Abstract
Five particle sizes and six dosages of polymerized aluminum chloride (PAC) residue were taken to prepare polymerized aluminum chloride residue cement mortar (PACRM). The compressive and flexural strengths were tested, and SEM analysis was performed. The results showed that the change pattern of strength of PAC waste residue mixed into cement mortar was similar to that of fly ash at the same substitution rate. However, the strength of PACRM decreased with the increase of substitution rate. The flexural and compressive strengths of PACRM reached their peak at all ages after replacing cement with PAC waste residue with particle size of 0-0.075 mm and 30% doping. Their strength ratios were greater than 62%. It does not contribute much to the early strength development of PACRM, but contributes more to the later strength development after the cement is replaced by PAC waste residue. PAC waste residue mainly acts as microaggregate and fills into the mortar pores at low substitution rate. The cracks of the matrix structure are filled by C-S-H, so the mortar structure is more dense and the material strength is enhanced. When the substitution rate increases, the water requirement of mortar increases, resulting in an uncompact structure. The internal pores increase after hardening, and the mortar strength decreases rapidly.
1. Introduction
Polymeric aluminum chloride is a commonly used water purifier with excellent flocculation properties [1, 2]. The raw material is mainly bauxite and other minerals with high aluminum content [3, 4]. However, a certain amount of PAC waste residue will be produced in the production process. The main components are SiO2 and Al2O3 after harmless treatment [5–7]. Its properties are similar to those of polymerized ferric sulfate aluminum waste slag and red mud, so it has a broad prospect of application in the field of construction materials.
In order to reduce pollution, numerous scholars have conducted a lot of research in the resource utilization of industrial waste and achieved some results. For example, red mud is currently used in magnesium phosphate cement [8, 9], ultra-high-performance concrete [10, 11], and environmental remediation agents [12, 13] after treatment. However, many studies have shown that the particle size and admixture of industrial waste sludge can have a significant effect on the performance of cement mortar [14–16]. Li et al. [17] showed that red mud could be effectively mechanically activated by means of particle size reduction. Venkatesh et al. [18] found that the workability of concrete decreases, but the stability of concrete increases with the increase of red mud. The particle size of red mud helps to improve the durability property of concrete. Zhang et al. [19] found that the compressive strength of cement mortar decreased and then increased with the increase in the average particle size of the mineral admixture. Bicer [20] investigated the effect of fly ash particle size on the mechanical properties of composites. It was found that the density of cement mortar increases as the particle size of fly ash incorporated decreases, and the porous structure of cement mortar is effectively filled by fly ash. However, the compressive strength of cement mortar decreases in a certain proportion with the increase of fly ash admixture. PAC waste residue has a large amount of active substances similar to red mud and fly ash, such as SiO2 and Al2O3, so it is more feasible to apply it in the construction field. At present, there is less research on PAC waste residue in the construction field, especially on the nature of PAC waste residue; the particle size and the amount of admixture applicable to the project have not been reported.
PAC waste residue was crushed and sieved for the purpose of investigating the effect law of variation of particle size and admixture of PAC waste residue on the strength of cement mortar. The compressive and flexural strength tests of cement mortar were carried out for different particle sizes and admixtures of PAC waste residue. The effect of PAC waste residue on the strength of cement mortar was revealed by using SEM scanning electron microscopy. It provides theoretical reference for the application of PAC waste residue in construction projects.
2. Materials and Test Design
2.1. Materials
The raw materials of cement mortar include cement, fly ash, standard sand, PAC waste residue, and polycarboxylic acid system high-efficiency water-reducing agent. The cement is ordinary Portland cement of P.O 42.5, which is produced in Jiaozuo, Henan Province, and its properties are shown in Table 1. The fly ash is produced in Gongyi City, Zhengzhou, Henan Province, and its physical properties are shown in Table 2. The standard sand used for the test complied with Chinese standard GB/T 17671-1999. PAC waste residue was dechlorinated by Henan Gongyi Xiangji Assembly Component Co., Ltd. before use, as shown in Figure 1. The water-reducing agent for the test is ST-01A polycarboxylic acid system high-efficiency water-reducing agent produced in Zhengzhou, Henan Province. It is light yellow liquid with 30% solid content and 40% water reduction rate.
