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Cement and Concrete Composites 74 (2016) 225e235

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Cement and Concrete Composites

journal homepage: www.elsevier .com/locate/cemconcomp

Performance and properties of mortar mixed with nano-CuO and ricehusk ash

Bahareh Mehdizadeh Miyandehi b, Abdosattar Feizbakhsh c, Mohammad Ali Yazdi a,Qing-feng Liu a, *, Jian Yang a, Pedram Alipour d

a State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, Chinab Department of Civil Engineering, University of Guilan, Rasht, Iranc Department of Civil Engineering, University of Birjand, Birjand, Irand Department of Civil Engineering, Islamic Azad University-UAE Branch, Dubai, United Arab Emirates

a r t i c l e i n f o

Article history:Received 17 July 2015Received in revised form5 September 2016Accepted 5 October 2016Available online 6 October 2016

Keywords:Nano-CuO (NC)Rice husk ash (RHA)DurabilityPermeabilityMicrostructureElectrical resistivityRapid chloride permeability test (RCPT)Water absorptionUltrasonic pulse velocity (UPV)Mercury intrusion porosimetry (MIP)Capillary water absorption

* Corresponding author.E-mail address: liuqf@sjtu.edu.cn (Q.-f. Liu).

http://dx.doi.org/10.1016/j.cemconcomp.2016.10.0060958-9465/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

This paper presents an experimental study on a new mixture scheme of mortar. Unlike most of existingwork, the present study investigates nano-CuO (NC), and its combined effects with cement replacementi.e., rice husk ash (RHA) on durability performance, as well as strength and permeability properties ofmortars. Comprehensive observations of both the performance and properties improvements on RHA-containing mortar specimens were determined with the addition of NC. To this end, a series of testsfor examining the strength both directly (compressive strength) and indirectly (Ultrasonic Pulse Ve-locity), electrical resistivity, chloride permeability, water absorption and microstructure characteristics(i.e., SEM micrographs, Mercury intrusion porosimetry (MIP) & capillary analyses) of mortar specimenswere performed. A relationship between the Rapid chloride permeability test (RCPT) and electrical re-sistivity was also studied in order to recommend an alternative method for quality control in thepresence of RHA and NC. Finally, a mixture scheme which provides relatively satisfactory propertiesimprovement with positive environment credential is suggested.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Concrete structures are usually exposed to harsh environmentalconditions, such as that involving chlorides, which may impair themechanical and durability properties of the reinforced concretestructures through rebar corrosion. A strong and durable concretecan be produced by improving the behavior of cement mortar. Tothis end, several investigations have been conducted to improvethe properties of cement mortar and paste. Meanwhile, the appli-cation of pozzolanic materials in cementitious materials has beenconsidered importantly since it can contribute to a serious ofbeneficial performances by improving the mechanical and dura-bility characteristics of mortar or concrete. Previous investigationsdemonstrated that the incorporation of fly ash can effectively

change the behavior of cement paste and mortar along with thereduction of cement consumption [1e3]. Nath et al. reported thepositive effects of fly ash as a partial replacement of cement on thedurability of concrete [4].

According to Li et al. the origination of nearly 5% of global carbonemissions are from the manufacturing process of cement [5].Various studies have been carried out for producing alternativematerials to cement in order to mitigate the greenhouse gasemissions from the cement production [6]. Rice husk ash (RHA)materials due to their pozzolanic activities are supposed to be anefficient alternative material [7]. Rice husk is one of the basicagricultural residues, which is obtained from the outer covering ofrice grains during the milling process. The achieved RHA can beuseful as a cement additive/replacement to increase the compres-sive and flexural strengths of cement mortar specimens [8]. In astudy by Ru-Shan Bie et al. on the effects of RHA and its burningconditions, it was revealed that the partially replacement of cementwith RHA reduces considerably the cost of concrete and also

Table 1Chemical composition and physical properties of cement and RHA.

