Assessment ofmechanical strengthof nano silica concrete (NSC)

subjected to elevated temperaturesHala Mohamed Elkady, Ahmed M. Yasien and Mohamed S. Elfeky

National Research Centre – Civil Engineering, Giza, Egypt, and

Mohamed E. SeragFaculty of Engineering, Cairo University, Cairo, Egypt

AbstractPurpose – This paper aims to inspect the effect of indirect elevated temperature on the mechanicalperformance of nano silica concrete (NSC). The effect on both compressive and bond strengths is studied. Pre-and post-exposure to elevated temperature ranges of 200 to 600°C is examined. A range covered by threepercentages of 1.5, 3 and 4.5 per cent nano silica (NS) in concrete mixes is tested.Design/methodology/approach – Pre-exposure mechanical tests (normal conditions – room temperature),using 3 per cent NS in the concrete mix, led to the highest increase in both compressive and bond strengths (43per cent and 38.5 per cent, respectively), compared to the control mix without NS (based on 28-day results). It isworth noticing that adding NS to the concrete mixes does not have a significant effect on improving early-agestrength. Besides, permeability tests are performed on NSC with different NS ratios. NS improved the concretepermeability for all tested percentages of NS. The maximum reduction is accompanied by the maximumpercentage used (4.5 per cent NS in the NSC mix), reducing permeability to half the value of the concrete mixwithout NS. As for post-exposure to elevated-temperature mechanical tests, NSC with 1.5 per cent NS exhibitedthe lowest loss in strength owing to indirect heat exposure of 600°C; the residual compressive and bond strengthsare 73 per cent and 35 per cent, respectively.Findings – The dispersion technique of NS has a key role in NSC-distinguished mechanical performancewith NSC having lower NS percentages. NS significantly improved bond strength. NS has a remarkable effecton elevated temperature endurance. The bond strength of NSC exposed to elevated temperatures sufferedfaster deterioration than compressive strength of the exposed NSC.Research limitations/implications – A special scale factor needs to be investigated for the NSC.Originality/value – Although a lot of effort is placed in evaluating the benefits of using nano materials instructural concrete, this paper presents one of the first outcomes of the thermal effects on concrete mixes withNS as a partial cement replacement.

Keywords Permeability, Elevated temperature, Compressive strength, Bond strength,Nano silica concrete

Paper type Research paper

1. IntroductionIn the past two decades, serious efforts have been performed in manufacturingmaterials at the nano scale, to serve different industries. Recently, the attention of civilengineers was drawn towards this minute level of materials, after dealing with the

The authors would like to express their deep appreciation to The STDF for supporting this researchthrough Grant No. 4354. Also, the authors extend their gratitude to The NRC administration andstaff, for facilitating and supporting us, through all the official and technical procedures.

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Received 6 October 2017Revised 9 April 2018Accepted 15May 2018

Journal of Structural FireEngineeringVol. 10 No. 1, 2019pp. 90-109© EmeraldPublishingLimited2040-2317DOI 10.1108/JSFE-10-2017-0041

The current issue and full text archive of this journal is available on Emerald Insight at:www.emeraldinsight.com/2040-2317.htm

microscale level for many years. Investigations undertaken to assess the effect ofnanoparticles on properties of cement-based materials owing to their fine particle sizeled to better packing (Li et al., 2004), high reactivity (Aly et al., 2011) and specificfunctional properties (Drg et al., 1998; Serag et al., 2014). The effect of nano-SiO2 oncement hydration was examined and it was concluded that early-age hydrationsignificantly increased (Serag et al., 2014).

Different problems were encountered when using NS in concrete mixes, including theproblem of overcoming re-agglomeration reported in previous works (Serag et al., 2014;Elkady et al., 2013). Enhancement of mixing methods led to improvement in compressivestrength (Serag et al., 2017; El-Feky et al., 2016).

More investigation is required to understand the behaviour of NS under different thermalconditions. This paper reports the effect of elevated temperatures on the compressive andbond strengths of NSC in addition to the permeability of NSC under normal conditions.Scanning electron microscopy and X-ray diffraction analysis will be introduced to helpinterpret the behaviour of NSC.

