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Citation: Dong, Y.; Pei, L.; Fu, J.;

Yang, Y.; Liu, T.; Liang, H.; Yang, H.

Investigating the Mechanical

Properties and Durability of

Metakaolin-Incorporated Mortar by

Different Curing Methods. Materials

2022, 15, 2035. https://doi.org/

10.3390/ma15062035

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Article

Investigating the Mechanical Properties and Durability ofMetakaolin-Incorporated Mortar by Different Curing MethodsYudong Dong 1 , Lianjun Pei 2, Jindong Fu 2, Yalong Yang 3, Tong Liu 1, Huihui Liang 1 and Hongjian Yang 1,*

1 School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China;[email protected] (Y.D.); [email protected] (T.L.); [email protected] (H.L.)

2 Tianjin Energy Investment Group Co., Ltd., Tianjin 300050, China; [email protected] (L.P.);[email protected] (J.F.)

3 Tianjin Cheng An Thermal Power Co., Ltd., Tianjin 300161, China; [email protected]* Correspondence: [email protected]

Abstract: In this paper, the traditional, silicate-based Portland cement (PC) was employed as thecontrol to explore the impact of adding varying amounts of metakaolin (MK) on the mechanicalproperties of cement mortar. In fact, as a mineral admixture, metakaolin (MK) has the ability tosignificantly improve the early strength and sulfate resistance of cement mortar in traditional, silicate-based Portland cement (PC). In addition to this, the performance of Portland cement mortar is greatlyaffected by the curing mode. The previous research mainly stays in the intermittent curing andalkaline excitation mode, and there are few studies on the influence of relatively humidity on it.Moreover, the paper investigated the impact of four different curing methods about humidity on themechanical properties and sulfate resistance. The results show that the best content of metakaolin inPortland cement is 10% (M10), and the best curing method is 95% humidity in the first three daysfollowed by 60% humidity in the later period (3#). Based on previous literature that suggests thatadding MK thickens water film layer on the surface of mortar, the mechanism of MK increasing theearly strength of cement was analyzed. The compressive strength of the Portland cement containing10% MK (M10) after 1 day curing is 3.18 times that of pristine PC mortar, and is comparable if PC iscured for three days under the same curing conditions. The traditional PC mortar is highly dependenton the wet curing time, and normally requires a curing time of at least seven days. However, theincorporation of MK can greatly reduce the sensitivity of Portland cement to water; MK cementmortar with only three days wet curing (3#M10) can reach 49.12 MPa after 28 days, which cangreatly shorten the otherwise lengthy wet curing time. Lastly, the cement specimens with MK alsodemonstrated excellent resistance against sulfate corrosion. The work will provide a strong theoreticalbasis for the early demolding of cement products in construction projects. At the same time, thisstudy can also provide a theoretical reference for the construction of climate drought and saline landareas, which has great reference value.

Keywords: cement; early strength; wet curing time; metakaolin; durability; sulfate erosion

1. Introduction

Ordinary Portland cement with its stable performance, less pollution, and high yieldcharacteristics has been the most important and the most widely cement used in thebuilding materials market. However, ordinary Portland cement also has the shortcomingsof low early strength and late strength, but most of the previous studies only focus onthe improvement of Portland cement late strength, rather than the early strength change.However, the early strength of ordinary Portland cement is a very important mechanicalproperty index of cement products, which determines the demolding time, productionefficiency and production cost of cement products. The early hydration process (in thefirst 7 days) of traditional Portland cement is highly dependent on water, and the required

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Materials 2022, 15, 2035 2 of 15

wet curing time is very lengthy [1,2]. In the drier areas of the north of China, especiallyin the northwestern of China region, the dry climate and large variation in the salineland elevates the currently high standard for early wet curing time of buildings and thesulfate resistance [3]. Therefore, it is necessary to find a suitable modifier to maintain themechanical properties of Portland cement in a low humidity environment. At the sametime, because the hydration process of Portland cement is highly dependent on water, it isalso very important to explore the influence of different curing methods on it.

Metakaolin (MK) is a category of mineral admixtures that possess a high pozzolanicactivity. It is primarily comprised of SiO2 and Al2O3, which can react with the cementhydration product, Ca(OH)2, to form the corresponding hydrated calcium silicate gel [4–7],and thereby enhance the compressive strength of the resulting compound. In addition, MKpossesses a strong micro-aggregation effect and nucleation effect, provides more nucleationsites and accelerates reaction rates [8]. The small particle size of MK can easily fill the voidgaps within cement mortar, and thereby, increases its density, which further improves themechanical properties of the mortar compound [9–11]. Furthermore, the small particlesize of MK enables a large specific surface area and water absorption capacity, which willincrease the homogeneity, yet, reduce the free water content within the mortar system [12].However, the water film on the mortar surface will thicken, which in turn, increases thelubrication between some interfaces, increasing the reaction rate and extent, and furtherincreasing the early strength of cement mortar [6,12,13]. Many scientists globally haveconfirmed that MK can improve the durability of cement mortar to large degrees [14–16].This improvement can be attributed to the significant depletion of Ca(OH)2 by SiO2 andAl2O3 in MK, which results in a weaker alkaline environment and improves the sulfateresistance [17,18]. In addition, saline land contains a large number of sulfate groups, thus,MK incorporation is crucial and significant especially in mortar that are mixed with salineland. In this paper, a Na2SO4 solution with a mass fraction of 5% was introduced as anerosion solvent to survey the sulfate resistance of the mortar specimen. The change in themass and compressive strength were employed to gauge the resistance of the specimens tosulfate corrosion.

There are indeed some previous studies on metakaolin improving the performanceof Portland cement, but there are not many studies on metakaolin cement mortar underdifferent curing conditions. However, there have been many previous studies on the effectof different curing methods on the properties of cement mortars. For example, Amr I.I.H.used the intermittent curing method to product the class F fly ash-based geopolymer. Theresult showed that the intermittent curing scheme at 70 ◦C for 4 steps for continuous 6 h ofheat curing in each step followed by 18 h of ambient temperature proved to improve thegeopolymer mortar compressive strength at the end of each curing step with no adverseeffect on the strength [19]. Hilal E.H. used the curing method of alkali-activated materialsto produce concrete. Results showed that the optimum curing regime for alkali-activatedblended concrete mixtures made with 0 and 25% fly ash was a combination of water andsubsequent air curing, while that for mixes made with 50% fly ash was continuous watercuring [20]. Athira et al. studied the influence of ambient, heat, water, and other curingmethods on the performance of slag, fly ash and a few other precursors based on alkali-activated binders and showed that a trend of reduction in the compressive strength iswitnessed with the increase in the replacement levels of alternative precursor materialswith slag/fly ash under heat curing, water curing, and ambient curing [21]. The previousresearch mainly stays in the intermittent curing and alkaline excitation mode, and there arefew studies on the influence of relative humidity on it. Therefore, the paper investigatedthe impact of four different curing methods about humidity on the mechanical propertiesand sulfate resistance.

However, most previous studies were carried out under standard curing conditionsand did not take into account the arid, low-humidity environment of actual production.Therefore, it is of practical significance to explore the effect of dry environment on cementmortar samples. At the same time, under the condition of dry and low humidity, it is

Materials 2022, 15, 2035 3 of 15

more meaningful to study the influence of different curing methods on the properties ofmetakaolin cement mortar in practical production.

