2.1.2. Cement

Ordinary Portland cement with a grade of 42.5 was used in this study. The physical and mechanical properties of the cement measured in accordance with the Chinese standards are presented in Table 2.


#### 2.1.3. Natural Sand and Crushed Stone

The natural sand and crushed stone used in this research were sampled from a construction site in Huai'an, China. Sieve testing was performed in accordance with JCJ 52-2006 [24]. The grain size distribution curves of the natural sand and crushed stone are presented in Figures 3 and 4, respectively. The flat–elongated particle content, crushing value, clay content, and other parameters were determined in accordance with Chinese standards, and the test results are presented in Table 3. Thewaterusedformanufacturingthemortar andconcreteinthisresearchwaslocaltapwater.

**Figure 4.** Grain size distribution curve of crushed stone.

**Table 3.** Material properties of natural sand and crushed stone.


#### 2.1.4. Mix Proportion Design

The mix proportion of mortar, determined according to GB/T 17671-199 [25], is shown in Table 4. The mix ratio of materials in mortar should be one cement, three standard sand, and one-half water by weight. Here, standard sand refers to natural quartz sea sand with SiO2 content greater than 96%. After washing and sieving, it was processed into standard sand that meets the ISO requirements.

Concrete with a strength grade of C30 was prepared in the laboratory. The mixture ratio design of the concrete was carried out in accordance with JGJ 55-2000 [26]. Firstly, the mixed strength of the concrete was determined according to the strength grade of the designed concrete. Then, the water–cement ratio (W/C) was calculated based on the mixed strength of the concrete and strength value of the cement. Next, according to the slump and particle size of the stone, the water consumption was determined, and the amount of cementing material was calculated. Finally, the sand ratio was selected, and the amount of sand and crushed stone was calculated according to the maximum particle size of the stone and fineness modulus of the sand.

The polycarboxylic water-reducing agen<sup>t</sup> mixed into the concrete accounted for 0.8% of the cementing material by volume. The water reducing rate was up to 29%. The final concrete mix proportions are shown in Table 4.


**Table 4.** Mix proportions of materials in the mortar and concrete.

#### *2.2. Combination Scheme*

Laboratory tests were performed to evaluate the effect of BA content on concrete under sulphate attack. In addition, strength tests were conducted on the mortar specimens to evaluate the hydration activity of BA. The combination scheme of cementitious materials for mortar and concrete is presented in Table 5. In this study, five groups of mortar and concrete samples with BA content from 10% to 30% were prepared. In addition, a group of samples without BA was prepared. Each group consisted of three specimens.


**Table 5.** Cementitious material combination scheme for mortar and concrete.

## *2.3. Experimental Methods*

#### 2.3.1. Mechanical Property Measurements

The specimen preparation and strength tests of the mortar were conducted in accordance with GB/T 17671-199 [25]. The well-stirred mixtures of cement, BA, sand, and water were put into a mold (40 × 40 × 160 mm) fixed on a vibrating table; after vibrating 120 times, the mixtures together with the mold were stored in a curing room (maintained at 20 ± 2 ◦C and no less than 95% RH). Then, form stripping was carried out. After 28 days of curing in standard conditions, the flexural and compressive strengths of the mortar were determined using a bending and compression tester.

The activity index of BA was determined with reference to GB T1596-2017 [27]. According to this standard, the compressive strengths of mortar with 30% and without fly ash were tested under a water-to-binder ratio of 0.5. Here, the activity index is defined as the compressive strength of the

mortar with BA divided by the compressive strength of the mortar without BA, and it can be calculated as follows:

$$H = \frac{R}{R\_0} \times 100\tag{1}$$

where *H* is the activity index (%); *R* is the compressive strength of mortar with BA at 28 days of curing (MPa); and *R*0 is the compressive strength of mortar without BA at 28 days of curing (MPa).

The concrete specimens were prepared and cured according to GB/T 50081-2002 [28]. All the materials were accurately weighed and put into the mixing pan. After stirring, specimens 100 × 100 × 100 mm in size were manufactured by the vibration molding method. Next, all specimens were left to stand for 24 h at a temperature of 20 ± 5 ◦C. In order to evaluate the strength property of the concrete, the specimens were cured in a curing room (maintained at 20 ± 2 ◦C and no less than 95% RH). After 28 days of curing, the compressive strength of the concrete was determined using a WAW-B Electro-hydraulic universal testing machine.

For durability tests, specimens 100 × 100 × 100 mm in size were divided into two series. The first one was cured under standard curing conditions. The second one was firstly cured at T = 20 ± 2 ◦C and RH ≥ 95% for 28 days and then totally immersed in a solution containing 10% sodium sulfate for 60 days; the solutions were renewed monthly. The sulfate damage was evaluated mechanically by determining the compressive strength loss of the specimens using the following equation:

$$Strength\,\log\left(^{\circ}\right) = \frac{R\_1 - R\_2}{R\_1} \times 100\,\tag{2}$$

where *R*1 is the compressive strength of concrete specimens under standard curing conditions (MPa) and *R*2 is the compressive strength of concrete specimens in sulfate solutions (MPa).

