Experimental study on ASR performance of concrete with nano-particles

ABSTRACT Alkali-silica reaction (ASR) is a serious durability problem, resulting in significant maintenance and reconstruction cost of concrete infrastructure all over the world, so more and more attention has been attracted to it. This paper tends to discuss whether nano-SiO2 and nano-Fe2O3 can mitigate the concrete deterioration caused by ASR. The test results show that the concretes with nano-particles have lower expansion and higher sonic velocity. Therefore, it is an available method for concrete to mix with nano-particles to mitigate ASR and the optimum content of nano-SiO2 and nano-Fe2O3 in concrete is 2.0% and 1.0%, respectively. The mitigating mechanism is revealed through the effect of mixing water on the porosity.


Introduction
Alkali-silica reaction (ASR), as the name implies, is a series of chemical reaction between the alkali and active silica in the interfacial transition zone (ITZ) or aggregate micro-crack.The product-ASR gel, a waterabsorbing swelling gel, will lead to the deterioration of concrete structure integrity owe to the internal swelling stress, sometimes even influence the normal service of the concrete structure.More seriously, ASR usually occurs in the entire concrete and the cracks caused by ASR will accelerate other corrosion (eg, chloride penetration and steel corrosion).
In the past several decades, a considerable amount of effort has been devoted to mitigate ASR damage, including using non-reactive aggregates, limiting the alkali content of concrete and mixing with additives (eg, supplementary cementitious materials and/or chemical admixture) and, to some extent, has made some achievements (Thomas 2011;Mohr and Bryant 2016).However, some researches represented different view about these approaches.For example, Stark (1978) stated that some of the highways and bridges in Illinois Rocky Mountain Area generated serious expansion crack due to ASR, even having used low-alkali cement and, finally, the soluble alkali content in the concrete is fourfold of that in the low-alkali cement.
Nano-materials, as a novel modified additive, have different physical and chemical properties due to their nanometer-level dimension.Study on the mechanical performance of concrete incorporated with nanoparticles as modified additives has been highlighted in previous literatures (Zhang, Wang, and Zhang 2011;Li et al. 2015;Silva et al. 2016;Ma et al. 2016).Currently, increasing attention has been devoted into the durability of concrete with nano-particles.However, relative studies on the ASR performance of concrete with nano-particles are limited.Therefore, this paper presents an analysis about the effect of nano-particles on ASR and attempts to reveal the mitigating mechanism.

Materials and mixture proportions
The Portland cement (P.O 42.5) is used as the binder, and additional NaOH solution is added into the cement to increase total alkali content of cement to 1.25% corresponding to GB/T50082-2009 (Standard for test methods of long-term performance and durability of ordinary concrete of China).The crushed andesite with a diameter of 5 ~31.5mm is used as coarse aggregate, and the river-sand with a fineness modulus of 2.40 is used as fine aggregate.Two kinds of nano-particles, nano-SiO 2 and nano-Fe 2 O 3 , are selected as the additives and their technical indexes are shown in Table 1.
A certain proportion superplasticizer (0.25% by mass of binder) is adopted to, on one hand, ensure the appropriate workability of the fresh concrete; on the other hand, act as a better dispersant to prevent the agglomeration of nano-particles (Xiao 2002).Notably, the superplasticizer is dissolved in mixing water, then the nano-particles is dispersed in the solution.Besides, the defoamer is mixed with a mass ratio of 4% to superplasticizer to eliminate air bubbles generated during the dispersion of nanoparticles.
The plain concrete is used as control specimens and marked as PC in this paper.The concrete mix is prepared with a W/C ratio of 0.45.The other two concrete types, the cement is equivalently replaced by different amounts of nano-particles (0.5%, 1.0%, 2.0% and 3.0% by mass), are respectively marked as NS and NF.Table 2 gives the detailed concrete mixture proportions.

