## 2 Experimental Section
### Microstructure Characterization  
A range of microstructural measurements was carried out on paste specimens of different ages. The reactivity of ultrafine slag particles and constitution of hydration products, as well as the pore solution chemistry and pore structure features, were determined to characterize the effects of slag addition on microstructure development.  
The reaction of the ultrafine slag particles was explored based on the EDTA dissolution method, which enables the extraction of components other than unreacted slag. The EDTA residues are composed primarily of amorphous silica phases and also contain hydrotalcite phases as well. For correction purposes, EDTA extractions were performed on the neat OPC paste and raw slag powders. For details regarding the EDTA dissolution and data analysis, a reference can be made to the previous work of [PERSON] et al. [19]. The decomposition of the hydration products was traced from thermogravimetric analysis (TGA) using an STA (TG-DTA-DSC) 449 F3 Jupiter. The heating temperature was increased from 40 to 1000 \({}^{\circ}\)C with a heating rate of 10 \({}^{\circ}\)C/min. The weight loss of each component occurring at a particular temperature range was captured. The mass of the specimen used in the ceramic crucible was around 50 mg, and argon was used as the protective gas.  
The pore solution of cylindrical paste specimens was collected according to the expression method proposed by [PERSON] and [PERSON] [20]. The collected pore solution was stored in plastic bottles and sealed immediately to avoid water evaporation and carbonation. An inductively coupled plasma optical emission spectrometer was employed to quantify the pore solution chemistry. The concentrations of ions Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), and SO\({}_{4}\)\({}^{2-}\) were figured out. The concentration of OH\({}^{-}\) and the pH value of the pore solution were determined based on titration against the 0.1 N HCl solution. In addition to hardened paste samples cured for 1-120 days, some paste samples, soon after casting at 2, 6, and 12 h, were  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Items** & **OPC** & **Slag** \\ \hline CaO & 64.32 & 41.52 \\ SiO\({}_{2}\) & 19.53 & 34.65 \\ Al\({}_{2}\)O\({}_{3}\) & 4.67 & 11.12 \\ Fe\({}_{2}\)O\({}_{3}\) & 3.45 & 0.53 \\ MgO & 1.98 & 8.12 \\ K\({}_{2}\)O & 0.51 & 0.45 \\ Na\({}_{2}\)O & 0.31 & 0.21 \\ SO\({}_{3}\) & 2.62 & 2.38 \\ TiO\({}_{2}\) & 0.32 & 1.05 \\ LOI & 2.13 & 1.52 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of OPC and slag, weight %.  
prepared, and the evolution of OH\({}^{-}\) in the pore solution was measured. Changes in the OH\({}^{-}\) at the very early stage were deemed as a reasonable index to trace the dissolution and hydration kinetics of slag particles.  
Mercury intrusion porosimetry (MIP) was applied to assess the influences of ultrafine slag particles on the pore features of cementitious pastes. It was assumed that the cementitious pores were cylindrical and equally accessible to mercury. The relationship between the pore size and external pressure can be described using the well-known Washburn equation [21]. Prior to the MIP tests, the specimens at the desired age were immersed in liquid at \(-195\)\({}^{\circ}\)C for 5 min and then moved to a freeze-dryer (\(-24\)\({}^{\circ}\)C and 0.1 Pa) until a constant mass of the specimen was attained.