Calcium Sulfate Hemihydrate on Setting, Compressive Strength, and Shrinkage Strain of Cement Mortar

materials

Article Inﬂuence of α-Calcium Sulfate Hemihydrate on Setting, Compressive Strength, and Shrinkage Strain of Cement Mortar

Bokyeong Lee 1, Gyuyong Kim 2,* , Jeongsoo Nam 2, Kyehyouk Lee 3, Gyeongtae Kim 2, Sangkyu Lee 2, Kyoungsu Shin 4 and Tomoyuki Koyama 5

1 Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Korea; [email protected] 2 Department of Architectural Engineering, Chungnam National University, Daejeon 34134, Korea; [email protected] (J.N.); [email protected] (G.K.); [email protected] (S.L.) 3 RDM Industrial Development Co., Ltd., Daejeon 34831, Korea; [email protected] 4 Corporate Research Institute, Yusung-Tech Co., Ltd., Okcheon 29057, Korea; [email protected] 5 Department of Architecture and Urban Design, Kyushu University, Fukuoka 812-0053, Japan; [email protected] * Correspondence: [email protected]; Tel.: +82-42-821-5623

Received: 4 December 2018; Accepted: 2 January 2019; Published: 7 January 2019

Abstract: This study focused on the quick initial setting time and the expansion strain that occurs during the early aging of α-calcium sulfate hemihydrate (αHH) and examined the setting, compressive strength, and shrinkage strain of αHH-replaced cement mortar. The results show that the initial setting time signiﬁcantly decreased with an increase in the αHH replacement ratio. Drastic occurrence of ettringite was observed early in the aging of cement mortar when αHH was substituted into the cement; however, the ettringite was not converted to monosulfate with increasing age and thus was not favorable for the development of the compressive strength. When αHH was substituted into cement, using Portland blast-furnace slag cement (PSC) was more advantageous than using ordinary Portland cement (OPC) for the development of the compressive strength. Meanwhile, the expansion of early age αHH can decrease the shrinkage strain of cement mortar. The generation of ettringite is more effective when αHH is substituted into PSC than into OPC and is thus more effective in suppressing the shrinkage strain.

Keywords: α-calcium sulfate hemihydrate; cement mortar; ettringite; initial setting time; compressive strength; shrinkage strain

1. Introduction Gypsum is a material in which the amount of water of crystallization varies with its surrounding conditions, such as temperature or humidity, and can be classiﬁed into three groups, namely, calcium sulfate anhydrite (CaSO4), calcium sulfate hemihydrate (CaSO4·1/2H2O), and calcium sulfate dihydrate (CaSO4·2H2O) [1]. Calcium sulfate hemihydrate can be further categorized into α and β forms, while calcium sulfate anhydrite can be further categorized into I, II, and III forms. Previous studies have focused on the production of calcium sulfate hemihydrate from calcium sulfate anhydrite and calcium sulfate dihydrate [2–6]. Both α-calcium sulfate hemihydrate (αHH) and β-calcium sulfate hemihydrate are soluble gypsums that are easily hydrated via the surrounding moisture, and they characteristically hydrate and set into calcium sulfate dihydrate in the presence of water. The solubility of calcium sulfate hemihydrate is higher than that of calcium sulfate anhydrite or calcium sulfate dihydrate; hence, excessive additions result in supersaturated solutions, which lead to

Materials 2019, 12, 163; doi:10.3390/ma12010163 www.mdpi.com/journal/materials Materials 2019, 12, x FOR PEER REVIEW 2 of 13 which lead to the precipitation and setting of calcium sulfate dihydrate. As previously mentioned, soluble gypsum calcium sulfate hemihydrate can easily absorb moisture and thus accelerate the rate of expansion and hardening. Figure 1 shows a conceptual diagram of the initial setting time and strain properties of αHH. In general, the setting time of calcium sulfate dihydrate becomes almost constant above a certain rate of addition. The setting of αHH is delayed by additions of up to approximately 2.0%, but further additions result in the immediate precipitation of calcium sulfate dihydrate from the supersaturated solution and, subsequently, a shorter setting time [7]. In addition, the growth pressure of the acicular ettringite crystals formed from the reactions between C3A and αHH increases the distance between the cement particles or hydrates, resulting in expansion strains early in the aging process. αHH is presently made from calcium sulfate dihydrate using an autoclave via pressurized steam or pressurized solution, or without an autoclave via atmospheric steam or atmospheric solution. However, due to the complex manufacturing steps, difficulties in mass production, and subsequent increase in material costs, these methods have not been very useful. On the other hand, the recent commercialization of a cost-effective αHH production technology that utilizes flue gas desulfurization gypsum has led to increased usability of αHH as a construction material [8–11]. αHH sets and hardens like cement when mixed with the appropriate amount of water. It has been reported that the strength of αHH is affected by its pore structure, the size and shape of its crystals, and the interactions among its microstructures [6]. Tang et al. [12] investigated the effects of different crystal sizes and shapes of αHH on its strength and microstructure. Guan et al. [13] and Ye et al. [14] evaluated the effects of particle size on the compressive strength of αHH. αHH significantly hardens at an early age, and after reaching maximum strength, the strength shows minimal change orMaterials a decreasing2019, 12, 163trend; this has been verified in previous studies [15,16]. Lewry and Williamson 2[1 of7 13] claimed that the strength development of calcium sulfate hemihydrate occurs as a three-step process, and Guan et al. [15] used this trend to propose a five-step hydration process of αHH, namely, the bondingthe precipitation of particles and in setting paste, offormation calcium sulfateof a crystal dihydrate. matrix, As release previously of internal mentioned, stress, solubleevaporation gypsum of freecalcium water, sulfate and joining hemihydrate of the instability can easily matrix absorb [1 moisture7–19]. In andother thus words, accelerate it can be the inferred rate of that expansion αHH isand converted hardening. to calcium sulfate dihydrate via hydration, and the structure is loosened and the strength decreasesFigure with1 shows age due a conceptualto recrystallization diagram of of the the precipitated initial setting calcium time andsulfate strain dihydrate properties crystals. of α HH. In general,Therefore, the settingthe objective time of of calcium this study sulfate is to dihydrate determine becomes fundamental almost constantdata for abovethe application a certain rateof αHHof addition. as a construction The setting material. of αHH The is study delayed focused by additions on the quick of up initial to approximately setting time and 2.0%, the butexpansion further strainadditions that resultoccurs in during the immediate the early precipitationaging of αHH of calciumand examined sulfate the dihydrate setting, from compressive the supersaturated strength, andsolution shrinkage and, subsequently, strain of αHH a shorter-replaced setting cement time mortar. [7]. In addition, In addition, the growth the crystalline pressure structure of the acicular and microstructureettringite crystals of formed αHH-replaced from the cement reactions mortar between were C3 A analyzed and αHH using increases X-ray the diffraction distance between (XRD), quantitativethe cement particlesX-ray diffraction or hydrates, (QXRD), resulting and inscanning expansion electron strains microscope early in the (SEM) aging observations. process.

