The widespread use of cement in concrete leads to increased carbon emissions, so the demand for supplementary cementitious materials increases significantly. Limestone powder and steel slag powder are widely used as supplementary cementitious materials in modern concrete. However, for UHPC and self-compacting concrete, an extremely low water/binder (W/B) ratio is on one hand a key factor in achieving ultra-high strength and ultra-low porosity of the materials, on the other hand, also leads to the deterioration of the rheological properties of the fresh paste. Meanwhile, the existing researches focus on the influence of single limestone powder or steel slag powder on the mechanical properties or microstructure of cement-based materials. Little work on the influence of steel slag powder or limestone powder on the rheological properties of composite paste at an extremely low water/binder ratio has been investigated quantitatively. The mechanism of the effect of steel slag powder or limestone powder on the rheological properties of composite paste at extremely low water/binder ratios is still unclear. In this work, the effects of steel slag powder and limestone powder on the rheological properties of composite paste at different low water/binder ratios were analyzed via determining the flow diameter, setting time, marsh cone flow time, rheological parameters, and total organic carbon content.

A composite paste was prepared with P.I 42.5 ordinary Portland cement, steel slag powder, limestone powder, blast furnace slag and silica fume as raw materials in a certain proportion. To achieve the preparation of composite paste with extremely low water/binder ratios, a polycarboxylate superplasticizer with a water/reducing rate of 40% (Jiangsu Subot New Materials Co., Ltd., China) was used. The dosage of polycarboxylate superplasticizer for the composite paste with different water/binder ratios of 0.16 and 0.21 was 2% and 0.8%, respectively.

Composite pastes with different proportions of steel slag powder or limestone powder at water/binder ratios of 0.16 and 0.21 were prepared. The flow diameter (i.e., the larger the flow diameter, the better the fluidity), setting time (i.e., the time when the fluidity is lost) and marsh cone flow time (i.e., the shorter the flow time, the better the fluidity) were determined to analyze the fluidity of the composite pastes. the rheological properties of composite paste at a water/binder ratio of 0.16 for rheological properties tests were determined, and the rheological parameters were obtained by the H-B model. The adsorption performance of the polycarboxylate superplasticizer was analyzed by testing the TOC content.

When the ratio of water/binder is 0.16, both limestone powder and steel slag powder initially increase the flow diameter of the composite paste. However, the fluidity of the composite paste decreases over time, and the reduction is more pronounced with steel slag powder. This is because the nucleation and hydration promotion effect of limestone powder can reduce the loss rate of flow diameter, while the rough particles of steel slag powder increase the internal friction force, resulting in a decrease in the flow diameter of composite paste. Adding limestone powder and steel slag powder both shortens the setting time and marsh cone flow time of the composite paste. However, the steel slag powder addition of 30 % delays the final setting time due to its delaying effect. Also, limestone powder can enhance the thixotropy and reduce the yield stress and plastic viscosity, thereby improving the rheological properties. In contrast, steel slag powder can increase the yield stress and plastic viscosity, thereby weakening the rheological properties and thixotropy. Steel slag powder and limestone powder both can enhance the adsorption effect of polycarboxylate superplasticizer. Steel slag powder has a stronger adsorption effect. The composite paste containing limestone powder has a higher free water content. This is because the rough and porous surface of steel slag itself and the uneven particle shape lead to the physical adsorption of polycarboxylate superplasticizer molecules on the surface of steel slag particles, thereby reducing the effective content of the water reducer dispersion. Increasing the water/binder ratio to 0.21 results in a decrease in the flow diameter of the composite paste. Furthermore, the setting time and marsh cone flow time can prolong due to the reduction in the dosage of polycarboxylate superplasticizer.

The results showed that the fluidity loss rate of composite paste with limestone powder could be lower than that of composite paste with steel slag powder. Compared to steel slag powder, the addition of limestone powder reduced the setting time and marsh cone flow time of the composite paste. The addition of limestone powder could shorten the yield stress and plastic viscosity of the composite paste. Therefore, the composite paste with limestone powder had better rheological properties and stronger thixotropy rather than that with steel slag powder. The addition of limestone powder could improve the rheological properties of the composite paste. Compared to composite paste with limestone powder, a better adsorption effect of polycarboxylate superplasticizer on the surface of the composite binder with steel slag powder could be obtained. The free water content of the composite paste with limestone powder was still higher than that of the composite paste with steel slag powder. The fluidity and rheological properties of the composite paste with limestone powder could be better. The comprehensive analysis indicated that a positive effect of limestone powder on rheological properties of composite paste at an extremely low water/binder ratio could be more dominant than that of steel slag powder.