Chemical composition Constituents (wt%) Cement RHA

SiO2 21.75 91.15Al2O3 5.15 0.41Fe2O3 3.23 0.21CaO 63.75 0.41MgO 1.15 0.45SO3 1.95 0.62K2O 0.56 6.25Na2O 0.33 0.05L.O.I 2.08 0.45

Physical properties Surface area (m2/g) 0.31 2.09Specific gravity 3.15 2.07

Table 2Properties of NC.

Nanoparticles Average diameter(nm)

Surface volume ratio(m2/g)

Purity(%)

Nano-CuO 20 ± 3 200 >99

Fig. 1. Nano-CuO particles of uniform distribution observed using Transmission elec-tron micrographs (TEM) (of size 50 nm).

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disposes of waste materials. In fact, the possible reasons thatexplain the pozzolanic activity of RHA are three-fold: (a) the ach-ieved silica content, (b) amorphous silica crystallization phase, and(c) size and surface area of ash particles obtained by grindingprocess [8].

Recently, nanotechnology has attracted a considerable scientificinterest due to the new potential use of particles in nanoscale,which can improve the properties of cementitious materials. Ma-jority of the already published literature, conducted on the appli-cation of nanoparticles in mortar or concrete, have focused onnano-SiO2 (NS) [9e25], carbon-based nanoparticles (e.g., carbonnano-tube, carbon nano-fiber and graphene) [26e28] and metal-containing nanoparticles (e.g., nano-TiO2, nano-Al2O3, nano-Fe2O3, ZnO2, ZrO2 and CaCO3) [10,11,29e37]. These studies suc-cessively demonstrated the improvements in various properties ofmortar/concrete made by nanoparticles. Regarding the binaryblended mortars containing nano-SiO2 and RHA, Sadrmomtazi andFasihi [38] proved a series of beneficial performances by demon-strating the high early age strength and the long-term strength,which are associated with the inclusion of nano-SiO2 and RHA.

Nano-CuO (NC) has been reported recently as an effective nano-powder which is considered as one of cheapest nanomaterials andshows a proper potential for enhancing the strength and thermalproperties of cementitious materials [22,39]. However, the binaryeffect of NC and RHA on performance and properties of cement-based materials such that RCPT, capillary absorption and electricalresistivity, MIP, UPV tests have been neglected in previous researchstudies. To further extend the binary application of NC and RHA,and explore the long-term durability performance, in this study, aninvestigation on RHA-containing mortar with NC has been carriedout. The ordinary Portland cement (OPC) herein was partiallyreplaced by RHA at various replacement ratios, along with addingdifferent dosages of CuO nanoparticles. By launching a series oftests on compressive strength, water absorption, electrical re-sistivity and chloride permeability, some detailed and quantitativediscussions between specimen groups and property performancewere presented. UPV test, SEM micrographs, capillary water ab-sorption and MIP data were also provided for better understandingof the mechanism of how the additives influenced the micro-structure and then affected the performance and properties ofmortar. A relationship between the RCPT and electrical resistivitywas also determined in order to recommend electrical resistivity asan alternative measurement for quality control in the presence ofRHA and NC. Finally, a mixture scheme which provides relativelysatisfactory properties improvement with positive environmentcredential is suggested.

2. Experimental program

2.1. Material

Natural river sand and ordinary Portland cement type I wereused conforming to ASTM C778 and ASTM C150, respectively. Thechemical composition and physical properties of cement and RHAare given in Table 1. NC in the form of dispersed suspension of 30%in water, with an average particle size of 20 nm and a nearlyspherical morphology were used as a partial replacement forcement. Table 2 presents some of the specific properties of the CuOnanoparticles, which were provided by suppliers. The RHA ismainly composed of silica, which constitutes 91% of the total mass.In this study, the husk provided from Rasht (a city in Iran) and anelectric kiln was utilized to burn the rice husk pellets. An electricsystem with a heating rate of 5 �C/min was also used in order toconduct the heating cycles. Each sample was held at temperaturesbetween 500 and 700 �C for 6 h.

