Characteristics of Ordinary Portland Cement Using the New Colloidal Nano-Silica Mixing Method

applied sciences

Article Characteristics of Ordinary Portland Cement Using the New Colloidal Nano-Silica Mixing Method

Taewan Kim 1 , Sungnam Hong 2, Ki-Young Seo 3 and Choonghyun Kang 2,*

1 Department of Civil Engineering, Pusan National University, Busan 46241, Korea; [email protected] 2 Department of Ocean Civil Engineering, Gyeongsang National University, Gyeongsangnam-do 53064, Korea; [email protected] 3 HK Engineering & Consultants, Busan 46220, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-55-772-9124

Received: 18 September 2019; Accepted: 12 October 2019; Published: 16 October 2019

Abstract: This study applies a new method of mixing colloidal nano-silica (CNS). Previous studies have used powdered nano-silica or colloidal nano-silica and applied a binder weight substitution method. In this study, we tried to use ordinary Portland cement (OPC) as a binder and replace CNS with weight of mixing water. CNS was replaced by 10%, 20%, 30%, 40%, and 50% of the mixing water weight. The flow value, setting time, compressive strength, hydration reactant (X-ray diffractometer; XRD), pore structure (mercury intrusion porosimetry; MIP), thermal analysis, and scanning electron microscopy (SEM) analysis were performed. Experimental results show that the new substitution method improves the mechanical and microstructural properties through two effects. One is that the weight substitution of the mixing water shows a homogeneous dispersion effect of the nano-silica particles. The other is the effect of decreasing the w/b ratio when the CNS is substituted because the CNS is more dense than the mixing water. Therefore, we confirmed the applicability of mixing water weight replacement method as a new method of mixing CNS.

Keywords: colloidal nano-silica; mixing water; ordinary Portland cement; mechanical and microstructural properties

1. Introduction Studies in the field of advanced construction materials, such as the use of nanoparticles as cement admixture, have reported significant improvements in the properties of traditional building products. In previous research, various nanoparticles were mixed with cement, and their performance was evaluated [1–4]. Nanoparticles, such as nanoclay, Al2O3, Fe2O3, TiO2, and CaCO3 were evaluated; however, SiO2 was the most commonly studied [5–18]. Nano-silica denotes that small particles comprise an amorphous SiO2 core with a hydroxylated surface, which makes the substance insoluble in water. The size of the particles can vary between 1–100 nm; therefore, they are small enough to remain suspended in a fluid medium without settling. Parameters, such as specific surface area, particle size, and size distribution can be controlled using the synthesis technique [3]. Nano-silica particles have a high surface area to volume ratio that provides high chemical reactivity. They behave as nucleation centers, contributing to the development of the hydration of ordinary Portland cement (OPC) or cementitious materials [19]. The addition of nano-silica into mortar and concrete mixtures accelerates the hydration process, improving the strength and microstructure characteristics of the OPC [8,13,19–25]. The increase in the initial hydration rate caused by the pozzolanic reaction increases the amount of calcium-silicate-hydrate (C-S-H) gel [26–28]. Cement-based materials containing nano-silica particles demonstrate an improved performance due to the combined effects of the filler, nucleation, and pozzolanic reaction [29,30].

Appl. Sci. 2019, 9, 4358; doi:10.3390/app9204358 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 4358 2 of 20

Most of the previous research have focused on the powdered nano-silica (PNS) replacement percentages of less than 5% by binder weight [8,9,21,22,31], whereas a few studies have increased this fraction to reach 12% [27]. The binder weight replacement method was used, which displayed a significant decrease in fluidity due to the high specific surface area of the PNS. A large amount of mixing water and/or superplasticizer was required to counteract this effect and ensure sufficient fluidity [19,22,24,32,33]. Therefore, previous studies using PNS and the binder weight replacement method are difficult to apply with a high substitution rate of more than 12% due to the ensuing rapid decrease of fluidity. Some studies have reported the results of binder weight replacement using CNS instead of PNS [7,19,26,28,34]. As a result, the mechanical behavior of OPC with higher percentages of PNS has not been well-reported. A study on the mixing method and mechanical and microstructural characteristics for replacing CNS by 15% or more is still insufficient. Therefore, a new mixing method is needed to increase the substitution rate of CNS. Following the development of the mixing method, the next step will be to study the mechanical and microstructural characteristics of the matrix of high CNS replacement rates. Therefore, for the development of cement using nano-silica particles and various nano-particles, a mixing method that can apply various substitution ratios is needed. In particular, a method to improve sudden initial fluidity degradation using nano particles is more important. This is important for solving problems such as a reduction in mechanical performance and durability degradation due to rapidly decreased fluidity. Binder weight replacement of nano-silica reduces the amount of binder. This may also affect the formation of hydration reactants and the pore structure. In addition, the rapid decrease in fluidity due to the mixing of nano-silica forms a dense matrix together with the binder reduction effect caused by the binder replacement method. Therefore, there is a need to consider how to apply the effects of nano-silica without reducing the amount of binder. In addition, it can be expected to be a more effective compounding method if it is considered together with methods to alleviate or improve the problem of abrupt fluidity reduction. The weight change method of mixing water to be applied in this study does not change the binder weight. In addition, because the liquid type of CNS is used, it is possible to replenish a part of the mixed water which is reduced by substitution. The CNS used in this study, rather than powdered nano-silica, is pre-mixed with mixed water, making it easier to stir nano-silica, and the dispersing effect of particles is excellent. Therefore, the new mixing method is expected to remedy the above-mentioned problem of rapid fluidity reduction. We will attempt to apply a higher mixing ratio of nano-silica particles through weight replacement of mixing water. This process can be used as basic research data to investigate the effect of high concentration of nano-silica particles on the hydration and mechanical properties of OPC. This study examines the mechanical properties of the OPC paste using CNS. The replacement method for the weight of mixing water was used instead of the binder replacement method, which was appropriate to be applied in the PNS. Additionally, the water-binder ratio was assumed to be 0.5 without the addition of superplasticizer.

2. Materials and Methods

2.1. Materials A commercial-grade CNS was used, which exhibited a mean particle size of 20 nm, a density of 3 0.0012 g/mm , and an aqueous solution having an alkaline pH of 10. The SiO2 content in the aqueous solution was 30%, while the viscosity was reported to be not greater than 20 cps at 20 ◦C. The chemical properties of the OPC that is used in this study are summarized in Table1, as determined by X-ray ﬂuorescence (XRF) analysis. Appl. Sci. 2019, 9, 4358 3 of 20

Table 1. The chemical properties of ordinary Portlandite cement (OPC).

