Characterization of Slag Cement Mortar Containing Nonthermally Treated Dried Red Mud

applied sciences

Article Characterization of Slag Cement Mortar Containing Nonthermally Treated Dried Red Mud

Gyeongcheol Choe 1 , Sukpyo Kang 2,* and Hyeju Kang 2 1 Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea; [email protected] 2 Department of Architecture, Woosuk University, Jincheon 27841, Republic of Korea; [email protected] * Correspondence: [email protected]

Received: 30 May 2019; Accepted: 18 June 2019; Published: 20 June 2019

Abstract: In this study, a method was suggested to produce dried powder from red mud (RM) sludge with 40%–60% water content without heating. The RM sludge is discharged from the Bayer process, which is used to produce alumina from bauxite ores. Nonthermally treated RM (NTRM) powder was produced by mixing RM sludge (50%), paper sludge ash (PSA, 35%), and high-calcium fly ash (HCFA, 15%). The physicochemical properties of NTRM were investigated by analyzing its water content, X-ray fluorescence spectra, X-ray diffraction patterns, and particle size. Moreover, to examine the applicability of NTRM as a construction material, slag cement mortar in which 20 wt% of the binder was replaced with NTRM was produced, and the compressive strength, porosity, and water absorption rate of the mortar were evaluated. Results indicated that NTRM of acceptable quality was produced when the water content in RM sludge decreased and CaO contained in PSA and HCFA reacted with moisture and formed portlandite. The NTRM-mixed mortar requires further examination in terms of durability because of the increased capillary voids and high water absorption rate, but its compressive strength is sufficient to enable its use in sidewalks, bike roads, and parking lots.

Keywords: red mud; bauxite; slag cement; paper sludge ash; high-calcium ﬂy ash

1. Introduction Sustainable development of an industry depends on effective treatment of industrial wastes and conservation of natural resources. Therefore, it is important to transform industrial wastes into raw materials that can replace natural resources [1]. Red mud (RM) is an industrial byproduct generated from the Bayer process, which is used to produce alumina, and is one of the most commonly generated industrial byproducts in modern society [2]. Production of 1 ton of alumina generates 1.0–1.5 tons of RM [3]. Growth of the aluminum industry has increased the number of factories, and it is estimated that 1.2 million tons of RM is being generated every year, and 4 billion tons of RM has been generated worldwide as of 2015. RM discharged from the Bayer process is an insoluble solid that exhibits high alkalinity (pH 10–12.5) because of the influence of caustic soda, which is added in the course of extracting alumina from bauxite ores [4]. Dumping of RM in the sea or in open yards may cause various environmental problems, and landfills can reach their limits because of such dumping [1,3]. Therefore, new RM treatment methods are required for sustainable growth of the aluminum industry [3,5] Use of industrial byproducts as construction materials is one of the good methods to reuse such byproducts in large quantities [6,7]. Efforts have been made in various fields to use RM as a construction material. For example, Tsakiridis et al. [8] produced cement through sintering at 1450 ◦C using RM as a raw material. Senff et al. [7] examined the rheology and curing characteristics of cement mortar containing RM. Villarejo et al. [9] attempted to produce ceramics by replacing clay with RM and

Appl. Sci. 2019, 9, 2510; doi:10.3390/app9122510 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2510 2 of 13 proposed optimal conditions for the RM substitution rate, sintering temperature, and sintering time. Ribeiro et al. [10] examined a method of utilizing RM in the production of ordinary Portland cement (OPC) mortar, and they reported that the workability of mortar decreased because of the addition of RM, but the water absorption rate could decrease and the compressive strength could increase when the RM substitution rate was 20% or less. Liu and Poon [11] confirmed that the addition of RM into self-compacting mortar had various benefits such as an increase in compressive and tensile strengths and a decrease in drying shrinkage. Moreover, Geng et al. [12] examined the production of geopolymers using RM. As described above, many researchers have made efforts to use RM as a construction material, but there are still several limitations in the production process used for such a material. In the production of ceramics using RM, an increase in the amount of RM leads to benefits such as an increase in the compressive strength of ceramics and a decrease in the water absorption rate because the presence of RM increases the vitreous phase in the ceramic matrix; however, the firing process used in production requires a high temperature of more than 900 ◦C[7]. In most studies that used RM as a base material for binders or geopolymers, dried RM powder was used. However, RM in the Bayer process is discharged in a slurry state with 40%–60% water content. Because water content in the RM sludge varies depending on the storage location or period, appropriate management of water content is necessary to use RM as a raw material [13]. Therefore, RM is turned into powder through a high-temperature drying process, which requires additional energy input. The cost increase resulting from this energy-intensive pretreatment process may act as another obstacle to the use of RM as a construction material [14]. Therefore, technology is required that can convert RM into a construction material using minimal energy input. This study attempted to develop a method of controlling the water content in RM sludge without heat treatment. The method of removing moisture without heating is based on the principle of using soil stabilizers to stabilize the soft ground. It is possible to reduce moisture in soil by inducing formation of hydration products through mixing of inorganic powder (e.g., Portland cement) that facilitates the hydration reaction with soil. Various industrial byproducts having MgO or CaO are currently being used as soil stabilizers. This study attempted to reduce the moisture in RM sludge without heat treatment by mixing CaO with the RM sludge. Nonthermally treated dried red mud (NTRM) was produced by mixing RM sludge, Paper sludge ash (PSA), and high-calcium fly ash (HCFA). PSA and HCFA were used to supply CaO. Changes in water content, X-ray fluorescence (XRF) spectra, X-ray diffraction (XRD) patterns, and particle size of NRTM were analyzed over time, and results were compared with those of the conventionally used dried RM powder. Moreover, NTRM was added to slag cement mortar, and the characteristics of this mortar were experimentally analyzed to examine the possibility of converting RM into a construction material.

