Value-added recycling of sludge and sludge ash into low-carbon construction materials: current status and perspectives

Sludge as the by-product of wastewater treatment usually accumulates toxic substances that require specific treatment before disposal. With the increase of sludge production, the safe and economic treatment of sludge has become a global issue. Construction filed provides a solution for consuming huge volumes of sludge and sludge ash owing to the similar chemical composition with cementitious materials. This paper critically reviewed the current status of recycling sludge and sludge ash into low-carbon construction materials and highlighted the future perspectives of sludge-derived construction materials. Furthermore, the immobilization mechanisms of heavy metals in sludge-derived construction materials were elaborated to promote the actualization of sustainable management of sludge and sludge ash. • Reviewed the valorization of different sludge and sludge ash in construction materials. • Sludge can be recycled as alternative fuels and raw materials to produce eco-cement clinkers. • Aluminosilicates in sludge/sludge ash improved the properties of construction materials. • Sludge/sludge ash-derived function concrete, bricks and ceramic exhibit good qualities. • Potential toxic elements in sludge/sludge ash could be effectively immobilized.


Introduction
Sludge is the semi-solid precipitated residue produced during the treatment of water and wastewater [1].With the acceleration of industrialization and rapid population growth, sludge production has increased rapidly worldwide.Depending on the source of wastewater, the sludge can be divided into drinking water treatment sludge (DWTS), sewage sludge and industrial sludge.The output, properties and environmental impacts of these sludges are quite different.Among them, DWTS is the sediment and filter residues produced during the drinking water purification process, consisting of dissolved colloids, suspended solids, organic matter and pathogens [2].Recent studies reported that the annual amount of DWTS is about 131,000 tons in the UK, and that in Italy even reaches 750,000 tons per year [3,4].Although the annual production of DWTS is very large, the treatment of DWTS presents a series of problems, such as low recycling rate, high cost and environmental pollution [5,6].Based on the available data, the resource recovery rate of DWTS is only 25% in most European countries.In Australia, the primary disposal method for DWTS is landfill treatment, which leads to a serious waste of land resources.Moreover, the average disposal cost of DWTS exceeds $150 per ton [7].Furthermore, due to the use of alum-based coagulant, aluminates are enriched in DWTS.When DWTS are under landfill disposal, aluminates can dissolve in the groundwater, leading to the increase in Al element content in the human body, which could increase the risk of diseases, e.g., Alzheimer's disease [8].Sewage sludge faces an even more severe disposal challenge due to its high production volume and potent toxicity [9].Sewage sludge is the sediment and scum produced during urban sewage treatment [10].For instance, approximately 240 million tons of wet sewage sludge is generated in China (2010).In the EU, the annual dry sewage sludge production is 1.85 million tons in Germany, and that in England is 1.03 million tons per year [11,12].In 2017, global dry sewage sludge production is recorded at 45 million tons per year [13].The disposal methods of sewage sludge include landfill, compost and incineration et al. [14,15].The status of sewage sludge disposal in selected countries (2015-2018) is shown in Fig. 1.In some developed countries, including the US, Spain, France and the UK, the main disposal method of sewage sludge is agricultural utilization.In Germany, the agricultural utilization of sewage sludge is limited by its current regulation, and most sewage sludge is treated by incineration [16].In Japan, approximately half of sewage sludge is recycled by the construction industry.However, some countries still use landfill as the main disposal method of sewage sludge, which could lead to serious environmental pollution.Incineration has gradually become the major disposal method of sewage sludge because it can completely eliminate pathogens and organic matter in the original sewage sludge [17].More importantly, incineration can save precious land resources.Furthermore, the residue ash of sewage sludge incineration, namely sewage sludge ash (SSA), exhibits a similar chemical composition to pozzolan with an aluminosilicate nature, which can be used as construction materials, such as mineral precursors, supplementary cementitious materials (SCMs), and fine aggregates [18][19][20].Industrial sludge is a general term for by-product from different industrial wastewater treatment [21].The properties of industrial sludge vary greatly, depending on its source.These differences are mainly reflected in the viscosity, hygroscopic properties, pollutant properties, oil content, water content, organic matter proportion, inorganic matter proportion and so on [21,22].Compared with other sludges, industrial sludge has the characteristics of high viscosity, high oil content and rich in potentially toxic elements (PTEs).Some of industrial sludge, are defined as hazardous waste if its toxic elements excess the standard limits [23].In China, the dehydrated industrial sludge reached 3.7 million tons in 2020.However, the treatment capacity of industrial sludge cannot fulfil its output.Although various disposal methods, including pyrolysis, biodegradation, stabilization/solidification (S/S), etc. have been documented in the literature, the treatment efficiency and environmental impact of these methods need to be further evaluated and improved [24][25][26].Among them, S/S has been gradually applied to the practical disposal of industrial sludge due to its advantages of high efficiency, low carbon and low cost [27,28].PTEs in industrial sludge are immobilized by hydration products of cement-based materials, significantly reducing the potential environmental risk [29].
In general, the harmless and resource disposal of sludge has excellent compatibility with the construction industry.A few review articles concentrated on the recycling of sludge in the construction field, most of which are focused on general aspects of cement or brick production, etc., however, the latest technologies of sludge disposal in the construction field still should be critical reviewed.In addition, there is a lack of studies to distinguish different types of sludge.For these purposes, this paper provides a critical review as follows: (i) the physical and chemical properties of different types of sludge, (ii) the latest application of these sludges in the construction field, such as low-carbon cementitious materials, lightweight aggregates, controlled low strength material, S/S block, etc., (iii) the durability and environmental impacts of sludge-based products, (iv) the perspectives of high value-added application of sludge in construction field.