Delay Acceleration Expansion Shrinkage section section strain strain

Strain days

time of setting oftime Start

(a) Replacement ratio of αHH (b)

FigureFigure 1. 1. ConceptualConceptual diagram diagram o off (a (a) )the the start start time time of of setting setting and and (b (b) )the the strain strain properties properties of of αα-calcium-calcium sulfatesulfate he hemihydratemihydrate (αHH (αHH).).

αHH is presently made from calcium sulfate dihydrate using an autoclave via pressurized steam or pressurized solution, or without an autoclave via atmospheric steam or atmospheric solution. However, due to the complex manufacturing steps, difficulties in mass production, and subsequent increase in material costs, these methods have not been very useful. On the other hand, the recent commercialization of a cost-effective αHH production technology that utilizes flue gas desulfurization gypsum has led to increased usability of αHH as a construction material [8–11]. αHH sets and hardens like cement when mixed with the appropriate amount of water. It has been reported that the strength of αHH is affected by its pore structure, the size and shape of its crystals, and the interactions among its microstructures [6]. Tang et al. [12] investigated the effects of different crystal sizes and shapes of αHH on its strength and microstructure. Guan et al. [13] and Ye et al. [14] evaluated the effects of particle size on the compressive strength of αHH. αHH significantly hardens at an early age, and after reaching maximum strength, the strength shows minimal change or a decreasing trend; this has been verified in previous studies [15,16]. Lewry and Williamson [17] claimed that the strength development of calcium sulfate hemihydrate occurs as a three-step process, and Guan et al. [15] used this trend to propose a five-step hydration process of αHH, namely, the bonding of particles in paste, formation of a crystal matrix, release of internal stress, evaporation of free water, and joining of the instability matrix [17–19]. In other words, it can be inferred that αHH is converted to calcium sulfate dihydrate via hydration, and the structure is loosened and the strength decreases with age due to recrystallization of the precipitated calcium sulfate dihydrate crystals. Therefore, the objective of this study is to determine fundamental data for the application of αHH as a construction material. The study focused on the quick initial setting time and the expansion strain that occurs during the early aging of αHH and examined the setting, compressive strength, and shrinkage strain of αHH-replaced cement mortar. In addition, the crystalline structure and microstructure of αHH-replaced cement mortar were analyzed using X-ray diffraction (XRD), quantitative X-ray diffraction (QXRD), and scanning electron microscope (SEM) observations. Materials 2019, 12, 163 3 of 13

Materials 2019, 12, x FOR PEER REVIEW 3 of 13 2. Experimental Procedures 2. Experimental Procedures 2.1. Materials 2.1. MaterialsTable1 lists the physical properties, while Table2 presents the chemical compositions of the materialsTable used 1 lists in thethe experiment.physical properties, To produce whileαHH-replaced Table 2 presents cement the mortar, chemical this compositions study used ordinary of the materialsPortland cementused in (OPC)the experiment. and Portland To produce blast-furnace αHH slag-replaced cement cement (PSC). mortar, Type I OPCthis study was used used according ordinary Portlandto the Standard cement Specification (OPC) and for Portland Portland blast Cement-furnace (ASTM slag C 150). cement The (P PSCSC). consisted Type I of OPC 62% was OPC used and according38% ground to granulatedthe Standard blast-furnace Specification slag for (GGBS). Portland The CementαHH used(ASTM in theC 150). experiment The PSC was consisted derived of from62% OPCflue gas and desulfurization 38% ground granulated gypsum using blast pressurized-furnace slag solution. (GGBS). For The the αHH fine aggregate,used in the ISO experiment standard sandwas wasderived used. from flue gas desulfurization gypsum using pressurized solution. For the fine aggregate, ISO standard sand was used. Table 1. Physical properties of materials used in the experiment. Figure 2 shows the particle size distribution of αHH measured using a Mastersizer 2000 particle size analyzer (MalvernMaterials Instruments (Sign) Ltd., Malvern, UK). The D50 Physical of the αHH Properties used in the experiment was 34.6 µm,ordinary while the Portland D90 was cement 62.8 (OPC) µm. The XRD patternsDensity: of 3.12the αHH g/cm 3are, Blaine: displayed 3500 cm in2 /gFigure 3. It was observedPortland that blast-furnace the diffraction slag cement pattern (PSC) of αHH includesDensity: most 3.05 of g/cm the calcium3, Blaine: sulfate 4000 cm hemihydrate2/g peaks. Figureα-calcium 4 shows sulfate a SEM hemihydrate micrograph (αHH) of αHH. Density: 2.72 g/cm3, Blaine: 1400 cm2/g sand (S) Density: 2.50 g/cm3, Absorption ratio: 1.00% Table 1. Physical properties of materials used in the experiment. Table 2. Chemical compositions of materials used in the experiment. Materials (Sign) Physical Properties ordinary Portland cement (OPC) ChemicalDensity: Composition 3.12 (%)g/cm3, Blaine: 3500 cm2/g MaterialsPortland blast-furnace slag cement (PSC) Density: 3.05 g/cm3, Blaine: 4000 cm2/g SiO2 Al2O3 Fe2O3 CaO MgO Na2OK2O SO3 Loss on Ignition α-calcium sulfate hemihydrate (αHH) Density: 2.72 g/cm3, Blaine: 1400 cm2/g OPC 20.70 6.20 3.10 62.20 2.80 0.10 0.84 2.10 1.96 3 PSC 27.11sand 9.84 (S) 1.88 52.66 3.40Density: 0.31 2.50 g/cm 0.66, Absorption 2.43 ratio: 1.711.00% αHH 2.57 0.88 0.41 39.99 0.32 - - 55.79 0.04 Table 2. Chemical compositions of materials used in the experiment.

Figure2 shows the particle size distributionChemical of α CompositionHH measured (%) using a Mastersizer 2000 particle Materials size analyzer (MalvernSiO2 InstrumentsAl2O3 Fe2O Ltd.,3 CaO Malvern, MgO UK). Na The2O D 50Kof2O the SOαHH3 usedLoss in o then Ignition experiment was 34.6OPCµm, while20.70 the D6.2090 was 3.10 62.8 µm.62.20 The XRD2.80 patterns0.10 of0.84 the αHH2.10 are displayed1.96 in Figure3. It was observedPSC 27.11 that the diffraction9.84 1.88 pattern 52.66 of αHH3.40 includes 0.31 most0.66 of the2.43 calcium sulfate1.71 hemihydrate peaks.αHH Figure 4 shows2.57 a SEM0.88 micrograph0.41 39.99 of α HH.0.32 - - 55.79 0.04

18 α-calcium sulfate hemihydrate 120

Volume 15 Cumulative volume 100

12 80

9 60

Volume, % Volume, 6 40

Cumulative volume, % volume, Cumulative 3 20

0 0 0.1 1 10 100 1000 Particle size, μm

Figure 2. Particle size distribution of αHH.αHH.