2. Materials and Methods2.1. Raw Materials

The ordinary Portland cement (PO 42.5) used in this research was purchased fromXinyue Brand of Tangshan Tianlu Cement Co., Ltd. (Tangshan, China) Metakaolin wasoriginated from the place of Inner Mongolia (China). The chemical composition of the rawmaterials is summarized in Table 1. The physical properties of the raw materials are shownin Figure 1. The chemical composition and particle size distribution were measured byX-ray fluorescence (XRF, ARLQUANT X, ThermoFisher Scientific, Shanghai, China) andlaser particle size analyzer (LSPA, Anton paar LitesizerTM500, Malvern, Worcs, UK). Fromthe Figure 1, the median particle size (D50) of PC and MK are 178.25 µm and 1.589 µm,respectively. At the same time, the BET surface area of PC and MK are 2.6659 m2/g and229.1912 m2/g, respectively. After the ordinary river sand was dried, it was passed througha 40-mesh sieve to obtain treated river sand. The physical properties of river sand aresummarized in Table 2. The water and superplasticizer needed for the experiment waslaboratory tap water and polycarboxylate, respectively.

Table 1. Chemical compositions of Portland cement and Metakaolin/%.

Material CaO SiO2 Fe2O3 Al2O3 MgO SO3 TiO2 K2O Others LOI

Cement 49.06 26.56 2.58 10.37 6.33 1.80 1.53 0.828 0.942 1.62MK 0.283 58.05 0.845 38.15 - - 2.17 0.224 0.278 0.40

Table 2. Physical properties of river sand.

Raw Material Water Absorption/% Specific Gravity Fineness Modulus Unit wt% (kg/m3)

River Sand 0.95 2.58 2.70 1588

Materials 2022, 15, x FOR PEER REVIEW 3 of 16

meaningful to study the influence of different curing methods on the properties of me-takaolin cement mortar in practical production.

2. Materials and Methods 2.1. Raw Materials

The ordinary Portland cement (PO 42.5) used in this research was purchased from Xinyue Brand of Tangshan Tianlu Cement Co., Ltd. (Tangshan, China) Metakaolin was originated from the place of Inner Mongolia (China). The chemical composition of the raw materials is summarized in Table 1. The physical properties of the raw materials are shown in Figure 1. The chemical composition and particle size distribution were meas-ured by X-ray fluorescence (XRF, ARLQUANT X, ThermoFisher Scientific, Shanghai, China) and laser particle size analyzer (LSPA, Anton paar LitesizerTM500, Malvern, Worcs, UK). From the Figure 1, the median particle size (D50) of PC and MK are 178.25 μm and 1.589 μm, respectively. At the same time, the BET surface area of PC and MK are 2.6659 m2/g and 229.1912 m2/g, respectively. After the ordinary river sand was dried, it was passed through a 40-mesh sieve to obtain treated river sand. The physical properties of river sand are summarized in Table 2. The water and superplasticizer needed for the experiment was laboratory tap water and polycarboxylate, respectively.

Table 1. Chemical compositions of Portland cement and Metakaolin/%.

Material CaO SiO2 Fe2O3 Al2O3 MgO SO3 TiO2 K2O Others LOI Cement 49.06 26.56 2.58 10.37 6.33 1.80 1.53 0.828 0.942 1.62

MK 0.283 58.05 0.845 38.15 - - 2.17 0.224 0.278 0.40

Table 2. Physical properties of river sand.

Raw Material Water Absorption/% Specific Gravity Fineness Modulus Unit wt% (kg/m3)

River Sand 0.95 2.58 2.70 1588

0.01 0.1 1 10 100 1000

0

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Median particle size: MK1.589μm PC178.25μm

(a)

Figure 1. Cont.

Materials 2022, 15, 2035 4 of 15Materials 2022, 15, x FOR PEER REVIEW 4 of 16

0.0 0.2 0.4 0.6 0.8 1.0

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(b) (c)

Figure 1. Physical properties of PC and MK. (a) Particle size distribution of PC and MK, (b,c) Ni-trogen isotherms of PC and MK.

2.2. Specimen Preparation The cement chosen for casting the test specimens was designed for a 28-day compres-

sive strength of about 43.7 MPa. The mix proportions were 0.45:1:1.5 (water: cement: sand by weight). The cement mortar mixer was used to uniformly stir the precursor materials, which were then sealed in 40 mm × 40 mm × 40 mm molds for natural curing for 24 h. Metakaolin was integrated at 0, 5, 10, 15, and 20 wt%, which were denoted as PC, M5, M10,

M15, and M20, respectively. Superplasticizers were added to the mixtures of each ratio. Due to the small particle size of MK, the fluidity of the mortar decreased with the increase of the replacement amount of MK. In order to ensure the workability of the mortar, it is necessary to gradually increase the amount of superplasticizer. The different amount of superplasticizer depends on that amount required to achieve the standard consistency for cement mortar sample [22]. Moreover, three blocks were prepared as replicates at each composition. Each block was prepared as summarized in Table 3.

Table 3. Proportions of mortar mixture.

Mix Samples W/b Cement (kg/m3)

MK (kg/m3)

Sand (kg/m3)

Water (kg/m3)

Superplasticizer (kg/m3)

1 PC 0.45 595.18 0 892.76 267.84 5.95 2 M5 0.45 565.42 29.76 889.83 267.84 7.14 3 M10 0.45 535.66 59.52 886.90 267.84 8.33 4 M15 0.45 505.9 89.28 883.96 267.84 9.52 5 M20 0.45 476.14 119.04 881.03 267.84 10.71

This paper focuses on addressing the aspect: To explore the influence of metakaolin on the mechanical properties (especially the intensity of the first 3 day and 28 day) and sulfate resistance of Portland cement mortar in each group under different curing meth-ods to provide a feasibility reference for the maintenance of cement products and the tim-ing of mold removal in engineering. Most previous studies were carried out under stand-ard curing conditions and did not take into account the arid, low-humidity environment of actual production. This paper focuses on the effect of different curing methods on the properties of metakaolin-incorporated cement mortar under a dry, natural environment. In the dry areas of northwest China, the relative humidity of the air in the natural envi-ronment is generally around 60%. Therefore, a combined curing method is adopted in this paper, that is, cured in the high humidity incubator in the early stage, and then cured in

Figure 1. Physical properties of PC and MK. (a) Particle size distribution of PC and MK, (b,c) Nitrogenisotherms of PC and MK.

2.2. Specimen Preparation

The cement chosen for casting the test specimens was designed for a 28-day compres-sive strength of about 43.7 MPa. The mix proportions were 0.45:1:1.5 (water: cement: sandby weight). The cement mortar mixer was used to uniformly stir the precursor materials,which were then sealed in 40 mm × 40 mm × 40 mm molds for natural curing for 24 h.Metakaolin was integrated at 0, 5, 10, 15, and 20 wt%, which were denoted as PC, M5, M10,M15, and M20, respectively. Superplasticizers were added to the mixtures of each ratio.Due to the small particle size of MK, the fluidity of the mortar decreased with the increaseof the replacement amount of MK. In order to ensure the workability of the mortar, it isnecessary to gradually increase the amount of superplasticizer. The different amount ofsuperplasticizer depends on that amount required to achieve the standard consistency forcement mortar sample [22]. Moreover, three blocks were prepared as replicates at eachcomposition. Each block was prepared as summarized in Table 3.

Table 3. Proportions of mortar mixture.