Sewage sludge
The illustration of sewage sludge production is shown in Fig. 2. Sewage sludge is composed of the primary sludge produced by the primary sedimentation tank and the residual activated sludge discharged from the secondary sedimentation tank [30].The compositions of sewage sludge are very complex due to various wastewater sources [31].Sewage sludge possesses an extremely high moisture content, which is defined as liquid or semiliquid waste (55-80% for dehydrated sewage sludge), whereas in the remaining solid, 60-80% is the organic matter [32].As for inorganic matters, the main oxide components of sewage sludge Fig. 1 Stats of sludge disposal in various countries [12] include SiO 2 , Al 2 O 3 , Fe 2 O 3 , P 2 O 5 and CaO [33,34].If sewage sludge is directly recycled as cementitious materials, the high moisture and organic matter contents of sewage sludge could exhibit an adverse influence on the cementitious properties, increasing the porosity and decreasing the bonding strength.Therefore, sewage sludge should be pretreated before being utilized in cement or concrete.Incineration is one of the most common pretreatment methods of sewage sludge [35,36].Sewage sludge as an organic solid waste possesses relatively high calorific value, which can be used for electricity production [37].In addition, the inorganic substances could be further enriched in the residue ash, sewage sludge ash (SSA), due to the evaporating of water and elimination of organic matter during incineration, leading to a greater potential of SSA utilized in the construction industry [30,38].Furthermore, the high treatment temperature contributes to stable immobilization of PTEs in original sewage sludge [39,40].The variability of the chemical compositions of SSA is shown in Fig. 3.The total amounts of major oxides inventing in the pozzolanic reaction (SiO 2 and Al 2 O 3 ) in SSA are nearly 50%, which is comparable to that in commonly used SCMs (e.g., pulverized fuel ash).The reactivity of the active substances in sewage sludge is further activated by the high treatment temperature, leading to a potential of SSA used to develop low-carbon cementitious materials.

Drinking water treatment sludge
Figure 4 shows the processes of drinking water treatment and DWTS generation.Inorganic coagulants are commonly used in drinking water treatment to remove impurities from the water.The commonly used coagulants are mainly composed of aluminum or iron salts, including aluminum sulfate (Al 2 (SO 4 ) 3 ), ferric chloride (FeCl 3 ) and ferric sulfate (Fe 2 (SO 4 ) 3 ) [6,42].Limebased coagulants are used when the water exhibits high levels of hardness and needs to be softened [43].Therefore, the final composition of DWTS is greatly influenced by the coagulant type.The chemical compositions of main types of DWTS are shown in Table 1 [7].The lime-based DWTS exhibits a high CaO content, which can be used as a substitute for limestone during cement production.The geotechnical properties of three types of DWTS are shown in Table 2, which contributes to predict the potential applications of DWTS.The aluminum-based and iron-based DWTS exhibit a certain similarity in the majority geotechnical characteristics with clayey soils.The difference between DWTS and clay mainly focuses on the organic matter and chemical contents [44].Due to the existence of coagulant and high organic matter contents, DWTS exhibits higher plasticity and compressibility than clay, while lower permeability.These characteristics of untreated DWTS make it not suitable for utilization in the construction field.Similar to SSA, the inorganic substances are enriched in the DWTS residual ash (DWTSA) after incineration.For instance, the aluminum-based DWTSA is rich in aluminates, which   can react with portlandite, and is usually used as an alternative to cement clinker in the production of mortars and concretes [45].

Industrial sludge
Based on the source of industrial wastewater, the industrial sludge can be divided into paper mill sludge, textile dyeing sludge, petrochemical sludge, electroplating sludge, etc.For instance, paper mill sludge is produced from the paper industry, which is composed of mineral components and cellulose lignin [49].Textile dyeing sludge, the by-product of dyeing industry, is composed of polycyclic aromatic hydrocarbons, mineral phases and dyes [50].Electroplating sludge is classified as a hazardous waste produced from the electroplating industry [51].Wastewater from the electroplating industry is mainly purified by the chemical precipitation method, leading to a large volume of PTEs-rich electroplating sludge [52].Table 3 illustrates the chemical composition of several typical industrial sludges.Although the composition of industrial sludge is greatly affected by its source, it is similar that all industrial sludges contain a certain content of PTEs, especially electroplating sludge.The utilization of industrial sludge in structural material is greatly limited due to the leaching behavior of PTEs.However, application as construction materials provides a low-carbon and efficiency method for the disposal of industrial sludge.The PTEs in industrial sludge can be stabilized and solidified by the hydration products of cementitious materials, and thus the S/S industrial sludge can be safely landfilled [53].
3 Recycling options for sludge/sludge ash into construction materials

Cement clinker
Cement is the most commonly used building material in the world.However, the cement industry often leads to excessive consumption of energy and serious environmental pollution [56].Especially in developing countries, the cement industry is the major contributor to energy and natural resource consumption.The cement production process consumes a large amount of coal resources due to a relatively high heating temperature.Using biomass or organic solid waste as an alternative fuel in cement production to reduce fossil fuel depletion seems to be a sustainable option [57].In addition, raw materials used in cement production are mainly natural limestone and clay, which increases the consumption of natural sources.Recently, the use of municipal solid waste or industrial by-products to produce ecological cement has aroused extensive research interest [58].Sludge can be recycled in the cement production process as an alternative to raw materials or conventional fuel, owing to its high calorific value and similar chemical compositions to ordinary Portland cement (OPC), as shown in Fig. 5.Most of organic pollutants and PTEs in sludge can be completely destroyed or immobilized under high temperature (approximately 1450 ℃) and alkalinity condition [59].For example, more than 20 wt% of total dried sludge is recycled for OPC production in Japan [12].

