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be utilized as solid fuel, low-cost adsorbent, catalyst, and contaminant remover [158,159]. Hydrochar resulting from paper mill sludge carbonization has lower ignition tempera- ture and higher burn temperatures compared to bituminous coal [160]. Hydrothermal carbonization has advantages over other treatments of a similar nature, such as pyrolysis, since it does not require prior drying of feedstock due to water acting as a solvent, reac- tant, and catalyst [161]. During hydrothermal carbonization, the organic compounds of waste hydrolyze into monomers, which can be turned into hydrochar and organic acids by undergoing several reactions such as dehydration, decarboxylation, condensation, and polymerization [162]. Temperature is the most critical parameter in hydrothermal car- bonization, but the type of biomass used also affects the chemical properties of produced hydrochar [163,164]. During hydrothermal carbonization, a fraction of carbon is lost as some of the organic material dissolves in the aqueous phase, and gas is produced in small quantities [165]. The waste undergoing hydrothermal carbonization requires minimal treatments such as dewatering and handling since hydrochar is sterilized during the pro- cess [166]. Hydrochar from paper waste can be subjected to a variety of modifications to widen the range of its applications [167,168]. The high ash content in feedstock reduces its suitability for hydrothermal conversion as it contributes to metal corrosion and operating complications in processing plants. In addition, the solid fuel produced by feedstock containing high ash content has low gross calorific value and possesses poor combustion properties. For example, sludge obtained from a primary clarifier at a paper mill is not suitable for hydrothermal carbonization due to the high ash content, whereas Kraft and recycling paper mill sludge has significantly lower ash content, increasing their preference for producing solid fuel [169,170]. 4.6. Construction Materials The increased importance of using environmentally friendly, low-cost, and lightweight materials in the construction industry has resulted in the implementation of waste materials in construction materials since it benefits the environment and maintains construction standards [25]. Therefore, cellulosic fibers obtained from natural sources such as plants are commonly explored for the production of construction materials. However, recycled fibers from paper waste or other sources have not been widely tested, as only a few studies have shown successful implementations of recycled fibers in cement-based composites. Products currently manufactured using recycled cellulosic fibers include plasterboard, insulation materials, and bricks [171]. Paper waste such as newspaper and cardboard can be mixed with cement to produce papercrete, which increases the bonding potential and strength of cement. The production of papercrete involves wastepaper soaked in water to soften the fibers and then added to cement to form blocks or used as mortar. However, there are no standard mixtures of papercrete, which makes it challenging to observe its properties from an engineering perspective. Papercrete production does not require excessive amounts of energy or release by-products that are harmful to the environment [172]. According to Akinwumi et al. (2014), papercrete produced from newspaper demonstrates significantly better structural qualities, such as compressive strength, compared to papercrete produced with office paper. Papercrete has high fire resistance but also absorbs more water. Therefore, it is not recommended to be used for outer walls or walls which are near the ground [173]. A study conducted by Sangrutsamee et al. (2018) investigated the usage of different types of re-pulped paper in cement composite production. Four types of re-pulped fibers obtained from newspaper, office paper, carton paper, and mixed papers were mixed with cement in different ratios, and properties such as compressive strength, bulk density, thermal conductivity, and water absorption were observed. Composites produced from re-pulped carton paper were found to be most effective as they were lightweight and had low thermal conductivity and density. However, they had the drawback of having low compressive strength and high water absorption [174]. A large fraction of inorganic substances in paper mill sludge makes it ideal for the production of several construction materials, such as bricks, cement, and insulation [90]. For example, cellulose fibers mixed with cement and
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