L-Arginine-Incorporated Cement Mortar As Sustainable Artificial Reefs

sustainability

Article L-Arginine-Incorporated Cement Mortar as Sustainable Artiﬁcial Reefs

Hyun-Min Yang 1,2, Nosang V. Myung 1, Han-Seung Lee 3,* and Jitendra Kumar Singh 2,*

1 Department of Chemical and Environmental Engineering, University of California-Riverside, Riverside, CA 92521, USA; [email protected] (H.-M.Y.); [email protected] (N.V.M.) 2 Innovative Durable Building and Infrastructure Research Center, Department of Architectural Engineering, Hanyang University, 1271 Sa-3-dong, Sangnok-gu, Ansan 15588, Korea 3 Department of Architectural Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan 15588, Korea * Correspondence: [email protected] (H.-S.L.); [email protected] (J.K.S.)

Received: 8 June 2020; Accepted: 4 August 2020; Published: 6 August 2020

Abstract: L-arginine is one of the amino acids found in plant seeds. In the present study, various amounts (i.e., 3%, 5%, 10%) of L-arginine were added to cement mortar to investigate the compressive strength, workability, leaching behavior, and pH change in distilled and natural seawater, as well as dissolved nitrogen and growth of chlorophyll-a (Chl-a) by immersion in natural seawater. The compressive strength of the cement mortar is decreased with increase in L-arginine content owing to the high flow/slump and air content. A concentration of 10% L-arginine significantly promoted the growth of Chl-a on the cement mortar for up to 56 days of immersion in natural seawater. This is due to the availability of high dissolved nitrogen and pH inside the pores. This study recommends the use of L-arginine in artificial reef structures where marine ecosystems can be maintained.

Keywords: L-arginine; cement mortar; artiﬁcial reef; chlorophyll-a; compressive strength; pH

1. Introduction Coastal ecosystems must be stabilized in underwater environments where marine algae, fish, and shellfish are found. Unfortunately, construction of artificial marine structures (e.g., artificial reef, seawall, tide embankment, tetrapod) causes extensive damage to the marine ecosystem [1]. They not only cause the loss of marine habitats and habitat fragmentation, but also restrict biodiversity more than natural rocks [2–6]. Artificial marine structures, which were constructed similarly to conventional structures, lead to maintenance of the alkalinity of concrete owing to the attached organism [7]. Moreover, freshly installed normal concrete may leach KOH, NaOH, and Ca(OH)2) into the sea, which inhibit the growth of marine life and decrease the mechanical properties of the concrete [8]. In addition, the pozzolanic materials can maintain the surface pH, mechanical properties [9,10], and chemical stability of the concretes. The concrete structures installed in mountainous terrain and river water are free from dissolved ions. Therefore, this water is aggressive for the health of immersed concrete where the hydration products of concrete, such as Ettringite and calcium silicate hydrate (C-H-S), are leached out from the pore system and transported away by diffusion or water flow [11]. Once the hydration products leach out, then ultimately, the porosity and permeability of the concrete would increase, resulting in a decrease in compressive strength. However, the leaching may influence the permeability, amount of calcium—i.e., Ca(OH)2—and CO2 content in the water, and hardness of the water. If all these are high, then leaching would be high. The leaching of pore water can be reduced by adding aluminum ferrite [11] and making dense concrete. Ekström (2001) has mentioned that pure water, which has

Sustainability 2020, 12, 6346; doi:10.3390/su12166346 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 6346 2 of 16

the ability to dissolve the Ca(OH)2, percolating into the concrete specimens results in leaching by diffusion and convection [11]. This depends on the water/cement ratio and curing conditions. A high water/cement ratio encourages one to make the concrete more porous, resulting in higher water permeability. However, incorporation of nano silica particles reduces the porosity, transforming portlandite into C-S-H gel through a pozzolanic reaction and increasing the average chain length of the silicate, resulting in durable concrete that reduces the leaching of calcium ions [12]. The addition of nano silica induces the pozzolanic reaction, which transforms the more susceptible Ca(OH)2 into C-S-H gel. Therefore, leaching of Ca(OH)2 is reduced. The diffusion of Ca(OH)2 can be reduced using additives such as wet biofilm, freeze-dried biofilm powder, and bacterial suspensions, which can prevent ingress of water into the concrete [13]. The bacteria can improve the durability of the concrete in a sulfate environment, which is attributed to the increase of the bulk density and lowering of the voids [14]. Sporosarcina pasteurii [15] and Bacillus megaterium [16] bacteria precipitate into the cement mortar, resulting in improvement of compressive strength. L-arginine is one of the naturally existing amino acids that is present in plant seeds [17–20]. Moreover, L-arginine can act as an inhibitor to mitigate the corrosion of steel [21]. Gowri et al. (2014) studied the synergistic effect of L-arginine with Zn2+ to enhance the corrosion resistance properties of steel exposed in seawater [22]. They found that 250 ppm L-arginine with 25 ppm Zn2+ showed 91% inhibition efficiency. This study revealed that L-arginine can be used as an inhibitor for the protection of embedded steel rebar in concrete. There have been many studies on leaching of inorganic elements from the concrete into water [23–26]. However, there are no studies on the leaching behavior of L-arginine-containing concrete in marine environments. Moreover, the effect of L-arginine on durability of the cement mortar has not yet been studied for application in an artificial reef. In this manuscript, systematic studies were conducted to investigate the effect of L-arginine on various materials and chemical properties of cement mortar, including the compressive strength, workability, leaching behavior, and pH change in distilled and natural sea water, as well as dissolved nitrogen and growth of chlorophyll-a (Chl-a) by immersing into an actual sea for an artificial reef.

2. Materials and Methods

2.1. Materials L-arginine ( 99.00%) was purchased from Sigma-Aldrich, and the properties are shown in Table1. ≥ The cement mortar was prepared using standard Portland cement [27]. The specific gravity and specific surface area of the cement were 3150 kg/m3 and 395 m2/kg, respectively. The chemical composition of cement is shown in Table2. Natural sand was used as the fine aggregate according to ASTM C33 [28]. The size, apparent density, and water absorption of the fine aggregate were less than 4.75 mm, 2461 kg/m3, and 2.6%, respectively.

Table 1. Properties of L-arginine.

Division Properties

Formula C6H14N4O2 Molecular weight 174.20 g/mol pH 12.0 at 25 ◦C Melting point 222 ◦C Water solubility 87.1 g/L at 20 ◦C - completely soluble Sustainability 2020, 12, 6346 3 of 16

Table 2. Chemical composition of cement.

