Properties of Cement Mortar Using Limestone Sludge Powder Modified

sustainability

Article Properties of Cement Mortar Using Limestone Sludge Powder Modiﬁed with Recycled Acetic Acid

Hwa-Sung Ryu 1, Deuck-Mo Kim 1, Sang-Heon Shin 1, Wan-Ki Kim 2, Seung-Min Lim 3 and Won-Jun Park 4,*

1 Hanyang Experiment and Consulting Co., Hanyang University, Ansan 15588, South Korea; [email protected] (H.-S.R.); [email protected] (D.-M.K.); [email protected] (S.-H.S.) 2 Department of Architectural Engineering, Hyupsung University, Hwaseong, 445-745, South Korea; [email protected] 3 Innovative Durable Building and Infrastructure Research Center, Hanyang University, Ansan 15588, South Korea; [email protected] 4 Department of Architectural Engineering, Kangwon National University, Samcheok 25913, South Korea * Correspondence: [email protected]; Tel.: +82-33-570-9529

Received: 19 November 2018; Accepted: 4 February 2019; Published: 8 February 2019

Abstract: One of the various methods of manufacturing low-carbon cement is substituting limestone powder as a raw material or admixture. Limestone sludge powder (LSSP) has the same composition as that of limestone powder. The surface characteristics of LSSP powder modified with recycled acetic acid (RAA) and the characteristics of cement using this modified LSSP as a substitute were investigated in this study. The surface of LSSP modified with RAA was converted into calcium acetate and had a large grain size. When conventional LSSP was used as a substitute for cement, the initial strength increased owing to improved pore filling; however, the strength after 28 days of aging was lower than that of non-substituted cement. In the case of modified LSSP being replaced with cement at up to 10% of the cement weight, however, the calcium acetate on its surface increased the amount of hydration products in the cement, thereby increasing both the initial and the long-term strength.

Keywords: cement mortar; limestone sludge; recycled acetic acid; surface modiﬁcation

1. Introduction One of the major issues in the construction industry in recent years has been the demand for cement materials that can reduce carbon emissions. The reality is that 1 t of CO2 is generated to produce 1 t of cement, and most studies have focused on reducing cement usage or using cement substitutes [1,2]. One of the most widely used materials in the cement production process is limestone powder. In Europe, limestone powder may be used at up to 35% according to the EN 197-1 standard, whereas in Canada and the United States, up to 5% Portland cement and 15% Portland limestone cement may be used [3–5]. Limestone powder contains a large amount of calcium carbonate. Calcium carbonate and silica or nanocarbon are used as fillers in organic and inorganic complexes. These fillers serve to improve the viscosity or physical performance of polydimethylsiloxane (PDMS) [6–10], which is used for penetrating water repellency and surface penetration coating. The functional groups on the surfaces of these fillers may be changed to improve their reactivity with the polymer or to optimize the dispersion ability. Compared with silica or nanocarbon, calcium carbonate is a less expensive functional filler [8–15]. In the steel industry, burnt lime is produced by washing and burning limestone. The sludge composed of powders washed from limestone is known as limestone sludge powder (LSSP). LSSP has the same composition as limestone powder and can be used as a substitute for cement and, therefore,

Sustainability 2019, 11, 879; doi:10.3390/su11030879 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 879 2 of 10 as an admixture material of cement mortars. Limestone powder is a raw material of cement, and the LSSP used in this study is an admixture for mixing with cement to reduce cement use. LSSP has the same composition as that of limestone powder and can be replaced with cement as an admixture material. The shape and particle size of limestone powder are relatively good compared to those of cement particles. Replacing a portion of cement with limestone powder improves the fluidity of the mortar [16,17]. It has been reported that it also contributes to quality enhancements such as improved resistance to material separation and enhanced strength, owing to the pore-filling effect. Studies have suggested that limestone powder accelerates the hydration of alite in cement. However, calcium carbonate, which is a major component of limestone powder, is mostly non-hydrated and, thus, it is possible that the initial strength can be lowered when a substantial amount of LSSP is used [16–21]. Similarly, using fly ash and blast furnace slag, which are mixed materials used to improve the performance of cement, may delay the hydration of cement and lower its initial strength. To induce initial hydration early, researchers have used strengthening agents such as compounds containing carboxyl groups, lithium compounds, and calcium nitrate [22–29]. Waste acetic acid, a byproduct of liquid crystal display (LCD) surface etching processes, has a low concentration and is restricted to reuse in industry. The calcium acetate produced by the reaction of calcium salts with acetic acid can promote the hydration of cement owing to the carboxyl groups of this acid. Calcium acetate also improves the initial strength and reduces internal voids. Hence, modification of the surfaces of limestone powders using acetic acid can improve the early strength properties of these powders when they are used as admixtures for cement. Therefore, in this study, an admixture material was developed by modifying LSSP using acetic acid to form reactive limestone powders to enhance the performance of cement mortar. The properties of the samples were investigated in terms of mortar setting time and compressive strength. They were also analyzed using X-ray diffraction (XRD), thermogravimetric/differential thermal analysis (TG-DTA), and scanning electron microscopy (SEM).

