Effect of a Nitrite/Nitrate-Based Accelerator on the Strength Development and Hydrate Formation in Cold-Weather Cementitious Materials

materials

Article Effect of a Nitrite/Nitrate-Based Accelerator on the Strength Development and Hydrate Formation in Cold-Weather Cementitious Materials

Akira Yoneyama 1, Heesup Choi 1,* , Masumi Inoue 1, Jihoon Kim 2, Myungkwan Lim 3,* and Yuhji Sudoh 4

1 Department of Civil and Environmental Engineering, Kitami Institute of Technology, Hokkaido 090-8507, Japan; [email protected] (A.Y.); [email protected] (M.I.) 2 Faculty of Environmental Technology, Muroran Institute of Technology, Hokkaido 090-8585, Japan; [email protected] 3 Department of Architectural Engineering, Songwon University, Gwangju 61756, Korea 4 Basic Chemicals Department Chemicals Division, Nissan Chemical Corporation, Tokyo 103-6119, Japan; [email protected] * Correspondence: [email protected] (H.C.); [email protected] (M.L.)

Abstract: Recently, there has been increased use of calcium-nitrite and calcium-nitrate as the main components of chloride- and alkali-free anti-freezing agents to promote concrete hydration in cold weather concreting. As the amount of nitrite/nitrate-based accelerators increases, the hydration

of tricalcium aluminate (C3A phase) and tricalcium silicate (C3S phase) in cement is accelerated, thereby improving the early strength of cement and effectively preventing initial frost damage. Nitrite/nitrate-based accelerators are used in larger amounts than usual in low temperature areas ◦ below −10 C. However, the correlation between the hydration process and strength development in concrete containing considerable nitrite/nitrate-based accelerators remains to be clearly identiﬁed. Citation: Yoneyama, A.; Choi, H.; In this study, the hydrate composition (via X-ray diffraction and nuclear magnetic resonance), pore Inoue, M.; Kim, J.; Lim, M.; Sudoh, Y. structures (via mercury intrusion porosimetry), and crystal form (via scanning electron microscopy) Effect of a Nitrite/Nitrate-Based were determined, and investigations were performed to elucidate the effect of nitrite/nitrate-based Accelerator on the Strength Development and Hydrate Formation accelerators on the initial strength development and hydrate formation of cement. Nitrite/nitrate- in Cold-Weather Cementitious AFm (aluminate-ferret-monosulfate; AFm) was produced in addition to ettringite at the initial Materials. Materials 2021, 14, 1006. stage of hydration of cement by adding a nitrite/nitrate-based accelerator. The amount of the − − https://doi.org/10.3390/ma14041006 hydrates was attributed to an increase in the absolute amounts of NO2 and NO3 ions reacting with Al2O3 in the tricalcium aluminate (C3A phase). Further, by effectively ﬁlling the pores, it greatly Academic Editor: Gyuyong Kim contributed to the enhancement of the strength of the hardened cement product, and the degree of the contribution tended to increase with the amount of addition. On the other hand, in addition to Received: 19 January 2021 the occurrence of cracks due to the release of a large amount of heat of hydration, the amount of Accepted: 13 February 2021 expansion and contraction may increase, and it is considered necessary to adjust the amount used for Published: 20 February 2021 each concrete work.

Publisher’s Note: MDPI stays neutral Keywords: anti-freezing agent; accelerator; cold-weather concrete; calcium nitrite; calcium nitrate; with regard to jurisdictional claims in strength development; pore structure; solid-state NMR; AFm phase; ettringite published maps and institutional afﬁl- iations.

1. Introduction When placing concrete in cold climates, it is necessary to control the temperature Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. until the strength of the concrete at an early age reaches the required initial strength to This article is an open access article prevent “initial frost damage” [1,2]. Currently, the most common method is to control the distributed under the terms and temperature using a heater and curing enclosure. However, this method cannot be used conditions of the Creative Commons on steep slopes, strong winds, and narrow spaces. In these situations, it is effective to use Attribution (CC BY) license (https:// anti-freezing agents. Generally, in concrete construction when the outside temperature ◦ creativecommons.org/licenses/by/ is below −10 C, the effect can be achieved by adding a large amount of anti-freezing 4.0/). agents [3–8].

Materials 2021, 14, 1006. https://doi.org/10.3390/ma14041006 https://www.mdpi.com/journal/materials Materials 2021, 14, 1006 2 of 14

At present, calcium nitrite (Ca(NO2)2) and calcium nitrate (Ca(NO3)2) are widely used as the main components of chloride- and alkali-free anti-freezing agents in cold- weather concreting [8,9], as well as the resistance to corrosion of reinforcing bar [10]. These accelerate the hydration reaction of the tricalcium aluminate phase (C3A) and tricalcium silicate phase (C3S) contained in cement, and contribute to the increase in hydrates such as calcium-silicate-hydrate (C–S–H; xCaO·SiO2·yH2O), ettringite (AFt; Ca3Al2[SO4]3[OH]12·25-27H2O), and monosulfate (AFm; Ca4Al2[SO4][OH]12·4-8H2O) at the initial stage of hydration [11,12]. Furthermore, it is known that hydrates, such as nitrite- AFm (Ca4Al2[OH]12[NO2]2·xH2O) and nitrate-AFm (Ca4Al2[OH]12[NO3]2·xH2O), are formed through reactions between the aluminate in cement and anions in the nitrite/nitrate- based accelerator [13–15]. These are effective for developing the initial strength of concrete and reducing the initial frost damage. However, when a large amount of nitrite/nitrate- based accelerator is used, problems are encountered, such as cracking due to an increase in expansion/contraction amount and a decrease in strength at later ages [14]. Choi et al. reported that a decrease in strength at later ages due to the consumption of a large amount of H2O during hydration occurs at an early age [14]. These are important problems to be solved in the use of nitrite/nitrate-based accelerators. Therefore, it is necessary to understand the correlation between hydration products and strength development. This study focuses on the early age (mixing ~24 h), clariﬁes the hydration reaction mechanism of cement when a large amount of a nitrite/nitrate-based accelerator is added, − − and examines the effect of NO2 and NO3 ions on strength development. The correlation between compressive strength properties and hydrate formation was examined by measuring the compressive strength and mercury intrusion porosimetry (MIP), and the effects of the internal structure involved in strength development were discussed. The hydrate formation process was evaluated and discussed based on the results of a calorimeter (hydration heat), thermogravimetric differential thermal analysis and differential ther- Materials 2021, 14, x FOR PEER REVIEWmogravimetric analysis (TG/DTG), X-ray diffraction (XRD), solid-state nuclear magnetic3 of 15

resonance (NMR), and scanning electron microscopy (SEM). Figure1 depicts the ﬂow chart of this study.

The Effect of Nitrite/Nitrate-Based Accelerator on the Strength Development and Hydrate Formation in Cold-weather Cementitious Materials

• Curing temp.; 10 ℃ const. CN addition amount 0, 7, 9, 11, 13 % to Cement Weight • Age; mixing ~ 24 hours

Evaluation of the strength development in early age cement

Strength development Condensation test Pore distribution Compressive strength Setting properties MIP

Correlation between Pore structure and Strength

Early hydration

(C3A, C3S) Correlation between Hydration products and Strength development by MicroAnalysis

Crystal form C3A hydration SEM Solid-State NMR

Calorimeter Hydration heat XRD TG/DTG Identification of hydrate Rate of hydration and amount of products

Elucidation of the mechanism of strength development using a nitrite/nitrate-based accelerator

Figure 1. Study FlowFigure Chart. 1. Study Flow Chart.