2. Materials and methods2.1 MaterialsThe followingmaterials were used:

� Cement: Ordinary Portland cement Type I, Grade CEM I 52.5 N.� NS: Commercial NS with silica content over 99 per cent, and size range of 20 to

80 nm.� Fine aggregate: Well-graded sand with particles size smaller than 0.5 mm and

specific gravity of 2.58 g/cm3; fineness modulus of 2.25 was used as fine aggregateas a 35 per cent of the total amount of aggregate.

� Coarse aggregate: Well-graded dolomite of maximum size of 12 mm and specificgravity of 2.96 g/cm3 was used as the coarse aggregate of 65 per cent of the totalamount of aggregate.

� Superplasticizer: Polycarboxylate as a polyethylene condensate defoamed-basedadmixture (Glenium C315 SCC).

2.2 Methods2.2.1 Nano silica characterization. Figures 1 and 2 show transmission electron microscope(TEM) and X-ray diffraction (XRD) for used NS, respectively. From the XRD patterns ofsilica nanoparticles, it is clear that the silica is observed at a peak centred at 2U = 23°, whichreveals the amorphous nature of the silica nanoparticles.

2.2.2 Mixing and nano-dispersion procedures. The mixtures are to be prepared withNS replacement ratios of 1.5 per cent, 3 per cent and 4.5 per cent by weight of cement(450 kg/m3) in the mix and constant water/binder ratio of 0.43. A number of differentdispersion methods and timings were investigated in previous works (Serag et al., 2014;Elkady et al., 2013; Serag et al., 2017; El-Feky et al., 2016) to detect the optimum methodand mixing time for the best dispersion of NS particles; indirect sonication using a bathsonicator should be applied. A modern ultra-sonication bath was used in NS dispersionwith different times according to the NS percentage: 9 min for 1.5 per cent, 12 min for 3per cent and 15 min for 4.5 per cent.

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Figure 1.TEM of NS particles

Figure 2.XRD of NS particles

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2.3 Tests and specimens2.3.1 Compressive strength. Cubes (100 mm) were cast for 7 and 28 days to conductcompressive strength tests as per ASTM C 39. The tests were performed at a rate of 0.5 N/mm2/s using universal testing machine SHIMADZU 1000 KN.

2.3.2 Pull out test. Concrete cubes of (150 � 150 � 150 mm3) were cast for the pull-outtest of steel rebars of 12 mm in diameter. Half of the length of the embedded part of the barwas de-bonded using polyvinylchloride tubing to avoid yielding of steel reinforcement. It isrecommended to cast the bars into concrete cubes, providing a clear cover of 4.5 times bardiameter from the bar to the centre of each side of the horizontal cross section (Serag et al.,2017). Figure 3 is a schematic of the pull-out test specimens.

2.3.3 Water permeability test. Cubes (150 mm) were cast for the permeability test. Limesolution was an alternative curing regime for better expected results with NSC. Thecoefficient of permeability K was determined from Darcy’s expression.

2.3.4 Effect of indirect fire (elevated temperatures). Temperature values of 200°C, 400°Cand 600°C are chosen to study the effect of high temperatures on the concrete compressivestrength. The specimens were held at the chosen temperature for approximately 2 h, andafter that, the furnace was turned off and the specimens were cooled to room temperature.Water vapour was allowed to freely escape during the heating period through an opening inthe top of the furnace.

2.3.5 Scanning electron microscope. A scanning electron microscope (SEM) was used forcharacterizing the concrete mixtures, and it helps to interpret the results of the samples after28 days of the two types of curing using Quanta FEG 250 (FEI, USA) with an imageprocessor up to 4,096� 3,536 pixels (approximately 14MP).

Figure 3.Pull-out testschematic

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2.3.6 X-ray diffraction. The samples were prepared by using the crushed concrete cubes ofeach mix (after testing) by finely grinding them. A sample from each mix was taken andground to a very fine powder.

3. Results and discussion3.1 Compressive strengthFigures 4 and 5 indicate that the addition of NS slightly improved the 7-day compressivestrength testing, reaching 385 kg/cm2 by the addition of 4.5 per cent NS versus 340 kg/cm2