Conventional fuel substitute
Dried sludge has been considered as a potential energy resource and can be recycled as a substitute for conventional fuel in the cement industry because it contains a large content of combustible matter and high molecular weight organic matter.The average energy demand for the production of cement clinker per ton is about 3.52 GJ, which is equivalent to consuming 0.12 tons of coal with a calorific value of 29.31 MJ/kg [60].Energy consumption contributes to nearly 35% of the total cost of cement production, and thus the utilization of alternative fuels to reduce energy bills is one of the major challenges for cement plants [61].Based on results from various research, the reuse of sludge as an alternative fuel during cement production is feasible.The physicochemical properties of cement clinker are not influenced by the addition of alternative sludge fuel.Sludge incorporation can reduce coal consumption by nearly 90% [62].Although the utilization of sludge as a fuel substitute in cement plants realizes the resource recovery of sludge, its potential human health risks and environmental impacts also deserve attention [63].The life-cycle assessment results reveal that the emissions of CO 2 and NO X are greatly reduced by the utilization of alternative sludge fuel [64,65].However, some studies reveal that the emission risk of volatile organic compounds (VOCs) increased with the addition level of sludge [66].In general, incorporating the appropriate amount of sludge as auxiliary fuel reduces energy costs and promotes the circular economy without additional environmental risks.

Alternative raw material
Dried sludge is rich in silicon, iron, aluminum and calcium, which can be recycled as part of raw materials for cement clinker production.Different types of sludge contain different contents of elements, and thus they can replace different raw materials in cement production, such as limestone, clay and gypsum, etc. Municipal sludge is rich in aluminosilicates, which could be utilized as a substitute for argillaceous materials to produce ecocement.The chemical compositions of eco-cement produced with the addition of 5-15 wt% municipal sludge is similar to that of OPC [67].However, the porosity structure of sludge leads to a decrease in the flowability of ecocement.The initial and final setting time of eco-cement is prolonged due to the existence of phosphates in sludge.
The phosphate (> 0.85%) from the original sludge promotes the formation of α-C 2 S and decreases the relative content of C 3 S, leading to the prolonged setting time [68].
Lime-based sludge and marble sludge possess a large amount of calcium, which can replace the calcareous raw materials.The burnability of cement clinker is improved with the addition of calcium-rich sludge, leading to a decrease in the temperatures for CaCO 3 decomposition and liquid phase formation [69].The trace elements of sludge acted as mineralizers and cosolvents could accelerate the liquid formation and reduce the eutectic point during the cement sintering.The mechanical properties and workability of eco-cement produced by lime-based sludge (< 15 wt%) is comparable to that of commercial OPC.However, the incorporation of excessive limebased sludge has an inhibition effect on the formation of main cement clinker phases.Shih et al. [70] reported that it is feasible to recycle industrial sludge as a substitute for raw materials in cement production.The maximum incorporation of industrial sludge reaches 15 wt% by controlling the technological parameters of cement sintering.
As the incorporation of industrial sludge within 15 wt%, the PTEs from sludge could promote the formation of C 3 S.During the sintering, 90% of the high volatile PTEs, e.g., Pb, are evaporated in the high temperature, while the low volatile PTEs, including Cu, Ni and Cr, are trapped in cement clinkers and reduced the leachability.Petroleum sludge possesses a large amount of gypsum, which could be used as an alternative to gypsum during cement production.In cement plants, gypsum is used to adjust the setting time of cement clinker [71].The addition of 5 wt% of petroleum sludge can achieve the same retarding effect as gypsum [72].However, when the petroleum sludge is blended with cement clinker without high-temperature treatment, the inherent PTEs could lead to environmental and human health risks.Except for traditional OPC, sludge can also be recycled as part of raw materials to produce green cement (sintering temperature < 1300℃).Calcium sulfoaluminatebelite (CSAB) cement as a typical low-energy cement is mainly composed of calcium sulfoaluminate and belite, Fig. 5 A schematic of recycling of sludge for OPC production produced from bauxite limestone and gypsum.El-Alfi et al. [73] successfully prepared CSAB cement in the laboratory by using marble sludge as a calcium source, where the dosage of marble sludge reaches 55% after 1-h calcination at 1250℃.Aluminum-based sludge is rich in aluminum, which can be recycled as an aluminum source during the production of CSAB cement.Rungchet et al. [74] reported that by utilizing the aluminumbased sludge, fly ash and flue gas desulfurization gypsum (FGDG), an industrial solid waste-based CSAB cement was produced after calcination at 1150℃ for 1 h.This utilization method realizes the complementary advantages of various solid wastes and further promotes the resource recovery of sludge and other solid wastes.PTEs-rich industrial sludge also can be recycled to produce CSAB cement [75,76].Most of PTEs accumulated in clinker phases under high-temperature sintering conditions and influence the stabilization and structure of clinker phases [77].Among them, Ni, Zn and Cr enhance the stability of β-C 2 S, while Cu exhibits an adverse effect [78].
Table 4 summarized the preparation process and performances of sludge-derived eco-cement in recent literature.Sludge incineration units can be combined with cement plants.On the one hand, the thermal energy generated by sludge incineration can be recycled for cement clinker production.On the other hand, the incineration residue ash can be reused as raw materials for eco-cement production.However, the effects of trace elements in sludge on the long-term mechanical properties and leaching behavior of eco-cement require further investigation.