2. Experimental Overview 2.1. Materials and procedures Table 1 lists the materials used in this experiment. The cement used is a commercially available ordinary Portland cement (Taiheiyo Cement Co., Ltd., Tokyo, Japan) according to JIS R 5210 [16]. Table 2 shows the components of the anti-freezing agent (Nissan Chem- ical Corporation, Tokyo, Japan), and Table 3 shows the chemical composition of cement used in this experiment. An aqueous solution containing calcium nitrite (Ca(NO2)2) and calcium nitrate (Ca(NO3)2) at a concentration of 45% (hereinafter referred to as CN) was used as a component of the cold resistance accelerator. Table 4 shows the mixing ratio of the cement paste used. In this experiment, cement paste with a water-cement ratio of 0.5 was used [14,17]. In the experiment to elucidate the mechanism of strength development when a CN is added, mixing and sealed curing were performed at 10 ± 1 °C, and the curing periods were 3 h, 6 h, 12 h, and 24 h after implantation [17,18]. Normally, the amount of standard addition of existing accelerators is approximately 3 ~ 5 L per 100 kg of cement (4 ~ 7% of the cement mass) depending on the ambient temperature [17]. In this study, an addition of CN at 7% or more was defined as a “large quantity” and the experiment was carried out with CN added at five different ratios: 0%, 7%, 9%, 11%, and 13%. According to the literature, when CN was added, Ca(NO2)2 and Ca(NO3)2 react with aluminate in cement to form a nitrite-nitrate hydrate within 24 h [14,19]. This study focuses on the early hydrate formation process from the perspective of preventing initial frost damage. Therefore, “early age” was defined as “within 24 h after mixing” [14]. The compressive strength of the cement paste was measured, and the correlation between the hydrate formation behavior and strength development was investigated. The compressive strength was measured at 24 h, and the size of the specimen was 5 cm diameter × 10 cm height. Since the pore structure greatly contributes to the strength development of the cured product, the distribution of capillary pores was confirmed by MIP. In order to understand the hydration characteristics of cement, the effect of addition of calcium nitrite on initial hydration was examined using XRD, TG/DTA, and solid-state NMR. In addition,

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2. Experimental Overview 2.1. Materials and Procedures Table1 lists the materials used in this experiment. The cement used is a commercially available ordinary Portland cement (Taiheiyo Cement Co., Ltd., Tokyo, Japan) according to JIS R 5210 [16]. Table2 shows the components of the anti-freezing agent (Nissan Chemical Corporation, Tokyo, Japan), and Table3 shows the chemical composition of cement used in this experiment. An aqueous solution containing calcium nitrite (Ca(NO2)2) and calcium nitrate (Ca(NO3)2) at a concentration of 45% (hereinafter referred to as CN) was used as a component of the cold resistance accelerator. Table4 shows the mixing ratio of the cement paste used. In this experiment, cement paste with a water-cement ratio of 0.5 was used [14,17]. In the experiment to elucidate the mechanism of strength development when a CN is added, mixing and sealed curing were performed at 10 ± 1 ◦C, and the curing periods were 3 h, 6 h, 12 h, and 24 h after implantation [17,18]. Normally, the amount of standard addition of existing accelerators is approximately 3~5 L per 100 kg of cement (4~7% of the cement mass) depending on the ambient temperature [17]. In this study, an addition of CN at 7% or more was deﬁned as a “large quantity” and the experiment was carried out with CN added at ﬁve different ratios: 0%, 7%, 9%, 11%, and 13%.

Table 1. Properties of the materials.

Materials (Code) Properties Cement (C) Ordinary Portland Cement, Density; 3.16 g/cm3 Main component; calcium nitrite, calcium nitrate Anti-freezing agent (CN) (45% water solution), Density; 1.43 g/cm3

Table 2. Properties of the anti-freezing agent (nitrite-nitrate based accelerator; CN).

Density of Aquarius pH Aquarius Component Component Ratio Solution (g/cm3) Solution Ca(NO ) 23.02% 2 2 1.43 9.3 Ca(NO3)2 22.81%

Table 3. Chemical composition of cement.

Chemical Composition (%) Alkali Ordinary Portland SiO Al O Fe O CaO MgO SO CaSO Ig.loss 2 2 3 2 3 3 4 Content Cement 21.4 5.5 2.8 64.3 2.1 1.9 - 0.56 0.25

Table 4. Properties of the cement-paste mix.

Unit Content (kg/m3) Anti-Freezing Agent (C × %) Type W/C (%) W C CN CN0 0 CN7 7 50 612 1225 CN9 9 CN11 11 CN13 13 Note: W/C; water-cement ratio, (C × %).

According to the literature, when CN was added, Ca(NO2)2 and Ca(NO3)2 react with aluminate in cement to form a nitrite-nitrate hydrate within 24 h [14,19]. This study focuses on the early hydrate formation process from the perspective of preventing initial frost damage. Therefore, “early age” was deﬁned as “within 24 h after mixing” [14]. The Materials 2021, 14, 1006 4 of 14

compressive strength of the cement paste was measured, and the correlation between the hydrate formation behavior and strength development was investigated. The compressive strength was measured at 24 h, and the size of the specimen was 5 cm diameter × 10 cm height. Since the pore structure greatly contributes to the strength development of the cured product, the distribution of capillary pores was conﬁrmed by MIP. In order to understand the hydration characteristics of cement, the effect of addition of calcium nitrite on initial hydration was examined using XRD, TG/DTA, and solid-state NMR. In addition, SEM (JEOL, Tokyo, Japan) observation was performed as a method for grasping the crystal structure and pore structure of the formed hydrate, and the relationship with the strength of the cured product was considered.

2.2. Compressive Strength The compressive strength was measured in accordance with JIS A 1108 [20]. The load was applied uniformly so that no impact was applied. The load speed was set to 0.6 ± 0.4 N/mm2 per second. Compressive strength was measured three times at each sample level, and the compressive strength results were expressed as the average of these values.

2.3. Mercury Intrusion Porosimetry (MIP) The Auto-pore III 9400 Series was used to measure the MIP, and the press-ﬁtting pressure was measured from 1.5 to 30 psia. The conditions of mercury used for measurement are as follows: Surface tension of mercury = 485 dynes/cm, contact angle = 130◦, density of mercury = determined by temperature at the time of measurement. The method for preparing the sample used for the measurement is as follows: A cured sample was prepared on a dice with a side of 5 mm using a diamond cutter, hydrated with acetone, and then dried in a vacuum environment.

2.4. Calorimeter The heat of hydration was measured using an isothermal calorimeter (TAM Air 8- channel; TAM Air, Tokyo, Japan), and the heat of the hydration curve was obtained. The temperature was set in a room temperature environment, and a reference (water) was created so that the heat capacity was the same as that of the measurement sample. In addition, ASTM (American Society for Testing and Materials; ASTM) 1702 was used as a reference for calculating the heat capacity. The sample used for the measurement was prepared so as to weigh 4.00 g with an electronic balance.

2.5. Condensation Test The setting test was performed by the same method as that specified in JIS A 1147 [21], and the setting characteristics of the cement paste were grasped. A penetration resistance tester was used to measure the resistance when penetrating into the sample. The bleeding water generated at that time was removed. In this experiment, the time when the penetration resistance reached 3.5 N/mm2 was defined as the Initial setting, and the time when it reached 28 N/mm2 was defined as the Final setting.

