Effects of Highly Crystalized Nano C-S-H Particles On

crystals

Article Eﬀects of Highly Crystalized Nano C-S-H Particles on Performances of Portland Cement Paste and Its Mechanism

Yuli Wang 1 , Huijuan Lu 1, Junjie Wang 2,* and Hang He 1 1 School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China; [email protected] (Y.W.); [email protected] (H.L.); [email protected] (H.H.) 2 Department of Civil Engineering, Tsinghua University, Beijing 100084, China * Correspondence: [email protected]

Received: 28 July 2020; Accepted: 7 September 2020; Published: 15 September 2020

Abstract: In order to improve the early age strength of ordinary Portland cement-based materials, many early strength agents were applied in different conditions. Different from previous research, the nano calcium silicate hydrate (C-S-H) particles used in this study were synthesized through the chemical reaction of CaO, SiO2, and H2O under 120 ◦C using the hydrothermal method, and the prepared nano C-S-H particles were highly crystalized. The influences of different amounts of nano C-S-H particles (0%, 0.5%, 1%, 2% and 3% by weight of cement) on the setting time, compressive strength, and hydration heat of cement paste were studied. The hydration products and microstructure of the cement paste with different additions of nano C-S-H particles were investigated through thermogravimetry-differential thermal analysis (TG-DTA), X-ray powder diffraction (XRD), and scanning electron microscope (SEM) tests. The results show that the nano C-S-H particles could be used as an early strength agent, and the early strength of cement paste can be increased by up to 43% through accelerating the hydration of tricalcium silicate (C3S). However, the addition of more than 2% nano C-S-H particles was unfavorable to the later strength development due to more space being left during the initial accelerated hydration process. It is suggested that the suitable content of the nano C-S-H particles is 0.5% 1% by weight of cement. − Keywords: highly crystalized nano C-S-H; early strength agent; setting time; compressive strength; microstructure

1. Introduction Additives, such as rapid setting agents and early strength agents, have been widely used in various applications of cement and concrete in different projects. Early strength agents are needed for the projects that require a rapid development of early age strength—for example, when the environmental temperature is low, and when the quick repairing and reinforcement work using cementitious materials are required [1]. The most frequently used early strength agents in concrete can be divided into inorganic, organic, and complexing agents. The inorganic early strength agents are mainly chloride salts, sulfate salts, and nitrate salts [2]. Hoang-Anh Nguyen et al. [3] reported that the sodium sulfate accelerated the early hydration rate and strength of slag cement paste. They also showed that the pore structure of concrete was refined and modified, and the crack resistance of the concrete at the early age was increased by adding sodium sulfate. P. Carballosa et al. [4] found that the addition of expansive calcium sulfoaluminate agent could shorten the setting time of self-compacting concretes and modify the early age physical properties. Heesup et al. [5] reported that the nitrate salts and nitrite salts could increase

Crystals 2020, 10, 816; doi:10.3390/cryst10090816 www.mdpi.com/journal/crystals Crystals 2020, 10, 816 2 of 17

the early age strength of concrete by accelerating the hydration of C3S and tricalcium aluminate (C3A), and they could also promote the formation of nitrate and nitrite hydrates and the ettringite (AFt) at the early age. Although chloride salts could accelerate the hydration of cement paste, they also bring chloride ions, which can cause the corrosion of steel in concrete [6–10]. Other early strength agents containing potassium and sodium salts could cause the alkali silica reaction [11]. The common organic early strength agents are diethanolamine, triethanolamine, triethanolamine, and some organic salts, such as formate, acetate, etc. Triethanolamine is the most commonly used one. Triethanolamine can accelerate the hydration of C3S in the induction period of cement hydration, and it has a positive impact on the later strength as well as increasing the early strength [12]. However, it is not suitable to use an organic agent such as triethanolamine for steam-curing concrete [13]. Complexing agents instead of a single agent could be used to modify any adverse effects caused by individual agents [14]. For example, Xiao et al. [15] reported that the combination of C6H15O3N, Al2(SO4)3, and Li2CO3 could increase the early age compressive strength of mortar by more than 200% without compromising other properties. As the development of nanotechnology has accelerated in recent years, nanoparticles such as nano SiO2 [16–19], CaCO3 [20,21], TiO2 [22], graphene oxide [23,24], core/shell TiO2@SiO2 [25], carbon nanotubes [26–32], and nano calcium silicate hydrate (C-S-H) [33,34] have been used in concrete. Wang et al. [20] showed that the addition of nano CaCO3 particles could improve the mechanical properties of plain cement paste and modify the microstructure. Musso et al. [32] reported that adding 0.5% (by weight of cement) of multi-wall carbon nanotubes could improve the mechanical strength of plain cement paste, and the treatments on the carbon nanotubes by temperature annealing and acid oxidative were able to enhance this effect. Jee et al. [35] found that the addition of 0.5% (by weight of cement) of titanium nanotubes could increase the compressive strength of cement paste by 12% and the flexural strength by 23% even at the later age, and they reported that the nanotubes were able to fill the nanopores and bridge the microcracks in cement paste. It was reported [36] that the addition of silica nanoparticles could increase the mechanical and durability of concrete by filling the cement pores and reacting with Ca(OH)2 to form additional C-S-H gel. Land and Stephan [37] found the main influencing factors of C-S-H seed as an early strength agent of cement were chemical composition and particle size, when they studied the effect of xonotlite and wollastonite C-S-H on the hydration and hardening of cement. Compared with nano SiO2, the influence of nano C-S-H is more obvious on the development of strength at age from the first day [34]. Meanwhile, nano SiO2 has pozzolanic reactivity, and nano C-S-H has filling and nucleation effects. Since the main hydration product in ordinary Portland cement paste is C-S-H gel, some studies have reported the effect of nano C-S-H particles on the hydration of cement paste. For example, it was presented [38] that nano C-S-H can accelerate cement hydration and improve its early compressive strength. Deyu Kong [39] found that nano C-S-H gel can mitigate the retardation caused by the sucrose. Kanchanason and Plank [40] synthesized nano C-S-H and polycarboxylate (PCE) comb polymer suspension, and they found that this suspension (solid content equals 2% by weight of cement) could accelerate the hydration of slag and calcined clay blended cements and increase the compressive strength by 240% after 16 h of hydration compared to the control groups. Wang et al. [38] synthesized nano C-S-H together with polycarboxylate (PCE) and polysulfonate (PSE), which were applied as stabilizer, and they found that the induction period of cement could be clearly shortened. Meanwhile, the specimen with PCE-type C-S-H showed a more obvious boosting effect than that with PSE type C-S-H. Das et al. [41] synthesized C-S-H nanocrystals by chemical reaction using bagasse as raw material, and they found that adding C-S-H seeds in concrete can accelerate the hydration process of cement within one day and increase the early strength by 30–50%, but it does not affect the later compressive strength. Almost all reported references mainly focused on the effect that amorphous state C-S-H seeds had on the hydration of cement, but the highly crystalized C-S-H particles were rarely reported elsewhere. The reason highly crystalized C-S-H particles were used was that the existence of C-S-H in nature Crystals 2020, 10, 816 3 of 17 was mostly found in crystalized form. In addition, the action mechanism of nano C-S-H remains unclear [19]. Investigations on the influence of highly crystalized nano C-S-H particles as an early strength agent on the hydration of cement paste with the addition of 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles (by weight of cement) were conducted through a series of tests including setting time, compressive strength, hydration heat, TG-DTA, XRD, and SEM. The results are analyzed and discussed, and a suggestion for the optimum content of the nano C-S-H particles is put forward.