Chemical Composition Cement (wt.%)

SiO2 23.20 Al2O3 5.03 Fe2O3 2.92 CaO 62.40 MgO 2.07 SO3 2.34 K2O 0.59 Na2O 0.26 Loss of ignition 1.19

2.2. Specimen Preparation Mortar specimens were prepared using a 0.5 water/binder (W/B) ratio with diﬀerent L-arginine contents (i.e., 0%, 3%, 5%, and 10%). The details of the mixing proportions of mortar with diﬀerent L-arginine replacement ratios are shown in Table3. L-arginine acts as a binder in cement mortar. In order to evaluate the mechanical properties and environmental activity of L-arginine mortars, the specimens were subjected to curing at 20 ( 2.5) C and 60% ( 5%) relative humidity for 24 h ± ◦ ± thereafter they were demolded and cured in tap water as well as natural seawater at 20 ( 2.5) C for ± ◦ up to 56 days.

Table 3. Mortal mixture proportions.

Mix Composition (kg/m3) W/B L-Arginine (%) Water Cement Sand L-Arginine 0 510 0 3 494.7 15.3 0.5 255 1530 5 484.5 25.5 10 459 51

2.3. Test Methods

2.3.1. Flow/Slump Value and Air Content In order to evaluate the workability of mortar with L-arginine, a ﬂow test was performed according to ASTM C1437 [29] and compared with ASTM C230 [30]. The same fresh cement mortars were measured three times, and the average value was reported. The evaluation of air content in the mortar with L-arginine was carried out by ASTM C185 [31].

2.3.2. Compressive Strength The cement mortar with 50 50 50 mm dimensions was prepared for the compressive strength × × measurement. The cement mortar specimen was cured in tap water and seawater. The compressive strength of the cement mortar at 3, 7, 14, 28, and 56 days cured at 20 ( 2) C was measured according ± ◦ to ASTM C109 [32]. At each curing duration, three identical specimens were measured and the average value was taken as a result.

2.3.3. Leaching Test for L-Arginine The cement mortar specimens with 50 50 50 mm dimensions were prepared to analyze the × × leaching behavior of L-arginine. The cement mortar was cured in a closed/dark chamber at 20 ◦C and 60% ( 5%) relative humidity for 28 days; thereafter, they were removed. The cured mortar was ± pulverized to less than 100 µm and dispersed by taking 1 g each in 25, 50, 100, 300, 500, and 1000 mL distilled water and stirring for 1 h. The obtained suspension was separated by glass ﬁber ﬁlter Sustainability 2020, 12, 6346 4 of 16

(GF/C, 1.2 µm); then, the L-arginine content of the liquid (solution) was analyzed by high-performance liquid chromatography (HPLC, Agilent 1100 High-Performance Liquid Chromatography, Waldbronn, Germany). In addition, the remaining part (residue or undissolved) of L-arginine on the filter paper wasSustainability washed 2020 with, 12, acid–alkali x FOR PEER solutionREVIEW (acetonitrile); thereafter, it was dissolved in 100, 300, and 5004 of mL 16 of 0.1, 0.5, and 1 M HCl solution for 1 h. 2.3.4. pH Determination 2.3.4.The pH DeterminationpH (Thermo Scientific Orion, Waltham, Massachusetts, United State America) determinationThe pH (Thermo of L-arginine-containing Scientific Orion, Waltham, cement Massachusetts, mortar was Unitedcarried Stateout America)in distilled determination water and ofseawater L-arginine-containing with different cementimmersion mortar times was at carried 20 (±2. out5) °C. in distilledThe pH watermeter andwas seawaterdirectly dipped with diff inerent the immersionabovementioned times atwater 20 ( 2.5)to measureC. The pHthe meter pH of was the directly solution dipped [33–35] in the at abovementioneddesignated durations water toof ± ◦ measureimmersion. the The pH ofcylindrical the solution cement [33– 35mortar] at designated specimen durationss of Ø 100 of× 200 immersion. mm were The directly cylindrical immersed cement in mortara 5-L polypropylene specimens of Øplastic 100 container200 mm werewith 300 directly × 400 immersed mm dimensions in a 5-L for polypropylene determination plastic of the container solution × withpH. 300 400 mm dimensions for determination of the solution pH. × 2.3.5.2.3.5. Dissolved Nitrogen TheThe amountamount ofof dissolveddissolved nitrogennitrogen inin L-arginine-containingL-arginine-containing cementcement mortarmortar waswas determineddetermined accordingaccording to ASTM ASTM D8083 D8083 [36]. [36]. In In this this procedure, procedure, 100 100 g homogenized g homogenized powder powder of each of each specimen specimen (0%, (0%,3%, 5%, 3%, and 5%, 10% and L-arginine) 10% L-arginine) was centrifuged was centrifuged in a 5-L in aglass 5-L glassbeaker beaker by adding by adding 1000 mL 1000 distilled mL distilled water waterfor 20 formin; 20 then, min; it then, was filtered it was filtered.. It was measured It was measured for up to for 56 updays. to 56The days. supernatant The supernatant was recovered was recoveredand stored and in a stored polypropylene in a polypropylene bottle. The bottle. total di Thessolved total nitrogen dissolved was nitrogen measured was measuredusing a Shimadzu using a ShimadzuTOC-V analyzer TOC-V (Shimadzu analyzer (ShimadzuCorp., Kyoto, Corp., Japan). Kyoto, This Japan).experiment This was experiment performed was with performed triplicate with sets triplicateof specimens sets ofand specimens the average and values the average were reported. values were reported.

2.3.6.2.3.6. Evaluation of Chlorophyll-A AmountAmount TheThe amountamount ofof chlorophyll-achlorophyll-a (Chl-a)(Chl-a) inin L-arginine-containingL-arginine-containing cementcement mortarmortar specimensspecimens waswas evaluatedevaluated byby immersingimmersing themthem inin aa naturalnatural seasea nearnear thethe SihwaSihwa tidetide embankmentembankment lakelake locatedlocated atat AnsanAnsan city,city, SouthSouth KoreaKorea accordingaccording toto ASTM ASTM D3731 D3731 [ 37[37].]. ItIt was observed observed that that after after immersion immersion of ofthe the ceme cementnt mortar mortar specimens specimens in the in sea, the generally, sea, generally, Chl- Chl-aa started started to be to attached be attached or orto tofloat. ﬂoat. The The attached attached Chl-a Chl-a was was removed removed from from the the surface surface of the cement mortarmortar immersedimmersed inin thethe seasea withwith distilleddistilled waterwater usingusing aa sterilizedsterilized semi-aromaticsemi-aromatic polyamide-basedpolyamide-based 2 brushbrush onon aa 55 cmcm2 surface area for didifferentﬀerent durationsdurations ofof immersion,immersion, asas shownshown inin FigureFigure1 .1.

FigureFigure 1.1. Removal of chlorophyll-a from thethe cementcement mortarmortar specimenspecimen immersedimmersed inin sea.sea.