2. Materials and Methods

2.1. Materials Table1 shows the chemical properties of the binders used in the experiments. The cement used was an ordinary type 1 Portland cement (OPC). Chemical analysis of LSSP showed that the CaO and MgO contents were 51.5% and 1.60%, respectively. This corresponds to about 90% of CaCO3, which corresponds to high-grade limestone, and is similar in chemical properties to reagent-grade CaCO3. Recycled acetic acid (RAA) was obtained after being extracted from the separation process of acids used in etching processes. For this study, RAA with 60% solid content was used. Figure1 shows the Fourier transform infrared (FT-IR) spectroscopy results for the RAA. Organic acids are denoted by the peaks in the range of 3300–2500 cm−1, where carboxylic groups are traditionally located. C=O groups (1760–1690 cm−1) and C–O groups (1320–1210 cm−1) are also present in this spectrum. Some examples of carboxylic acids are acetic acid, formic acid, glucosan, and propionic acid. The functional group of the RAA used for this study was conﬁrmed to be carboxylic acid.

Table 1. Chemical composition of the ordinary type 1 Portland cement (OPC), CaCO3, and limestone sludge powder (LSSP) (%).

CaO SiO2 MgO Al2O3 SO3 Fe2O3 K2O Na2O LOI OPC 63.35 21.09 3.32 4.34 3.09 2.39 1.13 0.29 1.0

CaCO3 52.53 1.18 2.47 0.47 0.02 0.43 0.13 0 42.77 LSSP 53.16 3.96 1.09 2.13 0.08 1.4 0.46 0.05 37.7 Sustainability 2019, 11, 879 3 of 10

SustainabilitySustainability 20182018,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 3 3 of of 10 10

Figure 1. Fourier transform infrared (FT-IR) spectrum of recycled acetic acid (RAA). Figure 1. Fourier transform infrared (FT-IR) spectrumtrum ofof recycledrecycled aceticacetic acidacid (RAA).(RAA). 2.2. Methods 2.2. Methods 2.2. MethodsFigure2 shows the surface modification process of the LSS. A 10% aqueous solution was made usingFigure RAA with2 shows 60% the solid surface content; modification then, 1 kg of proce the aqueousssss ofof thethe solutionLSS.LSS. AA 10%10% was aqueousaqueous mixed with solutionsolution 100 g ofwaswas LSS mademade and usingthe mixture RAA with was kneaded60% solid for content; about 10then, min. 1 kg The of modified thethe aqueousaqueous calcium solutionsolution carbonate waswas mixedmixed was collected withwith 100100 through gg ofof LSSLSS a andfilter. the The mixture reaction was with kneaded acetic acid for on about the surface 10 min. of The the LSSmodified is as follows: calcium carbonate was collected throughthrough aa filter.filter. TheThe reactionreaction withwith aceticacetic acidacid onon thethe surfacesurface ofof thethe LSSLSS isis asas follows:follows: 2CH3COOH + CaCO3 = Ca (CH3COO)2 + H2O + CO2 (1) 2CH3COOH + CaCO3 = Ca (CH3COO)2 + H2O + CO2 (1) 2CH3COOH + CaCO3 = Ca (CH3COO)2 + H2O + CO2 (1)

LimestoneLimestone SludgeSludge Acetic Acid

Reactor

Filtration/WashingFiltration/Washing

Modified Calcium Carbonate

FigureFigure 2. FlowFlow diagram diagram for for the the modified modiﬁed calcium carbonate.

