*2.2. Mix Proportions*

*2.2. Mix Proportions*  Table 2 shows the mix proportions of concrete. The total amount of binder was 400 kg/m3. A water/binder ratio of 0.4 and a sand ratio of 0.44 were selected. Two substitution rates of ultrafine metakaolin (9% and 15% by mass) were used, corresponding to sample M1 and sample M2. The plain cement concrete sample (sample C), concrete sample con-Table 2 shows the mix proportions of concrete. The total amount of binder was 400 kg/m<sup>3</sup> . A water/binder ratio of 0.4 and a sand ratio of 0.44 were selected. Two substitution rates of ultrafine metakaolin (9% and 15% by mass) were used, corresponding to sample M1 and sample M2. The plain cement concrete sample (sample C), concrete sample containing 9% silica fume (sample S1) and concrete sample containing 15% silica

fume (sample S2) were regarded as the reference samples. The superplasticizer dosages by mass percent of the total cementitious materials in samples C, M1 and M2 were 0.8%, 0.85% and 0.95%, respectively. The fresh concrete was poured into different sizes of molds, including 100 mm × 100 mm × 100 mm and 100 mm × 100 mm × 400 mm. Only three mix proportions (samples C, M1 and M2) were used for the mortar test. The water/binder ratio was the same as concrete (0.4), and the sand/binder ratio was 3.0. Fresh mortar was poured into steel molds with dimensions of 40 mm × 40 mm × 160 mm for sulphate attack test. After 24 h, all samples were unmolded and cured under scheduled regimes.


**Table 2.** Mix proportions of concretes/kg·m−<sup>3</sup> .

### *2.3. Curing Conditions and Test Methods*

In the fresh state, the workability of fresh concrete was determined according to Chinese National Standard GB/T 50080-2016. The fresh concrete was poured into the slump bucket (upper 100 mm, lower 200 mm, and height 300 mm). After each pouring, a tamper bar was used to hammer it evenly 25 times. After tamping, the bucket was pulled up, and the concrete collapsed due to its own weight. The height value after the slump was recorded. The slump value and loss ratio were calculated by the height difference between peak height and slump height. In this study, different methods were used to cure concrete and mortar. Concrete was cured under standard curing conditions at a constant temperature (20 ± 2 ◦C) and relative humidity (95 ± 1%). The compressive strength of concrete after 1 d, 3 d, 7 d, 28 d and 90 d and the splitting tensile strength after 28 d and 90 d were measured according to China National Standards GB/T 50081-2011. Tests of compressive strength and splitting tensile strength were performed after casting at a certain age by using three specimens for each test. The chloride ion penetrability resistance and freeze–thaw resistance of concrete were determined by the American Society of Testing Materials Standard ASTM C1202 and Chinese National Standard GB/T 50082- 2009, respectively. For the chloride ion penetrability resistance test, the cube specimen of 100 mm × 100 mm × 100 mm was used to cut one specimen of 50 mm × 100 mm × 100 from the middle. The charge passed in 6 h was used to evaluate the chloride ion penetrability resistance. Three test blocks were tested for each group of specimens. For the freeze–thaw resistance test, 100 mm × 100 mm × 100 mm cube blocks were used. The mass change and dynamic elasticity modulus were tested after 300 cycles. The average value for six specimens was obtained to ensure the accuracy of the test. For the connected porosity test, the cut specimen of 50 mm × 100 mm × 100 mm was used. The connected porosity was measured by the vacuum saturation–drying method. The connected porosity P was determined using the Equation (1):

$$\mathbf{P} = (\mathbf{m}\_1 - \mathbf{m}\_2) / \mathbf{m}\_1 \times 100\% \tag{1}$$

where m<sup>1</sup> is the mass of concrete that was completely saturated with water by the vacuum saturation method for 3 d and m<sup>2</sup> represents the mass of concrete that was dried at 40 ◦C for 14 d.

Two curing conditions for mortar were used: 3 d and 7 d initial moist curing. After initial moist curing, all mortars were placed in a natural environment. The compressive strength was tested after 28 d, and then the mortar was semi-immersed in a solution containing 10% sodium sulphate (by mass) for 28 d, 56 d and 90 d. The concentration of the sodium sulphate solution was maintained by periodically replacing the solution. Meanwhile, the reference specimens cured in water for the same lengths of time were tested for compressive strength and flexural strength. Therefore, the sulphate attack resistance was evaluated by the relative compressive strength and flexural strength loss for the same curing time.