2.6. Thermogravimetric Differential Thermal Analysis TG/DTG was measured using TG8121 (Thermo plus EVO2 TG-DTA; Rigaku, Tokyo, Japan). The TG/DTG conditions were as follows: measuring temperature = 20~1000 ◦C, ◦ rising speed = 20 C /min, atmosphere = N2-flow, sample weight = 15.00 mg, reference material = α-Al2O3. The method for preparing the sample used for TG / DTA measurement is as follows: Samples collected at a predetermined age are stopped hydrated with acetone, solid-liquid separated by suction filtration, and then dried in an RH11% (relative humidity; RH) environment. A sufficiently dried sample was pulverized to a particle size of 90 µm or less and used for the measurement. Materials 2021, 14, x FOR PEER REVIEW 6 of 15

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In this study, to evaluate the effect of the early hydration progress due to the addition 2.7. X-ray Diffraction of CN on the strength development, the pore volume and pore diameter were measured XRD was performed to identify the crystal phase. Rigaku Rint2000 powder diffrac- tometerusing mercury (Tokyo, Japan) intrusion was used porosimetry for the measurements. (MIP). Figure The XRD 2 shows conditions the 24 were h compressive as fol- strength lows:results Cu-K ofα radiationthe cement source paste = 40 kV/20specimens mA, scan cured range at = 5~6510 °C◦/2 forθ, scan each speed amount = 1 ◦/min, of CN added. The andcompressive step width =strength 0.02 ◦/step. results The samplein the experiment preparation method were recorded is as follows: as 2.53 A sample N/mm2 on CN0, 3.74 collectedN/mm2 at on a predetermined CN7, 4.66 N/mm age was2 on immersed CN9, 5.35 in acetone N/mm to stop2 onhydration, CN11, and and 7.07 suction N/mm2 on CN13, ﬁltration was performed for solid-liquid separation. Then, it was pulverized to a parti- cleindicating size of 90 µam proportional or less and dried correlation in an environment between ofthe 11% CN RH, amount which wasand used the forearly compressive XRDstrength. measurement. Furthermore, the rate at which the strength increased relative to that of CN0 in 24 h was almost proportional to the amount of CN added, with 148% on CN7, 184% on 2.8.CN9, Solid-State 211% Nuclearon CN11, Magnetic and Resonance279% on CN13. 27 AlFigure MAS NMR3 shows (Magic the angle 24 h spinning) MIP results spectra of were the cement collected paste at 156.4 specimens MHz on JEOL cured at 10 °C for ECA-600 (magnetic ﬁeld 14.1T, Tokyo, Japan) using a 3.2 mm probe. The 27Al NMR experimentseach amount employed of CN a spinningadded. speedFrom of the 16 kHz,MIP pulse resu widthlt, CN0 of 1.0 containsµs, relaxation the most delay pores in the di- ofameter 0.5 s, and range a total of of 1 1280 ~ 5 scans.μm (coarse Analysis capillary of the solid-state pores). NMR In spectracontrast, was it performed was confirmed that the onpore a JEOL size Delta decreased NMR processing as the andamount control of software CN addition (Delta 5.3.1). increased. As the sample In particular, used CN11 and forCN13 the measurement showed a distribution of solid-state NMR,of pore the diameters sample prepared in the in range the same of 0.1 manner ~ 1 μ asm, the and pore volume XRD sample was measured. tended to decrease. With regard to pore size distribution, there was no significant differ- 2.9.ence Scanning between Electron CN11 Microscope and CN13; (SEM) therefore, change of pore volume is presumed to be due to theIn SEM, difference a scanning in electronhydrate microscope composition (JSM-6510A, and rate JEOL, of Tokyo, hydration. Japan) was From used the to MIP results, it observewas confirmed the microstructure that when on the CN surface was of added, the hardened the pore cement. diameter The sample inside used forthe the hardened cement measurementbecame smaller was a sampleand a denser collected internal from a cured structure product, was dehydrated formed. with acetone, and dried in a vacuum environment. In addition, platinum vapor deposition was performed duringBased the measurement. on the results of these experiments, the effect of CN addition on the strength development and pore structure was discussed. In the case where CN was added, the 3.compressive Results and Discussion strength tended to increase in proportion to the added amount even after a 3.1.curing Evaluation time of at the 24 Strength h. Moreover, Development the in thepore Early st Ageructure Group became denser as the hydration progressed,In this study,and the to evaluate pore diameter the effect ofreduced the early as hydration the amount progress of due CN to added the addition increased. The den- of CN on the strength development, the pore volume and pore diameter were measured usingsification mercury of intrusion the pore porosimetry structure (MIP). with Figure hydrat2 showsion progression the 24 h compressive had a strength great influence on the resultsstrength of the development, cement paste specimens especially cured at atan 10 earl◦Cy for age, each and amount the ofcompressive CN added. The strength of CN13 compressivewas nearly strength three times results that in the of experiment CN0. From were th recordede aforementioned as 2.53 N/mm results,2 on CN0, it was found that 2 2 2 2 3.74the N/mmaccelerationon CN7, of 4.66hydration N/mm byon the CN9, addition 5.35 N/mm of CNon affected CN11, and the 7.07 increase N/mm in cement gel and on CN13, indicating a proportional correlation between the CN amount and the early compressiveformed a strength.fine pore Furthermore, structure the in rate the at cement which the matrix, strength and increased contributed relative to significantly that to the ofstrength CN0 in 24 development h was almost proportional at an early toage. the Additi amountonally, of CN added, the strength with 148% development on CN7, might have 184%been on useful CN9, 211% in preventing on CN11, and the 279% initial on CN13.frost damage.

8.00 Maximum 279% 300 ) 2 7.00 Average Minimum 250 6.00 211% 184% 5.00 200 148% 4.00 150 3.00 100% 100 2.00

1.00 50 Rate of strength increase (%) Compressive strength(N/mm 0.00 0 CN0 CN7 CN9 CN11 CN13

FigureFigure 2. Compressive2. Compressive Strength. Strength.

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Figure3 shows the 24 h MIP results of the cement paste specimens cured at 10 ◦C for each amount of CN added. From the MIP result, CN0 contains the most pores in the diameter range of 1~5 µm (coarse capillary pores). In contrast, it was confirmed that the pore size decreased as the amount of CN addition increased. In particular, CN11 and CN13 showed a distribution of pore diameters in the range of 0.1~1 µm, and pore volume tended to decrease. With regard to pore size distribution, there was no significant difference between CN11 and CN13; therefore, change of pore volume is presumed to be due to the difference in hydrate composition and rate of hydration. From the MIP results, it was Materials 2021, 14, x FOR PEER REVIEW 7 of 15 confirmed that when CN was added, the pore diameter inside the hardened cement became smaller and a denser internal structure was formed.

0.06 CN0 0.05 CN7 CN9 0.04 CN11 0.03 CN13

0.02 pore volume (mL/g) volume pore 0.01

0 0.01 0.1 1 10 100 diameter of pores (μm)

FigureFigure 3. Pore3. Pore Structure. Structure.

