1 Introduction

Supplementary cementitious materials (SCMs) are soluble siliceous, aluminosiliceous, or calcium aluminosiliceous powders, including a few industrial by-products and natural minerals. As the partial replacement of cement by SCMs can reduce energy consumption and improve properties, they have been widely used in cement-based materials for decades [1].

Fly ash (FA) and ground granulated blast slag (GGBS) are two commonly utilized SCMs. Some researchers have reported that adding FA and GGBS simultaneously to cement-based materials can not only reduce cement contents and production costs but also maintain good workability, strength, and durability. Li et al. [2] and Chen et al. [3] investigated the synergistic effect of FA and GGBS incorporated into concrete. They found that the joint replacement of 30 wt% cement by FA and GGBS was more effective in enhancing the 90 d compressive strength and chloride ion penetration resistance and reducing shrinkage. Therefore, the combination of FA and GGBS has been increasingly used in commercial concrete, bridge platforms, high-rise buildings, and other general engineering projects.

However, the construction industry has recently expanded to deeper, higher, and more challenging environments. As a result, cement-based materials with improved strength, volume stability, and durability are required, and the combination of FA and GGBS needs to be further enhanced [4]. In previous studies, the cement strength was characterized using the compressive strength, and drying shrinkage was used as an important indicator to evaluate the volume stability. In addition, the durability of cement-based materials is directly influenced by moisture transport, and capillary water absorption is widely used to evaluate the moisture transfer capacity [5]. Therefore, the optimization and improvement measures must increase the compressive strength while reducing drying shrinkage and capillary water absorption.

One way to achieve this is to add SF. Compared with FA and GGBS, SF has a larger specific surface area and higher pozzolanic activity. Further, it can enhance the mechanical properties and compactness of cement-based materials at lower admixture levels [6]. Consequently, SF is widely used in the production of high-performance cement-based materials. According to literature research, the addition of SF could increase the compressive strength of cement mortar by 5–20% [7] and reduce the capillary water absorption by 5–20% [8]. However, this results in increased drying shrinkage. For example, Wang et al. [9] found that upon the addition of 5% SF, the drying shrinkage after 90 d increased by up to 38% compared to the control (denoted JZ).

Nanomaterials can be used for further modification to compensate for the deficiencies of SF in terms of volume stability and to enhance other properties of cement-based materials. A commonly used nanomaterial is nanosilica (NS), which has a typical particle size of 10–200 nm and a large specific surface area (10–103 m2/g) [10]. Notably, the addition of NS improves the compressive strength of cement mortar by 10–20%, reduces capillary water absorption by 5–40%, and, most importantly, reduces drying shrinkage by 6–8% [11]. To date, studies on double doping with SF and NS have focused on strength enhancement. For instance, Wang et al. [12] found that the addition of 0.8 wt% NS and 12.5 wt% SF resulted in a 12.81% increase in compressive strength. However, studies on the effect of the addition of SF and NS on other properties such as drying shrinkage and capillary water absorption are few.

Meanwhile, previous research has proposed that calcium sulfate whisker (CSW) can be utilized as a replacement for the modification of cement-based materials because of the high cost of NS. Similar to NS, CSW can enhance mechanical properties by approximately 5–35% [13]. CSW is a low-carbon micron-sized fiber, and it is often used in papermaking, coatings, and composites. However, its utilization in cement-based materials has gained prominence only recently. There is currently a divided perspective on whether CSW can undergo reactions in cement-based materials. Wan et al. [14] showed that CSW had no chemical effect in cement, whereas Cao et al. [15] suggested that CSW can react to produce calcium aluminate hydrates (AFts) and thereby increase the volume of the solid phase; however, this requires experimental verification. In addition, many researchers have used CSW alone to modify cement-based materials or used CSW as one of the hybrid fibers. However, there are only a few studies on the effects of mixing SF and CSW and the synergistic mechanism, if any.