TheThe collectedcollected Chl-aChl-a waswas storedstored inin aa non-lightnon-light polypropylenepolypropylene bottlebottle withwith 11 mLmL ofof magnesiummagnesium µ carbonatecarbonate suspension.suspension. TheThe stored Chl-aChl-a was filteredfiltered throughthrough thethe glassglass fiberfiber filterfilter (GF(GF/C,/C, 1.21.2 μm)m) usingusing 15 KPa or less pressure for 20 min within 24 h of storing. The residue and filter paper were ground and homogenized in 5 mL acetone solution (acetone: 90%, distilled water: 10%) for 3 min. The ground specimen was placed in a centrifuge tube, sealed, and stored for 24 h in a light-free environment at 4 °C. The specimen was then centrifuged for 20 min to prepare a supernatant. This sample was transferred to a 10 mm absorption cell. Acetone was considered as a control solution, and the optical density (OD) of the specimen using a Cary® 50 UV-Vis Spectrophotometer (Varian Inc., California, Sustainability 2020, 12, 6346 5 of 16

15 KPa or less pressure for 20 min within 24 h of storing. The residue and ﬁlter paper were ground and homogenized in 5 mL acetone solution (acetone: 90%, distilled water: 10%) for 3 min. The ground specimen was placed in a centrifuge tube, sealed, and stored for 24 h in a light-free environment at 4 ◦C. The specimen was then centrifuged for 20 min to prepare a supernatant. This sample was transferred Sustainabilityto a 10 mm 2020 absorption, 12, x FOR PEER cell. REVIEW Acetone was considered as a control solution, and the optical density5 of 16 (OD) of the specimen using a Cary® 50 UV-Vis Spectrophotometer (Varian Inc., Palo Alto, CA, USA) USA)was measured was measured at 664, at 647, 664, 630, 647, and 630, 750 and nm. 750 Thenm. concentrationThe concentration of Chl-a of Chl-a on the on mortarthe mortar surface surface was wascalculated calculated using using the Jetheﬀrey Jeffrey and Humphreyand Humphrey equation equation [38]. [38].

(11.64𝑋 − 2.16𝑋 +0.01𝑋)×𝑉 Chl − a (μg/cm )= (11.64X1 2.16X2 + 0.01X3) V1 (1) Chl a µg/ cm2 = − 𝐴 × (1) − A where 𝑋 is OD 664–OD 750, 𝑋 is OD 647–OD 750, and 𝑋 is OD 630–OD 750. 𝑉 is the volume of where X is OD 664–OD 750, X is OD 647–OD 750, and X is OD 630–OD 750. V is the volume of supernatant1 (mL) and 𝐴 is the surface2 area of the cement mortar3 (cm2). 1 supernatant (mL) and A is the surface area of the cement mortar (cm2). 3. Results and Discussion 3. Results and Discussion

3.1.3.1. Properties Properties of of Cement Cement Mortar Mortar with with L-Arginine L-Arginine

3.1.1.3.1.1. Flow/Slump Flow/Slump and and Air Air Content Content of of Fresh Fresh Cement Mortar TheThe workability workability (flow/slump) (flow/slump) and and air air content content tests tests were were performed performed with with different different amounts amounts of L- of arginine.L-arginine. Figure Figure 2 2shows shows the the results results of of the the flow/s flow /lumpslump and and air air content content of of the the L-arginine L-arginine cement mortar.mortar. The The workability workability of of the the mortar mortar with with L-arginine L-arginine showed showed that that the the flow/slump flow/slump value value is is increased increased asas the the amount amount of of L-arginine L-arginine in in the the cement cement mortar mortar is isincreased increased owing owing to tothe the higher higher dissolution dissolution [39]. [39 It]. wasIt was observed observed that that 3%, 3%, 5%, 5%, and and 10% 10% L-arginine-containing L-arginine-containing cement cement mortar mortar exhibited exhibited increases increases in in slumpslump valuevalue ofof about about 3.5%, 3.5%, 10%, 10%, and and 21%, 21%, respectively, respectively, compared compared to without to without cement cement mortar L-arginine,mortar L- arginine,i.e., 0%. Thei.e., air0%. contentThe air alsocontent increased also increased as the amount as the ofamount L-arginine of L-arginine increased increased (Figure2) (Figure owing to2) owingthe characteristics to the characteristics of L-arginine, of L-arginine, which include which waterinclude absorption water absorption [40,41]. The[40,41]. increase The increase in air content in air contentvalues ofvalues L-arginine-containing of L-arginine-containing cement mortarcement ismortar due to is the due increase to the increase in the interfacial in the interfacial effect between effect betweennon-hydrated non-hydrated cement particles cement andparticles L-arginine. and L-arginine. L-arginine L-arginine acts as an acts acid–base as an cationicacid–base surfactant cationic surfactantwhere it reduces where theit reduces surface the tension surface and tension allows and the formationallows the offormation stable bubbles of stable [42 bubbles–45] on the[42–45] surface on theof the surface cement of andthe cement sand particles. and sand Thus, particles. flow andThus air, flow content and are airincreased content are with increased addition with of L-arginine addition ofin L-arginine cement mortar. in cement mortar.

Figure 2. Flow and air content of cement mortar with diﬀerent L-arginine replacement ratios. Figure 2. Flow and air content of cement mortar with different L-arginine replacement ratios.

3.1.2. Compressive Strength The compressive strength of L-arginine-containing cement mortar was measured by curing in tap water and seawater, and the results are shown in Figures3 and4, respectively. It is observed from these figures that the compressive strength of the cement mortar specimens is gradually increased with curing duration. The compressive strength of the specimens after 28 days of curing in tap water was found to be 28.6, 26.9, 24.6, and 22.9 MPa for 0%, 3%, 5%, and 10% L-arginine replacement, respectively (Figure3). The increase in compressive strength with curing duration could be attributed to the hydration reaction. The decrease in compressive strength of specimens (Figure3) with increase in Sustainability 2020, 12, x FOR PEER REVIEW 6 of 16 L-arginineSustainability 2020 replacement, 12, x FOR PEER can beREVIEW attributed to the higher flow/slump value and air content (Figure6 of2 16), contentwhich leads is higher, to the the increase compressive of the micro-cracks strength of/defects the specimen inside of would the cement be lower mortar. owing If the to airthe content inward is diffusionhigher,content the is of compressivehigher, water molecules.the compressive strength Thus, of the 10%strength specimen L-arginine-containing of wouldthe specimen be lower would owingcement be to mortar thelower inward (Figureowing diff tousion3) theshows ofinward water the lowestmolecules.diffusion compressive of Thus, water 10% molecules. strength L-arginine-containing compared Thus, 10% to L-arginine-containing the cement other specimens. mortar (Figure Moreover,cement3) shows mortar there the (Figure lowestis a possibility compressive3) shows that the thestrengthlowest softening compressive compared effect of to strength cement the other comparedmortar specimens. may to occurthe Moreover, other upon specimens. thereloading is a for possibility Moreover, compression, that there the and is softening a the possibility preexisting effect that of micro-crackscementthe softening mortar propagate effect may of occur cement and upon form mortar loading new one.may for Therefore, occur compression, upon the loading load and carrying the for preexisting compression, capacity micro-cracks of and the cementthe preexisting propagate mortar decreasesandmicro-cracks form as new the propagate one. peak Therefore, load and of form deformation the new load one. carrying increases Therefore, capacity in theL-arginine-containing ofload the carrying cement capacity mortar cement decreases of the mortar cement as the[46,47]. mortar peak loaddecreases of deformation as the peak increases load of deformation in L-arginine-containing increases in L-arginine-containing cement mortar [46,47]. cement mortar [46,47].