2. Nonthermally Treated Dried Red Mud (NTRM) Manufacturing

2.1. Raw Materials Table1 lists the chemical composition of raw materials used in the manufacturing of NTRM. The RM sludge used in this study was produced from the Bayer process. Its water content was 50%, and its viscosity was 10,000 cP. SiO2, Al2O3, Fe2O3, and Na2O, which are major oxides, accounted for more than 87% of the total mass. As previously stated, the water content of red mud sludge emitted in the Bayer process varies in the range of 40%–60%. The amount of water in RM sludge can aﬀect the water content in NTRM. Therefore, in this study, RM sludge with constant water content (50%) was used to avoid the inﬂuence of water content. Appl.Appl. Appl.Sci. Sci. 2019 Sci. 2019, 20199,, x9 ,FOR ,x 9 FOR, x PEERFOR PEER PEER REVIEW REVIEW REVIEW 3 of3 of313 of 13 13

SiOSiO2SiO, and2, and2, andCaO CaO CaO represent represent represent more more more than than than 50% 50% 50%of of the ofthe total the total total ma mass. mass. Forss. For PSAFor PSA PSA used used used in in this inthis study,this study, study, the the CaO the CaO CaO content content content waswas was59.70%, 59.70%, 59.70%, and and andthe the contentthe content content of of Al ofAl2O Al23O, 2SiO3O, SiO3,2 SiO, and2, and2, andCaO, CaO, CaO, which which which are are major are major major oxides, oxides, oxides, was was was86.53%. 86.53%. 86.53%. HCFAHCFAHCFA is isone isone oneof of the ofthe byproductsthe byproducts byproducts generated generated generated in in thermal inthermal thermal power power power plants plants plants that that thatuse use usea acirculating circulatinga circulating fluidizedfluidizedfluidized bed. bed. bed. It Itis isproducedIt producedis produced fr omfrom fr maintainingom maintaining maintaining the the combustion the combustion combustion temperature temperature temperature at atapproximately atapproximately approximately 850 850 °C850 °C °C andand andspraying spraying spraying ammonia ammonia ammonia to to prevent toprevent prevent discharge discharge discharge of of nitrogen ofnitrogen nitrogen oxides oxides oxides from from from power power power plants. plants. plants. In In South InSouth South Korea, Korea, Korea, 400,000400,000400,000 tons tons tons of of HCFA ofHCFA HCFA is isgenerated isgenerated generated each each each year, year, year, but but thebut the entirethe entire entire amount amount amount is isbeing isbeing being landfilled landfilled landfilled instead instead instead of of of beingbeingbeing used used used as as an asan admixture an admixture admixture for for concrete for concrete concrete and and andas as a asrawa raw a rawmaterial material material for for cement for cement cement because because because it failsit failsit failsto to meet tomeet meet the the the KoreanKoreanKorean industrial industrial industrial standard standard standard (KS (KS L0(KS L0 5405) L0 5405) 5405) [15,16]. [15,16]. [15,16]. For For HCFAFor HCFA HCFA used used used in in this inthis study,this study, study, the the major the major major oxides oxides oxides were were were AlAl2OAl23O, 2SiO3O, SiO3,2 SiO, and2, and2, andCaO CaO CaO as as in asin the inthe casethe case caseof of PSA, ofPSA, PSA, and and andthe the CaOthe CaO CaO content content content was was was41.20%. 41.20%. 41.20%.

Appl. Sci. 2019, 9, 2510 3 of 13 TableTableTable 1. 1.Chemical Chemical1. Chemical composition composition composition of ofraw ofraw materials.raw materials. materials.

ContentContentContent (mass%) (mass%) (mass%) ElementElementElement oxide oxide oxide Table 1. Chemical compositionPaperPaperPaper Sludge Sludge Sludge of rawAsh Ash materials. Ash High-CalciumHigh-CalciumHigh-Calcium Fly Fly AshFly Ash Ash RedRed RedMud Mud Mud (RM) (RM) (RM) sludge sludge sludge (PSA)(PSA)(PSA) (HCFA)(HCFA)(HCFA) Content (mass%) ElementSiOSiO2SiO 2 Oxide2 38.8038.8038.80 17.00 17.00 17.00 26.70 26.70 26.70 Red Mud (RM) Sludge Paper Sludge Ash (PSA) High-Calcium Fly Ash (HCFA) AlAl2OAl23O 23O 3 16.1016.1016.10 9.839.83 9.83 13.1013.1013.10 FeFe2OSiOFe23O 23O2 3 22.8022.8038.8022.80 6.77 17.00 6.77 6.77 8.71 8.71 26.70 8.71 Al O 16.10 9.83 13.10 MgOMgOMgO 2 3 0.200.20 0.20 4.054.05 4.05 4.404.40 4.40 Fe2O3 22.80 6.77 8.71 NaNa2OMgONa2 O2 O 10.0010.00 0.2010.00 4.05 - - - - 4.40 - - CaOCaONaCaO 2 O 3.403.40 10.00 3.40 59.70 59.70 - 59.70 41.20 41.20 41.20 - TiOTiOCaO2TiO 2 2 2.402.40 3.40 2.40 0.39 59.70 0.39 0.39 - 41.20 - - TiO 2.40 0.39 - SOSO3 SO3 23 0.240.24 0.24 1.801.80 1.80 5.285.28 5.28 SO3 0.24 1.80 5.28

