The Effects of CuO Nanoparticles on Properties of Self Compacting Concrete with GGBFS as Binder

In this work, strength assessments and percentage of water absorption of high performance self compacting concrete containing different amounts of ground granulated blast furnace slag and CuO nanoparticles as binder have been investigated. Portland cement was replaced by different amounts of ground granulated blast furnace slag and the properties of concrete specimens were investigated. Although it negatively impacts the physical and mechanical properties of concrete at early age of curing, ground granulated blast furnace slag was found to improve the physical and mechanical properties of concrete up to 45 wt. (%) at later ages. CuO nanoparticles with the average particle size of 15 nm were partially added to concrete with the optimum content of ground granulated blast furnace slag and physical and mechanical properties of the specimens were measured. CuO nanoparticle as a partial replacement of cement up to 3.0 wt. (%) could accelerate C-S-H gel formation as a result of increased crystalline Ca(OH) 2 amount at the early age of hydration and hence increase strength and improve the resistance to water permeability of concrete specimens. The increased the CuO nanoparticles’ content more than 3.0 wt. (%), causes the reduced the split tensile strength because of the decreased crystalline Ca(OH) 2 content required for C-S-H gel formation. Several empirical relationships have been presented to predict flexural and split tensile strength of the specimens by means of the corresponding compressive strength at a certain age of curing. More rapid appearance of the peaks related to hydrated products in X-ray diffraction results, all indicate that CuO nanoparticles could improve mechanical and physical properties of the concrete specimens.


Introduction
Self compacting concrete (SCC) is a fluid concrete that spreads through congested reinforcement, fills every corner of the formwork, and consolidated under its weight 1 . SCC necessitates excellent filling ability, good passing ability, and adequate segregation resistance. But it does not include high strength and good durability as significant performance criteria. Virtually all research has used SCC which includes active additions to satisfy the great demand for fines needed for this type of concrete, thereby improving their mechanical properties in comparison with NVC. Köning et al. 2 and Hauke 3 registered strength increase in SCCs made with different amount of fly ash. According to Fava et al. 4 , in SCCs with granulated blast furnace slag, this increase is also evident. On the other hand, when limestone filler is used, Fava et al. 4 and Daoud et al. 5 achieved a tensile strength in SCC lower than the other normal types of concrete.
Nowadays, most industrial slags are being used without taking full advantages of their characteristics or disposed rather than used. Ground granulated blast furnace (GGBFS) has been used for many years as a supplementary cementitious material in Portland cement concrete, either as a mineral admixture or a component of blended cement 6 . GGBFS typically replaces 35-65% Portland cement in concrete. Thus a 50% replacement of each ton of Portland cement would result in a reduction of approximately 500,000 t of CO 2 . Using GGBFS as a partial replacement takes advantage of the energy saving in Portland cement is governed by AASHTO M302 [7] . Three types of GGBFS are typically manufactured. They include Portland cement as covered by AASHTO M85 [8] , Portland blast furnace slag cement and slag cement as per AASHTO M240 [9] . Utilizing GGBFS as a partial replacement of ordinary Portland cement develops strength and durability of concrete by creating a denser matrix and thereby enhancing the service life of concrete structures. Grinding slag for cement replacement requires only about 25% energy needed to manufacture Portland cement 6 .
The use of these slags as cementitious components requires only grinding; it will save substantial amounts of energy compared with the production of Portland cement. The partial replacement may decrease the early strength, but increase the later strength and improve microstructure and durability of strengthened Portland cement and concrete considerably 10 . Research results have indicated that clinker less alkali-activated slags show higher strengths, denser structure and better durability than Portland cement under both normal and hydrothermal conditions [11][12][13][14][15] . Thus, the optimum content of these slags is as cementitious material components rather than as aggregates or for base stabilization. Blast furnace slag is a non-metallic material consisting essentially of silicates and aluminosilicates of calcium 16 . It is considerably used in the production of light weight aggregate. When the slag is allowed to cool slowly in the air, it solidifies into gray crystalline material known as crystallized slag. This slag is used as aggregates. When the slag is cooled very rapidly by water, it solidifies and granulates as a granulated slag. The chemical composition of slag can vary over a wide range depending on the nature of the ore, the composition of the limestone flux, coke consumption and the type of iron being made 17 .