Materials 2019, 12, x FOR PEER REVIEW 4 of 13 Materials 2019, 12, 163 4 of 13

Materials 2019,, 12,, xx FORFOR PEERPEER10,000 REVIEWREVIEWH : calcium sulfate hemihydrate 4 of 13

10,000 H : calcium sulfate hemihydrate 10,000 8,000

8,000 8,000 6,000 H H H Sample of α-calcium 6,000 4,000 H sulfate hemihydrate H H H

Intensity, Counts H H H Sample of α-calcium H H H H H H H sulfate hemihydrateH 4,000 2,000 HH H HH H HsulfateH hemihydrateH H H H H H

Intensity, Counts H

Intensity, Counts H H H H H H H H 2,000 0 H H H HHH H H H H 2,000 H H HH H H H HH H H H 5 10 15H 20 25 30 35 40H 45 50 55 60H 65 H70 75 Angle, 2-Theta° 0 5 10 15 Figure20 253. X-ray30 diffraction35 40 patterns45 50 of55 αHH.60 65 70 75 Angle, 2-Theta°

FigureFigure 3. 3. XX-ray--rayray diffractiondiffraction diffraction patternspatterns ofof αHH.αHH.αHH.

Figure 4. Scanning electron microscope micrograph of αHH.

2.2. Experimental Plan The experimentalFigureFigure plan4. 4. ScanningScanning of this electron electron study microscope microscope is summarized micrograph micrograph in Table of of αHH.α HH. 3. To evaluate the setting, compressive strength, and shrinkage strain of αHH-replaced cement mortar, the type of cement and 2.2.2.2. Experimental Experimentalthe αHH replacement Plan Plan ratio were selected as the experimental variables. The experiment was conducted using αHH replacement ratios of 0, 10, 20, and 30 wt % in OPC, as well as 0 and 10 wt % TheThe experimental experimental plan plan of ofthis this study study is summarized is summarized in Table in3. ToTable evaluate 3. To the evaluate setting, compressive the setting, in PSC. The setting time and compressive strength were measured along with the drying shrinkage compressivestrength, and strength, shrinkage and strain shrinkage of αHH-replaced strain of αHH cement--replacedreplaced mortar, cementcement the mortar,mortar, type of thethe cement typetype ofof and cementcement the α andandHH to assess the shrinkage strain. In addition, XRD and QXRD were used to analyze the crystalline thereplacement αHH replacement ratio were selected ratio were as the selected experimental as the variables. experimental The experiment variables. The was experimentconducted using was the αHHstructure, replacement while SEM ratio micrographs were selected were used as the to analyze experimental the microstructure. variables. TheThe mortar experiment was mixed was conductedαHH replacement using αHH ratios replacement of 0, 10, 20, ratios and 30 of wt 0, %10, in 20, OPC, and as 30 well wt % as in 0 andOPC, 10 as wt well % in as PSC. 0 and The 10 setting wt % conductedaccording using to αHH the Test replacement Methods forratios the ofDetermination 0, 10, 20, and of 30 Strength wt % in in OPC, Cement as well(ISO as679) 0 andwith 10 a waterwt % - intime PSC. and The compressive setting time strength and compressive were measured strength along were with measured the drying along shrinkage with to the assess drying the shrinkage shrinkage in PSC.to -Thebinder setting weight time ratio and (W/B) compressive of 0.5 and strength a binder -wereto-sand measured weight ratio along (B:S) with of the1:3. drying shrinkage totostrain. assess assess In the the addition, shrinkage shrinkage XRD strain. strain. and In InQXRD addition, addition, were XRD XRD used and and to analyze QXRD QXRD were werethe crystalline used used to to analyze analyze structure, the the while crystalline crystalline SEM structure,micrographs while were SEM used micrographs to analyze were the microstructure. usedTable to 3. analyzeExperimental The the mortar microstructure.plan. was mixed The according mortar w toas the mixed Test accordingMethods forto the Test Determination Methods for of the Strength Determination in Cement of (ISO Strength 679) with in Cement a water-to-binder (ISO 679) with weight a water ratio-- Experimental Variables and Level to(W/B)-binder ofSpecimen 0.5weight and ratioaID binder-to-sand (W/B) of 0.5 weightand a binder ratio (B:S)-to-sand of 1:3. weight ratio (B:S)Evaluation of 1:3. Items to-binder weight ratio (W/B)Cement of Type0.5 and Rep.a binder Ratio- toof- αHHsand weight ratio (B:S) of 1:3. OPC - ▪ Setting time Table 3. Experimental plan. OPC-αHH10 Table 3. Experimental10 plan.▪ Compressive strength OPC OPC-αHH20 20 ▪ Drying shrinkage ExperimentalExperimental Variables Variables and and Level Level SpecimenSpecimenOPC IDID- IDαHH30 30 ▪ X-ray diffractionEvaluationEvaluation analysis ItemsItems Items Cement Type Rep. Ratio of αHHα PSC CementCement Type Type Rep. Rep. Ratio Ratio of- αHH of HH ▪ Quantitative X-ray diffraction analysis OPC PSC - ▪ Setting time OOPCPCPSC -αHH10 - 10- ▪ ▪Setting SettingScanning time time electron microscope micrograph OOPC-PC--αHH10αHH10 10 10 ▪▪ CompressiveCompressive strength strength OPCOPC OOPC-PC--αHH20αHH20 20 20 ▪▪ DryingDrying shrinkage shrinkage OOPC-PC--αHH30αHH30 30 30 ▪▪ X--rayrayX-ray diffractiondiffraction diffraction analysisanalysis analysis PSC - ▪ Quantitative X-ray diffraction analysis PSC PSC -- ▪ Quantitative X--rayray diffractiondiffraction analysisanalysis PSC-αHH10PSC 10 Scanning electron microscope micrograph PSC --αHH10 10 ▪▪ Scanning electron microscope micrograph