Low-carbon cementitious materials
The production of cement accounts for the majority of carbon emissions from the construction industry.Approximately 8% of anthropogenic carbon emissions are from the cement production [80].Using SCMs to reduce clinker factors, and thus cement production is considered as a key step for construction industry to achieve low-carbon and environmental friendliness [81].Most commonly used SCMs, such as fly ash are byproducts of traditional industry, and their availability is limited with the development of clean energy.Due to the relatively high contents of aluminosilicates, it seems a promising way of recycling sludge/sludge ash to produce low-carbon cementitious materials [82].Due to the high moisture and organic matter contents, the original sludge has a serious adverse effect on the mechanical properties, workability and durability of the cementitious systems [83].The incineration process avoids these adverse impacts, aluminosilicates are further enriched in the incineration residual ash and activated during the high temperature.This part discusses and compares the application of sludge and sludge ash in the manufacturing of low-carbon cementitious materials: (i) as mineral precursors for geopolymer materials; (ii) as SCMs to partially replace OPC in cement-based materials.

Geopolymer materials
Geopolymer is considered as a kind of low-carbon cementitious material with a three-dimensional network structure by alkali-activation of precursors [84][85][86].The precursors are normally composed of solid waste materials enriched with aluminosilicate such as fly ash (FA), silica fume (SF) and ground granulated blast furnace slag (GGBS).Therefore, geopolymer exhibits a much lower carbon emission than OPC.Dehydrated sludge can be combined with other industrial solid wastes as precursors to form geopolymers under the action of alkali activators, such as NaOH and sodium silicate [87,88].In recent years, many researches have investigated the feasibility of preparing geopolymers with different sources of sludge, including sewage sludge, wastepaper sludge, tannery sludge and industrial sludge etc. Aluminosilicates in sludge can be dissolved in the pore solution under high alkalinity and form geopolymers with a three-dimensional network structure.PTEs dissolved from sludge can be captured and stabilized by geopolymers, reducing the overall PTEs leachability in geopolymer materials [89][90][91].Due to the high organic matter content and coarse pore structure, sludge exhibits an adverse impact on the strength development of geopolymers [92,93].Such a decrease in strength could be recovered through the adjustment of activator proportions under the premise of low sludge contents.When the precursors are 100% composed of sludge, the compressive strength of the geopolymer could not exceed 10 MPa after 28-day curing [94].Coagulants and other chemicals in sludge might reabsorb free water during mixing, increasing the water demanded.The strong water absorption capacity of sludge seems to exhibit well compatibility with geopolymers.Geopolymers with a high alkali content usually exhibit a great drying shrinkage, while sludge decreases the drying shrinkage of geopolymers due to its slowrelease effect on water [95,96].
Compared with sludge-based geopolymers, sludge ashbased geopolymers exhibit better mechanical properties because the high calcination temperature evaporates the moisture and changes the tetrahedral and octahedral structure of aluminosilicates in sludge.Zhao et al. [97] developed a low-carbon geopolymer material by utilizing SSA and GGBS as precursors.Both amorphous and crystal aluminosilicate in SSA participates in the geopolymerization process with the generation of amorphous phase, which further refines the pore structure and enhances the strength development.The maximum compressive strength of SSA-based geopolymers reached 48.5 MPa after 28-day curing.Furthermore, due to the use of plenty of solid wastes, the carbon emission of green geopolymers is much lower than that of OPC.In the manufacturing of geopolymers, alkali activators account for most of the carbon emissions.To further decrease the environmental impacts of geopolymers, silica-rich solid waste, rice husk ash, was utilized to produce alkali activators for the further production of SSA-based geopolymers [98].Similarly, Petrus et al. [99] prepared a green alkali activator from geothermal sludge for geopolymer production.The utilization of sludge-based green activators could further reduce carbon emissions by 23%.