nanoparticles is improved significantly and the sensitivity of their fatigue lives to the change of stress is also increased. In addition, the theoretic fatigue lives of concretes containing TiO 2 nanoparticles are enhanced in different extent. With increasing stress level, the enhanced extent of theoretic fatigue number is increased 35 . The abrasion resistance of concrete containing TiO 2 nanoparticles for pavement has been experimentally studied 36 . The abrasion resistance of concretes containing TiO 2 nanoparticles is significantly improved. The enhanced extent of the abrasion resistance of concrete is decreased by increasing the content of TiO 2 nanoparticles 36 . The hydration kinetics of titaniabearing tricalcium silicate phase has been studied 37 . Nano-TiO 2 -doped tricalcium silicate (C 3 S) was obtained by repeated firing of calcium carbonate and quartz in the stoichiometric ratio of 3:1 in the presence of varying amounts of titanium dioxide from 0.5 to 6% of mass. The study revealed that the presence of up to 2% TiO 2 has an inhibiting effect on the rate of hydration of C 3 S [37] .
Previously, the effects of different nanoparticles on physical and mechanical properties of concrete specimens have been studied [38][39][40][41][42][43][44][45][46][47][48][49] . It has been shown that utilizing nanoparticles in concrete improves the mechanical properties of the specimens besides the improvement in microstructure and pore structure of the concrete specimens. Nanoparticles can act as heterogeneous nuclei for cement pastes, further accelerating cement hydration because of their high reactivity, as nano-reinforcement, and as nano-filler, densifying the microstructure, thereby, leading to a reduced porosity. The most significant issue for all nanoparticles is that of effective dispersion.
Incorporating of other nanoparticles is rarely reported. Therefore, introducing some other nanoparticles which probably could improve the mechanical and physical properties of cementitious composites is inherent.
The aim of this study is investigating the effects of TiO 2 nanoparticles on concrete containing "ground granulated blast furnace slag" as binder. Using ground granulated blast furnace slag in concrete specimens as binder has been addressed in several works as mentioned above, but incorporating nanoparticles in the specimens containing ground granulated blast furnace slag to achieve high strength concrete has not been addressed in any work. Although management of waste materials is essential, achieving high strength component by using these materials seems necessary. Thus, combining the management of waste and hazardous materials and nanotechnology can lead to accessing both performance of structural components and reduction of the harmfulness of hazardous materials. The main part of the present study Investigate strength assessments and water permeability of self strength concrete incorporating CuO nanoparticles which instead of a part of its Portland cement, GGBFS has been used. In addition, pore structure and microstructure of the concrete specimens have been evaluated. Several specimens with a constant amount of polycarboxylate superplasticizer (PC) have been prepared and their physical and mechanical properties have been considered when, instead of cementitious materials, CuO nanoparticles were partially added to the cement paste.

Materials and Methods
Ordinary Portland cement (OPC) conforming to ASTM C150 [50] standard was used as received. The chemical and physical properties of the cement are shown in Table 1.
CuO nanoparticles with average particle size of 15 nm and 45 m 2 .g -1 Blaine fineness producing from Suzhou Fuer Import & Export Trade Co., Ltd was used as received. The properties of CuO nanoparticles are shown in Table 2.
Crushed limestone aggregates were used to produce self-compacting concretes, with gravel 4/12 and two types of sand: Detwiler et al. 18 investigated the effectiveness of using supplementary cementing materials to increase the chloride resistance of accelerated cured concrete and they found that concretes containing supplementary cementing materials performed better than the Portland cement concretes. As well, use of supplementary cementing materials can also prevent deleterious expansions related to both delayed ettringite formation 19 and alkali-silica reaction 20 .