Materials 2019, 12, 163 5 of 13

2.3. Methods The setting time was measured according to the Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance (ASTM C 403/C 403M), and regression analysis was used to determine the initial setting time and ﬁnal setting time as the times when the penetration resistances were 3.5 MPa and 27.6 MPa, respectively. The Standard Test Method for Compressive Strength of Hydraulic-Cement Mortars (ASTM C 349) was used to measure the compressive strength. Three specimens were fabricated for each age, each with dimensions of 40 × 40 × 160 mm3. The specimens were water cured at 20 ± 3 ◦C. To measure the compressive strength of each specimen, a dry surface was created by wiping away moisture and removing debris, and a mortar jig was used at ages of 1, 3, 7, and 28 days. A universal testing machine with a capacity of 2000 kN was used. Each specimen used for the measurement of the drying shrinkage was fabricated with dimensions of 100 × 100 × 400 mm3 and an embedded dual ﬂange strain gauge. After demolding, the strains were measured using a data logger in a room with constant temperature and humidity of 20 ± 3 ◦C and 60 ± 3%, respectively. To analyze the crystalline structure of each specimen, XRD analysis (D/MAX-2200 Ultima/PC, Rigaku International Corporation, Tokyo, Japan) and QXRD were carried out at ages of 3 and 28 days by collecting each sample, processing it into powder, carrying out pretreatment, and subsequently taking measurements. XRD analysis was carried out using a Cu target and a D8 Advance diffractometer (Bruker-AXS, Karlsruhe, Germany) equipped with a LynxEye position-sensitive detector. The diffraction patterns were obtained at 2θ from 5◦ to 65◦ in steps of 0.01◦, with each step lasting 1 s. The divergence slit was 0.3◦, while the soller slit was 2.5◦. The microstructure of each specimen was observed using SEM (JSM-6380, Jeol Ltd., Tokyo, Japan) at ages of 3 and 28 days by coating each specimen with platinum and applying an accelerating voltage of 15 kV. The crystalline structure and microstructure analyses were carried out on the OPC specimen, as well as the OPC-αHH20 and PSC-αHH10 specimens, which had the least strains due to drying shrinkage.

3. Experimental Results and Discussion

3.1. Setting Time The penetration resistance measurement results of cement mortar with αHH are shown in Figure5, and the setting times obtained from regression analysis are presented in Table4. When OPC was used, the setting time decreased with increasing replacement ratios of αHH. The lowest final setting time occurred in the OPC-αHH30 specimen, and unlike the trends observed in the initial setting time, the final setting times of the OPC-αHH10 and OPC-αHH20 specimens appeared to be longer than that of the OPC specimen. The initial setting time of the OPC-αHH30 specimen, which had an αHH replacement of 30 wt %, was 6 min, which is significantly lower than that of the OPC specimen without αHH replacement, while the final setting time was 350 min, thus confirming the trend of increasing time between the initial setting and the final setting. The coefficient of determination of the OPC-αHH30 specimen was calculated to be 0.7818, and the goodness of fit derived from regression analysis was relatively lower than those of other specimens. This trend is believed to be due to quick setting owing to excessive replacement of αHH. In the case of the PSC specimen, the initial setting time and the final setting time were longer than those of the OPC specimen due to the latent hydraulic property of the GGBS. The PSC-αHH10 specimen, which had an αHH replacement of 10 wt %, had a significantly lower initial setting time and final setting time compared to those of the PSC specimen. However, the PSC-αHH10 specimen had an initial setting time similar to that of the OPC specimen, while its final setting time was found to be longer. Materials 2019, 12, 163 6 of 13

Meanwhile, the coefficient of determination was 0.8853 due to delayed setting of the PSC specimen becauseMaterials 2019 of the, 12, latentx FOR PEER hydraulic REVIEW property of the GGBS, while the goodness of fit was relatively lower6 of 13 than that of the OPC specimen. However, the coefficient of determination of the PSC-αHH10 specimen wasαHH10 0.9727, specimen indicating was that 0.9727, the reduced indicating setting that time the reduced achieved setting by mixing timeα HH achieved resulted by in mixing a goodness αHH ofresulted fit that in was a goodness greater than of fit that that of was the greater PSC specimen. than that of the PSC specimen. At an ααHHHH replacement ratio of 10 wt % in cement mortar, setting occurs faster in OPC than in PSC. However, the rate of reductionreduction ofof thethe settingsetting time waswas higher when ααHHHH was substitutedsubstituted intointo PSC. These results indicate that ααHHHH affected the activation of the hydration reaction more in PSC than in OPC.

50 - W/B=0.5, B:S=1:3 50 - W/B=0.5, B:S=1:3 50 - W/B=0.5, B:S=1:3 - OPC - OPC-αHH10 - OPC-αHH20

2 40 2 40 2 40 R = 0.9497 R = 0.9605 R = 0.9789

30 Final setting 30 30

20 20 20

10 10 10

Penetration resistance, MPa resistance, Penetration MPa resistance, Penetration MPa resistance, Penetration Initial setting

0 0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Elapsed time, min. Elapsed time, min. Elapsed time, min.

50 - W/B=0.5, B:S=1:3 50 - W/B=0.5, B:S=1:3 50 - W/B=0.5, B:S=1:3 - OPC-αHH30 - PSC - PSC-αHH10

40 R2 = 0.7818 40 40 R2 = 0.8853 R2 = 0.9727

30 30 30

20 20 20

10 10 10

Penetration resistance, MPa resistance, Penetration MPa resistance, Penetration MPa resistance, Penetration

0 0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Elapsed time, min. Elapsed time, min. Elapsed time, min.

Figure 5. Penetration resistance measurement results of ααHHHH-replaced-replaced cement mortar.

Table 4. Setting times of αHH-replaced cement mortar. Table 4. Setting times of αHH-replaced cement mortar. Initial Setting Time Final Setting Time R2, Coefﬁcient of Specimen ID R2, Coefficient of Specimen ID Initial Setting Time(min) (min) Final Setting(min) Time (min) Determination Determination OPC 287 412 0.9605 OPCOPC- αHH10287 272 426412 0.97890.9605 OPC-αHH10OPC- αHH20272 137 455426 0.94970.9789 OPC-αHH20OPC- αHH30137 6 350455 0.78180.9497 OPC-αHH30PSC 6 352 521350 0.88530.7818 α PSC PSC- HH10352 284 493521 0.97270.8853 PSC-αHH10 284 493 0.9727

3.2. Compressive Strength The compressive strength measurement results of cement mortar with αHH are shown in Figure 6. The compressive strength of the cement mortar decreased with increasing replacement ratios of αHH. When OPC was used, the compressive strengths of the OPC-αHH10, OPC-αHH20, and OPC- αHH30 specimens at the age of 1 day were 10.43 MPa, 10.39 MPa, and 9.96 MPa, respectively, which

