Tensile and Fracture Properties of Silicon Carbide Whisker-Modified Cement-Based Materials

Table 5 gives the analysis results of the orthogonal experiment designed to determine the influencing factors of the SiCw dispersion.

In Table 5, K_i value represents the sum of test index values corresponding to the i level of a factor, and k_i = K_i/3. According to the characteristics of orthogonal design, an example is as follows: if factor A has no effect on the test index, k₁, k₂ and k₃ corresponding to A should be equal. If not, the optimal level of factor A can be obtained by comparing the sizes of k₁, k₂ and k₃. The optimal level combination of each factor can obtain an optimal combination.

R represents the range of a factor, which is calculated as follows: R = max (k_i) − min (k_i) reflects the variation range of the test index when the factor fluctuates. The greater the R corresponding to the factor, the greater the impact of the factor on the test index. By comparing the range, R, the influences of the various factors on the dispersion of SiCw were determined as follows: pH value of solution > water reducer dosage > magnetic stirring time > ultrasonic dispersion time. According to the experimental results, a solution pH value of 11 and a water reducer dosage of 0.3% was the most conducive for dispersing the SiCw. After the addition of SiCw, the optimal treatment method was magnetic stirring for 15 min and then subsequent ultrasonic stirring for 4 min.

In order to further verify the accuracy of the orthogonal test results, the two most important factors affecting the effectiveness of the dispersion, i.e., the solution pH value and water reducer dosage, were tested by a single control variable experiment. The influences of the solution pH value and water reducer on the dispersion of SiCw were studied.

According to the results in Sect. 3.1, the solution pH value had the greatest influence on the dispersion of the whiskers. Fig. 8 shows the absorbance and Zeta potential of SiCw dispersions prepared in solutions with different pH values.

When the pH value of the solution was 11, the absolute value of the Zeta potential on the surface of the SiCw was the largest. According to Stern double electric layer structure principle (Greenwood, 2003), when the solution pH value was 11, the repulsive force between the SiCw was large, and it was not easy for the whiskers to agglomerate, thus producing the best dispersion effect of the SiCw. This result was consistent with the absorbance results. When the pH value was 11, the absorbance was the highest because the superliquid of the SiCw dispersion prepared in a solution with a pH value of 11 had the highest whisker dosage which scattered or absorbed more UV light and led to low light transmission and a high UV absorbance value. In other words, after standing, it was more difficult for the SiCw dispersion prepared in a pH 11 solution to stratify and precipitate.

The influence of the water reducer dosage on the dispersion of SiCw is second only to the solution pH value. Similarly, we took the water reducer dosage as the variable to determine the absorbance and Zeta potential, and the results are shown in Fig. 9. As can be seen, the dispersion effect is the best when the dosage of the water reducer was 0.3%.

Fig. 10 shows the tensile strength of the cement mortar at different curing ages. The addition of SiCw improved the tensile strength of the cement mortar, the content of SiC whiskers with the content of 0.1%, 0.2% and 0.3% increased by 8.1%, 4.5% and 2.2%, respectively, at 28 days. There was an optimal SiCw dosage. The overall tensile performance of the cement mortar was best with a SiCw dosage of 0.1%. With an increase in the whisker dosage, the tensile performance of the cement mortar was not enhanced significantly. The reasons are summarized as follows. When the whisker dosage was low, the whiskers were dispersed evenly, and the toughening effect was significant (Wang et al., 2008; Zhu et al., 2021). As the SiCw dosage increased, the original defects in the material and the agglomeration entanglement of the SiCw had adverse effects on the tensile strength of mortar (Hambach et al., 2016). When the whisker dosage reached 0.3%, on the contrary, the tensile strength decreased significantly at 7 days, which may have been because the hydration products only wrapped the surface of the aggregates at an early age, but did not fill the gaps between the whiskers (Bentur & Mindess, 2006; Cuixiang et al., 2011).

From tensile tests of the 8-shaped mortars, the stress–displacement curves for different whisker dosages were obtained, and the results are shown in Fig. 11. For the blank group, with an increase in the curing age, the displacement of the stress peak increased first and then decreased. For the 7-day specimen group, the mortar strength was still very low, and the specimens broke easily. For the 14-day specimen group, further hydration of the cement enhanced the mortar strength to some extent, and the tensile strength also improved, so the displacement corresponding to stress peak value increased. After 28 days, the strength of mortar was further enhanced, and the mortar was significantly more brittle, so the displacement corresponding to the stress peak value decreased slightly.

The addition of SiCw delayed the specimens from reaching their peak stress value and increased their tensile strength to a certain extent. It can be seen that the tensile strengths of the test groups with dosages of 0.1% and 0.2% both improved to a certain extent, of which, the tensile performance for the specimen with a SiCw dosage of 0.1% was slightly better than that with 0.2%. However, due to the progress of the cement hydration, the mortar became more and more brittle, and the displacement corresponding to the peak stress value also gradually decreased. For the test group with an SiCw dosage of 0.3%, the strength deteriorated, and the peak displacement increased significantly. This may have been due to the high dosage of SiCw in the cement matrix leading to the formation of agglomerates and entanglements (Ren et al., 2013).

The work done during the tensile test was obtained by integrating the stress–displacement curves, as shown in Fig. 12. The experimental results showed that the addition of SiCw increased the energy consumption during the tensile process and effectively played a role in toughening the cement-based materials. For the 28-day specimens, the energy consumption of the experimental group with the SiCw dosage of 0.1% was 67.9% higher than that of the blank group.