Table2 2 shows shows the the mixing mixing proportions proportions of of samples, sample wheres,s, wherewhere OPC OPCOPC was waswas substituted substitutedsubstituted by conventional byby conventionalconventional and andmodified modified LSSP. LSSP. The specimens The specimens for evaluating for evaluating the characteristics thethe characteristicscharacteristics of the cement ofof thethe mortar cementcement were mortarmortar prepared werewere × × 3 preparedby mixing by the mixing mortar the and mortar placing and it intoplacing a beam-shaped it into a beam-shaped mold with mold dimensions with dimensions of 40 40 of 16040 × mm 40 × . The prepared3 specimens were cured for 18 h at room temperature and demolded for underwater 160 mm 3.. TheThe preparedprepared specimensspecimens werewere curedcured forfor 1818 hh atat roomroom temperaturetemperature andand demoldeddemolded forfor underwatercuring. The compressivecuring. The compressive strength was strength then measured was then after measuredmeasured different afterafter curing differentdifferent times: curingcuring 18 h, 1 times:times: day, 3 days,1818 h,h, 17 days,day, 3 and days, 28 days,7 days, according and 28 todays, ISO according 679 [30]. The to ISO mortar 679 fluidity [30]. The was mortar measured fluidity according was measured to ASTM accordingC 1437. The to mortar ASTM settingC 1437. time The wasmortar measured setting accordingtime waswas measured tomeasured the initial accordingaccording and final toto setting thethe initialinitial time andand specified finalfinal settinginsetting ASTM timetime C 191, specifiedspecified and the inin ASTM mortarASTM CC compressive 191,191, andand thethe strength momortarrtar compressivecompressive was measured strengthstrength according waswas to measuredmeasured ASTM C accordingaccording 109. toto ASTMASTM CC 109.109. The age of the sample for analysis is 28 days, and immersed in acetone for 24 hours to stop hydration of individual samples. The water was then removedremoved inin aa 6060 °C°C dryer.dryer. SampleSample preparationpreparation Sustainability 2018, 10, x FOR PEER REVIEW 4 of 10 Sustainability 2019, 11, 879 4 of 10 for each analysis is as follows. SEM samples were made of flake samples with a width of 0.5 cm × 0.5 cm and a thickness of 5 mm or less. TGA and XRD analysis specimens were prepared with paste, without aggregates, to suppress noiseTable 2.of Mixingsample. proportions After grinding (unit: g).to below 100 microns at 28 days of age, it was immersedOPC in an (Ordinary acetone solutionLSSP and (Limestone dried usingModified a vacuum distillation apparatus. Sample Silica sand Water Portland Cement) Sludge Powder) LSSP OPC 1000Table 2. Mixing - proportions (unit: - g). OPC-L5 950 50 OPC-L10OPC 900 LSSP 100 OPC-L15 850 150 (Ordinary (Limestone Modified SampleOPC-L20 800 200 Silica2000 sand 340Water OPC-ML5Portland 950 Sludge LSSP 50 OPC-ML10 900 100 OPC-ML15Cement) 850 Powder) 150 OPC-MLOPC 201000 800 - - 200 OPC-L5 950 50 OPC-L10 900 100 The age of the sample for analysis is 28 days, and immersed in acetone for 24 hours to stop OPC-L15 850 150 hydration of individual samples. The water was then removed in a 60 ◦C dryer. Sample preparation for OPC-L20 800 200 2000 340 eachOPC-ML5 analysis is as follows.950 SEM samples were made of flake50 samples with a width of 0.5 cm × 0.5 cm andOPC-ML10 a thickness of 5 mm900 or less. TGA and XRD analysis specimens100 were prepared with paste, without aggregates,OPC-ML15 to suppress noise850 of sample. After grinding to150 below 100 microns at 28 days of age, it was immersedOPC-ML in 20 an acetone 800 solution and dried using a vacuum200 distillation apparatus.

3. Results and Discussion

3.1. Characteristics of Modified LSSP 3.1. Characteristics of Modified LSSP Figure3 shows the SEM images of the surfaces of conventional and modified LSSP. On the Figure 3 shows the SEM images of the surfaces of conventional and modified LSSP. On the surface of the modified LSSP, calcium acetate crystals with relatively large grain sizes can be seen. surface of the modified LSSP, calcium acetate crystals with relatively large grain sizes can be seen. This is because the calcium carbonate in powder form was dissolved by acetic acid. The pyrolysis This is because the calcium carbonate in powder form was dissolved by acetic acid. The pyrolysis characteristics of conventional LSSP showed a rapid weight loss of approximately 84% between characteristics of conventional LSSP showed a rapid weight loss of approximately 84% between 700 700 ◦C and 800 ◦C, as shown in Figure4. The pyrolysis of the byproducts showed a heating curve °C and 800 °C, as shown in Figure 4. The pyrolysis of the byproducts showed a heating curve with a with a weight loss of approximately 1.476% at around 350 ◦C and 59% between 700 ◦C and 800 ◦C. weight loss of approximately 1.476% at around 350 °C◦ and 59% between 700 °C and 800 °C. The Themodified modified LSSP LSSP showed showed about about 30% 30% weight weight loss loss at at800 800 °C.C. The The result result of of the the particle particle size size distribution µ measurement of the LSS showed an average particle size of 18.5 μm. In In the the case case of of the the modified modified LSSP, LSSP, µ the average particleparticle sizesize waswas 17.217.2 μm.m.

