Study on the durability of concrete with FNS fine aggregate

Journal of Hazardous Materials 381 (2020) 120936

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Study on the durability of concrete with FNS fine aggregate a,⁎

a

b

a

Xiaoming Liu , Tingyu Li , Weiguang Tian , Yiqun Wang , Yanhu Chen a b

T

b

School of Civil Engineering, Central South University, Changsha, 410075, China Guangdong Guangqing Metal Technology Co. Ltd., Yangjiang, 529533, China

A R T I C LE I N FO

A B S T R A C T

Editor: Deyi Hou

As a by-product of nickel production, the ferronickel slag (FNS) puts a lot of pressure on the environment. It is becoming more and more urgent to deal with the increasing FNS. The aim of this study is to explore the durability of concrete with FNS fine aggregate. Two kinds of FNS with different storage time were selected. The radioactivity detection, XRD test and stability detection of FNS were conducted to ensure FNS can be used as construction materials. Then the durability of concrete with 13%, 27%, 40% and 50% FNS (by weight of fine aggregate) was investigated, respectively. It was found that the properties of concrete prepared from FNS with different storage time had little difference. The results indicated that 27% FNS replacement showed improvement in resistance to sulfate attack by 22% but the resistance to chloride ion penetration was not significantly influenced. Moreover, 40% FNS addition brought a 33% abrasion reduction than that of original concrete. SEM analysis showed that FNS produced more C-S-H gels and improved the microstructure of concrete. This study indicated that proper content of FNS can be used as fine aggregate and it was beneficial to the durability of concrete, especially to the abrasion resistance.

Keywords: FNS Fine aggregate Radioactivity Concrete Durability

1. Introduction Ferronickel alloy has better tensile strength and hardness than low carbon structural steel because it contains nickel, chromium and other mental elements. And the market demand is increasing due to its excellent performance. Ferronickel slag (FNS) is a by-product of the production of ferronickel alloy. It has 14 tons of FNS production for every ton of ferronickel steel manufactured (Saha and Sarker, 2016). Nowadays, the annual output of ferronickel alloy is about 11 million tons in the world, which means that more than 150 million tons of FNS are produced every year. FNS has been the fourth largest slag produced from smelting process after iron slag, steel slag and red mud (Xi et al., 2018). It is still a technical difficulty to effectively deal with these FNS. At present, the main methods of dealing with it are stacking and landfilling. The stacking of increasing FNS occupies a large area of farmland. It was reported (Futures, 2019) that FNS was temporarily piled up on the coast beside the factory which formed a "slag mountain" with a length of about 800 m and a width of about 500 m. The cumulative storage quantity has reached nearly 10 million tons. Also, as a hazard source, study (Kang et al., 2014) has pointed out that there were many harmful substances when FNS was exposed to acidic rain or seawater environment. Therefore, there is an urgent need to treat these

FNS or it would bring serious challenges to the environment and the continuous development of nickel steel smelting industry (Lu et al., 2018a; Liang et al., 2017). Many researches have focused on the resource utilization of industrial waste. Industrial slag mainly includes iron slag, steel slag, copper slag, lead-zinc slag, chromium slag, nickel slag, etc. The recycling technology of steel slag is very successful around the world (Liu and Wang, 2017). However, the recovery utilization rate of FNS is only 8% and the technology for treating FNS is still under development (Saha and Sarker, 2016; Piatak et al., 2015). At present, many researchers have begun to work on the resource utilization of FNS. It was reported that FNS may be utilized (Li et al., 2019) as an object for recovery of valuable metals (Zhu et al., 2018), powder for Portland cement (Mitrašinović and Wolf, 2015; Rahman et al., 2017), geopolymer binder (Lemonis et al., 2015; Komnitsas et al., 2009; Yang et al., 2017), filler materials (Yang et al., 2014), aggregate in concrete and hot-mix asphalt (Wang et al., 2013a, 2011). As for FNS powder, some researchers have evaluated the mechanistic properties of cement mortar when FNS was used to partially replace cement or fly ash. The results showed that, as a cement binder, FNS powder had positive effect on the compressive strength of cement mortar (Mitrašinović and Wolf, 2015; Saha and Sarker, 2017a; Wang et al.,