Chemical Components (%) Density Fineness L.O.I (g/mm3) (m2/kg) (%) SiO2 Al2O Fe2O MgO CaO K2O SO3 OPC 20.51 5.27 3.64 2.86 62.58 0.69 2.72 0.00315 330 0.76

2.2. Experiments Methods In this study, a replacement method was developed based on the replacement of mixing water weight rather than binder weight. Mixing CNS with mixing water is an attempt to increase the homogeneous dispersion of nano-silica particles. The water-to-binder ratio (w/b) of all the paste mixtures was 0.5; further, because a superplasticizer was not used, corrections to w/b were unnecessary. Six OPC mixtures with incremental CNS replacement ratios (0%, 10%, 20%, 30%, 40%, and 50% of the mixing water weight) were prepared by combining CNS with the mixing water that was followed by mechanically stirring prior to paste mixing. Table2 shows the substitution ratio and water-binder ratio of CNS according to mixing water and the binder weight substitution method. CNS has a higher density than mixing water. Therefore, substitution of CNS for the weight of mixing water reduces the w/b ratio.

Table 2. Summary of w/b ratios and the mixing ratio of colloidal nano-silica (CNS).

CNS Substitution Ratios w/b Ratios of Considering Water Including CNS Mixing Water Weight (%) Binder Weight (%) 0.5 0 0 0.485 10 5 0.47 20 10 0.45 30 15 0.44 40 20 0.425 50 25

The flow value was measured according to ASTM C230 [35] and the setting time was ASTM C266 [36] when using the Gillmore needle method. Paste mixing was performed by following the procedures described in ASTM C305 [37]. After mixing, the six different paste mixtures were kept in a temperature and humidity control chamber for 24 h at a temperature of 23 2 C and a relative ± ◦ humidity (RH) of 90 5%. The mold that was formed on the mixtures was then removed, and the ± mixtures were placed back in the chamber under identical humidity and temperature conditions until the age of measurement was reached. Compressive strengths were measured at 1-, 3-, 7-, and 28-days. The compressive strength of three specimens was measured and the mean value was used. The compressive strength was measured according to ASTM C109 [38] using a 50 50 50 mm × × cube mold. After measuring the compressive strength, the fractured pieces were immersed in acetone for 12 h and were then dried in a vacuum desiccator for 24 h. Thereafter, the samples were pulverized into fine particles and passed through a 7.5 µm sieve; further, the samples were analyzed with an X-ray diffractometer (XRD, Empyrean, PANalytical, Almelo, Netherlands). The purpose of this analysis was to investigate the type and morphology of hydration reaction materials by CNS using XRD analysis. The generator settings are 40 mA and 45 kV. The measurement range is from 5◦ to 60◦ (2θ) and the step size is 0.017◦ (2θ). XRD was performed for 1-day and 28-day samples. Separate specimens comprising small broker or cut pieces were also prepared after 28-days of aging for porosity analysis using mercury intrusion porosimetry (MIP, AutoPore IV 9500, Micromeritics, Norcross, GA, USA). The average contact angle was 130.0 degrees, the Hg surface tension was 485.0 dynes/cm, and the Hg density was 13.5291 g/mL. The microstructures were observed using scanning electron microscopy (SEM, SUPRATM 40, ZEISS, Oberkochen, Germany) after conducting the fracture toughness measurement. Measurements were made using a high vacuum mode using an Appl. Sci. 9 Appl. Sci. 20192019,, 9,, x 4358 FOR PEER REVIEW 44 of of 19 20 vacuum mode using an accelerating voltage of 15 kV. Further, thermogravimetric (TG) and accelerating voltage of 15 kV. Further, thermogravimetric (TG) and differential thermal analysis (DTG) differential thermal analysis (DTG) were performed on specimens at 28-days of age. Finally, a were performed on specimens at 28-days of age. Finally, a thermal analyzer (DSC800, Perkin Elmer, thermal analyzer (DSC800, Perkin Elmer, MA, USA) was used to evaluate the weight loss associated MA, USA) was used to evaluate the weight loss associated with water at 30 ◦C to 800 ◦C at 20 ◦C/min with water at 30 °C to 800 °C at 20 °C/min in a N2 gas environment. in a N2 gas environment.

3. Results Results and and Discussion Discussion

3.1. Flow Flow and and Setting Setting Times Times The measured flow flow values of the paste according to to the substitution ratio ratio of of CNS CNS are are shown shown in in Figure 11.. TheThe 0%0% CNS sample without CNSCNS substitutionsubstitution overflowsoverflows thethe flowflow tabletable duedue toto thethe highhigh ww/b/b ratio. However, However, from from 10% 10% CNS CNS paste, paste, flow flow rate rate decreases decreases steeply steeply as as the the substitution substitution ratio ratio of of CNS increases.increases. In In particular, particular, the the 50% 50% CNS CNS sample sample showed showed a low flow a low value flow of 127.5 value mm. of 127.5 This substitution mm. This substitutionof CNS promotes of CNS the promotes hydration the reactionhydration of r OPCeaction and of reducesOPC and the reduces flow. Thisthe flow. is similar This is to similar the flow to thecharacteristics flow characteristics of previous of previous studies using studies PNS. using PNS.

Figure 1. FlowFlow values values by by CNS CNS replacement replacement rate.

InIn a previous study using most nano-silicas, nano-silicas, a superplasticizer was used. This This is is due due to to the heheterogeneousterogeneous dispersion dispersion due due to to the the high high specific specific surface area area of the nano nano-silica-silica particles and the sudden drop drop in in fluidity. fluidity. Particularly, Particularly, in the in case the ofcase powdered of powdered nano-silica, nano- sincesilica, the since fluidity the is fluidity seriously is seriouslydeteriorated, deteriorated, a lot of experiments a lot of experiments using the superplasticizer using the super areplasticizer performed. are This performed. study used This CNS study with usedrelatively CNS homogeneous with relatively dispersibility homogeneous than dispersibility PNS. And a highthan w PNS./b ratio And of 0.5a high was chosenw/b ratio to excludeof 0.5 was the choseneffect of to the exclude superplasticizer. the effect However, of the superplasticizer. as shown in Figure However,1, the abrupt as shown decrease in Figure in fluidity 1, the is due abrupt to a decrease in fluidity the w/b is ratio due with to a andecrease increasing in the CNS w/b replacement ratio with an ratio. increasing CNS replacement ratio. The results results of of measuring measuring the the setting setting time time according according to to the the Gillmore Gillmore needle needle method method are are shown shown in Figurein Figure 2. As2. the As substitution the substitution rate of rate CNS of increased, CNS increased, initial initialand final and setting final settingtime decr timeeased. decreased. Chithra etChithra al. [19] et also al. [ 19showed] also showed the setting the settingtimes of times colloidal of colloidal nano-silica nano-silica substituted substituted for OPC for OPCby 0.5%, by 0.5%, 1%, 1.5%,1%, 1.5%, 2%, 2%, 2.5% 2.5%,, and and 3%. 3%. As As the the CNS CNS replacement replacement rate rate increases, increases, the the setting setting time time decreases. In particularparticular,, thethe reductionreduction of of the the final final setting setting time time is moreis more steep steep than than before. before Decreasing. Decreasing the settingthe setting time timecanbe can thought be thought of as of being as being caused caused by two by reasonstwo reasons in this in study.this study One. One is the ispozzolanic the pozzolanic reaction reaction and andnuclear nuclear effects effects of nano-silica of nano particles.-silica particles. This is a known This is eff ect a known from previous effect studiesfrom previous [8,19,21, 26 studies,39,40]. [8,19,21,2The other6, is39 the,40] e.ff Theect of other w/b reduction is the effect due to of the w/b mixing reduction water weight due to substitution the mixing of waterCNS. weight substitution of CNS. Appl. Sci. 2019, 9, 4358 5 of 20 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 19

FigureFigure 2. 2. SettingSetting time time of of the the CNS CNS replacement replacement rate rate..