2. Materials and Methods

2.1. Materials

2.1.1. Cement Type I Portland cement P.O. 42.5 conﬁrming to GB 175 2007 [42] was used in this study. The main − properties and chemical compositions of the cement are shown in Tables1 and2.

Table 1. Properties of Cement

Setting Time/min Flexural Strength/MPa Compressive Strength/MPa Fineness/% Stability Initial Final 3 Days 28 Days 3 Days 28 Days 1.6 Satisﬁed 169 356 5.9 9.7 32.2 51.4

Table 2. Chemical composition of Cement/%

SiO2 CaO Al2O3 Fe2O3 MgO Na2OK2O LOI 23.81 62.01 5.39 3.18 2.92 0.29 0.16 0.15

2.1.2. Synthesis of Nano C-S-H The nano C-S-H suspension was synthesized through the hydrothermal method [43,44] shown in Figure1. Calcium oxide (CaO, chemical purity more than 99.99%) and fumed silica (SiO 2) were mixed together with a molar ratio of 1:1. The mixed powders were put into an autoclave. De-ionized water was added into the autoclave with water/solids (mass ratio) at 10:1. The powders and water were mixed together for 5 min to form a suspension. The temperature inside the autoclave was maintained at 120 ◦C for 6 h with a stirring speed of 400 rpm to allow the reaction among CaO, SiO2, and H2O to form C-S-H. Then, the attained nano C-S-H suspension was allowed to cool down naturally to room temperature and sealed to avoid any further evaporation or carbonation. Meanwhile, nano C-S-H was dried in vacuum at 60 ◦C. The measured solid content in the nano C-S-H suspension was 22%. Figure2a shows the particle size distribution by laser di ﬀraction of the nano C-S-H particles from the suspension, and it can be seen that the particle size of the nano C-S-H particles ranges from 80 to 600 nm. Figure2b shows the XRD pattern of the nano C-S-H particles, and the peaks show that the mineral phases in the nano C-S-H particles are mainly tobermorite and jennite, which agrees well with previous literature [45,46], and there are also traces of SiO2 and wollastonite. Figure2c shows the SEM image of synthesized nano C-S-H particles, and the image shows that the shape of nano C-S-H particlesCrystals is polygon 2020, 10, x andFOR PEER the sizeREVIEW is consistent with the results in Figure2a. 4 of 17

Figure 1.FigureScheme 1. Scheme of theof the synthetic synthetic procedure procedure of highly of highly crystalized crystalized nano calcium nano silicate calcium hydrate silicate (C-S- hydrate (C-S-H) particles. H) particles.

16 Differential volume/% Cumulative volume/% 100

12 80

60 8

Differential volume/% 20 Cumulative volume/%

0 0 0.1 1 Particle diameter/μm

(a) 16000 Δ ♦ 14000

12000 ♦ Tobermorite Δ Jennite ♥ Wollastonite ♠ SiO 10000 2

8000

6000Intensity

4000 ♦ ♦ ♦ Δ 2000 ♥ ♠

0 10 20 30 40 50 60 70 θ 2 (°) (b)

(c)

Figure 2. (a) Particle distribution of synthesized nano C-S-H particles; (b) XRD pattern of the synthesized nano C-S-H particles; (c) SEM image of synthesized nano C-S-H particles.

Crystals 2020, 10, x FOR PEER REVIEW 4 of 17

Figure 1. Scheme of the synthetic procedure of highly crystalized nano calcium silicate hydrate (C-S- H) particles. Crystals 2020, 10, 816 4 of 17

16 Differential volume/% Cumulative volume/% 100

12 80

60 8

Differential volume/% 20 Cumulative volume/%

0 0 0.1 1 Particle diameter/μm

(a) 16000 Δ ♦ 14000

12000 ♦ Tobermorite Δ Jennite ♥ Wollastonite ♠ SiO 10000 2

8000

6000Intensity

4000 ♦ ♦ ♦ Δ 2000 ♥ ♠

0 10 20 30 40 50 60 70 θ 2 (°) (b)

(c)

FigureFigure 2. (a) Particle2. (a) Particle distribution distribution of synthesized of synthesized nano C-S-H nano particles; C-S-H particles; (b) XRD pattern(b) XRD of pattern the synthesized of the nano C-S-Hsynthesized particles; nano (c) SEM C-S-H image particles; of synthesized (c) SEM nanoimage C-S-H of synthesized particles. nano C-S-H particles.