⁎

Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (T. Li), [email protected] (W. Tian), [email protected] (Y. Wang), [email protected] (Y. Chen). https://doi.org/10.1016/j.jhazmat.2019.120936 Received 5 January 2019; Received in revised form 22 July 2019; Accepted 25 July 2019 Available online 29 July 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

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2018a). Considering the mechanical properties, FNS powder can be used as a bonding material in concrete or mortar. However, FNS powder has not yet been widely used in practical engineering. There are two main reasons for limiting the effective recycling of FNS: firstly, different from other steel slag, the higher content of Fe2O3 and SiO2 in FNS makes it hard to be ground (Zhang et al., 2017). The economic benefit is too low to recycle the FNS using traditional grinding method which limits its application; secondly, the existence form of MgO in FNS is related to the smelting temperature, grinding fineness, etc. MgO may not be hydrated for several years or even longer due to the different degrees of influence of these factors (Li et al., 2019). The high content of MgO in FNS is still a potential hazard to the stability of structures in the later stage (Lu et al., 2018a). There few researches were conducted on the use of FNS for fine aggregate. A.K. Saha and P.K. Sarker (Saha and Sarker, 2017a; Saha et al., 2019; Saha and Sarker, 2017b) have pointed out that the workability and compressive strength of mortar and concrete increased with the increase of FNS addition when the replacement of sand was within 50%. However, the in-depth and systematic research of long-term stability and durability of FNS fine aggregate concrete is also required to ensure long-term safety and reliability. From the above analysis, it can conclude that the technical difficulties of restricting the large-scale application of FNS are as follow: the existing researches mainly focus on grinding it into powder as a binder and the effect on the mechanical properties of mixture. The long-term properties of a wide range application of FNS in engineering structures are uncertain which limits the reusing of FNS. Therefore, it is significant to study the effect of FNS on the durability of cement-based mixture for the application of FNS. In addition, it is said that the FNS produced in different periods may also has different effects on concrete. So, two kinds of FNS with different storage time were selected as research objects. The aim of this study is to find an economic way to recycle and utilize FNS (produced in different periods) in large quantities, to turn waste into treasure, to deal with hazards. At the same time, this study is going to develop a kind of concrete with excellent abrasion resistance based on the characteristics of FNS. Furthermore, as a fine aggregate for concrete, the widely application of FNS is expected to relieve the pressure of natural sand resources. According to the fact that the particle size range of FNS is narrower than that of natural sand, FNS can only replace the natural sand partially. In this paper, the possibility of using FNS as a fine aggregate in concrete is studied first, based on this, the 0%, 13%, 27%, 40% and 50% replacement of FNS for sand was selected to explore its effect on the durability of concrete. This study provides a theoretical basis for the recycling of FNS produced at different periods.

Fig. 1. XRD diffraction pattern of FNS.

months, and 2# FNS was stored for more than one year. The chemical compositions of cement and FNS are provided in Table 1. X-ray diffraction (XRD) instrument with a model of D8 Advance, produced by Brook company, was used to detect the physical component of 1# FNS and 2# FNS. During the test, the step length was set to 0.05°, and 5 s/step for scan speed, 0°–70° for scan range (2θ). The physical component of FNS was determined by processing and indexing the diffraction data. XRD spectrums of 1# FNS and 2# FNS are shown in Fig. 1. From which we can see that the mineral compositions of FNS with different storage time are basically the same. There only iron forsterite was found in both 1# FNS and 2# FNS but no f-CaO and fMgO were discovered. It can be inferred that MgO existed in FNS in the form of forsterite. A shaker with a model of ZBSX 92A, produced by Shanghai dongxing building materials experimental equipment co., LTD. China, was used to determine the particle size distribution of aggregate. Particle size distribution was calculated by the residual weight on each pore size sieve. Fig. 2(a) shows the particle size distribution of FNS and natural sand. The particle size of FNS was larger than that of natural sand and its particle size doesn’t meet the permitted range of fine aggregate gradation for concrete according to Chinese Standard (GBT14684-2011). It can be seen from Fig. 2(b) that the grading of mixed fine aggregate can just meet the specification requirements when FNS and natural sand were mixed in a ratio of 1:1. 2.2. Sample preparation The mix proportions of concrete are given in Table 2. The watercement ratio of mixtures remained at 0.46. FNS was used to replace natural sand by weight of 0%, 13%, 27%, 40%, 50%, respectively.