In conclusion, research on the synergistic mechanism of SF and NS/CSW is significant. Moreover, SF and NS/CSW have the potential to further enhance the properties of cement-based materials containing FA and GGBS, but only a few systematic studies have been conducted. In this study, we investigated the effects of 3 wt% SF and the 1 wt% replacement of SF by NS/CSW on the strength, drying shrinkage, capillary water absorption, phase composition, pore structure, and micromorphology of cement mortar. This study aimed to summarize the effect of SF and NS/CSW, investigate whether CSW undergoes a chemical reaction, and analyze the relationships between the drying shrinkage, phase composition, and microstructure after the partial replacement of SF by NS/CSW.

2 Materials and methods

2.1 Materials

The raw materials used in this study included cement, FA, GGBS, SF, NS, CSW, superplasticizer (SP), sand, and water. Benchmark cement (P. I42.5) with a specific surface area of 358 m2/kg, supplied by Fushun Cement Co., Ltd., was used. The GB175-2020 standard supports its use. FA was obtained from the coal combustion process, and GGBS was obtained by drying and grinding quenched slag, both of which were produced in Xiangtan, Hunan Province of China, in compliance with GB/T 1596-2017 and GB/T 18046-2017, respectively. In addition, 940-grade SF (produced by Elkem ASA Co., Ltd., Shanghai, China) with a volume density of 262 kg/m3 was used in accordance with GB/T 27690-2011. The chemical compositions of the cement, FA, GGBS, and SF, which were tested with X-ray fluorescence spectrometry (Axiosm AX, PANalytical, Netherlands), are listed in Table 1. It can be seen that the main compositions of FA are SiO2 and Al2O3, those of GGBS are SiO2 and CaO, and that of SF is SiO2. The cumulative particle size distributions are shown in Fig. 1, and the particle size of SF is proven to be smaller than those of cement, FA, and GGBS. A laser particle size analyzer (Topsizer, OMEC, China) was used for the tests.

Table 1 Chemical compositions of cement, FA, GGBS, and SF (%)
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Fig. 1
figure 1

Cumulative particle size distributions of cement, FA, GGBS, and SF

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NS was produced by Zhongkehang Co., Ltd. (Shanghai, China) according to GB/T 20020-2013, and CSW was produced by Fengzhu Co., Ltd. (Shanghai, China), according to GB/T 35471-2017. The physical properties of NS and CSW are shown in Table 2. The particle size, specific surface area, density, pH, and purity were tested by a Topsizer, a BET Specific surface area tester (Kubo X1000, Beijing Builder, China), an automatic helium pycnometer (AccuPyc II 1345, Micromeritics, USA), a benchtop pH meter (PHSJ-6L, REX, China), and the acid–base titration, respectively. It can be seen that NS is nano-sized and has a large specific surface area, whereas CSW is micron-sized, both of which have a pH of ~ 7 and high purity. In addition, the micromorphologies of SF, NS, and CSW are shown in Fig. 2, which were obtained using scanning electron microscopy (SEM), which will be described in detail in Sect. 2.4.6. Similarly, the length and diameter in Table 2 were also measured using this equipment. It can be observed in Fig. 2 that both SF and NS are spherical particles, CSW are rod-like fibers, and NS is smaller than SF and CSW. SP, which was produced by Shanghai Sunrise Polymer Material Co., Ltd. (Shanghai, China), was a polycarboxylate water reducer with a water reduction ratio of ≥ 25% in compliance with GB8076-2008. The sand used in this study was ISO standard sand, and the water used was tap water, both in accordance with GB/T 17671-2021.

Table 2 Physical properties of NS and CSW
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Fig. 2
figure 2

Micromorphology of raw materials: a SF, b NS, and c CSW

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2.2 Mixing ratio

Table 3 shows the mixing ratio of the mortar. Briefly, the cement mortar was prepared using a combination of high-performance concrete proportions (70 wt% cement + 20 wt% FA + 10 wt% GGBS) commonly used in engineering. JZ denotes the control group without additives. The following labeling scheme was employed: SFxNSyCSWz, where x, y, and z are the weight percentages of SF, NS, and CSW, respectively. Thus, SF3 denotes a specimen containing 3 wt% SF, whereas SF2NS1 and SF2CSW1 indicate samples having 2 wt% SF and 1 wt% NS/CSW, respectively. The external doping of SF and NS/CSW was used to eliminate the influence of cement reduction, and a dosage of 3 wt% was utilized to prevent an excessive reduction in the water–binder ratio. In this study, the mortar samples were prepared with a fixed water–binder ratio (0.3, the ratio of water content to the sum of cement, FA, GGBS content) and sand–binder ratio (1.5, the ratio of sand content to the sum of cement, FA, GGBS content). Considering that the addition of NS significantly reduces fluidity [16], the amount of SP was varied to maintain the fluidity of cement mortar in the range of 170–180 mm.

