Print version ISSN 1516-1439
Mat. Res. vol.13 no.4 São Carlos Oct./Dec. 2010
Ali Nazari*; Shadi Riahi
Department of Technical and Engineering Sciences,Islamic Azad University (Saveh Branch), Saveh, Iran
In the present study, split tensile strength of self compacting concrete with different amount of ZrO2 nanoparticles has been investigated. ZrO2 nanoparticles with the average particle size of 15 nm were added partially to cement paste (Portland cement together with polycarboxylate superplasticizer) and split tensile strength of the specimens has been measured. The results indicate that ZrO2 nanoparticles are able to improve split tensile strength of concrete and recover the negative effects of polycarboxylate superplasticizer. ZrO2 nanoparticle as a partial replacement of cement up to 4 wt. (%) could accelerate C-S-H gel formation as a result of increased crystalline Ca(OH)2 amount at the early age of hydration. The increased the ZrO2 nanoparticles' content more than 4 wt. (%), causes the reduced the split tensile strength because of unsuitable dispersion of nanoparticles in the concrete matrix.
Keywords: A. Ceramic-matrix composites (CMCs), A. Nano particles, D. Scanning electron microscopy (SEM), D. Thermogravimetric analysis (TGA), D. X-ray diffraction (XRD)
Self-compacting concrete (SCC) is one of the most significant advances in concrete technology in recent years. SCC may be defined as a concrete with the capacity to flow inside the formwork, to pass around the reinforcements and through the narrow sections, consolidating simply under its own weight without needing additional vibration and without showing segregation or bleeding. This behavior is achieved in normally vibrated concretes (NVC) in which the same components are used with a higher content of fines and using very powerful superplasticizers. In addition, to increase the viscosity of the paste, viscosity-modifying admixtures can also be used. These are usually comprised of polymers made up of long-chain molecules which are capable to absorb and fix the free water content. This modification in the mix design may have an influence on the mechanical properties of materials; therefore it is important to ensure that all of the basic assumptions and test results for design models of NVC construction are also valid for SCC construction.
Most articles which are published until now show that for a certain compressive strength, SCC tends to reach strength slightly higher than type of NVC1-3. Nearly, all research has used SCC, which includes active additions to satisfy the great demand for fines are needed for this type of concrete, thereby, improving their mechanical properties in comparison with NVC. For instance, Köning et al.1 and Hauke2 registered strength increase in SCCs made with different amount of fly ash. According to Fava et al.3, in SCCs with granulated blast furnace slag, this increasing is also evident. On the other hand, when limestone filler is used, Fava et al.3 and Daoud et al.4 achieved a tensile strength in SCC lower than that type of NVC. Bolsjkov5 has illustrated the behavior of both types of concrete are similar. As for the modulus of elasticity, it is generally seen that this rises with age at a similar rate to that of NVCs1, though it seems that SCCs are a little more deformable6-9. These small differences in stiffness between the two types of concrete can be attributed to the SCCs' high paste content; although according to Su et al.10 increasing the fine aggregate/total aggregate ratio does not have a significant effect on the SCCs' modulus of elasticity. In any case, it should be pointed out that most of the results are available in the bibliography usually refer to highly strength SCCs, where high cement contents (higher than 400 kg.m-3) are used to be usually accompanied by active additions, such as fly ash or blast furnace slag. However, there are few studies that give results of low to medium compressive strength with SCCs.
As authors knowledge, there are few works on incorporating nanoparticles into SCCs to achieve improved physical and mechanical properties. Only, there are several reports on incorporation of nanoparticles in NVCs which most of them have focused on using SiO2 nanoparticles11-20. In addition, some of the works have conducted on utilizing nano-Al2O321,22, nano-Fe2O323 and zinc-iron oxide nanoparticles24. Previously, a series of works25-32 has been conducted on cementitious composites by adding different nanoparticles evaluating the mechanical properties of the composites.
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 incorporating ZrO2 nanoparticles into SCCs to study tensile strength and pore structure of the concrete. Several specimens with different amount of polycarboxylate superplasticizer (PC) have been prepared and their physical and mechanical properties have been considered when, instead of cement, ZrO2 nanoparticles were partially added to the cement paste.
2. Materials and Methods
Ordinary Portland Cement (OPC) conforming to ASTM C15033 standard was used as received. The chemical and physical properties of the cement are shown in Table 1. The particle size distribution pattern of the used OPC has been illustrated in Figure 1.