Mix Samples W/b Cement(kg/m3)

MK(kg/m3)

Sand(kg/m3)

Water(kg/m3)

Superplasticizer(kg/m3)

1 PC 0.45 595.18 0 892.76 267.84 5.952 M5 0.45 565.42 29.76 889.83 267.84 7.143 M10 0.45 535.66 59.52 886.90 267.84 8.334 M15 0.45 505.9 89.28 883.96 267.84 9.525 M20 0.45 476.14 119.04 881.03 267.84 10.71

This paper focuses on addressing the aspect: To explore the influence of metakaolinon the mechanical properties (especially the intensity of the first 3 day and 28 day) andsulfate resistance of Portland cement mortar in each group under different curing methodsto provide a feasibility reference for the maintenance of cement products and the timingof mold removal in engineering. Most previous studies were carried out under standardcuring conditions and did not take into account the arid, low-humidity environment ofactual production. This paper focuses on the effect of different curing methods on theproperties of metakaolin-incorporated cement mortar under a dry, natural environment. Inthe dry areas of northwest China, the relative humidity of the air in the natural environment

Materials 2022, 15, 2035 5 of 15

is generally around 60%. Therefore, a combined curing method is adopted in this paper,that is, cured in the high humidity incubator in the early stage, and then cured in the lowhumidity (60%) drying incubator. Therefore, this paper proposes four different curingmethods (all at 20 ◦C): 0# (humidity 60%); 1# (Standard Curing, humidity 95%); 2# (thefirst 7 days: humidity 95% followed by humidity 60% for remaining days); 3# (the first3 days: humidity 95% followed by humidity 60% for remaining days). This paper soughtto explore the performance of the above five groups of samples under each curing method.Three samples were selected from each group of samples for various performance tests,and the average value of the three values was taken as the final test result.

The specimens of each group were cured to the corresponding time, while the com-pressive strength of each group was measured. After 28 d curing, the durability of mortarspecimens in each group was investigated by immersing a (5 wt%) Na2SO4 solution for thecorresponding erosion durations (28 day, 90 day, 120 day, 180 day).

2.3. Test Methods

After each group of specimens were cured for the corresponding duration, theywere characterized according to the national standard GB/17671-1999 for the compressivestrength test [23]. The compressive strength of each specimen was assessed on a pressuretesting machine (CMT6104, Sans, Shenzhen, China) with a maximum load of 300 kN and aloading speed of 2.4 kN/s.

The specimens were cured in a constant temperature and humidity incubator for28 days according to the corresponding curing method above. Three samples were directlyused to test the compressive strength after 28 days of curing, and the other three sampleswere used to test the resistance to sulfate attack. After removal from the incubator, themass before erosion was measured (m1), and then the samples were placed into a (5 wt%)Na2SO4 solution. During this period, the erosion solution was replaced every 15 daysuntil the corresponding term was reached. Subsequently, the specimen was displaced ontoambient conditions for half a day, and the resulting mass was recorded (m2). The mass lossrate on a mortar block was calculated according to Equation (1). After the mass loss rate ofthe sample is tested, the compressive strength of the samples at the corresponding erosionage can be tested directly, so that the mass loss rate and compressive strength at differenterosion ages can be obtained.

∆m =m2 − m1

m1× 100% (1)

To determine the crystal phase composition of the hydration products, X-ray diffractionanalysis (XRD, Bruker D8 Discover, Karlsruhe, Germany) with Cu Kα radiation wasemployed to appraise each specimen with Cu Kα radiation. This included: setting scanningstep 0.019◦, scanning rate 0.2 s/step, and scanning range 5◦~70◦.

The microstructure within each specimen was examined by the field-emission scanningelectron microscope (SEM). The central section of each sample was extracted and analyzedafter grinding and gold-spraying. The working voltage used in the field-emission SEM(Nova Nano SEM450, FEI company, Hillsboro, OR, USA) was 10.00 kV.

3. Results and Discussion3.1. Effects of Four Curing Methods on the Mechanical Properties of Mortar

Figure 2 displays the compressive strength of each cement after aging for variousdurations under different curing methods. Figure 2a,b are the compressive strength data ofhydration 1 day, 2 day and 3 day for each sample under different curing methods. It can beseen from the figure that metakaolin-incorporated specimens with 10% metakaolin (M10)have the highest compressive strength under the same curing period and curing methods.It can be seen from Figure 2a that under the curing method of 0# (60% humidity), the com-pressive strength of cement specimen M10 in 1 day can reach 15.61 MPa, which is 3.18 timesthat of ordinary Portland cement under the same curing method. The compressive strength

Materials 2022, 15, 2035 6 of 15

is close to that of Portland cement for 3 days under the same curing condition. Similarly,the compressive strength of 3# M10 metakaolin-incorporated specimens after curing for3 days is close to that of ordinary Portland cement under the same curing method (3# PC)for seven days (conformed by Figure 2c). It can be seen that metakaolin can significantlyimprove the early strength of ordinary Portland cement [24,25]. Figure 2a,b shows thatthe compressive strength of the 1#PC, 2#PC and 3#PC specimens in the first three days issignificantly higher than that of the 0#PC specimen at the same age. As shown in Figure 2d,the 28 day compressive strength of PC specimen of 1#, 2# and 3#, was 42.37 MPa, 45.90 MPaand 43.08 MPa, respectively. Compared with that of the fully drying specimen (0#), thecompressive strength of 1#PC, 2#PC and 3#PC specimens increased by 19.3%, 29.2% and21.2%, respectively. This wholly confirms that ordinary Portland cement is highly sensitiveto water, and the importance of early wet curing of Portland cement [8,10,13]. On the con-trary, for the metakaolin-incorporated specimens under the four different curing methods,the early strength of the specimens has little difference, indicating that the addition ofmetakaolin can significantly reduce the sensitivity of silicate cement to water. Even in a dryenvironment, metakaolin cement can obtain high compressive strength. Figure 2 showsthat the optimum dosage of metakaolin is 10%, and the M10 specimens in each group showthe best mechanical properties. From the Figure 2d, we can see that the best curing methodis 3# and the 28-day compressive strength of 3# M10 specimen can reach 49.12 MPa, whichis about 1.16 times of that of ordinary Portland cement specimen under the best curingmethod (1# PC).


condition. Similarly, the compressive strength of 3# M10 metakaolin-incorporated speci-mens after curing for 3 days is close to that of ordinary Portland cement under the same curing method (3# PC) for seven days (conformed by Figure 2c). It can be seen that me-takaolin can significantly improve the early strength of ordinary Portland cement [24,25]. Figure 2a,b shows that the compressive strength of the 1#PC, 2#PC and 3#PC specimens in the first three days is significantly higher than that of the 0#PC specimen at the same age. As shown in Figure 2d, the 28 day compressive strength of PC specimen of 1#, 2# and 3#, was 42.37 MPa, 45.90 MPa and 43.08 MPa, respectively. Compared with that of the fully drying specimen (0#), the compressive strength of 1#PC, 2#PC and 3#PC specimens increased by 19.3%, 29.2% and 21.2%, respectively. This wholly confirms that ordinary Portland cement is highly sensitive to water, and the importance of early wet curing of Portland cement [8,10,13]. On the contrary, for the metakaolin-incorporated specimens under the four different curing methods, the early strength of the specimens has little dif-ference, indicating that the addition of metakaolin can significantly reduce the sensitivity of silicate cement to water. Even in a dry environment, metakaolin cement can obtain high compressive strength. Figure 2 shows that the optimum dosage of metakaolin is 10%, and the M10 specimens in each group show the best mechanical properties. From the Figure 2d, we can see that the best curing method is 3# and the 28-day compressive strength of 3# M10 specimen can reach 49.12 MPa, which is about 1.16 times of that of ordinary Port-land cement specimen under the best curing method (1# PC).

1d 2d 3d0

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Figure 2. The compressive strength of cement after aging for 1, 2, 3, 7 and 28 days under different curing methods; (a) 1, 2, 3 days of 0#, (b) 1,2,3 days of 1#, 2# and 3#, (c) 7 days of 0#, 1#, 2# and 3#, (d) 28 days of 0#, 1#, 2# and 3#.

Figure 2. The compressive strength of cement after aging for 1, 2, 3, 7 and 28 days under differentcuring methods; (a) 1, 2, 3 days of 0#, (b) 1, 2, 3 days of 1#, 2# and 3#, (c) 7 days of 0#, 1#, 2# and 3#,(d) 28 days of 0#, 1#, 2# and 3#.