2.2. Experimental2.2. Experimental Plan Plan to Manufacture to Manufacture NTRM NTRM 2.2.In Experimental South Korea, Plan the to Manufacture amount of NTRM paper sludge generated is approximately 1.6 million tons per year, andTableTableTable 20%–30% 2 2presents presents2 presents of the it the is experimentalthe estimatedexperimental experimental to plan beplan plan convertedfor for thfor the emanufacturingth manufacturinge into manufacturing PSA, which of of NTRM. ofNTRM. can NTRM. be It It clayconsisted Itconsisted consisted based of orof RM of limeRM RM based sludgesludgesludge (50%), (50%), (50%), PSA PSA PSA (35%), (35%), (35%), and and andHCFA HCFA HCFA (15%), (15%), (15%), which which which were were were mixed mixed mixed for for 4 for min4 min4 minusing using using an an asphalt an asphalt asphalt mixer mixer mixer for for for depending on the additive type. Because the additive used in South Korea is mainly limestone, Al2O3, thethe experiments.the experiments. experiments. As As the As the mixturethe mixture mixture that that thatwas was wasdischarg discharg dischargeded fromed from from the the asphaltthe asphalt asphalt mixer mixer mixer was was wascondensed condensed condensed in in the inthe the SiO2, and CaO represent more than 50% of the total mass. For PSA used in this study, the CaO content shapeshapeshape of of balls, ofballs, balls, they they they were were were crushed crushed crushed into into intopowder powder powder using using using a pina pin a millpin mill millat at1500 at1500 1500 rpm. rpm. rpm. was 59.70%, and the content of Al2O3, SiO2, and CaO, which are major oxides, was 86.53%. ToTo investigateTo investigate investigate the the characteristicsthe characteristics characteristics of of NTRM, ofNTRM, NTRM, the the waterthe water water content, content, content, particle particle particle size, size, size, XRF XRF XRF spectra, spectra, spectra, and and and HCFA is one of the byproducts generated in thermal power plants that use a circulating ﬂuidized XRDXRDXRD patterns patterns patterns were were were analyzed. analyzed. analyzed. Measurement Measurement Measurement results results results of of NTRM ofNTRM NTRM were were were compared compared compared with with with the the driedthe dried dried RM RM RM bed.throughthroughthrough It is heating. producedheating. heating. The The Thewater from water water content maintaining content content was was wasmeasured measured the measured combustion 2, 2,12, 12,2, 24, 12, 24, 48, 24, temperature48, 72, 48, 72, 96, 72, 96, and 96, and and120 at120 h120 approximately afterh afterh after completion completion completion 850 of of ◦ ofC and sprayingmixing.mixing.mixing. The ammoniaThe Themass mass mass of of tothe ofthe prevent samplethe sample sample before discharge before before and and andafter of after nitrogenafter drying drying drying was oxideswas wasmeasured, measured, measured, from powerand and andthe the plants.measuredthe measured measured In values Southvalues values Korea, 400,000werewerewere used tonsused used to ofto calculate to HCFAcalculate calculate the is the generated watethe wate water contentr contentr eachcontent using year,using using Equation but Equation Equation the entire(1). (1). (1). amount is being landﬁlled instead of being used as an admixture for concrete and as a raw material ()()()for cement ( because)()() it fails to meet the Korean 𝑊𝑎𝑡𝑒𝑟𝑊𝑎𝑡𝑒𝑟𝑊𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑐𝑜𝑛𝑡𝑒𝑛𝑡(%(%) =(%) =) = × 100× 100×, 100, , (1)(1) (1) ()()() industrial standard (KS L0 5405) [15,16]. For HCFA used in this study, the major oxides were Al2O3, SiO2,The andThe Theparticle CaO particle particle as size in size thesizedistributions distributions casedistributions of PSA, of of NTRM andofNTRM NTRM the and and CaO andRM RM contentRMwere were were examined examined was examined 41.20%. using using using a Particlea Particlea Particle size size sizeanalyzer analyzer analyzer (Mastersizer(Mastersizer(Mastersizer 3000-Maz6140, 3000-Maz6140, 3000-Maz6140, Malvern Malvern Malvern Instruments Instruments Instruments Lt Ltd., d.,Lt Malvern,d., Malvern, Malvern, UK). UK). UK). The The TheXRF XRF XRF spectrometer spectrometer spectrometer (ZSX (ZSX (ZSX 2.2.PrimusPrimus ExperimentalPrimus IV, IV, RIGAKU, IV, RIGAKU, RIGAKU, Plan Tokyo, toTokyo, ManufactureTokyo, Japan) Japan) Japan) was was NTRM wasused used used to to assess toassess assess the the chemical the chemical chemical compositions compositions compositions of of NTRM ofNTRM NTRM and and andRM. RM. RM. TheThe TheXRD XRD XRD patterns patterns patterns for for NTRMfor NTRM NTRM and and andRM RM RMwere were were collected collected collected using using using an an X-rayan X-ray X-ray Diffra Diffra Diffractometerctometerctometer (Smartlab, (Smartlab, (Smartlab, Table2 presents the experimental plan for the manufacturing of NTRM. It consisted of RM sludge RIGAKU,RIGAKU,RIGAKU, Tokyo, Tokyo, Tokyo, Japan) Japan) Japan) The The Thediffraction diffraction diffraction patterns patterns patterns were were were obtained obtained obtained at at2θ at2 θfrom 2 fromθ from 5° 5° to 5°to 65° to65° in65° in steps insteps steps of of 0.01°, of0.01°, 0.01°, (50%),withwithwith each PSAeach each step (35%),step steplasting lasting lasting and 1 s.1 HCFAs. The1 Thes. Thedivergence divergence (15%), divergence whichslit slit was slit was were was0.3°, 0.3°, 0.3°, while mixed while while the the for sollerthe soller 4 soller min slit slit was usingslit was was2.5°. 2.5°. an 2.5°. asphalt mixer for the experiments. As the mixture that was discharged from the asphalt mixer was condensed in the shape of balls,TableTable theyTable 2. 2.Experimental were Experimental2. Experimental crushed plan plan into planand and powdermixtureand mixture mixture proportion usingproportion proportion a pinto tomanufacture milltomanufacture manufacture at 1500 nonthermally nonthermally rpm. nonthermally treated treated treated dried dried dried red red red mudmudmud (NTRM). (NTRM). (NTRM). Table 2. Experimental plan and mixture proportion to manufacture nonthermally treated dried red mud (NTRM).

Materials Proportion (wt%) Mixing Method Measured Parameters Water content RM sludge 50 Mixer type: Particle size PSA 35 Asphalt mixer XRF spectra HCFA 15 Mixing time: 4 min XRD patterns

To investigate the characteristics of NTRM, the water content, particle size, XRF spectra, and XRD patterns were analyzed. Measurement results of NTRM were compared with the dried RM through heating. The water content was measured 2, 12, 24, 48, 72, 96, and 120 h after completion of mixing. Appl. Sci. 2019, 9, 2510 4 of 13

The mass of the sample before and after drying was measured, and the measured values were used to calculate the water content using Equation (1).

Weight o f undried NTRM(g) Weight o f dried NTRM(g) Water content(%) = − 100 (1) Appl. Sci. 2019, 9, x FOR PEER REVIEW Weight o f undried NTRM(g) × 4 of 13 The particle size distributions of NTRM and RM were examined using a Particle size analyzer Materials Proportion (wt%) Mixing method Measured parameters (Mastersizer 3000-Maz6140, Malvern Instruments Ltd., Malvern, UK). The XRF spectrometer Water content (ZSX PrimusRM sludge IV, RIGAKU, Tokyo,50 Japan) was used to assess the chemical compositions of NTRM and Mixer type: Asphalt mixer Particle size RM. ThePSA XRD patterns for NTRM35 and RM were collected using an X-ray Diﬀractometer (Smartlab, Mixing time: 4 min XRF spectra RIGAKU,HCFA Tokyo, Japan) The diﬀ15raction patterns were obtained at 2θ from 5 to 65 in steps of 0.01 , ◦ XRD◦ patterns ◦ with each step lasting 1 s. The divergence slit was 0.3◦, while the soller slit was 2.5◦.