2.1.3. Water The mixing water for cement paste used was the normal tap water conﬁrming to JGJ 63-2006 [47].

2.2. Sample Preparation The water–cement ratio was kept constant as 0.4. The nano C-S-H particles (0%, 0.5%, 1%, 2%, and 3% by weight of cement) were considered as the additional additive. The water content in the nano C-S-H suspension was considered as a part of the mixing water. The mixing procedure is as follows: the nano C-S-H suspension was treated with ultrasonic sonication for 5 min before mixing, the required amount of nano C-S-H suspension was mixed together with the additional water ﬁrst; then, cement powder was added in a mixer bowl, and the mixed suspension was added and mixed together with the cement powder at a speed of 120 rpm for 5 min using the automatic mixer. Then, the Crystals 2020, 10, 816 5 of 17 fresh paste was transferred into 40 mm 40 mm 40 mm molds. All samples were demolded after × × 24 h and stored in a standard curing room (20 ◦C, 97% relative humidity (R.H.)) until further tests.

2.3. Methods

2.3.1. Setting Time The initial and ﬁnal setting times of cement paste were tested according to GB/T1346-2011 [48].

2.3.2. Compressive Strength The compressive strength of cement paste (40 mm 40 mm 40 mm) was tested by the same × × loading procedure as described in GB/T17671-1999 [49]. The tests were conducted at the age of 1, 7, and 28 days. Each group was measured three times, and then the average value was taken.

2.3.3. Hydration Heat A multichannel thermal activity monitor (TAM) air micro calorimeter was used for the hydration heat monitoring. The chamber was stabilized for 0.5 h to achieve a table temperature of 20 ◦C before the isothermal test was started, and the test lasted for 72 h.

2.3.4. XRD The same powder sample as the TG-DTA test was used for XRD. An X-ray diﬀractometer of Rigaku SmartLab was used for the measurements. The test voltage was 40 kV, and the working current was 150 mA. The scanning was from 5◦ to 70◦ at a rate of 10◦/min.

2.3.5. TG-DTA After the compression tests, representative pieces of the samples from the inner part of the cubic paste sample were immersed into absolute ethyl alcohol for 24 h to stop the hydration [50]. Then, the pieces were dried in a vacuum oven at 60 ◦C for 24 h [51], followed by grounding into powder passing a sieve of 75 µm. TG-DTA tests were conducted in a simultaneous thermal analyzer in an N2 environment, and the heating temperature was from 20 to 900 ◦C by a heating rate of 10 ◦C/min.

2.3.6. SEM The piece samples after the termination of hydration and vacuum drying were used for SEM observation. Samples were gold coated, and the observations were conducted under high vacuum conditions.

3. Results and Discussion

3.1. Influence of Nano C-S-H on the Setting Time of Cement Paste Results of the effect of different contents of nano C-S-H particles on the initial and final setting times of the cement paste are shown in Figure3. It can be seen that both the initial and final setting times were shortened with the addition of nano C-S-H particles. Compared to the sample without nano C-S-H particles, the initial setting time was decreased by 7%, 12%, 19%, and 26% for the pastes with 0.5%, 1%, 2%, and 3% of nano C-S-H, respectively; and the final setting time was decreased by 6%, 13%, 23%, and 30% for the pastes with 0.5%, 1%, 2%, and 3% of nano C-S-H, respectively. Similar as the seeding effect of amorphous C-S-H particles, this effect could be mainly caused by the seeding effect [38,39] of the nanoparticles, which accelerated the initial hydration of cement. This was consistent with previous research results [41,52]. Crystals 2020, 10, x FOR PEER REVIEW 6 of 17

3. Results and Discussion

3.1. Influence of Nano C-S-H on the Setting Time of Cement Paste Results of the effect of different contents of nano C-S-H particles on the initial and final setting times of the cement paste are shown in Figure 3. It can be seen that both the initial and final setting times were shortened with the addition of nano C-S-H particles. Compared to the sample without nano C-S-H particles, the initial setting time was decreased by 7%, 12%, 19%, and 26% for the pastes with 0.5%, 1%, 2%, and 3% of nano C-S-H, respectively; and the final setting time was decreased by 6%, 13%, 23%, and 30% for the pastes with 0.5%, 1%, 2%, and 3% of nano C-S-H, respectively. Similar as the seeding effect of amorphous C-S-H particles, this effect could be mainly caused by the seeding effect [38,39] of the nanoparticles, which accelerated the initial hydration of cement. This was consistent with previous research results [41,52]. Crystals 2020, 10, 816 6 of 17

460 Initial setting time 440 Final setting time 440 420 400 400 380 360 360

340 320 320 300 Setting time/min Setting 280 280 260 240 240 0.00.51.01.52.02.53.0 Nano C-S-H content/%