## **3. Results and Discussion**

### *3.1. Workability*

Table 3 presents the values of slump and slump loss for all mixtures with different dosages of ultrafine metakaolin. No segregation or bleeding was observed during mixture experiments. Obviously, the workability of fresh concrete and replacement rate of ultrafine metakaolin are nonlinearly related in Table 3. Compared to plain cement concrete, adding 9% ultrafine metakaolin decreases workability, while adding 15% ultrafine metakaolin has little influence on workability. The addition of 9% ultrafine metakaolin results in a smaller average particle size and larger specific surface area of the composite binder, which leads to the availability of less free water in the concrete matrix. Therefore, sample M1 has poorer workability. However, sample M2 has the highest content of superplasticizer compared to sample C, which seriously weakens the water absorption effect of ultrafine metakaolin particles. Compared to silica fume concrete with the same replacement rate and superplasticizer content, ultrafine metakaolin concrete has a higher slump value. This indicates that the compatibility of the superplasticizer and ultrafine metakaolin concrete is better. After 0.5 h, the slump loss values show the same change tendency as the slump. However, the slump loss ratio is significantly changed. Compared to plain cement concrete, adding 9% ultrafine mineral admixtures and 6.25% polycarboxylate superplasticizer has little influence on the slump loss ratio. The addition of 15% ultrafine mineral admixtures and 18.75% polycarboxylate superplasticizer obviously decreases the slump loss ratio. Adding ultrafine metakaolin has a more effective impact.

**Table 3.** Slump and slump loss of fresh concretes.


### *3.2. Mechanical Strength*

The compressive strength and splitting tensile strength of all concrete are shown in Figure 2. Overall, adding mineral admixtures increases the compressive strength of concretes at all ages, as shown in Figure 2a. It is more obvious at the later ages. However, different admixture dosages have various effects on ultrafine metakaolin concrete. The compressive strength of sample M1 is slightly higher than that of sample M2 at 1 d, and it is obviously higher than that of sample M2 at 3 d and 7 d. At 1 d, the early compressive strengths of all concretes are approximately 15 MPa. At 3 d, the compressive strength of plain cement concrete is 30 MPa, while the maximum strength is nearly 40 MPa (sample M1). At 7 d, the compressive strengths of all concretes increase by approximately 10 MPa. However, the opposite trend occurs at 28 d and 90 d. At 28 d, the compressive strengths of sample C and sample M1 increase by approximately 10 MPa, but the compressive strength of sample M2 reaches 62 MPa, increasing by 17 MPa. Therefore, adding 15% ultrafine metakaolin can increase the strength of concrete to meet the strength requirements of C60. The compressive strength of the composite concrete increases slowly from 28 d to 90 d. Thus, sample M2 has the highest compressive strength at late ages. Compared

to plain cement concrete, adding 15% ultrafine metakaolin increases the compressive strength at 28 d and 90 d by 24% and 20%, respectively. There is not much difference in the compressive strengths of ultrafine metakaolin concrete and silica fume concrete with the same replacement rate and superplasticizer content at all ages. Remarkably, the 7 d compressive strengths of all concretes reached nearly 72–83% of the 28-d strength, which suggests that a high superplasticizer content has no obvious negative effect on the early strength of concrete. the same replacement rate and superplasticizer content at all ages. Remarkably, the 7 d compressive strengths of all concretes reached nearly 72–83% of the 28-d strength, which suggests that a high superplasticizer content has no obvious negative effect on the early strength of concrete.

**Figure 2.** Mechanical strength of concrete: (**a**) compressive strength; (**b**) splitting tensile strength. **Figure 2.** Mechanical strength of concrete: (**a**) compressive strength; (**b**) splitting tensile strength.