Table 3 The mix proportion of cement mortar (kg/m3)
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2.3 Mixing and curing methods

Previous study [17] showed that adding a certain amount of SP to an aqueous solution of NS and then using high-speed stirring and ultra-sonication for treatment had the effect of preventing agglomeration. Therefore, as shown in Figs. 3 and 4, in this study, we added 10 g NS to 300 g water with 0.8 g SP. First, the mixed solution was stirred at high speed (24,000 r/min) for 0, 1, 5, 10, and 30 min using a stirrer (SY-06, KORMES, Germany). It was found that the average particle size of NS decreased from 55 to 3 μm, and prolonging the stirring time did not have any distinct effect of reducing the particle size. Hence, the optimal time of high-speed stirring was selected as 1 min. Thereafter, ultra-sonication was carried out for 1, 5, 10, and 30 min with an ultrasonic generator (500 W, LS-500A, CWWIS, China) after high-speed stirring for 1 min. It was found that the average particle size of NS further decreased from 3 μm to approximately 150 nm. The best dispersion effect was obtained upon performing high-speed stirring for 1 min and ultra-sonication for 10 min, and the average particle size was 145 nm. In addition, the particle size of CSW was not accurately measured by Topsizer due to the rod-like structure, and the size of CSW was larger than that of NS shown in Fig. 2. As a result, high-speed stirring for 1 min and ultra-sonication for 10 min was also employed as the dispersion method for CSW to control the experimental variables.

Fig. 3
figure 3

Particle size distributions of NS under different dispersion methods

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Fig. 4
figure 4

The NS/CSW dispersion process and the cement mortar preparation process. *① represents JZ, ② represents SF3,③ represents SF2NS1, and ④ represents SF2CSW1. Add water with SP to the mixing pot when making specimens ① and ②, while add NS/CSW dispersion when making specimens ③ and ④. Furthermore, add SF when making the specimens ②, ③, and ④

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The mortar specimens used in this study were prepared and cured in accordance with GB/T 17671-2021 shown in Fig. 4. First, cement, FA, GGBS, SF, and water containing SP (or the NS/CSW dispersion) were added to the mixing pot. Second, the sand was added to the mixer, and the mixture was mixed slowly for 2 min and then quickly for 2 min. Third, the mixture was poured into molds. The specimen sizes for compressive and flexural strength, drying shrinkage, and capillary water absorption tests were 40 × 40 × 160 mm, 25 × 25 × 280 mm, and 40 × 40 × 80 mm, respectively. The specimen measuring 40 × 40 × 80 mm was cut from a specimen with dimensions of 40 × 40 × 160 mm. Following casting, the specimens were stored at 20 ± 5 °C for 24 h and then demolded. The 40 × 40 × 160 mm specimens were cured at 20 ± 2 °C and > 95% relative humidity (RH) to the specified age. The 25 × 25 × 280 mm specimens were immersed in water for 3 d to prevent self-shrinkage. Subsequently, the surface was wiped, and they were placed under a standard drying shrinkage environment (temperature: 20 ± 2 °C, RH: 60 ± 5%) for curing to the specified age.

2.4 Testing methods

2.4.1 Compressive strength tests

The compressive strengths of the specimens were measured after aging for 3, 7, and 28 d. The tests were carried out in accordance with GB/T 17671-2021. An electro-hydraulic pressure testing machine (TYA-300B, NEW LUDA, China) was used for the tests. The loading rate was 2.4 kN/s, and the average of the six samples was considered as the result.