3.2.3.2. Compressive Compressive Strength Strength FigureFigure 33 shows shows the the results results of of compressive compressive strength strength measurements measurements according according to the to the ages ages of the of testthe testspecimens. specimens As. the As replacement the replacement rate of rate CNS of increased CNS increased from 0% from to 50%, 0% the tostrength 50%, the was strength observed was to observedincrease at to all increase the measurement at all the measurement ages in Figure 3agesa. The in 1-dayFigure strength 3a. The increased 1-day strength from 15.04 increased MPa for from 0% 15.04CNS toMPa 33.37 for MPa 0% CNS for 50% to 33.37 CNS, MPa while for the 50% 3-day CNS, strength while increasedthe 3-day fromstrength 30.51 increased MPa for from 0% CNS 30.51 to MPa50.69 for MPa 0% for CNS 10% to CNS; 50.69 the MPa 7-day for 10% strength CNS; increased the 7-day from strength 33.72 increased to 61.33 MPa, from followed 33.72 to by61.33 a 28-day MPa, followedincrease fromby a 28 40.48-day MPa increase of 0% from CNS 40.48 to 75.41 MPa MPa of 0% of 10%CNS CNS. to 75.41 MPa of 10% CNS. ReplacementReplacement of CNS CNS exhibited exhibited a asignificant significant eff effectect on onthe the improvement improvement in strength, in strength, and the and e ff theect effectwas further was further increased increased according according to the CNSto the substitution CNS substitution ratio. In ratio. previous In previous PNS studies, PNS thestudies, strength the strengthenhancement enhancement effect of nano-silicaeffect of nano particles-silica particles in the early in the stages early of stages hydration of hydration is already is knownalready [ 6known,20,22]. [In6,20,22] the present. In the study,present in study, which in CNS which was CNS applied was applied as a weight as a weight substitution substitution method method for mixing for mixing water, water,the initial the strength initial strength improvement improvement effect was effect consistent was consistent with previous with previous PNS studies. PNS studies. This is becauseThis is becausenano-silica nano particles-silica particles promote thepromote hydration the hydration of cement of and ceme actnt as aand nucleation act as a site nucleation in the formation site in the of formationhydration of products hydration [5,20 products,26,41]. This [5,20,26, nucleating41]. This eff nucleatingect contributes effect to contributes the improvement to the improvement in strength by ingenerating strength by dense generating hydration dense products hydration during products the early during stage the of hydrationearly stage [ 20of, 42hydration,43]. In addition [20,42,43 to]. Inthe addition improvement to the improvement of the microstructure of the microstructure due to the filler due eff toect, the the filler pozzolanic effect, the reaction pozzolanic promotes reaction the promoteshydration the of thehydration cement, of thereby the cement, improving thereby the improving strength [ 5the,7,44 strength]. [5,7,44]. FigureFigure 33bb shows shows the the compressive compressive strength strength results results of previous of previous studies studies using mixing using mixing water weight water weigsubstitutionht substitution method method and binder and weight binder substitution weight substitution method. Said method. et al. Said [7] used et al. colloidal [7] used nano-silica colloidal nanowith- meansilica particle with mean size particleof 35 nm, size 50% of SiO 352 nm,contents, 50% and SiO2 pH contents of 9.5., Twoand kinds pH of of 9.5. binders, Two OPC kinds and of binders,OPC + Fly OPC ash, and were OPC used, + and Fly the ash compressive, were used, strengths and the of compressive 3-day and7-day strengt werehs ofmeasured 3-day and by adding 7-day wereCNS to measured each binder by (notadding the CNSsubstitution to each method). binder (not Both the the substitution OPC and OPC method).+ Fly ash Both binding the OPC materials and OPCincreased + Fly in ash 3-day binding and 7-daymaterials compressive increased strength in 3-day as and the 7 weight-day compressive substitution strength rate of CNSas the increased weight substitutionto 0%, 6%, and rate 12%. of CNS Chithra increased et al. to [19 0%,] used 6%, a a CNSnd 12%. with Chithra a particle et al. size [19] of used 5–40 a nm, CNS 40%–41.5% with a particle SiO2 sizecontents, of 5–40 and nm, a pH40% of–41.5% 10. CNS SiO2 was contents replaced, and bya pH 0.5%, of 10. 1%, CNS 1.5%, was 2%, replaced 2.5%, and by 0.5%, 3% of 1%, the 1.5%, OPC 2%, and 2.5%the 3-day,, and 7-day,3% of the and OPC 28-day and compressive the 3-day, 7 strengths-day, and were 28-day measured. compressive Experimental strengths resultswere measured. show that Experimenas the concentrationtal results of show CNS increases that as the from concentration 0% to 2%, the of compressive CNS increases strength from increases 0% to at 2%, all ages. the compressiveThe highest compressive strength increases strength at occurs all ages. at 2% The CNS, highest after whichcompressive the strength strength decreases occurs until at 2% 3% CNS, CNS afterconcentration which the is strength reached. decreases Nikravan until et al. 3% [45 CNS] is theconcentration result of compressive is reached. Nikravan strength using et al. CNS[45] is with the resultpH 10.23, of compressive mean particle strength size 2.1 using nm andCNS the with specific pH 10.23, gravity mean 1.21. particle CNS replaced size 2.1 nm 0%, and 1.5% the and specific 3% of gravity 1.21. CNS replaced 0%, 1.5% and 3% of cement weight. The 1-day, 3-day, 7-day, and 28-day compressive strengths increased with increasing concentrations of CNS. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 19

In previous studies using CNS, binder mass displacement methods have increased compressive strength as the concentration of CNS increases [7,45]. In other studies, however, there was a specific concentration of CNS that produced the highest compressive strength [19]. The specific CNS concentration with the highest intensity is the point where the nano-silica particles Appl.achieve Sci. 2019 a homogeneous, 9, 4358 dispersion and the pozzolanic reaction, nucleation effect and filler effect6 of 20 are optimal. However, exceeding the optimal CNS concentration, the compressive strength decreases due to the drastic loss of fluidity due to uneven dispersion of nano-silica particles and cement weight. The 1-day, 3-day, 7-day, and 28-day compressive strengths increased with increasing lack of units due to high specific surface area. concentrations of CNS.

(a)

(b)

Figure 3. 3. CNSCNS replacement replacement ratio ratio and and compressive compressive strength strength according according to age ( toa) this age study, (a) this (b ) study, compared (b) withcompared previous with studies. previous studies.

