S8W2 S7W3
80 70
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20
5of 11 30 Legend: AE—Alkali equivalent; Ms—Silicate modulus; W/B—Water-binder ratio; wt.%—weight percentage.
Sustainability 2022 , 14 , 13536
Figure 4. The compressive strength of SPW substitution proportion. Figure4. The compressive strength of SPW substitution proportion.
When paper sludge and wood chips were used as partial substitutes for BFS, the highest 28-day compressive strengths were 41.1 MPa and 28.8 MPa, respectively. This re- sult could be attributed to the high content of organic impurities, which disrupted the polymerization of the cement [25]. In addition, the highest compressive strength of bottom ash was 39.4 MPa, which substituted BFS after 28 days of curing. Compared to other wastes, bottom ash had some volume expansion except for the reduction in the w/b ratio of the slurry [21]. Although the volume expansion reduced the self-shrinkage of the slurry, cracks were also generated in this process, degrading the mechanical properties of the material [16]. In S8L2 and S8P2, the development of compressive strengths from 7 to 28 days ap- proached 20% of the final strength. Since the reactivity of the lime mud and the paper sludge is poorer than BFS, the weakening of compressive strength was reflected by the decreased development rate of compressive strength with an increasing ratio. Further- more, the strengths on day 28 showed a significant decrease of more than 10 MPa in the comparison between S8M2 and S7M3 or S8P2 and S7P3 (11.5 MPa and 14.0MPa, respec- tively). This result indicated that if the total proportion of lime mud and the paper sludge exceeded 20%, the polymerization in cementitious materials was incomplete [26], signifi- cantly decreasing the compressive strength. This result was similar to previous reports [11,12]. However, the other reports have expressed different opinions [27,28] due to dif- ferent pretreatment of slurry raw materials. Therefore, a total weight of 20% could be con- sidered a suitable proportion, which allowed the partial substitution of the lime mud or paper sludge for BFS. Considering the optimization of compressive strength, the stress-time curve of BFS replaced by 20% of the waste was selected as the study object. According to Figure 5, the stress-time curves for partial substitution of BFS with lime mud or bottom ash showed the characteristics of brittle materials. Moreover, the shapes of the curves were similar, with a linear increase in yield strength and a sharp decrease at the peak. This variation In S8L2 and S8P2, the development of compressive strengths from 7 to 28 days ap- proached 20% of the final strength. Since the reactivity of the lime mud and the paper sludge is poorer than BFS, the weakening of compressive strength was reflected by the decreased development rate of compressive strength with an increasing ratio. Furthermore, the strengths on day 28 showed a significant decrease of more than 10 MPa in the compari- son between S8M2 and S7M3 or S8P2 and S7P3 (11.5 MPa and 14.0 MPa, respectively). This result indicated that if the total proportion of lime mud and the paper sludge exceeded 20%, the polymerization in cementitious materials was incomplete [26], significantly decreasing the compressive strength. This result was similar to previous reports [11,12]. However, the other reports have expressed different opinions [27,28] due to different pretreatment of slurry raw materials. Therefore, a total weight of 20% could be considered a suitable proportion, which allowed the partial substitution of the lime mud or paper sludge for BFS. Considering the optimization of compressive strength, the stress-time curve of BFS replaced by 20% of the waste was selected as the study object. According to Figure 5, the stress-time curves for partial substitution of BFS with lime mud or bottom ash showed the characteristics of brittle materials. Moreover, the shapes of the curves were similar, with a linear increase in yield strength and a sharp decrease at the peak. This variation suggested that the samples were crushed and lost the stress intensity at the peak. Only the peak points of the curves differed between them, indicating that lime mud or bottom ash could not change the characteristics of brittle materials, resulting in the alkali activation of BFS. In the partial substitution of BFS with paper sludge or wood chips, the curves decreased slowly at the peak. This change indicated that the samples still exceeded the yield strength in some respects. Based on these results, paper sludge or wood chips could significantly change the properties of alkali-activated BFS from brittle to ductile [29]. When paper sludge and wood chips were used as partial substitutes for BFS, the highest 28-day compressive strengths were 41.1 MPa and 28.8 MPa, respectively. This result could be attributed to the high content of organic impurities, which disrupted the ash was 39.4 MPa, which substituted BFS after 28 days of curing. Compared to other of the slurry [21]. Although the volume expansion reduced the self-shrinkage of the slurry, cracks were also generated in this process, degrading the mechanical properties of the material [16].
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