Permeability of concrete is defined as the movement of liquid and/or gas through a mass of concrete under a constant pressure gradient. It is an inherent property of concrete that chiefly depends upon the geometric arrangement and characteristics of the constituent materials. The permeability of concrete is mainly controlled by the solidity and porosity of the hydrated paste present in bulk paste matrix and interfacial transition zone. In the hydrated paste, the capillary and gel pores can be distinguished. The gel pores are very small. Although they constitute a network of open pores, the permeability of this network is very low. Conversely, the capillary pores are relatively large spaces existing between the cement grains. It is the capillary porosity that greatly affects the permeability of concrete 21 . The permeability of SCC is typically lower than that of ordinary concrete. The previous research showed that SCC results in very low water and gas permeability 22,23 . This is mostly attributed to the superior flow properties, dense microstructure and refined pore. Good flow properties result in superb packing condition due to better consolidation, and thus contribute to reduce the permeability of concrete.
Since strength assessments and water permeability of concrete are joined together to affect the final performance of concrete, considering mechanical properties in terms of various types of strengths together with physical properties of concrete specimens seems essential. Hence, in this work, both physical and mechanical properties of concrete specimens have been studied.
There are several works on incorporating nanoparticles into concrete specimens to achieve improved physical and mechanical properties which most of them have focused on using SiO 2 nanoparticles [24][25][26][27][28][29][30][31][32][33] . In addition, some of the works have conducted on utilizing nano-Al 2 O 3 [34] and nano-TiO 2 [35][36][37] . SiO 2 nanoparticles have been found to improve concrete strength 25,26,32 , to increase resistance to water permeability 27 , and to help control the leaching of calcium 28 , which is closely associated with various types of concrete degradation. SiO 2 nanoparticles, in addition, have been shown to promote the hydration reactions of C 3 S as a result of the large and highly reactive surface of the nanoparticles 20,29 . SiO 2 nanoparticles have been found to be more efficient in enhancing strength than silica fume 30 . Adding 10% SiO 2 nanoparticles with dispersing agents has been observed to increase the compressive strength of cementitious composites at 28 days by as much as 26%, compared to only a 10% increase with adding 15% silica fume 33 . Even the addition of small amounts of SiO 2 nanoparticles has been observed to increase the strength results in improving the 28 day compressive strength by 10% and flexural strength by 25% [25] . However, these results depend on the production route and conditions of synthesis of SiO 2 nanoparticles (e.g., molar ratios of the reagents, the type of reaction media, and duration of the reaction for the sol-gel method) and that dispersion of SiO 2 nanoparticles in the paste plays an important role. SiO 2 nanoparticles not only behaved as nanofiller to improve the microstructure but also as an activator to accelerate pozzolanic reactions 30 .
Incorporating of TiO 2 nanoparticles has been addressed in some of the works considering the properties of normal concretes. The flexural fatigue performance of concrete containing TiO 2 nanoparticles for pavement has experimentally been studied by Li et al. 35 . They showed that the flexural fatigue performance of concretes containing TiO 2 were performed conforming to the ASTM C 293 [53] standard. After the specified curing period was over, the concrete cubes were subjected to related test by using universal testing machine. The tests were carried out triplicately and average strength values were obtained; b) Water permeability tests: Water permeability tests are performed with several methods such as coefficient of water absorption, rate of water absorption and percentage of water absorption. In this work, to evaluate the water permeability of the specimens, percentage of water absorption is an evaluation of the pore volume or porosity of concrete after hardening, which is occupied by water in saturated state. Water absorption values of CuO nanoparticle blended concrete samples were measured as per ASTM C 642 [54] after 7, 28 and 90 days of moisture curing; c) Mercury intrusion porosimetry: There are several methods generally used to measure the pore structure, such as optics method, mercury intrusion porosimetry (MIP), helium flow and gas adsorption 55 . MIP technique is extensively used to characterize the pore structure in porous material as a result of its simplicity, quickness and wide measuring range of pore one coarse 0/4, for fine aggregates and the other fine 0/2, with a very high fines content (particle size < 0.063 mm) of 19.2%, the main function of which was to provide a greater volume of fine materials to improve the stability of the fresh concrete. Ground granulated blast furnace slag was used as a replacement of Portland cement. The chemical composition of the utilized GGBFS has been illustrated in Table 1.
