Effect of nano-silica slurry on engineering, X-ray, and γ-ray attenuation characteristics of steel slag high-strength heavyweight concrete

2.1 Materials

2.1.2 Nano-silica slurry

MC-Bauchemie provided the NSS used in this study. The material was made from undensified nano-silica to ensure full dispersion. The suspension has a relative density of 1.38 g/cm³. The FE-SEM micrographs for NSS are shown in Figure 1(a) after 24 h of drying in an oven. The TEM images of NSS are shown in a liquid state in Figure 1(b). They confirm that the physical characteristics of the suspended particles are identical to those of the dried form. The results of the FE-SEM and TEM showed a particle size range of 60–100 nm [13]. The NSS XRD pattern is shown in Figure 2. The NSS consisted of silicon carbide in the mineral phase. The chemical composition of the NSS is presented in Table 1.

Figure 1

(a) FE-SEM and (b) TEM images of the NSS, adapted from ref. [13].

Figure 2

Mineralogy of NSS, adapted from ref. [13].

Table 1

The chemical composition of NSS (wt%)

Oxides	NSS
SiO₂	95.77
TiO₂	0.01
Al₂O₃	0.61
Fe₂O₃ (t)	0.11
MgO	0.35
CaO	0.27
Na₂O	0.06
K₂O	0.32
P₂O₅	0.04
Total	97.53

2.2 Mix design

The steel slag heavyweight concrete (SSHWC) was produced with ASFS (fine and coarse) to evaluate the impact of NSS on the engineering, gamma-ray, and X-ray protective properties of the concrete. The mix design was carried out using the approved procedures in ACI 211.1R. This study adopted the absolute volume mix design technique because it is the standard way of proportioning HWC material [8]. In terms of nomenclature, the mix S1.0 represents concrete mix containing 1.0% NSS as cement additive. The mix designs of the study are presented in Table 2.

The cement contents (fine and coarse) ASFS were fixed at a ratio of 1:2:3 [8]. The NSS was used to increase the percentage of C–S–H and to serve as a filler in the ITZ. In addition, the incorporation of the nanomaterial was aimed at increasing the atomic number of SSHWC. For the composite materials, the effective atomic number (Z _eff) is the average number of electrons in the materials that actively take part in the interaction [50]. The aggregate was used in the saturated dry surface condition to prevent the effect of water absorption during the mixing process. A fixed w/c ratio of 0.23 was used for all the mixes while adjusting the superplasticizer content to achieve 100–120 mm constant fresh concrete slump.

2.3 Mixing samples

The samples were mixed as normal concrete in the following manner. First, the ASFS (fine and coarse) was added to the mixer, followed by 2 min dry-mixing with cement. Then, the mixture was homogenized with water, followed by adding about 80% from the required mixing water volume and further mixing for 90 min. Finally, the remaining 20% of the amount of water needed was added gradually. The whole mixing time was 5 min for all the batches to prevent the segregation of aggregates [13]. After mixing, the resulting concrete was subjected to a slump test recommended by ASTM C143 for workability assessment [51].

3.1 X-ray attenuation measurement

The mass attenuation coefficient of the concretes was measured in the X-ray fluorescence instrument. The high purity metal plate (Tungsten; see Figure 3) was irradiated with a 59.54 keV (peak) of 100 mCi 241 Am to produce the characteristic X-ray with 59.32 keV kα energy as shown in Figure 4. The X-ray photons were attenuated by the absorber and detected by CANBERRA low energy Germanium (LEGe) detector with narrow beam geometry. The ORTEC 572 amplifier amplified the detector output pulses, and the spectrum was collected by a multichannel analyzer (MCA-3 series) for 10 min. The 20 mm thick test concrete samples were positioned, as shown in Figure 4. The average reading of the three samples was taken for each mix. Each sample was analyzed for 10 min at 7, 28, and 90 days of sample ages. The calculation of the test results was done using the same previous attenuation measurement of γ-ray [8,52].

Figure 3

Tungsten used in the study.

Figure 4

Schematic of the narrow-beam geometry experiment (X-ray experiment).

3.2 The γ-ray attenuation measurement

The attenuation properties of concretes for γ-ray were evaluated by calculating the linear attenuation coefficient (μ), tenth value layer (TVL), and half-value layer (HVL), as previously done in the study by ref. [8]. The experimental setup and stand device used for the γ-rays test in this study are shown in Figure 5. The 100 mm thick test samples were positioned between the γ-ray source and the detector. An average of three samples per mix was used for this test at 7, 28, and 90 days of age.

