Effect of chemical and thermal activation on the microstructural and mechanical properties of more sustainable UHPC

International audienceThis paper investigates the influence of the content of blast furnace slag (BFS) on the microstructural and mechanical properties of non-activated and activated ultra-high performance concrete (UHPC). Three volume-substitution rates of cement with BFS were explored (30% for UHPC2, 50% for UHPC3 and 80% for UHPC4) and two activation methods, chemical and thermal, were tested. Results show that with 30% of BFS, heterogeneous nucleation prevailed over dilution, accelerating the hydration reaction of cement and increasing the amount of C-S-H formed. C-S-H decreased the porosity of UHPC2 by 18% and 20% respectively at 3 and 90 days. The compressive strengths of UHPC1 (without BFS) and UHPC2 were very similar. For high BFS contents, the dilution effect prevailed and there was less portlandite, which decreased the amount of hydrated products, particularly at early age. As a result, the porosity of UHPC3 and UHPC4 was more than twofold higher than that of UHPC1.To boost the hydration reaction of blended UHPC with a high BFS content, KOH was added. The use of [KOH]3 significantly increased the amount of hydrated products, reducing the porosity of UHPC4 1.6-fold at 3 days and increasing its compressive strength by 42% at the same age. However, this activation mode was not enough to ensure the required compressive strength of UHPC1. Thermal activation at 90 °C for 2 days was therefore tested. Results showed the acceleration of the reaction of solid components, which increased the consumption of portlandite and hence the development of hydrated products. This resulted in improving the packing density of blended UHPC, decreasing its porosity and enhancing its compressive strength. In comparison with reference concrete at 90 days, the compressive strength of UHPC3-T increased by 7% and that of UHPC4 was 12.5% lower