A polycarboxylate with a polyethylene condensate defoamed based admixture (Glenium C303 SCC) produced from Muhu (China) Construction Materials Co., Ltd was used. Table 3 shows some of the physical and chemical properties of polycarboxylate admixture used in this study.
Totally, two series of mixtures were prepared in the laboratory trials. C0-GGBFS series mixtures were prepared by cement, fine and ultra-fine crushed limestone aggregates with 19.2% by weight of ultra-fine ones and 0, 15, 30, 45 and 60% by weight of GGBFS replaced by Portland cement. N-GGBFS series were prepared with different contents of CuO nanoparticles with average particle size of 15 nm. The mixtures were prepared with the cement replacement by CuO nanoparticles from 1 to 4 wt. (%). To improve workability of the fresh concrete, 1 wt. (%) of water was replaced by polycarboxylate admixture. The superplasticizer was dissolved in water, and then the nano-CuO was added and stirred at a high speed for 3 minutes. Though nano-CuO cannot be dissolved in water, a smaller amount of nano-CuO can be dispersed evenly by the superplasticizer. The water to binder ratio for all mixtures was set at 0.40. The binder content of all mixtures was 450 kg.m -3 . The proportions of the mixtures are presented in Table 4.
The mixing sequence for specimens was consisted of homogenizing the sand and cementitious materials for one minute in the mixer and then approximately 75% of the mixing water were added. The coarse aggregate was introduced and then the superplasticizer was pre-dissolved in the remaining water and was added at the end of the mixing sequence. The total mixing time including homogenizing was 5 minutes.
Several types of tests were carried out on the prepared specimens: a) Strength evaluation tests: Cubic specimens with 100 mm edge length for compressive tests. Cylindrical specimens with the diameter of 150 mm and the height of 300 mm for split tensile tests and Cubic specimens with 200 × 50 × 50 mm edges length for flexural tests were made. The moulds were covered with polyethylene sheets and moistened for 24 hours. Then the specimens were demoulded and cured in water at a temperature of 20 °C in the room condition prior to test days. The strength tests of the samples were determined at 7, 28 and 90 days of curing. Compressive tests were carried out according to the ASTM C 39 [51] standard, split tensile tests were done in accordance to the ASTM C 496 [52] standard and flexural tests    GGBFS up to 45%. Using more than 45% GGBFS has reduced the compressive strength of the specimens and it may be as a result of the reduced CaO content in GGBFS in comparison with Portland cement. This may be reduce the amount of crystalline Ca(OH) 2 and hence C-S-H gel. This fact may be due to various factors, such as using different superplasticizers or greater fines content in the concrete specimens. Roncero and Gettu 57 have pointed out the formation of large CH crystals by using polycarboxylate superplasticizers. These large crystals weaken the aggregate-paste transition zone and hence decrease the compressive strength of concrete by decreasing the aggregate-paste bond. As for the influence of the fines content, the bigger this is the greater the shrinkage becomes [58][59][60][61][62] , giving rise to the appearance of a greater number of micro-cracks in the aggregate paste interface which also reduce the compressive strength. Moreover, by increasing the volume of fines, the specific surface area of the aggregates increases, with the aggregate-paste transition zone is being precisely the weakest phase of the concrete. Table 5  CuO nanoparticles may be due to this fact that the quantity of CuO nanoparticles presented in the mix is higher than the amount required to combine with the liberated lime during the process of hydration thus leading to excess silica leaching out and causing a deficiency in strength as it replaces part of the cementitious material but does not contribute to strength. Also, it may be due to the defects generated in dispersion of nanoparticles that causes weak zones. The higher compressive strength in the N-GGBFS series mixtures with respect to C0-GGBFS series may be as a result of the rapid consumption of crystalline Ca(OH) 2 which are quickly