Materials 2022, 15, 2035 7 of 15

In construction engineering, the dual advantage of early demolding includes short-ening the curing cycle of the workpiece and improving the productivity in practical ap-plication. From the result above, for ordinary Portland cement, the compressive strengthshould reach at least 14 day to stabilize, while for M10, the compressive strength after7 day is sufficiently similar to the final strength. According to CB/T 70-2009 [26], the stentcan only be removed when the sample is cured for 24 h at 20 ◦C; however, for sampleM10, compressive strength requirement was achieved in less than one day, which wouldsignificantly shorten the time to demold each workpiece and greatly quicken constructionprojects that require cement.

From the particle size analysis of metakaolin (MK) and PC in Figure 1a, the particlesize of MK was considerably smaller than that of ordinary Portland cement. Figure 1b,cshows the nitrogen adsorption and desorption isotherms, and the specific surface diagramof PC and MK. The specific surface area of MK is approximately 96 times larger than thatof PC. Figure 3 depicts the filling effect of MK; MK increases the excess slurry above thecement mortar, which actually increases the compactness of the mortar [27]. Furthermore,the water film on the particle surface also thickens, which lubricates the particles, and thus,accelerates the reaction rate and extent, and improves the early strength. Due to the fillingeffect contributed by the small MK particle size, the cracks that typically form in cementspecimens are largely filled, which improves the density and compressive strength of theworkpiece. Moreover, the high pozzolanic activity enables the large number of Al2O3 andSiO2 within MK to react with Ca(OH)2 to generate a fibrous calcium silicate hydrate (C–S–H)gel, which further improves the compressive strength of the specimen [28,29]. However, ifmetakaolin exceeds 10% substitution, the compressive strength of the specimen insteaddeclines substantially. This behavior can be attributed to the fact that if the substitution rateis excessive, the content of silicate within the system would be too low. This also translatesto the content of both the generated Ca(OH)2 and the calcium silicate hydrate (C–S–H)gel, which is the primary contributor of the strength phase, would be insignificant. Lastly,the combination of a small particle size, large specific surface area, high water absorption,and high substitution rate of MK will increase the consistency, reduce free water contentand extent of hydration, and ultimately weaken the mortar. The filling effect from thesmall particle size would not suffice to compensate for the strength loss caused by theincomplete hydration.


In construction engineering, the dual advantage of early demolding includes short-ening the curing cycle of the workpiece and improving the productivity in practical ap-plication. From the result above, for ordinary Portland cement, the compressive strength should reach at least 14 day to stabilize, while for M10, the compressive strength after 7 day is sufficiently similar to the final strength. According to CB/T 70-2009 [26], the stent can only be removed when the sample is cured for 24 h at 20 °C; however, for sample M10, compressive strength requirement was achieved in less than one day, which would sig-nificantly shorten the time to demold each workpiece and greatly quicken construction projects that require cement.

From the particle size analysis of metakaolin (MK) and PC in Figure 1a, the particle size of MK was considerably smaller than that of ordinary Portland cement. Figure 1b,c shows the nitrogen adsorption and desorption isotherms, and the specific surface diagram of PC and MK. The specific surface area of MK is approximately 96 times larger than that of PC. Figure 3 depicts the filling effect of MK; MK increases the excess slurry above the cement mortar, which actually increases the compactness of the mortar [27]. Furthermore, the water film on the particle surface also thickens, which lubricates the particles, and thus, accelerates the reaction rate and extent, and improves the early strength. Due to the filling effect contributed by the small MK particle size, the cracks that typically form in cement specimens are largely filled, which improves the density and compressive strength of the workpiece. Moreover, the high pozzolanic activity enables the large num-ber of Al2O3 and SiO2 within MK to react with Ca(OH)2 to generate a fibrous calcium sili-cate hydrate (C–S–H) gel, which further improves the compressive strength of the speci-men [28,29]. However, if metakaolin exceeds 10% substitution, the compressive strength of the specimen instead declines substantially. This behavior can be attributed to the fact that if the substitution rate is excessive, the content of silicate within the system would be too low. This also translates to the content of both the generated Ca(OH)2 and the calcium silicate hydrate (C–S–H) gel, which is the primary contributor of the strength phase, would be insignificant. Lastly, the combination of a small particle size, large specific sur-face area, high water absorption, and high substitution rate of MK will increase the con-sistency, reduce free water content and extent of hydration, and ultimately weaken the mortar. The filling effect from the small particle size would not suffice to compensate for the strength loss caused by the incomplete hydration.

Figure 3. Principle Diagram of the Filling Effect of Metakaolin.

Figure 3. Principle Diagram of the Filling Effect of Metakaolin.

3.2. Effect of Four Curing Methods on the Durability of Mortar3.2.1. Compressive Strength after Sulfate Attack under Four Curing Methods

The four figures in Figure 4 above show the changes in compressive strength of eachspecimen after 28, 90, 120 and 180 days of erosion under four different curing methods. Thecompressive strength of the sample after sulfate attack will change with the extension of the

Materials 2022, 15, 2035 8 of 15

erosion period. For those samples with incomplete hydration, loose internal structure andpoor durability, the amplitude and rate of change of the compressive strength will fluctuategreatly. From Figure 4, M10 displayed the greatest compressive strength after 180 daysof erosion under any of the four different curing methods. Moreover, the compressivestrength of M10 exhibited the least fluctuation throughout the eroding period, whichattests to its incredible stability and resistance to erosion and sulfate attacks. For allof the specimens, the compressive strength initially rises, then after a period of time,declines. This can be attributed to the large number of sulfate that initially reacts with thehydration products resulting in swelling and the formation of ettringite and gypsum, whichmoderately compacts the internal structure of the specimen and improves the strength.As erosion progresses, the internal stress caused by the volume expansion, increases.Simultaneously, sulfate calcium silicate gel begins to form, which whittles down the mainstrength phase and gradually form cracks in the specimen. In each curing methods’ groupof specimens, the change range and rate of the strength of PC are the most obvious,indicating that PC is internally looser, possesses wider gaps and can be easily corroded ordamaged [30–34]. Comparatively to PC, the strength of M10 increased and decreased toa lesser extent, which can be ascribed to the denser structure, more complete hydration,and higher sulfate resistance. The erosion is not serious and is not easy to be eroded. Atthe same time, no matter which kind of curing method is used, the M10 shows the highestcompressive strength after 180 days of erosion, and the compressive strength of the sample3#M10 can reach the maximum value of 41.62 MPa. Therefore, the optimal amount ofMK is 10% (M10), when accounting for erosion due to sulfate. Despite dry conditions, theM10 specimens exhibit excellent strength and durability.


3.2. Effect of Four Curing Methods on the Durability of Mortar 3.2.1. Compressive Strength after Sulfate Attack under Four Curing Methods

The four figures in Figure 4 above show the changes in compressive strength of each specimen after 28, 90, 120 and 180 days of erosion under four different curing methods. The compressive strength of the sample after sulfate attack will change with the extension of the erosion period. For those samples with incomplete hydration, loose internal struc-ture and poor durability, the amplitude and rate of change of the compressive strength will fluctuate greatly. From Figure 4, M10 displayed the greatest compressive strength after 180 days of erosion under any of the four different curing methods. Moreover, the compressive strength of M10 exhibited the least fluctuation throughout the eroding pe-riod, which attests to its incredible stability and resistance to erosion and sulfate attacks. For all of the specimens, the compressive strength initially rises, then after a period of time, declines. This can be attributed to the large number of sulfate that initially reacts with the hydration products resulting in swelling and the formation of ettringite and gyp-sum, which moderately compacts the internal structure of the specimen and improves the strength. As erosion progresses, the internal stress caused by the volume expansion, in-creases. Simultaneously, sulfate calcium silicate gel begins to form, which whittles down the main strength phase and gradually form cracks in the specimen. In each curing meth-ods’ group of specimens, the change range and rate of the strength of PC are the most obvious, indicating that PC is internally looser, possesses wider gaps and can be easily corroded or damaged [30–34]. Comparatively to PC, the strength of M10 increased and decreased to a lesser extent, which can be ascribed to the denser structure, more complete hydration, and higher sulfate resistance. The erosion is not serious and is not easy to be eroded. At the same time, no matter which kind of curing method is used, the M10 shows the highest compressive strength after 180 days of erosion, and the compressive strength of the sample 3#M10 can reach the maximum value of 41.62 MPa. Therefore, the optimal amount of MK is 10% (M10), when accounting for erosion due to sulfate. Despite dry con-ditions, the M10 specimens exhibit excellent strength and durability.

