Yu-jia XIAO ,Michelle TIONG ,Kim Hung MO ,Ming-zhi GUO ,Tung-chai LING?

1College of Civil Engineering,Hunan University,Changsha 410082,China

2Department of Civil Engineering,Faculty of Engineering,Universiti Malaya,Kuala Lumpur 50603,Malaysia 3College of Mechanics and Materials,Hohai University,Nanjing 210098,China

Abstract:Red mud(RM)is a solid waste generated from the extraction of alumina from bauxite.It is estimated that more than 1.5 billion tonnes of RM are produced annually worldwide.Stockpiling and landfilling of RM is toxic for the environment due to the presence of high quantities of alkali-generating minerals and metal ions.The high cost of RM treatment and limited land availability encourage the recycling and reuse of RM as construction materials.In many developing countries,brick remains the ubiquitous building material.Thus,it is reasonable to use brick as a medium to accommodate a large amount of RM.In this review,the properties of RM are analyzed and their applications in bricks are discussed.Two common types of RM,namely Bayer red mud (BRM) and sintering red mud (SRM),are introduced along with the methods to produce bricks from them through firing,cementing,and geopolymerization.High alkaline-based BRM is particularly useful as a raw material to produce fired brick and geopolymer brick,whereas SRM,which contains more dicalcium silicate (C2S) and some cementitious phases,can be favorably used to produce cementing brick.RM geopolymer brick normally has more strength(up to 51 MPa)than fired and cementing bricks,and good durability(no efflorescence)with low energy consumption and CO2 emissions.Finally,several solutions have been suggested to resolve the issue of the radioactivity of RM brick,and strict regulation of its application has been imposed in some countries,such as China.

Key words:Bauxite residue;Red mud(RM);Firing brick;Cementing brick;Geopolymer brick;Construction materials

1 Introduction

Red mud (RM) (or bauxite residue) is a solid waste generated from alumina production with a total annual global production exceeding 120 million tonnes(Power et al.,2011).Usually,RM is stockpiled in landfills.This disposal practice causes great concern for the surrounding environment because RM has high alkalinity and contains heavy metals,which are harmful to the water,soil,groundwater,and air of the surround‐ing area(Liu et al.,2009b;Sutar et al.,2014).

For better environmental protection and sustain‐able development,many researchers have studied treat‐ments and uses of RM,including valuable metal recovery,preparation of construction materials,and as an absorbent for wastes or a catalyst in diverse processes.Among these applications,recycling of RM in building and construction materials appears to be a promising option to consume a large volume of RM.RM has been widely used for the production of cement clinker and various construction products,such as mortar,concrete,glass-ceramics,block,brick,and ceramics (Liu et al.,2009b;Liu and Zhang,2013;Khairul et al.,2019).

Since bricks have been a major construction and building material in many developing countries and are still in high demand,using RM to produce bricks is highly desirable.Usually,conventional bricks require natural resources like sand or gravel and clay as raw materials.RM can be a suitable alternative to clay due to its similar characteristics.For example,the presence of 4%–5% of alkali can provide good fluxing action,resulting in good plasticity and good bonding (Samal et al.,2013).Extensive studies have been published on the use of RM in bricks.However,a systematic review of different methods for processing RM and their effects on producing different types of brick is lacking.

In this review,we present an overview of the state-of-the-art use of different types of RM(i.e.,Bayer and sintering) and the various methods (i.e.,firing,geopolymerization,and cementing) used to produce bricks.Their advantages and disadvantages are dis‐cussed in detail to allow a better understanding of their applications in construction.The existing prob‐lems and future research need in response to major durability concerns are also highlighted.

2 Types of red muds:production processes and properties

RM is a by-product of alumina production,in which the bulk of bauxite sources is used as a feed for the manufacture of alumina.There are two types of production process,the Bayer process and the sintering process,which are defined according to the chemical nature of the bauxite sources (Liu et al.,2009b;Liu and Wu,2012).Fig.1 shows the production processes of these two approaches(Klauber et al.,2011),and Fig.2 shows the morphology of RM produced by each process(Zhang et al.,2018).

Fig.1 Production processes of alumina:(a) Bayer process;(b) sintering process.Reprinted from (Klauber et al.,2011),Copyright 2011,with permission from Elsevier

Fig.2 Morphology of RM produced from sintering and Bayer processes.Reprinted from (Zhang et al.,2018),Copyright 2018,with permission from Elsevier

2.1 Bayer production process and its properties

Normally,theBayerprocessisappliedtoaluminumbearing minerals in high-grade bauxite sources such as gibbsite (Al(OH)),boehmite (γ-AlO(OH)),and diaspore(α-AlO(OH)).These minerals are selectively extracted from the insoluble components (mostly oxides) by dissolving them in a solution of sodium hydroxide (NaOH),lime,and water at 140–280 ℃and 3.5-MPa pressure (Liu et al.,2009b;Sutar et al.,2014).A sodium aluminate (NaAlO) supersaturated solution or ‘pregnant liquor’ is obtained by the pres‐sure leaching step.The general chemical reactions in the Bayer process are given in the following equations.

