research Council of Turkey, and CSO - Israeli Ministry of Energy. CSP-ERA-Net is supported by the European Commission within the EU Framework Programme for Research and Innovation HORIZON 2020 (Cofund ERA-NET Action, N° 838311) and this work was partially funded by Agencia Estatal de Investigación (AEI) - Ministerio de Ciencia, Innovación y Universidades (PCI2020-120695-2/AEI/10.13039/501100011033), Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31 - MCIU/AEI/FEDER, UE), and Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) (RED2018-102431-T). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme.