Improvement Compressive Strength of Cementitious Composites in Different Curing Media by Incorporating ZrO 2 Nanoparticles

In the present work, the effect of curing medium on microstructure, physical, mechanical and thermal properties of ZrO 2 nanoparticles blended concrete has been investigated. ZrO 2 nanoparticles were partially used instead of cement by 0.5, 1.0, 1.5 and 2.0 weight percent. Curing of the specimens was carried out in water and saturated limewater for 7, 28 and 90 days. The results indicate that ZrO 2 nanoparticles up to maximum of 2.0% produces cementitious composite with improved compressive strength by curing in saturated limewater. The optimum level of replacement for the specimens cured in water was 1.0 weight percent. ZrO 2 nanoparticles can improve the filler effects and also the high activity of fine particles substantially increases the quantity of strengthening gel. Although the limewater reduces the strength of concrete without nanoparticles when compared with the specimens cured in water, curing the specimens in saturated limewater results in more strengthening gel formation around ZrO 2 nanoparticles blended concrete causes high strength.


Introduction
There are several reports on merging nanoparticles in concrete specimens which most of them have focused on using SiO 2 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and TiO 2 nanoparticles [17][18][19] .SiO 2 nanoparticles have been found to improve concrete strength 2,9,11 , to increase resistance to water permeability 12 , and to help control the leaching of calcium 13 , 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 10,14 .SiO 2 nanoparticles have been found to be more efficient in enhancing strength than silica fume 15,16 .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 10 .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% [2] .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 15 .
Incorporating TiO 2 nanoparticles has been addressed in some of the works considering the properties of NVCs.
The flexural fatigue performance of concrete containing TiO 2 nanoparticles for pavement has experimentally been studied by Li et al. 17 .They showed that the flexural fatigue performance of concretes containing TiO 2 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 17 .The abrasion resistance of concrete containing TiO 2 nanoparticles for pavement has been experimentally studied 18 .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 18 .The hydration kinetics of titania-bearing tricalcium silicate phase has been studied 19 .Nano-TiO 2doped 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% by weight.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 [19] .
The other nanoparticles (such as ZrO 2 nanoparticles in this work) have rarely investigated and some of researchers do not agree that the other nanoparticles could be pozzolans.It seems that several works are needed to introduce these new materials as partial cement replacement.

Materials and Methods
Ordinary Portland Cement (OPC) conforming to ASTM C150 [39] standard was used as received.The chemical and physical properties of the cement are shown in Table 1.
Nano-ZrO 2 with average particle size of 15 nm producing from Suzhou Fuer Import&Export Trade Co., Ltd was used as received.The properties of nano-ZrO 2 particles are shown in Table 2.
Locally available natural sand with particles smaller than 0.5 mm and fineness modulus of 2.25 and specific gravity of 2.58 g.cm -3 (loose form) was used as fine aggregate.Crushed basalt stored in the laboratory with maximum size of 15 mm and specific gravity of 2.96 g.cm -3 (loose form) was used as coarse aggregate.
Two series of mixtures were prepared in the laboratory trials.C0 Series mixtures were prepared as control specimens.The control mixtures were made of natural aggregates, cement and water.N series were prepared with different contents of nano-ZrO 2 particles with average particle size of 15 nm.The mixtures were prepared with the cement replacement of 0.5, 1.0, 1.5 and 2.0% by weight.The water to binder ratio for all mixtures was set at 0.40.The aggregates for the mixtures consisted of a combination of crushed basalt and of fine sand, with 30 wt% of sand.The binder content of all mixtures was 450 kg.m -3 .The proportions of the mixtures are presented in Table 3.
N series mixtures were prepared by mixing the course aggregates, fine aggregates and powder materials (cement and nano-ZrO 2 particles) in a laboratory concrete drum mixer.The powder material in the C0 series mixtures was only cement.They were mixed in dry condition for two minutes, and for another three minutes after adding the water.Cubes of 100 mm edge for compressive strength tests were cast and compacted in two layers on a vibrating table, where each layer was vibrated for 10 seconds.The moulds were covered with polyethylene sheets and moistened for 24 hours.Then the specimens were demoulded and cured in water (N-W series) and saturated limewater (N-LW series) at a temperature of 20 °C prior to test days.To produce saturated limewater, the lime powder was solved in fresh water and distilled until the excess lime powder was precipitated.The strength tests of the cementitious composite samples were determined at 7, 28 and 90 days after curing.After each time of the curing, the old limewater media was replaced by a new one to maintain saturated limewater.
Compressive strength of nano-ZrO 2 particles blended cement cementitious composite cubes was determined in accordance to the ASTM C 39 [40] after 7, 28 and 90 days of moisture curing.Tests were carried out on triplicate specimens and average compressive strength values were considered.
Mercury intrusion porosimetry (MIP) is extensively used to characterize the pore structure in porous material as a result of its simplicity, quickness and wide measuring range of pore diameter 41,42 .MIP provides information about the connectivity of pores 41 .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 are first broken into smaller pieces, and then the cement paste fragments selected from the center of prisms are used to measure pore structure.The samples are immersed in acetone to stop hydration as fast as possible.Before mercury intrusion test, the samples are 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 41,42 .Each pore size is quantitatively determined from the relationship between the volume of intruded mercury and the applied pressure 42 .The relationship between the pore diameter and applied pressure is generally described by Washburn Equation 1as follows 41,42 : 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.