For gibbsite(at 140 ℃),

After that,the solid/liquid separation of ‘preg‐nant liquor’ is carried out.The solid phase,namely the RM,is treated as waste residue,while the liquid phase then undergoes precipitation,filtration,and evap‐oration to obtain aluminum hydroxide (Al(OH))(Kelessidis et al.,2007)(Fig.1).

RM that undergoes the Bayer production process is known as Bayer red mud (BRM).FeOand AlOare the major components in BRM due to a higher FeOcontent in the bauxite sources and the low efficiency of the Bayer process(Klauber et al.,2011).In general,the main mineralogical phases of BRM are gibbsite (Al(OH)),boehmite (γ-AlOOH),hematite(FeO),goethite (FeO(OH)),quartz (SiO),ana‐tase (TiO),rutile (TiO),calcite (CaCO),dolomite(CaMg(CO)),hydrogarnet (CaAl(SiO)(OH)),sodalite (Na(AlSiO)Cl),anhydrite (CaSO),and cancrinite (NaCa(CO)AlSiO·2HO) (Yang et al.,2006;Sutar et al.,2014;Sun et al.,2019).Usually,the particle size of BRM falls within the range of 3–10 μm (Han et al.,2011),and depends heavily on the degree of grinding of the bauxite(Sutar et al.,2014).

Various modifications to the standard Bayer process have been attempted to reduce the high silica levels and some bauxite ores,and improve the alumina production efficiency (Yang,1993;Meyer,2004;Bi,2006;Liu et al.,2009b).On the other hand,low grade bauxite containing 8%–15% reactive silica can be treated by a sintering process at high temperatures.

2.2 Sintering production process and its properties

In the sintering process(Fig.1),low-grade bauxite is commonly sintered with limestone and soda ash at about 1200 ℃in rotary sintering kilns to form sodium aluminate (NaO·AlO),calcium silicate (CaO·SiO),sodium ferrite (NaO·FeO),and calcium titanate(CaO·TiO) (Liu et al.,2009b).The major reactions are listed as follows:

After sintering,water leaching of the sintered residue associated with RM occurs,followed by the carbonation of the resulting liquor to precipitate aluminum hydroxide(Al(OH)),which is then used to produce alumina.The phase separation and treatment are similar to those of the Bayer process.However,the sintering process requires a higher temperature to fire the bauxite so that the alumina-containing miner‐als can be effectively broken down and rendered into a soluble form of sodium aluminate.Therefore,the sintering process is relatively energy-and capitalintensive,as well as less environmentally-friendly(Klauber et al.,2011).

For sintering red mud (SRM),SiOand CaO are the major oxides,while the main mineralogical con‐stituents are β-2CaO·SiO,calcite (CaCO),aragonite(CaCO),hematite (FeO),gibbsite (Al(OH)),and perovskite (CaTiO) (Yang et al.,2006;Sutar et al.,2014).This is why SRM is dull grey in colour,while BRM is usually dark red (Zhang et al.,2018).Due to CaCOand SiOprecipitation and crystallization during the sintering process,the particle size of SRM usually falls within the range of 1–20 μm,which is expected to be larger than that of BRM (Han et al.,2011).

2.3 Comparison of red muds from the Bayer and sintering processes

The properties and characteristics of RMs can vary depending on the different bauxite resources and alumina production processes.The chemical composi‐tions of RMs used for brick production are listed in Table 1,and the basic properties of both SRM and BRM in Table 2.

Because RM derived from different sources and treatment methods varies considerably in its chemical and mineral contents,specific methods (e.g.,firing,cementing,and geopolymerization) should be tailored for each type of RM to fabricate bricks.The effects of different RMs on the production of bricks by different methods of firing,cementing,and geopolymerization are discussed in the following section.

3 Producing brick from Bayer red mud

3.1 Production of fired brick

Fired bricks require molding and high tempera‐ture baking,and commonly have high strength.They have been applied mainly in masonry bearing and non-bearing walls.Clay is the major raw material to produce fired bricks.Compared with clay,BRM has less silica but more alumina and fluxing oxides,making it perfect for combination with clay to manufacture bricks with less open porosity and a more vitrified structure (Rehman et al.,2020).Moreover,the high alkaline oxide content of BRM makes it a suitable material for the production of fired bricks due to the stabilization of soluble sodium during firing (Kalkan,2006;Samal et al.,2013).Table 3 lists previous pub‐lished studies regarding BRM fired bricks.