Figure 3. The initial and final setting times of cement paste with different contents of nano C-S-HFigure particles. 3. The initial and final setting times of cement paste with different contents of nano C-S-H particles. 3.2. Influence of Nano C-S-H on the Compressive Strength of Cement Paste 3.2.Figure Influence4 shows of Nano the C-S-H results on of the the Compressive compressive Strength strength of atCement the age Paste of 1, 7, and 28 days of the cement paste samplesFigure 4 withshows addition the results of 0%, of 0.5%,the compressive 1%, 2%, and strength 3% nano at the C-S-H. age Theof 1, average 7, and 28 compressive days of the strengthcement ofpaste the cementsamples paste with with addition no nano of C-S-H 0%, 0.5%, addition 1%, are 2%, 16.7, and 44.1, 3% and nano 66.8 C-S-H. MPa atThe the ageaverage of 1,compressive 7, and 28 days, strength respectively. of the cement Compared paste towith the no strength nano C-S-H results addition of the pasteare 16.7, with 44.1, no nanoand 66.8 C-S-H MPa addition,Crystalsat the 2020 age both, 10 of, x of 1,FOR the7, PEERand compressive REVIEW28 days, respectively. strength results Compared at the age to ofthe 1 strength and 7 days results were of improved, the paste butwith7 of the 17no strengthnano C-S-H results addition, at the age both of 28 of days the werecompressive decreased. strength Specifically, results the at compressive the age of strength1 and 7 atdays the agewere ofaccumulationimproved, 1 day was but increased of thehydration strength by 6%,products results 7%, 25%, ataround the and age the 43%, of nano 28 and days C-S-H the were compressive particles decreased. could strength Specifically, leave some at the the defects, age compressive of7 which days wasisstrength harmful increased atfor the the by age 10%,long-term of 11%,1 day 19%,strength was andincreased development; 14% in by the 6%, samples 7%, this 25%, will with beand thefurther 43%, additions and verified the of compressive 0.5%,by SEM 1%, observations. 2%, strength and 3% at nanothe Inage C-S-H consideration of 7 particles, days was of respectively. increased the compressive by This 10%, is strength 11%, consistent 19%, at alland with the 14% the 3 tested in previous the ages samples research (1, 7, with and results 28the days), additions [34 ,the50]. content Atof 0.5%, the ageof1%, nano of 2%, 28 C-S-H days, and particles the3% compressivenano should C-S-H be strengthparticles, no more was respectively. than decreased 1% by weight byThis 3%, is of 5%,consistent cement, 13%, and given with 14% thatthe in theitprevious could cement improve research paste withtheresults early the addition [34,50].age strength At of the 0.5%, without age 1%, of 2%,28significantly days, and 3%the nano comprecompromising C-S-Hssive particles, strength the laterrespectively, was age decreased strength. when by compared3%, 5%, 13%, to the and paste14% samplein the cement with no paste nano with C-S-H the addition.addition of 0.5%, 1%, 2%, and 3% nano C-S-H particles, respectively,

when compared to the paste sample with no nano C-S-H addition. It is indicated the nano C-S-H particles accelerated 1d the 7dearly age 28d hydration of cement paste and

increased the early age compressive60 strength (1 and 7 days). The reason for the decrease of the compressive strength at 28 days could be attributed to the acceleration effect caused by nano C-S-H particles becoming insignificant and disappearing after the first few days [39]. The initial rapid 40

20 Compressive strength/MPa

0 0123450.5 1 2 3 Nano C-S-H content/%

Figure 4. Compressive strength of cement pastes with the addition of different contents of nano C-S-H atFigure the age 4. Compressive of 1, 7, and 28 strength days. of cement pastes with the addition of different contents of nano C-S- H at the age of 1, 7, and 28 days. It is indicated the nano C-S-H particles accelerated the early age hydration of cement paste and3.3. Hydration increased Heat the earlyResults age compressive strength (1 and 7 days). The reason for the decrease of the compressive strength at 28 days could be attributed to the acceleration effect caused by nano Figure 5 shows the hydration heat rate results of the cement paste samples with the addition of C-S-H particles becoming insignificant and disappearing after the first few days [39]. The initial rapid different contents of nano C-S-H particles. There are two main peaks on the curves of the hydration accumulation of hydration products around the nano C-S-H particles could leave some defects, which heat, namely here the first and second peaks. The first peak happened at around 1.8 min, and the is harmful for the long-term strength development; this will be further verified by SEM observations. second one happened between 12 and 18 h. The first peak was mainly because of the dissolution of In consideration of the compressive strength at all the 3 tested ages (1, 7, and 28 days), the content cement powder and formation of AFt [51], and the second peak was mainly because of the hydration of nano C-S-H particles should be no more than 1% by weight of cement, given that it could improve of C3S in the cement. the early age strength without significantly compromising the later age strength. At the location of the first peak, the mixes with the addition of nano C-S-H particles showed an overall higher peak heat rate than that with no nano C-S-H. It is suggested that the addition of nano C-S-H contributed to a better dispersion effect of the cement powder. Wang et al. [33] also found that the addition of low-dose nano C-S-H significantly shortened the induction period and increased the height of the peak hydration rate. The hydration exothermic rate of cement with 0.5% nano C-S-H is slightly higher than that with 1% nano C-S-H. As for the second peak, it can be clearly seen that the peak heat rate was higher in the mix with a higher content of nano C-S-H particles, and the peak time was also earlier in the mix with a higher content of nano C-S-H particles. Specifically, the heat rate of second peak for the mixes with 0%, 0.5%, 1%, 2%, and 3% were 2.62, 2.79, 2.87, 3.16, and 3.54 mW/g, respectively; and the related peak time was 17.62, 16.28, 15.83, 14.57, and 12.96 h. Meanwhile, the accumulated heat generated of this period is shown in Table 3. The results show that the nano C-S-H particles obviously accelerated the hydration of C3S and promoted the formation of new C-S-H. This effect was more obvious when a higher content of nano C-S-H particles was added. The nano C-S-H particles act as nucleation centers and dispersing agents in cement paste. Due to the unique characteristics of nanoparticles, the hydration process is accelerated, including the increase of hydration rate peak and the decrease of induction period [53]. Figure 6 shows the accumulated heat generated in each mix, and the results indicate that the mixes with the addition of nano C-S-H particles showed a higher accumulated hydration heat than those without nano C-S-H particles. The mix with a higher content of nano C-S-H particles generally shows a higher accumulated hydration heat. In 1 day, as can be seen from Figure 6b and Table 3, nano C-S-H can increase the hydration heat of Portland cement. The results of the accumulated heat generated in cement pastes were consistent with the development of compressive strength. It is indicated that nano C-S-H particles obviously accelerated the hydration of ordinary Portland cement

Crystals 2020, 10, 816 7 of 17

Crystals3.3. 2020 Hydration, 10, x FOR Heat PEER Results REVIEW 8 of 17 Figure5 shows the hydration heat rate results of the cement paste samples with the addition of powders and increased the hydration heat. It is interesting to see that the accumulated heat at 3 days different contents of nano C-S-H particles. There are two main peaks on the curves of the hydration is very similar for 0.5%, 1.0%, 2.0%, and 3.0% mixes. By comparing with the data of 1 day, it could be heat, namely here the first and second peaks. The first peak happened at around 1.8 min, and the indicatedsecond that one nano happened C-S-H between mainly12 promoted and 18 h. the The hydration first peakwas of cement mainly becausein 1 day, of but the it dissolution has no obvious of effectcement on the powder total accumulated and formation heat of AFt at the [51 ],later and stage the second after peak3 days. was mainly because of the hydration of C S in the cement. 3