Materials Research
Improvement Compressive Strength of Cementitious Composites in Different Curing Media by Incorporating ZrO2 Nanoparticles 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).Conduction calorimetry test was run out on a Wexham Developments JAF model isothermal calorimeter, using IBM program AWCAL-4, at 22 °C for a maximum of 70 hours.Fifteen grams of cement was mixed with water and saturated limewater and admixture before introducing it into the calorimeter cell.
A Netzsch model STA 409 simultaneous thermal analyzer equipped with a Data Acquisition System 414/1 programmer was used for thermogravimetric analysis.Specimens which were cured in water and saturated limewater for 90 days were heated from 110 to 650 °C, at a heating rate of 4 °C/min in an inert N 2 atmosphere.
SEM investigations were conducted on a Hitachi 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.
A Philips PW-1730 unit was used for XRD analysis which was taken from 4 to 70°.

Compressive strength
The compressive strength results of C0-W and N-W series mixtures are shown in Table 4.By comparison the compressive strength of the specimens cured for 7, 28 and 90 days, it could be observed that the compressive strength increases with nano-ZrO 2 particles up to 1.0% replacement (N2-W) and then it decreases, although adding 2.0% nanoparticles (N4-W) results in strengths which are still higher than those of the plain cement concrete (C0-W).It was shown that using 2.0% nano-ZrO 2 particles in N-W series decreases the compressive strength to a value which is near to the control cementitious composite.This may be due to the defects generated in dispersion of nanoparticles that causes weak zones.The high enhancement of compressive strength in the N series blended cementitious composite are due to the rapid consuming of Ca(OH) 2 which was formed during hydration of Portland cement specially at early ages related to the high reactivity of nano-ZrO 2 particles.As a consequence, the hydration of cement is accelerated and larger volumes of reaction products are formed.Also nano-ZrO 2 particles recover the particle packing density of the blended cement, directing to a reduced volume of larger pores in the cement paste.Figures 1 and 2 respectively show the SEM micrograph of concrete without and with ZrO 2 nanoparticles after 7 days of curing in water.C-S-H gel which is existed in isolation is enclosed by some of needle-hydrates in the SEM micrograph of cement paste (Figure 1).On the other hand, the micrograph of the mixture containing nano-ZrO 2 revealed a compact formation of hydration products and a reduced content of Ca(OH) 2 crystals (Figure 2).In addition XRD results of the cement pastes with and without ZrO 2 nanoparticles after 15 hours of curing in water has been illustrated in Figure 3.The results show that after 15 hours of curing, Ca(OH) 2 crystals appears in concrete with ZrO 2 nanoparticles while in concrete specimens without ZrO 2 nanoparticles they do not appear indicating synergic effects of ZrO 2 nanoparticles on formation of subsequent C-S-H gel.Table 4 also shows the compressive strength of C0-LW and N-LW series.The results show that the replacement of cement by ZrO 2 nanoparticles up to 2.0 Wt% (N4-LW) in N-LW series produces cementitious composite with high strength with respect to N-LW cementitious composite.By comparison the compressive strength of C0-W and C0-LW series, it is seen that after 7, 28 and 90 days of curing, the compressive strength of the C0-LW series is smaller than the corresponding strength of C0-W series.This may be due to more formation of crystalline Ca(OH) 2 in limewater which reduces the compressive strength of C0-LW series with respect to C0-W series.On the other hand, the compressive strength of N-LW series is more than those of N-W series.Lime reacts with water and produces Ca(OH) 2 which needs to form strengthening gel 43 (Equation 2): when ZrO 2 nanoparticles react with Ca(OH) 2 produced from saturated limewater, the content of strengthening gel is increased because of high free energy of nanoparticles 28,29,38 .
The compressive strength of N-W and N-LW series should be compared from two viewpoints: 1) The compressive strength of N-LW series increases by partial replacement of cement with ZrO 2 nanoparticles up to 2.0 wt% (N4-LW) while for N-W series it increases by replacement level up to 1.0 wt% (N2-W) and then decreases.Once more this confirms the more strengthening gel formation in the presence of saturated limewater in which the quantity of nano-ZrO 2 particles present in the mix is close to the amount required to combine with the liberated lime during the process of hydration 28,29,38 .
2) The difference between compressive strengths of the N-W and N-LW series after 28 days of curing is relatively high while this difference after 90 days of curing is not high.This may be due to formation of crystalline Ca(OH) 2 in N-LW series after 28 days.In the other words, curing of the ZrO 2 nanoparticles blended cementitious composite in saturated limewater after 28 days is completely suitable to achieve high strength especially with high weight percent of nanoparticles.
Figures 4 and 5 show the SEM micrograph of concrete without and with ZrO 2 nanoparticles at 7 days of curing in saturated limewater.The micrograph of the mixture containing nano-ZrO 2 revealed a compact formation of hydration products and a reduced content of Ca(OH) 2 crystals.Moreover, No significant differences is evident between the microstructure of concrete without nano-ZrO 2 cured in water or saturated limewater.In addition XRD results of the cement pastes with ZrO 2 nanoparticles at 15 hours of curing in saturated limewater have been illustrated in Figure 2. The longer pick intensity show that the amount of Ca(OH) 2 crystals which needs to formation C-S-H gel is more for the specimens cured in limewater rather than the specimens cured in water when ZrO 2 nanoparticles are used.