2. Materials and methods 2.1. Raw materials

2.3. Experimental methods The used cement was the ordinary Portland cement with a strength grade of 42.5. The coarse aggregate was a crushed limestone between 5–25 mm and fine aggregate was natural river sand smaller than 5 mm. The FNS was supplied by Guangdong Guangqing Metal Technology Co., Ltd. Two kinds of FNS (1# and 2#) with different storage time were selected in this study. The storage time of 1# FNS was less than 3

The radioactivity test of FNS was proceeded by China National Building Materials Testing Center according to Chinese National Standard (GB6566-2011). The expansion and pulverization test of FNS were carried out in accordance to Chinese Standard (GB/T 241752009). The immersion expansion rate was calculated by formula (1).

Table 1 Chemical compositions of FNS and cement (%). Element Cement 1# 2#

SiO2 22.86 49.10 51.14

MgO

Al2O3

TFe

Cr2O3

CaO

TiO2

MnO

Else

Total

2.95 27.84 27.65

7.69 4.37 3.68

3.68 14.04 13.11

– 2.02 2.05

57.62 1.62 1.42

– 0.14 0.07

– 0.87 0.88

5.2 0.00 0.00

100 100 100

2

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Fig. 2. (a) Natural sand and FNS particle size distribution. (b) Natural sand, FNS and mixed grading.

γ=

di − d 0 × 100 120

abrasion area of specimen (m2). The abrasion resistance of FNS and sand was determined by Los Angeles Abrasion Test. Six samples with particle sizes between 2.36 mm and 4.75 mm were prepared, of which FNS content was 0%, 13%, 27%, 40%, 50% and 100%, respectively. The test was conducted according to JTG E42-2005″Test Methods of Aggregate for Highway Engineering" by DM-II Los Angeles Abrasion Test Machine. The chloride ion permeability of concrete was tested at the age of 56d using 100 mm × 50 mm cylindrical specimens. The charge passing through specimens during 6 h was measured to evaluate permeability grade according to Chinese Standard (GB/T 50082-2009). After the compressive strength test of 56d in the sulfate attack test, the destroyed concrete pieces with/without FNS were selected for SEM test. SEM tester was produced by JEOL with a model of JSM-IT300LA. The specific procedure of test was first put the sample into the vacuum chamber and vacuumed to 1–10 Pa. A layer of 25 nm copper film was deposited on the sample surface and then observed it by SEM tester.

(1)

Where: γ is the immersion expansion rate (%); 120 is the original height of specimen (mm); di is the dial indicator reading (mm); d0 is the initial reading of dial indicator (mm). Five groups (three samples for each group) of concrete specimens (100 mm × 100 mm × 100 mm cubes) with different FNS contents were prepared to proceed the sulfate attack test. At the age of 56d, one group of specimens were tested to identify the compressive strength. Among the remaining four groups, two were brought to conduct drywet cycle test for 30 cycles and 60 cycles, respectively, and the last two groups were continued to concurrent maintain. The specific procedures for the sulfate attack test referred to Chinese Standard (JTG E30-2005). The compressive strength of concretes was tested after treated with different conditions. The corrosion resistance coefficient of compressive strength (Kf) was then calculated by formula (2) according to Chinese Standard (JTG E30-2005).

Kf =

fcn fc0

3. Results and analysis

× 100

(2) 3.1. Radioactivity and stability

Where: Kf is the corrosion resistance coefficient of compressive strength (%); fcn is the compressive strength of concrete subjected to N dry-wet cycles (MPa); fc0 is the compressive strength of concrete given concurrent maintenance (MPa). Concrete specimens of 150 mm × 150 mm × 150 mm were molded for the abrasion resistance test. After 56 days of curing, the abrading test of concrete specimens was carried out by Cement Mortar /Concrete Wear Tester (TMS-400). Firstly, the surface of specimens was pretreated 30 turns to remove the mortar and then weighed the specimens to get initial weight m0 (kg). Continue abrading 60 turns and then weighed the weight m1 (kg). According to Chinese Standard (JTG E30-2005), the abrasion value per unit area is calculated by formula (3) to characterize the abrasion resistance.

Gc =

m 0 − m1 0.0125

Industrial waste usually contains a certain amount of hazardous substance (Oka and Uchida, 2018; Sarfo et al., 2017) which may be harmful to human body. It is necessary to investigate if the radioactivity meets the requirement of Chinese Standard (GB6566-2010) when industrial waste is used as a new construction material. In addition, the chemical composition analysis of FNS showed that the content of MgO was very high. Therefore, the tests for radioactivity and stability were conducted firstly before FNS can be considered to used in concrete as raw material. Radioactive test results are shown in Table 3. The internal and external radiation indices of FNS were close to zero which was much smaller than the maximum allowable value (1.0). The pulverization rate of FNS was 0.4% as shown in Table 4, which was less than 5% of the requirement of Chinese standard (YBT801-2008). The cumulative curves of immersion expansion of 1# FNS and 2#

(3) 2

Where: Gc is the abrasion value per unit area (kg/m ); 0.0125 is the Table 2 Mix proportions of FNS mixtures. FNS category

– 1#

2#

Number

Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8

Cement (kg/m3)

350 350 350 350 350 350 350 350 350

Fine aggregate (kg/m3) Sand

FNS

750 650 550 450 415 650 550 450 415

0 100 200 300 415 100 200 300 415

3

Coarse aggregate (kg/m3)

Water (kg/m3)

Superplasticizer (kg/m3)

980 980 980 980 900 980 980 980 900

160 160 160 160 160 160 160 160 160

1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7

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Table 3 Radioactivity detection results of FNS. 226 Ra Bq/kg

232

4.0

4.5

Th

40

K

18.7

IRa

Ir

0.0(+)

0.0(+)

FNS are shown in Fig. 3. As shown in Fig. 3, the expansion rates of both 1# FNS and 2# FNS were growing rapidly on the first day, then gradually slowed down and kept stable, which was attributed to the pozzolanic activity of FNS. The activity of FNS was quickly and fully exerted at the beginning of test and created a highest expansion value. As the test went on, the reaction became weaker and the expansion rates of both 1# FNS and 2# FNS were stabilized after 6 days. The immersion expansion rates of 1# FNS and 2# FNS for 10 days were 0.95% and 0.98%, respectively, which all met the requirements of not more than 2.0% specified in Chinese standard (GB/T25824-2010).

Fig. 3. Expansion rates of 1# FNS and 2# FNS.

3.2. Resistance to sulfate attack The anti-sulfate attack test can reflect the durability of concrete in the sulfate environment (Liu and Wang, 2016). The ability of concrete against sulfate attack may be affected by the addition of FNS. In this section, the resistance to sulfate attack of concretes with different FNS contents was studied. The compressive strength of concretes under different dry-wet cycles and different curing ages are shown in Fig. 4(a), according to which, the Kf of concrete specimens was calculated and drawn in Fig. 4(b). As shown in Fig. 4(a), the compressive strength at 56d presented a trend of increasing first and then decreasing with the increase of FNS content, and it has a similar changing trend for concretes treated under different conditions. The changing trend of Kf, in Fig. 4(b), was consistent with that of compressive strength, which means that compressive strength determined the sulfate resistance. It is believed that FNS contains a large amount of non-crystalline substance and therefore has potential pozzolanic activity (Ustabaş and Kaya, 2018). During the maintenance of concrete, FNS can bring a secondary hydration reaction to produce more gel products, which improved the weak interface transition zone. As a result, the compressive strength of concrete with proper content of FNS was increased, so was the sulfate resistance. For concrete with 50% FNS, discontinuous grading of aggregates caused a decrease in compressive strength. A similar phenomenon of concrete with other slag was also observed by He et al. (2018). It can be seen that the influences of 1# FNS and 2# FNS on the sulfate resistance of concrete were similar. From Fig. 4(b), the Kf of 30 cycles of the original concrete was 0.971, and it reached 1.098, 1.140, 1.124 and 1.061 for specimens with 13%, 27%, 40% and 50% FNS, respectively. However, when the cycles increased to 60, the Kf reduced to 0.855, 1.007, 1.025, 1.022 and 0.982 for concrete samples with 0%, 13%, 27%, 40% and 50% FNS, respectively. The Kf of 60 cycles was always smaller than that of 30 cycles. In the meanwhile, the sulfate resistance of FNS concrete was always better than that of original concrete. It can be concluded that the FNS can improve the sulfate resistance of concrete and the appropriate addition was 27–40%. It is interesting to note that the sulphate immersion seemed to be beneficial to the strength growth of FNS concrete in the initial stage. However, the effect of sulphate solution on compressive strength turned