LOI:loss on ignition;LOI=(?)/×100%;:weight before ignition;:weight after ignition

Sglavo et al.(2000a) pointed out that when the temperature is increased up to 900 ℃,BRM experi‐ences a loss of HO and an accompanying release of CO.This gives rise to the red colour appearance and the extremely fine particle size of BRM.Meanwhile,nepheline and NaSiOare formed at higher tempera‐tures.A ceramic body (a kind of fired building ma‐terial) with BRM achieves 20-MPa flexural strength and<40%porosity with a 102-nm average pore radius.Rai et al.(2013) proposed that BRM fired at 1200 ℃had a lower pH (within the permissible limit of 8.4),effectively tackling the caustic soda problem.Thus,BRM can be used as an alternative to clay for brick production with no efflorescence.Alekseev et al.(2019) found that in fired bricks containing 50%–100% BRM,there were strong cohesive forces be‐tween the clay and BRM particles,associated with the formation of a glassy phase at much lower tempera‐tures (900–1150 ℃),resulting in better mechanical properties than those of normal fired bricks.

To improve the performance of fired bricks,researchers have combined industry wastes such as fly ash,boron waste,and gold tailings with BRM.Singh et al.(2014) observed that a mixture of 40% BRM and 60% fly ash fired at 1200 ℃can produce excel‐lent fired lightweight foam bricks with a compressive strength of 160 MPa,48% porosity,and 1500-kg/mbulk density.Kavas (2006) found that adding 15%boron waste as a fluxing agent can enable the firing temperature to be reduced to 700 ℃and produce fired clay-BRM bricks with a compressive strength of about 300 MPa and bending strength of 10 MPa.Kim et al.(2019)discovered that adding 40%gold tailings to the fired BRM brick without clay can increase the thermal conductivity by 0.2 W/(m·K).They further produced a fired foam brick by adopting a new slurry gelation and foaming method,obtaining a brick with over 75%porosity within 30 min.Moreover,the addi‐tion of gold tailings can increase the pore diameter and produce a closed pore structure.Hence,there is high potential to use different industrial wastes in com‐bination with BRM to produce competitive fired bricks.

Different pre-treatment methods of raw materials have been adopted to improve the performance of BRM bricks,such as adding alkaline or using differ‐ent brick forming methods(e.g.,slip casting and press‐ing mixtures in three equal layers) (Table 3).Using slip casting can produce fired bricks with low shrink‐age upon drying (Sglavo et al.,2000a,2000b).Singh and Jawaid (2015) developed a new forming brick technique:each layer was given 25 blows from a 2.5-kg rammer dropped from a height of 30 cm,with a total of three layers formed.The produced fired BRM bricks showed better mechanical properties,fire retardancy,and abrasive wear resistance than ordinary bricks.The presence of 4%–5% alkali in BRM pro‐vides good fluxing action,which contributes to better plasticity and bonding in fired BRM bricks.Pretreat‐ment of BRM is important to improve the efficiency of brick manufacturing and reduce energy consumption and environmental pollution during the brick firing process.For example,prior to brick manufacture,dissolving BRM at 135–150 ℃and drying at 110 ℃for 48 h can reduce the firing temperature to 800,950,or 1100 ℃compared to that of normal fired brick(1150 ℃).This is because pretreatment can remove the moisture from the fired brick,allowing more uniform heating (Singh et al.,2014;Mandal et al.,2017;Alekseev et al.,2019).Annan et al.(2012)obtained BRM fired bricks with a bulk density of 1.3–1.8 g/cmand compressive strength of 3.2–12.5 MPa,meeting the standard brick requirement.In addition,certain burning conditions,such as controlled firing time and temperature,can enhance the performance of the produced brick.For example,Kim et al.(2019)froze the slurry mixture at–20 ℃for 24 h,followed by thawing at 20 ℃and heating the slurry gel at 60 ℃for 3 d.After that,the samples were sintered,first at 500 ℃for 2 h,and finally at 1250 ℃for 30 min.Arroyo et al.(2020) cured brick samples at 25 ℃for 48 h,then dried them at 105 ℃for 48 h.Afterwards,the samples were heated from 25 to 500 ℃at 100 ℃/h and then from 500 to 1100 ℃at 50 ℃/h,and then maintained at 1100 ℃for 8 h.The resulting bricks had a density of 1.5–2.0 g/cm,water absorp‐tion of 12%–35%,and compressive strength of 16 MPa with no environmental risk.Overall,by conducting pre-treatments and adopting appropriate forming methods to produce BRM fired bricks,a better perfor‐mance and more eco-friendly process can be expected due to the lower firing time and temperature.