160 First peak Second peak 160 1% 2% 3 120 1% -1 -1 3% 3% 2%

80 2 0.5% 120 0.5% Heat rate/mW.gHeat Heat rate/mW.g Heat

-1 0% 0% 40 1

80 0.0 0.1 0.2 0.3 5 101520 Hydration time/h Hydration time/h a b

Heat rate/mW.g 40 0% 0.5% 1% 2% 3%

0 0 10203040506070 Hydration time/h

Figure 5. Hydration heat rate of cement paste with diﬀerent amounts of nano C-S-H addition.

FigureAt the 5. location Hydration of theheat ﬁrst rate peak, of ceme thent mixes paste with different the addition amou ofnts nano of nano C-S-H C-S-H particles addition. showed an overall higher peak heat rate than that with no nano C-S-H. It is suggested that the addition of

nano C-S-H contributed to a better dispersion eﬀect of the cement powder. Wang et al. [33] also found that the 0% addition of low-dose nano C-S-H signiﬁcantly260 shortened the induction period and 400 0.5% increased the 1% height of the peak hydration rate. The hydration exothermic rate2% of cement with 0.5% 2% 3% 250 3%

nano-1 C-S-H is slightly higher than that with 1% nano C-S-H. As for the second peak, it can be clearly 300 3% -1 seen that the peak heat rate was higher in the mix with a higher content of nano C-S-H particles, and 2% 1% 240 0.5% the peak time was also earlier in the mix with a higher content of nano C-S-H particles. Speciﬁcally, 200 0.5% 1% the heat rate of second peak for the mixes with 0%, 0.5%, 1%, 2%, and 3% were 2.62, 2.79, 2.87, 3.16, Total heat/J.g Total heat/J.g 230 and 3.54 mW/g, respectively;0% and the related peak time was 17.62, 16.28, 15.83, 14.57, and 12.96 h. 0% Meanwhile,100 the accumulated heat generated of this period is shown in Table3. The results show that 220 the nano C-S-H particles obviously accelerated the hydration of C3S and promoted the formation of 0 new C-S-H.0 10203040506070 This eﬀect was more obvious when a higher content22 of nano 24 C-S-H particles 26 was added. Hydration time/h The nano C-S-H particlesHydration act time/h as nucleation centers and dispersing agents in cement paste. Due to the unique characteristics of nanoparticles, the hydration process is accelerated, including the increase of (a) (b) hydration rate peak and the decrease of induction period [53]. Figure 6. (a) Accumulated hydration heat of cement paste with different amounts of nano C-S-H addition; (b) Local enlarged view.

Table 3. Results of Accumulated Hydration Heat.

Nano C-S-H Content/% MHR/mW.g−1 AHH at MHR/J.g−1 1 d AHH/J.g-1 3 d AHH/J.g−1 0 2.62 179 228 345 0.5 2.79 184 244 359 1 2.87 178 239 355 2 3.16 177 246 360 3 3.54 169 250 361 Note: AHH—accumulated hydration heat, MHR—maximum heat rate.

Crystals 2020, 10, x FOR PEER REVIEW 8 of 17

powders and increased the hydration heat. It is interesting to see that the accumulated heat at 3 days is very similar for 0.5%, 1.0%, 2.0%, and 3.0% mixes. By comparing with the data of 1 day, it could be indicated that nano C-S-H mainly promoted the hydration of cement in 1 day, but it has no obvious effect on the total accumulated heat at the later stage after 3 days.

Crystals 2020, 10, 816 8 of 17

160 First peak Second peak 160 1% 2% Table 3. Results of Accumulated3 Hydration Heat. 120 1% -1 -1 3% 1 3%1 1 1 Nano C-S-H Content/% MHR/mW g− AHH at MHR/J g− 1 d AHH/J g− 3 d AHH/J g− 2%

0 80 2.62 1792 0.5% 228 345 120 0.5% 0.5 2.79 184 244 359 Heat rate/mW.gHeat Heat rate/mW.g Heat

-1 0% 0% 140 2.87 178 239 355 2 3.16 1771 246 360 3 3.54 169 250 361

80 0.0 0.1 0.2 0.3 5 101520 Note: AHH—accumulated hydration heat, MHR—maximumHydration heat time/h rate. Hydration time/h a b Figure6 shows the accumulated heat generated in each mix, and the results indicate that the mixes

with the additionHeat rate/mW.g 40 of nano C-S-H particles showed a higher accumulated hydration heat than those 0% 0.5% 1% 2% 3% without nano C-S-H particles. The mix with a higher content of nano C-S-H particles generally shows a higher accumulated hydration heat. In 1 day, as can be seen from Figure6b and Table3, nano C-S-H can increase the0 hydration heat of Portland cement. The results of the accumulated heat generated in cement pastes were consistent with the development of compressive strength. It is indicated that 0 10203040506070 nano C-S-H particles obviously accelerated the hydration of ordinary Portland cement powders and Hydration time/h increased the hydration heat. It is interesting to see that the accumulated heat at 3 days is very similar for 0.5%, 1.0%, 2.0%, and 3.0% mixes. By comparing with the data of 1 day, it could be indicated that nano C-S-H mainly promoted the hydration of cement in 1 day, but it has no obvious eﬀect on the total accumulatedFigure heat 5. Hydration at the later heat stage rate of after ceme 3nt days. paste with different amounts of nano C-S-H addition.