Figure 5

The test stand of γ rays, adapted from ref. [8].

3.3 Zeta-potential measurement

The nanomaterial was assessed for zeta-potential distribution. One milliliter of each sample was dispersed in 10 mL of distilled water; 5 mL of this solution was placed in a cuvette and investigated for particle stability on a dynamic light scattering equipment. The outcome of this assessment was taken as the zeta potential of the sample.

3.4 Physical and mechanical properties of SSHWC

Different tests were aimed at the physicomechanical properties of the produced SSHWC. The first test on the concrete samples was the slump test, as prescribed in BS EN 26-1-2 [53], which aimed at determining the workability of the fresh concrete. The slump level was classified as S3 in MS EN206, where it was controlled within 100–120 mm. The slump level was classified as S3 in BS EN 206, which was controlled within 100–120 mm [54]. After the mixing process, a density test on the fresh concrete was also carried out. For each batch of concrete, an average of three samples was tested. Then, the next test was the bulk density test of hardened concrete, which was done according to the BS EN 12390-7:2009 specification [55]. The samples were treated for 2, 7, 14, 28, 56 and 90 days, taking into consideration an average density of three samples.

The CS was established for 2, 7, 14, 28, 56, and 90 days after treatment, according to the BS EN 12390-3 recommendation [56]. The CS test was done on the samples after 30 min of removal from the water tank. In addition, the UPV test was performed on the samples according to the BS EN12504-4 specification [57]. Before the CS test, the pulse velocity of the test samples was noted at different sample ages. Splitting tensile strength test was also performed on the sample cylinders of height = 100 mm and diameter = 200 mm. After treating the concrete cylinders 7, 28, and 90 days, the test was performed according to the ASTM C496-96 specification [58].

3.5 Sorption properties measurement and characteristics of microstructural appraisal

The samples were tested for water absorption according to the BS 1881-122 specification [59]. Three sample cylinders of 75 mm diameter and 100 mm height were prepared for the test. They are tested at 2, 7, 14, 28, 56, and 90 days of age.

The capillary absorption test was performed as recommended by ref. [60] to determine the relative sample pore size distribution. The outcome of the test was reported as the coefficient of capillary absorption (C _a(t)) at (g/cm²). Three sample prisms of 40 × 40 × 160 mm were prepared and treated for 7 days before the test.

A scanning electron microscopy (SEM) was done to assess the quality of paste’s microstructure of the concrete. The results were used to explain the mechanical and pore structure properties of the concrete. The SEM of the hardened samples was tested for the microstructural evolution of concrete composites for the prepared mixes at 28 days of age [8].

4.1 Zeta potential

The surface charge of nanoparticles is an essential property, which shows the stability of the material in suspension, as well as its uniform dispersion in water devoid of agglomeration. It is important that nanomaterials show uniform dispersion when combined with other concrete components. In the concrete mix, the uniform distribution of nanomaterial improves the radiation attenuation property, and it can cover the cement particles, thereby increasing its radiation attenuation properties [61].

The surface zeta potential values (ZPVs) of NSS are given in Figure 6, which showed good ZPVs of the NSS (−29.5 mV). This value indicates that the NSS has a low potential to agglomerate. A negative ZPV means a negative organic molecule covering the metal nanoparticles; therefore, the stability is increased in suspension during the regarding of aggregation [13]. Nanoparticles that show surface ZPV of −30 mV to +30 mV are considered stable with low potential for agglomerating. The result is considered to be good within these limits, whereas it is an excellent result at >±30 mV. For medical applications, such results are necessary, as it takes longer stability time than using as a constituent concrete material [62].

Figure 6

Surface charge (zeta potential) value of NSS, adapted from ref. [13].

4.2 The SSHWC workability

The concrete workability refers to the ease of compacting and finishing fresh concrete. The slump test is commonly used to determine concrete workability. The slump values of all the concrete mixes were within 100–120 mm. The amount of cement used for all mixes is relatively high (486 kg/m³). Therefore, the minimum superplasticizer utilization is 1%; this could be attributed to the large surface area of the cement particles [63]. The amount of superplasticizer was about 1% lower for the S5.0 mix compared to 1.2% of mix S0.0, as presented in Table 2. The spherical shape of the nano-silica particles (as shown in Figure 1) provided a “ball bearing” effect that improved the workability of the fresh concrete. The positioning of HWC at the lowest slump level is commonly recommended. The specific factors to be considered during the designing of HWC mixes for use in construction works are the pumping requirements, the formwork specifics, the placement conditions, and the reinforcement spacing [44].