FigureIn previous 4 shows studies the using relative CNS, strength binder mass increase displacement ratios for methods each CNShave increased replacement compressive ratio as comparedstrength as to the the concentrationstrength of the of 0% CNS CNS increases specimen. [7, 45As]. the In su otherbstitution studies, rate however,of the CNS there increases, was a specific concentration of CNS that produced the highest compressive strength [19]. The specific CNS concentration with the highest intensity is the point where the nano-silica particles achieve a homogeneous dispersion and the pozzolanic reaction, nucleation effect and filler effect are optimal. However, exceeding the optimal CNS concentration, the compressive strength decreases due to the Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 19

the rate of relative strength increase increases. In Figure 4, the highest relative strength growth rate varies with age and CNS substitution rate. The 10% CNS sample had the highest relative intensity increase of 7-day, the 20% CNS sample was 28-day, and the 30% to 50% CNS sample was 1-day. If the CNS substitution rate was less than 20%, it affected the strength of late age from 7-days to 28-days, and the CNS substitution rate from 30% to 50% greatly affected the initial 1-day strength increase rate. In particular, the 1-day relative strength increase rates were 174.5% at a 30% CNS replacement rate, 205.3% at a 40% CNS replacement rate, and 221.8% at a 50% CNS replacement rate. The 1-day intensity increase of the 40% and 50% CNS replacement rate samples is about twice the increase compared to the sample that did not replace the CNS. If the substitution rate of the CNS exceeds 30%, the rate of increase in the daily strength is then sharply improved. The compressive strength results of previous studies using PNS are consistent with reports that nano-silica particles have a significant effect on the initial strength improvement [6,21,22,43,46]. However, after a rapid increase in intensity at 1-day, the rate of increase in strength from 3-days to 28-days decreases significantly. The decrease in strength increase after 1-day is similar to the trend in strength change shown in Appl. Sci. 2019, 9, 4358 7 of 20 previous studies using PNS [26]. Therefore, the strength-improving effect of nano-silica particles is known to have the greatest effect at the beginning of hydration within 1-day [28]. As a result, as drasticshown lossin Figure of fluidity 4, the due strength to uneven increase dispersion rate of of30% nano-silica to a 50% particles CNS substitution and lack of rate units was due highest to high at specific1-day. However, surface area. the remaining 3-, 7-, and 28-day strength increase rates were significantly lower thanFigure those 4at shows 1-day. the relative strength increase ratios for each CNS replacement ratio as compared to the strengthChithra of et the al. 0% [19] CNS analyzed specimen. the Ascompressive the substitution strength rate increase of the CNS rate increases, of the CNS the with rate of0.5%, relative 1%, strength1.5%, 2%, increase 2.5% and increases. 3% replacement In Figure of4, the highestbinder mass. relative As strengtha result, growththe 1-day rate strength varies withincresase age and rate CNSis the substitution largest and rate.decreases The 10%in the CNS order sample of 3-day hads, the 7-day highests, and relative 28-day intensitys. Also, as increase the substitution of 7-day, rate the 20%of CNS CNS increases, sample was the 28-day, rate of and the the increase 30% to in 50% strength CNS sample increases. was Therefore, 1-day. If the the CNS rate substitution of increase rate of wasstrength less than is high 20%, under it aff ectedthe condition the strength of early of late age age and from high 7-days CNS to replacement 28-days, and rate. the CNSThis is substitution consistent ratewith from the tendency 30% to 50% of increasing greatly affected the compressive the initial 1-day strength strength of this increase study. rate.

FigureFigure 4.4. Compression strengthstrength increaseincrease raterate accordingaccording toto thethe CNSCNS replacementreplacement ratio.ratio.

InStudies particular, have the also 1-day showed relative strength that, if increase the nano rates-silic werea particles 174.5% at are a 30% excessively CNS replacement mixed, rate, the 205.3%compressive at a 40% strength CNS replacement decreases [27]. rate, Therefore, and 221.8% the at aexcess 50% CNS SiO2replacement nanoparticles rate. can The leach 1-day out intensity without increaseany further of the chemical 40% and reaction 50% CNS and replacementserve only as rate filler samples and can is no about longer twice effectively the increase contribute compared to the to thecompressive sample that strength did not of replace binary theblended CNS. Ifconcrete. the substitution This situation rate of can the also CNS occur exceeds due 30%, to the the fact rate that of increasethe excess in theof silica daily can strength transform is then the sharply original improved. C-S-H gel The (lower compressive Ca/Si ratio) strength into tobermorite, results of previous which studiesexhibits using lower PNS strength are consistent properties with and reports increases that the nano-silica extent of particles silicate havepolymerization a significant [47 eff,48ect]. onIt may the initial strength improvement [6,21,22,43,46]. However, after a rapid increase in intensity at 1-day, the rate of increase in strength from 3-days to 28-days decreases significantly. The decrease in strength increase after 1-day is similar to the trend in strength change shown in previous studies using PNS [26]. Therefore, the strength-improving effect of nano-silica particles is known to have the greatest effect at the beginning of hydration within 1-day [28]. As a result, as shown in Figure4, the strength increase rate of 30% to a 50% CNS substitution rate was highest at 1-day. However, the remaining 3-, 7-, and 28-day strength increase rates were significantly lower than those at 1-day. Chithra et al. [19] analyzed the compressive strength increase rate of the CNS with 0.5%, 1%, 1.5%, 2%, 2.5% and 3% replacement of the binder mass. As a result, the 1-day strength incresase rate is the largest and decreases in the order of 3-days, 7-days, and 28-days. Also, as the substitution rate of CNS increases, the rate of the increase in strength increases. Therefore, the rate of increase of strength is high under the condition of early age and high CNS replacement rate. This is consistent with the tendency of increasing the compressive strength of this study. Studies have also showed that, if the nano-silica particles are excessively mixed, the compressive strength decreases [27]. Therefore, the excess SiO2 nanoparticles can leach out without any further Appl. Sci. 2019, 9, 4358 8 of 20 chemical reaction and serve only as filler and can no longer effectively contribute to the compressive strength of binary blended concrete. This situation can also occur due to the fact that the excess of silica can transform the original C-S-H gel (lower Ca/Si ratio) into tobermorite, which exhibits lower strength properties and increases the extent of silicate polymerization [47,48]. It may also be due to the defect in the dispersion of nano-silica particles that causes weak zones [22]. However, the weight replacement method of mixing water used in this study did not result in decreased strength, even with 50% CNS replacement ratio. This is because the dispersion of nano-silica particles is enhanced by liquid CNS. Compared with the previous studies using PNS, the strength improvement effect was excellent for the CNS ratios that were tested. In addition, the method of this study, which substituted CNS for the weight of mixing water instead of replacing the weight of the existing binder, meant that as the substitution ratio of CNS increased, the strength was improved because there was no weight change of the binder. In this study, the weight water substitution method of mixing water has the effect of reducing the w/b ratio according to the density difference of mixing water and CNS. Therefore, as the substitution ratio of CNS increased, the decrease of w/b ratio affected the increase of compressive strength.