0% 260 400 0.5% 1% 2% 2% 3% 250 3% -1 300 3% -1

2% 1% 240 0.5%

200 0.5% 1% Total heat/J.g 0% Total heat/J.g 230 100 0% 220

0 0 10203040506070 22 24 26 Hydration time/h Hydration time/h (a) (b)

Figure 6.6. ((a)) Accumulated hydration hydration heat heat of of cement pa pasteste with different different amounts of nano C-S-HC-S-H addition;addition; ((bb)) LocalLocal enlargedenlarged view.view. 3.4. XRD Results Table 3. Results of Accumulated Hydration Heat. Figure7 shows the XRD results of the cement paste samples with di fferent contents (0%, 0.5%, 1%,Nano 2%, C-S-H and 3%) Content/% of nano C-S-H MHR/mW.g particles−1 at theAHH di ffaterent MHR/J.g ages:−1 1, 7, 1 andd AHH/J.g 28 days.-1 The 3 d mainAHH/J.g phase−1 compositions0 in the cement paste samples2.62 reflected by179 XRD spectrum are Ca(OH)228 2, CaCO3,345 AFt, AFm, C2S, and C3S[0.551 ] As stated earlier,2.79 the formation of CaCO1843 could be due to the244 CO2 dissolved359 in mixing water, and the1 AFm here is CO2-AFm.2.87 At the age of 1 day,178 the peak height239 of AFt firstly increased355 and then decreased when the content of nano C-S-H particles increased from 0 to 3%; the highest peak 2 3.16 177 246 360 happened in the mix with 1% nano C-S-H particles. This was consistent with the results of quantitative 3 3.54 169 250 361 analysis by Zhang et al [34]. These results suggest that the addition of nano C-S-H particles facilitated Note: AHH—accumulated hydration heat, MHR—maximum heat rate. the formation of Ca(OH)2 and AFt. It should be noted that the comparison between peak heights does not actually give the precise quantitative analysis. It should be more precise to perform Rietveld analysis to quantify the amount of the different phases. Crystals 2020, 10, x FOR PEER REVIEW 9 of 17

3.4. XRD Results Figure 7 shows the XRD results of the cement paste samples with different contents (0%, 0.5%, 1%, 2%, and 3%) of nano C-S-H particles at the different ages: 1, 7, and 28 days. The main phase compositions in the cement paste samples reflected by XRD spectrum are Ca(OH)2, CaCO3, AFt, AFm, C2S, and C3S [51] As stated earlier, the formation of CaCO3 could be due to the CO2 dissolved in mixing water, and the AFm here is CO2-AFm. At the age of 1 day, the peak height of AFt firstly increased and then decreased when the content of nano C-S-H particles increased from 0 to 3%; the highest peak happened in the mix with 1% nano C-S-H particles. This was consistent with the results of quantitative analysis by Zhang et al [34]. These results suggest that the addition of nano C-S-H

particles facilitated the formation of Ca(OH)2 and AFt. It should be noted that the comparison between peak heights does not actually give the precise quantitative analysis. It should be more precise to perform Rietveld analysis to quantify the amount of the different phases. The changing patterns of the phase compositions at the age of 7 days are similar to those at 1 day. In the presence of C-S-H, the existence of a high peak value indicated a greater precipitation of hydration products [53], which could improve the microstructure and compressive strength of cement paste. The XRD result was consistent with that of compressive strength. With the increase of hydration time, the content of AFt first increases and then decreases, due to the transformation of AFt into AFm after the depletion of the sulfate phase, which was most obvious at 28 days of age. Crystals 2020, 10, 816 9 of 17

1d ♣ AFt ♦ Ca(OH) Φ AFm ♦ 2 ♦ ∇ CaCO ♠ C S ♥ C S ♦ ♠ 3 2 3 ♣ Φ ♣ ♥♠ ♥ ♦ ♦ ♦ ♣ ♥ ♠ ♥∇ ♦ 3%

1% Intensity

0.5%

10 20 30 40 50 60 70 2θ(°)

(a) 1 d.

♣ AFt ♦ Ca(OH) Φ AFm 7d ♦ 2 ♦ ∇ CaCO ♠ C S ♥ C S ♦ ♠ 3 2 3 ♣ ♣ ♠♥♥ ♦ Φ ♣ ♥ ♠ ♥∇ ♦ ♦ ♦ 3%

1% Intensity

0.5%

Crystals 2020, 10, x FOR PEER REVIEW 10 20 30 40 50 60 70 10 of 17 2θ(°)

(b) 7 d. ♣ AFt ♦ Ca(OH) Φ AFm 28d 2 ♦ ♦ ∇ CaCO ♠ C S ♥ C S ♦ ♠ 3 2 3 ♥♠ ♥ ♦ ♣Φ ♣ ♥ ♠♥∇ ♦ ♦ ♦ 3%

1% Intensity

0.5%

10 20 30 40 50 60 70 2θ(°)

Figure 7.FigureXRD 7. powder XRD powder diﬀraction diffraction patterns patterns of the of sample the sample with with diﬀ erentdifferent contents contents of nanoof nano C-S-H C-S-H particles at the age of 1, 7, and 28 days. particles at the age of 1, 7, and 28 days.

