Cements with supplementary cementitious materials for high-temperature geothermal wells

•Pozzolans help to hydrothermally self-heal damaged cementitious composites at 300 °C.•Different hydrates participate in strength recovery and cracks sealing.•Fly ash F improves cements’ strong acid resistance (pH 0.2) at 90 °C.•Micro-glass fibers allow reconstruction of fragmented samples for some formulations.•Young’s modulus can be used to classify cements failure type linked to cracks size. This paper discusses self-healing ability, characterized by the strength recovery and crack sealing, thermal shock and acid resistance of several cementitious composites in water, alkali carbonate or geothermal brine at 300 °C. The tested formulations included a common high-temperature OPC/SiO2 formulation, a calcium-aluminate cement with alkali activated FAF blend (TSRC), alkali-activated granulated blast furnace slag (GBFS/SiO2) and fly ash C/fly ash F (FAC/FAF) cementitious blends. Compressive strength recoveries and visual observations of cracks sealing using 3D imaging after the compressive damage and then after the 5-day healing period were used to evaluate the self-healing abilities of these composites. The strength recoveries were evaluated for composites after 1-, 5-, 10-, 15-, and 30- day initial curing periods at 300 °C and after repeated damage and 5-day healing treatments. The nature of the failure was classified depending on the Young’s modulus (YM) of the tested blends. Composites with moderate values of YM, such as OPC/SiO2 and TSRC developed slim cracks while brittle and very brittle FAC/FAF and GBFS/SiO2 cementitious blends produced multidirectional cracks or near-fragment failures resulting in poor strength recoveries. In comparison with the common high-temperature OPC/SiO2 blend, composites with alkali activated pozzolanic materials demonstrated a decrease in crack size during the short healing periods. Fly ash-containing composites showed better acid resistance surviving 28 days in sulfuric acid (pH 0.2) at 90 °C. The TSRC demonstrated the best thermal shock resistance losing only about 20 % of the original strength in five 350 °C heat→25 °C water thermal cycles. In all the tests the TSRC outperformed the rest of the composites with average strength recoveries of more than 85 %. Alkali carbonate was the most favorable for both strength recovery and crack sealing. The short-term strength recoveries could be further improved to above 100 % by addition of micro-glass fibers (MGF). However, the MGF raised the brittleness of the samples after longer