4.3 The fresh and hardened density of the SSHWC

The fresh SSHWC displayed values of density ranging between 3,000 and 3,004 kg/m³. The addition of NSS increases the density of concrete because the material that fills the voids and the interface at the ITZ is improved correspondingly. The control mix (S0.0) is the strongest combination of many mixtures that were tested in any earlier studies [8]. In this study, a fixed concrete mixing proportion was maintained for all mixes (1:2:3) (see Table 2).

However, as summarized in Figure 7, it is important to have differences in hardened concrete densities, especially at an early age. Such difference normally comes from the addition of relatively small amounts of nanomaterials relative to the total concrete weight. However, the concrete showed increased hardened densities because of the role of NSS, which facilitated the primary cement hydration by interacting with the cement at the nucleation sites. At the early concrete age, the C–S–H gel was further produced in the matrix of a produced concrete binder. An improved bonding was observed between the aggregates and cement paste, as the nanoparticles occupy the voids in the ITZ [64].

Figure 7

Fresh and hardened density of SSHWC.

The increase in NSS content up to 3% would improve the density of the concrete at an early age; it is essential for better protection from radiation. Besides, the higher NSS content of 4% resulted in negative effects, as depicted in Figure 7. It could be caused by the difficulty in homogenizing the NSS at higher contents. It could also be attributed to the larger amount of water contained in NSS, which makes the real w/c ratio higher. The presence of entrapped air reduced the rheological property; it also affected the density of the concrete [13]. The mechanical properties were also affected negatively at higher NSS content (>4.0%). The radiation shielding property of hardened concretes is affected by the presence of voids.

The concrete containing 5.0% and 3.0% NSS by weight of cement content showed density values of 3,010 and 3,015 kg/m³ respectively. The NSS content should be optimized to ensure an optimal concrete density and physicomechanical characteristics [65]. With the increasing curing period, the mass density of most mixes increased. In addition, the improvements were apparent until the 28th curing day. Under prolonged curing, the samples show a dense microstructure with continuous cement hydration [13]. The mixes S4.0 or S5.0 showed improved hardened density; however, higher amounts of NSS can inverse the effect of the micromorphology of the produced C–S–H gel [63].

4.4 Compressive strength

The CS reflects the quality of hardened concrete. The test result is a direct reflection of the quality of the concrete. It is believed that the physical and mechanical properties of the components of ASFS help in achieving high SSHWC packing density and strength performance. The low w/c ratios were attained because of the activity of the superplasticizer and the optimal cement content used. These factors are essential for SSHWC development for all mixtures, including the control mix [8].

Although the concretes generally showed good performances, the presence of NSS further improved their performance. Previously, the improved performance of concrete upon adding the nano-silica was believed because of the filler effect of the added nano-silica [66]. However, recent studies have shown that this improvement comes from the small size of the introduced nano-silica. It offered a large surface area that encouraged the rate of cement hydration and pozzolan reactions, which leads to an early higher strength [13]. According to the values in Figure 8, the CS of the SSHWC was directly at the 0.0–3.0% NSS content. Besides, there was a decline in the CS, as the NSS content (4.0–5.0%) exceeded the cement reaction limit [67].

Figure 8

The SSHWC CS.

As illustrated in Figure 8, the CS enhancement was evident at an early age compared to the control mix. The CS of concretes was improved by adding 3.0% NSS by cement weight. The CS of all hardened concrete increased until the 28th day of curing because of the increase in the cement paste efficacy during the primary cement hydration. However, there was a decline in the rate of CS development after 28 days; it was virtually nonexistent even when the curing period was extended [68].

The CS of the SSHWC mixes, namely S0.0, S1.0, S2.0, S3.0, S4.0, and S5.0, met the CS requirement of grade 100 HWC after 28 days of curing. Besides, the ideal SSHWC (S3.0) obtained the maximum CS value [8]. The small increase in CS with adding the NSS could make a significant difference in radiation isolation, which is the primary objective of this study [8].

4.5 Ultrasonic pulse velocity

The ultrasonic pulse velocity test is one of the nondestructive tests for the quality assessment of SSHWC. During this test, the longitudinal ultrasonic waves are propagated through the length of the concrete material. The UPV test aims to record the velocity value of this wave, as it is propagated throughout the material [69]. The UPV values of SSHWC prepared with different NSS contents are shown in Figure 9. The observed UPV values followed the same trend of the CS outcome. The UPV values of SSHWC after 2 days of age were significantly lower than those of samples at 28 days of age. It could be because of the increasing rate of cement hydration with age, which comes with greater microstructural compaction [70].