2.3. Characteristics Characteristics of of NTRM

2.3.1. Water Water Content Figure 11 showsshows thethe waterwater contentcontent inin NTRM NTRM with with aging aging time. time. TheThe waterwater contentcontent inin NTRMNTRM immediately afterafter mixing mixing could could be be estimated estimated to beto 25%be 25% of the of total the masstotal consideringmass considering the water the content water contentand mixture and mixture proportion proportion of the RM of the sludge. RM sludge The water. The water content content in NTRM in NTRM decreased decreased to 20% to 220% h after 2 h aftercompletion completion of mixing of mixing and toand 14% to after14% 24after h. 24 The h. waterThe water content content was signiﬁcantlywas significantly low forlow 24 for h after24 h aftercompletion completion of mixing, of mixing, and it decreasedand it decreased by approximately by approximately 3% from 3% 24 tofrom 96 h. 24 A to mere 96 0.4%h. A decreasemere 0.4% in waterdecrease content in water was content observed was in observed the 24 h periodin the 24 from h period 96 to 120from h, 96 and to the120 water h, and content the water exhibited content a exhibitedtendency a to tendency converge to to converge approximately to approximatel 11%. It wasy 11%. possible It was to possible produce to NTRM produce with NTRM an 11% with water an 11%content water from content RM sludge from with RM asludge 50% water with contenta 50% withoutwater content heating without by mixing heating the RM by sludgemixing with the PSARM sludgeand HCFA. with PSA and HCFA.

Figure 1. WaterWater content in NTRM with aging time.

2.3.2. Chemical Properties 2.3.2. Chemical properties Figure2 compares the oxide chemical compositions of NTRM obtained using XRF analyses with Figure 2 compares the oxide chemical compositions of NTRM obtained using XRF analyses with those of RM. In general, the components of RM are similar regardless of the production areas of the raw those of RM. In general, the components of RM are similar regardless of the production areas of the materials, but the component proportions in RM are known to be signiﬁcantly diﬀerent depending on raw materials, but the component proportions in RM are known to be significantly different the production method of aluminum or the production area of bauxite ores [4,17,18]. The RM used in depending on the production method of aluminum or the production area of bauxite ores [4,17,18]. this study consisted of SiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, TiO2,P2O5, and SO3, and its The RM used in this study consisted of SiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, TiO2, P2O5, major oxides were Fe2O3, Al2O3, SiO2, and CaO. These four oxides accounted for 87% of the total mass. and SO3, and its major oxides were Fe2O3, Al2O3, SiO2, and CaO. These four oxides accounted for 87% of the total mass. The chemical composition of NTRM was not considerably different from that of RM, but a slight difference in component proportions was observed. SiO2, Al2O3, Fe2O3, and Na2O proportions for NTRM were 28.00%, 5.50%, 10.00%, and 4.51% lower, respectively, than those for RM. However, the CaO proportion for NTRM was 21.60% higher than that for RM. It appeared that NTRM had a higher proportion of CaO because the PSA and HCFA used for production of NTRM had high proportions of CaO (59.70% and 41.20%, respectively).

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Figure 2. Oxide chemical compositions of RM and NTRM.

FigureThe chemical 3 shows composition the XRD patterns of NTRM of RM was and not considerablyNTRM. RM included different iron from oxides that of such RM, as but hematite a slight difference in component proportions was observed. SiO , Al O , Fe O , and Na O proportions for (Fe2O3); titanium oxides such as perovskite (CaTiO3), rutile2 (TiO2 2),3 and2 anatase3 (TiO2 2); and aluminum NTRM were 28.00%, 5.50%, 10.00%, and 4.51% lower, respectively, than those for RM. However, oxides such as boehmite (AlO(OH)). It also included quartz (SiO2) and calcite (CaCO3). The minerals the CaO proportion forFigure NTRM 2. Oxide was 21.60%chemical higher compositions than that of RM for and RM. NTRM. It appeared that NTRM had in NTRM, which were very similar to those in RM, included hematite (FeO3), boehmite (AlO(OH)), a higher proportion of CaO because the PSA and HCFA used for production of NTRM had high rutileFigure (TiO2 3), showsquartz the (SiO XRD2), calcite patterns (CaCO of RM3), and NTRM.portlandite RM (Ca(OH)included2 ).iron Note oxides that suchNTRM as hematiteincluded proportions of CaO (59.70% and 41.20%, respectively). portlandite, which was not found in RM. These observations confirmed that Ca(OH)2, which was a (Fe2O3); titanium oxides such as perovskite (CaTiO3), rutile (TiO2), and anatase (TiO2); and aluminum hydrationFigure product,3 shows was the XRDproduced patterns from of the RM reaction and NTRM. between RM the included moisture iron in oxidesRM sludge such and as hematite the CaO oxides such as boehmite (AlO(OH)). It also included quartz (SiO2) and calcite (CaCO3). The minerals (Fein PSA2O3); and titanium HCFA. oxides such as perovskite (CaTiO3), rutile (TiO2), and anatase (TiO2); and aluminum in NTRM, which were very similar to those in RM, included hematite (FeO3), boehmite (AlO(OH)), oxides such as boehmite (AlO(OH)). It also included quartz (SiO2) and calcite (CaCO3). The minerals rutile (TiO2), quartz (SiO2), calcite (CaCO3), and portlandite (Ca(OH)2). Note that NTRM included in NTRM, which were very similar to those in RM, included hematite (FeO3), boehmite (AlO(OH)), portlandite, which was not found in RM. These observations confirmed that Ca(OH)2, which was a rutile (TiO ), quartz (SiO ), calcite (CaCO ), and portlandite (Ca(OH) ). Note that NTRM included hydration product,2 was produced2 from the3 reaction between the moisture2 in RM sludge and the CaO portlandite, which was not found in RM. These observations confirmed that Ca(OH) , which was a in PSA and HCFA. 2 hydration product, was produced from the reaction between the moisture in RM sludge and the CaO in PSA and HCFA.

Figure 3. XRD patterns of RM and NTRM.