The Transmission electron micrographs (TEM) of NC and scan-ning electron micrograph (SEM) of RHA are given in Figs. 1 and 2,respectively. It can be seen from these figures that the NC particlesare spherical while the RHA particles are irregular in shape. In orderto achieve the desired fluidity and better dispersion of the nano-particles, a polycarboxylate type superplasticizer (SP) conformingto ASTM C494 with a density of 1.03 g/cm3 was utilized. Thesuperplasticizer content of the mixtures was adjusted to maintainsimilar fluidity in all specimens.

2.2. Mix proportions

Mixtures were prepared with different amounts of RHA, NC andSP. The amount of RHA was varied from 10 to 30% by weight of thetotal binder. The nanoparticle ratio was 0, 1, 2 and 3% of the binder.The amount of SP varied between 0.6 and 1% by weight of the

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binder, and a constant water to binder ratio (w/b) of 0.4 was used inall mixtures. The mortar mix proportion is given in Table 3. Thenumber after RHA or NC represents the percentage of rice husk ashand nano-CuO, respectively. The flowability of the samples isobserved visually to check the segregation and bleeding.

2.3. Production of specimens

The specimen production procedure used in this study wascarried out in accordance with the ASTMC305 standard. However,some changes weremade to themixing process due to the presenceof nanoparticles [40]. First, the cement and RHAwere dry mixed inthe mixer at a moderate speed for 1 min. Then, these componentswere mixed at a high speed for 90 s with the nanoparticles and 90%water. Sand was then gradually added over a period of 30 s whilethe mixer was running at a medium speed. Eventually, the super-plasticizer and remaining water were added and stirred at highspeed for 30 s. The mixture was allowed to rest for 90 s, and thenmixing was continued for 1 min at a high speed. This mixing pro-cedure was followed to facilitate the distribution of the nano-particles in the mortar.

Fresh mortar was cast into 50 � 50 � 50 mm cubes forcompressive strength, UPV, water absorption and electrical re-sistivity tests and in 100 � 100 mm cylindrical molds for RCPT.Afterward, specimens were compacted using a temping rod toexclude the air bubbles from the mortar. The specimens weredemolded after 24 h and cured inwater at 23 ± 3 �C until they weretested.

2.4. Test procedure

Compressive strength test was conducted in accordance withthe ASTM-C109 standard using a hydraulic testing machine at aloading rate of 1350N/s. The compressive results were determinedat 28 and 90 days of curing. Prior to the compression test, UltraPulse Velocity (UPV) measurement using a PUNDIT device was alsoconducted on the same samples.

The water absorption test was carried out at 90 days of age,corresponding to ASTM-C642-13. Saturated surface dry specimenswere kept in an oven at 110 �C for 48 h. After measuring the initialweight, the samples were immersed in water for 48 h. The finalweight was then measured, and the absorption was calculated toassess the permeability of the mortar specimens. The absorption

Fig. 2. SEM micrograph of RHA.

value was taken as the average of the test results of threespecimens.

The electrical resistivity test was conducted, since the electricalproperties of concrete can be related to the concrete's porousmicrostructure, water permeability, moisture and ion transport inconcrete. It can also be used to measure corrosion potentials inreinforced concrete. For electrical resistivity test, cubed sampleswith dimensions of 50� 50� 50 mmwere prepared andmeasuredat 90 days. The resistivity test involved an electrical resistancemeasurement device and two electrodes which were attached toboth sides of the samples, and the resistivity value (r) was calcu-lated using:

r ¼ RAL

(1)

where R is the measured resistance, A and L are the cross-sectionalarea and the length of the measured sample, respectively. The RCPTis one of the methods of evaluating the quality of the hardenedmortar or concrete with regards to its durability performance. Cy-lindrical samples with a diameter and length of 100mmwere cut tocylinders with a diameter of 100 mm and a thickness of 50 mm inaccordance with ASTM C-1202, which were used to conduct RCPT.The transmission charge was then recorded on a PC for approxi-mately 6 h. It is worth noting that the chloride ion permeability isobtained by using the charge passed through the samples and theranges set out in ASTM C-1202.