3.2.Based Effect on of the Calcium results Nitrit of thesee on experiments, Setting Characteristics the effect of CN addition on the strength development and pore structure was discussed. In the case where CN was added, the compressiveIn this strength experiment, tended tothe increase penetration in proportion resistan to thece value added of amount the cement even after paste prepared at aa curing mixing time temperature at 24 h. Moreover, of 10 °C the was pore calculated structure became according denser to asJIS the A hydration1147. The time when the progressed,penetration and resistance the pore diameter reached reduced 3.5 N/mm as the2 amount was defined of CN added as the increased. Initial setting, The and the time densification of the pore structure2 with hydration progression had a great influence on the strengthwhen it development, reached 28 especially N/mm atwas an earlydefined age, andas the the Final compressive setting. strength Figure of 4 CN13 shows the relation- wasship nearly between three times the thatpenetration of CN0. From resistance the aforementioned value and results, the curing it was found time that for each addition theamount. acceleration of hydration by the addition of CN affected the increase in cement gel and formedAs a aresult fine pore of structurethis test, in the the initial cement setting matrix, andperiods contributed were significantly11.46 h for toCN0, the 8.76 h for CN7, strength8.43 h for development CN9, 7.01 at h an for early CN11, age. Additionally, and 5.35 h fo ther strengthCN13. Furthermore, development might the havefinal setting periods been useful in preventing the initial frost damage. were 15.95 h for CN0, 12.1 h for CN7, 10.77 h for CN9, 9.17 h for CN11, and 7.27 h for 3.2.CN13, Effect all of Calcium of which Nitrite were on Setting proportional Characteristics to the amount added. From this tendency, it was foundIn this that experiment, the addition the penetration of CN accelerated resistance value hydration, of the cement and paste the preparedeffect increased at a in propor- ◦ mixingtion to temperature the amount of 10 of Caddition. was calculated In addition, according it to is JIS considered A 1147. The that time whenthis effect the increased the penetration resistance reached 3.5 N/mm2 was defined as the Initial setting, and the time whenstrength it reached at 24 28 h. N/mm 2 was defined as the Final setting. Figure4 shows the relationship 35 between the penetration resistance value21 and the curing time for each addition amount. As a result of this test, the initial setting periods were 11.46 h for CN0, 8.76 h forCN0 CN7, 30 8.43 h for CN9, 7.01 h for CN11, and 5.35 h18 for CN13. Furthermore, the final setting periodsCN7 CN9 were 15.95 h for CN0, 12.1 h for CN7, 10.77 h for CN9, 9.17 h for CN11, and 7.27 h for CN13, CN11

) 25 15 2 all of which were proportional to the amount added. From this tendency, it was foundCN13 that the addition of CN accelerated hydration, and the effect increased in proportion to the 20 12 amount of addition. In addition, it is considered that this effect increased the strength at 24 h. 15 CN0 9 CN7

10 CN9 elapsedtime(hours) 6 CN11 CN13

penetrationresistance(N/mm 5 3

0 0 3 6 9 12 15 18 21 Initial setting Final setting elapsed time (hours) Figure 4. Bond strength in cement paste with different amount of CN.

3.3. Effect of CN Addition on the Hydration of Portland Cement In this experiment, the rate of heat generation was measured with a calorimeter, and the effect of CN addition on the hydration reaction was examined. Figure 5 shows the calorimeter results for each amount of CN added. First, it was confirmed that the heat of hydration rapidly increased immediately after cement made contact with water. This peak is mainly due to the heat of wetting of cement, the heat of dissolution of aluminate (Al2O3) and sulfate (SO4) into solution, and the heat of hydration of calcium silicate (C3S) (first peak) [12,22,23]. It was approximately 53 J/h·g for CN0. In contrast, it was approximately 104 J/h·g, 128 J/h·g, 166 J/h·g, and 211 J/h·g for CN7, CN9, CN11, and CN13, respectively,

Materials 2021, 14, x FOR PEER REVIEW 7 of 15

0.06 CN0 0.05 CN7 CN9 0.04 CN11 0.03 CN13

0.02 pore volume (mL/g) volume pore 0.01

0 0.01 0.1 1 10 100 diameter of pores (μm) Figure 3. Pore Structure.

3.2. Effect of Calcium Nitrite on Setting Characteristics In this experiment, the penetration resistance value of the cement paste prepared at a mixing temperature of 10 °C was calculated according to JIS A 1147. The time when the penetration resistance reached 3.5 N/mm2 was defined as the Initial setting, and the time when it reached 28 N/mm2 was defined as the Final setting. Figure 4 shows the relationship between the penetration resistance value and the curing time for each addition amount. As a result of this test, the initial setting periods were 11.46 h for CN0, 8.76 h for CN7, 8.43 h for CN9, 7.01 h for CN11, and 5.35 h for CN13. Furthermore, the final setting periods were 15.95 h for CN0, 12.1 h for CN7, 10.77 h for CN9, 9.17 h for CN11, and 7.27 h for CN13, all of which were proportional to the amount added. From this tendency, it was

Materials 2021, 14, 1006 found that the addition of CN accelerated hydration, and the effect increased7 of 14 in proportion to the amount of addition. In addition, it is considered that this effect increased the strength at 24 h. 35 21 CN0 30 18 CN7 CN9 CN11

) 25 15 2 CN13 20 12

15 CN0 9 CN7

10 CN9 elapsedtime(hours) 6 CN11 CN13

penetrationresistance(N/mm 5 3

0 0 3 6 9 12 15 18 21 Initial setting Final setting elapsed time (hours) Figure 4.Figure Bond 4. Bondstrength strength in incement cement pastepaste with with different different amount amount of CN. of CN. 3.3. Effect of CN Addition on the Hydration of Portland Cement 3.3. Effect ofIn CN this experiment,Addition on the the rate Hydration of heat generation of Portland was measured Cement with a calorimeter, and the effect of CN addition on the hydration reaction was examined. Figure5 shows the Incalorimeter this experiment, results for the each rate amount of heat of CN genera added.tion First, was it was measured confirmed that with the a heat calorimeter, and the effectof hydration of CN rapidlyaddition increased on the immediately hydration after reaction cement made was contact examined. with water. Figure This 5 shows the calorimeterpeak is results mainly due for to each the heat amount of wetting of ofCN cement, added. the heatFirst, of dissolutionit was confirmed of aluminate that the heat of hydration(Al2O rapidly3) and sulfate increased (SO4) into immediately solution, and after the heat cement of hydration made contact of calcium with silicate water. This peak (C3S) (first peak) [12,22,23]. It was approximately 53 J/h·g for CN0. In contrast, it was is mainlyapproximately due to the 104 heat J/h· g,of 128 wetting J/h·g,166 of ceme J/h·g, andnt, the 211 J/hheat·g forof CN7,dissol CN9,ution CN11, of aluminate and (Al2O3) and sulfateCN13, respectively,(SO4) into and solution, the amount and of heat the generated heat of tended hydration to increase of ascalcium the amount silicate of (C3S) (first peak) [12,22,23].added CN increased. It was Afterapproximately the first peak, it53 was J/h·g confirmed for CN0. that, duringIn contrast, the dissolution it was of approximately aluminate and sulfate, the hydration reaction of C3S was stagnant for 2~3 h, and then the 104 J/h·g,second 128 largest J/h·g, peak 166 appeared J/h·g, and (second 211 peak) J/h·g [12 for,22 ].CN7, The second CN9, peak CN11, was attributedand CN13, to respectively, the hydration of tricalcium aluminate (formation of ettringite and conversion of ettringite to the AFm phase) in cement, the heat of dissolution of the tricalcium silicate phase (C3S), and the heat of formation of calcium-silicate-hydrate (C–S–H), which accounts for a large proportion of the total heat of hydration [13,22]. The second peak confirmed that hydration was promoted more than in the non-added case (CN0) in all cases where CN was added. The expression time was roughly proportional to the amount of CN added, and it was 4 h for CN7, 5 h for CN9, and 7 h for CN11, and the peak for CN13 was shifted to the left side by approximately 8 h at the maximum as compared with that of CN0. The local maximum of hydration of the second peak was larger when CN was added, which was approximately 9 J/h·g for CN0 and approximately 13 J/h·g for CN13. From the calorimeter results, it was confirmed that when CN was added, the rate of hydration reaction of C3A and C3S was increased, and the amount of total heat generation was increased by the hydration of C3A and C3S at an early age [9,22,23]. The tendency of the calorific value to increase according to the amount of CN added is the same as the improvement of the compressive strength, indicating that the initial acceleration of hydration greatly contributes to the development of the initial strength. Materials 2021, 14, x FOR PEER REVIEW 8 of 15