2.3.3. Particle Size Figure 4 shows the particle size distribution of RM and NTRM. For comparison purposes, the particle size distribution of OPCFigureFigure is also 3. 3. XRD XRDshown patterns patterns in the of of RMgraphs. RM and and NTRM.The NTRM. specific surface area and average grain diameter of RM were 2353 m3/kg and 2.75 μm, respectively, and its Dv10, Dv50, and Dv90 2.3.3. Particle Size 2.3.3.values Particle were 1.4, Size 5.5, and 43.0 μm, respectively. Its average particle diameter was smaller than that of Figure4 shows the particle size distribution of RM and NTRM. For comparison purposes, OPC.Figure In the 4 frequency shows the distribution particle size curve distribution for RM, ofth reeRM peaks and NTRM. were seen, For whichcomparison were notpurposes, seen in thethe correspondingthe particle size curve distribution for OPC. of OPC is also shown in the graphs. The speciﬁc surface area and average particle size distribution of OPC is also3 shown in the graphs. The specific surface area and average grain diameter of RM were 2353 m /kg and 2.75 µm, respectively, and its Dv10,2 Dv50, and Dv90 grain Thediameter specific of surfaceRM were area 2353 and m average3/kg and grain 2.75 diameterμm, respectively, of NTRM and were its 1260 Dv10, m Dv50,/kg and and 9.66 Dv90 μm, values were 1.4, 5.5, and 43.0 µm, respectively. Its average particle diameter was smaller than that of valuesrespectively, were 1.4, and 5.5, its and Dv10, 43.0 Dv50,μm, respectively. and Dv90 Itsvalues average were particle 1.9, 9.2, diameter and 29.1 was μ smallerm, respectively. than that ofIts OPC. In the frequency distribution curve for RM, three peaks were seen, which were not seen in the OPC.frequency In the distribution frequency distributioncurve had a verycurve similar for RM, shape three to peaks that of were OPC, seen, but itswhich average were particle not seen diameter in the corresponding curve for OPC. correspondingwas larger than curve that forof OPC.OPC. The specific surface area of NTRM was 50% smaller than that of RM, and Thethe averagespecific grainsurface diameter area and of averageNTRM was grain 3.5 diameter times that of of NTRM RM. were 1260 m2/kg and 9.66 μm, respectively, and its Dv10, Dv50, and Dv90 values were 1.9, 9.2, and 29.1 μm, respectively. Its frequency distribution curve had a very similar shape to that of OPC, but its average particle diameter was larger than that of OPC. The specific surface area of NTRM was 50% smaller than that of RM, and the average grain diameter of NTRM was 3.5 times that of RM.

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Figure 4. Particle size distribution of RM and NTRM. Figure 4. Particle size distribution of RM and NTRM. The specific surface area and average grain diameter of NTRM were 1260 m2/kg and 9.66 µm, 3.respectively, Characterization and its Dv10,of Slag Dv50, Cement and Dv90Mortar values Containing were 1.9, RM 9.2, and and NTRM 29.1 µm, respectively. Its frequency distribution curve had a very similar shape to that of OPC, but its average particle diameter was larger 3.1.than Materials that of OPC. The specific surface area of NTRM was 50% smaller than that of RM, and the average grain diameter of NTRM was 3.5 times that of RM. Previous studies [19–21] confirmed that RM that includes NaO2 may act as an activator of slag- based3. Characterization binders because of Slagof its Cement high alkalinity. Mortar Containing Therefore, in RM this and stud NTRMy, slag cement (SC) obtained by mixing type 1 OPC and ground-granulated blast-furnace slag at a 50:50 ratio was used as a mortar binder.3.1. Materials Table 3 lists the physical properties and chemical compositions of the type 1 OPC and ground granulated blast-furnace slag. ISO standard sand was used as the fine aggregate [22]. Previous studies [19–21] confirmed that RM that includes NaO2 may act as an activator of slag-based binders because of its high alkalinity. Therefore, in this study, slag cement (SC) obtained by Table 3. Physical properties and chemical compositions of binder. mixing type 1 OPC and ground-granulated blast-furnace slag at a 50:50 ratio was used as a mortar binder. TableSpecific3 lists the surface physical Density properties andLg. chemical compositionsChemical of composition the type 1 OPC (%) and ground Type 1 granulated blast-furnacearea (cm2/g) slag. ISO(g/cm standard3) loss sand wasSiO used2 asAl the2O3 fineFe aggregate2O3 CaO [22 ]. MgO SO3 OPC 3.144 3.15 2.44 21.7 5.7 3.2 63.1 2.8 2.2 GGBS 4196 2.90 1.32 32.75 15.61 0.5 43.51 4.41 - 1 OPC: ordinary Portland cement, GGBS: ground-granulated blast-furnace slag.

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Table 3. Physical properties and chemical compositions of binder.

Chemical Composition (%) 1 Speciﬁc Surface Density Type 2 3 Lg. Loss Area (cm /g) (g/cm ) SiO2 Al2O3 Fe2O3 CaO MgO SO3 OPC 3.144 3.15 2.44 21.7 5.7 3.2 63.1 2.8 2.2 GGBS 4196 2.90 1.32 32.75 15.61 0.5 43.51 4.41 - Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 13 1 OPC: ordinary Portland cement, GGBS: ground-granulated blast-furnace slag. 3.2. Experimental Outline 3.2. ExperimentalTable 4 presents Outline the experimental plan and mixture proportion of mortar to evaluate the possibility of NTRM as construction materials. Depending on the RM mixing conditions, three types Table4 presents the experimental plan and mixture proportion of mortar to evaluate the possibility of specimens (i.e., SC100, SC80RM20, and SC80NTRM20) were prepared. The binder and aggregate of NTRM as construction materials. Depending on the RM mixing conditions, three types of specimens were set at a 1:3 ratio based on the mass. NTRM and RM were added such that they replaced 20% of (i.e., SC100, SC80RM20, and SC80NTRM20) were prepared. The binder and aggregate were set the slag cement. Since the aim of this chapter was to review the possibility of NTRM as a construction at a 1:3 ratio based on the mass. NTRM and RM were added such that they replaced 20% of the material, a compressive strength test was conducted to assess mechanical properties. Porosity and slag cement. Since the aim of this chapter was to review the possibility of NTRM as a construction water absorption tests were conducted to estimate the microstructure formation of mortar. Figure 5 material, a compressive strength test was conducted to assess mechanical properties. Porosity and shows test setups for compressive strength and water absorption. Three specimens of each type were water absorption tests were conducted to estimate the microstructure formation of mortar. Figure5 prepared for each test. shows test setups for compressive strength and water absorption. Three specimens of each type were prepared for each test. Table 4. Experimental plan and mixture proportion of mortar.