Pore structure of concrete
The pore structure of concrete is the general embodiment of porosity, pore size distribution, pore scale and pore geometry.The test results of MIP in this study include the pore structure parameters such as total specific pore volume,

Improvement Compressive Strength of Cementitious Composites in Different
Curing Media by Incorporating ZrO2 Nanoparticles most probable pore diameter, pore size distribution, porosity, average diameter, and median diameter (volume).
In terms of the different effect of pore size on concrete performance, the pore in concrete is classified as harmless pore (<20 nm), few-harm pore (20~50 nm), harmful pore (50~200 nm) and multi-harm pore (>200 nm) [44] .In order to analyze and compare conveniently, the pore structure of concrete is divided into four ranges according to this sort method in this work.
3.2.1.Total specific pore volume and most probable pore diameter of concrete Table 5 shows that with the addition of ZrO 2 nanoparticles, the total specific pore volumes of concretes is 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 nanoparticles refines the pore structure of concretes.
The effectiveness of nano-ZrO 2 in reducing the total specific pore volumes and most probable pore diameters of concretes increases in the order N4-W < N1-W < N3-W < N2-W series for the specimens cured in water and N1-LW <N2-LW < N3-LW < N4-LW for the specimens cured in saturated limewater.
With increasing the content of nanoparticles, the reduced extent of total specific pore volume and most probable pore diameter increases, and the refinement on the pore structure of concretes is improved.
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.Therefore, it is no longer necessary to analyze one by one herein.

Pore size distribution of concrete
The pore size distribution of concretes is shown in Table 7.It can be seen that by the addition of nano-particles, the amounts of pores decrease, which shows that the density of concretes is increased and the pore structure is improved.
The effectiveness of nano-ZrO 2 in improving the pore structure of concretes increases in the order N4-W < N1-W < N3-W < N2-W series for the specimens cured in water and N1-LW < N2-LW < N3-LW < N4-LW for the specimens cured in saturated limewater.
The harmless and few-harm pores in N2-W and N4-LW series increase by the largest extent, while its harmful and multi-harm pores decrease by the largest extent, which indicates that the pore structure of N2-W and N4-LW series is most significantly improved.With increasing of nanoparticles' content, the extent pores in concretes are all decreased, and the improvement on the pore structure of concretes is evident.
The mechanism that the nanoparticles improve the pore structure of concrete can be interpreted as follows.Supposed that nanoparticles are uniformly dispersed in concrete and each particle is contained in a cube pattern, the distance between nanoparticles can be determined.After hydration begins, hydrate products diffuse and envelop nanoparticles as kernel.If the content of nanoparticles and 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-ZrO 2 in the amount of 2.0% by weight of nanoparticles.
With increasing content of nanoparticles, the improvement on the pore structure of concrete is evident.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 is decreased 44 , thus the pore structure of cement matrix is better relatively.
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.

