*2.2. Test Method*

### 2.2.1. Compressive Strength

Mortar samples were prepared to test the compressive strength by mixing the raw materials with a planet mixer. The pastes were mixed for 30 s at low speed after the solution was added. The sands were added for another 4 min. The fresh mortar was cast in 40 mm × 40 mm × 160 mm molds and cured in standard curing conditions of 20 ± 2 ◦C and 95% RH until testing. The specimens were tested at 1 d, 3 d, 7 d and 28 d.

### 2.2.2. Isothermal Calorimetry

The hydration heat of alkali-activated BBFS with a water/binder ratio of 0.5 was measured by isothermal conduction calorimetry (TA instrument, TAM Air) at 25 ◦C. Two slags were weighed and placed in plastic bowls. The bowls were set in the calorimeter until thermal balance was reached. After 10 h of thermal equilibration, the activator was mixed with the precursors for 3 min. The heat evolution was recorded. Compared with the reaction heat, the heat disturbance caused by mixing can be neglected.

### 2.2.3. X-ray Diffraction (XRD)

The alkali-activated pastes were mixed with a planet mixer for 2 min. The water-toslag ratio for all paste in this study is 0.5 after mixing, the suspensions were cast into 50-mL sealed centrifuge tubes and cured at 25 ◦C. After a specified duration, hydration of the sample was terminated using the isopropanol replacement method. The detailed method is described in previous studies [29,30]. The sample powder was ground to <100 µm for XRD. XRD analysis was performed with a Bruker D8 advance using CuKα radiation (scanning range from 5◦ to 65◦ for 40 min).

## 2.2.4. Mercury Intrusion Porosimeter

A mercury intrusion porosimeter (MIP) was used to characterize the pore structure of the paste specimens. The paste was obtained from the same sample analyzed by XRD. The surface tension and contact angle were 135◦ and 0.485 N/m, respectively. The paste samples for the MIP were cut into small cubes with a maximum size of 0.5 mm.

### 2.2.5. ATR–FTIR Spectroscopy

Attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR– FTIR) was used to determine the chemical microstructure of the paste. The measurements were performed in the mid-infrared spectral region of 550–4000 cm−<sup>1</sup> with a spectral resolution of 2 cm−<sup>1</sup> using a BRUKER Tensor 27 spectrometer equipped with a diamond crystal as the ATR element (PIKE Miracle™). The paste was obtained from the sample analyzed by XRD. Same powder sample analyzed by XRD was used for ATR-FTIR.
