Influence of silica fume on properties of fresh and hardened ultra-high performance concrete based on alkali-activated slag

Abstract Based on the principles of ultra-high performance concrete (UHPC) a Portland cement free, alkali-activated material was optimized in order to enhance strength and durability. The formulation is based on ground granulated blast furnace slag and as an activator a combination of potassium water-glass and potassium hydroxide was used. Furthermore, silica fume and metakaolin as inorganic fines were used to increase the packing density of the mixture. Quartz sand (0–2 mm) and quartz powder were added as aggregates. The results comprise an enhancement of the rheological properties by the stepwise increase of silica fume. A w/b ratio of less than 0.25 was realised using a certain mixing procedure with a high intensity mixer. The compressive strength reaches values comparable with the strength range of an UHPC. The already low capillary porosity could further be decreased by the substitution of slag with a small amount of metakaolin, which is leading to a higher polymerisation degree due to an incorporation of aluminium in the reaction products. The enhanced polymerisation degree was estimated by FTIR and XRD. Although no effective superplasticizer can be used in this high alkaline material, the low w/b ratio and a good workability can be achieved by using certain amounts of silica fume.