Figure 3. Compressive strength of mortar with L-arginine cured in tap water. Figure 3. CompressiveCompressive strength of mortar with L-arginine cured in tap water.

FigureFigure 4. 4. CompressiveCompressive strength strength of of mortar mortar with with L-arginine L-arginine cured cured in in seawater. seawater. Figure 4. Compressive strength of mortar with L-arginine cured in seawater. In the case of seawater curing, the compressive strength of 0%, 3%, 5%, and 10% In the case of seawater curing, the compressive strength of 0%, 3%, 5%, and 10% L-arginine- L-arginine-containing cement mortar after 28 days of curing was found to be 29.9, 28.2, 27.7, and 26.9 MPa, containingIn the cement case of mortarseawater after curing, 28 days the ofcompressive curing was strength found toof be0%, 29.9, 3%, 28.2, 5%, 27.7,and 10%and L-arginine-26.9 MPa, respectively (Figure4). As the amount of L-arginine replacement increased, the compressive strength of respectivelycontaining cement(Figure 4).mortar As the after amount 28 days of L-argini of curingne replacementwas found to increased, be 29.9, the28.2, compressive 27.7, and 26.9 strength MPa, ofrespectively the cement (Figure mortar 4). decreased. As the amount The ofreduction L-argini nein replacementcompressive increased,strength of the L-arginine-containing compressive strength cementof the cementmortar mortaris due decreased.to the higher The contentreduction of inentrained compressive air duringstrength curing of L-arginine-containing in seawater. The compressivecement mortar strength is due of toseawater-cured the higher content cement ofmo entrainedrtar was higherair during compared curing to intap seawater. water for Theall curingcompressive durations. strength This isof attributedseawater-cured to the cementformation mo ofrtar Friedel’s was higher salt bycompared the hydration to tap productswater for i.e. all curing durations. This is attributed to the formation of Friedel’s salt by the hydration products i.e. Al2O3–Fe2O3-mono (AFm) or –tri (AFt) [48]. Moreover, the Cl- ions from seawater can infiltrate during - curingAl2O3–Fe [49–51].2O3-mono Thus, (AFm) the filling or –tri ability (AFt) of [48]. pores Moreover, by Friedel’s the Cl salt ions evidently from seawater improved can the infiltrate compressive during strengthcuring [49–51]. of the cement Thus, the mortar filling [47,52]. ability of pores by Friedel’s salt evidently improved the compressive strength of the cement mortar [47,52]. 3.2. L-Arginine Extraction from Hardened Cement Mortar 3.2. L-Arginine Extraction from Hardened Cement Mortar 3.2.1. Extraction of L-Arginine in Distilled Water 3.2.1. Extraction of L-Arginine in Distilled Water Sustainability 2020, 12, 6346 7 of 16 the cement mortar decreased. The reduction in compressive strength of L-arginine-containing cement mortar is due to the higher content of entrained air during curing in seawater. The compressive strength of seawater-cured cement mortar was higher compared to tap water for all curing durations. This is attributed to the formation of Friedel’s salt by the hydration products i.e., Al2O3–Fe2O3-mono (AFm) or –tri (AFt) [48]. Moreover, the Cl- ions from seawater can infiltrate during curing [49–51]. Thus, the filling ability of pores by Friedel’s salt evidently improved the compressive strength of the cement mortar [47,52].

3.2. L-Arginine Extraction from Hardened Cement Mortar

3.2.1.Sustainability Extraction 2020, 12 of, x L-ArginineFOR PEER REVIEW in Distilled Water 7 of 16

FigureFigure5 5 shows shows thethe recoveryrecovery ofof L-arginineL-arginine leachedleached fromfrom 11 gg ofof cementcement mortarmortar inin distilleddistilled water.water. FromFrom this figure, figure, it can can be be seen seen that that the the recovery recovery of of L-arginine L-arginine started started to tooccur occur from from 65% 65% for for 3% 3% L- L-arginine-containingarginine-containing cement cement mortar mortar in in 25 25 mL mL dist distilledilled water. water. As As the the amount amount of L-arginine waswas increasedincreased upup toto 10%,10%, thethe recoveryrecovery waswas foundfound toto bebe aroundaround 69%69% inin 2525 mLmL distilleddistilled water.water. In addition, whenwhen thethe volumevolume ofof distilleddistilled waterwater reachedreached upup toto 500500 mL,mL, the recovery waswas found to be 78% and 84% forfor 3%3% andand 10% 10% L-arginine-containing L-arginine-containing cement cement mortar, mortar, respectively. respectively. Moreover, Moreover, it can it becan observed be observed from Figurefrom Figure5 that once5 that the once volume the ofvolume distilled of waterdistilled is increasedwater is increased beyond 500 beyond mL, there 500 ismL, no significantthere is no recoverysignificant of recovery L-arginine of inL-arginine any L-arginine-containing in any L-arginine-containing cement mortars. cement As mortars. a result, As 500 a mLresult, of distilled 500 mL waterof distilled was suwaterfficient was to sufficient extract L-arginine. to extract L-argi Moreover,nine. whenMoreover, the volume when the of distilledvolume of water distilled was same,water i.e.,was 25, same, 50, 100,i.e., 300,25, 50, 500, 100, and 300, 1000 500, mL and for a1000 particular mL for L-arginine-containing a particular L-arginine-containing mortar, the recovery mortar, rate the increasedrecovery owingrate increased to the solubility owing to of the L-arginine solubility in distilledof L-arginine water. in Since distilled L-arginine water. is Since soluble L-arginine and mostly is remainedsoluble and in themostly pore remained water, it could in the be pore extracted water, with it could distilled be extracted water. It iswith not possibledistilled towater. recover It is 100% not L-arginine;possible to recover therefore, 100% another L-arginine; solution therefore, is required anothe to recoverr solution it completely. is required to Therefore, recover it we completely. used 100, 300,Therefore, and 500 we mL used of 0.1, 100, 0.5, 300, and and 1 M 500 HCl mL solution. of 0.1, In 0.5, the and subsequent 1 M HCl paragraph, solution. weIn willthe discusssubsequent the recoveryparagraph, of L-argininewe will discuss in HCl the solution. recovery of L-arginine in HCl solution.

