African Fusion July 2020

Figure 7: Water repellent function of a polymer sealed alumina plasma spray coating. plasma sprayed alumina coatings. Only two out of sixteen coatings that were sprayed with the GTV Penta torch at 300 g/min powder feed rate showed an average bond strength of less than 2.4 N/mm 2 and failed in the interface between the plasma sprayed coating and concrete substrate (Figure 8). These coatings show a thickness of about 200 µmandwere depositedwith amaximumspray distance of 200 mm and, therefore, with relatively high heat input to the concrete substrate. All the other coatings showed higher bond strength to the concrete substrate compared to the substrate´s cohesive strength and failure occurred at tensile stresses of be‑ tween 2.4 and 3.3 N/mm 2 . Summary and conclusions In-situ fusion during plasma spraying of glass coatings on basalt mortar could not be achieved within the limits of the performed tests. Basalt mortar substrates were thermally damaged before any relevant coating formation could be realised. However, well adhering alumina coatings can be deposited on the concrete substrates at a powder feed rate of 300 g/min and 70‑80% deposition efficiency by use of high-power plasma spray torch type GTV Penta. If a coating thickness of 100 µm is not exceeded, bond strength between plasma sprayed coatings and basalt mortar substrates exceeds the cohesive strength of the basalt mortar. Due to their pore and crack network, however, the plasma sprayed alumina coatings are not an effective barrier between con‑ crete substrates anddrinkingwater. But sealingwitha conventional polymer sealer that is approved for tapwater applications enables perfect water tightness to be achieved. Accordingly, sealed plasma sprayed alumina coatings represent a promising solution for the long term protection of tap water reservoirs – and these coating can also be deposited as restoration measures. Further investigations will be carried out on the resistance of sealed plasma sprayed alumina coatings against wastewater con‑ taining chlorides and sulphates. Also, complex shaped geometries suchas stairs andbendswill be coated toprove the full applicability of this technology for the restoration of tap water reservoirs and sewage ductwork. Besides fairly temperature resistant basalt mortars, different qualities of conventional quartzite-based concrete and clay bricks will also be coated to explore the spectrumof applicable basema‑ terials and the need to apply basaltmortars as intermediate layers.

Figure 8: Bond strength test failures of the alumina coating/concrete substrate interface (top); and the concrete substrate (bottom). Acknowledgement Project ZF4672701 SU8 was funded in the frame of programme ZIM (Zentrales Innovationsprogramm Mittelstand) by the Federal Ministry of Economy and Energy based on a resolution of the Fed‑ eral German Parliament. The authors express their gratitude to the mentioned institutions for the funding. References [1] Legoux jG, Dallaire S: Adhesion mechanisms of arc- sprayed zinc on concrete. Journal of Thermal Spray Technology, 4 (1995), pp. 395-400. [2] Arcondéguy A, Grimaud A, Denoirjean A, Gasgnier G, Huguet C, Pateyron B, Montavon G: Flame-Sprayed Glaze Coatings: Effects of Operating Parameters and Feedstock Characteristics Onto Coating Structures. Journal of Ther‑ mal Spray Technology, 16 (2007) 5-6, pp. 978-990 [3] Eiwen F, Pfeiler J, Roos C: Corrosion resistance of CaO- Al2O3-SiO2 glasses used for flame spraying on concrete. Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B, 61 (2020) 1, pp. 11‑20. [4] Schulte-Holthausen R, Otto S, Thiele S, Eiwen F, Conradt R, RaupachM,WeicholdR: FlammgespritzteSchutzschich‑ ten aus Glas f r Beton - Erste Untersuchungsergebnisse. Proceedings of 19th Int. Baustofftagung, Vol. 2, September 16 th -18 th , 2015. [5] Hellmann, K: AnwendungsorientierteGlasentwicklungam Beispiel des Systems CaO-Al2O3-SiO2. Dissertation, RWTH Aachen University: https://d-nb.info/1130327140/34.

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July 2020

AFRICAN FUSION