Document Type : Research Paper
Authors
1 Department of Wood and Paper Science and Technology, Faculty of Natural Resources, University College of Agriculture & Natural Resources, University of Tehran, Karaj, Iran
2 Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran.
3 Department of Biosystems, Faculty of New Technologies and Aerospace Engineering, Zirab campus, Shahid Beheshti University, Tehran, Iran.
Abstract
Background and Objectives: The pulp and paper industry generate substantial amounts of paper sludge annually, the disposal of which imposes considerable economic costs and environmental challenges. Recycling this waste in cementitious materials offers a sustainable approach to reducing cement consumption, mitigating CO₂ emissions associated with cement production, and promoting industrial waste valorization. Although numerous studies have investigated the incorporation of paper sludge into cement-based materials, limited information is available regarding the combined effects of paper sludge replacement ratio and particle size on the physical and mechanical properties of cementitious composites, as well as the capability of statistical models to predict these effects. Therefore, the objective of this study was to evaluate the influence of paper sludge replacement ratio and particle size on the compressive strength, modulus of rupture, modulus of elasticity, water absorption, density, and porosity of cementitious composites, and to assess the applicability of Response Surface Methodology (RSM) for modeling and analyzing these responses.
Materials and Methods: In this study, cement-based composites were produced using paper mill sludge at three replacement levels of 10%, 20%, and 30% by weight of cement, along with a control sample (0% sludge), and four particle-size ranges of 0.5, 1, 1.5, and 2 mm. The experiments were conducted based on a full factorial design with three replicates, resulting in a total of 48 specimens. After curing, compressive strength, modulus of rupture, modulus of elasticity, 72-h water absorption, density, and porosity were measured. To investigate the effects of replacement percentage and particle size and to evaluate their interaction effects, analysis of variance (ANOVA) and a second-order regression model within the response surface methodology (RSM) framework were employed. The significance of the models and factor effects was evaluated at the 0.05 significance level, while the goodness of fit of the models was assessed using the coefficients of determination (R²) and adjusted coefficients of determination (Adjusted R²).
Results: The results showed that the percentage of paper mill sludge replacement was the dominant factor for most of the responses investigated, whereas the effect of particle size was generally limited within the studied range and was significant only for the modulus of elasticity (P = 0.023). The interaction effect between replacement percentage and particle size was not statistically significant for any of the responses (P > 0.05). As the replacement percentage increased, compressive strength and density exhibited decreasing trends, whereas water absorption and porosity increased. Compressive strength decreased from approximately 18.34 to 10.26 MPa, while water absorption increased from 8.85% to 38.21% and porosity from 11.89% to 35.93%. The density of the composites also decreased with increasing sludge content, indicating a reduction in the specific gravity of the resulting composites. At low replacement levels, particularly around 10%, a relative increase in compressive strength and modulus of rupture was observed; however, this trend was not statistically significant for the modulus of rupture. At higher replacement levels, increasing sludge content was associated with increased porosity and water absorption, accompanied by an overall decline in mechanical performance. In the RSM analysis, the water absorption model showed the best fit (R² = 83.9% and Adjusted R² = 72.5%), whereas the models for some responses were not statistically significant and therefore had limited predictive capability. Overall, the results demonstrated that the percentage of paper mill sludge replacement was a more influential factor than particle size in determining most of the properties of the cement-based composites.
Conclusions: The findings demonstrate that the paper sludge replacement ratio is the dominant parameter controlling the physical and mechanical performance of cementitious composites, whereas the influence of particle size between 0.5 and 2 mm is relatively limited. Incorporating low amounts of paper sludge, particularly around 10% replacement, maintained satisfactory mechanical performance while reducing cement consumption. In contrast, higher replacement levels increased water absorption and porosity, resulting in lower strength and stiffness. Overall, paper sludge shows considerable potential as a sustainable cement replacement material and can contribute to reducing cement consumption, promoting the valorization of paper industry waste, and supporting the development of environmentally friendly cementitious materials, particularly for non-structural applications.
Keywords: Response Surface Methodology; paper sludge; cementitious composites; mechanical properties; sustainable development; optimization.
Keywords
- Response Surface Methodology
- Industrial waste
- cementitious composites
- mechanical properties
- sustainable development
- optimization
Main Subjects
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