3.2 Production of cementing brick

Cementing brick does not need kiln firing,but re‐lies on the cementing properties of the incorporated materials.Cement,gypsum,and lime are usually needed for the development of mechanical strength through the formation of calcium silicate hydrate(C-S-H).BRM has a low cementitious property,and its application in cementing brick is therefore limited.

Table 4 lists the few studies using BRM in ce‐menting brick.In most studies,up to 70% BRM was used with the addition of a small amount of ce‐ment,fly ash,and lime.Mymrin et al.(2003) used BRM and waste ferrous slag as raw materials to prepare cementing brick,which met the required strength of 11 MPa.Liu et al.(2009a) prepared low-iron(Fe)BRM upon recovery of Fe from BRM.They noticed that the prepared cementing brick in‐corporated with the low-iron (Fe) BRM yielded higher strength (24.1 MPa) and was more environ‐mentally friendly than ordinary cementing brick.Xu et al.(2019) combined BRM with fly ash as key in‐gredients for making cementing brick and found that with a CaO/SiOmass ratio of 1.23,the optimal me‐chanical strength(32 MPa)and water resistance(soft‐ening coefficient greater than 0.8) were achieved.They explained that the improved performance was associated with the growth of amorphous gel com‐pounds from BRM,and wastes (slag and fly ash)transitioned to a harder and denser condition due to a slow self-hydration.To accelerate the cementing brick hydration and hardening process,high humidi‐ty (95%),elevated temperature (nearly 100 ℃),and pressure (1–5 atm) are usually required for sample curing (1 atm=101.325 kPa) (Mymrin et al.,2003;Liu et al.,2009a;Xu et al.,2019).The cementitious activity of BRM has been proven to be activated by calcination.Zhou et al.(2021) activated BRM (baux‐ite tailing) by calcining it at 800 ℃for 1 h.They found that in the activated BRM-lime-gypsum ternary system,BRM shortened the ending time of the in‐duction and acceleration periods,as well as the time of occurrence of the main reaction.Moreover,the values of the main exothermic peak and the cumulative hydration heat were increased for the activated BRM.

However,due to the presence of soluble salts,such as Mg(OH),CaAl(SiO),and KAlSiOin BRM (Wang,2010),the cementing brick may have efflorescence issues.Fang(2010)presented the efflo‐rescence areas of cementing brick with 20%–80%BRM.Efflorescence on the surfaces became more obvious with increasing BRM content.This is due to the ionic compound (Na) in the alkaline BRM,which is easy to ionize,but difficult to solidify.In particular,in a humid environment (e.g.,within the cementing brick),it will dissolve in water and pene‐trate outward,resulting in the formation of sodium salt on the brick surface.Fang (2010) found that the sodium salt content was increased by 4% in BRM cementing brick.Therefore,more attention should be paid to the durability of RM brick,especially for BRM cementing bricks.The methods and mecha‐nisms for reducing the efflorescence will be dis‐cussed in detail in Section 5.

3.3 Production of geopolymer brick

High energy consumption and greenhouse gas emissions are the major drawbacks of firing and cementing methods to produce brick(Zhang,2013).As an attractive alternative material,geopolymer (alkali activated material) relies on the chemical reaction of solids rich in aluminosilicate and a highly alkaline solution to form semi-crystalline aluminosilicate inor‐ganic polymers or geopolymers (Shi et al.,2006;Duxson et al.,2007;Majidi,2009;He et al.,2012).Since BRM is abundant in silica and alumina oxides and has a high alkaline content,it can act as a raw material (precursor or activator) in geopolymer brick.Normally,BRM geopolymer brick is manufactured using 20%–80% BRM with alkaline activators and a small amount of other aluminosilicate materials,like fly ash,metakaolin (MK),slag,and rice husk ash(RHA),depending on the chemical composition,to achieve effective geopolymerization.Table 5 summa‐rizes the studies that used BRM in geopolymer brick.

As an alkaline activator in geopolymer brick,BRM can accelerate the dissolution of the aluminosili‐cate precursor,thereby increasing the polymerization rate.Choo et al.(2016) produced geopolymer (using fly ash as precursor)using BRM(high alkalinity:pH around 11) as an NaOH supplier.The 60% BRM geopolymer product had a compressive strength of 1.7 MPa,comparable to that of the control samples without BRM.Ayg?rmez (2021) found that a 25%BRM geopolymer brick(using metazeolite as precur‐sor) had good performance (compressive strength of above 65 MPa,flexural strength of 11 MPa,and strength stability after the durability tests),and showed about a 12%increase in compressive strength compared with control samples without BRM.In essence,the geopolymerization reaction is the chemical reaction of pozzolanic phases,while the oxides(CaO,SiO,and AlO) in raw materials play a role in the formation of hydration products and strength development in geopolymer bricks(Mendes et al.,2021).