3.3. Hydration Products Figure5 shows the XRD results for the 0% CNS and 50% CNS specimens. Representative reaction products included portlandite, calcite, ettringite, and C-S-H gel. 0% CNS hydration reactants are portlandite, ettringite, C-S-H gel, calcite, quartz, and gypsum. The hydration reactants of 50% CNS were similar to 0% CNS. The 50% CNS samples showed little change in peak size, for portlandite, ettringite, and C-S-H gel, which are major hydration reactants in the 1-day and 28-day periods. The gypsum was observed in the 0% CNS specimen; however, it was not observed in the 50% CNS specimen. The nano-silica particles were not observed in 50% CNS gypsum because gypsum and portlandite were consumed by the formation of ettringite by the pozzolanic reaction [8,20,27,31]. The pozzolanic reaction is further promoted as the number of nano-silica particles increased [49–52]. However, as shown in Figure5b, there was no change in the size of the C-S-H peak from 1-day to 28-days, even in case of the 50% CNS specimen. This is because the reaction between CNS and OPC occurs mostly before 1-day, which is the initial hydration stage, and then the hydration reaction proceeded very slowly. The reason why the initial hydration reaction is performed rapidly is explained as follows. It is known that the particles in CNS are characteristically composed of mono-dispersed spherical particles consisting of an amorphous SiO2 core with a hydroxylated surface, making them small enough to remain suspended in a liquid medium without settling [39,53,54]. Additionally, the nano-silica particles exhibit a high surface energy, which is known to react easily with other substances. Therefore, the pozzolanic reaction of nano-silica particles is improved during the initial hydration reaction step [6]. This affects the reduction of flow value and setting time as the amount of CNS increases, as in the results already mentioned in Figures1 and2. In addition, the increase in the relative compressive strength shown in the Figure4 is also markedly increased in the 1-day period, so that the initial hydration reaction promoting effect can be confirmed. Figure5c shows the XRD results after 28-day according to the CNS replacement percentages that ranges from 0% to 50%. The amount of portlandite decreased as CNS increased to 50%. The gypsum has the highest peak at 10% CNS and then decreases to 50% CNS. Thus, gypsum peaks are rarely observed in 50% CNS. Peaks of ettringite from 0% to 50% CNS showed no significant change in size and were almost similar. The decreasing tendency between the ettingite and gypsum peaks is similar. At a substitution rate of less than 20% CNS, nano-silica particles promotes the formation of ettringite and C-S-H gel by the action of pozzolanic reaction and nucleation effects [5–7,20,22,26,34,55]. However, at replacement rates above 20% CNS, nano-silica particles have a major impact on filler effects [5,6]. Thus, there is little change in ettringite or C-S-H peaks in XRD above 20% CNS. The pozzolanic reaction of these nano-silica particles is most apparent during the initial stage of hydration, which reduces the Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 19

Appl. Sci. 2019, 9, 4358 9 of 20 A previous study related to PNS reported that portlandite decreased rapidly during the first 7-day, which then declined slowly after 7-days. This indicates that most of the pozzolanic reactions coagulationoccurred during time and the improvesfirst 7-day thes [29]. initial There strength is almost [8,19 no,21 change,26,39 ,40in ].the The ettringite eﬀectof and these C-S nano-silica-H peaks particlesfrom 3 has-day alreadys to 28-day beens, as mentioned shown in inFigure theﬂowability 5b. However, (Figure CNS1 has) and a faster setting hydration time (Figure reaction2). effect than PNS.

(a)

(b)

Figure 5. Cont. Appl. Sci. 2019, 9, 4358 10 of 20

Appl. Sci. 2019, 9, x FOR PEER REVIEW 10 of 19

(c)

FigureFigu 5.re XRD5. XRD results results after after 1-day 1-day and and 28-day: 28-day: (a) ( 0%a) 0% CNS, CNS, (b) ( 50%b) 50% CNS, CNS, (c) compared(c) compared XRD XRD with with CNS, P: portlandite,CNS, P: portlandite, E: ettringite, E: ettringite, G: gypsum, G: gypsum, C: calcite, C: calcite, M: magnesium M: magnesium oxide, oxide, S: quartz, S: quartz, A: alite, A: alite, B: belite, B: CSH:belite, calcium CSH: silicatecalcium hydrate. silicate hydrate.

A previousPrevious studystudies related using toCN PNSS have reported shown that that portlandite the generation decreased of C-S rapidly-H gel is during accelerated the first in 7-day,the whichinitial then hydration declined stage slowly before after 12 7-days. h [28]. This This means indicates that that the initial most ofhydration the pozzolanic reaction reactions of CNS is occurred much duringfaster the than first the 7-days PNS.[ 29In]. view There of is the almost changes no change in these in the reactants, ettringite CNS and accelerates C-S-H peaks hydr fromation 3-days more to 28-days,rapidly as as shown compared in Figure to the5b. However, powdered CNS nano has-silica a faster particles hydration in previous reaction studies. e ffect Therefore,than PNS. the substitution of CNS during the initial hydration stage before 1-day reacts with portlandite to cause Previous studies using CNS have shown that the generation of C-S-H gel is accelerated in the a rapid pozzolanic reaction. Therefore, portlandite consumption is accelerated during the first initial hydration stage before 12 h [28]. This means that the initial hydration reaction of CNS is much 1-day, and the reaction with nano-silica particles gradually slows as the amount of portlandite faster than the PNS. In view of the changes in these reactants, CNS accelerates hydration more rapidly decreases. The peaks of ettringite and C-S-H did not show a sharp increase, even when the mixing as compared to the powdered nano-silica particles in previous studies. Therefore, the substitution of ratio of CNS increased from 0% to 50%. This is because the amount of OPC (the main source of CNScalcium during and the aluminum) initial hydration required stage to form before ettringite, 1-day reacts and withC-S-H portlandite does not change. to cause Therefore, a rapid pozzolanic most of reaction.the portlandite Therefore, is portlandite consumed consumption in the initial hydrationis accelerated stage during and the the firstamount 1-day, of and hydrat theio reactionn reactant with nano-silicachanges little particles as the gradually age increases. slows as the amount of portlandite decreases. The peaks of ettringite and C-S-HThese did results not show are a supported sharp increase, by previous even when studies the mixing that have ratio reported of CNS increased that the hydration from 0% to 50%.acceleration This is because caused the by amountthe PNS ofexhibits OPC (the a high main correlation source of with calcium sufficient and aluminum)calcium-adsorption required ability to form ettringite,[39,42]. In and the C-S-H present does study, not CNS change. and mixing Therefore, water most weight of the substitution portlandite methods is consumed show little in the change initial hydrationin the hydration stage and reactant the amount peak in of XRD hydration analysis reactant after 1- changesday due to little the as rapid the agehydration increases. reaction in the initialThese hydration results are stage. supported by previous studies that have reported that the hydration acceleration caused by the PNS exhibits a high correlation with sufficient calcium-adsorption ability [39,42]. In the3.4. present Pore Structure study, CNS and mixing water weight substitution methods show little change in the hydrationFigure reactant 6 shows peak the in XRD pore analysis structure after of the 1-day specimens due to the with rapid 0%, hydration 30%, and reaction 50% CNS in bythe MIP initial hydrationanalysis stage.at 1-day and 28-days. Figure 6a shows the pore structure analysis of the 1-day samples. As the amount of CNS increases, the graph gradually moves to the left. This indicates that the diameter and amount of pores gradually decrease. The 28-day samples in Figure 6b also show pore changes Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 20 thatAppl. were Sci. 2019 similar, 9, 4358 to that for 1-day. In addition, the MIP results shown in Figure 6 confirm that11 ofthe 20 CNS mixing method applied in this study has the effect of reducing the diameter and amount of pores. 3.4. Pore Structure Previous researchers have reported that fine nano-silica particles form a dense matrix by filling poresFigure between6 shows hydration the pore reactants, structure thereby of the improving specimens withmechanical 0%, 30%, performance and 50% CNS [5,6] by. The MIP results analysis of thisat 1-day study and applyin 28-days.g the Figure new 6mixinga shows-method the pore using structure CNS analysisshow the of same the 1-day effect samples. as the previous As the amountstudies. Thisof CNS is because increases, nano the-silica graph particles gradually reduce moves the to porosity the left. Thisof the indicates matrix through that the diameterthe nucleation and amount effect, pozzolanicof pores gradually reaction, decrease. and filler The effect 28-day in samplescement inhydration Figure6b reaction. also show This pore effect changes is further that were enhanced similar withto that increasing for 1-day. amounts In addition, of nano the MIP-silica results particles shown [7,8,21,29 in Figure–31,43,44,566 confirm] that. Figure the CNS6 also mixing shows method that as theapplied amount in this of CNS study increases, has the e theffect pore of reducing size and theamount diameter decrease and amountdue to the of effects pores. mentioned above.