Figure 9

UPV of SSHWC mixes.

The UPV values of the 28 days showed the role of NSS in increasing the density of mixes S2.0 and S3.0. It is evidenced by the UPV values of the mixes, which exceeded 5,000 m/s. The mix S5.0 exhibited lower UPV value because of its greater porous microstructure, as a result of the larger quantity of added NSS. At 56 and 90 days of age, there was no noticeable improvement in the UPV values of SSHWC, when matched to the values at 28 days of age. It could be because of primary cement hydration, which was near completion by the 28th day [71].

In general, at the age of 7 days, the mixes S1.0, S2.0, S3.0, S4.0, and S5.0 showed excellent UPV values of >4,650 m/s. However, the UPV performances were excellent (>4,850 m/s) at the age of 28 days for all mixes (S0.0, S1.0, S2.0, S3.0, S4.0, and S5.0). In an earlier study, the UPV values ranging from 4,300 to 4,650 ms⁻¹ are considered good. The quality of the concrete is considered as excellent when its value exceeded 4,650 ms⁻¹ [45]. All the concrete mixes used in the study were considered to exhibit excellent quality at the age of 28 days and beyond.

4.6 Split tensile strength

The split tensile strength (STS) of SSHWC at different ages is shown in Figure 10. Observably, there was a slight increase in the STS value with increases in the NSS content from 1.0% to 3.0% by weight of cement. The observed increase in the STS value of SSHWC indicates improved bond strength between aggregates and cement. Besides, a decline was observed in the STS value of mixes S4.0 and S5.0 from the optimum. It was because of the effect of high NSS amount, which negatively impacted the reaction of the cement and caused the development of voids and gaps in the mix [13].

Figure 10

Splitting tensile strength of SSHWC mixes.

The properties of ASFS contribute to a high STS performance [72] as its irregular shape is necessary for the production of concretes with good tensile strength [47]. However, the decline in the STS of S5.0 could be caused by the weakness of the ITZ between the aggregate and the paste matrix (Figure 10). On the contrary, the rest of the mixes performed well in terms of tensile strength with the inclusion of NSS. The trend may also be because of a stronger developed ITZ [67]. The STS behavior of SSHWC is similar to the CS of SSHWC and the UPV results [8].

4.7 Water absorption

SSHWC mixes prepared with different NSS contents were evaluated for water absorption capability at different ages, as depicted in Figure 11. The samples showed improved water absorption capacity with age (from 2 to 28 days). However, it showed no discernible improvement in water absorption at older ages (28–90 days). It could be because of the high rate of cement hydration at an early age, and it was near to completion by 28 days [73,74]. The water absorption values of the mixes S3.0 and S4.0 were below 0.65% by the age of 7 days. At the same age, the control mix recorded 0.8% water absorption test value. These good results agreed with those of the mechanical tests. Besides, the water absorption test values could be caused by the irregular shape of the aggregate, as it interacted well with the superplasticizer to improve the structure of the paste-aggregate pore. The aggregates’ mechanical properties also improved the durability of the concrete. Water absorption was impeded by the impermeable voids present in the aggregates.

Figure 11

Water absorption of SSHWC mixes.

Concrete mixes S2.0, S3.0, and S4.0 showed significantly lower water absorption test value. This is because of the filler action of NSS which filled the voids (action physically) and accelerated primary cement hydration (action chemically) to form a dense network of C–S–H gel [75]. This observation is consistent with previous research. Zareei et al. [76] observed that the inclusion of nano-silica led to a 35% reduction in the water absorption of the concrete because of the micro-filling effect of nano-silica particles. The water absorption rate of the mix S5.0 mix was significantly higher than that of mix S3.0 because of internal voids and gaps that weakened the ITZ [71]. The pessimistic effect of NSS with 5% could be attributed to the extra water content of the stabilizer of the NSS. It adds more water to the total amount of the mixture of water content. The addition of nano-silica beyond the optimum limit has been reported to negatively affect the mechanical strength of the resulting concrete [77].

4.8 Capillary absorption

SSHWC mixes produced with different NSS contents were tested for capillary action. The results are presented in Figure 12. The reason for this test is to determine the relative volume of capillary pores present in the mixtures. This test also explains the resistivity of the concrete to the penetration of harmful ions, such as sulfate and chloride, through capillary actions. The deleterious ions will affect the long-term concrete durability upon penetration [71]. From the capillary test results, it was observed that within the first 8 h, the capillary absorption rate was higher, as the whole mixes achieved a steep linear gradient. It was because of the influx of water saturated of the large capillary voids in the matrix through capillary action. The control mix (S0.0) and mix S5.0 showed the maximum capillary absorption rate within the first 8 h.