2.4.2 Drying shrinkage and mass loss tests

Tests were performed using a length comparator (BC156-300, ZHONGKE LUDA, China) and an electronic balance referring to JCT 603-2004. Before testing, a calibration rod was used to calibrate the comparator. Subsequently, the nail head on one end of the specimen was placed into the spherical hole on the comparator, and the nail head on the other end was gently placed into the under-spherical hole. The specimen was rotated counterclockwise slowly, and the value on the dial was read after the data stabilized. Finally, an electronic balance was used to weigh the sample. The drying shrinkage and mass loss were calculated using Eqs. (1) and (2), respectively, and they are reported as the average of those of three specimens.

$$S_{i} = \frac{{L_{0} - L_{i} }}{{L_{0} }} \times 100\%$$
(1)
$$W_{i} = \frac{{M_{0} - M_{i} }}{{M_{0} }} \times 100\%$$
(2)

Here, Si is the drying shrinkage at the test age i, L0 is the initial value recorded by the comparator after 3 d, Li is the value recorded by the comparator at test age i, Wi is the mass loss at test age i, M0 is the initial weight recorded by the electronic balance after 3 d, and Mi is the weight recorded by the electronic balance at test age i.

2.4.3 Capillary water absorption tests

The tests were conducted based on ASTM C1585-2013. After 28 d of curing, the specimens were placed in an oven at 60 °C for 48 h and then actively cooled to 20 °C. The 40 × 80 mm surfaces were sealed with silicone rubber, and the top side of the specimen was fixed with plastic wrap and a rubber band. Before saturation with water, the original weights of the specimens were recorded using an electronic balance (precision: 0.01 g). The bottom side of the specimen was then immersed in a water tank. The water surface was 2 ± 1 mm higher than the contact surface of the specimen, as shown in Fig. 5. Finally, the sample weight was measured from 1 min to 8 d. The average of four specimens was considered the capillary water absorption height.

Fig. 5
figure 5

Schematic diagram of capillary water absorption test

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The capillary water absorption height was calculated using Eq. (3) [18].

$$I = \frac{{m_{t} }}{a \cdot d}$$
(3)

Here, I denotes the capillary water absorption height, mt denotes the change in sample mass at time t, a denotes the contact area of the sample with water, and d is the density of water.

The capillary water absorption coefficient can be derived by substituting the measured data with the ISAT [19] method using Eq. (4).

$$I = S \cdot t^{0.5} + b$$
(4)

Here, S is the capillary water absorption coefficient, and b is the intercept of the line, which is a constant.

2.4.4 Thermogravimetric analysis

Thermogravimetric/differential thermogravimetric (TG/DTG) analysis was performed using a simultaneous thermal analyzer (TGA/DSC 2, Mettler, Switzerland) to investigate the sample phase composition. After 28 d, the samples were soaked in isopropanol for 7 d to stop further hydration and then placed in a vacuum drier for an additional 6 d. Each dried sample was milled until the particle size was no more than 200 mesh before the tests. The powdered samples were heated from room temperature to 1000 °C at a heating rate of 10 °C/min under a N2 atmosphere. In the TG tests, the Ca(OH)2 content of the mortar was obtained using Eq. (5) [20].

$${\text{Ca}}({\text{OH}})_{2} (\% ) = \frac{{M(400\,^\circ {\text{C}}) - M(500\,^\circ {\text{C}})}}{M} \times \frac{74.09}{{18.01}} \times 100\%$$
(5)

Here, M is the weight of the specimen before combustion, M (400 °C) is the remaining mass of the specimen at 400 °C, and M (500 °C) is the remaining mass of the specimen at 500 °C.

2.4.5 Mercury intrusion porosimetry analysis

Mercury intrusion porosimetry (MIP) was used to investigate the pore structure. MIP measurements were carried out using an automatic aperture analyzer (PoreMaster GT-60, Quantachrome, USA) with minimum and maximum pressures of 0.5 and 33,000 psi, respectively, corresponding to minimum and maximum pore sizes of 5 nm and 350 μm, respectively. In the MIP tests, based on the Menger sponge model [21], the relationship between the volume of intruded mercury and the pore diameter was determined using Eq. (6), and the fractal dimension was determined from the slope of the curve.

$$- \ln V\sim (3 - D)\ln r$$
(6)

Here, V is the volume of intruded mercury during the MIP test, r is the pore diameter into which mercury can intrude, and D is the fractal dimension, having values of 2–3.