for the control mix (0 per cent NS). The gain in the early-age compressive strength reached 3,4.5 and 13.5 per cent by the addition of 1.5, 3 and 4.5 per cent NS, respectively. The slightincrease in the early-age compressive strength indicates the presence of a relatively largenumber of small-size agglomerates in the NSCmixes. Such agglomerates need longer time toreact with the excess calcium hydroxide (CH) to form additional calcium silicate hydrate (C-S-H) gel which is the main factor affecting the compressive strength increase. Despite its lowreactivity in the early age, the agglomerated NS particles acted as fillers, leading to adecrease in the porosity of the matrix and resulting in a relatively better compaction andconsequently a higher compressive strength. As for the 28-day results, adding NSsignificantly increased the compressive strength. The strength reached 610 kg/cm2 by theaddition of 3 per cent NS compared to 426 kg/cm2 for the control mix. This can be attributedto the action of NS as nuclei for the cement phase to promote cement hydration owing to itspozzolanic reactivity with CH increasing the production of C-S-H gel that has a significantpositive effect on the cohesion between aggregates and the mechanical properties ofconcrete. Late-age compressive strength was improved by increasing NS percentage up to

Figure 4.NSC’s seven-daycompressive strength

Figure 5.NSC’s 28-daycompressive strength

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3 per cent and then a decrease in strength was observed by the addition of 4.5 per cent NS.The gain in the 28-day compressive strength reached 17.5, 43.5 and 29 per cent by theaddition of 1.5, 3 and 4.5 per cent NS, respectively. It is worth noting here that the 4.5per cent NS mix is still higher in strength than the control mix. This can be attributed to thefact that the more the number of NS particles, the higher the capability of these particles togather around each other and be stuck together within the concrete matrix without anychemical reaction owing to the van der Waals force, contributing to the agglomeration ofnanoparticles. Although relatively larger agglomerates need longer time than the 28 days toreact with the excess CH resulting from cement hydration process to form C-S-H gel, thepresent large NS agglomerates acted as fillers to the nanopores, which were the main reasonfor the observed increase in compressive strength as compared to the control mix. It wasstated that nanoparticles can fill part of the cement paste matrix voids even in anagglomerated manner owing to their ultrafine dimension (Serag et al., 2017; El-Feky et al.,2016). Although the agglomerated NS that had not completely dissolved in the mix solutioncan affect the gain in early-age strength significantly, those particles will reduce porosity byfilling some of the pores/capillaries of the C-S-H gel. Therefore, the density of the CSH willincrease in the plastic form, leading to a denser compressive bearing structure after 28 daysof hydration (Serag et al., 2017). This explains the slight increase in early strength and thesignificant increase in late strength even with the increase in the amount of agglomeratedNS particles.

3.2 Bond strength of nano silica concreteAs mentioned earlier, 12-mm rebar specimens have enough cover to guarantee the slippagebehaviour (Al-Negheimish and Al-Zaid, 2004; Ahmed et al., 2008) (Figure 6), so the dominantfactor to affect the bond strength was the compressive strength. As mentioned before, NS

Figure 6.Slippage of 12-mm

rebars

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significantly enhanced the compressive strength owing to the production of more C-S-H gelwhich increases the bond between concrete and reinforcement and subsequently itsignificantly enhanced the bond strength of the 12-mm rebars. From Figure 7 it can be seenthat the bond strength value reached 301 kg/cm2 by the addition of 3 per cent NS instead of217.5 kg/cm2 for the control mix (containing 0 per cent NS). The gain in bond strengthreached 12, 38.5 and 2.5 per cent for 1.5, 3 and 4.5 per cent NSC, respectively. Finally,Figure 8 emphasized that 12-mm rebar bond strength results followed the same trend ofcompressive strength.

3.3 Permeability of nano silica concretePermeability of cement concrete is a very significant indicator for durable concrete. Itindicates the ease with which water or other ions or fluids can move or diffuse throughconcrete, leading to transporting aggressive agents and durability issues. As it can be seenfrom Figure 9, generally, increasing the NS percentage significantly decreased thepermeability of concrete. By using 1.5, 3 and 4.5 per cent NS, the percentage of passing flowreduced to be 82.5, 69.5 and 48.5 per cent of these passing through control mixes (containing0 per cent NS), respectively. This can be attributed to the filling effect of NS owing to itsparticles’ small size as compared with cement particles even if NS particles wereagglomerated. In addition to the above, NS reacts with CH to form more C-S-H gel, whichmakes the matrix more dense and homogeneous and decreases the permeability of concrete.In addition, NS can react with CH crystals, resulting in a reduction in the amount and size ofthese crystals, and here the interfacial transition zone (ITZ) of aggregates and paste will bedenser. Moreover, NS particles can fill the voids of the C-S-H gel structure and act as a

Figure 8.Strength versuspercentage of NS

Figure 7.Bond strength andgain in NSC

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nucleus to tightly bond with C-S-H gel particles, densifying the microstructure, increasingthe tortuosity of the pore system and densifying the binding paste matrix (Serag et al., 2017;Arefi et al., 2011; Sobolev and Shah, 2008). The reduction in concrete permeability reached17.5, 31.5 and 51.5 per cent by the addition of 1.5, 3 and 4.5 per cent NS, respectively.