Supplementary cementitious materials
SCMs can be used as a partial replacement of cement clinker in mortars or concrete due to their self-cementing behavior or pozzolanic activity [100,101].Aluminosilicates in sludge could react with portlandite generated from cement hydration to form additional hydration products.In addition, the trace elements in sludge could influence the hydration kinetics of cement clinker.Valls et al. [102] evaluated the compatibility between OPC and sludge, the results revealed that aluminates dissolved from sludge promoted the formation of aluminate products, while the hydration process of blended cement was inhibited by organic matters and trace elements in sludge.Both workability and mechanical properties of blended cements are decreased sharply with the sludge incorporation.Interestingly, the organic matter in sludge are gradually decomposed due to the high alkalinity of pore solution in blended cements.To enhance the compatibility between sludge and OPC, researchers explored using activators or accelerators, including CaCl 2 , Ca(OH) 2 and biomass ash, to promote the pozzolanic reactivity of sludge [103,104].The performance of blended cements with sludge is greatly enhanced with the addition of activators, which can meet the application requirements of landfill covering and road construction.
In order to further expand the utilization value of sludge in low-carbon cementitious materials, pre-treated sludge is used as an SCM in most studies.The most common pre-treatment method is incineration, which can remove organic matter and stimulate the reaction activity of aluminosilicate in sludge.The pozzolanic activity of sludge ash is influenced by the incineration temperature and grinding process.The organic matters are burnt at 300-500℃ and the inert minerals such as kaolinite are transformed to active metakaolin at 500-900℃.When the combustion temperature exceeds 900℃, active metakaolin continues to dehydrate, resulting in the formation of inert mullite, and thus the most suitable incineration temperature of sludge is around 800℃ [105,106].The fineness of sludge ash also influences the strength development of blended cements.Mechanical milling can further improve the pozzolanic reactivity of sludge ash [107].Figure 6 shows the effects of sludge ash on the compressive strength of concrete.The incorporation of sludge ash exhibits adverse impacts on the strength development of concrete, especially with a high dosage.Noted that the compressive strength of blended concretes within a 10% dosage of sludge ash is comparable to that of pure cement concrete.The amorphous silicon in sludge ash can react with portlandite, increasing the length of silica chain of C-S-H gels.In aluminum-based sludge ash, aluminates promote the generation of aluminate products [105,108].Similarly, iron dissolved from iron-rich sludge ash can replace the sites of aluminum in aluminate products, promoting the participation of AFt and AFm [109].The types of aluminate products are determined by the relative contents of aluminates and sulfates in sludge ash.When sludge ash is rich in sulfates, the synergistic effect of aluminates and sulfates leads to the generation of a large amount of AFt.On the contrary, when the blended cements lack sulfates, aluminates dissolved from sludge ash lead to the conversion of AFt to AFm.Phosphates in sludge ash lead to the formation of whitlockite (Ca 3 (PO 4 ) 2 ) [31].The formation of these additional hydration products refines the pore structure and promotes the strength development of sludge ash-based cementitious materials.However, the porous structure of sludge ash particles increases the water demand of blended cements, as reported by Gu et al. [110] that the fluidity of specimen was sharply decreased from 106 to 89 mm when 15 wt% of sludge ash was incorporated.In addition, the phosphate in sludge ash inhibits the dissolution and hydration of cement clinker, prolonging the setting time of blended cements.Furthermore, the pozzolanic reactivity of sludge ash is lower than that of other SCMs, which limits the replacement level of sludge ash on cement.When the dosage of sludge ash is up to 30 wt%, the strength reduction exceeds 35% [111].Therefore, to ensure the mechanical properties, the substitution level of sludge is limited to 15 wt% to meet the requirements of the relevant regulation.
In order to further increase the substitution percentage of sludge ash in cement or geopolymer, researchers have explored various modification methods.Alkaline hydroxides, sulfates and chloride salts are commonly used as early strength agents and activators for blended cements with a high volume of SCMs [109,112,113].Alkaline hydroxides can increase the alkalinity of pore solution, which promotes the hydration of cement clinker and the dissolution of sludge ash.Sulfates such as Na 2 SO 4 can react with aluminates and ferrites in sludge ash, leading to the generation of AFt, which refines the pore structure and promotes the strength development of blended cements.Chloride salts especially NaCl promote the generation of Friedel's salt and accelerate the early hydration kinetics [114].In addition to using chemical activators, some researchers aimed to enhance the pozzolanic reactivity of sludge ash by the improvement of sludge incineration process [115,116].The co-combustion of sludge and rice husk seems to be a promising solution [117,118].The incineration of sludge without additional energy consumption is achieved due to the high calorific value of rice husk.In addition, the co-combustion ash of sewage sludge and rice husk possesses higher amorphous silicon than sludge ash, leading to a higher pozzolanic reactivity.Under the requirement of strength, the maximum dosage of co-combustion ash can reach 30 wt%.The combination of multiple SCMs could also increase the disposal capacity of sludge ash in blended Fig. 6 Effects of sludge ash recycled as SCM on the compressive strength of concrete (adapted from [12]) cements.The synergistic effect of sludge ash with other SCMs could maximize their potential reactivity, promoting the strength development [119,120].For instance, the aluminates and ferrites in sludge ash can react with limestone, generating carboaluminate phases (hemicarboaluminate (Hc) and monocarboaluminate (Mc)) [121,122].These carboaluminate phases are intermixed with the surrounding C-S-H matrix, effectively contributing to pore structure refinement and strength development of blended cements.Except chemical effect, the particle packing structure of blended cements was refined by the combination between sludge ash and other SCMs due to the filling effect of fine SCM particles into the voids between OPC particles and sludge ash particles [123].
In summary, sludge ash is a more suitable material than original sludge to be utilized as SCM due to the pyrolysis of organic matter and the generation of activated minerals during the incineration process.However, excessive substitution of OPC with sludge ash leads to a decrease in the various performance of blended cements.Modification means may be the key to solve this problem, therefore further research is needed to focus on the compatibility of chemical activators and blended cements, as well as the synergistic effect of sludge ash with other SCMs.

Functional concrete
The low potential reactivity of sludge/sludge ash leads to adverse impacts on the strength development and durability of blended cements, and thus recycling sludge/ sludge ash to produce structural concrete with high strength requirements is still in the laboratory research stage.In view of the lightweight and porous characteristics, sludge/sludge ash exhibits great potential to produce functional concrete with low strength requirements, including controlled low strength material (CLSM), lightweight aerated materials etc. [124,125].In addition, S/S is a safe and effective treatment method for industrial sludge enriched with PTEs.PTEs in industrial sludge are immobilized by the hydration products of low-carbon cementitious materials [126].Therefore, the S/S sludge block meets the leaching standards and can be used to backfill or prepare paving blocks to realize high valueadded utilization of industrial sludge.