FigureFigure 5.5. Recovery of L-arginine in distilled water.

3.2.2.3.2.2. Recovery of Undissolved L-Arginine inin HClHCl TheThe residueresidue (remaining(remaining partpart ofof undissolvedundissolved L-arginine-containingL-arginine-containing cementcement mortarmortar inin distilleddistilled water)water) cancan bebe dissolveddissolved in in 0.1 0.1 M M HCl HCl solution. solution. L-arginine L-arginine can can dissolve dissolve in in distilled distilled water, water, but but owing owing to theto the cement cement and and sand sand in cement in cement mortar, mortar, some L-argininesome L-arginine was adsorbed was adsorbed on the surface on the and surface not dissolved and not indissolved distilled in water; distilled therefore, water; HCl therefore, was used HCl to dissolvewas used the to remaining dissolve the L-arginine. remaining It wasL-arginine. observed It from was Figureobserved5 that from the Figure recovery 5 that of L-arginine the recovery is optimum of L-argi innine 500 is mLoptimum distilled in water;500 mL thus, distilled we selected water; onlythus, 500we selected mL of 0.1 only M HCl 500 solutionmL of 0.1 to M recover HCl solution the remaining to recover L-arginine the remaining from the L-arginine residue. from the residue. Figure 6a shows the recovery of L-arginine leached into the 0.1 M HCl solution. It can be seen in this figure that, as the volume of 0.1 M HCl solution increased, the recovery of L-arginine increased. The maximum recovery was found to be 8.27% by the 10% L-arginine-containing cement mortar specimen, followed by 5% and 3% L-arginine around 8.15% and 8.05%, respectively. Figure 6b shows the recovery of 5% L-arginine with respect to the concentration of HCl solution. It can be observed from Figure 6a that 5% L-arginine exhibited the optimum amount of recovery. Thus, it was our prudent thought to consider only this specimen for the recovery of L-arginine with different concentrations of HCl solution. It can be observed from Figure 6b that, as the concentration of HCl increased from 0.1 to 1 M, the recovery was almost constant. Therefore, from this result, it is suggested that the leaching of L-arginine into the mortar is found to be more sensitive to the volume of HCl solution rather than concentration. Sustainability 2020, 12, 6346 8 of 16

Figure6a shows the recovery of L-arginine leached into the 0.1 M HCl solution. It can be seen in this ﬁgure that, as the volume of 0.1 M HCl solution increased, the recovery of L-arginine increased. The maximum recovery was found to be 8.27% by the 10% L-arginine-containing cement mortar specimen, followed by 5% and 3% L-arginine around 8.15% and 8.05%, respectively. Figure6b shows the recovery of 5% L-arginine with respect to the concentration of HCl solution. It can be observed from Figure6a that 5% L-arginine exhibited the optimum amount of recovery. Thus, it was our prudent thought to consider only this specimen for the recovery of L-arginine with diﬀerent concentrations of HCl solution. It can be observed from Figure6b that, as the concentration of HCl increased from 0.1 to 1 M, the recovery was almost constant. Therefore, from this result, it is suggested that the leaching of L-arginine into the mortar is found to be more sensitive to the volume of HCl solution ratherSustainability than 2020 concentration., 12, x FOR PEER REVIEW 8 of 16

Figure 6.6. Recovery ofof L-arginineL-arginine withwith ( a(a)) di differentﬀerent amounts amounts of of L-arginine-containing L-arginine-containing cement cement mortar mortar in 0.1in 0.1 M HClM HCl and and (b) ( 5%b) 5% L-arginine L-arginine with with concentration concentration and an volumed volume of HClof HCl from from the the residue. residue. 3.3. Evaluation of the Impact of the Environmental Activity of L-Arginine-Containing Mortar 3.3. Evaluation of the Impact of the Environmental Activity of L-Arginine-Containing Mortar 3.3.1. Change of pH Value According to the Type of Water Used to Immerse Cement Mortar with L-Arginine 3.3.1. Change of pH Value According to the Type of Water Used to Immerse Cement Mortar with L- ArginineThe changes in pH of the cement mortar immersed in distilled water and seawater are shown in Figure7a,b, respectively. The pH of each data point is normally an average of three measurements. The changes in pH of the cement mortar immersed in distilled water and seawater are shown in Figure7a shows the change in pH of L-arginine-containing cement mortar immersed in distilled water Figure 7a,b, respectively. The pH of each data point is normally an average of three measurements. according to duration. During immersion of L-arginine cement mortar in distilled water, the pH of the Figure 7a shows the change in pH of L-arginine-containing cement mortar immersed in distilled distilled water was measured, and it was found to be 7.3. It can be observed from Figure7a that, as the water according to duration. During immersion of L-arginine cement mortar in distilled water, the amount of L-arginine-containing cement mortar is increased, the pH is increased. The pH value of pH of the distilled water was measured, and it was found to be 7.3. It can be observed from Figure each specimen initially increased for up to seven days; thereafter, it was marginally reduced. Initially, 7a that, as the amount of L-arginine-containing cement mortar is increased, the pH is increased. The i.e., on day 0, L-arginine did not react with water; thus, it shows tap water pH, but as the immersion pH value of each specimen initially increased for up to seven days; thereafter, it was marginally time increased, L-arginine started to dissolve and pH was increased (Figure7a). After seven days’ reduced. Initially, i.e., on day 0, L-arginine did not react with water; thus, it shows tap water pH, but immersion of 0%, 3%, 5%, and 10% L-arginine-containing cement mortar, the pH values of distilled as the immersion time increased, L-arginine started to dissolve and pH was increased (Figure 7a). water are found to be 12.13, 12.55, 12.71, and 12.76, respectively. It is observed from Figure7a that the After seven days’ immersion of 0%, 3%, 5%, and 10% L-arginine-containing cement mortar, the pH pH was gradually reduced after seven days of immersion owing to several factors, such as carbonation values of distilled water are found to be 12.13, 12.55, 12.71, and 12.76, respectively. It is observed from from the external atmosphere, slow equilibrium, dilution from the excessive volume of water in the Figure 7a that the pH was gradually reduced after seven days of immersion owing to several factors, pores, and leaching of pore water, i.e., KOH, NaOH, and Ca(OH)2 [53]. such as carbonation from the external atmosphere, slow equilibrium, dilution from the excessive Figure7b shows the change in pH of L-arginine-containing cement mortar immersed in seawater. volume of water in the pores, and leaching of pore water, i.e., KOH, NaOH, and Ca(OH)2 [53]. It can be seen from this ﬁgure that the pH gradually increased for up to seven days, as similarly observed in tap water (Figure7a), but after seven days of immersion, it decreased. The pH value of the seawater was 8.14. Therefore, initially, all specimens showed identical pH, i.e., 8.14. For seven days of immersion, the pH of seawater was found to be 10.45, 9.78, 9.69, and 9.36 for 0%, 3%, 5%, and 10% L-arginine-containing cement mortar, respectively. It can be seen from Figure7b that without L-arginine-containing cement mortar, i.e., 0% L-arginine, the cement mortar exhibited higher pH values compared to L-arginine-containing cement mortar. This is a contradictory result in the pH of

(a) (b)

Figure 7. pH values of (a) distilled water and (b) seawater used to immerse test specimens.