Materials 2019, 12, x FOR PEER REVIEW 7 of 13 are approximately 57–59% that of the OPC specimen (17.62 MPa). At 28 days, the compressive strengths of the OPC-αHH10, OPC-αHH20, and OPC-αHH30 specimens were 22.31 MPa, 18.79 MPa, and 17.81 MPa, respectively, which are approximately 44–55% that of the OPC specimen (40.88 MPa). When PSC was used, the compressive strength of the PSC specimen was 14.54 MPa at the age of 1Materials day and2019 36.51, 12, 163 MPa at the age of 28 days. The compressive strength of the PSC-αHH10 specimen7 of 13 was measured to be 8.56 MPa—the lowest observed compressive strength at an early age in this study. These results indicate that αHH did not have a significant effect as a stimulant for the hydration of 3.2. Compressive Strength PSC at an early age. However, the compressive strength of the PSC-αHH10 specimen at the age of 28 days Thewas compressive27.90 MPa, which strength is measurementapproximately results 68% that of cement of the mortarOPC specimen. with αHH Therefore, are shown among in Figure the6. cementThe compressive mortars with strength αHH ofreplacements, the cement mortar the PSC decreased-αHH10 specimen, with increasing which replacement used PSC, was ratios confirmed of αHH. toWhen be most OPC favorable was used, for the the compressive development strengths of the compressive of the OPC-α strength.HH10, OPC- αHH20, and OPC-αHH30 specimensOverall, at replacement the age of 1 with day wereαHH 10.43resulted MPa, in 10.39a decrease MPa, in and the 9.96 compressive MPa, respectively, strength in which cement are mortar,approximately which was57–59% also that reported of the OPCin a previous specimen study (17.62 [1 MPa).6]. Meanwhile, At 28 days, a thedecrease compressive in compressive strengths strengthof the OPC- owingαHH10, to the OPC-use ofα HH20,only αHH and was OPC- notαHH30 observed specimens within the were αHH 22.31 replacement MPa, 18.79 ratio MPa, range and used17.81 in MPa, the respectively,current experiment. which are approximately 44–55% that of the OPC specimen (40.88 MPa).

50 - W/B=0.5, B:S=1:3 - water curing (20±3°C)

40 OPC PSC

30 PSC-αHH10

OPC-αHH10

20 OPC-αHH20

OPC-αHH30 10

Compressive strength, MPa Compressivestrength,

0 0 5 10 15 20 25 30 35 Age, days

FigureFigure 6. 6. CompressiveCompressive strength strength measurement measurement results results of of αHHαHH-replaced-replaced cement cement mortar. mortar.

3.3. DryingWhen Shrinkage PSC was used, the compressive strength of the PSC specimen was 14.54 MPa at the age of 1 day and 36.51 MPa at the age of 28 days. The compressive strength of the PSC-αHH10 specimen was measuredFigure 7 shows to be the 8.56 drying MPa—the shrinkage lowest measurement observed compressive results of strengthcement mortar at an early with ageαHH. in thisThe study.OPC andThese PSC results specimens indicate that that didαHH not did have not αHH have replacementsa signiﬁcant effect showed as a drastic stimulant increases for the in hydration shrinkage of strainsPSC at at an their early early age. ages, However, with thethe compressiveslope of the c strengthurve decreasing of the PSC- withαHH10 time. specimenHowever, atwhen the ageαHH of was28 days substituted was 27.90 in MPa,, the which drastic is approximately increase in the 68% shrinkage that of the strain OPC observed specimen. in Therefore, the OPC among and PSC the specimenscement mortars did not with occur.αHH This replacements, can be explained the PSC- α byHH10 the growth specimen, pressure which usedof the PSC, acicular was conﬁrmed ettringite crystalsto be most formed favorable from formixing the development with αHH, which of the increases compressive the strength.distance between the cement particles or hydrates,Overall, resulting replacement in expansion with αHH strains resulted in the in early a decrease ages. in the compressive strength in cement mortar,When which OPC was was also used reported with αHH in a replacement, previous study the [strain16]. Meanwhile, curve appeared a decrease to be linear in compressive up to the agestrength of approximately owing to the 40 use days, of only withα theHH slope was gradually not observed decreasing within afterward. the αHH replacement In particular, ratio the OPC range- αHH20used in specimen the current displayed experiment. expansion behavior at an early age. The shrinkage strain at the early age of the αHH-replaced cement mortar was relatively less than that of cement mortar but increased with progressing3.3. Drying Shrinkageage, while some specimens even displayed shrinkage strains that were greater than that of cement mortar. Figure7 shows the drying shrinkage measurement results of cement mortar with αHH. The OPC The PSC-αHH10 specimen, which had αHH replacement in PSC, showed expansion behavior at and PSC specimens that did not have αHH replacements showed drastic increases in shrinkage strains its early age, as can be observed in the OPC-αHH20 specimen. The PSC-αHH10 specimen showed a at their early ages, with the slope of the curve decreasing with time. However, when αHH was more drastic increase in shrinkage strain than did the OPC-αHH10, OPC-αHH20, and OPC-αHH30 substituted in, the drastic increase in the shrinkage strain observed in the OPC and PSC specimens did specimens; however, the slope of the strain curve decreased with age, resulting in the least shrinkage not occur. This can be explained by the growth pressure of the acicular ettringite crystals formed from strain in the current experiment. Therefore, it was determined that replacing αHH in PSC was more mixing with αHH, which increases the distance between the cement particles or hydrates, resulting in effective for suppressing the shrinkage strain than replacing αHH in OPC. expansion strains in the early ages. When OPC was used with αHH replacement, the strain curve appeared to be linear up to the age of approximately 40 days, with the slope gradually decreasing afterward. In particular, the OPC-αHH20 specimen displayed expansion behavior at an early age. The shrinkage strain at the early age of the αHH-replaced cement mortar was relatively less than that of cement mortar but increased Materials 2019, 12, 163 8 of 13 with progressing age, while some specimens even displayed shrinkage strains that were greater than that of cement mortar. The PSC-αHH10 specimen, which had αHH replacement in PSC, showed expansion behavior at its early age, as can be observed in the OPC-αHH20 specimen. The PSC-αHH10 specimen showed a more drastic increase in shrinkage strain than did the OPC-αHH10, OPC-αHH20, and OPC-αHH30 specimens; however, the slope of the strain curve decreased with age, resulting in the least shrinkage strain in the current experiment. Therefore, it was determined that replacing αHH in PSC was more effective for suppressing the shrinkage strain than replacing αHH in OPC. Materials 2019, 12, x FOR PEER REVIEW 8 of 13

20 OPC-αHH20 0 -20 OPC-αHH30 -40 200 - W/B=0.5, B:S=1:3 OPC-αHH10 -60 PSC-αHH10 -80 0 OPC -100 OPC PSC -120 -200 0 1 2 3 4 5 6 7