Lightweight aggregates
Lightweight aggregates (LWAs) as a porous material could enhance the thermal insulation performance of buildings and decrease the unit mass of concrete [127].Sludge mixed with clay and other materials can be utilized to manufacture eco-LWAs after pelletizing and firing.During the firing process, the organic matter in the sludge decomposes and produces exhaust gases, leading to an increase in the porosity and reduce the density of LWAs prepared with sludge.The heating temperature as one of the most critical parameters greatly influences the pore structure, mechanical properties and PTEs leaching behavior of eco-LWAs.When the heating temperature is below 1020 ℃, the eco-LWAs is rough in surface, irregular in shape and existing extensive intergranular porosity, which results in their low mechanical properties.Under the heating temperature of 1060℃, the pores become discontinuous and isolated, leading to a compact matrix, which enhances the mechanical properties of eco-LWAs.The excessive firing temperature leads to the appearance of larger isolated pores due to the aggregation and consolidation of small pores, thus resulting in the reduction in the mechanical properties of eco-LWAs [128].The high heating temperature contributes to the reduction in the leach ability of PTEs, because PTEs from original sludge can be well immobilized in the glass or crystalline phases during the manufacturing of eco-LWAs [129].Another critical parameter is the proportion of oxides (SiO 2 , Al 2 O 3 ) in the raw materials [130].Liu et al. [131] found that the prepared LWA exhibited the optimal mechanical properties and pore structure with SiO 2 content of 30-45% and Al 2 O 3 content of 11-19%.Compared with traditional LWAs, eco-LWAs prepared with sludge exhibit high porosity and strong moisture absorption capacity, and when added to concrete, it could lead to a decrease in the fluidity of concrete.Suchorab et al. [132] reported that using a hydrophobic agent to impregnate eco-LWAs could reduce the high moisture absorption characteristic of eco-LWAs, improving the compatibility between eco-LWAs and concrete.In general, the pyrolysis of organic matter in sludge promotes the formation of pores and further decreases the density of LWAs.The calorific value of sludge contributes to the reduction in sintering temperature, which decreases energy consumption.The high treatment temperature improves the stability of PTEs in sludge.However, further investigation into the long-term performance of eco-LWAs prepared with sludge should be carried out.

Controlled low strength material
Controlled low strength material (CLSM) is defined as a self-compacting and self-levelling cementitious material for backfill.Due to the strict requirements of application scenarios, high-quality CLSM should be characterized by high fluidity for reducing labour requirement (> 200 mm) and fast stiffening for accelerating engineering works (3-5 h).However, the extremely strong water absorption capacity of sludge leads to a decrease in fluidity.Besides, the setting time is greatly prolonged due to the coating layer formed with the organic matter in sludge and calcium.Therefore, sludge is usually combined with pulverized fuel ash to produce CLSM.The spherical fly ash particles contribute to the decrease in the friction between particles and thereby increase the fluidity of CLSM.In addition, accelerators are also commonly added to CLSM prepared with sludge to accelerate the reaction kinetics and shorten the setting time.Wang et al. [133] investigated the compatibility of different types of accelerators, including sodium-based, calcium-based and organic accelerators with alum sludge-based CLSM.Among them, triethanolamine significantly promoted the reaction between aluminates and sulfates, which was the most effective accelerator.Alkali activation is also an efficient method to enhance the performance of CLSM.The setting time of CLSM can be controlled by adjusting the alkali equivalent.Fang et al. [134] demonstrated the feasibility of sludge-based CLSM as a backfill material through field test, and the sludge-based CLSM were easily excavated after three months.Therefore, CLSM provides a new approach for sludge disposal.Large amounts of sludge can be transformed into building materials with low strength requirements.Undehydrated liquid sludge also could be utilized in CLSM, the moisture in the sludge can be used as mixing water.However, due to the low strength and loose structure of CLSM, the long-term leaching behavior of PTEs needs to be further evaluated.

Lightweight aerated materials
Lightweight aerated material (LAM), also known as foam concrete, is a kind of building material made by introducing air voids to the cementitious matrix by mechanical foaming or chemical foaming.It possesses the advantages of low density, controllable strength and excellent thermal insulation, which can be applied as semi-structural materials in the construction filed.The porous structure of sludge ash is suitable for the production of LAM.Besides, the pozzolanic reactivity of sludge ash guarantees favourable mechanical properties.Wang et al. [135] used aluminum powder as a foaming agent to prepare sludge ash-based LAM.The results revealed that the porous structure and irregular properties of sludge ash particles exhibited low thermal conductivity, and thus the incorporation of sludge ash in LAM contributed to better heat insulation properties.The main heat conduction modes in sludge ash-based LAM are solid and gas conduction.Therefore, the thermal conductivity of LAM is determined by its pore structure.High water-to-binder ratio and high foaming agent content could lead to the formation of open and connecting pores in LAM, which exhibits adverse impacts on its mechanical properties and heat insulation.The disadvantages of sludge ash-based LAM, including poor mechanical properties and high water absorption, could be improved by sintering at a high temperature.The mechanical properties of sludge ash based-LAM could be enhanced with the dosage of sludge ash under the sintering temperature between 1000℃ and 1093℃ [136].Zhang et al. [137] explored using biochar to modify sludge ash-based LAM to achieve long-term carbon sequestration, and the results indicated that biochar not only improved the mechanical properties of sludge ash-based LAM, but also contributed to the immobilization of PTEs.The porous structure of sludge ash makes it very suitable for LAM production.However, the matrix of LAM may not provide adequate physical coating for PTEs due to its porous structure.Therefore, the leaching behavior of sludge ash-based LAM should be further investigated.In addition, the combination of various industrial wastes could contribute to the further reduce in the carbon emissions from LAM production.

Stabilization/solidification blocks
S/S is a widely applied technology for the treatment of hazardous wastes, such as industrial sludge.During the treatment, the hydration products of the cementitious materials form a relatively dense matrix, which exhibits a physical sequestration effect on PTEs and organic matter in sludge.In addition, PTEs are further immobilized in the matrix via chemical bonding, including adsorption, complexation and precipitation.Due to the high efficiency and low cost of S/S for hazardous waste treatment, the treated blocks can be used as building materials, including filling materials, road bed materials and mine filling materials.For decades, OPC is the most commonly used cementitious material for S/S.PTEs are precipitated under the high alkalinity of pore solution or adsorbed by the hydration products of C-S-H gels and AFt.However, the compatibility between OPC and PTEs is poor, because most of PTEs such as Zn greatly inhibit the hydration of OPC.Besides, the production of OPC causes serious carbon emissions.To avoid these issues, researchers explored using low-carbon cementitious materials for S/S disposal of sludge, such as geopolymer and magnesium cement.Previous research demonstrated that geopolymers exhibited extremely high efficiency for PTEs immobilization.Bivalent metal cations can be immobilized in geopolymer framework by means of substitution and adsorption [138].Another research showed that geopolymer could convent Cr 6+ to Cr 3+ without additional reductants, thus achieving the immobilization of Cr [139].Zhang et al. [55] found that magnesium phosphate cement has excellent compatibility with industrial sludge.PTEs such as Zn precipitate with phosphate, thereby reducing the leachability of S/S blocks.The S/S method can efficiently treat a large amount of hazardous waste with very low cementitious material consumption.However, the durability of S/S blocks and the life-cycle of S/S method should be further assessed in various scenarios.