(a)

(b)

FigureFigure 6. 6. MMIPIP results, results, ( (aa)) 1 1-day,-day, (b)) 28 28-day.-day. Appl. Sci. 2019, 9, 4358 12 of 20

Previous researchers have reported that fine nano-silica particles form a dense matrix by filling pores between hydration reactants, thereby improving mechanical performance [5,6]. The results of this study applying the new mixing-method using CNS show the same effect as the previous studies. This is because nano-silica particles reduce the porosity of the matrix through the nucleation effect, pozzolanic reaction, and filler effect in cement hydration reaction. This effect is further enhanced with increasing amounts of nano-silica particles [7,8,21,29–31,43,44,56]. Figure6 also shows that as the amount of CNS increases, the pore size and amount decrease due to the effects mentioned above. There are various studies on the mechanism and effect on the change of pore structure of nano-silica particles. Once a small amount of nano-silica particles is homogeneously mixed with the cement paste, the cement hydration products can react with these particles due to their ultra-large surface area and high energy, and a continuous solid mass forms around the nano-silica particles. The nano-silica particles that were embedded in the cement paste can further promote and accelerate the hydration process due to their ultrahigh reactivity, causing the formation of a compact microstructure with uniformly distributed mass [22]. Therefore, the pores in the reaction products are reduced. However, increases in the amount of trapped air can be expected in PNS that contain pastes in fresh state conditions. Substitution of nano-silica particles has implications for adversely affecting the pores reduction effects. This is due to the fact that nano-silica particles introduce a high gelling tendency, thereby increasing the entrapped air content, as observed through experimental results in Senff et al. [32]. This increase in porosity is caused by the non-uniform dispersion of the nano-silica particles and the abrupt decrease of the fluidity due to the lack of mixing water as the amount of nano-silica particles increases. Therefore, the use of CNS reduces the rapid deterioration of fluidity and induces dispersion of nano-silica particles better than PNS. Mindess et al. [57] classified the pores of cement paste into large capillary pores (10-0.05 µm), medium capillary pores (0.05-0.01 µm), and gel pores (<0.01 µm). The gel pores represent the intrinsic porosity of C-S-H gel. That is, an increase in gel pores means an increase in C-S-H gel, indicating that a dense hydration matrix has been produced. Table3 summarizes the pore size distribution in the 0%, 30%, and 50% CNS specimens in Figure6. As the amount of CNS increases, the large capillary pores gradually decrease, while the middle capillary pores and the gel pores increase. This is due to the hydration promoting action and the filler effect associated with the pozzolanic reaction of CPC and OPC particles. On the 28-day, large capillary pores decreased and gel pores increased than 1-day. Thus, the CNS forms a dense OPC mixture matrix. These results are consistent with those observed in previous studies using PNS [29,30,43,56] or CNS [7,34].

Table 3. Pore size distribution according to the CNS replacement ratio.

CNS Contents 0% 30% 50% 1-Day 28-Day 1-Day 28-Day 1-Day 28-Day Large capillary pores 61.51 48.33 67.25 42.72 32.65 26.2 (10-0.05 µm), (%) Medium capillary pores 18.55 19.94 11.6 23.51 33.09 32.65 (0.05-0.01 µm), (%) Gel pores 19.91 31.73 21.15 33.77 34.26 41.16 (<0.01 µm), (%) Total porosity, (%) 22.21 19.95 17.79 16.43 17.1 14.11

The total porosity also decreased as the amount of CNS increased, with 28-day total porosity being smaller than 1-day. Said et al. [7] reported the total porosity of samples with 0%, 6%, and 12% addition of CNS to OPC and OPC + Fly ash weights. As a result, OPC samples decreased to 10.13%, 6.91%, and 6.44%, and OPC + Fly ash samples decreased to 12.56%, 9.30%, and 8.21%, respectively. The total porosity decreased with increasing amount of CNS, consistent with the results of this study. Appl. Sci. 2019, 9, 4358 13 of 20