(a) (b)

Figure 3. ScanningScanning electron electron microscopy microscopy (SEM) (SEM) images images of oflimestone limestone sludge sludge powder powder (LSSP): (LSSP): (a) (conventionala) conventional LSSP LSSP and and (b) (modifiedb) modiﬁed LSSP. LSSP. Sustainability 2019, 11, 879 5 of 10 Sustainability 2018, 10, x FOR PEER REVIEW 5 of 10

Sustainability 2018, 10, x FOR PEER REVIEW 5 of 10 TGA (%) TGA TGA (%) TGA DTA DTA (um) DTA DTA (um) TGA (%) TGA TGA (%) TGA DTA DTA (um) DTA DTA (um)

Temperature (℃) Temperature (℃) (a) (b)

FigureFigure 4. 4. ResultsResults of of the the thermogravimetric/diffe thermogravimetric/differentialrential thermal thermal analysis analysis (TG-DTA): (TG-DTA): (a (a) )conventional conventional Temperature (℃) Temperature (℃) LSSP and (b) modified LSSP. LSSP and (b) modiﬁed LSSP.(a) (b)

3.2.3.2. Mortar Mortar ApplicatFigure Application 4. ionResults Characteristics Characteristics of the thermogravimetric/diffe rential thermal analysis (TG-DTA): (a) conventional LSSP and (b) modified LSSP. 3.2.1. Setting Time 3.2.1. Setting3.2. Mortar Time Applicat ion Characteristics FigureFigure 55 showsshows thethe measured measured setting setting time time of mortarof mortar mixed mixed with with conventional conventional and modifiedand modified LSSP. 3.2.1. Setting Time LSSP.The use The of use conventional of conventional LSSP showedLSSP showed no noticeable no noticeable difference difference in both in the both initial the andinitial the and final the setting final settingtime. In time. theFigure caseIn the of5 shows thecase modified ofthe the measured modified LSSP, setting however, LSSP, time ho theof wever,mortar initial mixed the setting initial with time conventional setting decreased time and asdecreased modified the substitution as the LSSP. The use of conventional LSSP showed no noticeable difference in both the initial and the final substitutionratio increased. ratio The increased. final setting The final time decreasedsetting time by decreased up to 10%, by but up increased to 10%, but for LSSPincreased content for aboveLSSP 15%. Itsetting is possible time. In that the thecase initialof the settingmodified time LSSP, decreased however, the owing initial to setting hydration time decreased reactions as inducedthe by content substitutionabove 15%. ratio It is increased. possible The that final the setting initial time setting decreased time decreasedby up to 10%, owing but increased to hydration for LSSP reactions calcium acetate on the surface of the modified LSSP; however, the presence of excessive organic calcium inducedcontent by calcium above 15%.acetate It is onpossible the surface that the ofinitial the se mottingdified time LSSP;decreased however, owing to the hydration presence reactions of excessive organicfor higher inducedcalcium contents by for calcium ofhigher LSSP acetate contents caused on the the ofsurface increaseLSSP of caused the in mo valuesdified the ofincreaseLSSP; the however, final in settingvalues the presence time.of the Acetic finalof excessive acid,setting used time. for AceticLSS modification, acid,organic used calcium for has LSSfor been higher modification, reported contents to of has increase LSSP been caused therepo the conductivityrted increase to increase in values in the the of initial theconductivity final cement setting paste, in time. the thereby initial increasingAcetic the acid, ionic used concentration for LSS modification, of Ca2+ hasand been Al repo (OH)rted −to. increase However, the anconductivity excessive in amountthe initial of acetic cement paste, thereby increasing the ionic concentration of4 Ca2+ and Al (OH)4−. However, an excessive acid maycement cause paste, hydration thereby increasing delay with the ionic increasing concentration adsorption of Ca2+ and amount Al (OH) on4−. theHowever, surface an excessive of cement paste amountamount of acetic of aceticacid mayacid may cause cause hydration hydration delay delay withth increasing increasing adsorption adsorption amount amount on the on surface the surface particles [31]. of cementof cement paste particlespaste particles [31]. [31].