Figure 7b shows the change in pH of L-arginine-containing cement mortar immersed in seawater. It can be seen from this figure that the pH gradually increased for up to seven days, as similarly observed in tap water (Figure 7a), but after seven days of immersion, it decreased. The pH value of the seawater was 8.14. Therefore, initially, all specimens showed identical pH, i.e., 8.14. For Sustainability 2020, 12, x FOR PEER REVIEW 8 of 16

Figure 6. Recovery of L-arginine with (a) different amounts of L-arginine-containing cement mortar in 0.1 M HCl and (b) 5% L-arginine with concentration and volume of HCl from the residue.

3.3. Evaluation of the Impact of the Environmental Activity of L-Arginine-Containing Mortar

3.3.1.Sustainability Change2020 of, 12 pH, 6346 Value According to the Type of Water Used to Immerse Cement Mortar with9 of L- 16 Arginine seawater-immersedThe changes in pH specimens of the cement compared mortar to tap-water-immersedimmersed in distilled specimens.water and seawater There is are a probability shown in Figurethat Ca(OH) 7a,b, respectively.2 can leach outThe frompH of the each pores data of po 0%int L-arginineis normally cement an average mortar of three into seawater;measurements. thus, Figurehigher in7a pHshows is observed the change (Figure in 7pHb). Onof L-arginine-containing the other hand, L-arginine-containing cement mortar mortarimmersed attracts in distilled marine waterorganisms, according which to attach duration. to the During surface immersion of the cement of L-arginine mortar specimen. cement Themortar marine in distilled organisms water, prevent the pHthe of ingress the distilled of water water molecules was measured, from the seawaterand it was and found maintain to be the7.3. insideIt can pHbe observed of the cement from mortar.Figure 7aMoreover, that, as the there amount are fewer of L-arginine-containing chances to diﬀuse the cement Ca(OH) mortar2 from is the increased, cement mortarthe pH intois increased. the seawater; The pHthus, value it the of seawatereach specimen pH is shown.initially increased However, for there up wereto seven some days; bacteria thereafter, present it inwas seawater marginally that reduced.catalyzed Initially, the hydrolysis i.e., on day of urea 0, L-arginine or amino did acids no intot react ammonia with water; and thus, caused it shows the increase tap water in pH pH, up but to asseven the daysimmersion of immersion time increased, [13]. In the L-arginine meantime, star theyted also to dissolve produce and CO2 pHor carbonicwas increased acid, which (Figure induce 7a). Aftercalcite seven (CaCO days’3) formation. immersion Thus,of 0%, reduction 3%, 5%, and in pH10% was L-arginine-containing observed from 14 cement to 28 days mortar, (Figure the7 pHb). valuesThere isof anotherdistilled possibilitywater are found that atmospheric to be 12.13, 12.55, CO2 can12.71, cause and reduction12.76, respectively. in the pH It ofis observed seawater from after Figureseven days7a that of immersionthe pH was for gradually all specimens. reduced This after experiment seven days was of conductedimmersion in owing the absence to several of sunlight; factors, suchthus, as the carbonation microorganisms from of the seawater external could atmosphere not photosynthesize,, slow equilibrium, resulting dilution in higher from amounts the excessive of CO2 . volumeTherefore, of water lower in pH the was pores, observed and leaching compared of topore distilled water, water. i.e., KOH, NaOH, and Ca(OH)2 [53].

(a) (b)

FigureFigure 7. pHpH values values of of ( (aa)) distilled distilled water water and and ( (bb)) seawater seawater used used to to immerse immerse test test specimens. specimens.

3.3.2.Figure Time Required7b shows to the Change change the Colorin pH of of River L-arginine-containing Water after Immersion cement of L-Arginine-Containing mortar immersed in Cement Mortar seawater. It can be seen from this figure that the pH gradually increased for up to seven days, as similarlyFigure observed8 shows in the tap change water in (Figure color of 7a), L-arginine-containing but after seven days cement of immersion, mortar immersed it decreased. in seawater The pH valueaccording of the to seawater duration. was It 8.14. is believed Therefore, that initiall mariney, organismsall specimens can showed consume identical nutrients, pH, e.g.,i.e., 8.14. nitrogen For in the presence of CO2 and sunlight from the atmosphere by photosynthesis. Therefore, they float on the surface of water to obtain the nutrients from the air. It can be seen from Figure8a,b that initially, i.e., three days and seven days of immersion, there was no significant difference in the color of seawater for all concentrations of L-arginine-containing cement mortars. However, after seven days of immersion, change in the color of seawater was observed. Therefore, it is required to provide oxygen for survival of marine lives. We artificially provided air bubbles in the seawater to make oxygen available. Once the immersion duration was extended up to 14 days, the change in color of seawater from colorless to green (Figure8c) with low amounts, i.e., 0% and 3%, of L-arginine in cement mortar was observed owing to the lack of nitrogen (an essential element for growth of organisms), which cannot significantly diffuse from the pores of cement mortar. Thus, the chlorophyll-a (green color) floats in seawater to take nutrients (nitrogen) and light from the atmosphere. However, once the amount of L-arginine in the cement mortar was increased from 5% to 10% (high amount of nitrogen), after 14 days of immersion, there was no change in the color of seawater (Figure8c), which can be attributed to the consumption of nutrients by chlorophyll-a in the water. Sustainability 2020, 12, 6346 10 of 16 Sustainability 2020, 12, x FOR PEER REVIEW 10 of 16

Figure 8. Cont. SustainabilitySustainability 20202020,, 1212,, 6346x FOR PEER REVIEW 1111 ofof 1616 Sustainability 2020, 12, x FOR PEER REVIEW 11 of 16