formed during hydration of Portland cement specially at the early ages as a result of high reactivity of CuO nanoparticles. As a consequence, the hydration of cement is accelerated and larger volumes of reaction products are formed. Also CuO nanoparticles recover the particle packing density of the blended cement, directing to a reduced volume of larger pores in the cement paste. However, as indicated, the larger volume of CuO nanoparticles than 3.0 wt. (%) reduces the compressive strength due to reduction of hydrated lime with respect to the CuO nanoparticle content in addition to the deficiency occurred during dispersion of CuO nanoparticles in the cement paste. diameter 55,56 . MIP provides information about the connectivity of pores 55 . In this study, the pore structure of concrete is evaluated by using MIP. To prepare the samples for MIP measurement, the concrete specimens after 90 days of curing were first broken into smaller pieces, and then the cement paste fragments selected from the center of prisms were used to measure pore structure. The samples were immersed in acetone to stop hydration as fast as possible. Before mercury intrusion test, the samples were dried in an oven at about 110 °C until constant weight to remove moisture in the pores. MIP is based on the assumption that the non-wetting liquid mercury (the contact angle between mercury and solid is greater than 90°) will only intrude in the pores of porous material under pressure 55,56 . Each pore size is quantitatively determined from the relationship between the volume of intruded mercury and the applied pressure 56 . The relationship between the pore diameter and applied pressure is generally described by Washburn equation as follows 55,56 : where, D is the pore diameter (nm), γ is the surface tension of mercury (dyne/cm), θ is the contact angle between mercury and solid (°) and P is the applied pressure (MPa). The test apparatus used for pore structure measurement is Auto Pore III mercury porosimeter. Mercury density is 13.5335 g.mL -1 . The surface tension of mercury is taken as 485 dynes.cm -1 , and the contact angle selected is 130°. The maximum measuring pressure applied is 200 MPa (30000 psi), which means that the smallest pore diameter that can be measured reaches about 6 nm (on the assumption that all pores have cylindrical shape); d) Scanning electron microscopy (SEM): SEM investigations were conducted on a Hitachi Model S-9260 CD-SEM apparatus. Backscattered electron (BSE) and secondary electron (SE) imaging was used to study the samples, which were prepared under conditions that ensured their subsequent viability for analytical purposes; e) X-ray diffraction (XRD): A Philips PW-1730 unit was used for XRD analysis which was taken from 4 to 70°. Table 5 shows the compressive strength of C0-GGBFS specimens after 7, 28 and 90 days of curing which are all increased by increasing  Table 5 also shows the split tensile strength and the flexural strength of C0-GGBFS and N-GGBFS series concretes. Similar to the compressive strength, the split tensile strength and the flexural strength of all N-GGBFS specimens is more than those of C0-GGBFS specimens. In addition, the split tensile strength and the flexural strength of N-GGBFS series is increased by adding CuO nanoparticles up to 3.0 wt. (%) and then it is decreased, similar to the compressive strength results. Since evaluations of strength with different tests are not affordable, here, the relationship between compressive strength and split tensile strength, and the relationship between compressive strength and flexural strength is presented. Figure 1a, b and c show the relationship between the splitting tensile strength and compressive strength of all mixes cured for 7, 28 and 90 days, respectively. In addition, Figure 2a, b and c show the relationship between the flexural strength and compressive strength of all mixes cured for 7, 28 and 90 days, respectively. In all curves, a logarithmic relation has been adopted to show this relationship. The R-squared values are also given in the figures and show a good compatibility between two specified strength. As figures show, at every age of curing, one may predict a specified strength by testing at least one of the specimens' strength.