Other recycling options
Although the majority of research on sludge recovery is focused on cement, cementitious material and function concrete, researcher have developed some alternative recycling options for sludge and sludge ash.These potential applications of sludge include the production of bricks and ceramics and the substitution of sand.

Bricks and ceramic production
Sludge can be recycled as a substitute of clay during the production of bricks and ceramics due to the similarity in chemical composition.However, the high moisture content of sludge influenced the formation of blocks during the mixing, decreasing the production efficiency.Besides, the decomposition of organic matter in sludge during the sintering greatly affects the quality of bricks.When the sludge content reaches 10 wt%, the mechanical properties of bricks decrease by more than 30%.In order to eliminate this adverse impact, eco-bricks in most research are prepared by the incorporation of pre-treated sludge.Incineration is the most effective and common pretreatment method for sludge.The incorporation of sludge ash within 20 wt% could prepare bricks with high mechanical properties.Granulation seems to be a promising method of sludge pretreatment.Chen et al. [140] compared the properties of environmentally-friendly bricks produced with raw sludge or granulated sludge, and the results revealed that sludge granulation treatment significantly reduces the negative effects of sludge introduction on the physical and mechanical properties of eco-brick.The combination of industrial solid wastes and sludge can further reduce the cost of eco-brick.Zhang et al. [141] produced an eco-brick that meets Chinese specifications by using sewage sludge, lake sediment and cinder.Waste glass also can be recycled as an additive to improve the properties of sludge-based bricks [142].The incorporation of waste glass increases the mechanical properties of bricks and decreases the leaching capacity of PTEs.
Montero et al. [143] investigated the feasibility of recycling sewage sludge and marble sludge in ceramic tile production, and the results showed that the addition of sludge affected the mechanical properties and water absorption of ceramic tile, thus the maximum dosage of sludge was limited to 20%.Lin et al. [144] investigated the compatibility between glazes with different colorants and ceramic tiles prepared by incineration of sludge ash and clay, including Fe 2 O 3 (red), CoCO 3 (blue), MnO 2 (purple), and V 2 O 5 (yellow).The glaze formed a film on the surface of the tile, thus protecting the surface and reducing water absorption.Fe 2 O 3 was helpful to reduce the sintering temperature, which increases the interface adhesion between the red glaze and the matrix, thus improving the mechanical properties of tiles.
Similar to the application of sludge in sintered aggregate, the high moisture and organic matter content of sludge exhibit an adverse impact on the mechanical properties of bricks and ceramic.The quality of sintered bricks and ceramics can be improved by the pretreatment of sludge and the synergistic effect between sludge and other solid wastes.

Substitution of sand
The particle size distribution of dried sludge is similar to that of sand, and thus the sludge exhibits the potential to be recycled as fine aggregates for the preparation of mortar and concrete.Liu et al. [145] prepared paving blocks with DWTS as aggregates, and the results revealed that aluminates and sulfates in DWTS contributed to the formation of AFt in the matrix.However, due to the high content of organic matter, DWTS exhibited an adverse impact on the pore structure, leading to a decrease in compressive strength.In order to meet the mechanical properties requirements, the dosage of DWTS was limited to 10%.The leaching results showed that PTEs in DWTS were immobilized in the matrix and the final products were safe for using as paving blocks.The replacement of sand with sludge also influenced the durability of blocks.For instance, aluminates dissolved from sludge promote the generation of AFt under sulfate attack conditions.CO 2 curing of blocks can compensate for this adverse effect with the help of the formation of crystalline CaCO 3 during CO 2 curing, which refines the pore structure and prevents the diffusion of sulfates [146].Another research comparatively analyzed the potential application of sludge ash after acid-washed to replace cement or sand in concrete, and the results indicated that the sludge ash used as sand substitute exhibited extremely adverse effects on both mechanical properties and workability of concrete [147].The application of sludge as a substitute for sand still faces many problems that need to be discussed and solved by future research.Surface treatment of sludge particles to reduce the adverse effects of the original sludge on concrete may be a feasible approach.