Pore structure measurements were also conducted using theMIPmethod and capillary water absorption analysis at the age of 90days. In order to get information on the porous characteristics ofsolid materials such as porosity, pore volume, pore size distributionand density, mercury porosimetry uses the non-wetting propertiesof mercury. Prior to the mercury porosimetry analysis, vacuumdegassing was used to remove moisture from the porous structure.MIP test was conducted using a Micromeritics Autopore IV 9505porosimeter. A 5 mm cubic sample was used for both MIP and BET.The capillary absorption coefficient was calculated using the mea-surements for 10 and 90 min. Note that the capillary water ab-sorption test herein was conducted according to ASTM C1585-13.

3. Result and discussion

3.1. Compressive strength

The average compressive strength results of various mixes areshown in Fig. 3. As can be seen from the figure, at both 28 and 90days, the compressive strength decreased as the amount of RHAwas increased. According to the previous research studies, RHA didnot showa significant improvement in the strength development ofcementitious materials, although highly reactive RHA produced bycontrol incineration or other treatments on RHA showed its posi-tive effects on the strength at 28 and 90 days [41e43]. In the pre-sent study, the inclusion of less reactive particles of 10% RHAwith ahigher specific surface area than cement (see Table 1), and a loweramount of amorphous CuO might also be attributed to a lower rateof strength development.

In contrast, it was observed that increasing NC up to 3 wt%increased the compressive strength of RHA cement mortars, exceptfor mixtures containing 30 wt% RHA. The decrease in strength witha relatively large content of pozzolans stands from the reducedamount of ordinary Portland cement, which leads to a reduction inhydration products. The incorporation of 3% NC augmented thestrength of samples with 10% RHA up to 17% and 17.5% at 28 and 90days, respectively, which showed the highest level of compressivestrength in comparison with the other mortar specimens. A 3%

Table 3Mixture proportions of the mortars.

Sample ID Cement (kg/m3) RHA (kg/m3) Nano-CuO (kg/m3) Water (kg/m3) Sand (kg/m3) SP (kg/m3)

CO 700 0 0 280 1190 4.2

RHA10 630 70 0 280 1090 5RHA10NC1 623 70 7 280 1080 5RHA10NC2 616 70 14 280 1070 5RHA10NC3 609 70 21 280 1060 5

RHA20 560 140 0 280 1070 6RHA20NC1 553 140 7 280 1055 6RHA20NC2 546 140 14 280 1045 6RHA20NC3 539 140 21 280 1035 6

RHA30 490 210 0 280 1040 7RHA30NC1 483 210 7 280 1030 7RHA30NC2 476 210 14 280 1020 7RHA30NC3 469 210 21 280 1010 7

Fig. 3. (a) Variation of compressive strengths of RHA-containing mortars with NC at 28 and 90 days, and (b) Normalized compressive strength results relative to the control mix.

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addition of NC to mortars as the optimum limit was also reportedby the other published studies [44].

In general, the improvement of compressive strength achievedby the presence of NC could be explained as follows:

First of all, it is supposed that nanoparticles are uniformlydispersed in the cement, in which the distributed nanoparticles actas a nucleus with cement hydrate and promote the cement hy-dration due to their high reactivity, which is favorable for thestrength of cement mortar [34].

Secondly, the addition of NC acts as a kernel in cement paste,

which facilitates to smaller sized Ca(OH)2 crystals and a morestochastic tropism, resulting in the improvement of the porestructure of mortar. It is therefore considered that NC can also act asa filler to improve the density of mortar, which reduces the porosityof mortar significantly [44].

3.2. Ultrasonic pulse velocity

In this study UPV test was carried out to further monitor thestrength development of the control samples and RHA-containing

Fig. 4. Comparison between UPV and compressive strength.

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mortars with and without NC at 90-day. Fig. 4 indicates a similartrend in compressive strength and UPV. It can be seen that allsamples with 10%, 20% and 30% RHA showed a higher pulse velocitythan that of the control sample. The maximum strength develop-ment was achieved by 20% RHA sample among the mortars con-taining only RHA. The replacement of NC up to 3% increased theUPV dramatically. As it is evident, samples incorporating 3% NCfostered the UPV about 200% compared with the control sample.