Table 4. Experimental planBinder and mixture (wt%) proportion of mortar. Specimen ID W/B 1 B:S 2 Measured parameters SC BinderRM (wt%) NTRM Specimen ID W/B 1 B:S 2 Measured Parameters SC100 SC100 RM- NTRM - Compressive strength (MPa) SC80RM20SC100 0.6 1:3 10080 -20 -- Compressive strengthPorosity (MPa) SC80NTRM20SC80RM20 0.6 1:3 8080 20- -20 Water absorption Porosity coefficient SC80NTRM20 80 - 20 Water absorption coeﬃcient 1 water to binder ratio, 2 Binder to sand (fine aggregate) ratio. 1 2 water to binder ratio, Binder to sand (ﬁne aggregate) ratio.

(a) (b)

FigureFigure 5. 5.Test Test setup setup of of (a) ( compressivea) compressive strength strength and and (b )(b water) water absorption. absorption. 3.3. Experimental Method 3.3. Experimental Method 3.3.1. Compressive Strength 3.3.1. Compressive Strength Beam mortar samples (40 mm 40 mm 160 mm) were prepared and cured under room Beam mortar samples (40 mm× × 40 mm× × 160 mm) were prepared and cured under room conditions (temperature: 20 2 ◦C and relative humidity: 60 5%). After curing for 1, 3, 7, and 28 days, conditions (temperature: ±20 ± 2 °C and relative humidity:± 60 ± 5%). After curing for 1, 3, 7, and 28 the compressive strength was evaluated according to ASTM C 349 [23]. days, the compressive strength was evaluated according to ASTM C 349 [23].

3.3.2. Porosity Mercury intrusion porosimetry (MIP) measurements were performed using cubic pieces (5 mm × 5 mm × 5 mm, volume: 125 mm3) obtained from the central inner part of hardened samples. These pieces were immersed in isopropyl alcohol for 4 weeks to prevent further hydration. After 4 weeks, the samples were dried in a vacuum desiccator for 2 days to eliminate any residual solvent inside the samples. A mercury porosimeter (Autopore IV, Micromeritics, GA, USA) was used for MIP measurements.

3.3.3. Water Absorption Coefficient

Appl.Appl. Sci. 2019,, 9,, 2510x FOR PEER REVIEW 88 of of 1313

Construction materials with a porosity like that of cement mortar can absorb water through 3.3.2.capillary Porosity suction. Initially, a large amount of water is absorbed, and the absorption later becomes steady.Mercury The absorption intrusion coefficient porosimetry was (MIP)obtained measurements based on KS wereF 2609 performed [24] according using to the cubic following pieces (5equation: mm 5 mm 5 mm, volume: 125 mm3) obtained from the central inner part of hardened × × samples. These pieces were immersed in isopropyl𝑚 = 𝑤 alcohol √ 𝑡, for 4 weeks to prevent further hydration.(2) After 4 weeks, the samples were dried in a vacuum desiccator for 2 days to eliminate any residual 2 2 1/2 solventwhere m inside is the the amount samples. of water A mercury absorption porosimeter (kg/m (Autopore), t is the test IV, period Micromeritics, (h), and GA,w (kg/m USA)·h was) is used the forwater MIP absorption measurements. coefficient.

3.3.3.3.4. Experimental Water Absorption Results Coeand ﬃDiscussioncient

3.4.1.Construction Compressive materialsstrength with a porosity like that of cement mortar can absorb water through capillary suction. Initially, a large amount of water is absorbed, and the absorption later becomes steady. The absorptionFigure 6 and coe Tablefficient 5 show was obtained the compressive based on strengths KS F 2609 of [24 SC100,] according SC80RM20, to the followingand SC80NTRM20. equation: For all curing ages, SC100 had the highest compressive strength, and SC80NTRM20 had the lowest value. On the first day, the measured compressivem = w strength√ t, was 6.26 MPa for SC100, 5.02 MPa for (2) · SC80RM20, and 3.95 MPa for SC80NTRM20. After 28 days, the compressive strengths of SC100, whereSC80RM20,m is the and amount SC80NTRM20 of water were absorption 30.14, (kg25.91,/m2 ),antdis 19.98 the test MPa, period respectively, (h), and w indicating(kg/m2 h1 /that2) is thethe · waterdifferences absorption in the coecompressivefficient. strength increased as the age increased. This tendency of decreasing compressive strength owing to the addition of RM was the same as the tendency reported in other 3.4.previous Experimental studies Results[25–29]. and However, Discussion this tendency was significantly different from that reported by 3.4.1.Ribeiro Compressive et al. [10], who Strength found that the density and compressive strength increased as the RM addition amount increased. This difference may have occurred because the RM used in this study had larger particleFigure sizes6 and than Table the RM5 show used the in compressivethe study of strengthsRibeiro et of al. SC100, [10], and SC80RM20, thus, the andpore SC80NTRM20. filling was not Forsufficient all curing to increase ages, SC100 the compressive had the highest strength. compressive strength, and SC80NTRM20 had the lowest value.Yao On et theal. [30] first reported day, the that measured mortar compressive based on a bi strengthnder, in which was 6.26 RM, MPa coal forrefuse, SC100, fly ash, 5.02 and MPa OPC for SC80RM20,were mixed, and could 3.95 achieve MPa for a higher SC80NTRM20. compressive After strength 28 days, than the OPC compressive mortar on strengths a long-term of SC100, basis SC80RM20,because of pozzolanic and SC80NTRM20 reactions. wereThe PSA 30.14, and 25.91, HCFA and used 19.98 in this MPa, study respectively, contained indicating a large amount that the of diCaO,fferences and thus, in the the compressive compressive strength strength increased could be as theexpected age increased. to increase This because tendency of the of decreasing hydration compressivereaction. However, strength it appears owing to that the the addition strength of of RM the was mortar the samecouldas not the be tendency increased reported because the in other CaO previousalready present studies in [25 PSA–29]. and However, HCFA this reacted tendency with wasmoisture significantly in the diRMfferent sludge from during that reportedthe NTRM by Ribeiroproduction et al. process [10], who and found gene thatrated the portlandite density and (Ca(OH) compressive2), as strength shown increasedin Figure as 7. the In RM this addition study, amountalthough increased. SC80NTRM20 This di hadfference a lower may havecompressive occurred strength because than the RM SC100 used and in this SC80RM20, study had it larger was particleconfirmed sizes that than it had the sufficient RM used strength in the study to be of used Ribeiro as a etraw al. material [10], and for thus, sidewalks, the pore bike filling roads, was and not suparkingfficient lots, to increase as suggested the compressive by Kim et al. strength. [21].