Results and Discussion
In addition, Table 5 shows the percentage of water absorption of the specimens. The results indicate that the percentage of water absorption of C0-GGBFS specimens at 7 days of curing is increased  by increasing GGBFS content while at the later ages (28 and 90 days) it is increased. This behaviour is similar to the specimens' compressive strength in which GGBFS has negative effects at the early age of hydration. As Table 5 shows, the percentage of water absorption in C0-GGBFS specimens at 7 days of is lower than that of N-GGBFS series while at 28 and 90 days of curing, this value is lower for N-GGBFS series concrete. This may be due to more formation of hydrated products in N-GGBFS series at the early ages of curing. As mentioned above, CuO nanoparticles accelerate formation of cement hydrates and hence the specimens needs more water to produce these products. Therefore, at 7 days of curing, the consumption of water in N-GGBFS series is more than in C0-GGBFS series concrete. At 28 and 90 days of curing, the pore structure of N-GGBFS series concrete is improved and water permeability of these series is decreased with respect to the C0-GGBFS series concrete. Table 5 also shows that the percentage of water absorption in N-GGBFS series at 28 and 90 ages of curing is decreased by increasing the CuO nanoparticles content up to 4.0 wt. (%) and then it is increased. Once again, this may be due to unsuitable dispersion of the nanoparticles in the cement paste when the content of the nanoparticles goes beyond 3.0 wt. (%). On the other hand, at 7 days of curing, more water requirement by increasing nanoparticles content up to 3.0 wt. (%) results in the decreased percentage of water absorption. Therefore, it can be suggested that with prolonged curing, increasing the ages and percentages of CuO nanoparticles can lead to reduction in permeable voids. This is due to the high action and filler effects of CuO nanoparticles. Another finding is that the interfacial transition zone in concrete is improved due to high reactivity as well as filler effect of the CuO nanoparticles. This finding is partially in confirmation of the results of the study by Bui et al. 63 . Table 6 shows that with increasing GGBFS content, the total specific pore volumes of concretes are decreased and the most probable pore diameters of concretes shift to smaller pores and fall in the range of few-harm pore, which indicates that the addition of GGBFS refines the pore structure of concretes. Table 6 also gives the porosities, average diameters and median diameters (volume) of various concretes. The regularity of porosity is similar to that of total specific pore volume. The regularity of average diameter and median diameter (volume) is similar to that of most probable pore diameter. The pore size distribution of concretes is shown in Table 6. It is seen that by increasing GGBFS content, the amounts of pores decrease, which shows that the density of concretes is increased and the pore structure is improved. Table 6 shows that with increasing CuO nanoparticles up to 3.0 wt. (%), the total specific pore volumes of concretes are decreased, and the most probable pore diameters of concretes shift to smaller pores and fall in the range of few-harm pore, which indicates that the addition of PC refines the pore structure of concretes. Table 6 gives the porosities, average diameters and median diameters (volume) of various concretes. The regularity of porosity is similar to that of total specific pore volume. The regularity of average diameter and median diameter (volume) is similar to that of most probable pore diameter.
The pore size distribution of concretes is shown in Table 6. It is observed that by adding nanoparticles, the amounts of is pores decreased, which shows that the density of concretes is increased and the pore structure is improved.
The effectiveness of nano-CuO in improving the pore structure of concretes increases in the order: N1-GGBFS <N2-GGBFS <N4-GGBFS <N3-GGBFS. With increasing the nanoparticles' content, the reduced extent of pores in concretes is all decreased, and the improvement on the pore structure of concretes is weakening.
The mechanism that the nanoparticles improve the pore structure of concrete can be interpreted as follows 64 : Suppose that nanoparticles are uniformly dispersed in concrete and each particle is contained in a cube pattern, therefore the distance between nanoparticles can be determined. After the hydration begins, hydrate products diffuse and envelop nanoparticles as kernel 64 . If the content of nanoparticles and    On the whole, the addition of nanoparticles improves the pore structure of concrete. On the one hand, nanoparticles can act as a filler to enhance the density of concrete, which leads to the porosity of concrete reduced significantly. On the other hand, nanoparticles can not only act as an activator to accelerate cement hydration due to their high activity, but also act as a kernel in cement paste which makes the size of Ca(OH) 2 crystal smaller and the tropism more stochastic. Figure 3-1 shows XRD analysis of C0-GGBFS specimens at different times after curing. As Figure 3-1 also shows, the peak related to formation of the hydrated products shifts to appear in earlier times indicating the positive impact of PC on formation of Ca(OH) 2 and C-S-H gel. Figure 4a and b show SEM micrographs of C0-GGBFS specimens without and with GGBFS, respectively. The morphological analysis evinced no significant differences in the form and the texture of the different reaction products in pastes with and without admixtures. This may be due to using superplasticizer which helps self compactibility the distance between them are appropriate, the crystallization will be controlled to be a suitable state through restricting the growth of Ca(OH) 2 crystal by nanoparticles. Moreover, the nanoparticles located in cement paste as kernel can further promote cement hydration due to their high activity. This makes the cement matrix more homogeneous and compact. Consequently, the pore structure of concrete is improved evidently such as the concrete containing nano-CuO in the amount of 1% by weight of binder 64 .