-6 -400 OPC-αHH30

PSC -600 PSC-αHH10 OPC-αHH20

Strain, ×10 Strain, -800 OPC OPC-αHH10 OPC-αHH20 -1000 OPC-αHH30 PSC OPC-αHH10 PSC-αHH10 -1200 0 20 40 60 80 100 120 140 160 180 Age, days

FigureFigure 7. 7. DryingDrying shrinkage shrinkage measurement measurement results results of of αHHαHH-replaced-replaced cement cement mortar. mortar. 3.4. X-ray Diffraction and Quantitative X-ray Diffraction Analysis 3.4. X-ray Diffraction and Quantitative X-ray Diffraction Analysis Figure8 shows the XRD analysis results of the OPC, OPC- αHH20, and PSC-αHH10 specimens at Figure 8 shows the XRD analysis results of the OPC, OPC-αHH20, and PSC-αHH10 specimens the ages of 3 and 28 days. Quartz peaks with high intensities were observed due to the mixing of fine at the ages of 3 and 28 days. Quartz peaks with high intensities were observed due to the mixing of aggregate, and similar diffraction patterns were observed in the specimens, including the occurrence fine aggregate, and similar diffraction patterns were observed in the specimens, including the of portlandite and ettringite. The intensity of the gypsum peak in the αHH-replaced OPC-αHH20 occurrence of portlandite and ettringite. The intensity of the gypsum peak in the αHH-replaced OPC- specimen was higher than those of the other specimens, and it increased with progressing age. αHH20 specimen was higher than those of the other specimens, and it increased with progressing Table5 presents the QXRD results of the OPC, OPC- αHH20, and PSC-αHH10 specimens at the age. ages of 3 and 28 days. As mentioned above, the OPC, OPC-αHH20, and PSC-αHH10 specimens all Table 5 presents the QXRD results of the OPC, OPC-αHH20, and PSC-αHH10 specimens at the contained 83–86% quartz at the age of 3 days owing to the mixing of fine aggregate, and the quartz ages of 3 and 28 days. As mentioned above, the OPC, OPC-αHH20, and PSC-αHH10 specimens all contents gradually decreased with progressing age. On the other hand, the αHH-replaced OPC-αHH20 contained 83–86% quartz at the age of 3 days owing to the mixing of fine aggregate, and the quartz and PSC-αHH10 specimens showed increasing ettringite contents compared to the OPC specimen. contents gradually decreased with progressing age. On the other hand, the αHH-replaced OPC- In particular, it was observed that the PSC-αHH10 had the highest ettringite content. Overall, the αHH20 and PSC-αHH10 specimens showed increasing ettringite contents compared to the OPC ettringite contents decreased with time, being less at the age of 28 days than at the age of 3 days. It is specimen. In particular, it was observed that the PSC-αHH10 had the highest ettringite content. clear that replacement with αHH and the subsequent drastic production of ettringite at an early age Overall, the ettringite contents decreased with time, being less at the age of 28 days than at the age of accelerated the hydration reaction and decreased the setting time; however, the residual ettringite that 3 days. It is clear that replacement with αHH and the subsequent drastic production of ettringite at was not converted to monosulfate, even with progressing time, appeared to have negatively impacted an early age accelerated the hydration reaction and decreased the setting time; however, the residual the development of the compressive strength. ettringite that was not converted to monosulfate, even with progressing time, appeared to have As mentioned in the introduction, the growth pressure of the acicular ettringite crystals formed negatively impacted the development of the compressive strength. from the reactions between C A and αHH increases the distance between the cement particles or As mentioned in the introduction,3 the growth pressure of the acicular ettringite crystals formed hydrates, resulting in expansion strains early in the aging process. In other words, replacement from the reactions between C3A and αHH increases the distance between the cement particles or with αHH can suppress the shrinkage strain in cement mortar, and this effect is believed to be at a hydrates, resulting in expansion strains early in the aging process. In other words, replacement with maximum at the early age of the material. However, as confirmed from the QXRD results, the ettringite αHH can suppress the shrinkage strain in cement mortar, and this effect is believed to be at a content decreases with age, which diminishes the effects of suppressing the shrinkage strain. Therefore, maximum at the early age of the material. However, as confirmed from the QXRD results, the ettringite content decreases with age, which diminishes the effects of suppressing the shrinkage strain. Therefore, as shown in Figure 7, even with replacement with αHH, the effect of suppressing the shrinkage strain was not very significant with age progression. Consequently, when αHH was substituted into PSC, the production of ettringite was higher than in OPC; thus, the suppression of the shrinkage strain was more effective.

Materials 2019, 12, 163 9 of 13 as shown in Figure7, even with replacement with αHH, the effect of suppressing the shrinkage strain was not very signiﬁcant with age progression. Consequently, when αHH was substituted into PSC, the production of ettringite was higher than in OPC; thus, the suppression of the shrinkage strain was moreMaterials effective. 2019, 12, x FOR PEER REVIEW 9 of 13

10,000 - W/B=0.5, B:S=1:3 Q C3S : C3S monoclinic - OPC C2S : C2S beta Q Q : Quartz 8,000 C : Calcite Q P : Portlandite G : Gypsum 6,000 E : Ettringite

C2S Q Q Q Q C S 3 Q Q Q 4,000 E P C G E G P P Q Q 28 days

Intensity, Counts

2,000 3 days

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 (a) Angle, 2-Theta°

10,000 - W/B=0.5, B:S=1:3 Q C3S : C3S monoclinic - OPC-αHH20 C2S : C2S beta Q Q : Quartz 8,000 C : Calcite P : Portlandite G : Gypsum 6,000 E : Ettringite Q Q C S Q P 2 Q Q C S G 3 Q Q Q 4,000 E C E G P P Q Q 28 days

Intensity, Counts

2,000 3 days

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 (b) Angle, 2-Theta°

10,000 - W/B=0.5, B:S=1:3 Q C3S : C3S monoclinic - PSC-αHH10 C2S : C2S beta Q : Quartz 8,000 Q C : Calcite Q P : Portlandite G : Gypsum 6,000 E : Ettringite Q C2S Q Q C S 3 Q Q Q Q 4,000 E G P E C G P Q Q P 28 days

Intensity, Counts

2,000 3 days

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 (c) Angle, 2-Theta°

Figure 8.8. X-rayX-ray diffraction analysis ofof ((aa)) OPC,OPC, ((bb)) OPC-OPC-ααHH20HH20,, andand ((cc)) PSC-PSC-ααHH10HH10 specimens.specimens.