3.4 Effect of elevated temperature on compressive strengthStructural elements are damaged because of being subjected to continual high-temperatureconditions, such as during a fire (Rahel et al., 2011). The concrete thermal properties areconstant at ambient temperatures, but at high temperatures, those properties change owingto the decomposition of the hydration products and the change in moisture content. Theseprocesses are mainly dependent on the maximum temperature of exposure, rate oftemperature increase and the time. Whereas, the response of hydrated cement pastes totemperature depends on the moisture content and degree of hydration. A well-hydratedcement paste mainly consists of CH, C-S-H and calcium–sulphate–aluminate-hydrate.Moreover, the saturated paste also contains a large amount of capillary water and gel water(chemically bonded water). A phenomenon called surface spalling occurs when a high-performance, low-permeable mix is exposed to a high rate of heating due to developedstresses, which is higher than concrete tensile strength, coming from the vapour pressure inthe pores (Buchanan, 2002).

The strength deterioration at elevated temperatures for high-strength concrete isconcluded to be higher than it is in normal strength concrete (Behnood and Ziari, 2008;Yamazaki et al., 1995), while some researchers have stated that normal concrete has lowerperformance than high-strength concrete at elevated temperatures (Buchanan, 2002).

After cooling, the residual compressive strength was determined by an unstressedcompression test in which the specimens were tested after being cooled down gradually for24 h.

According to the residual compressive strength of control concrete, the heating regimecan be divided into two regions as 0°C-400°C and 400°C-600°C.

� For the zone of 0°C-400°C, NSC specimens lost about 16, 25 and 22 per cent of theirinitial compressive strength using 1.5, 3 and 4.5 per cent NS, respectively, and theloss in compressive strength of control concrete (30 per cent) was higher than that ofNSC. The initial decreases were believe to be caused by the loss of evaporable waterin concrete which affected negatively the cohesive forces between the layers of C-S-H gel in hardened concrete, and the formation of microcracks in mortar phase andthe ITZ between aggregate and cement paste (Arefi et al., 2011; Noumowe et al.,1996; Khoury, 1992). According to Arefi et al. (2011), this decrease is mainly becauseof the reduction in the cohesion of van der Waal forces between the C-S-H layers(shrinkage), consequently reducing the surface energy of C-S-H gel and leading tothe formation of silanol groups (Si-OH:OH-Si) that have much less bonding strength.

Figure 9.Percentage of passing

flow of water curedNSCmixes versus

control

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� For the zone from 400°C-600°C, NS specimens lost about 27, 37 and 62 per cent oftheir initial compressive strength using 1.5, 3 and 4.5 per cent NS, respectively, andthe compressive strength loss was lower in the control concrete than in NSCs. Thissevere strength loss can be attributed to the very dense pore structure of NSC whichimproved the build-up of vapour pressure upon heating and led to spalling andcracking. The 3 per cent NSC performed the best.

� At 600°C between all NS mixes, the mixes with 1.5 and 3 per cent cementsubstitution resulted in maintaining compressive strength in a very close range tothe strength of control sample prior to fire exposure.

� No visible effect was observed on any specimen surface which were exposed to atemperature of 200°C and 400°C, while after reaching 600°C, all concrete mixessuffered from a sharp reduction in compressive strength, severe cracks andexplosive spalling (Figure 10). The concrete damage after the exposure to elevatedtemperatures was assessed based on visual inspection.

From Figures 11 and 12, the following can be observed:� For the control mix, the residual compressive strength reached 387, 300 and 218 kg/

cm2 after being subjected to 200°C, 400°C and 600°C, respectively. The residualcompressive strength is about 91, 70 and 51 per cent of the unheated specimens’compressive strength of 426 kg/cm2.