600 600 600600

500 500 500 500 400 400 400 400 300 300 300 300 Time (min) 200 (min) Time 200 Initial setting Initial setting Time (min) 200 (min) Time 200 100 100 Initial settingFinal setting FinalInitial setting setting 100 0 1000 Final OPCsetting OPC-L5 OPC-L10 OPC-L15 OPC-L20 OPCFinal OPC-ML5 setting OPC-ML10 OPC-ML15 OPC-ML20 0 (a) 0 (b) OPC OPC-L5 OPC-L10 OPC-L15 OPC-L20 OPC OPC-ML5 OPC-ML10 OPC-ML15 OPC-ML20 Figure 5. Setting time of cement mortar: a) Mortar using conventional LSSP; (b) Mortar using Figure 5. Setting time(a) of cement mortar: (a) Mortar using conventional( LSSP;b) (b) Mortar using modified LSSP. modified LSSP. Figure 5. Setting time of cement mortar: a) Mortar using conventional LSSP; (b) Mortar using 3.2.2. Compressive Strength 3.2.2.modified Compressive LSSP. Strength Figure 6a shows the compressive strength of the cement mortar according to the substitution Figure6a shows the compressive strength of the cement mortar according to the substitution 3.2.2. Compressiveratio of conventional Strength LSSP for different curing times. Overall, the compressive strength of the mortar ratio ofwas conventional not significantly LSSP different for different from that curing of OPC, times. even Overall, when the the replacement compressive ratio strengthof conventional of the mortar wasFigure notLSSP significantly 6aincreased shows at the different all curingcompressive fromages. Figure that strength of6b OPC,shows of eventhe graphcement when of the themortar compressive replacement according strength ratio to theaccording of conventionalsubstitution ratioLSSP of increased toconventional the substitution at all LSSP curing ratio for of ages. differentthe modified Figure curing 6LSSP.b shows time Substitutions. the Overall, graph with the of the the compressive modified compressive LSSP strength produced strength of lower the according mortar wasto the not substitutioninitial significantly compressive ratio different strength of the from modifiedthan that that of LSSP. ofOPC OPC, for Substitution all even specimens. when with Afterthe thereplacement 1 day modified of aging, LSSPratiothe substitution of produced conventional lower LSSPinitial increased compressive at all strength curing ages. than thatFigure of OPC6b shows for all the specimens. graph of Afterthe compressive 1 day of aging, strength the substitution according to the substitution ratio of the modified LSSP. Substitution with the modified LSSP produced lower initial compressive strength than that of OPC for all specimens. After 1 day of aging, the substitution Sustainability 2018, 10, x FOR PEER REVIEW 6 of 10

proportions of 5% and 10% modified LSSP showed higher strength than that of OPC. After 28 days of aging, the 5% substitution resulted in strength superior to that of OPC. We believe that the introduction of modified LSSP into the space between cement particles improved pore filling and Sustainabilityinduced2019 ,hydration11, 879 reactions around the unreactive calcium carbonate particles, thereby increasing 6 of 10 the strength. At a replacement rate of up to 15%, the mortar using LSSP and modified LSSP showed a performance equal to or higher than that of the mortar using OPC. proportions of 5% and 10% modified LSSP showed higher strength than that of OPC. After 28 days of SustainabilityIt has 2018 been, 10, x reportedFOR PEER that REVIEW the rheological properties are improved when limestone powder 6 ofis 10 aging, thereplaced 5% substitution with cement. resultedIn addition, in limestone strength powder superior is reported to that of to OPC.be effective We believe in reducing that hydration the introduction of modifiedproportionsheat and LSSP in of increasing into5% and the 10% space the modifiedinitial between strength. LSSP cement However,showed particles hi itgher is expected improved strength that than pore both that filling the of long-term OPC. and inducedAfter strength 28 days hydration reactionsof andaging, around the thechlorine 5% the substitutionion unreactive penetration resulted calcium resistance in carbonate arestreng decreasedth superior particles, [32]. When to therebythat the of modified OPC. increasing WeLSSP believe was the replaced strength. that the At a replacementintroductionwith cement rate of of at modified up up toto 15%,10% LSSP of the the into mortar cement the space usingweight, be LSSP thetween initial and cement modified(up to particles 3 days LSSP of improvedaging) showed strength pore a performance wasfilling low. and equal On the other hand, the compressive strength of the mortar was as good as that of OPC after 28 days to orinduced higher thanhydration that ofreactions the mortar around using the unreactive OPC. calcium carbonate particles, thereby increasing theof strength. aging, and At aa 5%replacement substitution rate showed of up higher to 15%, st rengththe mortar than usingthose ofLSSP OPC and and modified conventional LSSP LSSP. showed

a performance50 equal to or higher than that of the mortar50 using OPC. OPC OPC It has beenOPC-L5 reported that the rheological properties areOPC-ML5 improved when limestone powder is replaced40 withOPC-L10 cement. In addition, limestone powder is40 reportedOPC-ML10 to be effective in reducing hydration heat and in increasingOPC-L15 the initial strength. However, it is expectedOPC-ML15 that both the long-term strength OPC-L20 OPC-ML20 and the30 chlorine ion penetration resistance are decreased30 [32]. When the modified LSSP was replaced

with cement20 at up to 10% of the cement weight, the initial20 (up to 3 days of aging) strength was low. On the other hand, the compressive strength of the mortar was as good as that of OPC after 28 days of aging,10 and a 5% substitution showed higher strength10 than those of OPC and conventional LSSP. Compressive strength (MPa) strength Compressive Compressivestrength (MPa) 50 0 500 OPC OPC 18 hours 1 day 3 days 7 days 28 days 18 hours 1 day 3 days 7 days 28 days OPC-L5 OPC-ML5 40 OPC-L10 (a) 40 OPC-ML10 (b) OPC-L15 OPC-ML15 Figure 6. Compressive strength of cement mortar: (a) mortar using conventional LSSP and (b) mortar Figure 6. CompressiveOPC-L20 strength of cement mortar: (a) mortarOPC-ML20 using conventional LSSP and (b) mortar 30 using modified LSSP. 30 using modiﬁed LSSP. 3.3.20 X-Ray Diffraction Analysis 20 It has been reported that the rheological properties are improved when limestone powder is replaced10 withFigure cement. 7 shows In the addition, results of limestone the XRD analysis powder acco10 isrding reported to the tocement be effective substitution in reducing ratios of the hydration