Figure 6. Compressive strength of didifferentﬀerent RM-typeRM-type mortarmortar specimens.specimens.

Appl. Sci. 2019, 9, 2510 9 of 13

Table 5. Compressive strength data of diﬀerent RM-type mortar specimens.

Compressive Strength (MPa) Specimen ID Age (d) no.1 no.2 no.3 Average CV 1 1 6.24 6.34 6.19 6.26 0.07 3 13.36 13.99 14.49 13.95 0.56 SC100 7 23.55 18.63 21.96 21.38 2.50 28 30.08 30.53 29.81 30.14 0.35 Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13 1 5.18 4.87 5.01 5.02 0.15 3 11.81 11.83 11.87 11.84 0.03 SC80RM20 Table 5. Compressive7 strength 20.18 data of different 17.62 RM-type 18.71 mortar 18.83 specimens. 1.28 28 24.74 27.67 25.33 25.91 1.55 Age Compressive strength (MPa) Specimen ID 1 4.19 3.82 3.84 3.95 0.21 (d) 3no.1 9.31no.2 10.28 no.3 10.26 average 9.95 0.55CV 1 SC80NTRM20 7 15.92 13.24 15.70 14.95 1.49 1 286.24 20.846.34 18.78 6.19 20.33 19.986.26 1.070.07 3 13.361 13.99 14.49 13.95 0.56 SC100 Coefficient of variation. 7 23.55 18.63 21.96 21.38 2.50 28 30.08 30.53 29.81 30.14 0.35 Yao et al. [30] reported that1 mortar5.18 based on4.87 a binder, in5.01 which RM, coal5.02 refuse, fly0.15 ash, and OPC were mixed, could achieve a higher3 compressive11.81 strength11.83 than11.87 OPC mortar11.84 on a long-term 0.03 basis because SC80RM20 of pozzolanic reactions. The7 PSA and20.18 HCFA used17.62 in this study18.71 contained18.83 a large 1.28amount of CaO, and thus, the compressive strength28 could24.74 be expected27.67 to increase 25.33 because 25.91 of the hydration 1.55 reaction. However, it appears that the1 strength 4.19 of the mortar3.82 could not3.84 be increased3.95 because the0.21 CaO already present in PSA and HCFA reacted3 with9.31 moisture in10.28 the RM sludge10.26 during the9.95 NTRM production 0.55 process SC80NTRM20 and generated portlandite (Ca(OH)7 215.92), as shown13.24 in Figure 7.15.70 In this study,14.95 although 1.49 SC80NTRM20 had a lower compressive strength28 than20.84 SC100 and18.78 SC80RM20, 20.33 it was confirmed 19.98 that 1.07 it had sufficient strength to be used as a raw material for1 Coefficient sidewalks, of variation. bike roads, and parking lots, as suggested by Kim et al. [21].

(a) (b) (c)

Figure 7. SEMSEM images images of of ( (aa)) SC100, SC100, ( (bb)) SC80RM20, SC80RM20, and and ( (c)) SC80NTRM20 SC80NTRM20 on the first ﬁrst day.

3.4.2. Porosity Figure 88 shows shows thethe porepore size size characteristics characteristics of of SC100, SC100, SC80RM20, SC80RM20, and and SC80NTRM20. SC80NTRM20. AsAs porepore characteristics, the the total total porosity porosity and and pore pore size size di distributionstribution were were measured. measured. The The total total porosities porosities of ofSC100, SC100, SC80RM20, SC80RM20, and and SC80NTRM20 SC80NTRM20 were were 17.16%, 17.16%, 20.78%, 20.78%, and and 22.66%, 22.66%, respectively. respectively. The The total porosities of SC80RM20 and SC80NTRM20, which included RM, were 3%–5% higher than that of SC100. Among the samples, SC80NTRM20 had the highest total porosity. In the pore size distribution, it was found that there were relatively moremore pores with the three sizes of 10–100, 1000, and 100,000 nm. In In general, pores with 0–1000 nm sizes represent capillary voids, and and pores pores with with 100,000 100,000 nm nm sizes sizes represent represent air bubbles air bubbles [31]. [ 31Pores]. Pores with 10–100 with 10–100 nm sizes, nm which sizes, whichcorrespond correspond to small to capillary small capillary pores, pores,were found were found to have to havethe largest the largest distribution distribution in SC80RM20 in SC80RM20 and andthe smallest the smallest distribution distribution in SC100. in SC100. Pores Poreswith 100 with0 nm 1000 sizes, nm sizes,which which correspond correspond to relatively to relatively large largecapillary capillary voids, voids, were werefound found to have to have the the larges largestt distribution distribution in in SC80NTRM20 SC80NTRM20 and and the the smallestsmallest distribution in SC100. A previous study [32] reported that pores with sizes larger than 50 nm affected the compressive strength of mortar. Pores with sizes larger than 50 nm were found to have the largest distribution in SC80NTRM20, followed by SC80RM20 and SC100. This result was found to be closely related to the results obtained while testing the compressive strength of mortar. The pore volume of SC80RM20 was the lowest in the 1000–10,000 nm range. As RM had many particles with 1000–10,000 nm sizes, it appeared that these particles exhibited the pore filling effect [4,10]. SC80NTRM20 also had the lowest pore volume among the three mortar specimens in the 10,000–100,000 nm range, where the particle size distribution was the largest. Kim et al. [21] reported that the porosity of mortar decreased because of the pore filling effect of RM microparticles, and this effect was larger for bigger pores.

Appl. Sci. 2019, 9, 2510 10 of 13 distribution in SC100. A previous study [32] reported that pores with sizes larger than 50 nm aﬀected the compressive strength of mortar. Pores with sizes larger than 50 nm were found to have the largest distribution in SC80NTRM20, followed by SC80RM20 and SC100. This result was found to be closely Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 13 related to the results obtained while testing the compressive strength of mortar.

FigureFigure 8. 8.Pore Pore size size distributions distributions of of SC100, SC100, SC80RM20, SC80RM20, and and SC80NTRM20. SC80NTRM20.