With increasing the content of CuO nanoparticles more than 3.0 wt. (%), the improvement on the pore structure of concrete is weakened. This can be attributed to that the distance between nanoparticles decreases with increasing content of nanoparticles, and Ca(OH) 2 crystal cannot grow up enough due to limited space and the crystal quantity is decreased, which leads to the ratio of crystal to strengthening gel small and the shrinkage and creep of cement matrix increased 65 , thus the pore structure of cement matrix is looser relatively.
6) The pore structure of self compacting concrete containing CuO nanoparticles is improved and the content of all mesopores and macropores is increased.
of the specimens. The beneficial effects of GGBFS in concrete results from the modified microstructure of cementitious paste, which has more capillary pores, filled with low density C-S-H gel than Portland cement paste [66][67][68][69] . It can be observed that GGBFS can be effectively used to reduce the pore sizes and cumulative pore volume 70 . It appears that higher GGBFS replacement percentage has denser structure and prevents concrete from water penetration. The GGBFS reacts with water in alkali environment and then with calcium hydroxide to form cement hydration product through pozzolanic reaction to form extra C-S-H gel in the paste and slow down the strength development at early age. Denser microstructure or lower porosity results from higher C-S-H content that represents higher GGBFS replacement percentage and higher durability of concrete. Portland cement is usually used with GGBFS and the hydration product of Ca(OH) 2 activates the slag hydration to from a mixture of low CaO/SiO 2 (C/S) ratio CaO-SiO 2 -H 2 O (C-S-H) and AF m (cementitious product from the reaction of reactive alumina and calcium hydroxide) phases. Pozzolanic reaction is also found to increase the C/S ratio to a value of about 1.7 in slag-cement blends due to unstable low calcium C-S-H and Ca(OH) 2 mixture. When supplementary cementitious material like GGBFS is used in concrete, they do not only reduce the porosity but also the pores become finer and the change in mineralogy of the cement hydrates leads to the reduction in mobility of chloride ions.
Since it has been found that using 45% GGBFS instead of Portland cement produces a suitable specimen with the optimum properties, CuO nanoparticles with different amounts of replacement by Portland cement were added to the specimens containing 45 wt. (%) GGBFS and their properties have been investigated in the following section. Figure 3-2 shows XRD analysis of N-GGBFS specimens at different times after curing. As Figure 3-2 also shows, the peak related to formation of the hydrated products shifts to appear in earlier times indicating the positive impact of PC on formation of Ca(OH) 2 and C-S-H gel at early age of cement hydration.
Finally, Figure 4c show SEM micrographs of N-GGBFS specimens containing 3.0 wt. (%) of CuO nanoparticles. Figure 4c shows a more compact mixture after all days of curing which indicate rapid formation of C-S-H gel in presence of CuO nanoparticles.

Conclusions
The results obtained in this study can be summarized as follows: 1) The increased the GGBFS content up to 45.0 wt. (%) results in the increased the split tensile strength. It has been argued that utilizing GGBFS content more than 45.0 wt. (%) reduces the amount of CaO which is required for Ca(OH) 2 and subsequent C-S-H gel; 2) The pore structure of concrete specimens is found to improve with adding up to 45.0 wt. (%) GGBFS; 3) As the content of CuO nanoparticles is increased up to 3.0 wt. (%), the compressive strength, split tensile strength and flexural strength of the specimens is increased. This is due to more formation of hydrated products in presence of CuO nanoparticles; 4) CuO nanoparticles could act as nanofillers and improve the resistance to water permeability of concrete at 7 and 28 days and curing. At 7 days of curing, the percentage of water absorption is increased by increasing the nanoparticles content up to 3.0 wt. (%) since the specimens require more water to rapid forming of hydrated products; 5) Some empirical relationships in terms of logarithmic equations were provided to correlate the split tensile strength and flexural strength of a certain mixture to its compressive strength;