At an age of 7 days, for SiCw dosages of 0.2% and 0.3%, the whisker entanglement caused by the higher SiCw dosage significantly contributed to the energy consumption during the tensile tests at the initial stages of cement hydration. In contrast, for the SiCw dosage of 0.1%, at the initial stage of cement hydration, the whisker dispersion was relatively homogeneous, but the insufficient cement hydration led to a weak holding force on the whiskers. As a result, the whiskers were pulled out easily rather than breaking, and their energy consumption was not obvious.

At 14 days of aging, for the SiCw dosages of 0.2% and 0.3%, the adverse effects of cement hardening on the toughness were much higher than the improvements from the inhomogeneous dispersion of the whiskers. For the SiCw dosage of 0.1%, with further cement hydration, the hydration products filled the gaps between the whiskers, and the whiskers were wrapped by the cement matrix, significantly enhancing the toughness.

Table 6 shows the results of the three-point bending tests on the specimens. The addition of the SiCw effectively increased the initial cracking load (P_ini) of the material and delayed cracking of the crack tip. The P_ini value of the specimen with 0.1% SiCw dosage was 9.8% higher than that of the blank group, and the deformation of the specimen with 0.1% SiCw dosage before the crack tip cracked was much higher than that of the blank group (Fig. 13). It can also be seen from the CMOD of the group with 0.1% SiCw content increased by 78.7%. Obviously, the brittleness of the cement mortar modified with the SiCw decreased. The maximum load (P_max) at fracture did not significantly increase. The specimen with 0.1% SiCw was only 6.1% higher than that of the blank group, but the unstable fracture toughness (\(K_{{{\text{IC}}}}^{{{\text{un}}}}\)) and CMOD were significantly enhanced, compared with the blank group, the group with 0.1% SiCw increased by 31.88% and 78.7%, respectively, which indicating that the addition of SiCw had an obvious effect on increasing the fracture toughness of the cement mortar. Among the different addition dosages of SiCw, the toughening effect was the best when the SiCw dosage was 0.1%.

The P-CMOD curves of all test groups are shown in Fig. 14. When the whisker dosage was 0.1%, the discreteness of the P_max values decreased significantly, which may have been due to the whiskers being evenly dispersed, and their filling effect improved the initial defects in the cement-based materials. With an increase in the whisker dosage, the discreteness of the P_max values also increased. When the whisker dosage reached 0.3%, the P_max value decreased, indicating that too high of a whisker dosage led to an inhomogeneous SiCW dispersion, which affected the strength of the material. The addition of SiCw improved the CMOD, and the specimens achieved an increased maximum deformation during failure, indicating that the whiskers improved the brittleness of the cement-based materials and thus consumed more energy during the fracture process.

In order to further study the fracture process of the specimens, the displacement and strain cloud of the FPZ measured during the DIC treatment are provided in Fig. 15. Since the mechanical properties of the SiCw and cement-based materials were different, and SiCw have bridging, deflection, and pull-out effects on the cement-based materials (Becher et al., 2005), the stress state of the cement-based materials tended to be more complicated after the addition of the SiCw. It can be seen from Fig. 15 that, compared with the blank group, the specimens with added whiskers had more complex crack propagation paths and consumed more energy during the fracture process.

The images from left to right correspond to P = 0.6P_max, P = 0.9P_max, P = P_max, P = 0.8P_max, and P = 0.2P_max, respectively. As can be seen from the SiCw-0 group, at P = 0.9P_max, even though the loading force was close to the maximum load that the specimen could bear, there was no obvious crack propagation at the tip of the pre-crack, but obvious cracks suddenly appeared at the tip of the pre-crack at P = P_max. However, after the addition of SiCw, there was an obvious toughening effect. Taking the SiCw-0.1 group with the best toughening effect as an example, the crack began to develop at P = 0.9P_max, and the deformation was more obvious than that of the blank group. When the load reached P = P_max, although the crack continued to develop, the crack length was far smaller than that of the blank group under the maximum load. As the loading process continued, the crack development in the blank group was basically stable at P = 0.8P_max, but at this moment, the height of the unfractured zone of the specimen doped with SiCw was still very large, and the crack almost ran through the specimen until P = 0.2P_max. These results indicated that the addition of SiCw obviously improved the brittleness of cement-based materials. Because cement-based materials are quasi-brittle materials, there was no significant deformation under stress, and fracture failure occurred suddenly once the stress surpassed the tensile strength. However, after the addition of SiCw, the deformation of the specimens under stress increased obviously. From the point of view of crack development, the cracks in the specimen modified by the SiCw developed slowly, and the failure was no longer sudden.

Fig. 16 shows the SEM results and EDS spectra measured at the crack in the SiCw-modified cement mortar. As can be seen, there were needle-like structures in the fractured positions of the specimens, which were confirmed to be SiCw by the EDS results.

The effectiveness of whiskers in enhancing the mechanical performance of mortar is dependent to a large extent on the whiskers–mortar interactions. There were bridging (Fig. 16a) and pull-out (Fig. 16b) phenomena from the SiCw in the fractured surface of the material. The pulled out SiCw surface was not bonded with a lot of cement matrix, the adhesional and frictional bonding between a SiC whisker and cementitious matrix may be relatively weak. They had however significant contribution and practical significance because of SiCw have a large surface area (Bentur & Mindess, 2006).When there was a shear stress at the interface between the cementitious matrix and SiCw, the crack deflected significantly, and the whiskers fractured or pulled out of the matrix, thus consuming extra strain energy during the stress process. When the cement mortar was under tension, bridging of the whiskers effectively prevented the micro-crack propagation. This may have been the main mechanism of the SiCw reinforcement of the cement-based materials.