Attempts have been made to mix BRM with a wide variety of wastes,such as fly ash,RHA,munici‐pal solid waste incineration fly ash(MSWIFA),waste ferrous slags,and bottom ash,as the precursor in the production of geopolymers.Different oxides present in these wastes can react with the silica and alumina oxides as well as the alkaline fraction in BRM.The reaction product is usually an amorphous to semicrystalline polymeric structure,binding the individual particles of BRM and thereby transforming the initial granular material into a compact matrix(Dimas et al.,2009).Kumar and Kumar (2013) and Kumar (2015)used BRM and fly ash to make geopolymer bricks with a strength of 30 MPa.Komnitsas and Zaharaki(2015) and Singh et al.(2020) added granulated blast furnace slag (GBFS) and electric arc furnace (EAF)slag to produce geopolymer bricks (30% BRM) with a strength of more than 50 MPa.However,some researchers found that the brick had poor flexural strength and freeze-thaw resistance,which was attrib‐uted to the 2D layered internal structure of the synthe‐sized inorganic polymers.The layers were weakly cross-linked,probably due to an insufficient amount of aluminum in the polymeric phase (Dimas et al.,2009).To address this issue,Panias et al.(2014)added ferronickel slag and MK,which provided sufficient Al in the BRM geopolymer bricks to strengthen the bonding of the layered structure,resulting in a substan‐tial improvement in the flexural strength and durability of the brick under moderate weathering conditions.

4 Producing brick from sintering red mud

Compared with BRM,SRM has lower alkaline leaching due to the abundance of insoluble CaCO(Wang,2010).Bricks made by SRM are normally immune to efflorescence and thereby have better dura‐bility.Therefore,SRM bricks are safer for applications in construction industries.Studies of the application of SRM in producing fired,cementing,and geopolymer bricks are summarized in Tables 6–8.

4.1 Production of fired brick

Table 6 lists published studies regarding the use of SRM in fired bricks.Wu et al.(2010) produced simple ceramic bricks (a kind of wall insulation material) by a normal firing method using SRM,fly ash,and clay shale as the main raw materials.The bricks exhibited good performance:45.64%water ab‐sorption,58.91% porosity,1.29-g/cmbulk density,54.91-MPa compressive strength,29.52-MPa bend‐ing strength,as well as obtaining 29.28-MPa com‐pressive strength after freeze-thaw cycles.Lv et al.(2013) prepared fired bricks using a combination of SRM (10%–70%),shale,and sintering activation ma‐terials.The results showed that when the ratio of SRM：shale：industrial wastes was 50：35：15,the sin‐tered bricks could meet the requirements of grade-A brick according to the GB 5101-2003 standard(GAQSIQ,2003),and yielded 25-MPa compressive strength,3.71-MPa flexural strength,1.34-g/cmdensity,and 17.57% water absorption.Gehlenite(CaAlSiO),wollastonite (CaO·SiO),nepheline(NaAlSiO),glass phase,and remained quartz were present in the SRM fired bricks,contributing to the denser structure and higher strength of the finished brick.Ma et al.(2014) prepared a lightweight insula‐tion brick using SRM,fly ash,and bentonite as the main raw materials along with foaming and fluxing agents.The results showed that the brick had low density(<0.67 g/cm)and water absorption(<5%).It was also fairly stable in preserving heat over time,implying that it had good heat insulation.

4.2 Production of cementing brick

Table 7 summarizes previous studies on the use of SRM in cementing bricks.SRM contains abun‐dant amorphous silicon aluminates,has fine granu‐larity and high plasticity (Feng et al.,2015),and therefore has potential cementitious activity andexcellent properties for cementing brick preparation.Zhao and Gou (2021) prepared cementing bricks by incorporating SRM in calcium sulfoaluminate cement(binary blends),and found that the 70% SRM bricks had a compressive strength of 30–35 MPa.The solu‐ble Na in SRM promoted the hydration of CAand CS.SRM also reduced the porosity of the hydration products of the binary blends.On the other hand,the soluble Na and F could be immobilized by the precipi‐tation of AlFand CaF,respectively.Note that the dissolution of heavy metals was prevented,which satisfies the requirements detailed in GB 5749-2006(NHC,2006) and GB 6566-2010 (GAQSIQ,2010;Zhao and Gou,2021).