The3.5. TG-DTA changing Results patterns of the phase compositions at the age of 7 days are similar to those at 1 day. In theFigure presence 8 shows of the C-S-H, TG-DTA the existenceresults of ofthe a diffe highrent peak paste value samples indicated with 0%, a greater 0.5%, 1%, precipitation 2%, and of hydration3% nano products C-S-H [particles53], which at the could age improveof 1, 7, and the 28 microstructure days. There are and3 main compressive peaks on the strength DTA curves: of cement paste.the The first XRD peak result is atwas around consistent 110 ℃, withwhich that is the of compressiveweight loss of strength. water from With AFt the and increase C-S-H of[51]; hydration the time,second the content peak ofis at AFt around ﬁrst increases460 °C which and thenis the decreases,decomposition due of to Ca(OH) the transformation2; and the third of AFtpeak into is at AFm after thearound depletion 700 °C ofwhich the sulfateis the decomposition phase, which of was CaCO most3. Additionally, obvious at 28there days is a of small age. peak at around 160 °C which is the water loss from the AFm (CO2-AFm) [21]. At the age of 1 day, the weight losses at the location of the first peak for the paste samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles were 5.8%, 5.9%, 6.0%, 5.0%, and 5.1%; the weight losses at the location of Ca(OH)2 based on a tangential model for the paste samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles were 1.73%, 1.57%, 1.75%, 1.77%, and 2.04%, respectively, as shown in Table 4. At the age of 7 days, the weight losses at the location of the first peak for the paste samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles were 6.7%, 7.0%, 6.5%, 6.0%, and 5.7%; the weight losses at the location of Ca(OH)2 for the paste samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles were 1.87%, 1.98%, 2.07%, 1.94%, and 1.98%, respectively. At the age of 28 days, the weight losses at the location of the first peak (4.6%) and location of Ca(OH)2 (2.0%) were almost the same for different paste samples, since that their TG curves were similar to each other. The weight loss rate in TG curves is consistent with that in the DTA diagram. These results show that at the early ages (1 and 7 days), the nano C-S-H particles accelerated the formation of Ca(OH)2, which is the hydration product from C3S at early age. As it is known that the formation of Ca(OH)2 is associated with the new C-S-H gel, the nano C-S-H particles also promoted the formation of C-S-H gel. At the later age (28 days), the effect of nano C-S-H particles on the hydration products is not very significant. From the compressive strength results at 28 days, it can be seen that the different strength results could be caused by the different layout structure of the hydration products in different mixes [51] from 7 to 28 days, the consumption of Ca(OH)2 was higher in the mixes with nano C-S-H particles.

Crystals 2020, 10, x FOR PEER REVIEW 11 of 17 Crystals 2020, 10, 816 10 of 17

Table 4. The Weight Loss of Ca(OH)2 at Different Ages. 3.5. TG-DTA Results Nano C-S-H/% 1 d Weight loss/% 7 d Weight 28 d Weight loss/% Figure8 shows the TG-DTA results of the di ﬀerent pasteloss/% samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles0 at the age of 1, 7, and1.73 28 days. There are1.87 3 main peaks on the DTA2.00 curves: the ﬁrst peak is at around 1100.5°C , which is the weight1.57 loss of water from1.98 AFt and C-S-H [51]; 2.08 the second peak is at around 460 ◦C which1 is the decomposition1.75 of Ca(OH)2; and2.07 the third peak is at around2.00 700 ◦C which is the decomposition2 of CaCO3. Additionally,1.77 there is a small1.94 peak at around 160 ◦C1.95 which is the water loss from the AFm (CO3 2-AFm) [21]. 2.04 1.98 1.84

12 1d 0% 0.5% 1% 100 2% 3% 2

8 TG 90 -1 -1 0 1% 4 0.5% 80 0.5% v.mg v.mg μ 1% 2% μ 2%

0% 70 -2 0 3% DTA/ DTA/

60 Mass/% 0% DTA -4 -4 AFm CaCO3 50 3% AFt Ca(OH)2 -8 40 -6 100 200 300 400 500 600 700 800 430 440 450 460 470 480 490 500 Temperature /°C Temperature /°C (a) 1 d.

7d 0% 0.5% 1% 100 4 2% 3% 12 TG 90

-1 2 8

-1 0% 80 2% v.mg

3% μ 4 v.mg 1% 0 0.5%

μ 70 2% DTA/ 0 Mass/% DTA/ 0.5% -2 3% 60 0% CaCO 3 1% -4 AFm DTA Ca(OH)2 AFt 50 -4 -8 100 200 300 400 500 600 700 800 420 440 460 480 500 Temperature /°C Temperature /°C (b) 7 d.

28d 0 % 0.5% 1% 100 12 2% 3% 4 90 8 TG -1 80 -1 2 2% 0.5% v.mg μ 4 0% v.mg 1% μ 1% 70

Mass/% 0 DTA/ 3% 0 DTA/ 0% 60 CaCO 3% 3 -2 0.5% -4 DTA Ca(OH) 50 2 2% AFt AFm 100 200 300 400 500 600 700 800 420 430 440 450 460 470 480 490 Temperature /°C Temperature /°C (c) 28 d.

FigureFigure 8. 8.Thermogravimetry-di Thermogravimetry-differentialfferential thermalthermal analysis (TG-DTA) (TG-DTA) results results of of cement cement paste paste with with diffdifferenterent contents contents of nanoof nano C-S-H C-S-H at theat the age age of 1,of 7,1, and7, and 28 days;28 days; the the sub-figure sub-figure on on the the right right was was a local a

enlarged view of the peaklocal of enlarged Ca(OH)2 viewon the of left. the peak of Ca(OH)2 on the left.

Crystals 2020, 10, 816 11 of 17

At the age of 1 day, the weight losses at the location of the first peak for the paste samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles were 5.8%, 5.9%, 6.0%, 5.0%, and 5.1%; the weight losses at the location of Ca(OH)2 based on a tangential model for the paste samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles were 1.73%, 1.57%, 1.75%, 1.77%, and 2.04%, respectively, as shown in Table4. At the age of 7 days, the weight losses at the location of the first peak for the paste samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles were 6.7%, 7.0%, 6.5%, 6.0%, and 5.7%; the weight losses at the location of Ca(OH)2 for the paste samples with 0%, 0.5%, 1%, 2%, and 3% nano C-S-H particles were 1.87%, 1.98%, 2.07%, 1.94%, and 1.98%, respectively. At the age of 28 days, the weight losses at the location of the first peak (4.6%) and location of Ca(OH)2 (2.0%) were almost the same for different paste samples, since that their TG curves were similar to each other. The weight loss rate in TG curves is consistent with that in the DTA diagram. These results show that at the early ages (1 and 7 days), the nano C-S-H particles accelerated the formation of Ca(OH)2, which is the hydration product from C3S at early age. As it is known that the formation of Ca(OH)2 is associated with the new C-S-H gel, the nano C-S-H particles also promoted the formation of C-S-H gel. At the later age (28 days), the effect of nano C-S-H particles on the hydration products is not very significant. From the compressive strength results at 28 days, it can be seen that the different strength results could be caused by the different layout structure of the hydration products in different mixes [51] from 7 to 28 days, the consumption of Ca(OH)2 was higher in the mixes with nano C-S-H particles.