2.4.6 SEM analysis

SEM (JSM-IT500, JEOL, Japan) was used to observe the micromorphologies of the samples. The measurements were carried out at an accelerating voltage of 20 kV. The surfaces of the samples were sprayed with gold in vacuum before analysis.

3 Results and discussion

3.1 Compressive strength

Figure 6 shows the compressive strength of the cement mortar. Clearly, the compressive strength of all groups increases with age. Further, a comparison between the compressive strengths of the different samples revealed that those of the SF3, SF2NS1, and SF2CSW1 samples are all higher than that of the control (JZ). This indicates that SF, NS, and CSW enhance the compressive strength. To illustrate the strengthening effect, the extents of increase in compressive strength with respect to that of JZ are listed in Table 4. As shown, at 3 and 7 d, SF2NS1 showed the highest increase, and the SF2NS1 sample showed an increase at early and medium curing periods [22]. At 28 d, the increase in the compressive strength for the SF2CSW1 sample was greater than those of the SF3 and SF2NS1 samples, and the compressive strength of the SF2CSW1 sample continued to increase from 7 to 28 d. Thus, the strength is enhanced by the partial replacement of SF by NS/CSW.

Fig. 6
figure 6

Compressive strength of cement mortar

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Table 4 Compressive strength enhancement rate of mortar relative to JZ
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3.2 Drying shrinkage and mass loss

3.2.1 Drying shrinkage and mass loss values

Figure 7a shows the drying shrinkage from 0 to 70 d for the different samples. The drying shrinkage increased gradually with age. In addition, the increase in drying shrinkage was marked before 14 d, accounting for approximately 60% of drying shrinkage at 70 d. Subsequently, the growth rate slowly decreased, and after 56 d, the change in drying shrinkage was negligible. Notably, the addition of 3 wt% SF increased the drying shrinkage of cement mortar. However, after the partial replacement of SF by NS, the drying shrinkage was reduced by 5.93% and 2.63% compared to those of SF3 and JZ, respectively, after 70 d. In addition, after the partial replacement of SF by CSW, the drying shrinkage was reduced by 11.33% and 8.22% compared to those of SF3 and JZ, respectively, after 70 d.

Fig. 7
figure 7

Drying shrinkage and mass loss of cement mortar: a drying shrinkage, and b mass loss

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The mass losses from 0 to 70 d for different groups are shown in Fig. 7b. Clearly, the mass loss gradually increases with age, whereas the growth rate slowly decreases. A comparison of the mass loss of the different groups shows that the incorporation of SF mitigates water evaporation [23]. In addition, the partial replacement of SF by NS/CSW further enhances these effects, and the lowest mass loss is observed for SF2CSW1.

3.2.2 Relationship between drying shrinkage and mass loss

Figure 8 shows the relationship between the mass loss and drying shrinkage of different groups from 0 to 70 d. The drying shrinkage increases with an increase in mass loss arising from water evaporation. The data were fitted to Eq. (7) [24].

$$y = A(1 - {\text{e}}^{ - bx} )$$
(7)
Fig. 8
figure 8

The relationship between drying shrinkage and mass loss of mortar

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Here, y is the drying shrinkage value, x is the mass loss, and A and b are constants.

The coefficients of determination (R2) for fitting to this equation were all above 0.9, indicating a good fit to the data. Further, the gradient of the curve increased with an increase in mass loss, as reported in previous study [25]. This is because of the presence of abundant free water in capillary pores and the large difference between the water content of the sample and the environment at an early curing age, which results in a faster loss of free water from the paste. Subsequently, as the free water content and water content gradient decrease, the degree of mass loss decreases [26]. As shown, the gradient of SF2NS1 is steepest, followed by those of SF2CSW1 and SF3, whereas that of JZ is the gentlest. This result indicates that the mass loss decreases in the order of JZ > SF3 > SF2CSW1 > SF2NS1 when the drying shrinkages are the same. Based on this result, the SF2NS1 sample should have the lowest number of capillary pores and amount of free water [27].