and the amount of heat generated tended to increase as the amount of added CN increased. After the first peak, it was confirmed that, during the dissolution of aluminate and sulfate, the hydration reaction of C3S was stagnant for 2 ~ 3 h, and then the second largest peak appeared (second peak) [12,22]. The second peak was attributed to the hydration of tricalcium aluminate (formation of ettringite and conversion of ettringite to the AFm phase) in cement, the heat of dissolution of the tricalcium silicate phase (C3S), and the heat of formation of calcium-silicate-hydrate (C–S–H), which accounts for a large proportion of the total heat of hydration [13,22]. The second peak confirmed that hydration was promoted more than in the non-added case (CN0) in all cases where CN was added. The expression time was roughly proportional to the amount of CN added, and it was 4 h for CN7, 5 h for CN9, and 7 h for CN11, and the peak for CN13 was shifted to the left side by approximately 8 h at the maximum as compared with that of CN0. The local maximum of hydration of the second peak was larger when CN was added, which was approximately 9 J/h·g for CN0 and approximately 13 J/h·g for CN13. From the calorimeter results, it was confirmed that when CN was added, the rate of hydration reaction of C3A and C3S was increased, and the amount of total heat generation was increased by the hydration of C3A and C3S at an early age [9,22,23]. The tendency of the calorific value to Materials 2021, 14, 1006 increase according to the amount of CN added is the same as the improvement8 of 14 of the compressive strength, indicating that the initial acceleration of hydration greatly contributes to the development of the initial strength.

240 CN0 250 CN0 CN7 CN7

g) 210

・ 100 CN9 200 CN9 180 CN11 CN11 CN13 CN13 150 150 120 10 90 100 Energy (J/g) 60 50

Rate of heat generation (J/h 30

0 1 0 0 0.2 0.4 0.6 0 5 10 15 20 25 30 0 5 10 15 20 25 30 time (hours) time (hours) time (hours) (a) (b)

FigureFigure 5. Calorimeter 5. Calorimeter (( ((aa):): Hydration Hydration heatheat rate rate curve, curve, (b ):(b Cumulative): Cumulative heat heat of hydration). of hydration).

3.4.3.4. Thermogravimetric Thermogravimetric DifferentialDifferential Thermal Thermal analysis analysis In this experiment, the thermal weight change and the derivative mass loss for each In this experiment, the thermal weight change and the derivative mass loss for each case were measured, the amount of bound water in each case was compared, and the casehydrate were productsmeasured, were the identified. amount of Figure bound6 shows water an in example each case of thewas TG/DTG compared, graph and at the hy- dratecuring products ages of 3were h, 12 identified. h, and 24 h Figure and the 6 amount shows an of CHexample produced. of the TG/DTG graph at curing ages ofBalonis 3 h, 12 et h, al. and (2011) 24 h [and13] reported the amount that of when CH CNproduced. is added, the mass loss is in the rangeBalonis of 200 et ~ 300al. (2011)◦C due [13] to nitrite/nitrate-AFm reported that when formed CN is by added, the decomposition the mass loss of OH-is in andthe range ofNO 2002 -/NO~ 3003 °C-groups due to [13 nitrite/nitrate-AFm]. Also in this study, formed in the case by the of the decomposition sample to which of OH- CN was and NO2- ◦ /NOadded,3-groups a peak [13]. of mass Also reduction in this study, was observed in the case at approximately of the sample 250~300 to whichC, CN and was it was added, a peakconfirmed of mass that reduction nitrite/nitrate-AFm was observed was at produced. approximately Moreover, 250 the ~ 300 amount °C, and of mass it was loss confirmed at approximately 250~300 ◦C showed a slightly increasing trend with each curing time. In that nitrite/nitrate-AFm was produced. Moreover, the amount of mass loss at approxi- addition, the amount of mass loss in this range increases in proportion to the amount of mately 250 ~ 300 °C showed a slightly increasing trend− with each− curing time. In addition, CN added, and it is estimated that the increase in NO2 and NO3 ion concentration in thethe amount solution of contributes mass loss to in the this production range increases of nitrite/nitrate-AFm. in proportion Basedto the onamount this result, of CN it is added, andspeculated it is estimated that nitrite/nitrate-AFm that the increase is generated in NO2− immediatelyand NO3− ion within concentration 3 h. For other in peaks, the solution it contributeswas confirmed to the that production the decomposition of nitrite/nitrate- peaks increasedAFm. in Based the range on of this ~100 result,◦C in it each is speculated case, thatand nitrite/nitrate-AFm the mass loss in this rangeis generated was larger immediat in the caseely where within CN 3 wash. For added. other With peaks, regard it was to con- ◦ firmedthe results that atthe ~100 decompositionC, this decomposition peaks increased peak was in due the to therange evaporation of ~100 °C ofbound in each water case, and theor mass interphase loss in water this range incorporated was larger in AFt, in the AFm, case and where C–S–H, CN and was it isadded. speculated With that regard the to the addition of CN accelerated the hydration of C A and C S at an early age [18]. The mass loss results at ~100 °C, this decomposition peak3 was due3 to the evaporation of bound water or in the temperature range of 400~480 ◦C indicates that decomposition of CH had occurred. The amount of CH was calculated from the inflection point of the DTA curve near 400 ◦C. The amount of CH generated calculated from TG is as shown in Figure6. Based on the quantitative evaluation of CH, it can be observed that the addition of CN slightly affects the formation of CH. Looking at the amount of CH produced at each age, production tended to promotion with the acceleration of hydration by the addition of CN at the age of 12 h. On the other hand, the tendency in the case of CN addition was reversed by the time the material age passed 24 h, and the tendency was inversely proportional to the addition amount at the material age of 3 days. It has been reported that CH may contribute to the formation of nitrite/nitrate-AFm when Ca(NO2)2 and Ca(NO3)2 are added [13]. Therefore, in this experiment, CH might contribute to the formation of nitrite/nitrate-AFm, which slows down the rate of CH crystal (Portlandite) precipitation, resulting in a decrease in the amount produced at an early age. In addition, the total mass loss amount increased as CN was added, indicating that the amount of bound water due to accelerated hydration of cement was increased. However, the increase in the amount of total bound water tended to increase with time in the CN0 group. In the group to which CN was added, we confirmed that the amount of bound water increased due to rapid hydration within 3 h. After that, the rate of increase in the amount of bound water between 12 h and 24 h tended to decrease Materials 2021, 14, x FOR PEER REVIEW 9 of 15

interphase water incorporated in AFt, AFm, and C–S–H, and it is speculated that the addition of CN accelerated the hydration of C3A and C3S at an early age [18]. The mass loss in the temperature range of 400 ~ 480 °C indicates that decomposition of CH had occurred. The amount of CH was calculated from the inflection point of the DTA curve near 400 °C. The amount of CH generated calculated from TG is as shown in Figure 6. Based on the quantitative evaluation of CH, it can be observed that the addition of CN slightly affects the formation of CH. Looking at the amount of CH produced at each age, production tended to promotion with the acceleration of hydration by the addition of CN at the age of 12 h. On the other hand, the tendency in the case of CN addition was reversed by the time the material age passed 24 h, and the tendency was inversely proportional to the addition amount at the material age of 3 days. It has been reported that CH may contribute to the formation of nitrite/nitrate-AFm when Ca(NO2)2 and Ca(NO3)2 are added [13]. Therefore, in this experiment, CH might contribute to the formation of nitrite/nitrate- AFm, which slows down the rate of CH crystal (Portlandite) precipitation, resulting in a decrease in the amount produced at an early age. In addition, the total mass loss amount increased as CN was added, indicating that the amount of bound water due to accelerated hydration of cement was increased. However, the increase in the amount of total bound Materials 2021, 14, 1006 water tended to increase with time in the CN0 group. In the group to which CN9 was of 14 added, we confirmed that the amount of bound water increased due to rapid hydration within 3 h. After that, the rate of increase in the amount of bound water between 12 h and 24compared h tended with to decrease that of CN0.compared This iswith thought that of to CN0. be due This to theis thought difference to be in due the hydrationto the dif- ferenceexothermic in the rate, hydration and there exothermic was some rate, correlation and there with was the some expression correlation position with of the the expres- second sionpeak. position These results of the indicatesecond that,peak. when These CN result is added,s indicate the hydrationthat, when of CN cement is added, is accelerated the hy- drationwithin 12of hcement and contributes is accelerated greatly within to the 12 strengthh and contributes development greatly in earlyto the age strength cement. development in early age cement. 0 －0.25 0 －0.25 0 －0.25 TG CN0 TG CN7 TG CN9 －0.20 －0.20 －0.20 －5 －5 －5