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Figure 4. Compressive strength of cement mortar after a different erosion period under different curing methods; (a) 0#, (b) 1#, (c) 2#, (d) 3#.

To see the changes of compressive strength of PC and M10 under various curing con-ditions more clearly and intuitively, we drew Figure 5. Figure 5 reveals that for the PC and M10, the best curing method is 1# and 3#. As can be seen from the Figure 5, for PC and M10 samples, compared with other curing methods, the variation range of compres-sive strength of sample 1#PC and 3#M10 in the whole erosion process fluctuates very little, and the maximum compressive strength reaches 37.98 MPa and 41.61 MPa respectively, after 180 days of erosion. In addition, the compressive strength of ordinary Portland ce-ment samples changed rapidly after erosion, but that of M10 samples change little. Com-paring 3#M10 to 0#M10, the strength increased by a mere 10.6% after 180 days of erosion, which is quite insignificant. In other words, for the M10 sample, the dry curing method (0#) and optimal curing method (3#) show little difference in sulfate erosion resistance. Therefore, adding MK can actually reduce the great dependence of PC towards water in the early stages. Therefore, in the dry areas of northwest China where wet or early wet curing may not be available, the loss of strength due to insufficient early wet curing con-ditions can be largely mitigated by adding MK to PC, optimally at 10%.

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Figure 5. Compressive strength of PC and M10 after a different erosion period under various curing methods; (a) PC, (b) M10.

Figure 4. Compressive strength of cement mortar after a different erosion period under differentcuring methods; (a) 0#, (b) 1#, (c) 2#, (d) 3#.

Materials 2022, 15, 2035 9 of 15

To see the changes of compressive strength of PC and M10 under various curingconditions more clearly and intuitively, we drew Figure 5. Figure 5 reveals that for the PCand M10, the best curing method is 1# and 3#. As can be seen from the Figure 5, for PC andM10 samples, compared with other curing methods, the variation range of compressivestrength of sample 1#PC and 3#M10 in the whole erosion process fluctuates very little, andthe maximum compressive strength reaches 37.98 MPa and 41.61 MPa respectively, after180 days of erosion. In addition, the compressive strength of ordinary Portland cementsamples changed rapidly after erosion, but that of M10 samples change little. Comparing3#M10 to 0#M10, the strength increased by a mere 10.6% after 180 days of erosion, which isquite insignificant. In other words, for the M10 sample, the dry curing method (0#) andoptimal curing method (3#) show little difference in sulfate erosion resistance. Therefore,adding MK can actually reduce the great dependence of PC towards water in the earlystages. Therefore, in the dry areas of northwest China where wet or early wet curing maynot be available, the loss of strength due to insufficient early wet curing conditions can belargely mitigated by adding MK to PC, optimally at 10%.


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Figure 4. Compressive strength of cement mortar after a different erosion period under different curing methods; (a) 0#, (b) 1#, (c) 2#, (d) 3#.

To see the changes of compressive strength of PC and M10 under various curing con-ditions more clearly and intuitively, we drew Figure 5. Figure 5 reveals that for the PC and M10, the best curing method is 1# and 3#. As can be seen from the Figure 5, for PC and M10 samples, compared with other curing methods, the variation range of compres-sive strength of sample 1#PC and 3#M10 in the whole erosion process fluctuates very little, and the maximum compressive strength reaches 37.98 MPa and 41.61 MPa respectively, after 180 days of erosion. In addition, the compressive strength of ordinary Portland ce-ment samples changed rapidly after erosion, but that of M10 samples change little. Com-paring 3#M10 to 0#M10, the strength increased by a mere 10.6% after 180 days of erosion, which is quite insignificant. In other words, for the M10 sample, the dry curing method (0#) and optimal curing method (3#) show little difference in sulfate erosion resistance. Therefore, adding MK can actually reduce the great dependence of PC towards water in the early stages. Therefore, in the dry areas of northwest China where wet or early wet curing may not be available, the loss of strength due to insufficient early wet curing con-ditions can be largely mitigated by adding MK to PC, optimally at 10%.

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Figure 5. Compressive strength of PC and M10 after a different erosion period under various curing methods; (a) PC, (b) M10.

Figure 5. Compressive strength of PC and M10 after a different erosion period under various curingmethods; (a) PC, (b) M10.

3.2.2. Mass Loss Rate after Sulfate Attack under Four Curing Methods

The mass loss ratio of samples under differing curing methods is summarized inFigure 6. From Figure 6, the quality of all specimens under different curing methodsincreases first and then decreases. Briefly, sulfate permeates the specimen and reacts withhydration products to form ettringite and gypsum. Volume expansion from the formationof ettringite and gypsum fills the void and increases the compactness of the specimens.Moreover, the quality of all specimens are increasing, firstly. As the erosion progresses,cracks in the specimens begin to appear and then quality reduction gradually occurs untilthe quality loss is compared with the mass at 28 days. Similar to the compressive strengths,the rate and extent of the mass change of PC are the quickest and largest compared tothe other specimens. However, the MK compound with a substitution rate of 10% (M10)shows the least mass change and the mass loss rate of 3#M10 is only 0.18% after 180 daysof erosion, which attests to its superior stability and corrosion resistance.

To see the mass loss rate of PC and M10 under different curing conditions more clearlyand intuitively, we drew the Figure 7. As shown in Figure 7, the longer the initial wetcuring time, the better the sulfate resistance of the specimen is. The mass loss rate of 1#PCafter 180 days of erosion was 0.57%, while the mass loss rate of 3#M10 was 0.18%, which isclose to 0.14% of 1#M10. Therefore, the 3#M10 specimens showed a strong advantage insulfate attack durability.

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Figure 6. The mass loss rate of cement after a different erosion period under different curing methods;(a) 0#, (b) 1#, (c) 2#, (d) 3#.


which is close to 0.14% of 1#M10. Therefore, the 3#M10 specimens showed a strong ad-vantage in sulfate attack durability.

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Figure 7. Mass loss rate of PC and M10 after a different erosion period under different curing con-ditions; (a) PC, (b) M10.

In order to more clearly see the relationship between the compressive strength and sulfate resistance of the sample, Figure 8 was drawn. As we have explained in the previ-ous discussion, for metakaolin cement mortar, the 3# curing method is the best curing method, so Figure 8 shows the relationship between the compressive strength and the mass loss rate under the 3# curing method. Figure 8a shows the relationship between the initial compressive strength of the sample and the mass loss rate eroded for 180 days. Figure 8b shows the relationship between the compressive strength and the mass loss rate of the sample after 180 days of erosion. We can see that there are very similar changes in the two figures. Both the initial compressive strength and the residual compressive strength are negatively correlated with the mass loss rate. In other words, the greater the compressive strength of the specimens, the smaller the mass loss rate after 180 days of erosion, which indicates that the specimen’s resistance to sulfate attack is better. In the figure, we can also see that the overall performance of the sample M10 is the best.

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Figure 8. Relationship between compressive strength and mass loss rate; (a) Relationship between initial compressive strength and mass loss rate, (b) Relationship between residual strength and mass loss rate.

3.3. X-ray Diffraction (XRD) Analysis

Figure 7. Mass loss rate of PC and M10 after a different erosion period under different curingconditions; (a) PC, (b) M10.