3.3 Capillary water absorption

Figure 9 shows the capillary water absorption height after 28 d (calculated using Eq. (3)) with respect to time for the different samples. There is one inflection point in the curve at 6 h. Using this point, the capillary water absorption height curves can be divided into two stages. The period from 1 min (8 s1/2) to 6 h (147 s1/2) is the first stage, whereas that from 6 h (147 s1/2) to 8 d (831 s1/2) is the second stage [28]. Using Eq. (4), the S values of two stages were calculated via linear fitting (Table 5). The high R2 values indicate a good fit to this equation.

Fig. 9
figure 9

Capillary water absorption height of cement mortar

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Table 5 Capillary water absorption coefficient and correlation coefficient
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The capillary water absorption heights increased rapidly in the initial stage and then slowed down in the second stage. This result might be attributable to the decrease in capillary potential as the capillary force drives water toward the top of the sample. In addition, the water enters smaller pores from the capillary pores. Both these factors increase the resistance for capillary water absorption. The incorporation of SF reduced the capillary water absorption height, and the capillary water absorption height was further reduced by 24.4% for SF2NS1 and 17.8% for SF2CSW1 compared to JZ. This result indicates that NS/CSW promotes the densification of the structure and reduces the porosity and pore connectivity. Moreover, NS reduces capillary water absorption to a greater extent than does CSW.

3.4 Phase composition analysis

The results of TG/DTG analysis are shown in Fig. 10. As shown in Fig. 10a, an exothermic peak corresponding to AFt is observed at 90–150 °C, and the AFt content in SF2CSW1 is the greatest, followed by SF3, SF2NS1, and JZ. Therefore, CSW can react to produce AFt, as reported by Cao et al. [15]. In addition, SF and NS can promote the hydration of tricalcium aluminate (C3A) to generate AFt.

Fig. 10
figure 10

Thermogravimetric analysis test results: a TG/DTG, and b Ca(OH)2 content

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As shown in Fig. 10b, the Ca(OH)2 contents in JZ, SF3, and SF2NS1 calculated using Eq. (5) are 12.37%, 10.89%, and 9.68%, respectively. This result suggests that SF and NS exert a pozzolanic effect to consume Ca(OH)2, and the reactivity of NS is higher than that of SF [29]. A similar phenomenon is observed after the partial replacement of SF by CSW. However, the weakening effect is not as distinct as that of NS: specifically, the content of Ca(OH)2 is 10.73%. Thus, although CSW has no pozzolanic activity, it can promote the reaction of SF.

3.5 Pore structure analysis

3.5.1 Pore volume distribution

The pore volume distribution and porosity of the cement mortar samples are shown in Fig. 11. Clearly, SF3, SF2NS1, and SF2CSW1 have refined pore structures. Relative to that of JZ, the total porosities of SF3, SF2NS1, and SF2CSW1 after 28 d are reduced by 4.5%, 17.4%, and 14.2%, respectively. This is likely related to the filler effect of SF, NS, CSW, and their reaction products.

Fig. 11
figure 11

Percentage for pore volume distribution and porosity of cement mortar

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The pore size distribution in Fig. 11 can be divided into four sections: 5–10, 10–50, 50–200, and > 200 nm [30]. These pore size ranges affect different macroscopic properties. In particular, the 5–10 nm pores are gel pores, and a higher content of such pores indicates the presence of more C–S–H gels [31]. Notably, the addition of 3 wt% SF resulted in a slight increase in the number of gel pores, and the partial replacement of SF by NS/CSW resulted in a further increase, consistent with the results shown in Sect. 3.4. In addition, the 10–50 nm pores are capillary pores, and a higher content of these pores suggests a greater drying shrinkage of the sample [32]. Compared to that in JZ, the number of capillary pores increased by 3% in SF3 and decreased by 2% and 3% in SF2NS1 and SF2CSW1, respectively. Pores larger than 50 nm are large pores, and a greater content of these pores corresponds to a lower strength [33]. The large pore contents decreased in the order of JZ > SF3 > SF2CSW1 > SF2NS1.