Figure 2b shows the same influence of ultrafine mineral admixtures on the splitting tensile strength and compressive strength. The results show that the splitting tensile strength of concrete increases as the ultrafine admixture content increases. There is little difference in the splitting tensile strength of ultrafine metakaolin concrete and silica fume concrete with the same replacement rate and superplasticizer content at 28 d and 90 d. Adding 9% ultrafine metakaolin increases the splitting tensile strength by approximately 17% at 28 d and 90 d. Adding 15% ultrafine metakaolin increases the splitting tensile Figure 2b shows the same influence of ultrafine mineral admixtures on the splitting tensile strength and compressive strength. The results show that the splitting tensile strength of concrete increases as the ultrafine admixture content increases. There is little difference in the splitting tensile strength of ultrafine metakaolin concrete and silica fume concrete with the same replacement rate and superplasticizer content at 28 d and 90 d. Adding 9% ultrafine metakaolin increases the splitting tensile strength by approximately 17% at 28 d and 90 d. Adding 15% ultrafine metakaolin increases the splitting tensile strength at 28 d and 90 d by approximately 33% and 30%, respectively. The maximum splitting tensile strength is close to 7 MPa at 90 d (samples M2 and S2).

### strength at 28 d and 90 d by approximately 33% and 30%, respectively. The maximum *3.3. Connected Porosity*

splitting tensile strength is close to 7 MPa at 90 d (samples M2 and S2). *3.3. Connected Porosity*  The connected porosity of concrete is an index used to measure the transport capacity of water and solution erosion, which is closely related to permeability. The connected porosities of all concretes at 28 d are shown in Figure 3. Remarkably, the 28-d connected porosity values of all concretes are in the 11–14% range. As shown in Figure 3, the connected porosity of concrete obviously decreases with increasing ultrafine metakaolin content. The connected porosity of silica fume concrete is relatively lower than that of ultrafine metakaolin concrete at the same replacement rate and superplasticizer content. This indicates that the addition of ultrafine mineral admixtures refines the pore structure The connected porosity of concrete is an index used to measure the transport capacity of water and solution erosion, which is closely related to permeability. The connected porosities of all concretes at 28 d are shown in Figure 3. Remarkably, the 28-d connected porosity values of all concretes are in the 11–14% range. As shown in Figure 3, the connected porosity of concrete obviously decreases with increasing ultrafine metakaolin content. The connected porosity of silica fume concrete is relatively lower than that of ultrafine metakaolin concrete at the same replacement rate and superplasticizer content. This indicates that the addition of ultrafine mineral admixtures refines the pore structure of concrete and makes the matrix denser, which improves the durability of concrete. This result is consistent with the findings of Erhan et al. [22], who found that ultrafine metakaolin substantially enhanced the pore structure of concrete and reduced the presence of harmful large pores, especially at a high replacement level. The beneficial effect of adding silica fume is slightly better than that of adding ultrafine metakaolin.

ing silica fume is slightly better than that of adding ultrafine metakaolin.

of concrete and makes the matrix denser, which improves the durability of concrete. This result is consistent with the findings of Erhan et al. [22], who found that ultrafine metakaolin substantially enhanced the pore structure of concrete and reduced the presence of harmful large pores, especially at a high replacement level. The beneficial effect of add-

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**Figure 3.** Connected porosity of concrete. According to ASTM C1202, the penetrability grades of concrete at 28 d and 90 d are shown in Figure 4. Apparently, Figure 4 shows that the penetrability grades of samples

### **Figure 3.** Connected porosity of concrete. *3.4. Chloride Ion Penetrability of Concrete* C, M1 and M2 at 28 d are ''moderate", "low" and "very low", respectively. At 90 d, alt-

*3.4. Chloride Ion Penetrability of Concrete*  According to ASTM C1202, the penetrability grades of concrete at 28 d and 90 d are shown in Figure 4. Apparently, Figure 4 shows that the penetrability grades of samples C, M1 and M2 at 28 d are ''moderate", "low" and "very low", respectively. At 90 d, although the charge passed by all concretes decreases due to lower connected porosity, there is no change in penetrability levels. Meanwhile, the penetrability grades of ultrafine me-According to ASTM C1202, the penetrability grades of concrete at 28 d and 90 d are shown in Figure 4. Apparently, Figure 4 shows that the penetrability grades of samples C, M1 and M2 at 28 d are "moderate", "low" and "very low", respectively. At 90 d, although the charge passed by all concretes decreases due to lower connected porosity, there is no change in penetrability levels. Meanwhile, the penetrability grades of ultrafine metakaolin concrete and silica fume concrete with the same replacement rate show little difference at 28 d and 90 d for the same superplasticizer content. This result is attributed to similar pore structures. Therefore, adding ultrafine mineral admixtures can improve the resistance to chloride ion penetration of concretes at 28 d and 90 d. Ultrafine metakaolin has the same effect as silica fume. hough the charge passed by all concretes decreases due to lower connected porosity, there is no change in penetrability levels. Meanwhile, the penetrability grades of ultrafine metakaolin concrete and silica fume concrete with the same replacement rate show little difference at 28 d and 90 d for the same superplasticizer content. This result is attributed to similar pore structures. Therefore, adding ultrafine mineral admixtures can improve the resistance to chloride ion penetration of concretes at 28 d and 90 d. Ultrafine metakaolin has the same effect as silica fume.