In order to more clearly see the relationship between the compressive strength andsulfate resistance of the sample, Figure 8 was drawn. As we have explained in the previousdiscussion, for metakaolin cement mortar, the 3# curing method is the best curing method,so Figure 8 shows the relationship between the compressive strength and the mass lossrate under the 3# curing method. Figure 8a shows the relationship between the initialcompressive strength of the sample and the mass loss rate eroded for 180 days. Figure 8bshows the relationship between the compressive strength and the mass loss rate of thesample after 180 days of erosion. We can see that there are very similar changes in the two

Materials 2022, 15, 2035 11 of 15

figures. Both the initial compressive strength and the residual compressive strength arenegatively correlated with the mass loss rate. In other words, the greater the compressivestrength of the specimens, the smaller the mass loss rate after 180 days of erosion, whichindicates that the specimen’s resistance to sulfate attack is better. In the figure, we can alsosee that the overall performance of the sample M10 is the best.


which is close to 0.14% of 1#M10. Therefore, the 3#M10 specimens showed a strong ad-vantage in sulfate attack durability.

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Figure 7. Mass loss rate of PC and M10 after a different erosion period under different curing con-ditions; (a) PC, (b) M10.

In order to more clearly see the relationship between the compressive strength and sulfate resistance of the sample, Figure 8 was drawn. As we have explained in the previ-ous discussion, for metakaolin cement mortar, the 3# curing method is the best curing method, so Figure 8 shows the relationship between the compressive strength and the mass loss rate under the 3# curing method. Figure 8a shows the relationship between the initial compressive strength of the sample and the mass loss rate eroded for 180 days. Figure 8b shows the relationship between the compressive strength and the mass loss rate of the sample after 180 days of erosion. We can see that there are very similar changes in the two figures. Both the initial compressive strength and the residual compressive strength are negatively correlated with the mass loss rate. In other words, the greater the compressive strength of the specimens, the smaller the mass loss rate after 180 days of erosion, which indicates that the specimen’s resistance to sulfate attack is better. In the figure, we can also see that the overall performance of the sample M10 is the best.

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Figure 8. Relationship between compressive strength and mass loss rate; (a) Relationship between initial compressive strength and mass loss rate, (b) Relationship between residual strength and mass loss rate.


Figure 8. Relationship between compressive strength and mass loss rate; (a) Relationship betweeninitial compressive strength and mass loss rate, (b) Relationship between residual strength and massloss rate.


Figure 9a presents the XRD spectra of PC mortar specimens under the four differentcuring methods: 0#PC, 1#PC, 2#PC, and 3#PC. Comparing PC under dry curing conditions(0#PC) to PC under standard curing conditions (1#PC), 2#PC and 3#PC, the content ofcalcium hydroxide (C–H), hydrated calcium silicate gel (C–S–H), calcium aluminate hydrate(C4AH13) and ettringite (AFm) are significantly lower. Therefore, for PC specimens, themechanical properties of 0#PC at each curing age are all smaller than those of other groups,which is consistent with the rule obtained above. This shows once again that adequatewet curing time in the early stage of hydration is very necessary. The lesser content can beascribed to 0#PC lacking the recommended pre-wet curing, which resulted in an incompletehydration [35,36]. Moreover, Figure 8b presents the XRD spectra of PC mortar specimensand M10 specimens under different curing methods: 0#PC, 0#M10, 3#M10, and 3#M10erosion. Among them, the sulfate erosion age of the 3#M10 sample was 180 days. 3#M10shows that there are fewer amounts of calcium hydroxide (C–H) and higher amounts oftetracalcium aluminate hydrate (C4AH13), ettringite (AFm) and hydrated calcium silicategel (C–S–H) compared with other specimens 0#PC and 0#M10 displayed in Figure 9b.The result is due to the SiO2 and Al2O3 in MK that react with the hydration product andconsumes the calcium hydroxide (C–H) in the system to generate AFm. However, higheramounts of calcium aluminate hydrate can be attributed to the fact that the hydrationprocess of 3#M10 is more thorough and more calcium aluminate hydrate (C4AH13) andC–S–H are generated than that of 0#M10 and 0#PC. At the same time, ettringite crystals arethe most abundant and calcium hydroxide has few in the sample system after 3#M10 sulfateerosion. This is because sulfate ions enter the sample and react with calcium hydroxide inthe system, consuming calcium hydroxide and producing more ettringite.

Materials 2022, 15, 2035 12 of 15


Figure 9a presents the XRD spectra of PC mortar specimens under the four different curing methods: 0#PC, 1#PC, 2#PC, and 3#PC. Comparing PC under dry curing conditions (0#PC) to PC under standard curing conditions (1#PC), 2#PC and 3#PC, the content of calcium hydroxide (C–H), hydrated calcium silicate gel (C–S–H), calcium aluminate hy-drate (C4AH13) and ettringite (AFm) are significantly lower. Therefore, for PC specimens, the mechanical properties of 0#PC at each curing age are all smaller than those of other groups, which is consistent with the rule obtained above. This shows once again that ad-equate wet curing time in the early stage of hydration is very necessary. The lesser content can be ascribed to 0#PC lacking the recommended pre-wet curing, which resulted in an incomplete hydration [35,36]. Moreover, Figure 8b presents the XRD spectra of PC mortar specimens and M10 specimens under different curing methods: 0#PC, 0#M10, 3#M10, and 3#M10 erosion. Among them, the sulfate erosion age of the 3#M10 sample was 180 days. 3#M10 shows that there are fewer amounts of calcium hydroxide (C–H) and higher amounts of tetracalcium aluminate hydrate (C4AH13), ettringite (AFm) and hydrated cal-cium silicate gel (C–S–H) compared with other specimens 0#PC and 0#M10 displayed in Figure 9b. The result is due to the SiO2 and Al2O3 in MK that react with the hydration product and consumes the calcium hydroxide (C–H) in the system to generate AFm. How-ever, higher amounts of calcium aluminate hydrate can be attributed to the fact that the hydration process of 3#M10 is more thorough and more calcium aluminate hydrate (C4AH13) and C–S–H are generated than that of 0#M10 and 0#PC. At the same time, ettringite crystals are the most abundant and calcium hydroxide has few in the sample system after 3#M10 sulfate erosion. This is because sulfate ions enter the sample and react with calcium hydroxide in the system, consuming calcium hydroxide and producing more ettringite.

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Figure 9. XRD patterns of PC and M10 after curing 28 days under various curing methods; (a) PC (0#,1#,2#,3#), (b) PC (0#) and M10 (0#, 3#).

3.4. SEM Analysis As shown in Figure 10, the 0#M10 specimen is comprised of a small amount of scat-

tered hexagonal, plate-like crystal calcium hydroxide (C–H), acicular prism crystal ettringite (AFm), and a large number of fibrous hydrated calcium silicate gels (C–S–H) [37]. Apparently, the surface is quite compact and without large voids. However, a small number of cracks can be seen. The surface morphology can be assigned to the incomplete hydration of the M10 cement under dry conditions. Figure 11 reveals a number of large macropores, and unreacted raw and clinker particles in 0#PC [38]. The specimen also con-tains a large amount of calcium hydroxide (C–H) and ettringite (AFm) generated by ce-ment hydration. However, the surface does not seem compacted and is dispersed with a number of large cracks. As shown in Figure 12, the structural density of 3#M10 seems

Figure 9. XRD patterns of PC and M10 after curing 28 days under various curing methods; (a) PC(0#, 1#, 2#, 3#), (b) PC (0#) and M10 (0#, 3#).