Figure 8. Change in color of 0%, 3%, 5%, and 10% L-arginine cement mortar immersed in sea water after (a) 3 days, (b) 7 days, and (c) 14 days. FigureFigure 8. 8. ChangeChange in in color color of of 0%, 0%, 3%, 3%, 5%, 5%, and and 10% 10% L-arginine L-arginine cement cement mortar mortar immersed immersed in in sea sea water water afterafter (a ()a 3) 3days, days, (b ()b 7) 7days, days, and and (c ()c 14) 14 days. days. 3.3.3. Total Dissolved Nitrogen in Distilled Water 3.3.3.3.3.3. TotalL-arginine Total Dissolved Dissolved can leachNitrogen Nitrogen out in in Distilledwater Distilled from Water Water cement mortar; therefore, it is of utmost importance to determineL-arginineL-arginine the cancontent can leach leach of out nitrogen out in in water water with from fromimmersion cement cement mortar;duration. mortar; therefore, therefore,Figure 9 itshows itis is of of utmostthe utmost results importance importance of dissolved to to determinedeterminenitrogen inthe the distilled content content waterof of nitrogen nitrogen with L-argininewith with immersion immersion replacemen duration. duration.t. From Figure Figure this 9 9figure, shows shows itthe thecan results results be seen of of dissolvedthat, dissolved as the nitrogennitrogenamount in inof distilled distilledL-arginine waterwater in withthewith cement L-arginine L-arginine mortar replacement. replacemen increased, Fromt. theFrom thistotal this figure, dissolved figure, it can it nitrogen becan seen be that,seen alsoas that,increased the amountas the in amountofdistilled L-arginine of waterL-arginine in with the cementin different the cement mortar immersion mortar increased, increaperiods. thesed, total This the dissolved totalresult dissolved correlates nitrogen nitrogen alsowith increased the also recovery increased in distilled of in L- distilledwaterarginine with water in different distilled with immersion differentwater (Figure immersion periods. 5). It This can periods. resultbe inferred correlatesThis resultfrom with Figure correlates the recovery 9 that with the of L-argininetheamount recovery of indissolved distilledof L- argininewaternitrogen (Figure in is distilled not5). increased It canwater be inferred(Figuresignificantly from5). It Figureaftercan be 289 inferred thatdays the of amountimmersion.from Figure of dissolved This 9 that result the nitrogen amountcan be is correlated notof dissolved increased with nitrogensignificantlythe change is not afterin increased pH 28 (Figure days significantly of immersion.7a) of distilled after This 28 water, result days can whereof beimmersion. correlated after 14 This withdays result the of changeimmersion, can be in correlated pH there (Figure was with7a) ofno thedistilledsignificant change water, in change pH where (Figure inafter value. 7a) 14 daysUpof distilled to of seven immersion, water, days therewhereof immersion, was after no significant14 thedays pH of change increasedimmersion, in value.gradually there Up was to due seven no to significantdaysleaching of immersion, ofchange nitrogen. in the value.Therefore, pH increased Up toregardless seven gradually days of the dueof L-arginineimmersion, to leaching replacement ofthe nitrogen. pH increased ratio, Therefore, maximum gradually regardless L-arginine due of to the leachingL-argininewas leached of nitrogen. replacement within Therefore, 28 ratio,days maximumof regardless immersion. L-arginine of There the L-arginine waswas leachedno dissolved replacement within nitrogen 28 daysratio, of foundmaximum immersion. in 0% L-arginine L-arginine There was wasnocement dissolvedleached mortar within nitrogen owing 28 founddays to the of inabsence immersion. 0% L-arginine of L-arginine. There cement was mortarno dissolved owing tonitrogen the absence found of in L-arginine. 0% L-arginine cement mortar owing to the absence of L-arginine.

FigureFigure 9.9.Total Total dissolved dissolved nitrogen nitrogen of L-arginine of L-arginine cement mortar cement in distilled mortar water in accordingdistilled towater immersion according duration. to Figureimmersion 9. Total duration. dissolved nitrogen of L-arginine cement mortar in distilled water according to immersion duration. Sustainability 2020, 12, x 6346 FOR PEER REVIEW 1212 of of 16 16

3.3.4. Growth of Chlorophyll-A on Mortar Surface 3.3.4. Growth of Chlorophyll-A on Mortar Surface In order to examine the effect of L-arginine cement mortars on growth of chlorophyll-a, the cementIn ordermortar to specimens examine the were effect immersed of L-arginine in the cement natural mortars sea near on growth the tide of chlorophyll-a,embankment thelocated cement at Ansanmortar city, specimens South Korea. were immersed The amount in the of Chl-a natural on sea the near cement the tidemortar embankment surface was located measured at Ansan for up city, to 56South days Korea. of exposure. The amount Figure of 10 Chl-a shows on thethe cementdigital image mortar taken surface during was measuredremoval of for Chl-a up to from 56 days the surfaceof exposure. of the Figurecement 10 mortar. shows Figure the digital 10a,b imageshows takenthe digital during images removal of Chl-a of Chl-a removed from on the filter surface paper of afterthe cement 7 and 28 mortar. days of Figure immersion 10a,b showsin sea water, the digital respectively. images ofIt Chl-acan be removed seen that, on as filter the amount paper after of L- 7 arginineand 28 days increased, of immersion the color in seabecame water, gray respectively. with the exposure It can be period. seen that, The as 10% the amountL-arginine ofL-arginine specimen showsincreased, deep the gray color color became (specimen gray with dipped the exposure in sea water period. where The 10%Chl-a L-arginine was attached specimen on the shows surface), deep whereasgray color the (specimen 0% specimen, dipped i.e., in that sea without water where L-argi Chl-anine, exhibited was attached a light on yellow the surface), color owing whereas to the scarcity0% specimen, of nitrogen; i.e., that thus, without Chl-a floats L-arginine, in water exhibited rather than a light being yellow deposited color owingonto the to cement the scarcity mortar of afternitrogen; 28 days thus, of Chl-a immersion. floats in This water result rather suggests than being that, deposited as the amounts onto the of cement L-arginine mortar and after immersion 28 days durationof immersion. are increased, This result the suggests formation that, of as Chl-a theamounts on the surface of L-arginine of the andcement immersion mortar durationspecimen are is increased.increased, the formation of Chl-a on the surface of the cement mortar specimen is increased.

(a) (b)

Figure 10. DigitalDigital imageimage of of removed removed chlorophyll-a chlorophyll-a (Chl-a) (Chl-a) on ﬁlteron filter paper paper from thefrom surface the surface of L-arginine of L- argininecement mortar cement after mortar (a) 7after days (a and) 7 days (b) 28 and days (b) of28 immersion days of immersion in a sea. in a sea.