Figure 10.Concrete crackingand spalling afterexposure to 600°C

Figure 11.Post-elevatedtemperature exposurecompressive strengthand concrete spalling

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� For 1.5 per cent NSC, the residual compressive strength reached 448.5, 421 and 367kg/cm2 after being subjected to 200°C, 400°C and 600°C, respectively. The residualcompressive strength is about 90, 84 and 73 per cent of the unheated specimens’compressive strength of 500 kg/cm2.

� For 3 per cent NSC, the residual compressive strength is 84, 75 and 65 per cent fromthe unheated specimens.

� For 4.5 per cent NSC, the residual compressive strength reached 493, 430 and 210kg/cm2 after being subjected to 200°C, 400°C and 600°C, respectively. The residualcompressive strength is about 90, 78 and 38 per cent of the unheated specimens’compressive strength of 550 kg/cm2.

� The 1.5 per cent of NSC mix could maintain its compressive strength (73 per cent at600°C) more than any other percentage of NSC. This can be attributed to the abilityof the mix to release vapour pressure due to the high relative voids in the matrix ascompared to the other percentages of NS.

� The 3 per cent NSC maintained 90 per cent of the control (0 per cent NS specimen)strength after exposure to 600°C. This can be attributed to the high strength of themix that could handle vapour pressure in addition to the relative high permeabilityof the matrix compared to the 4.5 per cent mix.

� NSC with 4.5 per cent NS gave the lowest residual compressive strength after beingsubjected to 600°C which equals the 49 per cent of control unheated concrete zero percent NS. This can be attributed to the low permeability of the matrix compared to otherNS percentages which prevent the vapour release from the specimens.

� Previous study has reported that using 5 per cent of NS in concrete led to maximumloss in compressive strength (40 per cent at 500° and 80 per cent at 700°). Suchbehaviour was believed to be due to the excessive build-up of vapour pressurewhich led to extensive cracking in NS samples (Shah et al., 2013).

3.5 Effect of elevated temperature on bond strengthAt first it should be mentioned that the extended steel rebars were insulated so thegenerated stresses in steel during the pull-out test is expected to be well below the yieldstrength and the effect of heating on the mechanical properties and the bond between steel

Figure 12.Residual compressivestrength after thermal

exposure versuscontrol

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and concrete of steel can be neglected. From Figures 13 and 14, the following can beobserved:

� For the control mix, the residual bond strength reached 191, 138 and 35 kg/cm2 afterbeing subjected to 200°C, 400°C and 600°C, respectively. The residual bondstrength is about 88, 63 and 16 per cent of the unheated specimens’ bond strength(217.5 kg/cm2).

� For 1.5 per cent NSC, the residual bond strength reached 205, 173 and 85 kg/cm2

after being subjected to 200°C, 400°C and 600°C, respectively. The residual bondstrength is about 85, 71 and 35 per cent of the unheated specimens’ bond strength(244 kg/cm2).

� For 3 per cent NSC, the residual bond strength reached 152, 81.5 and 64 kg/cm2 afterbeing subjected to 200°C, 400°C and 600°C, respectively. The residual bond strengthis about 51, 27 and 21 per cent of the unheated specimens’ bond strength (301 kg/cm2).

� For 4.5 per cent NSC, the residual bond strength reached 191, 166 and 32 kg/cm2

after being subjected to 200°C, 400°C and 600°C, respectively. The residual bondstrength is about 86, 74 and 14 per cent of the unheated specimens’ bond strength(223 kg/cm2).

� 1.5 per cent NS is the best percentage to resist high temperatures due to thereduction in the loss of the bond strength. This can be attributed to relatively highair content rather than other percentages of NS.

Figure 14.Post-thermalexposure residualbond strengthpercentages for NSCversus control

Figure 13.Post-thermalexposure bondstrength of NSC.

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� 1.5 per cent NS gave residual bond strength after being subjected to 600°C whichequal 39 per cent of control unheated concrete 0 per cent W.

� 3 per cent NS nearly led to loss most of the bond strength after being subjected to200°C (residual bond strength equals 51 per cent as compared to unheated specimenbond strength). This can be attributed to the C-S-H shrinkage gel that led to theincrease in the voids within the matrix and to the weakening of the bond betweenrebars and concrete. Hence 3 per cent was the best mix in mechanical propertiesowing to the formation of more C-S-H gel, and it was the first mix to lose the bondstrength owing to the shrinkage of the C-S-H gel.