3.4. SEM Analysis

As shown in Figure 10, the 0#M10 specimen is comprised of a small amount ofscattered hexagonal, plate-like crystal calcium hydroxide (C–H), acicular prism crystalettringite (AFm), and a large number of fibrous hydrated calcium silicate gels (C–S–H) [37].Apparently, the surface is quite compact and without large voids. However, a smallnumber of cracks can be seen. The surface morphology can be assigned to the incompletehydration of the M10 cement under dry conditions. Figure 11 reveals a number of largemacropores, and unreacted raw and clinker particles in 0#PC [38]. The specimen alsocontains a large amount of calcium hydroxide (C–H) and ettringite (AFm) generated bycement hydration. However, the surface does not seem compacted and is dispersed with anumber of large cracks. As shown in Figure 12, the structural density of 3#M10 seems quitehigh and free of large pores. Apparently, there is an abundance of fibrous C–S–H gels and ascarcity of calcium hydroxide crystals on the surface of 3#M10, which can be ascribed tothe reaction between calcium hydroxide, and SiO2 and Al2O3 in MK, to form the C–S–Hgels [15,39,40]. As a result, the content of calcium hydroxide is very low, which attests tothe completeness of the hydration, dense internal structure and very high strength anddurability. In Figure 13, the eroded 3#M10 specimen manifests a loose internal structuredue to erosion, as marked by the many pores and fine cracks. This can be explained by thelarge amount of sulfate that entered the specimen and consumed the calcium hydroxide,which reduces the alkaline environment in the specimen. As we know, proper alkalineenvironment has important resistance to sulfate attack in the Portland cement system.Concurrently, the erosion degenerates the calcium silicate gel which is the main strengthphase in the specimen. Eventually, the volume expansion from the resulting ettringiteand gypsum accumulates internal stress, which produces fine cracks and destroys thecompactness and compressive strength of the specimen.

Materials 2022, 15, 0 12 of 15


Figure 9a presents the XRD spectra of PC mortar specimens under the four different curing methods: 0#PC, 1#PC, 2#PC, and 3#PC. Comparing PC under dry curing conditions (0#PC) to PC under standard curing conditions (1#PC), 2#PC and 3#PC, the content of calcium hydroxide (C–H), hydrated calcium silicate gel (C–S–H), calcium aluminate hy-drate (C4AH13) and ettringite (AFm) are significantly lower. Therefore, for PC specimens, the mechanical properties of 0#PC at each curing age are all smaller than those of other groups, which is consistent with the rule obtained above. This shows once again that ad-equate wet curing time in the early stage of hydration is very necessary. The lesser content can be ascribed to 0#PC lacking the recommended pre-wet curing, which resulted in an incomplete hydration [35,36]. Moreover, Figure 8b presents the XRD spectra of PC mortar specimens and M10 specimens under different curing methods: 0#PC, 0#M10, 3#M10, and 3#M10 erosion. Among them, the sulfate erosion age of the 3#M10 sample was 180 days. 3#M10 shows that there are fewer amounts of calcium hydroxide (C–H) and higher amounts of tetracalcium aluminate hydrate (C4AH13), ettringite (AFm) and hydrated cal-cium silicate gel (C–S–H) compared with other specimens 0#PC and 0#M10 displayed in Figure 9b. The result is due to the SiO2 and Al2O3 in MK that react with the hydration product and consumes the calcium hydroxide (C–H) in the system to generate AFm. How-ever, higher amounts of calcium aluminate hydrate can be attributed to the fact that the hydration process of 3#M10 is more thorough and more calcium aluminate hydrate (C4AH13) and C–S–H are generated than that of 0#M10 and 0#PC. At the same time, ettringite crystals are the most abundant and calcium hydroxide has few in the sample system after 3#M10 sulfate erosion. This is because sulfate ions enter the sample and react with calcium hydroxide in the system, consuming calcium hydroxide and producing more ettringite.

0 10 20 30 40 50 60 70

3#PC

2#PC

0#PC

1#PC

Inte

nsity

/a.u

.

2θ/degree

•

••

♦ ♦▲▲♦

▲-C-S-H ♦-CH •-AFt -quartz-C4AH13

♣

♣-C3A

0 10 20 30 40 50 60 70

♦▲♦•

Inte

nsity

/a.u

.

2 Theta/degree

0#PC

0#M10

3#M10

3#M10 Erosion

▲-C-S-H ♦-CH •-AFt -C4AH13 ♣-C3A -quartz•

▲♣

(a) (b)

Figure 9. XRD patterns of PC and M10 after curing 28 days under various curing methods; (a) PC (0#,1#,2#,3#), (b) PC (0#) and M10 (0#, 3#).

3.4. SEM Analysis As shown in Figure 10, the 0#M10 specimen is comprised of a small amount of scat-

tered hexagonal, plate-like crystal calcium hydroxide (C–H), acicular prism crystal ettringite (AFm), and a large number of fibrous hydrated calcium silicate gels (C–S–H) [37]. Apparently, the surface is quite compact and without large voids. However, a small number of cracks can be seen. The surface morphology can be assigned to the incomplete hydration of the M10 cement under dry conditions. Figure 11 reveals a number of large macropores, and unreacted raw and clinker particles in 0#PC [38]. The specimen also con-tains a large amount of calcium hydroxide (C–H) and ettringite (AFm) generated by ce-ment hydration. However, the surface does not seem compacted and is dispersed with a number of large cracks. As shown in Figure 12, the structural density of 3#M10 seems

Figure 9. XRD patterns of PC and M10 after curing 28 days under various curing methods; (a) PC(0#,1#,2#,3#), (b) PC (0#) and M10 (0#, 3#).

3.4. SEM Analysis

As shown in Figure 10, the 0#M10 specimen is comprised of a small amount ofscattered hexagonal, plate-like crystal calcium hydroxide (C–H), acicular prism crystalettringite (AFm), and a large number of fibrous hydrated calcium silicate gels (C–S–H) [37].Apparently, the surface is quite compact and without large voids. However, a smallnumber of cracks can be seen. The surface morphology can be assigned to the incompletehydration of the M10 cement under dry conditions. Figure 11 reveals a number of largemacropores, and unreacted raw and clinker particles in 0#PC [38]. The specimen alsocontains a large amount of calcium hydroxide (C–H) and ettringite (AFm) generated bycement hydration. However, the surface does not seem compacted and is dispersed with anumber of large cracks. As shown in Figure 12, the structural density of 3#M10 seems quitehigh and free of large pores. Apparently, there is an abundance of fibrous C–S–H gels and ascarcity of calcium hydroxide crystals on the surface of 3#M10, which can be ascribed tothe reaction between calcium hydroxide, and SiO2 and Al2O3 in MK, to form the C–S–Hgels [15,39,40]. As a result, the content of calcium hydroxide is very low, which attests tothe completeness of the hydration, dense internal structure and very high strength anddurability. In Figure 13, the eroded 3#M10 specimen manifests a loose internal structuredue to erosion, as marked by the many pores and fine cracks. This can be explained by thelarge amount of sulfate that entered the specimen and consumed the calcium hydroxide,which reduces the alkaline environment in the specimen. As we know, proper alkalineenvironment has important resistance to sulfate attack in the Portland cement system.Concurrently, the erosion degenerates the calcium silicate gel which is the main strengthphase in the specimen. Eventually, the volume expansion from the resulting ettringiteand gypsum accumulates internal stress, which produces fine cracks and destroys thecompactness and compressive strength of the specimen.


quite high and free of large pores. Apparently, there is an abundance of fibrous C–S–H gels and a scarcity of calcium hydroxide crystals on the surface of 3#M10, which can be ascribed to the reaction between calcium hydroxide, and SiO2 and Al2O3 in MK, to form the C–S–H gels [15,39,40]. As a result, the content of calcium hydroxide is very low, which attests to the completeness of the hydration, dense internal structure and very high strength and durability. In Figure 13, the eroded 3#M10 specimen manifests a loose inter-nal structure due to erosion, as marked by the many pores and fine cracks. This can be explained by the large amount of sulfate that entered the specimen and consumed the calcium hydroxide, which reduces the alkaline environment in the specimen. As we know, proper alkaline environment has important resistance to sulfate attack in the Portland ce-ment system. Concurrently, the erosion degenerates the calcium silicate gel which is the main strength phase in the specimen. Eventually, the volume expansion from the result-ing ettringite and gypsum accumulates internal stress, which produces fine cracks and destroys the compactness and compressive strength of the specimen.