3.5.Appl. Thermal Sci. 2019, Analysis9, x FOR PEER REVIEW 13 of 20

ettringite.The results As the of age thermal gradually analysis increased of the 0% to CNS1-, 3 and-, and 50% 28 CNS-days specimens, weight loss are also showed increased. in Figure This7. Theindicates TG/DTG that resultsthe formation for the of 0% ettringite CNS specimen and C-S are-H gel presented were increased in Figure with7a. the The increasing weight loss age. in the temperatureAt 100 °C range–200 of °C 50 [61◦C–200], the weight◦C[58] orloss 120 of ◦gypsumC–145 ◦C[ was2] wasconfirmed. due to theThis water was supported loss of the C-S-Hby the gel.gypsum The weightpeak observed loss observed in the at XRD 80 ◦ C–130of Figure◦C[ 595a.] orIn 110Figure◦C–150 5a, the◦C[ gypsum60] was peak due toshowed the water a larger loss ofpeak ettringite. height at As 28 the-day ages than gradually at 3-day increaseds. In Figure to 1-, 7 3-,a, 28 and-day 28-days,s shows weight more lossweight also loss increased. than 3-day Thiss. indicatesThis means that that the the formation hydration of ettringiteof OPC particles and C-S-H continues gel were to increased28-days. with the increasing age. AtThe 100 weight◦C–200 loss◦C[ at61 400], the°C– weight450 °C loss[62] ofis gypsumdue to the was water conﬁrmed. loss of portlandite. This was supported In the 0% by CNS the gypsumspecimen, peak as the observed age increased, in the XRD the ofweight Figure loss5a. Inrate Figure of portlandite5a, the gypsum also increased. peak showed This aindicated larger peak that heightthe cement at 28-days particles than were at 3-days. continuously In Figure hydrating7a, 28-days after shows the moreinitialweight hydration loss stage than 3-days.and that This the meansweight that loss the at 665 hydration °C–800 of°C OPC [63] or particles 720 °C– continues760 °C [64 to] was 28-days. due to the water loss of calcite. TheFigure weight 7b shows loss atthe 400 results◦C–450 of TG/DTG◦C[62] is analysis due to theof the water 50% loss CNS of specimen. portlandite. The In weight the 0% loss CNS of specimen,C-S-H gel aswas the observed age increased, at 50 °C the–200 weight °C [58 loss] or rate 120 of °C portlandite–145 °C [64 also]. Water increased. loss of Thisettringite indicated was thatalso theobserved cement at particles 80 °C–130 were °C [ continuously59] or 110 °C– hydrating150 °C [58]. after However, the initial the hydration weight loss stage of gypsum and that (100 the weight°C–200 loss°C [61 at]) 665 observed◦C–800 in◦C[ Figure63] or 7a 720 did◦C–760 not occur.◦C[64 Unlike] was duethe 0% to the CNS water sample, loss ofthe calcite. weight loss rate due to waterFigure loss 7ofb portlandite shows the resultsobserved of TGat 400/DTG °C–450 analysis °C [62 of] was the 50%similar CNS at 1 specimen.-, 3-, and 28 The-day weights. Further, loss ofthe C-S-H weight gel reduction was observed rate of at portlandite 50 ◦C–200 in◦C[ the58 50%] or 120CNS◦ C–145specimen◦C[ was64]. less Water than loss that of of ettringite the 0% CNS was alsospecimen. observed This at indicates 80 ◦C–130 that◦C[ most59] or of 110 the◦C–150 portlandite◦C[58 ].was However, consumed the weightin hydration loss of and gypsum that (100pozzolanic◦C–200 reacted◦C[61]) during observed the inearly Figure stage7a didof hydration not occur. (initial Unlike 24 the h 0%). This CNS is sample,consistent the with weight the lossobservations rate due toof Björnström water loss ofet portlanditeal. [28] that colloidal observed nano at 400-silica◦C–450 rapidly◦C[ forms62] was the similar C-S-H atgel 1-, within 3-, and 12 28-days.h. Therefore, Further, the thereplacement weight reduction of CNS ratewith of the portlandite weight of in mixing the 50% water CNS accelerates specimen was the lesshydration than that of ofcement the 0% particles, CNS specimen. and this Thisphenomenon indicates thatis mostly most completed of the portlandite during wasthe initial consumed 24 h. in hydration and that pozzolanicWhen the CNS reacted is replaced during the by earlythe mixing stage ofwater hydration weighing (initial method 24 h). of This this is study, consistent portlandite with the is observationsrapidly consumed of Björnström before 1-day et al. to [28 form] that a dense colloidal hydration nano-silica reaction rapidly matrix. forms Therefore the C-S-H, it is gel a withinmixing 12method h. Therefore, that can the improve replacement the mechanical of CNS with performance the weight of sufficiently mixing water compared accelerates with the the hydration PNS and of cementbinder weight particles, substitution and this phenomenon method used is in mostly the previous completed research. during the initial 24 h.

(a)

Figure 7. Cont. Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 20

Appl. Sci. 2019, 9, 4358 14 of 20

Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 20

(b)

Figure 7. Thermal analysis: (a) 0% CNS, (b) 50% CNS.

3.6. Microstructure (b) Figure 8 shows the SEM images of the specimens with 0%, 30%, and 50% CNS replacement. FigureFigure 7. 7.Thermal Thermal analysis: analysis: (a(a)) 0% 0% CNS, CNS, ( b(b)) 50% 50% CNS. CNS. The presence of ettringite and C-S-H gel observed in the XRD results of Figure 5 was confirmed. The CNS-substituted specimens exhibit a needle-like shape ettringite in the reaction products 3.6. WhenMicrostruc the CNSture is replaced by the mixing water weighing method of this study, portlandite is rapidly[26]. As consumedshown in beforeFigure 1-day8a, this to ettringite form a dense is also hydration observed reaction inside matrix.the pores. Therefore, However, it is when a mixing the Figure 8 shows the SEM images of the specimens with 0%, 30%, and 50% CNS replacement. methodCNS replacement that can improve rate increased the mechanical to 30%, performance a thin bed suettringiteﬃciently was compared rarely withobserved, the PNS as andshown binder in The presence of ettringite and C-S-H gel observed in the XRD results of Figure 5 was confirmed. weightFigure 8b. substitution As showed method in Figure used 8c, in thelittle previous ettringite research. was observed in 50% CNS specimens. However, the reactionThe CNS product-substituted matrix specimens exhibited aexhibit dense astate needle with-like almost shape no ettringite pores. The in theincrease reaction in theproducts CNS 3.6.substitution[26] Microstructure. As show raten inpromoted Figure 8a,the thisformation ettringite of hydrationis also observed reactants, inside such the as pores.C-S-H However,gel, by the when reaction the ofCNS nano replacement-silica particles rate withincreased portlandite. to 30%, Therefore, a thin bed the ettringite nano-silica was particles rarely observed,of CNS improved as shown the in Figure8 shows the SEM images of the specimens with 0%, 30%, and 50% CNS replacement. The microstructureFigure 8b. As showeddencification in Figure by increasing 8c, little ettringite the reactivity was observed with the in cement 50% CNS particles specimens. using However,the filler presence of ettringite and C-S-H gel observed in the XRD results of Figure5 was conﬁrmed. effectthe reaction and the productpozzolanic matrix reaction exhibited [22,31]. a dense state with almost no pores. The increase in the CNS substitution rate promoted the formation of hydration reactants, such as C-S-H gel, by the reaction of nano-silica particles with portlandite. Therefore, the nano-silica particles of CNS improved the microstructure dencification by increasing the reactivity with the cement particles using the filler effect and the pozzolanic reaction [22,31].

(a)

Figure 8. Cont.

(a) Appl. Sci. 2019, 9, 4358 15 of 20

Appl. Sci. 2019, 9, x FOR PEER REVIEW 15 of 20

(b)

(c)

Figure 8. SEM images at 28 28-day:-day: ( (aa)) 0% 0% CNS, CNS, ( (bb)) 30% 30% CNS, CNS, ( (cc)) 50% 50% CNS. CNS.

