The BFNS used in this paper came from Luoyuan County, Fujian Province. The chemical composition of the BFNS was determined by X-ray fluorescence spectroscopy, as shown in Table 1. Compared to common GBFS, the CaO content is lower in BFNS. A laser particle size analyzer (MasterSizer 2000) was used to measure the particle size distribution of the BFNS, as shown in Figure 1. The average particle size was D₅₀ = 24.740 μm for BFNS particles. The mineral composition of the BFNS obtained is shown in Figure 2. A hump appears in the range of 25°–35° (2θ) in the XRD spectrum, which indicates that there is an activated vitreous phase in the BFNS. It is worth noting that the Mg in the BFNS mainly exists as a spinel (MgAl₂O₄) and magnesium olivine (MgSiO₃).

Sodium hydroxide (NaOH) of 99% purity was obtained from Beijing Tongguang Fine Chemicals Company, China. Commercial liquid sodium silicate (Na₂O·nSiO₂) with a silicate modulus (SiO₂/Na₂O molar ratio) of 3.36 was also used. Water glass was prepared with NaOH and Na₂O·nSiO₂ as an alkaline activator. Natural river sand with a particle size less than 5 mm and limestone with a particle size between 5 and 25 mm were selected as coarse and fine aggregates, respectively.

The mix proportions of concrete are shown in Table 2. The total amount of cementitious material was 400 kg/m³, and the water-binder ratio was 0.5 which involves the water in water glass solution. The BFNS was activated by water glass at a constant activator to slag ratio of 10% (water glass: BFNS in mass). And the modulus (molar ratio between SiO₂ and Na₂O) of the water glass was adjusted to 0.5, 1.0, 1.5 and 2.0 by adding NaOH. The alkaline activator was first mixed with water before 2 h and cooled to room temperature to avoid temperature interference, and subsequently mixed with solid materials. Concrete samples (100 × 100 × 100 mm) were prepared according to GB/T 50082-2009. The hardened paste and fresh mortar of sample BFNS-M1.0 are shown in Figure 3. According to the pre-preparation test, the initial setting time of sample BFNS-M1.0 was very short, and the moisture on the surface disappeared rapidly after stirring for 5 min. Meanwhile, cracks began to appear after 3 h, which indicated that sample BFNS-M1.0 underwent quick and large shrinkage. In terms of fresh mortar, sample BFNS-M1.0 could not be molded due to rapid setting. Therefore, only samples BFNS-M0.5, BFNS-M1.5 and BFNS-M2.0 were studied in this paper. The water-binder ratio of paste was the same as that of the concrete, and the fresh paste was put into the plastic tube. All samples were cured under the same conditions (20 ± 2°C, >90% RH) until testing.

After 28 days and 90 days, the middle part of the hardened paste was taken, crushed and immersed in anhydrous ethanol to stop hydration and then dried at 60°C. The dried pastes were treated in two ways. First, the block pastes were directly tested as follows: an FEI Quanta-200 scanning electron microscope (SEM) was used to observe the morphology of alkali-activated BFNS under different conditions. The composition of the hydration products was also analyzed using an EDAX Genesis 2000 X-ray energy spectrometer (EDX). The pore structure of the hardened paste was tested by an Autopore IV 9500 mercury injection apparatus (MIP). Then, the other pastes were ground. A Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer was used to analyze the structure of the gel product. The types of hydration products were measured by XRD analysis with a scanning range from 5° to 70° and a measuring speed of 8°/min. At 1, 3, 7, 28 and 90 days, the compressive strength and splitting tensile strength of concrete were obtained by using three specimens for each test according to the Chinese National Standard GB/T 50081-2002.

Under different hydration conditions, the XRD spectra of the alkali-activated BFNS pastes at 28 days are shown in Figure 4. Compared to the XRD spectrum of BFNS in Figure 2, diffraction peaks of MgAl₂O₄ and MgSiO₃ of alkali-activated BFNS pastes in Figure 4 are very clearly detected. There is almost no change in the peaks, which means that MgAl₂O₄ and MgSiO₃ do not participate in the reaction. This result is similar to the use of ferronickel slag as a mineral admixture in cement.