(a) (b)

Figure 10. SEM images for 0#M10; (a) and (b) are SEM images of 0#M10.

(a) (b) (c)

Figure 11. SEM images for 0#PC; (a), (b) and (c) are SEM images of 0#PC.

(a) (b) (c)

Figure 10. SEM images for 0#M10; (a,b) are SEM images of 0#M10.

Figure 10. SEM images for 0#M10.

Materials 2022, 15, 2035 13 of 15Materials 2022, 15, 0 13 of 15



(a) (b)


(a) (b) (c)


(a) (b) (c)

Figure 11. SEM images for 0#PC; (a–c) are SEM images of 0#PC.



(a) (b)


(a) (b) (c)


(a) (b) (c)

Figure 12. SEM images for 3#M10; (a–c) are SEM images of 3#M10.


Figure 12. SEM images for 3#M10; (a), (b) and (c) are SEM images of 3#M10.

(a) (b)

Figure 13. SEM images for 3#M10 erosion; (a) and (b) are SEM images of 3#M10 erosion.

4. Conclusions 1. In this paper, the influence of the addition of metakaolin on mechanical properties

and durability of Portland cement mortar was studied. The results show that: The filling effect of metakaolin (MK) can effectively increase the density of Portland ce-ment (PC). Moreover, the large surface area and strong water absorption of MK will thicken the water film on the surface of the cement, which lubricates the particles, accelerating the reaction speed and extent, and the optimal dosage of metakaolin (MK) is 10%. The sample can significantly improve the compressive strength and du-rability of Portland cement mortar, especially the early strength. The compressive strength (at 28 day) and durability of MK10 are superior to that of the other samples.

2. In this paper, four different curing methods were designed by changing the early wet curing time, and the influences of the four curing methods on each sample were ex-plored. The results showed that: The traditional silicate-based PC is highly depend-ent on the wet curing time in the initial curing stages, and typically requires a wet curing time of at least seven days. However, the incorporation of MK can greatly reduce the dependency of PC to water. Moreover, the compressive strength of MK-incorporated cement (3#M10) that has been wet cured for as short as three days can reach 49.12 MPa after 28 days, which can greatly shorten the curing time that require humid conditions. Therefore, the addition of metakaolin can alleviate the problem of high maintenance cost caused by the long wet curing time of Portland cement prod-ucts in the actual production. Especially in areas with low humidity and when the early wet curing time cannot be guaranteed, the addition of metakaolin has more practical production significance.

3. The optimized amount of MK in cement was found to be at 10% (M10), which demon-strated an excellent, comprehensive performance. Under four different curing meth-ods, the best condition for optimizing compressive strength involved the first three days: humidity 95%, followed by humidity 60% for remaining days (3#). From the microstructure of 3#M10, the surface was densely compacted and the hydration was more thorough. Regardless of the 28 day compressive strength, the sulfate resistance and mass retention rate were outstanding.

Author Contributions: Conceptualization, Y.D., L.P., J.F., Y.Y., T.L., H.L. and H.Y.; Methodology, Y.D., L.P., J.F.,Y.Y., T.L., H.L. and H.Y.; Formal analysis, Y.D., L.P., J.F., Y.Y., T.L., H.L. and H.Y.; Investigation, Y.D.,T.L., H.L. and H.Y.; Resources, H.Y. and L.P., J.F. Writing—original draft prep-aration, T.L. and H.Y.; Writing—review and editing, T.L., L.P., J.F., and H.Y.; Supervision, Y.D., L.P.,

Figure 13. SEM images for 3#M10 erosion; (a,b) are SEM images of 3#M10 erosion.

4. Conclusions

1. In this paper, the influence of the addition of metakaolin on mechanical properties anddurability of Portland cement mortar was studied. The results show that: The fillingeffect of metakaolin (MK) can effectively increase the density of Portland cement (PC).Moreover, the large surface area and strong water absorption of MK will thicken thewater film on the surface of the cement, which lubricates the particles, accelerating thereaction speed and extent, and the optimal dosage of metakaolin (MK) is 10%. Thesample can significantly improve the compressive strength and durability of Portland

Figure 11. SEM images for 0#PC.

Materials 2022, 15, 0 13 of 15



(a) (b)


(a) (b) (c)


(a) (b) (c)




(a) (b)


(a) (b) (c)


(a) (b) (c)




(a) (b)








4. Conclusions


Figure 12. SEM images for 3#M10.

Materials 2022, 15, 0 13 of 15



(a) (b)


(a) (b) (c)


(a) (b) (c)




(a) (b)


(a) (b) (c)


(a) (b) (c)




(a) (b)








4. Conclusions


Figure 13. SEM images for 3#M10 erosion.

4. Conclusions

1. In this paper, the influence of the addition of metakaolin on mechanical properties anddurability of Portland cement mortar was studied. The results show that: The fillingeffect of metakaolin (MK) can effectively increase the density of Portland cement (PC).Moreover, the large surface area and strong water absorption of MK will thicken thewater film on the surface of the cement, which lubricates the particles, accelerating thereaction speed and extent, and the optimal dosage of metakaolin (MK) is 10%. Thesample can significantly improve the compressive strength and durability of Portlandcement mortar, especially the early strength. The compressive strength (at 28 day)and durability of MK10 are superior to that of the other samples.

2. In this paper, four different curing methods were designed by changing the earlywet curing time, and the influences of the four curing methods on each sample

Materials 2022, 15, 2035 14 of 15

were explored. The results showed that: The traditional silicate-based PC is highlydependent on the wet curing time in the initial curing stages, and typically requiresa wet curing time of at least seven days. However, the incorporation of MK cangreatly reduce the dependency of PC to water. Moreover, the compressive strengthof MK-incorporated cement (3#M10) that has been wet cured for as short as threedays can reach 49.12 MPa after 28 days, which can greatly shorten the curing timethat require humid conditions. Therefore, the addition of metakaolin can alleviate theproblem of high maintenance cost caused by the long wet curing time of Portlandcement products in the actual production. Especially in areas with low humidity andwhen the early wet curing time cannot be guaranteed, the addition of metakaolin hasmore practical production significance.

3. The optimized amount of MK in cement was found to be at 10% (M10), which demon-strated an excellent, comprehensive performance. Under four different curing meth-ods, the best condition for optimizing compressive strength involved the first threedays: humidity 95%, followed by humidity 60% for remaining days (3#). From themicrostructure of 3#M10, the surface was densely compacted and the hydration wasmore thorough. Regardless of the 28 day compressive strength, the sulfate resistanceand mass retention rate were outstanding.

Author Contributions: Conceptualization, Y.D., L.P., J.F., Y.Y., T.L., H.L. and H.Y.; Methodology, Y.D.,L.P., J.F.,Y.Y., T.L., H.L. and H.Y.; Formal analysis, Y.D., L.P., J.F., Y.Y., T.L., H.L. and H.Y.; Investigation,Y.D.,T.L., H.L. and H.Y.; Resources, H.Y. and L.P., J.F. Writing—original draft preparation, T.L. andH.Y.; Writing—review and editing, T.L., L.P., J.F. and H.Y.; Supervision, Y.D., L.P., J.F.,Y.Y., T.L., H.L.and H.Y. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the Tianjin science and technology planning project withGrant No. 18YFCZZC00200.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The study did not report any data.

Conflicts of Interest: The authors declare no conflict of interest.

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