Figure 11 shows the amount of Chl-a extracted from the surface of L-arginine cement mortar according toto immersionimmersion duration. duration. The The amount amount of Chl-aof Chl-a extracted extracted from from 0%, 3%, 0%, 5%, 3%, and 5%, 10% and L-arginine 10% L- argininecement mortarscement surfacemortars after surface seven after days seven of immersion days of immersion in sea water in sea was water found was to befound 0.47, to 0.53, be 0.47, 1.25, 0.53,and 1.581.25,µ andg/cm 1.582, respectively. μg/cm2, respectively. It can be It observed can be observed in Figure in 11 Figure that, as11 thethat, amount as the amount of L-arginine of L- arginineincreased increased in cement in mortar, cement the mortar, amount the of amount Chl-a increased of Chl-a increased with immersion with immersion duration, duration, which correlates which correlates with the visual observation (Figure 10), where the 10% L-arginine specimen shows a deep Sustainability 2020, 12, 6346x FOR PEER REVIEW 13 of 16 gray color on the paper. It can be seen after 56 days of immersion that the amount of Chl-a extracted withfrom the0%,visual 3%, 5%, observation and 10% L-arginine (Figure 10 cement), where mortar the 10% was L-arginine found to be specimen 1.15, 1.84, shows 3.81, a and deep 4.82 gray μg/cm color2, respectively.on the paper. It can be seen after 56 days of immersion that the amount of Chl-a extracted from 0%, 3%, 2 Sustainability5%, and 10% 2020 L-arginine, 12, x FOR PEER cement REVIEW mortar was found to be 1.15, 1.84, 3.81, and 4.82 µg/cm , respectively.13 of 16 Figure 12 shows the Chl-a growth rate on the L-arginine cement mortars versus the mortar without grayL-arginine. color on This the resultpaper. shows It can thatbe seen 3% L-arginineafter 56 days mortar of immersion increases that the the rate amount of Chl-a of growth Chl-a extracted by 112%, whereasfrom 0%, 5%3%, and5%, 10%and 10% L-arginine L-arginine show cement 226% mortar and 336%, was found respectively, to be 1.15, after 1.84, seven 3.81, days and of4.82 exposure. μg/cm2, respectively.It was observed after 14 days of immersion that the rate of Chl-a slightly decreased.

Figure 11. Surface chlorophyll-a content on the surface of L-arginine cement mortar according to exposure periods in the sea.

Figure 12 shows the Chl-a growth rate on the L-arginine cement mortars versus the mortar without L-arginine. This result shows that 3% L-arginine mortar increases the rate of Chl-a growth by 112%,Figure whereas 11. SurfaceSurface 5% chlorophyll-a chlorophyll-aand 10% L-arginine content content on onshow the surface 226% andof L-arginine 336%, respectively, cement mortar after according seven days to of exposure.exposure It was periods observed in the sea.after 14 days of immersion that the rate of Chl-a slightly decreased.

Figure 12 shows the Chl-a growth rate on the L-arginine cement mortars versus the mortar without L-arginine. This result shows that 3% L-arginine mortar increases the rate of Chl-a growth by 112%, whereas 5% and 10% L-arginine show 226% and 336%, respectively, after seven days of exposure. It was observed after 14 days of immersion that the rate of Chl-a slightly decreased.

Figure 12.12. Chl-aChl-a growthgrowth rate rate on on the the surface surface of L-arginine of L-arginine cement cement mortar mortar according according to exposure to exposure periods periodsin the sea. in the sea. 4. Conclusions 4. Conclusions Since L-arginine is water soluble, it can easily mix with cement and sand to make cement mortar. Since L-arginine is water soluble, it can easily mix with cement and sand to make cement mortar. Therefore,Figure ﬂow12. Chl-a and airgrowth content rate are on higherthe surface once of the L-argi L-argininenine cement amount mortar is increased according in to cement exposure mortar, Therefore, flow and air content are higher once the L-arginine amount is increased in cement mortar, whichperiods leads in to the a decreasesea. in compressive strength. L-arginine in cement mortar is mostly soluble whichin a pore leads solution, to a decrease which canin compressive easily leach strength. out from L- cementarginine mortar. in cement Thus, mortar the pH is is mostly increased soluble for upin a pore solution, which can easily leach out from cement mortar. Thus, the pH is increased for up to 4.to Conclusions seven days of exposure in distilled water. As the amount of L-arginine is increased in a cement sevenmortar, days the amountof exposure of dissolved in distilled nitrogen water. increases, As the which amount encourages of L-arginine the deposition is increased of Chl-a in a ontocement the Since L-arginine is water soluble, it can easily mix with cement and sand to make cement mortar. mortar,cement mortarthe amount surface. of dissolved The amount nitrogen of Chl-a increase formeds, which on 10% encourages L-arginine the cement deposition mortar of wasChl-a found onto Therefore, flow and air content are higher once the L-arginine amount is increased in cement mortar, theto be cement 4.82 µ mortarg/cm2, surface. which was The highestamount amongof Chl-a all formed specimens on 10% after L-arginine 56 days cement of immersion mortar was in natural found which leads to a 2decrease in compressive strength. L-arginine in cement mortar is mostly soluble in tosea. be This 4.82 showedμg/cm , awhich growth was rate highest of Chl-a among 4.2 times all specimens higher than after that 56 founddays of with immersion 0% L-arginine in natural cement sea. aThis pore showed solution, a growthwhich can rate easily of Chl-a leach 4.2 out times from higher cement than mortar. that Thus,found the with pH 0% is increasedL-arginine for cement up to seven days of exposure in distilled water. As the amount of L-arginine is increased in a cement mortar, the amount of dissolved nitrogen increases, which encourages the deposition of Chl-a onto the cement mortar surface. The amount of Chl-a formed on 10% L-arginine cement mortar was found to be 4.82 μg/cm2, which was highest among all specimens after 56 days of immersion in natural sea. This showed a growth rate of Chl-a 4.2 times higher than that found with 0% L-arginine cement Sustainability 2020, 12, 6346 14 of 16 mortar. Therefore, this process can be recommended for artiﬁcial reefs. In addition, the optimum performance can be achieved by controlling the surface roughness, area, and porosity of the cement mortar or concrete.

Author Contributions: Data curation, H.-M.Y.; Formal analysis, H.-M.Y., N.V.M., and J.K.S.; Funding acquisition, H.-S.L.; Investigation, H.-M.Y.; Methodology, H.-M.Y., H.-S.L., and J.K.S.; Supervision, H.-S.L. and J.K.S.; Writing—original draft, H.-M.Y., N.V.M., H.-S.L., and J.K.S.; Writing—review and editing, H.-M.Y., N.V.M., H.-S.L., and J.K.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This research was supported by a basic science research program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Science, ICT, and Future Planning (No. 2015R1A5A1037548). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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