－0.15 －0.15 －0.15 －10 －10 －10 －0.10 －0.10 －0.10 Mass loss (%) Mass loss (%) －15 －15 Mass loss (%) －15 －0.05 －0.05 －0.05 DerivativeTG (%/℃) DerivativeTG (%/℃) DerivativeTG (%/℃) DTG DTG DTG －20 0 －20 0 －20 0 0 200 400 600 800 1000 0 200 400 600 800 1000 0 200 400 600 800 1000 Temperature (℃) Temperature (℃) Temperature (℃) (a) (b) (c) 0 －0.25 0 －0.25 15 TG CN11 TG CN13 CN0 CN7 CN9 CN11 CN13 －0.20 －0.20 －5 －5 10 －0.15 －0.15 －10 －10 －0.10 －0.10 5 Mass loss (%) Mass loss (%) －15 －15 －0.05 －0.05 Derivative TG (%/℃) TG Derivative (%/℃) TG Derivative Amount of produced CH by TG (%) DTG DTG －20 0 －20 0 0 0.5 1 3 0 200 400 600 800 1000 0 200 400 600 800 1000 Age (days) Temperature (℃) Temperature (℃) (d) (e) (f)

FigureFigure 6. 6.TG/DTG TG/DTG (TG/DTG (TG/DTG = Green/Blue, 3 3h h= =Dotted Dotted line line,, 1212 h = h Broken = Broken line, line, 24 h 24 = hSolid = Solid line). line). (a)CN0; (a)CN0; (b) (CN7;b) CN7; (c)CN9; (d) CN11; (e)CN13;(c)CN9; (f) amount (d) CN11; of CH produced(e)CN13; by(f) TG. amount of CH produced by TG.

3.5.3.5. X-Ray X-ray DiffractionDiffraction InIn this experiment, to confirmconfirm thethe formationformation ofof thethe nitrite/nitrate-AFmnitrite/nitrate-AFm reported reported by BalonisBalonis et et al. al. (2011) (2011) and and the the effect effect of ofCN CN addition addition on onthe theformation formation of other of other hydrates, hydrates, the hydratesthe hydrates were were identified identified from from the XRD the XRD results. results. Figure Figure 7 shows7 shows the theXRD XRD results results for forthe ◦ diffractionthe diffraction angle angle range range of 5 ~ of 25° 5~25 for thefor five the ca fiveses casesfor each for curing each curing age. Balonis age. Balonis et al. (2011) et al. (2011) [13] reported that, through reaction experiments using synthetic AFm, produces nitrite (nitrate) type AFm by incorporating various anions between the layers of the 2− − − AFm phase, and the ion exchange also occurs between SO4 and NO2 (NO3 )[13]. According to Balonis et al. (2011), nitrite-AFm and nitrate-AFm have diffraction peaks around 10~11◦ [13]. Based on this report, this study focused on diffraction at low angles. At the curing age of 3 h for CN0, ettringite (2Theta/Theta= 9.1◦, 15.8◦, 18.9◦, 22.9◦) and ◦ ◦ gypsum (CaSO4·2H2O; 2Theta/Theta= 11.6 , 20.8 ) peaks were visible [24,25]. In contrast, the peak of gypsum was observed in the CN0 samples but not after CN addition. From the XRD results, in the case where CN was added, it was confirmed that gypsum was rapidly consumed because of its reaction with C3A and ettringite. According to the contents reported by Cheung et al. (2011), it was reported that the addition of calcium nitrite markedly increased the consumption rate of gypsum and at the same time increased the production of ettringite [11]. Based on this report, in this experiment, we speculated that gypsum was rapidly consumed, and the amount of ettringite produced also increased as the amount of added CN increased. In addition, nitrite/nitrate-AFm (2Theta/Theta = 10.5◦) peaks are also visible along with ettringite for all ages for the cases of CN addition. Therefore, we speculated that the addition of CN accelerated the C3A reaction and that the formation of AFt and nitrite/nitrate-AFm proceed simultaneously. Furthermore, the intensity of the peak for calcium hydroxide (CH; 2θ ◦ 18.1◦) at 24 h was higher for CN0 than for the CN addition group [19,26]. It is speculated that this is because calcium hydroxide in the solution is consumed in the process of producing a large amount of ettringite and AFm by Materials 2021, 14, x FOR PEER REVIEW 10 of 15

[13] reported that, through reaction experiments using synthetic AFm, produces nitrite (nitrate) type AFm by incorporating various anions between the layers of the AFm phase, and the ion exchange also occurs between SO42− and NO2−(NO3−) [13]. According to Balonis et al. (2011), nitrite-AFm and nitrate-AFm have diffraction peaks around 10 ~ 11° [13]. Based on this report, this study focused on diffraction at low angles. At the curing age of 3 h for CN0, ettringite (2Theta/Theta= 9.1°, 15.8°, 18.9°, 22.9°) and gypsum (CaSO4·2H2O; 2Theta/Theta= 11.6°, 20.8°) peaks were visible [24,25]. In contrast, the peak of gypsum was observed in the CN0 samples but not after CN addition. From the XRD results, in the case where CN was added, it was confirmed that gypsum was rapidly consumed because of its reaction with C3A and ettringite. According to the contents reported by Cheung et al. (2011), it was reported that the addition of calcium nitrite markedly increased the consumption rate of gypsum and at the same time increased the production of ettringite [11]. Based on this report, in this experiment, we speculated that gypsum was rapidly consumed, and the amount of ettringite produced also increased as the amount of added CN increased. In addition, nitrite/nitrate-AFm (2Theta/Theta = 10.5°) peaks are also visible along with ettringite for all ages for the cases of CN addition. There- fore, we speculated that the addition of CN accelerated the C3A reaction and that the for- Materials 2021, 14, 1006 mation of AFt and nitrite/nitrate-AFm proceed simultaneously. Furthermore, the intensity10 of 14 of the peak for calcium hydroxide (CH; 2θ ° 18.1°) at 24 h was higher for CN0 than for the CN addition group [19,26]. It is speculated that this is because calcium hydroxide in the solutionadding CN.is consumed This result in is the consistent process withof prod theucing TG/DTG a large results, amount and of it isettringite plausible and that AFm the byaddition adding of CN. CN This has anresult effect is onconsistent the crystallinity with the and TG/DTG production results, of and CH atit is an plausible early age. that the addition of CN has an effect on the crystallinity and production of CH at an early age.