Environmental impacts
The environmental impacts of sludge utilization in the construction field should be concerned.The dissolution of PTEs from sludge derived construction materials could lead to secondary pollution and severe threat to human health.It is noteworthy that the leaching concentration of PTEs in almost all sludge-derived building materials in the literature is below the regulatory threshold.However, in most of these researches, the long-time leaching risks of PTEs from sludge-derived construction materials is not deeply analyzed and discussed.Besides, the credible evidence should be provided to elucidate the immobilization of PTEs in sludge-derived construction materials.These mechanisms could address public concerns about the safety of sludge-derived construction materials and further promote the use of low-carbon sustainable construction materials.
Thermal treatment and S/S treatment are efficient method for PTEs-rich sludge treatment [148].Interestingly, all sludge-derived construction materials contain the above treatments in the production process.During S/S treatment, PTEs are immobilized in the matrix by physical sequestration effect and chemical bonding effect [149].Physical sequestration effect refers to the fact that the dense matrix formed by hydration of the cementitious material limits the migration and leach ability of PTEs.Chemical binding refers to the interaction between cement hydration products and PTEs, including precipitation, complexation and adsorption.In cement-based materials, the main hydration products are C-S-H gels, AFt, AFm and portlandite.C-S-H gel has strong binding ability to metal cations.The binding mechanism of PTEs in C-S-H gel mainly includes surface complexation, interlayer adsorption, and incorporation into silicon-oxygen chain [150].Aluminate products including AFt and AFm have a strong binding ability to oxyanions.CrO 4 2− , AsO 4 3− , SeO 4 2− , and VO 4 2− can occupy the sites of SO 4 2− in the ettringite structure [151].The AFm crystal has a bilayer plate-like structure, which is considered to be a layered double hydroxide and the sulphate group in the middle layer can be replaced by oxyanions [152].Portlandite increases the alkalinity of pore solution, resulting in part of metal cations to be precipitated.In geopolymers, PTEs cations (Zn 2+ , Cd 2+ and Cu 2+ ) can replace alkali metal cations in the 3D network structure of geopolymers [90,153,154].During the heating treatment process, PTEs undergo a phase transformation, thus form a new crystal phase or incorporating into the crystal phase formed by sludge [155].Inorganic substances in sludge, such as SiO 2 , Al 2 O 3 and Fe 2 O 3 transform into a large amount of amorphous and crystal phases during the sintering.PTEs could be chemically incorporated into the void of the crystal phases and amorphous network.
Toxic characteristic leaching product (TCLP) is the most widely used method to evaluate the leachability and toxicity characteristics of sludge products.TCLP is tested under the worst conditions for leaching behavior of PTEs in sludge-derived construction materials [156].TCLP has limitations because the leaching behavior of PTEs in sludge-derived construction materials is affected by environmental erosion and service time.For instance, in cement-based materials, the carbonization degree increases with the extension of the service time of sludgederived construction materials due to the erosion by CO 2 in atmosphere.This leads to a decrease in the alkalinity of pore solution in cement-based materials.Therefore, part of PTEs adsorbed or precipitated by hydration products may be converted to the unbound state.Natural disasters such as acid rain can also cause this phenomenon.Therefore, the long-term durability and safety of sludgederived construction materials should be a priority in future research.

Perspectives
Although the value-added recycling of sludge as construction materials has been extensively investigated in the previous literature, these approaches still face many challenges in the process of industrialization.The future directions and practical requirements of sludge treatment can be summarized as follows: (1) Co-treatment of various solid wastes is a promising sludge treatment method.The suitable industrial solid waste could be selected based on the combination of chemical components.The mixture ratio also could be scientifically designed based on the particle packing behavior of the granular materials to promote the synergistic effect between different materials.The synergistic effect between sludge and selected industrial solid waste greatly enhances the performance of sludgederived construction materials.(2) Under the consumption of additional energy, the thermal treatment of sludge achieves the resource recycling of sludge.The research about co-combustion of sludge and biomass to improve combustion process has been widely reported, however the special incineration residual ash treatment and recycling should be further investigated.In most sludge incinerators, sludge is usually co-combustion with high calorific value materials to save costs.Therefore, in future research, more attention should be paid to the recycling of actual incineration ash produced from sludge incinerator as construction materials.
(3) The porous structure of sludge ash particles demands further exploitation in potential application areas.For instance, sludge could be incorporated in ultra-high performance concrete as an internal curing agent, which can absorb free water during the mixing and release water at later curing ages to promote the further cement hydration.
In addition, this slow-release effect of water from sludge ash could alleviate the shrinkage of ultrahigh performance concrete.(4) In most potential application of sludge, the dosage of sludge is limited to 10 wt% in order to ensure the performance of sludge-derived construction materials.In future researches, the chemical modification of sludge should be strengthened to elevate the amount of sludge added to construction materials.(5) The long-term performance of sludge-derived construction materials should be further investigated.
The short-term mechanical properties of sludgederived construction materials could be comparable to the traditional construction materials.However, the long-term performance in sludge-derived construction materials is influenced by environmental erosion and service time.(6) Life-cycle assessment and cost-benefit analysis of sludge-derived construction materials should be conducted.Part of sludge-derived construction materials exhibits excellent performance, while the complex pre-treatment steps and associated high costs limit the development of these practical applications.(7) This review mainly focuses on the recycling of sludge in the construction field, while the recovery of phosphorus from sludge is also an important way for the disposal of sludge, which needs to be further reviewed and investigated.

Conclusions
Sludge as the by-product from wastewater treatment is rich in pathogen, microorganisms, heavy metals and other toxic substances.Sustainable disposal of sludge has been a global environmental issue.This paper critically evaluated the recycling methods of sludge from different sources as low-carbon construction materials, including cement production, supplementary cementitious materials and function concrete.The sludge-derived construction materials exhibit comparable physicochemical performance to conventional construction materials, and PTEs in sludge could be effectively immobilized in the structure of sludge-derived construction materials.The chemical modification method and the co-treatment of sludge with other wastes should be further explored to increase the maximum allowable sludge content in sludge-derived building materials.In addition, the longterm performance of sludge-derived construction materials under aggressive environments should be considered.
The life-cycle assessment should be combined with the cost-benefit analysis to demonstrate and quantify the sustainability of sludge-derived construction materials, achieving the maximum environmental and economic benefits.

Table 1
Chemical compositions of main types of DWTS

Table 2
Geotechnical properties of three types of DWTS

Table 3
Chemical composition of several typical industrial sludges

Table 4
Performances of eco-cement with sludge as raw materials