3.5.2 Fractal dimensions

The pore volume fractal dimension is a comprehensive representation of the pore shape and pore volume distribution [34]. Using Eq. (6), the relationship between the pore diameter and intruded mercury volume was determined, as shown in Fig. 12. The fractal dimensions of the red and blue zones are between 2 and 3, indicating that these two zones have fractal characteristics [35]. Further, the fractal dimensions of the red zone are smaller than those of the blue zone, and those of the blue zone are the backbone fractal dimensions; this is determined by external factors such as the mixing and molding method, which were not investigated in this study.

Fig. 12
figure 12

The fractal dimension: a JZ, b SF3, c SF2NS1, and d SF2CSW1. *Ds is the fractal dimension in the percolation region, and Dk is the fractal dimension in the backbone formation region

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The red zone represents percolation fractal dimensions, which correspond to a pressure range of 2500–10,000 psi and a pore diameter range of 18–69 nm. The percolation fractal dimension is mainly related to the composition and pore structure of the cement paste [36], and it increased in the order of JZ < SF3 < SF2CSW1 < SF2NS1. Therefore, as SF is partially replaced by NS/CSW, the fractal dimensions increase, indicating the pore structure becomes more complex. Further, the average pore diameter and pore contents are reduced, and the contents of large pores decrease while the contents of small pores increase [37].

3.6 Micromorphological analysis

Figure 13 shows the SEM images of different samples after 28 d. As shown in Fig. 13a, the connection between the hydrates of JZ is loose, and many apparent pores exist. In addition, the FA surface is relatively smooth. As shown in Fig. 13b, after the addition of 3 wt% SF, as the content of pores decreases, the hydrate content increases. For SF2NS1, as shown in Fig. 13c, more hydrates are attached to the surface of FA, suggesting that the filler effects of SF and NS at the micron and nano scales enhance the compactness and reduce the visible porosity. For SF2CSW1, unhydrated CSW can be observed, as shown in Fig. 13d. Clearly, whiskers fill the pores, and the rough surface indicates that the whiskers also provide nucleation sites for cement hydration, which is consistent with the findings of Wan et al. [14].

Fig. 13
figure 13

Micromorphology of specimens: a JZ, b SF3, c SF2NS1, and d SF2CSW1

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3.7 Discussion

3.7.1 Mechanism of improvement after the partial replacement of SF by NS

Figure 14 shows the microscopic mechanism behind the enhancing effect after the partial replacement of SF by NS. Clearly, NS exerts filler, pozzolanic, and nucleation effects. In particular, NS has a large specific surface area for contact with Ca(OH)2. Thus, the pozzolanic reaction is more intense compared to that of SF, and consequently, more Ca(OH)2 is consumed to generate C–S–H gels. The generated C–S–H gels are wrapped around the NS surface, which fill the small pores between cement, FA, GGBS, and SF. Therefore, SF2NS1 had the lowest Ca(OH)2 content and the lowest porosity, as described in Sects. 3.4–3.6.

Fig. 14
figure 14

Micromechanisms in the partial replacement of SF by NS: a cement mortar incorporated SF, and b cement mortar of the partial replacement of SF by NS

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Mechanical properties and durability depend on the microstructure [38]. In terms of the phase composition of SF2NS1, the content of C–S–H gels is higher than in SF3. Crucially, C–S–H gels are one of the primary hydration products that resist external load. As a result, the compressive strength of SF2NS1 is higher than those of the other samples. Further, as shown by the pore structure analysis, SF2NS1 has the lowest total porosity and the fewest pores larger than 50 nm, which is consistent with its higher compressive strength relative to SF3 [39]. In addition, the content of pores measuring 10–50 nm was less than that of JZ, indicating a denser structure, enhanced resistance to water migration, and reduced drying shrinkage [40].