#### 2.3.2. Concrete Porosity Measurements

The concrete porosity was determined in accordance with the standard test method [29]. The porosity of concrete can be obtained indirectly from the water loss rate of saturated concrete specimens under certain conditions. The concrete specimens were prepared and cured for 28 days in standard conditions. After vacuum saturation, the water on the specimen surface was wiped off using a dry cloth. The mass of the specimen, measured using an electronic balance, was noted down. Then the specimen was cured at RH = 90% for 30 days. When water diffusion in the concrete reached equilibrium states, the mass of the same specimen was measured again. Next, the specimen was oven-dried to a constant weight at T = 105 ◦C. The specimen was finally weighed after cooling. The concrete porosity was calculated using the following equations:

$$P\_{fin} = \frac{(M\_0 - M\_1) \times \rho\_{\overline{C}}}{M\_0 \times \rho\_w} \times 100\% \tag{3}$$

$$P\_{\text{total}} = \frac{(M\_0 - M\_2) \times \rho\_{\text{C}}}{M\_0 \times \rho\_{\text{w}}} \times 100\% \tag{4}$$

$$P\_{\text{carrsse}} = P\_{\text{total}} - P\_{\text{finc}} \tag{5}$$

where *Ptotal* is the total porosity of the concrete specimen (%); *Pfine* is the fine capillary porosity of the concrete specimen (%); *Pcoarse* is the coarse capillary porosity of the concrete specimen (%); *M*0 is the mass of the saturated concrete specimen (kg); *M*1 is the mass of the concrete specimen cured at RH = 90% for 30 days (kg); *M*2 is the mass of the dried concrete specimen; ρ*c* is the density of the concrete (kg/m3); and ρ*w* is the density of the water (kg/m3).

#### 2.3.3. Capillary Rise and Crystallization Tests

A concrete specimen 150 × 150 × 150 mm in size was prepared and cured by following the standard method. After 28 days of curing, a cylinder (100 mm in diameter and 150 mm in length) was drilled from the specimen (Figure 5a). The cylinder part was oven-dried to a constant weight at T = 60 ◦C. Then, this part was partially immersed in water to measure the rising height of capillary water in the concrete; the test method is shown in Figure 5c. The hollow part was used to measure the capillary crystallization rate of sodium sulfate solution in the concrete. A schematic diagram of the capillary crystallization tests is shown in Figure 5b. The bottom surface was firstly sealed. Then, a solution containing 5% sodium sulfate was poured into the cavity. The crystallization of sodium sulfate solution from the side wall and the thickness of concrete wall were observed and recorded every half hour. The above tests were performed at T = 20 ± 2 ◦C and RH = 60% ± 5%. The capillary crystallization rate was calculated using the following equation:

$$V\_{\sf s} = L/T \tag{6}$$

where *V*s is the capillary crystallization rate (cm/s); *L* is the thickness of the sidewall (cm); and *T* is the time of original crystallization on the concrete surface (s).

**Figure 5.** Schematic diagram of the capillary rise and crystallization tests (**a**) Concrete specimen without core sample; (**b**) 1-1 Profile of Figure 5a; (**c**) Core sample.

#### 2.3.4. Solution Absorption Measurements

A specimen (100 mm in diameter and 100 mm in length) was prepared and cured for 28 days. Next, the solution absorption was measured in the laboratory; a schematic diagram is shown in Figure 6. All the side faces of the concrete specimen were sealed using epoxy resin. The specimen was oven-dried to a constant weight at T = 60 ◦C for no less than 12 h and was weighed after cooling. Then, the specimen was placed in a solution containing 5% sodium sulfate, keeping the water surface 5 mm above the bottom surface. The solution temperature was kept constant at 20 ◦C. The water on the specimen surface was wiped off using a dry cloth, and the weight of the specimen was measured at regular intervals.

**Figure 6.** Schematic diagram of solution absorption measurements.

#### **3. Results and Discussion**

#### *3.1. Strength Properties of Mortar and Concrete*

The flexural and compressive strengths of the mortar samples with various contents of BA at 28 days of curing are shown in Table 6. Both the compressive and flexural strengths of the mortar decreased with increasing BA content, which is similar to the results of previous research [30–32]. The compressive strength of mortar showed a large decrease at the addition of BA from 10% to 20%. There was not a significant decrease in compressive strength at the addition of BA from 20% to 25%. Beyond 25%, the compressive strength decreased significantly again. Similar to the compressive strength, with increasing BA, the flexural strength also showed a rapid reduction first and then a gentle reduction, followed by another significant reduction.

Low hydration activity of BA was obtained, and the activity index was only 43% with the addition of 30% BA. The hydration activity of BA was apparently smaller in comparison with the records in the literature [33]. In the literature, a low W/C value of 0.38 was used. However, a similar industrial mineral admixture, electric arc furnace dust, has high hydration activity with W/C values from 0.35 to 0.7 [34]. Hence, the W/C of 0.5 used in this study is not the main cause of the low hydration activity. The samples in previous research were prepared by melting the MSWI fly ash at a high temperature and then water-quenching [33]. In this study, the BA was prepared by artificially removing the impurities. In addition, the particle size of the BA sample was controlled under 180 μm, making it hard to densify the microscopic structure of the mortar. In addition, our BA sample had a higher content of SiO2 and a lower amount of CaO in comparison with the finer samples [35], and CaO may participate in the cement hydration process.


**Table 6.** Flexural strength and compressive strength of mortar samples with various contents of BA.

Figure 7 shows the compressive strength values of the concrete samples with di fferent contents of BA at 28 days of curing. The compressive strength of the concrete decreased gradually with increasing addition of BA. In addition, the water/cement ratio (W/C) had a significant influence on the compressive strength: the larger the W/C, the smaller the compressive strength. The concrete samples with the addition of 10%, 15%, and 20% BA at a W/C of 0.35 met the strength requirement of C30, at 37 MPa, 34 MPa, and 32 MPa, respectively. When the W/C was 0.40, only the concrete samples with the addition of 10% and 15% BA met the strength requirement, at 35 MPa and 31 MPa, respectively. Unfortunately, the compressive strengths of all concrete specimens at a W/C of 0.35 were less than 30 MPa. This shows that the addition of BA could not improve the strength performance of the concrete due to its relatively low hydration activity.

**Figure 7.** Influence of BA content on the compressive strength of concrete.