Methods for assessment and analysis
The cubic specimens (100 × 100 × 100mm) and the prisms (100 × 100 × 400mm) are cast in advance.After 28 days standard curing, according to JTG E30-2009 (test methods of cement and concrete for highway engineering of China), the cubic specimens are tested for compressive strength.As for the prisms, periodic axial length measurements are taken to calculate the expansion, which is used as a reference parameter to assess ASR development by a vernier caliper (division value is equal to 0.01 mm).The ultrasonic pulse velocity (UPV) measurements are carried out to determine the sonic time, further, to calculate the sonic velocity.The goal of performing UPV measurements is to detect the internal microscopic deterioration caused by ASR.Then, the prisms are removed into a thermostatic water bath at 70°C and test ages for length measurements and UPV are 0, 3, 7, 14 and 28 days, respectively.
In this paper, the compressive strength is used to verify the mixture proportions of concrete and the enhanced effect on compressive strength of nano-particles.The expansion and sonic velocity are selected as the indexes to assess the effect of nano-particles on the ASR.

Compressive strength
Figure 1 shows the compressive strength of concrete with nano-particles at 28 days.After mixed with nanoparticles, the compressive strength of concrete has increased obviously and the increased orders are NS30 < NS05 < NS10 < NS20 and NF30 < NF20 < NF05 < NF10.When the amount of nano-SiO 2 and nano-Fe 2 O 3 are 1.0% and 2.0%, respectively, the compressive strength of concrete (NS20 and NF10) increases most significantly.Compared with PC, the compressive strength of concrete with 2% nano-SiO 2 and concrete with 1% nano-Fe 2 O 3 increases by 14.65 % and 12.06%, respectively.The above results can demonstrate that the mixture proportions of concrete are valid and the nano-particles can improve the compressive strength of concrete.

Expansion
Figure 2 indicates the relationship between the expansion of concrete with nano-particles and test age.It can be concluded that the expansion of all concretes increases rapidly in early test ages, then this trend eases obviously.In general, the expansion of concretes with nano-SiO 2 or nano-Fe 2 O 3 increase slower than that of plain concretes, especially in first 14 days.Whereas, except the content of nanoparticles is 1% by the mass of cement, the expansion of concretes with nano-SiO 2 increases slower than that of nano-Fe 2 O 3 with the same content of nanoparticles.That means, compared with nano-Fe 2 O 3 , nano-SiO 2 may be more effective for limiting the expansion of concrete.For example, followed by NF10, the expansion of NS20 is the smallest in all test ages.In fact, compared with other concretes, the expansion of NS20 and NF10 begin to increase more slower after 7 days.these all prove that the nano-particles can mitigate the ASR, and NS20 and NF10 have better effect on mitigating ASR, in other words, the optimum content of nano-SiO 2 and nano-F 2 O 3 in concrete is 2.0% and 1.0%, respectively.Figure 3 indicates the relationship between the expansion of concrete and the content of nanoparticles at different ages.It can be seen that the expansion of concrete decreases gradually with the increase of the content of nano-particles, when the contents of nano-SiO 2 and nano-Fe 2 O 3 are 2.0% and 1.0% respectively, the expansion is minimum, then the expansion increases again with the growth of the content of nano-particles.The expansion of concretes with nanoparticles is less than that of the plain concrete, which means the addition of nano-particles can mitigate the ASR to limit the expansion of concrete.The effectiveness of nano-SiO 2 on mitigating the ASR increases in the order: NS05 < NS10 < NS30 < NS20; while that of nano-Fe 2 O 3 increases in the order: NF05 < NF30 < NF20 < NF10.Same as above, the optimum content of nano-SiO 2 and nano-F 2 O 3 are 2.0% and 1.0%, respectively in this test.