A cement-fly ash-lime system was proven to be effective in stabilizing SRM.SRM cementing bricks were prepared with 20%–70% SRM,18%–30% fly ash,6%–10%lime,and 1%–15%cement.The C-S-H gel and ettringite present in the bricks were responsible for the improved performance.Feng et al.(2015)produced the cementing bricks by mixing 44%–55% SRM,19%–30% fly ash,6%–7% lime,4%–6% α-hemihydrate gypsum,25%–35% Yellow River silt,and 2% ordinary cement,with a water to binder ratio of 14%–18%.Then,the mixtures were molded under 15-MPa pressure and cured at room tempera‐ture for 12 h.The 28-d compressive strength of the produced brick was up to 18 MPa,meeting the stan‐dard requirements of bricks.Yang and Xiao (2008)proposed that the optimal proportions of the materials in SRM bricks were 25%–40% SRM,18%–28% fly ash,30%–35% sand,8%–10% lime,1%–3% gyp‐sum,and about 1% Portland cement.The produced SRM cementing brick could achieve 29.5-MPa com‐pressive strength,22.39% water absorption,1.62-g/cmdensity,and 0.63% shrinkage.However,Zhao et al.(2020) reported that the optimal proportions of SRM brick constituents were 35% RM,14% fly ash,8% lime,15% cement,and 28% sand.These bricks had the best performance with a compressive strength of more than 19 MPa after 25 freeze-thaw cycles,water absorption of 15.8%,and shrinkage of 0.04%.

Ma and Li(2013,2014)studied the effect of acti‐vator and foaming agents on the properties of light‐weight cementing bricks prepared with SRM.The re‐sults showed that by mixing 1% activator and 10-mL foaming agent per 390-g mix,the bricks attained low density (423 kg/m) and gained a 64.4% and 44.1% increase in the flexural strength (0.49 MPa)and compressive strength (1.87 MPa),respectively,compared to samples without additives.The improve‐ment in properties was attributed to the additives inducing an even distribution of closed pores in the SRM cementing bricks.

Overall,previous studies showed that it is feasi‐ble to use SRM to produce cementing bricks,due to its higher CaO content and abundant CS phase,which give rise to its cementitious activity(Yang et al.,2006;Sutar et al.,2014).The properties of the SRM cement‐ing bricks are influenced by the formation and micro‐structure of C-S-H,ettringite,and hydration products.

4.3 Production of geopolymer brick

Table 8 lists studies on the use of SRM in geo‐polymer bricks.Zhang et al.(2011) investigated the pozzolanic behavior of the compound-activated SRMcoal gangue-lime system and pointed out that the hydration products were mainly aluminous C-S-H,CaAlO·

x

HO,and a high content of Ca(OH).Thus,it is feasible to use SRM as a raw material in produc‐ing geopolymer bricks.Fu and Xiong (2014) mixed 30% gypsum,10% RM and fly ash,and 20% lime to prepare an SRM-based geopolymer with a compres‐sive strength of 15.44 MPa.Zhou et al.(2020) used 30%–70% refuse mudstone,GBFS,and SRM as the precursor materials,and water glass as the alkaline activator,to produce geopolymer composites contain‐ing 21% SRM.The end products had the highest unconfined compressive strength of 23.48 MPa and the highest Brazil tensile strength of 2.98 MPa.Scanning electron microscopy (SEM) observations revealed that the SRM geopolymer composites had a compact mi‐crostructure.However,larger-scale studies are needed to explore the mix proportion,manufacturing methods,performance,and mechanisms of SRM geopolymer bricks.

5 Discussion

5.1 Comparison between BRM and SRM in RMmodified brick

The advantages and disadvantages of differ‐ent types of RM-modified bricks are reviewed and summarized in Table 9.Regardless of the brick type,20%–80%RM can be used to produce bricks.Differentmaterials were used as the additive/activator/precursor for RM bricks to stimulate the RM activity.For exam‐ple,clay and fly ash were added for RM fired bricks,and cement,lime,fly ash,and gypsum were mixed for RM cementing bricks.Coal gangue,fly ash,MK,slag,and RHA were used as the precursors for RM geopolymer bricks.To produce a more environmentally-friendly brick,metal minerals can first be recovered from RM.Alternatively,innovative technology,such as accelerated carbonation,can be used to improve the RM reactivity.

Fired bricks have been the most used product because of their high strength(up to 54 MPa)(Nivetha et al.,2017).From an environmental protection per‐spective,however,they are undesirable due to their high energy consumption and COemissions during the firing process.Instead,RM cementing bricks are more environmental-friendly,although they have a lower performance in terms of their mechanical prop‐erties (e.g.,lower compressive strength) and durability(e.g.,higher water absorption and potential efflores‐cence) (Wang et al.,2011;Yin et al.,2011).RM geo‐polymer bricks seem to represent a promising new approach to overcome some of the above-mentioned drawbacks,with high strength (up to 51 MPa),good durability (no efflorescence),and lower energy con‐sumption and COemissions(Singh et al.,2016).

The RMs produced from different processes(Bayer and sintering) have different salient features for each type of brick.BRM is usually used to produce fired and geopolymer bricks since it contains a higher amount of AlO,SiO,and alkaline phases (higher pH),which are more favorable for calcination and geopolymerization.On the other hand,SRM is more suitable for cementing brick manufacture due to its higher CaO content and abundant CS phase,which give rise to better cementitious activity.