3.7.2 Mechanism of improvement after the partial replacement of SF by CSW

Figure 15 shows the microscopic mechanism behind the enhancing effect obtained after the partial replacement of SF by CSW. As shown, CSW provides filler and carrier effects and can undergo chemical reactions. CSW has the same chemical composition as gypsum and can react with C3A or monosulfates (Ms) to produce AFt, as shown in Eqs. (8) and (9) and Fig. 15 and reported in the literature [15]. Meanwhile, because of its special fibrous structure, CSW can be used as a carrier for cement hydration and secondary hydration to increase the degree of hydration. Further, CSW and its reaction product AFt can fill pores, but the filler effect is weaker than that of NS because of size limitations. Therefore, as discussed in Sects. 3.4–3.6, SF2CSW1 has the highest AFt content and lower porosity.

$$\text{C}_{3} A + \text{CaSO}_{4} + \text{H}_{2}\text{O} \to \text{C}_{3} \text{A} \cdot 3 \text{C}\overline{S} \cdot \text{H}_{32} (AFt)$$
(8)
$$\text{C}_{3} \text{A} \cdot \text{C} \overline{S} \cdot \text{H}_{12} ({\text{Ms}}) + {\text{Ca}}\text{SO}_{4} + \text{H}_{2}\text{O} \to \text{C}_{3} \text{A} \cdot 3\text{C}\overline{S} \cdot \text{H}_{32} (\text{AFt})$$
(9)
Fig. 15
figure 15

Micromechanisms in the partial replacement of SF by CSW: a cement mortar incorporated SF, and b cement mortar of the partial replacement of SF by CSW

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As described in Sect. 3.7.1, the partial replacement of SF by CSW resulted in changes to the microstructure, which also affected the macroscopic properties. In terms of the phase composition of SF2CSW1, the consumption of Ca(OH)2 by the pozzolanic reaction of SF and the framework structure produced by fibrous AFt increase the compressive strength. In particular, the AFt generated by CSW contains 32 mol of chemically bound water, which increases the volume of the solid phase, and the skeletal structure of AFt limits the drying deformation of the cement paste. In addition, pore structure analysis revealed that the total porosity and percentage of pores larger than 50 nm were slightly higher for SF2CSW1 than for SF2NS1, with the former exhibiting a slightly lower compressive strength and higher capillary water absorption compared to those of SF2NS1. Finally, the percentage of pores in the 10–50 nm range was 54%, which is consistent with the lowest drying shrinkage of SF2CSW1 of all samples.

4 Conclusions

In this study, we investigated the effects and corresponding mechanisms of the partial replacement of SF by NS/CSW on the mechanical strength, drying shrinkage, and capillary water absorption of cement mortar containing SF. The following conclusions were obtained:

  1. (1)

    Compared with that of pure cement mortar, the early compressive strengths of samples with SF and NS increased, and after 28 d, the compressive strength increased by 10.83%. Additionally, after the partial replacement of SF by CSW, the later compressive strength increased and the compressive strength after 28 d increased by 12.10%.

  2. (2)

    Compared with those of pure cement mortar, the 70 d drying shrinkage and 28 d capillary water absorption decreased by 2.63% and 24.4%, respectively, after the partial replacement of SF by NS. Further, after the partial replacement of SF by CSW, the 70 d drying shrinkage and 28 d capillary water absorption decreased by 8.22% and 17.8%, respectively.

  3. (3)

    Compared with that of pure cement mortar, the degree of pozzolanic reaction increased, Ca(OH)2 content decreased, and C–S–H gel content increased after the partial replacement of SF by NS. Moreover, after the partial replacement of SF by CSW, the Ca(OH)2 content decreased, whereas the AFt content increased. Thus, CSW underwent a chemical reaction with C3A or Ms in the cement mortar to generate AFt, and the additional AFt formed a framework and increased the solid phase volume.

  4. (4)

    Compared with those of pure cement mortar, the total porosity and mesoporosity (10–50 nm) decreased after the partial replacement of SF by NS/CSW, which may be the primary reason for the decrease in drying shrinkage. By contrast, the additional AFt generated by CSW compensated for the shrinkage.