Sonic velocity
Figure 4 illustrates the relationship between sonic velocity and test age.According to this figure, initial sonic velocity of concrete with nano-particles is higher than that of plain concrete, and the sonic velocity of all concretes begin to decrease, then this downward trend tends to level off after 7 days.The sonic velocity of concrete with nano-particles is always higher than that of plain concrete at all test ages, and the difference of sonic velocity between concretes with nano-particles and plain concrete increases with test age.For example, the sonic velocity of concrete with 2% nano-SiO 2 , compared with plain concrete, has increased 5.80% at 28 days,  corresponding to 4.47% before test.This means nanoparticles can improve the internal porosity of concrete to form a compact structure.Even affected by ASR, the nano-particles can mitigate the ASR to limit the deterioration of this compact structure.
Figure 5 shows the relationship between the sonic velocity of concrete and the content of nano-particles.
From Figure 5, it can be learned that the sonic velocity of concrete increases gradually with the increase of the content of nano-particles, when the contents of nano-SiO 2 and nano-Fe 2 O 3 are 2.0% and 1.0%, respectively, the sonic velocity of concrete is maximum, then the sonic velocity of concrete decrease with the increase of the content of nano-particles.But, affected by ASR, the sonic velocity of all concretes at 28 days is lower than initial velocity.Additionally, the sonic velocity of concrete has only decreased by 1.61% and 1.89% for concrete with 2% nano-SiO 2 and concrete with 1% nano-Fe 2 O 3 at 28 days.So, it can be concluded that nano-particles can not only improve the internal porosity to form a compact structure, but also help to mitigate ASR process to maintain this compact structure.Based on above test results, it can be known that nano-particles can mitigate ASR and the optimum content is 2.0% and 1.0% for nano-SiO 2 and nano-Fe 2 O 3 , respectively.