(a) 2.36~4.75 mm
(b) 0.6~1.18 mm
(c) 0.15~0.3 mm
(d) 0~0.15 mm
(e) 0~0.075 mm
2.2. PAC Waste Residue
2.2.1. Particle Size Analysis
The dried PAC waste residue powder and ordinary Portland cement were taken and their particle size and cumulative distribution were measured by laser particle size meter, as shown in Figure 2. The test instrument is BT-9300SE laser particle size meter of Dandong Baxter Instrument Co., Ltd. with the test range of 0.1~1200 μm. From Figure 2, it can be seen that the average particle size of PAC waste residue powder is larger than that of ordinary Portland cement. The finer PAC waste residue particles can fill the pores of the cement slurry and reduce the porosity of the material.
(a) The cumulative values
(b) Particle size range
2.2.2. XRF Testing
The main chemical composition of PAC waste residue powder material was analyzed and tested by X-ray fluorescence spectrometer of Bruker-S8 TIGER type, Germany. The dried PAC waste residue powder was taken and tested by XRF. The test results are shown in Table 3. SiO2 and Al2O3 accounted for 45.72% and 27.80% in the oxides of the PAC waste residue, respectively. These two substances are similar to the main components in fly ash and have potential hydration activity, so it is feasible to use them to replace cement as cementitious material.
2.2.3. SEM Testing
The microscopic morphology of the dried PAC waste residue was observed by SEM, and the test results are shown in Figure 3. From Figure 3(a), it can be seen that the particles of PAC waste residue are of different sizes and have irregular lumpy structure. As can be seen from Figure 3(b), the surface of individual PAC waste residue is rough with more microcracks and micropores, which can make PAC waste residue better wrapped by the cement slurry. At the same time, the rough surface increases the friction with the cement paste and reduces the workability of the mortar.
(a) 100 μm
(b) 2 μm
2.3. Specimen Preparation and Testing
The dimensions of the specimens were . Portland cement mortar is used as the reference group and is denoted by P1. The polymerized aluminum chloride residue cement mortar (PACRM) was prepared by mass substitution of 30% of cement by PAC waste residue as a control group. P2, P3, P4, P5, and P6 denote PACRM with particle sizes ranging 0~0.075 mm, 0~0.15 mm, 0.15 mm~0.3 mm, 0.6 mm~1.18 mm, and 2.36 mm~4.75 mm, respectively, and the mix proportion is shown in Table 4. In the particle size range of 0-0.075 mm, PACRM was prepared by replacing 5%, 10%, 15%, 20%, 25%, and 30% of cement with equal amounts of PAC slag, denoted as PACRM5, PACRM10, PACRM15, PACRM20, PACRM25, and PACRM30, respectively. FM5, FM10, FM15, FM20, FM25, and FM30 denote mortar specimens prepared with 5%, 10%, 15%, 20%, 25%, and 30% of fly ash instead of silicate cement. The mix proportion is shown in Table 5. PAC5 and FM5 denote the test block with 5% PAC waste residue and 5% fly ash, respectively.
The demolded specimens were placed in a constant temperature maintenance chamber and maintained until day 7, day 14, and day 28 when they were removed. The specimens were then subjected to flexural strength, compressive strength, and SEM tests.
3. Results and Analysis
3.1. Effect of Different Particle Sizes of PAC Waste Residue on the Mechanical Properties of PACRM
3.1.1. Compressive Strength
Figure 4 shows the trend of compressive strength and strength ratio of each group of PACRM specimens. The compressive strength ratio is the ratio of the compressive strength of each group of specimens mixed with PAC slag to that of group P1. The main purpose is to reflect the effect of PAC slag on the compressive strength of concrete more visually. For the PAC waste residue powder materials in groups P2, P3, P4, P5, and P6, the compressive strength of PACRM specimens at all ages was lower than that of group P1, and the strength decreased with the increase of waste particle size. The compressive strengths of the P2 and P3 groups were significantly higher than those of the P4, P5, and P6 groups. The compressive strength of PACRM specimens reached a peak of 37.98 MPa at day 28 with a compressive strength ratio of 74.96% when the PAC waste residue particle size ranged from 0 to 0.075 mm. The P3 group was the next highest, with compressive strength and strength ratio of 35.71 MPa and 70.48%, which were greater than 62%, respectively. The compressive strength of the PAC waste residue was increased to 28.18 MPa, and the compressive strength ratio was only 55.62%. In summary, the size of 0~0.075 mm PAC waste residue is more suitable for use as admixture for cement.
