# Experimental Study on the Workability and Stability of Steel Slag Self-Compacting Concrete
## 3 Experimental Results and Workability Evaluation
### Compressive Strength of SSCC  
The compressive strength of SSCC with different VPS was tested according to Chinese standard GB 50081-2002 [36]. The average value of the measured compressive strengths of three cubic specimens sized \(150\times 150\times 150\) mm in a group was taken as the final compressive strength result. If either of the differences between the maximum value or the minimum value and the intermediate value exceeds 15%, the intermediate value shall be taken as the strength result, if both the differences exceed 15%, the test results of this group were invalid. The compressive strengths at different age are shown in Figure 6. The microscopic pictures of several different groups of SSCC are shown in Figure 7.  
As shown in Figure 6, compared with the group of S\({}_{0}\)SCC, the addition of steel slag sand improves the compressive strength of self-compacting concrete at 28 days when the water-binder ratio is constant and the working performance is ideal after the optimization of concrete mix proportion based on workability of fresh SSCC. The difference of compressive strength, at the same age, between SSCC specimen groups with different VPS. With the increase of VPS, the compressive strength of SSCC at 7 days shows a decreasing trend (up to 11% at 100% VPS), and the compressive strength at 28 days decreases slightly when VPS exceeds 60%. Combined with the results of the workability test results, it is not recommended to exceed 60% VPS in the preparation of SSCC.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Group** & **Total Sampling/kg** & **Separation Quantity/kg** & **SR/\%** \\ \hline VPS = 0\% & 5.056 & 0.350 & 6.92 \\ VPS = 20\% & 4.982 & 0.298 & 5.98 \\ VPS = 40\% & 5.040 & 0.380 & 7.54 \\ VPS = 60\% & 4.960 & 0.270 & 5.44 \\ VPS = 80\% & 5.092 & 0.430 & 8.44 \\ VPS = 100\% & 5.032 & 0.438 & 8.70 \\ \hline \hline \end{tabular}
\end{table}
Table 13: Sieve analysis test results.  
As shown in Figure 7, there is a directional alignment of calcium hydroxide crystal around aggregates in the specimen of Group S\({}_{0}\)SCC, and there is less C-S-H gel surrounding the coarse aggregate surface (see Figure 7a); the main characteristic features observed are the reduction of porosity accompanied by the continuous denser matrix of C-S-H along with VPS increase (see Figure 7b,c) and the less obvious alignment of calcium hydroxide crystal, the interface transition zone (ITZ) in concrete is gradually improved and, therefore, concrete shows high compressive strength.  
The microcosmic appearance of hydration products as shown in Figure 7d appears in Group S100 SCC, presumably caused by the calcium hydroxide produced by f-CaO hydration. The calcium  
Figure 6: Grading of fine aggregates according to sieve analysis test.  
Figure 7: Scanning electron micrograph of hardened SSCC samples with different VPS: (**a**) S\({}_{0}\)SCC; (**b**) S\({}_{20}\)SCC; (**c**) S\({}_{60}\)SCC; (**d**) S\({}_{100}\)SCC.  
hydroxic crystal from hydration of f-CaO has a large volume and occupies a large space, when compared with that from the hydration of cement clinker, resulting in a large expansion force and cracking.  
Therefore, it can be considered that the microstructure and ITZ of SSCC improves, under the same water binder ratio, with the increase of VPS. This may be one of the main reasons for the improvement of concrete strength by the addition of steel slag in the SSCC. The improvement effect of steel slag on concrete strength may also be related to the surface roughness of steel slag aggregate and certain water absorption capacity, and the detailed mechanism needs to be further studied.