Discussion
The mechanism that nano-particles can mitigate the ASR damage can be illustrated as follows.
There are four prerequisites for ASR: (1) metastable silica contained in aggregates, (2) OH − ions to attack silica, (3) a source of soluble Ca (eg, portlandite (CH)) to react with dissolved silica and form deleterious gel, and (4) access to moisture to allow gel expansion (Rajabipour et al. 2015).Actually, except for the prerequisite (1), the other three prerequisites can only take place in aqueous ambient in the capillary voids or ITZ voids.Therefore, the porosity, in some sense, is the determinant factor of ASR.Mehta and Monteiro (2006) had stated that cement hydration may be looked upon as a process during which the space originally occupied by cement and water was being replaced more and more by the hydration products.The water-cement ratio for completely hydration of cement is only 0.32, that is to say, the space taken up by water will finally form capillary voids.The volume and size of capillary voids were determined by the original distance between the anhydrous cement particles in the freshly mixed cement paste and the degree of cement hydration.So, this part tends to  reveal the mitigating mechanism of nano-particles by the influence on water at micro level.
First, when the cementitious compounds are dissolved by the mixing water (the nano-particles are dispersed in the mixing water in advance), lots of water is absorbed by nano-particles due to the highspecific surface and water affinity (Senff et al. 2010).Thus, the water between anhydrous cement particles and on the surface of aggregates decreases obviously, which can close the distance of two anhydrous cement particles and reduce the thickness of the water film on the aggregates to form a compact cement paste and ITZ during hydration.
Second, due to the nano-scale size, there are a lot of free bonds and unsaturated bonds or residual valence forces in the surface of nano-particles, so that the nano-particles are in an unstable state of thermodynamics (Ye et al. 2007).Moreover, with the reduction of particle size, there exist many uneven atom steps which increase chemical reaction area (Zhang and Li 2002).So, the nano-particles, such as nano-SiO 2 and nano-Fe 2 O 3 , have high-surface energy, and atoms in the surface have a high activity to promote hydration of cement to generate more hydrated calcium silicate (C-S-H) and other hydration products (eg, ettringite).Consequently, when well dispersed, the nano-particles can act as a nucleus to tightly bond with cement hydration products and further promote cement hydration and form a network structure.On one hand, with more C-S-H and other hydration products, there will be more monomolecular water layer between the layers of C-S-H and more water chemically combined with the hydration products as an integrity, which can reduce the space occupied by the water to form voids (Mehta and Monteiro 2006); on the other hand, this network structure can confine the propagation of CH crystal to enhance interface cohesion between cement paste and aggregates which can limit the exposure of alkaline solution to aggregates.
Finally, nano-particles can also fill the micro-and nano-metric pores in the cement matrix particularly in the ITZ (Najigivi et al. 2013).Because of the existence of nano-particles, some of the water and CH, essential for expansion, can be consumed and/or absorbed which can limit the ASR.
Based on the above analysis, we can see that the ultra-micro size, high activity, high-specific surface and surface hydrophilicity, as the common mechanism, are beneficial to form a homogeneous and compact concrete matrix.When mixed with nano-particles, from the dissolution to the hydration of cementitious compound, more water is physically absorbed onto the surface of solids in the hydrated cement paste, associated with the C-S-H, or chemically combined with the cement hydration products.So, the space occupied by capillary water decreases, which helps to form less voids after the evaporation of capillary water.Thus, both the volume and size of the voids reduce so obviously that less void space can be provided for the moisture migration and ASR to mitigate ASR.
Additionally, above discussion can also explain why there exists optimum content for nano-particles.Nano-particles can promote the hydration which is called "positive effect", while nano-particles can also reduce the workability of fresh concrete which is called "negative effect", and if the content of nanoparticles is relatively too much, the "negative effect" will limit the dispersion of nano-particles and cement particles.When the content of nano-particles is less than optimum content, the "negative effect" is small which has little effect on hydration.So, the performance of concrete with nano-particles is enhanced in a small range.When the content of nano-particles is more than optimum content, the "negative effect" play a decisive role.At this time, both nano-particles and cement particles cannot disperse well in mixing water to limit the hydration (Zhang 2007).Only when the content of nano-particles is optimum, the "positive effect" plays a decisive role and the "negative effect" just begins to limit the dispersion of nanoparticles and cement particles because the watercement ratio for complete hydration of cement is only 0.32 as mentioned above.Here, the concrete with nano-particles has the best performance.The decrease in concrete durability of ASR, when the content of nano-particles is more than the optimum content, is perhaps the result of following influence of nano-particles incorporation: it is difficult to ensure whether the nano-particles can be dispersed uniformly; and the flowability of fresh concrete will decrease which leads to more entrapped air exist in fresh concrete during the concrete fabrication to make the permeability decrease (Yu, Spiesz, and Brouwers 2014).
However, besides the small size effect and the surface effect, nano-SiO 2 and nano-Fe 2 O 3 also have their own special role in mitigating ASR damage.In the process of cement hydration, the formation of C-S-H gel is accompanied with the release of CH which can provide essential OH − and Ca 2+ for alkalisilica reaction and increase the PH value of pore solution.At high PH value, the OH − can promote the dissolution of SiO 2 (Sjöberg 1996); the Ca 2+ can link aqueous silica species and finally form ASR gel, as a catalyst, will be further ejected (Boehm 1979;Gaboriaud et al. 1998).Because of the high pozzolanic activity of nano-SiO 2 , the released CH can be consumed to form additional C-S-H gel.Without sufficient CH, the dissolved silica species with negative charges remain in solution and the resulting electrostatic repulsion prevents gelation.In such a case, SiO 2 dissolution slows and eventually stops when aqueous

Figure 2 .
Figure 2. The expansion of concrete with nano-particles at different test ages.

Figure 3 .
Figure 3.The expansion of concrete with different contents of nano-particles.

Figure 4 .
Figure 4.The sonic velocity of concrete with nano-particles at different test ages.

Figure 5 .
Figure 5.The sonic velocity of concrete with different contents of nano-particles.

Table 1 .
The main technical indexes of nano-particles.

Table 2 .
The mixture proportions of concrete.
Figure 1.Compressive strength of concretes with nano-particles.