3500 CN0 3500 3500 CN7 CN9 3000 3000 3000

2500 2500 2500

2000 2000 2000

1500 1500 1500 24 h 24 h 24 h Intensity Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) 12 h 12 h 12 h 1000 1000 1000 6 h 6 h 6 h 3 h 3 h 3 h 500 500 500 1 h 1 h 1 h 0 0 0 5 10152025 5 10152025 5 10152025 2θ/θ; CuKα (degrees) 2θ/θ; CuKα (degrees) 2θ/θ; CuKα (degrees)

3500 3500 CN11 CN13

3000 3000

2500 2500

2000 2000

1500 24 h 1500 24 h

Intensity (a.u.) 12 h Intensity (a.u.) 12 h 1000 1000 6 h 6 h 3 h 3 h 500 500 1 h 1 h

0 0 5 10152025 5 10152025 2θ/θ; CuKα (degrees) 2θ/θ; CuKα (degrees) Ettringite (AFt phase) Nitrite/Nitrate AFm Portlandite (Ca(OH) )Gypsum (CaSO) 2 4

FigureFigure 7. 7. X-RayX-ray Diffraction.

3.6.3.6. 2727AlAl MAS MAS NMR NMR

In this experiment, the progress of hydration of C3A and the formation of nitrite/nitrate- AFm were confirmed using 27Al NMR, and the NMR results were compared with the aforementioned experimental results. Figure8 shows the solid 27AlNMR integrated surface integral ratio for each case from mixing ~72 h, and Figure9 shows the results of the solid 27AlNMR spectrum at a material age of 24 h (−10 ppm to 100 ppm). The chemical shift region of 27Al NMR can be explained as follows: tetrahedral coordination (Al[IV]) = 50~100 ppm, pentahedral coordination (Al[V]) = 30~40 ppm, and octahedral coordination (Al[VI]) = 10~20 ppm [27,28]. Commonly, it is known that resonance in the 50~100 ppm range is due to poor crystalline structures [29,30]. The broad range of resonance from to 70 ~ 100 ppm confirmed in this study is due to the Al contained in the unhydrated cement (anhydrous material area) [19]. The NMR results reveal that the cases in which CN was added had lower peak heights and a smaller peaks area in the range of 70~100 ppm (Al[IV] area) in comparison with CN0. Based on this result, the addition of CN, which accelerated the hydration of C3A, was confirmed, similar to the aforementioned experimental result (Sections 3.1–3.5). Furthermore, the peak in the range of 0~20 ppm (Al[VI] area) is due to the resonance of Al[VI] derived from the hydration product, and AFt, 3− (3+x)− AFm, and third aluminate hydrate (TAH; Al(OH)6 ,OxAl(OH)6−x ) peaks are mainly observed [30,31]. In addition, it has been already reported that calcium aluminoferrite (C4AF; Ca2(Al,Fe)2O5), which constitutes the clinker and overlaps with the peak of AFm, is observed at an early age [19]. In this range, ettringite (=AFt), Afm, or C4AF, and TAH peaks were confirmed in each case in which CN added was. Even in the case of CN0, the peaks Materials 2021, 14, 1006 11 of 14

of AFt, AFm, or C4AF, and TAH were conﬁrmed as a result of the waveform separation. However, the peak position of AFm differs between CN non-added (9.6~10 ppm) and CN added (10.3~10.5 ppm), and it is considered that nitrite/nitrate-AFm was produced at an early age. In addition, the peak of the AFm phase was higher and the peak area was larger in the case where CN was added. This result conﬁrmed that the amount of AFm produced had increased. The AFt peak tended to increase with the amount of CN added, but it was the same in all cases at the age of 24 h. It is considered that this is because almost all of the dihydrate gypsum contained in the cement was consumed. From the results of 27Al NMR, the addition of CN accelerates the production of AFt in addition to the − − Materials 2021, 14, x FOR PEER REVIEWproduction of nitrite/nitrate-AFm by the reaction of NO2 , NO3 ions and C3A. Moreover,12 of 15 Materials 2021, 14, x FOR PEER REVIEWby accelerating the hydration of C A, a large number of these calcium aluminate hydrates12 of 15 3 are produced, which contributes to strength development at an early age.

100 anhydrous material 100 anhydrous material 100 anhydrous material 100 C-[A]-S-Hanhydrous material 100 C-[A]-S-Hanhydrous material 100 C-[A]-S-Hanhydrous material 90 90 90 AFtC-[A]-S-H AFtC-[A]-S-H AFtC-[A]-S-H 90 90 90 80 C4AFAFt and AFm 80 C4AFAFt and AFm 80 C4AFAFt and AFm 80 TAHC4AF and AFm 80 TAHC4AF and AFm 80 TAHC4AF and AFm 70 TAH 70 TAH 70 TAH 70 70 70 60 60 60 60 60 60 50 50 50 50 50 50 40 40 40 40 40 40 Al NMR Integratedratio 30 Al NMR Integratedratio 30 Al NMR Integratedratio 30 27 27 27 Al NMR Integratedratio 30 Al NMR Integratedratio 30 Al NMR Integratedratio 30 27 20 27 20 27 20 20 20 20 10 10 10 10 10 10 0 0 0 00.01 0.1 1 10 00.01 0.1 1 10 00.01 0.1 1 10 100 1000 0.01 0.1time (hours) 1 10 0.01 0.1time (hours) 1 10 0.01 0.1time 1 (hours) 10 100 1000 time (hours) time (hours) time (hours) (a) CN0 (b) CN7 (c) CN9 (a) CN0 (b) CN7 (c) CN9 100 anhydrous material 100 anhydrous material 100 C-[A]-S-Hanhydrous material 100 C-[A]-S-Hanhydrous material 90 90 AFtC-[A]-S-H AFtC-[A]-S-H 90 90 80 C4AFAFt and AFm 80 C4AFAFt and AFm 80 TAHC4AF and AFm 80 TAHC4AF and AFm 70 TAH 70 TAH 70 70 60 60 60 60 50 50 50 50 40 40 40 40 Al NMR Integratedratio 30 Al NMR Integratedratio 30 27 27 Al NMR Integratedratio 30 Al NMR Integratedratio 30 27 20 27 20 20 20 10 10 10 10 0 0 00.01 0.1 1 10 100 1000 00.01 0.1 1 10 100 1000 0.01 0.1time 1 (hours) 10 100 1000 0.01 0.1time 1 (hours) 10 100 1000 time (hours) time (hours) (d) CN11 (e) CN13 (d) CN11 (e) CN13 Figure 8. 27Al MAS NMR integrated area ratio (mixing ~72 h). 27 Figure 8. 27AlAl MAS MAS NMR NMR integrated integrated area area ratio ratio (mixing (mixing ~72 ~72 h). h).

AFt in Al[VI] AFt in Al[VI] AFt in Al[VI] AFt in Al[VI] AFt in Al[VI] CN0 AFt in Al[VI] CN7CN0 CN7 AFm phases CN9 AFmin Al phases[VI] CN11CN9 AFm phases AFm phases in Al[VI] [VI] in Al[VI] CN11 AFmin Al phases AFm phases CN13 in Al[VI] in Al[VI] CN13 Nitrite(nitrate)-AFm Nitrite(nitrate)-AFm AFm phases (CN0 peak positon) AFm phases TAH in Al[VI] (CN0 peak positon) Anhydrous material TAH in Al[VI] in Al[IV] TAH in Al[VI] TAH in Al[VI] Anhydrous material TAH in Al[VI] in Al[IV] TAH in Al[VI]