(a) Compressive strength with ages
(b) Compressive strength ratio of PACRM
3.1.2. Flexural Strength
The trend of flexural strength and strength ratio from P1 to P6 is shown in Figure 5. The flexural strength of PACRM specimens at all ages was lower than that of group P1 compared with groups P2, P3, P4, P5, and P6, and the strength showed a decreasing trend with the increase of the particle size range of slag. For the flexural strength of PACRM mixed with different particle size range of slag, the flexural strengths of P2 and P3 groups at all ages were significantly higher than those of P4, P5, and P6 groups, with flexural strengths of 6.57 MPa and 6.24 MPa at day 28, respectively, and flexural strength ratios of 85.77% and 81.46%. The flexural strength ratio decreases rapidly when the PAC waste residue particle size range is greater than 0.15 mm, and the lowest value is 67.49%, indicating that the size of PAC waste residue particle size has a more obvious effect on the flexural strength of cement mortar.
(a) Flexural strength with ages
(b) Flexural strength ratio of PACRM
Figure 6 shows the damage section of the test block after the flexural test. There are only a few tiny holes in the test block cross section of P1 and P2 groups, and the mortar structure is denser, while there are more tiny holes in the cross section of P3, P4, P5, and P6 groups, and the internal structure of the test block is loose. Because PAC waste residue particles have no water hardness, they do not have strength by themselves. When the particle size of PAC waste residue becomes small, it can be better mixed with the cement slurry and filled into the cement mortar pores, thus improving the mortar strength. In addition, PAC slag as a cementitious material contains reactive substances such as SiO2 and Al2O3 which also undergo hydration reactions together with cement, and the reaction generates C-S-H gels which also have a strengthening effect on the mortar. At the same time, yellow PAC waste residue particles can be clearly observed in the cross section of specimens from groups P4, P5, and P6. PAC waste residue did not mix and react with cement mortar sufficiently, but was only wrapped in the cement mortar. It indicates that the PAC waste residue with larger particle size is not mixed with the cement when the particle size range of PAC waste residue increases, but acts as the skeleton role of the aggregate filled in the cement mortar. Since the strength of the waste slag is lower than that of the standard sand, the bearing capacity of the cement mortar specimen at the time of destruction is reduced. As a result, the strength of the specimen decreases rapidly with the increase of the particle size of the PAC waste residue.
(a) P1
(b) P2
(c) P3
(d) P4
(e) P5
(f) P6
3.2. Effect of Different Dosing of PAC Waste Residue on the Mechanical Properties of PACRM
3.2.1. Compressive Strength
Figure 7(a) shows the compressive strength of each group of cement mortar at day 7. It was found that the strength of mortar specimens continued to decrease with the increase of PAC waste residue and fly ash admixture. The compressive strength of the P1 group specimens reached the maximum value at day 7. And when the substitution rates were 5%, 10%, and 15%, the compressive strength of the specimens in the PACRM group increased by 4.5%, 13.6%, and 5.9%, respectively, compared to the FM group for the same substitution rates. It is shown that the early strength enhancement effect of PAC waste residue on cement mortar is better than that of fly ash when the mass replacement ratio is within 15%, which is due to the water-reducing effect played by smooth spherical particles in fly ash [21]. And the particle surface of PAC waste residue is rough and porous, which is very easy to bond with cement slurry. The smaller particle size can fill into the voids of the slurry to reduce the porosity and thus improve the early strength of the cement mortar. When the substitution rate was higher than 20%, the strength of the FM group decreased more than that of the PACRM group. The compressive strength of the PACRM30 group was 16% higher than that of the FM30 group. Because a large amount of PAC waste residue is mixed in place of cement, it does not fully participate in the hydration reaction due to its low activity. It more just acts as a filler of fine aggregates, thus reducing the rate of compressive strength loss.