Table 4. The Weight Loss of Ca(OH)2 at Diﬀerent Ages.

Nano C-S-H/% 1 d Weight Loss/% 7 d Weight Loss/% 28 d Weight Loss/% 0 1.73 1.87 2.00 0.5 1.57 1.98 2.08 1 1.75 2.07 2.00 2 1.77 1.94 1.95 3 2.04 1.98 1.84

3.6. SEM Results Figure9 shows the images of the SEM observations at di fferent ages (1, 7, and 28 days) for the cement pastes with the addition of different contents (0%, 0.5%, 1%, 2%, and 3%) of nano C-S-H particles. The hexagonal plates are Ca(OH)2, the needle-shaped hydration products are AFt, and the gel in irregular shape is the C-S-H gel. It is difficult to find the previously added nano C-S-H particles, which could be wrapped with the newly formed C-S-H gel. Wang et al. [33] reported C-S-H gel is a substance that grows on nano C-S-H particles, and fibrous C-S-H was found at the age of 1 and 7 days; the content increased first and then decreased. At the age of 1 and 7 days (Figure9a,b), Ca(OH) 2 is more commonly found in the cement pastes with the previously added nano C-S-H particles compared to the control group with no nano C-S-H addition. This phenomenon was not found at the age of 28 days. This should not be caused by the carbonation, since the sampling location is from the center part. Meanwhile, the pore size of needle-like material with nano C-S-H content of 0.5% and 1% is smaller than others, at about 3 µm. When comparing the 28-days results of the compressive strength, XRD, TG-DTA, and the SEM images, it is indicated that the continuous decrease of the compressive strength of the cement paste with the increase of nano C-S-H additions is not caused by the overall amount of hydration products, which is similar in different mixes at 28 days based on the confirmation from the TG-DTA results, but it could be due to the microstructure of these hydration products. From Figure9c, it can be seen that the microstructure of the hydration products in the mixes with the previous addition of nano C-S-H particles is less compact compared to the paste with no nano C-S-H additions, and the reason could be that the initial rapid accumulation of Ca(OH)2 and new C-S-H gels around the added nano C-S-H particles left more unfavorable spaces, which could facilitate the development and propagation of microcracks. In addition, the later higher consumption rate of Ca(OH)2 from 7 to 28 days in the Crystals 2020, 10, 816 12 of 17 mixes with nano C-S-H additions could leave more frail spaces, which could also compromise the compressive strength. Some studies [34,54] reported that the addition of nano C-S-H nucleating agent actually increased the gel pores, the formed C-S-H from the nucleation of nano C-S-H was loosely packed, and the porosity of the hydration products was increased with the amount of nano-C-S-H crystal seeds. These agree with our finding that the addition of a higher content of nucleating agents makes the C-S-H gel structure become more porous at the later age. So, when there is too much nano C-S-H content, this may cause adverse reactions at the later age, which corresponds well to the results in this paper.

(a) 1 d.

Figure 9. Cont.

Crystals 2020, 10, 816 13 of 17

(b) 7 d.

Figure 9. SEM images of cement pastes with diﬀerent contents of nano C-S-H particles at the age of 1, 7, and 28 days. 2 Crystals 2020, 10, 816 14 of 17

4. Conclusions This study investigated the effect of synthesized highly crystalized nano C-S-H particles on the hydration of Portland cement paste. The setting time and compressive strength of different mixes with 0%, 0.5%, 1%, 2%, and 3% synthesized C-S-H particles are reported and discussed. The related mechanisms were analyzed through hydration heat, XRD, TG-DTA, and SEM tests. From the above results and analysis, the following conclusions can be drawn: (1) Similar to the amorphous C-S-H particles, the synthesized high crystalized nano C-S-H particles could be used as an early strength agent for ordinary Portland cement, but the long-term performance still needs further investigation. The final setting time can be shortened by up to 30% and the 1-day compressive strength can be increased by 43% with the addition of 3% nano C-S-H particles. (2) Although the addition of synthesized nano C-S-H particles could increase the early age strength (especially the 1-day strength), the later age strength (28 days) was decreased compared to that with no synthesized nano C-S-H particles. The reason could be that the initial accelerated hydration of cement caused by the addition of nano C-S-H particles did not form a compact structure, which was evidenced by the SEM observations. (3) The mechanism of the effect of nano C-S-H particles on the ordinary Portland cement paste has been confirmed to be the accelerated hydration of C3S and promoted formation of C-S-H gel and Ca(OH)2 at early ages. (4) In consideration of the results on both of the early age and later age strength development, 0.5% 1% synthesized nano C-S-H particles could be a suitable addition content in ordinary Portland − cement. Compared to the cement paste with no addition of C-S-H particles, the final setting time can be shortened by 13%, while the compressive strength at the age of 1 and 7 days can be increased by 7% and 11%, respectively.

Author Contributions: Conceptualization, J.W. and Y.W.; methodology, J.W. and Y.W.; formal analysis, Y.W., H.H. and H.L.; investigation, H.L. and H.H.; writing—original draft preparation, H.L., J.W. and Y.W.; writing—review and editing, J.W. and Y.W.; supervision, J.W. and Y.W.; project administration, J.W. and Y.W.; funding acquisition, Y.W. All authors have read and agree to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China (U1904188, 51678220), the Program for Innovation Scientists and Technicians Troop Construction Projects of Henan Province in China (CXTD2017088) and the Program for Innovative Research Team (in Science and Technology) (19IRTSTHN027). Acknowledgments: The authors want to express their gratitude towards the support from Department of Civil Engineering, Tsinghua University, China. Conflicts of Interest: The authors declare no conflict of interest. Data Availability: Data used to support the findings of this study are available from the corresponding author upon request.

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