conventional (MPa) strength Compressive and modified LSSP after curing for 28 days. For the substitution with the conventional heat and in increasing the initial strength. However, itCompressivestrength (MPa) is expected that both the long-term strength and LSSP,0 the XRD analysis showed that the peak of calcium0 carbonate gradually increased and that the the chlorine ion penetration resistance are decreased [32]. When the modiﬁed LSSP was replaced with peak 18of hourscalcium 1 dayhydroxide 3 days decreased. 7 days This 28 days phenom enon 18has hours also been 1 day shown 3 days to reduce 7 days peak 28 days of cementettringite at up to and 10% C–S–H. of the (Thisa cement) might weight, have been the the initial cause (up of the to initial 3 days strength of aging)(b reduction) strength in the was case low. of On the other hand,substitutionFigure the 6. compressiveCompressive with the conventional strength strength of cementLSSP. of the In mortar:mortar this case (a) was ,mortar however, as using good it isconventionalas believed that of that OPCLSSP high-density and after (b) 28 mortar daysfilling of aging, by calcium carbonate increased the strength, and that the initial strength reduction was not and a 5%using substitution modified LSSP. showed higher strength than those of OPC and conventional LSSP. significant.

3.3. X-ray3.3. X-Ray Diffraction Diffraction Analysis Analysis C-S-H C-S-H C-A-S-H C-A-S-H Ca(OH)2 Ca(OH)2 Ettringite FigureFigure7 OPC-L10shows 7 shows the the results results of of the the XRDEttringite XRD analysisanalysis acco accordingrdingOPC-ML10 to to the the cement cement substitution substitutionCaCO3 ratios ratios of the of the CaCO3 conventionalconventional and and modified modified LSSP LSSP after after curingcuring for 28 28 days. days. For For the the substitution substitution with with the conventional the conventional LSSP,LSSP, the XRDthe XRD analysis analysis showed showed that that the the peakpeak of calcium calcium carbonate carbonate gradually gradually increased increased and that and the that the OPC-L5 OPC-ML5 peakpeak of calcium of calcium hydroxide hydroxide decreased. decreased. This phenom phenomenonenon has has also also been been shown shown to reduce to reduce peak peakof of ettringite and C–S–H. This might have been the cause of the initial strength reduction in the case of ettringite and C–S–H. This might have been the cause of the initial strength reduction in the case of substitution with the conventional LSSP. In this case, however, it is believed that high-density filling OPC OPC substitutionby calcium with carbonate the conventional increased LSSP.the strength, In this case,and that however, the initial it is believedstrength thatreduction high-density was not filling by calciumsignificant. carbonate increased the strength, and that the initial strength reduction was not significant.

0 102030405060 0 10 20 30 40 50 60 0 102030405060 0 10 20 30 40 50C-S-H 60 2 Theta (degree) C-S-H 2 Theta (degree) C-A-S-H C-A-S-H Ca(OH)2 Ca(OH)2 Ettringite OPC-L10 (a) Ettringite OPC-ML10 (b) CaCO3 CaCO3

OPC-L5 OPC-ML5

OPC OPC

0 102030405060 0 10 20 30 40 50 60 0 102030405060 0 10 20 30 40 50 60 2 Theta (degree) 2 Theta (degree) (a) (b)

Figure 7. X-ray diffraction (XRD) patterns of samples with different contents: (a) conventional LSSP and (b) modiﬁed LSSP. Sustainability 2018, 10, x FOR PEER REVIEW 7 of 10

Figure 7. X-ray diffraction (XRD) patterns of samples with different contents: (a) conventional LSSP and (b) modified LSSP. Sustainability 2019, 11, 879 7 of 10 3.4. Thermogravimetric/Differential Thermal Analysis