Fig. 4. (a) Compressive strength of concretes under different dry-wet cycles and different curing ages. (b) Corrosion resistance coefficient of concretes with different FNS contents under different cycles.

Table 4 Results of pulverization rate. Weight after steaming/g

Weight that smaller than 1.18 mm after steaming/g

Pulverization rate/%

Average value/%

264 266 268

1.1 1.1 1.2

0.4 0.4 0.4

0.4

4

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Fig. 5. Typical failure interfaces of concretes with different FNS contents.

to be negative after 30 dry-wet cycles. This phenomenon can be explained by the fact that the ettringite formed in the reaction of Ca(OH)2 and SO42− filled the pores in concrete and thus improved the density of concrete in the initial stage of sulphate immersion. However, as the reaction proceeded, the excessive ettringite squeezed and destroyed the interior structure of concrete. This also explained why Kf of 60 cycles was less than that of 30 cycles. Moreover, with the addition of FNS, the formed ettringite blocked the penetrating path of sulfate ions, so the concrete exhibited higher resistance to sulfate attack than that of original concrete (Zhu et al., 2018; Lu et al., 2018b). Concrete without FNS had many interconnected voids which made it easily corroded by sulfate ions and led to a decrease in compressive strength. To explain the results from a macro perspective, the typical failure interfaces of concrete with different contents of FNS are shown in Fig. 5. As can be seen from Fig. 5(a), there were obvious air voids on the bond surface between aggregates and mortars. However, Fig. 5(b) and (c) show that the failure interface of concrete with 27% and 50% FNS had less air voids and denser structure. From Fig. 5, it can be well explained the difference in sulfate resistance of concrete with different FNS contents.

3.3. Abrasion resistance The abrasion resistance of concrete is an important indicator to characterize its durability (Kumar and Sharma, 2014), especially for the concrete structure with heavy traffic such as airport runways, expressway intersection and so on. FNS is difficult to be ground into powder compared with other steel slag. In order to make full use of this characteristics of FNS, this study conducted an abrading test to explore the effect of FNS on the abrasion resistance of concrete. The abrasion of concrete varied with different FNS contents is presented by Fig. 6(a). As can be seen from the illustration, the influences of 1# FNS and 2# FNS on the abrasion resistance of concrete were similar. The abrasion of concrete ranged from 2.336 kg/m2 to 1.740 kg/m2. They were 2.640, 2.274, 4.071, 1.782 and 2.209 kg/m2 for 0%, 13%, 27%, 40% and 50% FNS addition, respectively. Fig. 6(b) shows that the abrasion resistance of concrete increased basically linearly with increasing FNS contents when the content of FNS was within 40%. The abrasion of concrete was the lowest when the content of FNS was 40% and it was 33% smaller than that of original concrete. However, when the content of FNS reached 50%, the abrasion resistance had a large decrease but still better than that of original concrete. It can be concluded that FNS can improve the abrasion resistance. The abrasion resistance of concrete is usually depended on three important factors (Tripathi et al., 2013). The first is the abrasion resistance of aggregates. And the second is the adhesion between aggregates and cement binders, which is related to the shape and the surface texture of aggregates. The third one is the density of cement binder. In order to explore the specific reasons for the improvement in abrasion resistance of FNS concrete. A Los Angeles abrading test was