� Using high percentages of NS (4.5 per cent) led to cracking and spalling ofconcrete after being subjected to 600°C. This can be attributed to the failure ofthe C-S-H gel and low permeability of the mix that prevents the specimens torelease vapour which led to generate pressure more than the tensile strength ofthe mixes.

3.6 Microstructure analysis3.6.1 Scanning electron microscopy. SEM images were taken to investigate themicrostructure of NSC. The SEM images are shown in Figures 15 to 19 for specimens

Figure 15.SEMmicrograph of0 per cent NSmixes

Figure 16.SEMmicrograph of

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before and after fire exposure. As for the SEM plates, the structure in control samples,when compared with NS systems, is in agreement with the results. C-S-H plates, as wellas CH crystals, and Aft needles were clearly identifiable in the control specimen as well asin the porous structure of paste (Figure 15). While for low percentages of NS (1.5 per cent

Figure 19.SEMmicrograph of4.5 per cent NSCmixes

Figure 17.SEMmicrograph of3 per cent NSCmixes

Figure 18.SEMmicrograph of3 per cent NSCmixesafter fire exposure

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and 3 per cent) specimens (Figures 16 and 17), the C-S-H plates were clearly dominatingwith a well-compacted structure, as for the high-volume NS (4.5 per cent), C-S-H platescan also be clearly found but with some agglomerated NS surrounding the hydrationproducts (Figure 19). As a conclusion, NS presence led to producing higher levels of C-S-Hin the system (Ji, 2005). The NS’s high reactivity acted as a nucleating point (Li et al.,2004) to bind the hydration products together (Jo et al., 2007). On the other side, thisphenomenon may explain the high strength of specimens containing NS. NS can absorbthe CH crystals and reduce the CH crystals’ size and amount, therefore making the ITZ ofaggregates and cement denser. Moreover, NS particles have the ability to fill the C-S-H gelstructure voids and act as a nucleus to tightly bond with C-S-H gel particles, leading tohigher durability and long-term mechanical properties of concrete.

Micrograph of the NSCs, which reached the highest compressive and bond strengths,revealed that these samples have an ITZ denser than the control sample. The ITZ isdenser because of the filling effect of the NS [Figure 17(b)], so they can improve thebehaviour of cement matrix in two different ways packing effect and reacting with CH toform more C-S-H gel. The observations from the SEM images also showed that thenanoparticles acted as an activator to promote the hydration process and to enhance thecement paste microstructure in addition to acting as a filler if the nanoparticles were goodand uniformly dispersed. The micrograph also showed that the microstructure ofconcrete modified with NS was more homogeneous, uniform and compacted than plainnormal concrete.

The performed SEM examination verified the mechanism discussed in the results, andNS particles were found to have an effect on the hydration behaviour and lead to theimprovement in the microstructure of the hardened paste. The microstructure of the mixtureincorporating NS revealed a dense, compact formation of hydration products and a reducednumber of CH crystals.

Through SEM observation, an obvious improvement in hardened cement paste and theITZmicrostructure in concrete by adding NS can be concluded. It was discovered that C-S-Hgel from pozzolanic reaction of the agglomerates cannot function as a binder. NS reduced theporosity of the hardened concrete owing to the pozzolanic reactions and introducing of moreC-S-H gel to the system. Moreover, the microstructure was enhanced because of the filling

Figure 20.XRD of 0 per cent

Wmixes

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effect of the nanoparticles. The quantity and size of portlandite crystals were reduced as aconsequence of the pozzolanic reaction and crystal growth prevention by NS. Theagglomerated NS appearing in the micrographs of the 3 per cent NS mix explains the fillingperformance of the NS that significantly affected the permeability of concrete [Figure 17(b)].For 3 per cent NS mixes, the SEM results showed the best compacted mixes and a higheramount of the C-S-H than those found in the other mixes. This confirms the gain incompressive and bond strengths that was reported earlier by using 3 per cent NS. Moreover,after exposure to 600°C, the specimen with 3 per cent NS appeared to have more stable C-S-Hwhich proves the ability of NSC to resist high temperatures as compared to normal concreteas shown in Figure 18.

Figure 21.XRD of 1.5 per centWmixes

Figure 22.XRD of 3 per centWmixes

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3.6.2 X-ray diffraction. XRD is one of the best ways to detect the effect of NS and theexposure to high temperature on the hydration products. Figures 20 to 27 show the XRDresults for the control and that of NS before and after exposure to elevated temperatures,respectively.