Materials 2019, 12, x FOR PEER REVIEW 10 of 13 Materials 2019, 12, 163 10 of 13

Table 5. Quantitative X-ray diffraction analysis of OPC, OPC-αHH20, and PSC-αHH10 specimens. Table 5. Quantitative X-ray diffraction analysis of OPC, OPC-αHH20, and PSC-αHH10 specimens. OPC OPC-αHH20 PSC-αHH10 Phases (%) 3 DaysOPC 28 Days 3 Days OPC- 28αHH20 Days 3 Days PSC- 28α DaysHH10 Phases (%) C3S monoclinic 3 Days2.36 283.05 Days 2.44 3 Days 2.30 28 Days 1.41 3Days 1.82 28 Days C2S beta 4.60 5.76 0.88 3.09 2.36 5.82 C3S monoclinic 2.36 3.05 2.44 2.30 1.41 1.82 3 C2CS betaA cubic 4.60- 0.02 5.76 0.88- 0.13 3.09 - 2.36- 5.82 C3AC Na3A cubicorthorhombic 0.55 - 0.02- 1.85 - 1.46 0.13 1.11 -1.13 - C3A Na orthorhombicC4AF 0.551.75 2.95 - 2.70 1.85 3.16 1.46 1.84 1.11 1.90 1.13 CP4ericlaseAF 1.750.38 0.42 2.95 0.27 2.70 0.88 3.16 0.26 1.84 0.30 1.90 Periclase 0.38 0.42 0.27 0.88 0.26 0.30 LimeLime 0.120.12 - - - -0.02 0.02 0.02 0.02 - - ArcaniteArcanite 0.350.35 0.30 0.30 0.39 0.39 1.03 1.03 0.12 0.12 0.36 0.36 GypsumGypsum 0.440.44 0.61 0.61 0.07 0.07 1.47 1.47 0.76 0.76 0.48 0.48 BassaniteBassanite -- 0.09 0.09 0.09 0.09 - -0.12 0.12 0.12 0.12 Calcite 0.27 2.58 0.85 3.73 0.55 3.07 QuartzCalcite 83.850.27 2.58 80.38 0.85 86.08 3.73 79.12 0.55 88.22 3.07 80.26 PortlanditeQuartz 83.85 3.03 80.38 2.10 86.08 1.07 79.12 0.93 88.22 1.14 80.26 0.59 EttringitePortlandite 2.303.03 2.10 1.74 1.07 3.30 0.93 2.67 1.14 5.10 0.59 4.15 Ettringite 2.30 1.74 3.30 2.67 5.10 4.15 3.5. Scanning Electron Microscope Micrographs 3.5. Scanning Electron Microscope Micrographs Figure9 shows SEM micrographs of the OPC, OPC- αHH20, and PSC-αHH10 specimens at the age ofFigure 3 days. 9 shows Compared SEM micrographs to the OPC specimen,of the OPC, the OPCαHH-replaced-αHH20, and OPC- PSC-ααHH10HH20 andspecimens PSC-α HH10at the agespecimens of 3 days. had Compared wider distributions to the OPC of specimen, acicular ettringite the αHH crystals.-replaced In OPC general,-αHH20 ettringite and PSC causes-αHH10 the specimensinternal structure had wider to become distributions dilatated of when acicular widely ettring distributedite crystals. within In the general, matrix and ettringite creates causes a negative the internaleffect of structuredrastically to decreasing become dilatated the compressive when widely strength distributed when αHH within is substituted the matrix into and the creates cement. a negativeMoreover, effect as previously of drastically mentioned, decreasing the the growth compressive pressure strength of the ettringite when αHH acicular is substituted crystals increases into the cementthe distance. Moreo betweenver, as thepreviously cement mentioned, particles or hydrates;the growth thus, pressure replacement of the ettringite with αHH acicular is effective crystals for increasessuppressing the the distance shrinkage between strain the early cement in the particles aging process. or hydrates; thus, replacement with αHH is effective for suppressing the shrinkage strain early in the aging process. (a) (b)

(c)

FigureFigure 9. 9. ScanningScanning electron electron microscope microscope micrographs micrographs of ( a) OPC, OPC, ( (bb)) OPC OPC--αHH20αHH20,, and and ( (cc)) PSC PSC--αHH10αHH10 specimensspecimens at at age age of 3 3 days.

Materials 2019, 12, 163 11 of 13

Materials 2019, 12, x FOR PEER REVIEW 11 of 13 Meanwhile, as shown in Figure 10, ettringite was found in the αHH-replaced OPC-αHH20 at the ageMeanwhile, of 28 days, as similar shown to in the Figure observation 10, ettringite at the agewas offound 3 days, in butthe theαHH effect-replaced on the OPC suppression-αHH20 ofat the shrinkageage of 28 days, strain similar was not to asthe significant observation as thatat the during age of the 3 days, early but age. the However, effect on the the ettringite suppression found of inthe the shrinkageαHH-replaced strain was PSC- notα HH10as significant specimen as that was during significantly the early coarser age. However, than that inthe the ettringite OPC-α HH20found specimen,in the αHH which-replaced has aPSC significantly-αHH10 specimen higher suppression was significantly effect on coarser the shrinkage than that strain in the compared OPC-αHH20 to the OPC-specimen,αHH20 which specimen. has a significantly higher suppression effect on the shrinkage strain compared to the OPC-αHH20 specimen. (a) (b)

(c)

Figure 10. 10. ScanningScanning electron electron microscope microscope micrographs micrographs of ( a of) OPC, (a) OPC, (b) OPC- (b) α OPCHH20,-αHH20 and (,c )and PSC- (cα)HH10 PSC- specimensαHH10 specimens at age of at 28 age days. of 28 days. 4. Conclusions 4. Conclusions This study evaluated the effects of αHH, which is characterized by a quick initial setting time and This study evaluated the effects of αHH, which is characterized by a quick initial setting time expansion strain that occurs during its early age, on the setting, compressive strength, and shrinkage and expansion strain that occurs during its early age, on the setting, compressive strength, and strain of cement mortar, and the following conclusions were made. shrinkage strain of cement mortar, and the following conclusions were made. (1) The substitution of αHH into cement mortar decreases the initial setting time: the initial (1) The substitution of αHH into cement mortar decreases the initial setting time: the initial setting time clearly decreased with increasing replacement ratios of αHH. Meanwhile, when αHH was setting time clearly decreased with increasing replacement ratios of αHH. Meanwhile, when αHH substituted in, the setting time of OPC was lower than that of PSC; however, the rate of decrease of was substituted in, the setting time of OPC was lower than that of PSC; however, the rate of decrease the setting time of PSC was higher. This is due to the greater effect of αHH on the activation of the of the setting time of PSC was higher. This is due to the greater effect of αHH on the activation of the hydration reaction in PSC than in OPC. hydration reaction in PSC than in OPC. (2) The compressive strength of cement mortar drastically decreased when αHH was substituted (2) The compressive strength of cement mortar drastically decreased when αHH was substituted in and further decreased with increasing replacement ratios of αHH. It is clear that the substitution of in and further decreased with increasing replacement ratios of αHH. It is clear that the substitution αHH and the subsequent signiﬁcant production of ettringite at an early age accelerated the hydration of αHH and the subsequent significant production of ettringite at an early age accelerated the reaction; however, the residual ettringite that was not converted to monosulfate, even with progressing hydration reaction; however, the residual ettringite that was not converted to monosulfate, even with time, appeared to have negatively impacted the development of the compressive strength. Furthermore, progressing time, appeared to have negatively impacted the development of the compressive the development of the compressive strength was more favorable when substituting αHH into PSC strength. Furthermore, the development of the compressive strength was more favorable when than into OPC. substituting αHH into PSC than into OPC. (3) It was conﬁrmed that expansion at the early age of αHH can decrease the shrinkage strain early (3) It was confirmed that expansion at the early age of αHH can decrease the shrinkage strain in the aging of cement mortar. This is due to the growth pressure of the ettringite crystals formed from early in the aging of cement mortar. This is due to the growth pressure of the ettringite crystals mixing with αHH. Meanwhile, the effect of substituting in αHH and suppressing the shrinkage strain formed from mixing with αHH. Meanwhile, the effect of substituting in αHH and suppressing the shrinkage strain in cement mortar was at a maximum at the early age of the material, and the