3.4.Calcium Thermogravimetric/Differential carbonate, which is a Thermal major component Analysis of limestone powder, is mostly non-hydrated, and, thus, it is possible that the initial strength can be lowered when a substantial amount of LSSP is used. CalciumFigures 8 carbonate, and 9 show which the results is a major of the component TG-DTAs of for limestone different powder, contents is mostlyof conventional non-hydrated, and modifiedand, thus, LSSP, it is possiblerespectively. that theThe initial temperature strength rang cane be of lowered the cured when specimens a substantial were amountmeasured of LSSPat a maximumis used. Figurestemperature8 and 9of show 900 °C. the In results Figure of 8, the comparison TG-DTAs of for the different dehydration contents amounts of conventional at 450 °C showsand modified that the amount LSSP, respectively. of calcium hydroxide The temperature decrease ranged with of increasing the cured specimensLSSP content. were In measuredthe case of at ◦ ◦ theamaximum modified LSSP, temperature Figure of9 shows 900 C. that In Figurethe reduction8, comparison in the amount of the dehydration of calcium hydroxide amounts atwas 450 notC significant,shows that even the amountwhen the of calciumsubstitution hydroxide ratio decreasedincreased. withThis increasingsuggests that LSSP the content. calcium In theacetate case of the modified LSSP, Figure9 shows that the reduction in the amount of calcium hydroxide was component on the surface of the modified LSSP was absorbed by tricalcium aluminate (C3A) and convertednot significant, into calcium even whenhydroxide, the substitution thereby fixing ratio the increased. internal amount This suggests of calcium that hydroxide. the calcium Calcium acetate chloridecomponent and calcium on the surface nitrate ofare the known modified to produce LSSP wascalcium absorbed aluminate by tricalcium hydrate combined aluminate with (C3 A)anion and converted into calcium hydroxide, thereby fixing the internal amount of calcium hydroxide. Calcium through hydration reaction with C3A in cement and to release calcium ions. When anions substitute OHchloride− in the and cement calcium to produce nitrate arehydrates, known the to produceorder of calciumanions affecting aluminate the hydrate hydration combined of cement with is anionOH− through hydration reaction with C A in cement and to release calcium ions. When anions substitute < Cl− < NO3− < Br− < acetate. Acetate3 ions are expected to produce hydrates of C3A and to produce − − calciumOH in hydroxide. the cement The to produce amount hydrates, of calcium the hydroxide order of anions determined affecting by theTGA hydration is shown of in cement Table is3 [33– OH − − − 35].< Cl < NO3 < Br < acetate. Acetate ions are expected to produce hydrates of C3A and to produce calcium hydroxide. The amount of calcium hydroxide determined by TGA is shown in Table3[ 33–35].

300 100 300 100

95 95 200 200 DTA(uV) 90 90 TGA(%) 85 85 100 TGA 100 DTA(uV) TGA TGA(%) TGA(%) TGA(%)

80 80

(%) (%) 0 0 DTA(uV) DTA(uV) 75

DTA (um) 75 DTA (um)

70 70 -100 -100 65 65

-200 60 -200 60 0 200 400 600 800 1000 0 200 400 600 800 1000 TemperatureTTTTTTTTuTT( (℃)℃ ) TemperatureTTTTTTTTuTT( ℃(℃) ) (a) (b) 300 100

95 200 90

100 (%) TGA

DTA(uV) TGA(%) TGA(%) 80

0 DTA(uV)

DTA (um) 75

70 -100 65

-200 60 0 200 400 600 800 1000 TemperatureTTTTTTTTuTT( (℃)℃ ) (c)

FigureFigure 8. 8. TG-DTATG-DTA results results for for samples samples cured cured for for 28 28 days: days: (a ()a OPC,) OPC, (b ()b OPC) OPC with with 5% 5% conventional conventional LSSP, LSSP, andand (c ()c )OPC OPC with with 10% 10% conventional conventional LSSP. LSSP.

Table 3. Quantiﬁcation of the calcium hydroxide (28 days).

Sample OPC (Plain) OPC-L5 OPC-L10 OPC-ML5 OPC-ML10 Ca(OH) Weight 2 2.33 1.01 0.985 1.771 1.876 content (%) Sustainability 2018, 10, x FOR PEER REVIEW 8 of 10

Sustainability 2019, 11, 879 8 of 10 85 TGA TGA Sustainability 2018, 10, x FOR PEER REVIEW DTA(uV) 8 of 10TGA(%)

TGA(%) 80

(%) (%) 300 100 300 100

DTA (um) 75 DTA DTA (um) 95 95 70 200 200 90 90 65

85 85 100 100 TGA (%) TGA (%) 60 TGA(%) TGA(%) 200 400 600 DTA(uV) 800 DTA(uV) 000 TGA(%) 80 TGA(%) 80 TemperatureTTTTTTTTuTT( (℃)℃ ) Temperature (℃) 0

DTA(uV) 0 DTA(uV)

75 (um) DTA 75 DTA (um) DTA (a) (b) 70 70 -100Figure 9. Results of the TG-DTA for samples cured for-100 28 days: (a) OPC with 5% modified LSSP and 65 (b) OPC with 10% modified LSSP. 65 -200 60 -200 60 0 200 400 600 800 1000 0 200 400 600 800 1000 Table℃ 3.) Quantification of the calcium hydroxide (28Temperature days). (℃℃)) TemperatureTemperature( (℃) Temperature( Sample (a) OPC (Plain) OPC-L5 OPC-L10 OPC-ML5(b) OPC-ML10 Ca(OH)2 Weight FigureFigure 9. 9. ResultsResults of of the the TG TG-DTA-DTA2.33 for for samples samples1.01 cured cured for for 28 28 0.985day days:s: (a ()a )OPC OPC with with 1.771 5% 5% modified modiﬁed LSSP LSSP 1.876 and and (b(b) )OPCcontent OPC with with (%) 10% 10% modified modiﬁed LSSP. LSSP.