3.4.3.The Moisture pore volume Absorption of SC80RM20 Coefficient was the lowest in the 1000–10,000 nm range. As RM had many particles with 1000–10,000 nm sizes, it appeared that these particles exhibited the pore filling effect [4,10]. Figure 9 shows water absorption rates of the mortar specimens. The water absorption rate SC80NTRM20 also had the lowest pore volume among the three mortar specimens in the 10,000–100,000 linearly increased over time for up to 2 h for all specimens, but this increase showed a tendency to nm range, where the particle size distribution was the largest. Kim et al. [21] reported that the porosity decrease after 6 h. The water absorption rate was the lowest for SC100 and the highest for of mortar decreased because of the pore filling effect of RM microparticles, and this effect was larger SC80NTRM20 over the entire time period. for bigger pores. Figure 10 shows the water absorption coefficients calculated for the quantification of the increase 3.4.3.in the Moisture water absorption Absorption rate Coe overfficient time. The water absorption coefficients of SC100, SC80RM20, and SC80NTRM20 were 0.57, 0.62, and 0.69, respectively, indicating that the specimens with RM had Figure9 shows water absorption rates of the mortar specimens. The water absorption rate linearly higher water absorption coefficients than the specimen with only slag cement, and the specimen with increased over time for up to 2 h for all specimens, but this increase showed a tendency to decrease NTRM had a higher water absorption coefficient than the specimen with RM. Such differences in the after 6 h. The water absorption rate was the lowest for SC100 and the highest for SC80NTRM20 over water absorption coefficient can be explained using the differences in porosity analyzed previously. the entire time period. Because of the increase in the number of capillary voids, owing to the mixing of RM, the amount of Figure 10 shows the water absorption coefficients calculated for the quantification of the increase moisture absorbed by the capillary voids increased. in the water absorption rate over time. The water absorption coefficients of SC100, SC80RM20, Zhang and Zong [33] reported that it was difficult to find a correlation between the water and SC80NTRM20 were 0.57, 0.62, and 0.69, respectively, indicating that the specimens with RM had absorption rate of mortar and its strength. However, according to the present study, results of the higher water absorption coefficients than the specimen with only slag cement, and the specimen with capillary void distribution, water absorption rate, and compressive strength of mortar were NTRM had a higher water absorption coefficient than the specimen with RM. Such differences in the consistent. Therefore, the increase in the number of capillary voids in the mortar can be considered water absorption coefficient can be explained using the differences in porosity analyzed previously. to be the cause of the reduction in the compressive strength and the increase in the water absorption Because of the increase in the number of capillary voids, owing to the mixing of RM, the amount of rate. moisture absorbed by the capillary voids increased. Pores can be considered to be either the spaces in the gap between hydrates or the spaces created Zhang and Zong [33] reported that it was difficult to find a correlation between the water by water bubbles, and the presence of a number of large pores is known to significantly affect the absorption rate of mortar and its strength. However, according to the present study, results of the strength as well as durability of a material. Therefore, it is necessary to examine the durability of RM capillary void distribution, water absorption rate, and compressive strength of mortar were consistent. and NTRM when they are used as construction materials.

Appl. Sci. 2019, 9, 2510 11 of 13

Therefore, the increase in the number of capillary voids in the mortar can be considered to be the cause ofAppl. the Sci. reduction 2019, 9, x FOR in the PEER compressive REVIEW strength and the increase in the water absorption rate. 11 of 13

Figure 9. Water absorption rates of SC100,SC100, SC80RM20, and SC80NTRM20.

Figure 10. Water absorption coecoefficientsﬃcients of SC100,SC100, SC80RM20,SC80RM20, andand SC80NTRM20.SC80NTRM20.

4. ConclusionsPores can be considered to be either the spaces in the gap between hydrates or the spaces created by water bubbles, and the presence of a number of large pores is known to significantly affect the In this study, a method of producing nonthermally treated red mud (NTRM) powder was strength as well as durability of a material. Therefore, it is necessary to examine the durability of RM proposed for economical and efficient recycling of red mud (RM). In the course of mixing RM and NTRM when they are used as construction materials. sludge (50%), paper sludge ash (PSA, 35%), and high-calcium fly ash (HCFA, 15%), CaO 4.contained Conclusions in the ash-based byproduct reacted with moisture in the RM sludge to form portlandite. Therefore, it was possible to produce RM-based dried powder without heating. InThe this compressive study, a method strength of producing of the slag nonthermally cement mortar treated mixed red mudwith (NTRM)NTRM was powder evaluated was proposed to forbe approximately economical and 65% effi ofcient that recycling of the mortar of red with mud only (RM). slag In cement. the course Although of mixing the RMcompressive sludge (50%), paperstrength sludge of the ash slag (PSA, cement 35%), mortar and high-calcium could not be fly expected ash (HCFA, to increase 15%), CaO because contained of the in mixing the ash-based of byproductNTRM, it was reacted confirmed with moisture that the inperformance the RM sludge of the to formmortar portlandite. mixed with Therefore, NTRM was it was sufficient possible to produceto enable RM-based its use in parking dried powder lots and without road pavements. heating. Therefore, the possibility of using NTRM as a construction material was confirmed. Because NTRM had only a small number of particles with sizes that could fill capillary voids, which have a significant impact on the compressive strength and durability of a material, the mortar mixed with NTRM had a low compressive strength and a high water absorption rate. Therefore, it is necessary to additionally examine durability when NTRM is used as a construction material.

Appl. Sci. 2019, 9, 2510 12 of 13

The compressive strength of the slag cement mortar mixed with NTRM was evaluated to be approximately 65% of that of the mortar with only slag cement. Although the compressive strength of the slag cement mortar could not be expected to increase because of the mixing of NTRM, it was confirmed that the performance of the mortar mixed with NTRM was sufficient to enable its use in parking lots and road pavements. Therefore, the possibility of using NTRM as a construction material was confirmed. Because NTRM had only a small number of particles with sizes that could fill capillary voids, which have a significant impact on the compressive strength and durability of a material, the mortar mixed with NTRM had a low compressive strength and a high water absorption rate. Therefore, it is necessary to additionally examine durability when NTRM is used as a construction material.

Author Contributions: Conceptualization, G.C. and S.K.; methodology, H.K.; validation, S.K., G.C., and H.K.; formal analysis, G.C.; investigation, H.K.; resources, H.K.; data curation, G.C.; writing—original draft preparation, G.C.; writing—review and editing, G.C.; visualization, G.C.; supervision, S.K.; project administration, S.K.; funding acquisition, S.K.” Funding: This research was supported by a grant (19CTAP-C142091-02) from Infrastructure and transportation technology promotion research Program funded by Ministry of Land, Infrastructure and Transport of Korean government. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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