Sustainability 2023 , 15 , 6915
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some other additives produce fiber cement products that have high strength, durability, fire resistance, and good appearance. Fiber cement products are used in different construction applications, such as the production of floors, walls, and decorations [175]. The usage of cellulose fibers in construction materials such as cement composites has some drawbacks, such as low durability in alkaline or mineral-rich environments and degradation due to aging mechanisms (mainly alkaline hydrolysis and cell wall mineralization), affecting the reinforcing role of cellulose fibers when used as cement composites [171]. The construction industry consumes clay bricks in large amounts for buildings, and the improvement of these bricks, such as their thermal conductivity, can reduce heat loss through walls. Brickmaking involves the usage of pore-making additives, which help in producing bricks with low thermal conductivity and decreased density while saving clay material. Paper waste can be used as an additive in brickmaking to manufacture bricks, which are lightweight, porous, and have decent strength along with low thermal conductivity [176]. In a study conducted by Goel et al. (2021), paper mill sludge compost was used in brickmaking instead of paper mill sludge due to its low moisture content and ease of shredding. While this concept has not been used until now, this study provides guidelines that can be useful in the future to elevate the concept at a commercial level in brickmaking [177]. Recycled aggregated cement obtained from demolished buildings and paper mill sludge can be used together to produce a control low-strength material, which is not considered concrete or cement but is suitable for backfilling and has certain advantages such as quicker construction time and reduced dependency on equipment due to its self-leveling properties [178]. The incineration of paper waste and paper sludge results in two different types of residues, which are bottom ash and fly ash. Fly ash accounts for less than one-quarter of all residues and contains toxic substances. Bottom ash, however, contains fewer toxic substances in comparison to fly ash and, due to its large solid particles, is suitable for usage in construction. Bottom ash has been previously used as a road filler and for cement production, but it presents a major drawback since it releases heavy metals into the environment [179]. Paper sludge ash can also be used as a clay stabilizer, which was previously done using commercially produced limes or cement. It does not require any energy for production and provides several economic and environmental benefits [180]. 4.7. Soil Amendment by Composting The manufacturing of commercial chemical fertilizers consumes large amounts of resources and degrades the environment due to mineral extraction and calcination [181]. Being an active organic material, paper mill sludge can be potentially beneficial for crops as a source of nutrition [182]. The utilization of paper mill sludge on tropical acidic soils decreases its acidity by neutralizing the soil and increases organic matter and essential nutrients [55]. However, spreading paper mill sludge on agricultural land is challenging, and the impact of physical and chemical properties of organic residues must be tested on soil fertility and site quality. Moreover, paper sludge has high carbon-to-nitrogen ratios and BOD, which causes short-term nitrogen sequestration. In addition, paper sludge is difficult to handle, transport, store, and control the odors released from it. Composting paper sludge minimizes these issues [183]. Composting occurs by mixing the waste with the microorganisms that are naturally found in the soil, such as bacteria, fungi, and protozoa. The microorganisms convert the waste into biologically stable humic substances, which constitute the resultant compost. It is the most advantageous disposal method since it returns the macro and micronutrients back into the ecosystem [184]. Ahmed et al. (2018) studied the biodegradation of different types of papers when composted. Newspaper with ink took the longest (21 days) to degrade when buried in soil, followed by newspaper without ink (18 days). Glossy paper and recycled paper fully degraded the quickest among all four, as they took 14 and 16 days, respectively. The advantages of using paper as compost include the absorption of free water from food materials, reducing odors, and providing a source of carbon, which benefits soils lacking organic matter [185]. Composting paper
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