Conduction calorimetry
Two signals can be distinguished on all test results: a peak corresponding to the acceleration or post-induction period, associated with the precipitation of C-S-H gel and CH, and a shoulder related to a second, weaker signal with a later peak time, associated with the transformation from the ettringite (AFt) to the calcium monosulphoaluminate (AFm) phase via dissolution and reaction with Al(OH) 4-[45] .The numerical values corresponding to these two signals (heat release rate, peak times) and the total released heat are shown in Table 8.The time period over the total heat was measured until the heat release rate was below 1% of the maximum of the second peak.The heat release rate values in Table 8 show that decreasing the percentage of nanoparticles in the pastes retards peak times and raises heat release rate values.In addition, the specimens which have been cured in saturated

Conclusions
The results show that the specimens containing ZrO 2 nanoparticles have significantly higher compressive strength with respect to the specimens without nano-ZrO 2 at every age of curing.It is found that the cement could be advantageously replaced with nano-ZrO 2 particles up to maximum limit of 2.0% with the average particle size of 15 nm when the specimens cured at saturated limewater.The optimum level of nano-ZrO 2 particles content was achieved 1.0% for the specimens cured in water.ZrO 2 nanoparticles could accelerate the appearance of the first peak in conduction calorimetry test which is related to the acceleration in formation of hydrated cement products.Thermogravimetric analysis shows that ZrO 2 nanoparticles could increase the weight loss of the specimens when partially added to cement paste.More rapid formation of hydrated products in presence of ZrO 2 nanoparticles which was confirmed by XRD results could be the reason of more weight loss.The pore structure of concrete containing ZrO 2 nanoparticles is improved and the content of all mesopores and macropores is increased.limewater show a decreased peak time and heat release rate values with respect to the corresponding specimens cured in water.This is indicative of a delay in initial cement hydration due to lesser content of nanoparticles and the lesser content of Ca(OH) 2 in the specimens cured in water.The retardation is much less marked in the second peak.The total heat released under identical conditions (at times when the heat release rate is less than 1% of the maximum amount of heat released in the first peak) decreases with higher percentages of ZrO 2 nanoparticles in the mix.

Thermogravimetric analysis results
Table 9 shows the weight loss measured in the 110-650 °C range in which dehydration of the hydrated products occurred.The results show that after 90 days of curing, the loss in weight of the specimens is increased by increasing the nanoparticles content in concretes.This is more evident in N2-W and N4-LW series.This may be due to more formation of hydrated C-S-H gel in N2-W and N4-LW series.

Figure 2 .
Figure 2. SEM micrograph of cement paste with ZrO 2 nanoparticles cured in water.

Figure 3 .
Figure 3. XRD results of cement pastes at 15 hours of curing for specimens a) without ZrO 2 nanoparticles cured in water; b) with ZrO 2 nanoparticles cured in water; c) without ZrO 2 nanoparticles cured in saturated limewater and d) with ZrO 2 nanoparticles cured in saturated limewater.

Table 1 .
Chemical and physical properties of Portland cement (Wt%).

designation nano-ZrO 2 particle (%) Compressive strength, MPa 7 days 28 days 90 days
Water to binder [cement + nano-ZrO 2 ] ratio of 0.40.W denotes the specimens cured in water and LW denotes to those cured in saturated limewater.

Table 5 .
Total specific pore volumes and most probable pore diameters of concretes.
Water to binder [cement + nano-ZrO 2 ] ratio of 0.40.W denotes the specimens cured in water and LW denotes to those cured in saturated limewater.

Table 6 .
Prosities, average diameters and median diameters (volume) of concretes.Water to binder [cement + nano-ZrO 2 ] ratio of 0.40.W denotes the specimens cured in water and LW denotes to those cured in saturated limewater.

Table 9 .
Weight loss (%) of the pastes in the range of 110-650 °C after 90 days of curing.
Water to binder [cement + nano-ZrO 2 ] ratio of 0.40.W denotes the specimens cured in water and LW denotes to those cured in saturated limewater.

Table 7 .
Pore size distribution of concretes.ZrO 2 ] ratio of 0.40.W denotes the specimens cured in water and LW denotes to those cured in saturated limewater.

Table 8 .
Calorimetric results of cement pastes.