ZrO2 nanoparticles with average particle size of 15 nm and 45 m2.g-1 Blaine fineness producing from Suzhou Fuer Import & Export Trade Co., Ltd was used as received. The properties of ZrO2 nanoparticles are shown in Table 2. Scanning electron micrographs (SEM) and powder X-ray diffraction (XRD) diagrams of ZrO2 nanoparticles are shown in Figures 2 and 3.
Crushed limestone aggregates were used to produce self-compacting concretes, with gravel 4/12 and two types of sand: 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.
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-SCC series mixtures were prepared by cement, fine and ultra-fine crushed limestone aggregates with 19.2% by weight of ultra-fine ones and 0, 0.3, 0.5, 0.7 and 1.0% by weight of polycarboxylate admixture replaced by water. N-SCC series were prepared with different contents of ZrO2 nanoparticles with average particle size of 15 nm. The mixtures were prepared with the cement replacement by ZrO2 nanoparticles from 1 to 5 wt. (%) and 1 wt. (%) polycarboxylate admixture. The water to binder ratio for all mixtures was set at 0.4034. 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 SCCs was consisted of homogenizing the sand and cementitious materials for one minute in the mixer and then approximately 75% of the mixing water was 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:
Split tensile strength: Cylindrical specimens with the diameter of 150 mm and height of 300 mm were made for split tensile tests. The moulds were covered with polyethylene sheets and moistened for 24 h. Then the specimens were demoulded and cured in water at a temperature of 20 ºC in the room condition prior to test days. The split tensile strength tests of the samples were determined at 2, 7 and 28 days of curing. Split tensile tests were carried out according to the ASTM C 49635 standard. The tests were carried out triplicately and average split tensile strength values were obtained;
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 adsorption36. 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 diameter36,37. MIP provides information about the connectivity of pores36; and
In this study, the pore structure of concrete is evaluated by using MIP. To prepare the samples for MIP measurement, the concrete specimens after 28 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 ºC 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 90º) will only intrude in the pores of porous material under pressure36,37. Each pore size is quantitatively determined from the relationship between the volume of intruded mercury and the applied pressure37. The relationship between the pore diameter and applied pressure is generally described by Washburn equation as follows36,37:
D is the pore diameter (nm);
γ is the surface tension of mercury (dyne.cm-1);
θ is the contact angle between mercury and solid (º); 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 130º. 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: The test was run out on a Wexham Developments JAF model isothermal calorimeter, using IBM program AWCAL-4, at 22 ºC 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.
Thermogravimetric analysis (TGA): A Netzsch model STA 409 simultaneous thermal analyzer equipped with a Data Acquisition System 414/1 programmer was used for the tests. Specimens which were cured for 28 days were heated from 110 to 650ºC, at a heating rate of 4ºC/min and in an inert N2 atmosphere.
Scanning electron microscopy (SEM): 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.
X-ray diffraction (XRD): A Philips PW-1730 unit was used for XRD analysis which was taken from 4 to 70º.
3. Results and Discussion
3.1. Strength analysis of C0-SCC specimens
Figure 4 shows the split tensile strength of C0-SCC specimens after 2, 7 and 28 days of curing which are all reduced by increasing PC amount especially at early age of curing. This fact may be due to various factors, such as using different superplasticizers or greater fines content in the SCCs. Roncero and Gettu38 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 tensile 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 becomes8,39-43, giving rise to the appearance of a greater number of micro-cracks in the aggregate paste interface which also reduce the tensile 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.
During the early days of hydration, the strength is affected by two opposing effects: on one hand, the limestone fines raise the rate of hydration of some clinker compounds since the fines act as nucleation sites of the hydrates formed in the hydration reactions5,44. On the other hand, PC has a delaying effect on hydration of CH crystals and formation of C3H[45,46].
At higher ages, 28 days, the two aforementioned effects disappear and it can clearly be seen that there is less effects on reducing the split tensile strength in SCCs by increasing PC. This is due to a longer development over time for the cement's hydration processes in the SCCs with higher content of PC as a result of the SCCs' greater capacity to retain water47, which allows pozzolanic additions to continue reacting at higher ages with the lime resulting from the cement hydration. Furthermore, although PC retards the initial hydration reactions, according to Puertas et al.47 these reactions are intensified in later stages as a result of particle dispersion.
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, 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)48. 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.
Table 5 shows that with increasing PC 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 PC refines the pore structure of concretes.
Table 5 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 5. It is seen that by increasing PC content, the amounts of pores decrease, which shows that the density of concretes is increased and the pore structure is improved.
Table 6 shows the conduction calorimetry of C0-SCC specimens. 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- 49. The numerical values corresponding to these two signals (heat release rate, peak times) and the total released heat are shown in Table 6. 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 6 show that increasing the percentage of PC in the pastes retards peak times and raises heat release rate values. This is indicative of a delay in initial cement hydration because of higher content of PC. 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 PC in the mix.
Table 7 shows the thermogravimetric analysis results of C0-SCC specimens measured in the 110-650ºC range in which dehydration of the hydrated products occurred. The results show that after 28 days of curing, the loss in weight of the specimens is increased by decreasing the PC content in concretes.
Figure 5 shows XRD analysis of C0-SCC specimens at different times after curing. As Figure 5 also shows, the peak related to formation of the hydrated products shifts to appear in later times indicating the negative impact of PC on formation of Ca(OH)2 and C-S-H gel at early ages of cement hydration.
Finally, Figure 6 shows SEM micrographs of C0-SCC specimens without and with PC. The morphological analysis evinced no substantial differences in either the form or the texture of the different reaction products in pastes with and without admixtures. The micrographs corresponding to paste cured for 2 and 7 days show anhydrous cement that has not yet reacted, along with a relatively porous mass analogous to the reaction products. This region is more compact and less porous in the paste with admixture. After 28 days, the reaction is observed to progress, with a considerable decrease in the amount of anhydrous cement particles. The atomic ratios found in an analysis performed on the reaction products of all the pastes studied which are shown in Table 6.
The results obtained with respect to the effect of PC on cement hydration show that at early ages PC retards the initial cement hydration. This effect is more evident at higher doses of superplasticizer. This phenomenon is confirmed by the results obtained in conduction calorimetry, with a retardation of the peak time for the first peak in the heat release rate, associated with C3H and CH formation. This lower initial formation of reaction products is further corroborated by the smaller weight loss detected in 2 days cured pastes with admixtures, when subjected to temperatures of 110-650 ºC. Such weight loss is related to the partial and total dehydration of C3H and CH.
The delay in hydration reactions is expressed as a very extensive lengthening of the initial and final setting times of the pastes that have high dosages of PC admixture. SNF and SMF superplasticizers also produce retardations in initial cement hydration reactions, a development that is closely associated to the adsorption of the compounds to the surface of cement particles or some of the hydration products50. PC admixtures likewise adsorb on the surface of cement particles, with a dispersive capability that is dependent upon the amount adsorbed. According to Yamada and Hanehara51 such adsorption is affected by two important factors: the specific area of the solid phases and possible competition with other anionic species, such as sulphate ions, in the adsorption process52.
In summary, in its interactions with the reactive species, the organic admixture affects hydrated phase diffusion, nucleation and growth and therefore the hydration process. Setting is related to the concentration of Ca2+ ions in the liquid phase: delayed setting is attributed to a decrease in the concentration of such Ca2+ ions. Using X-ray photoelectron spectroscopy or electron spectroscopy for chemical analysis (ESCA), Uchikawa et al.53 showed that a chelate forms in pastes with PC admixtures as a result of the reaction between the Ca2+ ions and the admixture molecules. The formation of this chelate would inferior the Ca2+ concentration and thereby retards setting and hinders solid phase nucleation.
Differences of some significance were observed, however, in the porosity data: after 2 days of hydration, as the PC admixture content is increased, total porosity is declined and the pore structure became more refined, with a rise in the percentage of mesopores and a decline in the macropore contents. Roncero et al.54 have reported similar results, observing a decrease in total porosity and in the distribution of pore size in cement pastes which contains different types of superplasticizer admixtures (a polycarboxylate admixture among them). Their results refer to pastes cured at 7 and 28 days.
This, in turn, would explain the lengthening of the induction period observed in the heat release rate when the content of the PC admixture is increased (Table 6). Jolicoeur and Simard55 have found similar results with more conventional superplasticizers. This retardation of C3H and CH nucleation and growth is the reason for the higher weight loss obtained with thermogravimetric analysis (Table 7) in 28-day pastes containing more PC admixture.