takaolin concrete and silica fume concrete with the same replacement rate show little dif-

Freeze–thaw damage is the main factor affecting the instability of concrete structures

in cold areas, which seriously threatens the safety and service life of concrete structures. The relative dynamic elasticity modulus and mass loss of all concretes after 300 freeze– thaw cycles are presented in Figure 5a,b, respectively. Figure 5a shows that the relative dynamic elasticity moduli of samples M1–S2 are all 81–86% after 300 cycles; therefore, sample M2 has the best freeze–thaw resistance, and its relative dynamic elasticity modulus is 85.5%. However, the relative dynamic elasticity modulus of plain cement concrete

sample M2 has the best freeze–thaw resistance, and its relative dynamic elasticity modulus is 85.5%. However, the relative dynamic elasticity modulus of plain cement concrete

**Figure 4.** Chloride ion penetrability of concrete. **Figure 4.** Chloride ion penetrability of concrete.

### *3.5. Freeze–Thaw Resistance*

*3.5. Freeze–Thaw Resistance* 

*3.5. Freeze–Thaw Resistance*  Freeze–thaw damage is the main factor affecting the instability of concrete structures in cold areas, which seriously threatens the safety and service life of concrete structures. The relative dynamic elasticity modulus and mass loss of all concretes after 300 freeze– Freeze–thaw damage is the main factor affecting the instability of concrete structures in cold areas, which seriously threatens the safety and service life of concrete structures. The relative dynamic elasticity modulus and mass loss of all concretes after 300 freeze–thaw cycles are presented in Figure 5a,b, respectively. Figure 5a shows that the relative dynamic

**Figure 4.** Chloride ion penetrability of concrete.

mass loss.

elasticity moduli of samples M1–S2 are all 81–86% after 300 cycles; therefore, sample M2 has the best freeze–thaw resistance, and its relative dynamic elasticity modulus is 85.5%. However, the relative dynamic elasticity modulus of plain cement concrete is only 62.8%, which is much less than those of composite concretes. Figure 5b shows that the mass loss of plain cement concrete exceeds 5%. However, the mass loss of samples M1–S2 is very small, less than 4% after 300 cycles in Figure 5b. Therefore, ultrafine metakaolin concrete and silica fume concrete can meet the frost resistance requirements in cold areas; they have the lowest frost resistance grade of F300. With increasing ultrafine metakaolin or silica fume content, the relative dynamic elasticity modulus of concrete tends to increase, and the mass loss decreases obviously. Meanwhile, it can hardly observe surface denudation of samples M1, S1, M2 and S2, and the surface damage layers of these samples above are very thin. Thus, composite concrete has excellent apparent performance. Therefore, adding ultrafine metakaolin or silica fume has a positive influence on the freeze–thaw resistance of concrete. M1–S2 is very small, less than 4% after 300 cycles in Figure 5b. Therefore, ultrafine metakaolin concrete and silica fume concrete can meet the frost resistance requirements in cold areas; they have the lowest frost resistance grade of F300. With increasing ultrafine metakaolin or silica fume content, the relative dynamic elasticity modulus of concrete tends to increase, and the mass loss decreases obviously. Meanwhile, it can hardly observe surface denudation of samples M1, S1, M2 and S2, and the surface damage layers of these samples above are very thin. Thus, composite concrete has excellent apparent performance. Therefore, adding ultrafine metakaolin or silica fume has a positive influence on the freeze–thaw resistance of concrete.

is only 62.8%, which is much less than those of composite concretes. Figure 5b shows that the mass loss of plain cement concrete exceeds 5%. However, the mass loss of samples

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 **Figure 5.** Results of fast freeze-thaw cycle tests of concretes after 300 cycles: (**a**) relative dynamic elasticity modulus; (**b**) mass loss.