In addition to the three types of bricks discussed,an innovative brick has been developed by simply mixing RM with water cured by accelerated carbon‐ation at room temperature (Liu et al.,2017b).This SRM-based ceramic brick was prepared as follows:SRM and water were mixed for 10 min at a constant ratio of 1：10,followed by extrusion into a mold with 20-MPa extruding pressure.The semi-finished brick was then immediately placed into a sealed carbonation chamber with 99.9% CO,at 0.3-MPa COpressure,98% relative humidity,and (23±2) ℃.The brick was carbonated for 5 and 24 h to obtain 23.1-and 25.0-MPa compressive strengths,respectively.In addi‐tion,up to 100 kg of COwas sequestrated per ton of SRM used.The strength enhancement was attributed to the formation of crystalline calcium carbonate and amorphous silica gels during the carbonation process.Therefore,as a raw material for manufacturing bricks,RM can be used not only with other solid wastes,but also with agricultural and industrial wastes,such as gas waste and liquid waste,in an attempt to maximize environmental protection in a cost-effective way.

5.2 Efflorescence consideration

The main reason that RM has not been widely used to produce bricks is the potential efflorescence issue (Fang,2010).Besides negatively affecting the brick’s appearance,it also deteriorates the bonding between the brick and mortar.As a result,the brick strength and durability are impaired,leading to serious damage to the masonry structure.Therefore,it is nec‐essary to understand the efflorescence mechanism and find a viable method to solve this vexing problem.

In a humid environment,sodium alkali or salt(e.g.,NaOH or NaCO) in RM will easily dissolve and ionize to produce OH,which reacts with oxides such as CaO or MgO.Afterwards,it penetrates out‐wards and migrates to the brick surface,forming efflo‐rescence (Chen,2016).The main reactions are de‐scribed as follows(Fang,2010):

Compared with insoluble salts,soluble salts have a much bigger impact on the efflorescence level due to their higher alkalinity(Xing,2010).SRM consists mainly of insoluble CaCO,followed by NaCl,KCl,CaTiO,FeN,and KF.In contrast,the main alkali con‐stituents of BRM are soluble salts,such as NaNO,Mg(OH),CaAl(SiO),and KAlSiO(Wang,2010).Thus,compared with SRM bricks,BRM bricks are prone to more severe efflorescence.Compared with fired bricks and geopolymers,BRM cementing bricks experience the greatest formation of efflorescence because almost all of the alkaline phases in BRM dissolve in water,penetrate outwards,and migrate to the brick surface.Geopolymer has a lower propensity for efflorescence due to the use of parts of alkaline phases as an activator.The fired bricks are the least affected by efflorescence.This is because the soluble alkaline substances in BRM are converted into insol‐uble salts during the firing process (Xing,2010;Rai et al.,2013).

To solve the efflorescence issue,many ap‐proaches have been used to reduce the alkalinity of cementing bricks,including double reverse washing(Wang,2010),adding lime to dissolve the BRM at 170 ℃and 0.9 MPa (Zhong et al.,2009),a suspen‐sion carbonation method (Wang et al.,2009),and a slurry multi-stage cycle method (Yang et al.,2012).However,viable approaches with satisfactory results remain elusive.Further studies are needed to ascer‐tain the applicability of de-alkaline processes in a larger-scale industrial setting.

5.3 Radioactivity concerns

One of the major concerns of using RM in brick production is its radioactivity,requiring strict compli‐ance with regulations (Brunori et al.,2005;Santona et al.,2006;Somlai et al.,2008;Gu et al.,2011;Luo et al.,2013).Bauxite is associated with high radionu‐clides and trace elements,such as uranium,thorium,radium-226,and potassium-40.These elements exist in some minerals,such as zircon and monazite,which are difficult to remove during alumina production(Luo et al.,2013).Usually,these radioactive elements can penetrate several meters thickness of cement concrete,producing ionizing radiation that can greatly harm human health (Gu et al.,2011).Unlike stabilizing or solidifying heavy metals,radioactivity cannot be con‐tained easily in solid materials(Heged?s et al.,2016).