In addition, the only newly generated crystalline phase detected is 2CaO·Al₂O₃·SiO₂ (C₂AS, peak 4). This crystalline phase can also be found in alkali-activated GBFS paste. However, there are differences in the specific composition and content, which should be attributed to the similar chemical composition of GBFS and BFNS. It is worth noting that the diffraction peak of C₂AS is obvious and high, indicating that the crystallinity is high. When the modulus of water glass increases from 0.5 to 1.5, the XRD spectra of alkali-activated BFNS pastes change significantly. The number and intensity of C₂AS diffraction peaks are significantly reduced with increasing modulus. This indicates that increasing the modulus significantly reduces the amount of C₂AS. Moreover, with the increase in the modulus of water glass, the “hump” in the XRD spectra becomes more obvious, which indicates that increasing the modulus is beneficial to the formation of amorphous products. However, it is worth noting that there is no significant difference between the XRD patterns of samples BFNS-M1.5 and BFNS-M2.0. This indicates that when the modulus of water glass is more than 1.5, increasing the content of silicate will not have a significant impact on the hydration products of alkali-activated BFNS pastes.

The microstructure of sample BFNS-M0.5 at 90 days is shown in Figure 5. The microstructure of the water glass-activated BFNS hardened paste is very dense, but even at 90 days, unreacted BFNS particles can still be obviously detected (point B in Figure 5). In the EDX spectrum at point A, a small amount of Mg was detected in the gel products. According to the XRD results, the Mg-containing phase in the BFNS almost did not participate in the reaction, which suggests that the vitreous components of the BFNS may contain Mg. According to the EDX analysis, the gel products of water glass-activated BFNS can be written as (N,C)-A(M)-S-H. Na ions may replace some of the Ca ions to form a gel, which is similar to the reaction products of alkali-activated GBFS-based cementitious materials. Mg in the vitreous of BFNS may enter the gel structure or adsorb on the surface of the gel, which is similar to the result from previous research on alkali-activated low Ca and high Mg ferronickel slag (Yang et al., 2014; Yang et al., 2017).

To further determine the effect of the modulus on the composition of the gel, a total of 50 micro-scale areas of the (N,C)-A(M)-S-H gel were analyzed by EDX. The Ca/Si and Al/Si ratios of each sample were calculated according to the EDX test results, as shown in Figure 6 and Figure 7, respectively.

When the modulus of water glass is 0.5, the Ca/Si ratio of the gel varies from 0.52 to 0.94, with an average value of 0.70. When the modulus of water glass is 1.5, the Ca/Si ratio of the gel varies from 0.45 to 0.91, with an average value of 0.61. When the modulus of water glass is 2.0, the Ca/Si ratio of the gel varies from 0.42 to 0.81, with an average value of 0.57. The Ca/Si ratio of the gel decreases with increasing modulus, but the amplitude reduction is not significant. Since the increase in modulus means a significant increase in silicate content, the change in Ca/Si ratios in the gels is very small, which indicates that more silicate due to the increase in modulus does not participate in the later reaction. Previous studies show that the Ca/Si ratio of gel in pure cement systems generally varies in the range of 1.2–2.3 (Richardson, 1999). Another study shows that the Ca/Si ratio in alkali-activated GBFS is 1.29 ± 0.33 (Kovtun et al., 2015). Therefore, the Ca/Si ratio in the gels generated by alkali-activated BFNS is significantly lower than that generated by cement and alkali-activated GBFS.