Based on the results obtained from both compressive strengthand UPV tests, one may find that the samples of RHA10NC3 havethe best mechanical capacity at all ages. Later on in section 3.6.1(Fig. 8-c), it can be observed that the addition of 3% NC and 10%RHA increased the density and uniformity of specimens. It might bedue to the accelerated cement hydration products which areassociated with the high silica content in RHA and high free energyof NC. However, those of RHA20NC3 also show a decent mechanicalperformance, especially given that they are more environmentally

Fig. 5. Variation of electrical resistivity of R

friendly by consuming more RHA and using less cement content inconcrete.

3.3. Electrical resistivity

The results of the electrical resistance of binary mixes at 90 daysare presented in Fig. 5. As the figure shows, increasing the contentof RHA up to 20% leads to a spectacular increase in electrical re-sistivity, at the age of 90 days. However, compared to the binaryeffect of RHA and NC, the presence of only RHA did not improve theresistivity significantly.

As shown in Fig. 5, the addition of NC with the high specific areahas improved the electrical resistance of mortars compared tomixes containing only RHA. The explanation of this is that the totalpore volume of mortar is not reduced by the pozzolanic reactionsassociatedwith the presence of only RHA. In general, the increase inelectrical resistivity in samples including RHA pozzolans and NC

HA mortars containing NC at 90 days.

Table 5RCPT results.

Mixture Passing charge (C) Permeability class (ASTM C 1202)

CO 4023 HighRHA10 3045 ModerateRHA10NC1 2749 ModerateRHA10NC2 2501 ModerateRHA10NC3 1958 LowRHA20 2610 ModerateRHA20NC1 2546 ModerateRHA20NC2 2287 ModerateRHA20NC3 1840 LowRHA30 2650 ModerateRHA30NC1 2365 ModerateRHA30NC2 2215 ModerateRHA30NC3 2010 Moderate

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materials is associated with the decreased ratio of porosity(Fig. 8aec), which reduces the pore water solution in specimens(see Fig. 7). However, Zahedi et al. reported the negative influenceof the addition of nano-SiO2 on the electrical resistivity of RHA-containing mortars [45].

The higher electrical resistivity of the mixtures with thereplacement of mineral additives, such as RHAwas also reported byother researchers, particularly when the curing time and the con-tent of the substitution were increased [46e50]. They attributedthis higher resistivity to the importance of (OH-) ions in the elec-trical conductivity of mortar and indicated that the reduction of(OH-) ions in pore solution of the mortars containing supplemen-tary cementitious materials could increase the electrical resistanceof mortars. This might make the construction engineers concernthat due to the reduced alkalinity of concrete, the addition ofpozzolans to reinforced and prestressed concrete might bedangerous for corrosion of steel. However, from one hand, it isknown that the resistance to the carbonation of concrete is notdependent on the amount of Ca(OH)2 in the pore solution sincethere is little or no calcium in the pore solutions in mature Portlandcement pastes. From the other hand, the resistance to carbonationis mainly in physical factors such as permeability of the cementpaste. In this case, the presence of RHA and NC has reduced porosityand has shown an effective reduction in the size of large pores after90-days curing (see Fig. 8-b). In brief, the pozzolanic and cemen-titious reactions associated with RHA reduce the free lime presentin the cement paste as well as decrease the permeability of thesystem, which leads to the improvement of the overall resistance toCO2 attack. This suggests that even when very active SiO2-richpozzolans are used the danger due to carbonation should be of lessconcern in concretes containing pozzolans. Therefore, improvedresistance to corrosion of steel in concrete can be anticipated [51].