Two main types of methods have been proposed to reduce the risks from radioactivity:reducing the self-radioactivity of RM and adopting radioactive shielding measures.Qin and Wu(2011)found that the radioactivity levels of RM(6360 Bq)could be reduced to the natural radioactive background level in Guilin,China(3600 Bq)when producing self-glazing RMbased ceramic materials,owing to the sintering process.This also can be achieved by removing the radioactive minerals from RM using magnetic separation.For radioactive shielding,spraying surfactant such as anti-radiation functional mortar or paint has also been proven to be effective (Luo et al.,2013).Tian et al.(2015) and Wang et al.(2016) suggested that using barite,which contains barium(Ba)atomic energy and boron elements,could activate the attenuation of radi‐ation energy,particularly for X-ray absorption (Mann et al.,2013;Wang et al.,2016).Boron-rich slag can also offer effective radiation shielding (Tian et al.,2016).However,to ascertain safeness for construction and building applications,tests of the internal expo‐sure index (

I

) and external exposure index (

I

) should be carried out.In China,RM bricks must be tested and meet the Chinese standard GB 6566-2010 (GAQSIQ,2010)before they can be introduced into the construc‐tion industry(Zhang et al.,2018,2021).

6 Conclusions

In this paper,the characteristics of BRM and SRM were compared and their applications in the production of different bricks including fired,cement‐ing,and geopolymer bricks were thoroughly reviewed.Several conclusions can be drawn:

BRM with high alkalinity is generated from caustic soda in the alumina production process and has a high AlOcontent derived from the bauxite resource,whereas SRM has lower alkalinity with a certain cementitious property because of the amorphous aluminosilicate generated from the high temperature sintering process.The addition of limestone in the sintering process results in the presence of a high CaO content in SRM.As a result,efflorescence is the main issue for BRM cementing bricks due to high alkalinity and content of free oxides,as explained in Section 5.To date,there is no effective way to solve this problem.Compared with BRM cementing bricks,BRM fired bricks and geopolymer bricks are more durable and stronger.Therefore,BRM is considered as a good raw material to produce these two types of bricks.

SRM has received less research attention than BRM due to the complexity of the sintering process.Only a few studies have investigated the feasibility of using SRM in bricks.However,SRM is not suitable for manufacturing fired bricks because an intensive manufacturing process is required to solidify and stabilize the alkalinity.Instead,SRM is considered as a better option in the production of cementing bricks due to the presence of abundant amorphous alumino‐silicate.Compared with fired and cementing bricks,geopolymer bricks usually have better strength and durability because of the high alkalinity of RM,which is favorable for the geopolymerization reaction.

There is an urgent need to address the efflores‐cence issue in RM-modified bricks,especially BRM cementing bricks,because efflorescence is the key issue that greatly limits the wide use of RM in brick production.Regarding the potential radiation issue,several solutions have been suggested in the literature,though it is not considered a great concern as long as the use of RM meets the standard regulations.There‐fore,recycling and reusing RM in the production of bricks are still recommended to reduce environmental contamination and enhance the brick properties.Pro‐cess optimization of RM brick production,especially geopolymer bricks,should be systematically investi‐gated to broaden its application.More importantly,innovative and economically viable methods,such as carbonation of RM,should be given attention to develop new types of environmentally-friendly RM bricks at an industry scale.Last,but not least,to have larger-scale RM applications in the future,it is neces‐sary to analyze the economic benefits and costs of RM bricks.

Acknowledgments

This work is supported by the Hunan Provincial Program for Innovative Talents of Excellent Postdoctoral Fellow (No.2020RC2031),China.

Author contributions

Yu-jia XIAO wrote the first draft of the manuscript.Michelle TIONG,Kim Hung MO,Ming-zhi GUO,and Tungchai LING helped to organize the manuscript.Tung-chai LING,Michelle TIONG,and Yu-jia XIAO revised and edited the final version.

Conflict of interest

Yu-jia XIAO,Michelle TIONG,Kim Hung MO,Ming-zhi GUO,and Tung-chai LING declare that they have no conflict of interest.

Introducing Editorial Board Member

Prof.Tung-Chai LING has been the editorial board member of

Journal of Zhejiang University-SCIENCE A (Applied Physics &Engineering)

since 2016.He is a professor at College of Civil Engineering,Hunan University,China.Prior to the appointment,he was worked at the USA,UK,HK SAR of China,and Malaysia,focusing on the resource utilization of solid wastes and carbon sequestration technology in the field of construction materials.His works have been published in 129 SCI journal papers (100 as first/corresponding author),garnering over 4400 citations and an h-index of 41.He presided over 18 competitive national and provincial research projects.He has been elected as an associate fellow of the ASEAN Academy of Engineering and Technology (AAET),and fellow of the Hong Kong Concrete Institute (HKCI),China;serving as the associate editor of the ASCE

Journal of Materials in Civil Engineering

and the editorial board member of several international journals.He has been listed as one of the top 2% of global scientists (Career-long impact),and was awarded the“Robert L’Hermite Medal”by International Union of Laboratories and Experts in Construction Materials,Systems and Structures(RILEM)in 2020.The medal is awarded to one young outstanding scientist globally every year,and he is the first scholar from a Chinese university to receive this prestigious award since the inception of the medal in 1967.