(a) Day 7
(b) Day 28
From Figure 7(b), it can be seen that the compressive strength of PACRM and FM specimens at day 28 showed a trend of increasing and then decreasing with the increase of substitution rate. The compressive strength of specimens in the P1 group was 60.4 MPa. At 5% substitution rate, the compressive strength of PACRM increased to 62.3 MPa, which was similar to that of FM5 and higher than that of the P1 group. This is because the water-cement ratio of cement mortar does not change much at the lower substitution rate. PAC waste residue is mixed in and mainly acts as a microaggregate, filling into the pores of the mortar. The compactness of the mortar structure was increased, which was beneficial to the later strength development. The strength of cement mortar started to decrease with the increase of substitution rate at day 28. It can be seen from the figure that the strength of the PACRM group decreases sharply, and the strength is only 44.5 MPa at 30% substitution rate. It may be due to the relatively large water requirement of PAC waste residue. The mortar is not easily compacted as the amount of PAC waste residue replaces cement increases. Therefore, the specimens had more internal pores after hardening and the strength decreased rapidly. At the same time, the quantity of hydration products decreases with the decrease of cement dosage, which is also the reason for the direct decrease of strength.
3.2.2. Flexural Strength
From Figure 8(a), it can be seen that the flexural strength of cement mortar on day 7 decreases with the increase of PAC waste residue and fly ash. The flexural strength of the benchmark group P1 reached a maximum value of 6.9 MPa on the seventh day. The flexural strengths of the test blocks were 6.5 MPa, 6.2 MPa, and 5.7 MPa for 5%, 10%, and 15% substitution rates of PAC waste residue. The flexural strengths of the test blocks in the fly ash group rate were 6.3 MPa, 6.1 MPa, and 5.6 MPa at the same substitution. It indicates that when the substitution rate is within 15%, the PAC waste residue has a better effect than fly ash in improving the flexural strength of cement mortar on the seventh day. The flexural strength of the specimens in the PACRM group continued to decrease after the dosage was continued to increase. The flexural strength was 4.3 MPa at the substitution rate of 30%, which was lower than the strength of the reference group and the FM group.
(a) Day 7
(b) Day 28
From Figure 8(b), it can be seen that the flexural strength increases and then decreases with the increase of the substitution rate of PAC waste residue and fly ash. The peak strengths of 9.0 MPa and 9.3 MPa were reached in the PACRM5 and FM5 groups, respectively, with a small increase in the flexural strength of the mortar at 5% replacement rate of PAC waste residue. The filling of the pores is responsible for the increase in strength that occurs, which is similar to the principle of compressive strength increase. The flexural strength of the PACRM group decreased rapidly when the substitution rate increased from 10% to 30%. The flexural strength of the PACRM30 group was 6.4 MPa, which was 20.5% loss of strength compared to the baseline group.
The flexural strength of cement mortar showed an overall decreasing trend after the replacement of cement by PAC waste residue. The difference in flexural strength between the PACRM and FM groups was not obvious when the substitution rate was within 10%. The flexural strength decreased with the increase of substitution rate on day 7, and the strength was better when the substitution rate was within 15%. The flexural strength of PAC waste residue at 5% substitution rate has 2.3% increase compared with the reference group, and the flexural strength decreases rapidly after greater than 5% at day 28. It can be found that the flexural and compressive strengths of PACRM have similar trends with the replacement rate of waste slag at different ages. The strength of the PACRM group has some improvement in a certain range, sometimes higher than the FM group, which indicates that PAC waste residue material has some research value to be utilized as an external admixture.
The compressive strength ratio and strength contribution of cement mortars at different substitution rates are shown in Figure 9. It can be seen that the compressive strength ratio of PACRM is low with a value close to 1 at day 7. It indicates that the incorporation of PAC waste residue has no contribution to make the hydration process of cementitious materials in cement mortar. And the specific strength coefficient decreases with the increase of substitution rate. The compressive strength ratio of PACRM was all positive at day 28, reaching a maximum value at 5% substitution. It indicates that PAC waste residue promotes the hydration process in cement mortar. The compressive strength ratio was all greater than 1 with the continued increase in the replacement rate of PAC waste residue. It can be seen that PAC waste residue has a contributing effect on the later strength of cement mortar. From the compressive strength contribution of PACRM and FM, it is found that the change pattern is similar to the development pattern of compressive strength ratio. It indicates that PAC waste residue, like fly ash, also has a certain potential volcanic ash effect, which contributes to both the internal hydration process of mortar and the development of mortar strength.