The relationship between corrosion rate and electrical resistivityis given in Table 4, according to the ACI Committee 222 [52]. As it isevident, the addition of NC gives rise to a lower corrosion. Mixturescontaining 1%, 2%, and 3% NC are in a low to moderate range ofcorrosion (see Table 4). In contrast with the control specimens, anincrease in the electrical resistivity can be seen in all specimens,whereas the specimens containing 3% NC achieved the highestresistance and had a substantial effect on the improvement of theelectrical resistivity of specimens. The lowest probability of corro-sion was observed by the RHA20NC3 sample.

3.4. Rapid chloride permeability test

RCPT is one of the most common methods for investigating thedurability of the hardened mortar [53e55]. The RCPT results of themixtures at the age of 90 days are presented in Table 5, with regardto the permeability rate in accordance with ASTM C1202.

The results show a substantial reduction in passed charge due tothe incorporation of CuO nanoparticles. The binary use of RHA andNC has led to a noticeable reduction in the charge passed throughmortars. The incorporation of 10%, 20% and 30% RHA reduced thepassing charge to 24%, 35% and 34%, respectively, compared to thecontrol specimen. Since RHA is finer than cement, it can diminishporosity and refine pores, which result in a lower permeability (see

Table 4Relationship between electrical resistivity and corrosion rate [52].

Electrical resistivity (KU cm) Corrosion rate

>20 Low10e20 Low to moderate5e10 High<5 Very high

Fig. 8-b). Based on Table 5, it can be seen that most specimens canbe sorted into a category with moderate chloride permeability.From the records, binary samples incorporating the replacement ofcement with a combination of 10%, 20% and 30% RHA, and 1%, 2%and 3% NC had the average transmission charge in all RHA fractions,which reduced the permeability of the samples by 36.53%, 41.97%,and 51.88%, respectively, compared with the control mortar. It isobserved that chloride permeability of the mixtures with CuOnanoparticles is declined with the increase in NC content, whichindicates the important role of nanoparticles in dropping the ratioof chloride permeability with different weight fractions and soincreasing the durability of mixtures. In brief, the RHA20NC3sample experiences the best performance during RCPT.

Since the charge passed through cement-based materials in theRCPT test is dependent on both the microstructure of the paste andthe chemical composition of pore solution particularly (OH�), thediscussion which was presented for the observed trends in theelectrical resistance section is also applicable here.

In order to demonstrate the relationship between RCPT andelectrical resistivity test results at the age of 90 days, a scatter di-agram is presented in Fig. 6. The parabolic curve is chosen as themost accurate relationship between the two variables, resulting inthe highest value of R2 (correlation coefficient). As shown in Fig. 6,R2 is equal to 0.8975 which indicates a close correlation betweenthe results of RCPT and electrical resistivity test. It means thatelectrical resistivity can be utilized as an alternative measurementfor quality control and assurance with the same confidence as theRCPT in the presence of RHA and NC. Since electrical resistivityoffers many benefits over RCPT, such as reduced testing time,reduced sample preparation time, and no risk of inaccurate resultsdue to heating. Further studies are still required in order torecommend ranges of resistivity to determine the risk of chloridepenetration for difference type of nanomaterials and pozzolans.

3.5. Water absorption

The water absorption results of the mortar samples aredemonstrated in Fig. 7. As it is observed, water absorption ofsamples was decreased by replacing up to 10% RHA, above which itwas then increased. Moreover, Fig. 7 shows a constant trend whenthe content of nanoparticles and RHA are increased.

It was generally observed that replacing CuO nanoparticles hasdropped water absorption compared with the ordinary sample inall mixtures. This might be due to the expectation of nanoparticlesfilling the voids in mixtures and so declines the water absorption(See Fig. 8-c).

The replacement of 3% NC decreased the water absorption by13.25% as an average, respectively. Creating a denser and more

Fig. 6. Relationship between the RCPT and electrical resistivity test results at the age of 90 days.

Fig. 7. Variation of water absorption of RHA mortars containing NC at 28 days.