When the modulus of water glass is 0.5, the Al/Si ratio of the gels varies from 0.43 to 1.10, with an average value of 0.66. When the modulus of water glass is 1.5, the Al/Si ratio varies from 0.32 to 0.84, with an average value of 0.53. When the modulus of water glass is 2.0, the Al/Si ratio varies from 0.31 to 0.63, with an average value of 0.46. With the increase in the modulus of water glass, the Al/Si ratio gradually decreases. On the one hand, an increase in the modulus leads to an increase in silicate. On the other hand, the increase in silicate makes it easier for Al to form a gel, resulting in a reduction in the Al/Si ratio (Pardal et al., 2009; García-Lodeiro et al., 2013).

FTIR patterns of hydration products of water glass-activated BFNS at 28 and 90 days are shown in Figure 8A and Figure 8B, respectively. In general, the peak at approximately 950 cm⁻¹ is the Q² units of the stretching vibration peak of the Si-O bond, and some peaks at approximately 600–700 cm⁻¹ are the deformation vibration peaks of Si-O-Si(Al) (Wang et al., 2018a). The narrow and sharp peak indicates better crystallization of the corresponding phase, and the wide and rounded peak indicates worse crystallization (Wang et al., 2018a). It can be seen from Figure 8 that the crystallization of Si-O and Al-O connection structures in BFNS is poor, and there is no obvious stretching vibration peak for Si-O bonds and no deformation vibration peak of Si-O-Si(Al). However, after activation, a series of deformation vibration peaks of Si-O bonds of Q² units and Si-O-Si(Al) appear in alkali-activated BFNS. With the increase in the modulus of water glass, the stretching vibration peak of the Si-O bond of Q² units tends to shift toward a higher frequency. This indicates a higher polymerization degree in the alkali-activated BFNS pastes. The FTIR results clearly reveal the polymerization process of alkali-activated BFNS. The variation after 90 days was basically the same as that after 28 days, which further verified the possibility of the above speculation.

Pore structure has a very important effect on strength and permeability resistance of concrete. According to previous studies, the pores in cementitious materials are classified as follows: harmless pores (<20 nm), little harmful pores (20–50 nm), harmful pores (50–200 nm) and much harmful pores (>200 nm) (Zhou et al., 2020). In this study, the pore structure distributions of different kinds of hardened pastes at 28 and 90 days are shown in Figure 9.

Obviously, with increasing modulus, the total pore volume increases. Compared to samples BFNS-M1.5 and BFNS-M2.0, sample BFNS-M0.5 has an extremely low pore volume. This indicates that increasing the modulus of water glass has an adverse impact on the pore structure. Moreover, the pores in alkali-activated BFNS hardened pastes are mainly composed of small pores, and the proportion of large pores is very small. With the prolongation of curing time, there is no obvious change in the total pore volume. The main reason is that the pore structures of alkali-activated BFNS hardened pastes at 28 days are very dense and prolonging the curing time has little influence on improving the pore structure. However, in terms of sample BFNS-M2.0, with prolonged curing age, the pore distribution obviously changes. The harmless pore volume increases, little harmful pores and harmful pores volume decreases, and much harmful pore volume change slightly.

The changes in the compressive strength and splitting tensile strength of all concrete with age under different conditions are shown in Figure 10A and Figure 10B, respectively. As illustrated in Figure 10, the compressive strengths show a similar tendency to the splitting tensile strength at all ages. Apparently, increasing the modulus of water glass has an adverse impact on the development of compressive strength and splitting tensile strength, resulting from deteriorated pore structure. At 1 day, samples BFNS-M1.5 and BFNS-M2.0 have no strengths. Meanwhile, the late strength increases slowly. Thus, these two materials e cannot be used in practical engineering applications due to their low strengths. In terms of sample BFNS-M0.5, the compressive strength and splitting tensile strength rapidly increase before 7 days. However, after 7 days, the growth rates of the strength decrease. Sample BFNS-M0.5 can achieve 60 MPa compressive strength and 5.5 MPa splitting tensile strength. In this study, when the concentration of water glass is 10% in alkali-activated BFNS cementitious material, the optimum modulus for compressive strength and splitting tensile strength is 0.5.