Fig. 6. (a) Abrasion variation (b) Percentage reduction in abrasion of concrete with different FNS contents.

conducted for the mixtures with different proportions of natural sand and FNS. Since there was no difference between the effect of 1# FNS and 2# FNS on the abrasion resistance. The 1# FNS was selected for Los Angeles abrading test. Fig. 7 shows that the abrasion rate of 100% natural sand and 100% FNS were 40.04% and 72.36%, respectively. When the natural sand was replaced by FNS partially, the abrasion rate was linearly proportional to the content of FNS and the correlation coefficient reached 0.97. 5

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Fig. 7. Abrasion variation of the fine aggregate with different FNS contents.

3.5. Relationship between durability parameters and compressive strength

From the results it can be found that the abrasion resistance of FNS was not as good as that of natural sand. This is because FNS is angular and contained some voids. However, when added to the concrete, the multi-edges and rough surface of FNS enhanced the bonding force between it and cement binders. And the characteristic of difficult to be ground of FNS can be better utilized when it was wrapped in cement mortar. In addition, the hydration products of FNS’s pozzolanic reaction could repair the air voids and micro-cracks in concrete partially (Pyo et al., 2018; Wang et al., 2018b). Therefore, the pozzolanic activity of FNS and the strong adhesion between it and cement binder improved the abrasion resistance of concrete.

It is generally believed that the durability of concrete has a close relation with the compressive strength. For concrete prepared with different contents of FNS (Saxena and Tembhurkar, 2018), the relationship between compressive strength and durability parameters are observed from Fig. 9. The relation between compressive strength and corrosion resistance under 60 cycles is shown in Fig. 9(a). The trends of two curves were consistent approximately which means that the compressive strength can roughly reflect the sulfate resistance. Low porosity and high density are conducive to the improvement of compressive strength and sulfate resistance. Fig. 9(b) shows the comparison results between abrasion and compressive strength of concrete. It can be seen that the abrasion resistance was related to the compressive strength. But the higher compressive strength did not always mean better abrasion resistance. This can be contributed to the pozzolanic activity and surface roughness of FNS. Fig. 9(c) shows that the electric flux of FNS concrete was decreased with the increase of compressive strength, which means that the density of concrete was the dominant factor against chloride ions penetration. So, it has a strong positive correlation between compressive strength and durability when concrete prepared with different contents of FNS fine aggregate.

3.4. Resistance to chloride ions penetration Chloride ions corrosion is one of the main failure modes that leads to a decrease in mechanical properties and durability of concrete (Wang et al., 2013b; Biskri et al., 2017). The porosity, density and character of air void in concrete are the main factors affecting the permeability of chloride ions. It was also said that the increased content of alumina (Al2O3) can decrease the charge passed in concrete (Uysal and Akyuncu, 2012) but the content of iron may have negative effect on the performance of concrete in chlorine environment (Lu et al., 2018b). There was 4% alumina in both 1# FNS and 2# FNS and the content of iron was 14%. In this section, the electric flux test was conducted to investigate the effect of FNS with different storage time on the resistance to chloride ions penetration of concrete. Fig. 8 shows the electric flux and chloride ions penetration grade at 56d age for all concretes. There was no significant difference between the effect of 1# FNS and 2# FNS on the resistance to chloride ion penetration of concrete. The electric flux of concrete with 50% FNS was greater than that of other groups relatively. The samples with 27% FNS obtained the lowest electric flux which reduced 21% compared with that of original concrete. This can be attributed to the blocking effect on the connected pores by the hydration products of FNS. And it is noted that the resistance to chloride ions penetration of concrete was greatly reduced when the addition of FNS was more than 50%. As can be seen from Table 1, the high content of iron in FNS may be the reason for the high electric flux (Liu and Wang, 2016). When the positive effect of hydration reaction of FNS was not enough to compensate for the adverse effects caused by iron, the resistance to chloride ions penetration of FNS concrete was poorer than that of original concrete.