The presence of admixtures does not seem to affect the mechanical strength of the paste at either 2 or 28 days of hydration (Figure 4). After 2 days, the pastes with superplasticizer have smaller amounts of reaction products (C-S-H gel and Ch crystals), their mechanical behavior appears to be more tightly is related to the pore structure (decrease in the size of the pores) and, very likely, to a better distribution of the different components. Legrand and Warquin56 have found that in the presence of superplasticizers, despite a decrease in hydrate formation, strengths were comparable, and explained this development by the better dispersion of cement particles. Therefore, only cement paste with 1 wt. (%) PC admixture was selected because of its high workability and cement was partially replaced by different amount of ZrO2 nanoparticles. The results are discussed in the following section.
3.2. Strength analysis of N-SCC specimens
Figure 7 shows the split tensile strength of N-SCC specimens after 2, 7 and 28 days of curing. The results show that the split tensile strength increases by adding ZrO2 nanoparticles up to 4 wt. (%) replacement (N4-SCC series) and then it decreases, although adding 5% ZrO2 nanoparticles produces specimens with much higher split tensile strength with respect to the all other C0-SCC concretes. The reduced split tensile strength by adding more than 4 wt. (%) ZrO2 nanoparticles may be due to this fact that the quantity of ZrO2 nanoparticles present 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 split tensile strength in the N-SCC series mixtures with respect to C0-SCC series is due to the rapid consumption of crystalline Ca(OH)2 which quickly are formed during hydration of Portland cement specially at early ages as a result of high reactivity of ZrO2 nanoparticles. As a consequence, the hydration of cement is accelerated and larger volumes of reaction products are formed. Also ZrO2 nanoparticles recover the particle packing density of the blended cement, directing to a reduced volume of larger pores in the cement paste.
Table 5 shows that with increasing ZrO2 nanoparticles up to 4 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 5 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 5. It is observed that by adding nanoparticles, the amounts of pores decreased, which shows that the density of concretes is increased and the pore structure is improved.
The effectiveness of nano-ZrO2 in improving the pore structure of concretes increases in the order: N1-SCC <N2-SCC <N3-SCC <N5-SCC <N4-SCC. 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 follows57: 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 kernel57. 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-ZrO2 in the amount of 1% by weight of binder57.
With increasing the content of ZrO2 nanoparticles more than 4 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 increased58, thus the pore structure of cement matrix is looser 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.
The heat release rate values in Table 6 show that increasing the percentage of ZrO2 nanoparticles up to 4 wt. (%) in the pastes accelerates peak times and drops heat release rate values. This is indicative of acceleration in initial cement hydration due to higher content of ZrO2 nanoparticles. ZrO2 nanoparticles as a foreign nucleation site can accelerate the cement hydration and hence increase the heat release rate. As it is stated above, the appearance of the peaks in conduction calorimetry tests are due to CH and C3H compounds formation in the cement paste. When ZrO2 nanoparticles partially added to cement paste, the acceleration in formation of CH and C3H would result in more rapid appearance of the related peaks.
Table 7 shows the thermogravimetric analysis results of N-SCC specimens measured in the 110-650ºC range in which dehydration of the hydrated products occurred. The results show that after 28 days of curing, the loss in weight of the specimens is increased by increasing ZrO2 nanoparticles in concretes up to 4 wt. (%). Again, such as the results obtained for conduction calorimetry, the increase in loss weight is due to more formation of CH and C3H compounds in the cement paste.
Figure 8 shows XRD analysis of N-SCC specimens at different times after curing. As Figure 8 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 9 shows SEM micrographs of N-SCC specimens containing 4 wt. (%) of ZrO2 nanoparticles. Figure 9 shows a more compact mixture after all days of curing which indicate rapid formation of C-S-H gel in presence of ZrO2 nanoparticles.
In this paper, the strength enhancement under compression force has been investigated energetically:
Both nanoparticles and aggregates are exterior components but, why the nanoparticles are able to make a stronger composite is as a result of the free energy of nucleation sites. The driving force for the nucleation is the reduction of interfacial free energy between the nucleus (C-S-H gel) and nucleation site (nanoparticle or aggregates). In comparison between these two approximately spherical particles (nanoparticle and aggregates), the ratio between surface area to volume of nanoparticle is much larger than that of aggregates. Therefore, the abiltity of the nanoparticle to formation of C-S-H gel is more than this for aggregates.