(a) Compressive strength ratio
(b) Compressive strength contribution
3.3. SEM Analysis
Figure 10 shows the microscopic morphology of PACRM in the P1, P2, and P3 groups at different magnifications at day 7. From Figures 10(a)–10(c), it can be seen that a large amount of C-S-H (hydrated calcium silicate) and a small amount of Ca(OH)2 appear in the P1 group. Because the hydration rate of C3S (tricalcium silicate) in cement composition is the fastest, a large amount of C-S-H and Ca(OH)2 is generated after its hydration. The cracks in the matrix structure are filled by C-S-H, so the mortar structure is dense as a whole. It indicates that the hydration process of the cementitious material in group P1 is normal, and the hydration process is as follows.
(a) P1 at day 7
(b) P2 at day 7
(c) P3 at day 7
(d) P1 at day 28
(e) P2 at day 28
(f) P3 at day 28
The microstructure of PACRM changed significantly when the PAC waste residue substitution rate was 30% and the particle size ranged from 0 to 0.075 mm and 0 to 0.15 mm. The amount of C-S-H, as the main major hydration product, was reduced in the P2 and P3 groups. The microstructure of PACRM had more gaps, loose and porous structure, and increased amount of Ca(OH)2 at day 7. It indicates that the internal structure of cement mortar was deteriorated more after the particle size of PAC waste residue increased. The microstructure of the PACRM in group P2 was more dense than that in group P3 at day 7. Meanwhile, there was a small amount of Ca(OH)2 on the surface of PAC waste residue particles.
Figures 10(d)–10(f) show the microstructure of PACRM at day 28. The hydration process of the cementation system has been completed at this point. There are no obvious cracks and pores in the microstructure of the matrix of group P1, and the matrix is a continuous dense whole. The matrix structure of group P2 showed a few cracks and holes, and the matrix was in a continuous block structure, while the matrix of group P3 was in a scattered block structure. The strength is lower than that of group P2 because of the larger gap between matrix and matrix and the porous and loose matrix structure. It may be due to the fact that the PAC scrap with smaller particle size mainly acts as microaggregate. The smaller size PAC waste residue particles fill into the pores of the mortar and the porosity of the structure is reduced, thus enhancing the strength of the cement mortar. Therefore, the strength of the test block decreases rapidly when the range of PAC waste residue particle size increases.
In summary, the mechanical properties of cement mortar are better when the particle size range of PAC waste residue is less than 0.15 mm. Among them, when the particle size of PAC waste residue was 0-0.075 mm and the dosing amount was 30%, the flexural and compressive strength of cement mortar reached the peak at all ages. The strength ratio was greater than 62%, which could be applied to prepare cement mortar.
4. Conclusion
In this paper, the strength, specific strength coefficient, and strength contribution of cement mortar with different PAC waste residue particle sizes and admixtures were analyzed. The conclusions are obtained as follows. (1)The mechanical properties of cement mortar with waste slag material are better when the particle size of PAC waste residue is less than 0.15 mm. Among them, when the particle size of PAC waste residue is 0~0.075 mm and the admixture amount is 30%, the flexural and compressive strength of cement mortar reaches the peak at all ages, and the strength ratio is greater than 62%(2)After the cement mortar was incorporated with PAC waste residue, its strength development change pattern was similar to that of fly ash after substitution incorporation at the same substitution rate. However, the strength loss of PACRM mortar was greater as the substitution rate increased(3)The mortar strength decreased with the increase of PAC waste residue substitution rate at day 7, while the mortar strength showed a first increase and then decrease with the increase of substitution rate at day 28. PAC waste residue mainly plays the role of microaggregate, filling into the mortar pores and improving the material strength at low substitution rate(4)PAC waste residue did not contribute much to the early strength development of cement mortar, but it contributed more to the later strength development. The variation of specific strength coefficient and strength contribution of the PACRM group were similar to those of the FM group, which indicated that PAC waste residue had better potential activity
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
Financial supports from the Key Science and Technology Program of Henan Province, China (No. 202102310253), the Doctor Foundation of Henan Polytechnic University (No. B2016-67), the Science and Technology Project of Henan Provincial Department of Transportation, China (No. 2019J-2-13), and the Science and Technology Project of Henan Province, China (No. 222102320262) are gratefully appreciated.