3.6. SEM analysis The above results indicate that concrete with FNS had higher compressive strength and better durability than that of original concrete. The addition of FNS can produce more hydration products and form a denser internal structure. These conclusions can be supported by SEM analysis. The influences of 1# FNS and 2# FNS on the durability of concrete were similar, so 1# FNS concrete samples were selected as the representative for SEM test in this paper. Fig. 10 shows the SEM images magnified 100 and 5000 times of specimens containing 0%, 27%, 50% FNS at the age of 7d. It can be clearly seen that there were still unhydrated cement particles in Fig. 10(a). From Fig. 10(a), there were only a small amount of acicular ettringite and layered hexagonal plate shaped Ca(OH)2 (Zhu et al., 2018) with the size of about 2 μm growing on the surface of aggregates, no obvious gelatinous hydration product can be observed. Moreover, many obvious air voids can be seen in Fig. 10(a). However, in the concrete containing FNS as shown in Fig. 10(b) and Fig. 10(c), 6

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Fig. 8. Charge passed of concrete with different FNS contents.

compacted, and there only few small air voids could be observed; however, in Fig. 10(c), the distribution of hydration products was not uniform and contained many air voids; moreover, the structure was looser and the density was obviously not as good as that of Fig. 10(b). It is worth noting that there’s lots of laminar Ca(OH)2 with size of approximately 100 μm in the SEM image in Fig. 10(b), which could be the result of cement hydration. With the increase of curing time, the Ca (OH)2 was consumed due to the secondary hydration of FNS and more flocculated C-S-H gels were generated. This further improved the inner structure and thereby helped to obtain better durability for concretes.

there were more hydration products and most of them were needleshaped AFt crystals (Guo et al., 2017) and amorphous C-S-H gels (Zhu et al., 2018; Guo et al., 2017), in addition, a number of Ca(OH)2 crystals growing in the voids between the cement binders and aggregates. It can be distinguished that the morphological feature of C-S-H gels in Fig. 10(b) was main reticular different from the spherical C-S-H gels in Fig. 10(c). The reason is that the Ca/Si ratio in C-S-H gels was decreased when the replacement of FNS exceeded 30% (Li et al., 2019). The microstructure of different specimens is summarized as follows: in Fig. 10(b), the distribution of hydration products was very uniform and

Fig. 9. Relationship between (a) sulfate resistance. (b) abrasion resistance. (c) electric flux and compressive strength of concretes with different FNS contents. 7

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Fig. 10. Micromorphology of concretes with (a) 0% FNS (b) 27% FNS (c) 50% FNS at the age of 7d.

Through the SEM analysis, the influence mechanism of FNS on concrete performance is well explained.

expansion rates of FNS were about 0.4% and 0.96%, respectively, which all met the requirements of Chinese Standard. As a kind of fine aggregate, the influences of FNS with different storage time on the properties of concrete were similar. The lowest abrasion of concrete was obtained when the content of FNS was 40%, which was less than that of original concrete by 33%. All FNS concrete had Kf value greater than 1.00 at 60 cycles while that of original concrete is 0.855. This means that FNS can improve the sulfate resistance of concrete. However, FNS fine aggregate had no significant influence on the resistance to chloride ion penetration of concrete when the content was less than 50%. The results show that the durability of concrete had a

4. Conclusions Based on the tests results for eight concretes with different contents of FNS and one control, through a series of standard tests such as radioactivity test, stability test and durability test, several conclusions can be drawn. The radioactivity test results show that the internal and external radiation indices of FNS were close to zero, far smaller than the specified limit (≤1.0). And the pulverization rate and immersion 8

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great correlation with the compressive strength. SEM tests show that there were more hydration products in FNS concrete than that of original concrete. The reaction activity of FNS strengthened the internal structure of concrete, so the concrete had better durability. It can be concluded that FNS fine aggregate can be used to replace natural sand within 40% to improve the durability of concrete.

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