3.5. Scanning Electron MicroscopyTable 3. Quantification Analysis of the calcium hydroxide (28 days).

FigureSample 10 shows the SEMOPC ( imagesPlain) of OPC samples-L5 withOPC 5%- L10 substitution OPC of-ML5 conventional OPC and-ML10 modifiedmodified LSSP,Ca(OH) respectively.respectively.2 Weight When reagent-grade calcium carbonate was mixed, it was observed that the 2.33 1.01 0.985 1.771 1.876 calciumcontent carbonate (%) was held between the hydrates. Hydrates, such as ettringite, were formed on the surface, which enabled compact fillingfilling through physicalphysical fillingfilling and induced hydrate formation in the 3.5.cement Scanning mortar. Electron Microscopy Analysis Figure 10 shows the SEM images of samples with 5% substitution of conventional and modified LSSP, respectively. When reagent-grade calcium carbonate was mixed, it was observed that the calcium carbonate was held between the hydrates. Hydrates, such as ettringite, were formed on the surface, which enabled compact filling through physical filling and induced hydrate formation in the cement mortar.

Calcium Hydroxide Calcium Hydroxide Calcium Hydroxide Tobermorite

Calcium Carbonate

Calcium Carbonate Ettringite

Ettringite C-S-H

C-S-H (a) (b) C-S-H Calcium Carbonate Figure 10.10. SEM imagesimages ofof samples:samples: ((aa))C 5%-5%S-H conventionalconventional LSSPLSSP (10,000(10,000×)×) and and ( b) 5% modiﬁedmodified LSSPLSSP (10,000(10,000×).×).

4. Conclusions

4. Conclusion (a) (b) The conclusions of this study cancan bebe summarizedsummarized asas follows.follows. Limestone powder is used as a raw material or admixture of cement. In this study, various FigureLimestone 10. SEM powder images is of used samples: as a ( araw) 5% materialconventional or admixtureLSSP (10,000×) of cement.and (b) 5%In modifiedthis study, LSSP various properties(10,000×). of limestone sludge powder (LSSP) were evaluated by using recycled acetic acid (RAA), which isis oneone ofof thethe industrialindustrial byproducts,byproducts, asas aa cementcement admixtureadmixture afterafter surfacesurface modificationmodification ofof LSSP.LSSP. 4. ConclusionThe surfacesurface ofof LSSP LSSP modified modified with with RAA RAA was was converted converted into into calcium calcium acetate acetate and hadand ahad large a grainlarge size. Calcium acetate has effects on mortar properties. The setting time of mortars with conventional grainThe size. conclusions Calcium ofacetate this study has caneffects be summarized on mortar properties.as follows. The setting time of mortars with LSSPconventional was independent LSSP was of independent the mixing ratiosof the and mixing was equalratios toand that was of ordinaryequal to that type of 1 Portlandordinary cement type 1 (OPC),Limestone whereas powder the initial is compressiveused as a raw strength material slightly or admixture increased. of When cement. conventional In this study, LSSP isvarious used as properties of limestone sludge powder (LSSP) were evaluated by using recycled acetic acid (RAA), a cement admixture material, it is possible that physical performance may deteriorate in the long term. which is one of the industrial byproducts, as a cement admixture after surface modification of LSSP. Nevertheless, the use of modified LSSP with RAA can promote and continuously induce hydration in The surface of LSSP modified with RAA was converted into calcium acetate and had a large the long term, thereby improving the performance of the cement mortar. When the modified LSSP grain size. Calcium acetate has effects on mortar properties. The setting time of mortars with conventional LSSP was independent of the mixing ratios and was equal to that of ordinary type 1 Sustainability 2019, 11, 879 9 of 10 was replaced with cement up to 10% of the cement weight, the initial (up to 3 days of aging) strength was low. On the other hand, the compressive strength of mortar was as good as that of OPC after 28 days of aging, and 5% substitution showed higher strength than those of OPC and conventional LSSP. Therefore, the use of cement admixture through modification of LSSP using RAA in this study is expected to be useful as a sustainable construction material.

Author Contributions: For research articles, author contributions are as follows; conceptualization, H.-S.R.; methodology and investigation, D.-M.K.; data curation, S.-H.S.; writing—original draft preparation, H.-S.R.; writing and editing, W.-J.P.; review and editing, S.-M.L. and W.-K.K. Acknowledgments: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (no. NRF-2018R1D1A3B07045700). Conﬂicts of Interest: The authors declare no conﬂict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.

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