FigureThe CNS-substituted 9 shows the Ca/Si specimens and Al/Si exhibit ratios a needle-like by performing shape energy ettringite dispersive in the reaction spectroscopy products (EDS) [26]. analysisAs shown on in the Figure hydration8a, this products ettringite that is alsoare observedexhibited insidein Figure the 8. pores. The Ca/Si However, ratio whenof the the reaction CNS productreplacement C-S-H rate gel increased was 2.76, to2.31, 30%, and a thin1.68 bedin the ettringite 0%, 30%, was and rarely 50% observed,CNS specimens, as shown respectively. in Figure 8Asb. theAs showedsubstitution in Figure ratio8 ofc, littleCNS ettringiteincreased, was the observed nano-silica in 50%particles CNS increased, specimens. and However, the Si of the C reaction-S-H gel alsoproduct increased. matrix As exhibited a result, a densethe Ca/ stateSi ratio with of almost the reactants no pores. decreased. The increase Previous in the studies CNS substitution with PNS haverate promoted shown that the the formation Ca/Si ratio of hydration decreases reactants, with increasing such as C-S-H nano gel,-silica by concentrations the reaction of nano-silica[21,29,34]. Furthermore,particles with in portlandite. the previous Therefore, study, the the increase nano-silica in the particles amount of CNSof nano improved-silica increases the microstructure the Al/Ca ratiodencification of the hydration by increasing reactant the [27] reactivity and the withtendency the cementof increasing particles the usingAl/Ca theratio filler shown effect in Figure and the 9 ispozzolanic similar. The reaction method [22 of,31 substituting]. CNS for the weight of mixing water in this study showed that the Ca/SiFigure and9 shows Al/Ca theratios Ca /ofSi the and hydration Al/Si ratios reactant by performings were similar energy to dispersive those of the spectroscopy previous studies (EDS) usinganalysis PNS on or the CNS. hydration products that are exhibited in Figure8. The Ca /Si ratio of the reaction product C-S-H gel was 2.76, 2.31, and 1.68 in the 0%, 30%, and 50% CNS specimens, respectively. As the substitution ratio of CNS increased, the nano-silica particles increased, and the Si of C-S-H gel also increased. As a result, the Ca/Si ratio of the reactants decreased. Previous studies with PNS have shown that the Ca/Si ratio decreases with increasing nano-silica concentrations [21,29,34]. Furthermore, in the previous study, the increase in the amount of nano-silica increases the Al/Ca ratio of the hydration Appl. Sci. 2019, 9, 4358 16 of 20 reactant [27] and the tendency of increasing the Al/Ca ratio shown in Figure9 is similar. The method of substituting CNS for the weight of mixing water in this study showed that the Ca/Si and Al/Ca ratios ofAppl. the Sci. hydration 2019, 9, x FOR reactants PEER REVIEW weresimilar to those of the previous studies using PNS or CNS. 16 of 20

Figure 9. Atomic ratios of the hydration products.products.

4. Conclusions The experimentalexperimental data data that that was was evaluated evaluated as a partas a of part this studyof this led study to the followingled to the conclusions. following conclusions.As the substitution rate of colloidal nano-silica (CNS) increased, flow value and setting time decreased.As the Also,substitution the compressive rate of colloidal strength nano increased-silica ( atCNS all) measurement increased, flow ages. value Especially, and setting the poretime structuredecreased. increased Also, the the compressive amount of strength gel pores increased and affected at all the measurement increase of largeages. capillaryEspecially, pore. the Thispore tendencystructure wasincreased clearly the evident amount at a CNSof gel replacement pores and rateaffected of 50%. the CNS increase promoted of large the capilla hydrationry pore. of cement This andtendency affected was the clearly ettringite, evident calcium-silicate-hydrate at a CNS replacement (C-S-H), rate of gypsum50%. CNS and promoted portlandite. the Inhydration particular, of portlandite showed a tendency to decrease as the substitution rate of CNS increased. Through the cement and affected the ettringite, calcium-silicate-hydrate (C-S-H), gypsum and portlandite. In thermalparticular analysis, portlandite (DT/DTG), showed it was a confirmedtendency thatto decreas the amounte as the of ettringite substitution and C-S-Hrate of gel CNS was increased. increased andThrough the portlandite the thermal was analysis changed (DT/DTG), as the amount it was of confirmed CNS was increased. that the amount The SEM of showedettringite a longand needleC-S-H typegel was ettringite increased and and a dense the hydrationportlandite reactant. was changed The EDS as analysisthe amount showed of CNS a decreasing was increased. tendency The of SEM the Cashowed/Si ratio. a long needle type ettringite and a dense hydration reactant. The EDS analysis showed a decreasingThe eff ectstendency of CNS of mixing the Ca/Si water ratio. weight replacement method applied in this study are summarized as beingThe causedeffects byof two CNS main mixing reasons. water One weight is that thereplacement substitution method of the CNSapplied with in the this weight study of theare mixingsummarized water as promotes being caused the homogeneous by two main reasons. dispersion One eff isect tha oft the substitution nano-silicaparticles, of the CNS promoting with the theweight pozzolanic of the mixing reaction, water the promotes nucleation the eff ect,homogeneous and the filler dispersion effect. The effect other of isthe the nano reduction-silica particles, effect of wpromoting/b caused bythe replacingpozzolanic the reaction dense CNS, the than nucleation the mixing effect water., and Thisthe resultsfiller effect. in a decrease The other in wis/b the by reducingreduction only effect the of mixing w/b caused water withoutby replacing changing the dense the amount CNS than of binder. the mixing As a result, water. the This CNS results was ablein a todecrease replace in up w/b to 50% by reducing of the mixing only water. the mixing This substitution water without rate changing was at a permissible the amount level of binder. at up to As 25% a ofresult, thebinder the CNS weight. was able to replace up to 50% of the mixing water. This substitution rate was at a Authorpermissible Contributions: level at upConceptualization, to 25% of the binder T.K.; methodology,weight. T.K. and C.K.; validation, C.K., S.H. and K.-Y.S.; formal analysis, T.K. and C.K.; investigation, C.K. and S.H.; resources, T.K. and K.-Y.S.; data curation, T.K. and C.K.;Author writing—original Contributions: draftConceptualization preparation, T.K.;, T.K writing—review.; methodology, T.K and. editing,and C.K C.K.;.; validation, visualization, C.K., S.H C.K.. and S.H.;K.S.; supervision,formal analysis, T.K.; T.K project. and administration, C.K.; investigation, T.K.; fundingC.K. and acquisition, S.H.; resources, T.K. and T.K C.K.. and K.S.; data curation, T.K. and Funding:C.K.; writingThis— workoriginal was draft supported preparation, by the T.K National.; writing Research—review Foundation and editing, of Korea C.K.; (NRF) visualization, through aC.K grant. and funded S.H.; bysupervision, the Government T.K.; project of Korea administration, (MOE) (NRF-2015R1D1A3A01019583 T.K.; funding acquisition, and T.K. NRF-2017R1D1A1B03034470). and C.K. ConflictsFunding: ofThis Interest: work Thewas authorssupported declare by the no conflictNational of interest.Research Foundation of Korea (NRF) through a grant funded by the Government of Korea (MOE) (NRF-2015R1D1A3A01019583 and NRF-2017R1D1A1B03034470).

Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2019, 9, 4358 17 of 20

Abbreviation

CNS colloidal nano-silica OPC ordinary Portland cement PNS powdered nano-silica XRD X-ray diﬀractometer MIP mercury intrusion porosimetry SEM scanning electron microscopy TG/DTG thermogravimetric/ diﬀerential thermal analysis C-S-H calcium-silicate-hydrate EDS energy dispersive spectroscopy

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