Owing to their crystalline nature, silica calcium silicate and CH peaks clearly appear inthe XRD diagrams. However, C-S-H cannot be detected by this technique because it is anamorphous material (Arandigoyen andAlvarez, 2006).

Before the exposure to high temperature, the use of NS led to a noticeable decrease in theCH peaks compared to control specimens. As it can be confirmed from the semi-quantitativeanalysis where the CH content decreased from 4 per cent for the control mix to reach3 per cent by using 3 per cent of NS in the concrete mix. While, some peaks disappeared

Figure 23.XRD of 4.5 per cent

Wmixes

Figure 24.XRD of 0 per centW

mixes after beingsubjected to 600°C

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because of the very high pozzolanic reactivity of NS (Aly et al., 2011) that produced moreamounts of C-S-H gel in the system, resulting in high-strength results for these specimens.However, exposure to 600°C caused a clear decrease in the CH peaks in the controlspecimens (Peng and Huang, 2008), while these peaks disappeared in the NSC specimens (3,4.5 per cent). Moreover, the increase in calcium silicate peaks (Bakhtiyari et al., 2011)indicated the C-S-H decomposition which resulted in the obvious loss in strength for thecontrol concrete specimens. The decrease in calcium silicate and silica peaks for the NSCspecimens was thought to be due to new compounds formation from the reaction betweenthe two materials (Peng and Huang, 2008) which might interpret the higher residualstrength in specimens with NS after being subjected to temperature of 600°C.

Figure 25.XRD of 1.5 per centWmixes after beingsubjected to 600°C

Figure 26.XRD of 3 per centWmixes after beingsubjected to 600°C

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4. Conclusions� Generally, adding NS improved concrete bond and compressive strengths. The

optimum percentage of NS is 3 per cent which led to an increase in concretecompressive and bond strengths by 43.5 per cent and 38.5 per cent, respectively.

� Generally, the increasing NS percentage led to a continuous reduction inpermeability (within the tested range up to 4.5 per cent). The reduction reached 51.5per cent using 4.5 per cent NS compared to control mix (0 per cent NS).

� High percentages of NS in NSC expressed better mechanical behaviour than lowpercentages to resist indirect temperature between 200°C and 400°C, while lowpercentages have better resistance to higher temperatures (up to 600°C).

� Post indirect fire tests from 0°C to 400°C indicate that specimens lost about 30, 16,25 and 22 per cent of their initial compressive strength using 0, 1.5, 3 and 4.5per cent NS, respectively. While for the range between 400°C and 600°C, specimenslost about 49, 27, 37 and 62 per cent of their initial compressive strength using 0, 1.5,3 and 4.5 per cent NS, respectively.

� For the zone of 0°C-400°C, specimens lost about 37, 29, 49 and 26 per cent of theirinitial bond strength using 0, 1.5, 3 and 4.5 per cent NS, respectively.

� From 400°C-600°C, specimens lost about 84, 65, 79 and 86 per cent of their initialbond strength using 0, 1.5, 3 and 4.5 per cent NS, respectively.

� NSC with 1.5 per cent NS gave residual bond strength after being subjected to 600°Cequals 39 per cent of control unheated concrete of 0 per cent NS.

� No visible effects were observed on the surface of all the specimens, which wereexposed to 200°C and 400°C, while after 600°C, all concrete mixes showed a highreduction in addition to explosive spalling and severe cracks for high percentages of NSmixes.

Figure 27.XRD of 4.5 per centWmixes after beingsubjected to 600°C

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� As for the SEM plates, the structure of control samples, when compared with NSsamples, is in agreement with the poor properties in the fresh and hardened states. Itwas proved that NS enhanced NSC performance by acting as a filler, and reactingwith CH forming C-S-H gel.

� XRD results complied very well with the strength results. Hence before beingexposed to high temperature, XRD analysis showed that the use of NS led to anobvious decrease in the CH peaks in comparison with control specimens. Moreover,some peaks totally disappeared because of the high pozzolanic reactivity of NS. Inaddition, the XRD tests revealed that there was a reaction between the calciumsilicate produced by the dehydration of C-S-H and the silica from the NS for NSspecimens after being exposed to 600°C, producing a new binding material thatmight have been able to contribute to concrete mechanical properties and result inhigh residual strength after exposure to 600°C.

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Corresponding authorHala Mohamed Elkady can be contacted at: hala.elkady@gmail.com

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