In addition, the C-S-H gel-nanoparticle interface is probably coherent or semi-coherent. This is due to this fact that C-S-H gel formed around the nanoparticle could maintain its coherency with nanoparticle because both C-S-H gel and nanoparticle have nano-scale dimensions. But the C-S-H gel formed around the aggregate has a completely incoherent interface with aggregate. In the other words, the sand particle is large enough which could not maintain its coherency with nanoscale C-S-H gel. Furthermore, in the vicinity of the aggregates, the probability of void formation and presence of un-reacted cement, other aggregates and even nanoparticles is much more with respect to the nanoparticle causes more weak zones in the vicinity of sand. Although sands act as reinforcement in cementitious matrix but as a result of incoherency under split tensile loading, fracture occurrence and crack propagation from the C-S-H gel formed at the surface of the sand is more probable with respect to the nanoparticle. This phenomenon is tightly like to the inclusions and precipitations in metallic alloys where inclusions with incoherent interface (like aggregate in concrete) could not improve the mechanical properties of the alloy while precipitations with coherent or semi-coherent interface (like nanoparticle in concrete) could improve mechanical properties of the alloy mainly. Therefore, the smaller the nanoparticle size, the more the heterogeneous nucleation sites results in shorter early age according to Equation 159,60:
I [nucleus/s] is the nucleation rate;
ΔG* [J] is the critical free energy for nucleation;
T [K] is the absolute temperature;
K [J/K] is the Bultzman's constant;
A is a constant; and
NT is the number of nucleation sites.
In a constant temperature, by increasing NT (the smaller nanoparticles) the rate of C-S-H gel formation is increased causes shorter early age.
In general, the free energy of heterogeneous nucleation from gel on the surface of a foreign particle (ΔGhet which here is ZrO2 in the form of aggregate or nanoparticle) could be obtained from Equation 261,62;
VS [cm3] is the volume of solid nucleus;
ΔGV [J.cm-3] is the volume energy of solid nucleus;
Asg [cm2] is the interface between solid nucleus and gel;
Asp [cm2] is the interface between solid nucleus and nanoparticle;
γsg [J.cm-2] is the surface free energy between solid nucleus and gel;
γsp [J.cm-2] is the interface between solid nucleus and nanoparticle; and
γpg [J.cm-2] is the surface free energy between nanoparticle and gel.
The negative signs are due to conversion of gel to solid nucleus. In heterogeneous nucleation, to minimize ΔG*, the nucleus shape must be a part of hemisphere (Figure 10). The values of VS, Asg and Asp could be obtained as61,63:
r [cm] is the nucleus radius; and
θ is the wetting angle and is constant during the growth of nucleus (Figure 10).
Figure 10 shows that the equilibrium condition is obtained when61:
By substituting of Equations 3 through 6 into Equation 2 one may write61:
where S(θ) is the shape factor and could be written as61:
The critical radius of nucleation (r*) could be obtained from the first derivative of Equation 7 with respect to r and equaling to zero61,64:
By substituting Equation 9 into Equation 7 one may write61:
γsg and δGV depend on the particle composition and are equal for different particle sizes. Equation 10 shows that by decreasing θ, ΔG* is also decreased. If both nucleus and nucleation site have the same crystalline structure and have approximately equal cell parameter, then γsp could be maintained at its minimum amount and according to Equation 6, θ is minimized.
From this point of view, ΔG* is equal for both nanoparticles and aggregates, but Equation 10 could be written as61;
V* [cm3] is the critical volume of nucleus.
The difference between nucleation on aggregates and nanoparticles is in the volume of nucleated material for reaching to r*. C-S-H gel which is formed around the sand propagates over the time and makes a large amount of C-S-H gel with incoherent interface since its critical volume probably reaches to r*. On the other hand, nanoparticles do not grow sufficiently and make many dispersed coherent C-S-H gel results in high strength concrete.
The results obtained in this study can be summarized as follows:
The increased the PC content, results in the decreased the split tensile strength. It has been argued that PC retards cement hydration especially at early ages. However no evident differences between split tensile strength of specimens with and without PC.
As the content of ZrO2 nanoparticles is increased up to 4 wt. (%), the split tensile strength of SCC specimens is increased. This is due to more formation of hydrated products in presence of ZrO2 nanoparticles.
The pore structure of self compacting concrete containing ZrO2 nanoparticles is improved and the content of all mesopores and macropores is increased.
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Received: July 18, 2010; Revised: September 9, 2010