100 90 80 70 60 50 40 30 20 10 0 ー 10 20 18 16 14 12 10 8 6 4 2 0 20 18 16 14 12 10 8 6 4 2 0 100 90 80 70 60Chemical50 Shift/ppm40 30 20 10 0 ー 10 20 18 16Chemical14 12 Shift/ppm10 8 6 4 2 0 20 18 16 14Chemical12 10 Shift8 (ppm)6 4 2 0 Chemical Shift/ppm Chemical Shift/ppm Chemical Shift (ppm) (a) (b) (a) (b) Figure 9. 27Al MAS NMR spectra of each cases after 24 h. (a) NMR spectrum in the range of -10 to 100 ppm; (b) NMR FigureFigure 9. 9.27 27AlAl MAS MAS NMR NMR spectraspectra ofof eacheach cases after 24 h. ( (aa)) NMR NMR spectrum spectrum in in the the range range of of -10 -10 to to 100 100 ppm; ppm; (b) ( bNMR) NMR spectrum derived from hydrate. spectrum derived from hydrate. spectrum derived from hydrate. 3.7. Crystal Form 3.7. Crystal Form Comparative analysis of nitrite-nitrate crystals of the hydrates produced as a result Comparative analysis of nitrite-nitrate crystals of the hydrates produced as a result of the addition of CN was observed at 24 h for CN0 and CN13. Figure 10a,b shows the of the addition of CN was observed at 24 h for CN0 and CN13. Figure 10a,b shows the acquired SEM images. The identification of the observed crystals was determined by the acquired SEM images. The identification of the observed crystals was determined by the crystal form, crystal size reported in previous studies, and XRD (Section 3.5) and 27Al crystal form, crystal size reported in previous studies, and XRD (Section 3.5) and 27Al NMR (Section 3.6) results in this study [13,32,33]. From the SEM images of CN0, hydration NMR (Section 3.6) results in this study [13,32,33]. From the SEM images of CN0, hydration products that formed in the early stage of hydration, such as AFt, AFm(hexagonal plate- products that formed in the early stage of hydration, such as AFt, AFm(hexagonal plate- like crystals), CH, and C–S–H gel(Type I), were confirmed [33]. It is known that ettringite like crystals), CH, and C–S–H gel(Type I), were confirmed [33]. It is known that ettringite (AFt) is formed by the reaction between C3A and gypsum, and is formed early in the hy- (AFt) is formed by the reaction between C3A and gypsum, and is formed early in the hydration reaction in cement. Further, they greatly contribute to the strength development dration reaction in cement. Further, they greatly contribute to the strength development of cement at an early age. In the case of CN13, AFm and C–S–H gel(Type Ⅲ, Ⅳ) were con- of cement at an early age. In the case of CN13, AFm and C–S–H gel(Type Ⅲ, Ⅳ) were confirmed [33], and the crystal size was larger than that of CN0. These are usually hydrates firmed [33], and the crystal size was larger than that of CN0. These are usually hydrates

Materials 2021, 14, 1006 12 of 14

3.7. Crystal Form Comparative analysis of nitrite-nitrate crystals of the hydrates produced as a result of the addition of CN was observed at 24 h for CN0 and CN13. Figure 10a,b shows the acquired SEM images. The identiﬁcation of the observed crystals was determined by the crystal form, crystal size reported in previous studies, and XRD (Section 3.5) and 27Al NMR (Section 3.6) results in this study [13,32,33]. From the SEM images of CN0, hydration products that formed in the early stage of hydration, such as AFt, AFm(hexagonal plate-like crystals), CH, and C–S–H gel(Type I), were conﬁrmed [33]. It is known that ettringite (AFt) is formed by the reaction between C3A and gypsum, and is formed early in the hydration Materials 2021, 14, x FOR PEER REVIEWreaction in cement. Further, they greatly contribute to the strength development of cement13 of 15

at an early age. In the case of CN13, AFm and C–S–H gel(Type III, IV) were conﬁrmed [33], and the crystal size was larger than that of CN0. These are usually hydrates formed after the middle stage (approximately 24 h) of the hydration reaction, and it can be seen that the formed after the middle stage (approximately 24 h) of the hydration reaction, and it can addition of CN accelerated the hydration reaction in the cement [22]. These results suggest be seen that the addition of CN accelerated the hydration reaction in the cement [22]. that, when CN is added, the hydration of cement in the initial stage is accelerated, and These results suggest that, when CN is added, the hydration of cement in the initial stage the hydrate ﬁlls the pores effectively, thereby contributing to strength development at an is accelerated, and the hydrate fills the pores effectively, thereby contributing to strength early age. development at an early age.

(a)

(b)

Figure Figure10. SEM 10. imagesSEM imagesof (a) CN0 of ( aand) CN0 (b) andCN13. (b) CN13.

4.4. Conclusions Conclusions WeWe examined examined thethe effecteffect of internal internal struct structureure on on the the strength strength development development when when CN CNwas was added added and and conducted conducted various various experimental experimental and and chemical chemical studies studies to clarify to clarify the the hy- hydrationdration mechanism mechanism at at an an early early age. age. The The results results of of the the study study are are summarized summarized as as follows: follows: (1)1) When CN was added, thethe raterate ofof ionion elutionelution fromfrom thethe cementcement clinker clinker increased increased owing owing − − to the effect of NO 2− andand NO NO3−3 ions,ions, and and the the rate rate of ofhydrate hydrate formation formation increased increased (es- (especiallypecially within within 12 12h). h). (2)2) The addition of CN accelerates the the usual usual hydration hydration reactions reactions (C (C33A,A, C C3S)3S) that that occur occur in thein the cement cement matrix, matrix, while while additionally additionally form forminging nitrite/nitrate-AFm nitrite/nitrate-AFm through through the reac- the tion between C3A and NO2−, NO3− ions. The hydrate generated effectively contributes to strength development by filling the pores. 3) Nitrite/nitrate-AFm was rapidly deposited as hexagonal-plate crystals immediately after contact with water, and the production amount tended to increase as the amount of added CN increased. 4) By adding CN, hexagonal plate-like crystals, which are presumed to be nitrite/nitrate- AFm, were confirmed in a wide range, and these are considered to contribute significantly to the strength development at an early age. 5) Furthermore, the calorimeter, TG/DTG, and SEM results reveal that the hydration acceleration of C3S also contributes to the filling of pores and strength development. 6) When nitrite/nitrate-based accelerator is added, it accelerates the hydration reaction of the initial cement and greatly contributes to the development of strength, so that problems such as delay in setting and initial frost damage in cold-weather concrete

Materials 2021, 14, 1006 13 of 14

− − reaction between C3A and NO2 , NO3 ions. The hydrate generated effectively contributes to strength development by filling the pores. (3) Nitrite/nitrate-AFm was rapidly deposited as hexagonal-plate crystals immediately after contact with water, and the production amount tended to increase as the amount of added CN increased. (4) By adding CN, hexagonal plate-like crystals, which are presumed to be nitrite/nitrate- AFm, were confirmed in a wide range, and these are considered to contribute significantly to the strength development at an early age. (5) Furthermore, the calorimeter, TG/DTG, and SEM results reveal that the hydration acceleration of C3S also contributes to the filling of pores and strength development. (6) When nitrite/nitrate-based accelerator is added, it accelerates the hydration reaction of the initial cement and greatly contributes to the development of strength, so that problems such as delay in setting and initial frost damage in cold-weather concrete works can be improved. Furthermore, by enabling early demolding, the overall construction period can be shortened. However, there is a concern that the amount of expansion and contraction will increase due to the promotion of hydration, and there may be problems such as cracking due to heat of hydration, so it is necessary to adjust the amount used.

Author Contributions: Conceptualization, H.C., M.L. and J.K.; Data curation, J.K., A.Y.; Investigation, M.I., M.L. and Y.S.; Project administration, A.Y.; Supervision, H.C.; Supervision, M.L.; Writing— original draft, A.Y.; Writing—review & editing, H.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: Data available on request due to restrictions eg privacy or ethical. Acknowledgments: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF-2018R1D1A1B07049390). And a series of experiments described herein was sponsored by Nissan Chemical Co. of JAPAN. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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