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compact microstructure of matrix might be the reason for thisslightly reduced amount of water absorption. The effective role ofthe pozzolanic materials such as rice husk ash and CuO nano-particles on the water absorption characteristics of mixtures can beexplained by the following factors. First and foremost, the pozzo-lanic materials (RHA) act as a filler, hence reducing the average poresize and resulting in a less permeable paste [56,57]. In addition, theimprovement of the interfacial transition zone (ITZ) in mortars dueto the pozzolanic reaction as well as filler effect of both RHA and NCparticles.

Existing studies have indicated similar results [58], in which thewater absorption level for RHA as a partial cement replacement wasless than the control sample. This may be related to prolongedhydration, resulting in a less porous matrix.

The other trend can be observed from the results is the increasein the water absorption with the addition of more RHA content(except for the individual sample RHA20NC3). The same trendcould also be observed from the capillary water absorption results,

Fig. 10. In fact, angular shape of RHA particles as well as its porousstructure might contribute to trapping more water [59]. Besides,the excessive content of absorbed water by replacing a higheramount of RHA might be due to the decrease in the rate of hydra-tion reaction as well as the formation of hydration products, whichfinally results in the arrangement of higher permeable voids in thesystem. MIP results are also indicator of a higher porosity achievedwith RHA30 compared to the control sample.

3.6. Microstructure

3.6.1. SEM micrographsMicrostructure affects the mechanical and durability perfor-

mance of mortar and can be examined through the SEM micro-graphs. The SEM micrographs of specimens with and withoutnanoparticles (after 90 days of curing in water) are presented inFig. 8. Fig. 8-a shows a SEM photo taken for the control sample,which is consistent with the former study [60]. Fig. 8-b shows a

Fig. 8. SEM micrograph of a) control b) RHA10 and c) RHA10NC3 specimens.

Fig. 9. Pore volume in hardened mortars containing a)10% RHA, b) 20%RHA and c) 30% RHA measured by MIP.

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SEM micrograph of the mixture containing 10% replacement ofRHA. In fact, the combination of RHA with mortar to some extentprovided densermicrostructure and reduced the size of large pores.This result was also reported by Refs. [60,61], the addition of fineparticles of RHA caused segmentation of large pores and increasednucleation sites for precipitation of hydration products in cementpaste. This could be due to the fineness of RHA, or a modification ofthe pore structure through pozzolanic reactions occurring that fillthe pores with hydration products (CSH, CH, etc.). However, asFig. 8-b shows, there are still some small pores, when 10% RHAwasused as a cement replacement.

The replacement of NC gives rises to a higher density due to itsimportant role as filler, leading to a lower porosity of mixtures (seeFig. 8-c). This also results in higher durability performance ofspecimens. In other words, the addition of 3% NC to samples con-taining 10% RHA, as can be seen in section 3.5., decreased the waterabsorption by 13% compared to RHA10. As shown in Fig. 8-a, somelarge pores in the control specimen can be seen; whereas by addingnanoparticles these pores are filled, resulting in a denser micro-structure (see Fig. 8-c). In fact, a compact formation of CSH gel wasobserved in the presence of NC. As indicated in Fig. 8-c, (A) showshow CSH filled in the voids and played a bonding role in themicrostructure of specimens, and also (B) is indicator of a denseCSH gel. It shows the mechanism by which a more compact

Fig. 10. Capillary water absorption

Table 6MIP results of the mortars.

Sample ID MIP analysis

Volume of cumulative mercury intruded (cm3/g) Pore pro

4e10

CO 0.0412 48

RHA10 0.0401 51RHA10NC1 0.0351 61RHA10NC2 0.0209 60RHA10NC3 0.0018 87

RHA20 0.0387 58RHA20NC1 0.0368 55RHA20NC2 0.025 68RHA20NC3 0.0159 79

RHA30 0.0514 37RHA30NC1 0.036 51RHA30NC2 0.0324 61RHA30NC3 0.0254 60

microstructure can be formed. It also indicates the rapid formationof CeSeH gel in the presence of dispersed nanoparticles. Thus, thecement hydration is accelerated, and the porosity is reduced. Inother words, CuO nanoparticles promote and accelerate thepozzolanic reactions, resulting in the consumption of Ca(OH)2 andthe formation of more CeSeH gel.

