Effect of dosage of silica fume on the macro-performance and micro/nanostructure of seawater Portland cement pastes prepared with an ultra-low water-to-binder ratio

Seawater and sea-sand based ultra-high performance concrete (UHPC) is currently regarded as one of the most innovative and promising construction materials. To provide design guidance for the usage of silica fume (SF) during the production of seawater and sea-sand based UHPC, this work investigated the effect of SF on the later-age macro-performances and micro/nanostructure of seawater ordinary Portland cement (OPC) pastes prepared with a low water-to-binder (W/B) ratio. The results showed that the optimal SF dosage in seawater OPC-SF system was 10 wt% while that for the deionized (DI) water OPC-SF system was 20 wt%. This showed that the usage of SF could be reduced for the production of the seawater based UHPC system. It was found that the later-age compressive strength of seawater OPC-SF system was increasingly degraded with increasing SF dosage when compared with the corresponding DI water OPC-SF system. The difference in the distribution of unhydrated clinkers in the matrix as SF addition increased was the main cause. The seawater OPC-10%SF system exhibited the highest compressive strength and the lowest pore content among all the seawater-OPC systems. Additionally, with the usage of this ultra-low W/B ratio, Friedel's salt was only present in the seawater OPC pastes prepared without SF, but absent in the seawater OPC-SF pastes.