Materials 2019, 12, 163 12 of 13 in cement mortar was at a maximum at the early age of the material, and the production of ettringite was higher in PSC than in OPC; therefore, the suppression of the shrinkage strain was more effective.

Author Contributions: Conceptualization, B.L.; Formal analysis, B.L.; Funding acquisition, G.K. (Gyuyong Kim); Investigation, K.L., G.K. (Gyeongtae Kim), S.L. and K.S.; Project administration, G.K. (Gyuyong Kim) and J.N.; Resources, K.S.; Supervision, G.K. (Gyuyong Kim), J.N. and T.K.; Validation, T.K.; Writing—original draft, B.L.; Writing—review & editing, B.L., K.L., G.K. (Gyeongtae Kim) and S.L. Funding: This research received no external funding. Acknowledgments: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2015R1A5A1037548). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2018R1A6A3A01011913). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Hand, R.J. Calcium sulphate hydrates: A review. Br. Ceram. Trans. 1997, 96, 116–120. 2. Deutsch, Y.; Nathan, Y.; Sarig, S. Thermogravimetric evaluation of the kinetics of the gypsum-hemihydrate- soluble anhydrite transitions. J. Therm. Anal. Calorim. 1994, 42, 159–174. [CrossRef] 3. Amathieu, L.; Boistelle, R. Crystallization kinetics of gypsum from dense suspension of hemihydrate in water. J. Cryst. Growth 1988, 88, 183–192. [CrossRef]

4. Rinaudo, C.; Boistelle, R. Gypsum grown under pressure from dense suspension of CaSO4.0.67H2O. J. Appl. Crystallogr. 1991, 24, 129–134. [CrossRef] 5. Badens, E.; Veesler, S.; Boistelle, R. Crystallization of gypsum from hemihydrate in presence of additives. J. Cryst. Growth 1999, 198, 704–709. [CrossRef] 6. Singh, N.B.; Middendorf, B. Calcium sulphate hemihydrate hydration leading to gypsum crystallization. Prog. Cryst. Growth Charact. Mater. 2007, 53, 57–77. [CrossRef] 7. Lea, F.M. The Chemistry of Cement and Concrete, 3rd ed.; Edward Arnold: London, UK, 1970. 8. Jiang, G.; Wang, H.; Chen, Q.; Zhang, X.; Wu, Z.; Guan, B. Preparation of alpha-calcium sulfate hemihydrate

from FGD gypsum in chloride-free Ca(NO3)2 solution under mild conditions. Fuel 2016, 174, 235–241. [CrossRef] 9. Guan, B.; Yang, L.; Wu, Z.; Shen, Z.; Ma, X.; Ye, Q. Preparation of α-calcium sulfate hemihydrate from FGD

gypsum in K, Mg-containing concentrated CaCl2 solution under mild conditions. Fuel 2009, 88, 1286–1293. [CrossRef] 10. Guan, B.; Yang, L.; Fu, H.; Kong, B.; Li, T.; Yang, L. α-calcium sulfate hemihydrate preparation from FGD gypsum in recycling mixed salt solutions. Chem. Eng. J. 2011, 174, 296–303. [CrossRef] 11. Miao, M.; Feng, X.; Wang, G.; Cao, S.; Shi, W.; Shi, L. Direct transformation of FGD gypsum to calcium sulfate hemihydrate whiskers: Preparation, simulations, and process analysis. Particuology 2015, 19, 53–59. [CrossRef] 12. Tang, M.; Shen, X.; Huang, H. Inﬂuence of α-calcium sulfate hemihydrate particle characteristics on the performance of calcium sulfate-based medical materials. Mater. Sci. Eng. C 2010, 30, 1107–1111. [CrossRef] 13. Guan, B.; Ye, Q.; Wu, Z.; Lou, W.; Yang, L. Analysis of the relationship between particle size distribution of α-calcium sulfate hemihydrate and compressive strength of set plaster—Using grey model. Powder Technol. 2010, 200, 136–143. [CrossRef] 14. Ye, Q.; Guan, B.; Lou, W.; Yang, L.; Kong, B. Effect of particle size distribution on the hydration and compressive strength development of α-calcium sulfate hemihydrate paste. Powder Technol. 2011, 207, 208–214. [CrossRef] 15. Guan, B.; Ye, Q.; Zhang, J.; Lou, W.; Wu, Z. Interaction between α-calcium sulfate hemihydrate and superplasticizer from the point of adsorption characteristics, hydration and hardening process. Cem. Concr. Res. 2010, 40, 253–259. [CrossRef] 16. Lee, K.H.; Yang, K.H. Development of a neutral cementitious material to promote vegetation concrete. Constr. Build. Mater. 2016, 127, 442–449. [CrossRef] 17. Lewry, A.J.; Williamson, J. The setting of gypsum plaster. J. Mater. Sci. 1994, 29, 5524–5528. [CrossRef] Materials 2019, 12, 163 13 of 13

18. Chappuis, J. A new model for a better understanding of the cohesion of hardened hydraulic materials. Colloids Surf. A Physicochem. Eng. Asp. 1999, 156, 223–241. [CrossRef] 19. Reynaud, P.; Saâdaoui, M.; Meille, S.; Fantozzi, G. Water effect on internal friction of set plaster. Mater. Sci. Eng. A 2006, 442, 500–503. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).