3.6.2. MIP and capillary analysesFig. 9aec and 10 show the changes in the porosity measured

using MIP and capillary tests, respectively. According to the results,the porosity decreased with increasing NC. In fact, the lowest porevolume was achieved for the specimens containing 3% NC in allgroups. This might be the reason why the presence of NC signifi-cantly improved the durability performance of mortars. However,the beneficial effect of RHA was not that pronounced compare toNC. From the MIP results, it can be seen that the addition of RHA upto 20% decreased the pore volume by 6% compared to the controlsample. However, RHA30 achieved even a higher porosity than thecontrol sample (Fig. 9-c). The increase in pore volume with a rela-tively large amount of RHA might be due to the fact that thereduced amount of OPC results in lower hydration products [62].This result is in agreement with those of Chindaprasirt et al. [61].According to Fig. 10, the sorptivity increased with increase in RHAratio, which shows the higher possibility of chloride ion

results of hardened mortars.

portions (%) classified by diameter (nm)

10e50 50e100 100e500 500e1000 >1000

39 6 2 5

27 7 7 6 233 5 136 48 3 2

20 12 6 438 724 7 115 3 3

36 14 1 3 941 7 129 2 5 331 6 3

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penetration at higher rates of RHA. In fact, based on the MIP results,Table 6, the replacement of RHA from 10% to 20% with cementrelatively increased the large capillary pores. Beyond, an increase inboth the medium and large capillary pores was observed.

Most of the pores in all groups are narrower than 100 nm in poreradius (see MIP figures). In parallel to the MIP and capillary results,the fact that NC produced a finer pore structure also displayed itseffect on the other properties of the specimens such as compressivestrength, electrical resistivity and water absorption. For instance,according to results in previous sections, the increases in NC con-tent could improve both the performance and properties of mortarssignificantly.

4. Conclusions

This study has carried out a series of tests to examine both singleand combine effect of NC and RHA on mortar properties. Based onthe obtained results and discussions on compressive strength, ul-trasonic pulse velocity, water absorption, electrical resistivity,chloride permeability and microstructure characteristics of thespecimens, the following conclusions can be drawn:

a. Addition of RHA decreased the compressive strength of samplesup to 13% when cement was replaced with 30% of RHA at 28days. The compressive strength of RHA-containing mortarspecimens tended to increase when the CuO nanoparticle con-tent was increased and reached the highest amount when 3% ofNC was replaced. As an average, the most advantageously op-timum content of NC was 3% by weight of the binder.

b. The replacement of NC also influenced the electrical resistivityand chloride permeability of mixtures dramatically. In fact,chloride penetration decreased significantly by the addition ofnanoparticles, which could be as a result of more packedmicrostructure achieved from adding nanoparticles andenlarging the volume of paste.

c. Water absorption and chloride penetration decreased by lowervolume RHA replacement. Rather significant decline by theaddition of CuO nanoparticles in water absorption resultsshowed the important role of nanoparticles as nanofillers andimproved the resistance to water permeability of concrete.

d. NC as a partial cement replacement of and filler could accelerateCeSeH gel formation associated with the increased crystallineCa(OH)2 amount of hydration. It also reduced the porosity ofcementmatrix, which led to an increase in both the compressivestrengths and durability of mortar specimens.

e. Electrical resistivity can be considered as an alternative mea-surement for quality control instead of RCPT in the presence ofRHA and NC. However, further studies are still necessary.

f. The opposite effects caused by RHA and nano-CuO on porosityand sorptivity are quantitatively discussed by MIP test andcapillary analyses, respectively. The sorptivity and porosity ofmortars increased at high ratios of RHA, which means thepenetration of chloride ion rises at high rates of RHA. However,the presence of NC decreased both the porosity and sorptivitynoticeably and improved the performance and properties ofspecimens.

g. RHA20NC3 appeared to offer great advantages regarding thedurability and cement replacement rate, which balances bothperformance and environmental goals.

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