Figure 12

Capillary absorption of SSHWC mixes.

Besides, the capillary absorption rate was lowest in S1.0, S2.0, S3.0, and S4.0 mixes. The results indicated that within the cement paste matrix, S0.0 and S5.0 had the maximum relative volume of the continuous capillary pores. Because of the high capillary absorption rate, the resistance of S0.0 and S5.0 to aggressive ion penetration would be lower than the other NSS mixes. It was because of the role of the NSS in improving the properties of the resulting concrete. The NSS content is not recommended to exceed the limits, which may affect the properties of the resulting concrete as in the case with mix S5.0 [78]. Throughout the test, the capillary absorption rates for mixes S1.0, S2.0, S3.0, and S4.0 were lower than mixes S0.0 and S5.0.

The capillary absorption of mix S3.0 was significantly improved compared to the control mix (S0.0). The effect was optimistic, with an increase of 5.2% in capillary values at 1 h and 1% after 28 days of testing [79,80]. These results agreed with previous studies that reported improvement in capillary absorption property upon addition of 1.5% and 3% of nano-silica and micro-silica, respectively [26]. These results also indicated the important role of NSS in C–S–H gel nucleation and capillary pores filling in the concrete. The irregular shape of the aggregates and the superplasticizer can also influence the capillary absorption of SSHWC, as the excellent cement paste–aggregates interaction will reduce the voids [71].

As the water absorption and capillary absorption tests reveal the fluid transport capabilities of concrete, they are considered the most two significant tests for HWC. Both tests also show the extent to which the concrete contains continuous and discontinuous pores and voids. In addition, the test results indicate the ability of the concrete to attenuate radiation in the same context [12]. Besides, the distribution of pore sizes showed narrowing of the large capillary pores by the nano-silica because of the combined effect of the nano-filler and the pozzolanic reaction [81]. The capillary absorption test presented similar results as the water absorption test. Both findings were in line with the trend of mechanical tests.

4.9 SEM analysis

One of the most important tests in HWC evaluation is the SEM test. This test provides a clear understanding of the concrete microstructure. The best way for spreading harmful radiation through concrete is the air void. Therefore, the serious consideration that was taken to ensure all voids is filled in the internal structure of the concrete. This is to make sure that the radiation attenuation property can be improved [8]. The reason for the SEM analysis of the microstructure of the concrete was to check the ITZ and binder phase performances. The analysis was carried out on the control mix (S0.0), the optimum concrete mix (S3.0), and mix with NSS 5.0%. The comparison between the SEM observations of these samples is shown in Figure 13. The framework of the C–S–H gel was also evaluated at 28 days of age. For this study, the backscatter SEM mode was used to ensure a clear observation of the internal microstructure of the samples. In Figure 13(a–c), the internal structure of S0.0, S3.0, and S5.0 is displayed at 500× magnification. The general sample morphology was examined at the magnification level of 500×. From the results of the SEM, the mix S3.0 showed a significantly denser internal microstructure compared to S0.0 and S5.0 mixtures. This is consistent with the observations of Zhang et al. [82] who reported that nano-SiO₂ particle improved the microscopic structure of cementitious composites. On the contrary, the more nanomaterials content as in mix S5.0 will effect inversely on the produced concrete microstructure [13,83].

Figure 13

SEM images of SSHWC at 28 days curing age. (a) S0.0 SSHWC at 500× [13]. (b) S3.0 SSHWC at 500×. (c) S5.0 SSHWC at 500×. (d) S0.0 SSHWC at 50,000× [13]. (e) S3.0 SSHWC at 50,000×. (f) S5.0 SSHWC at 50,000×.

In contrast, the microstructure of mixes S0.0 and S5.0 (Figure 13a and c) had microcracks in the ITZ and cement paste matrix. Although microcracks were also present in S3.0, as shown in Figure 13(b), the number and width of cracks were significantly less than S0.0 and S5.0. Consequently, the NSS particles are considered useful in improving binder phase microstructural density for the content used in S3.0 [84]. Furthermore, the SEM evaluation was carried out at higher magnifications (10,000× and 50,000×) to have a more unobstructed view for the sample micromorphology at the binder phase. As evidenced in Figure 13(d–f), the micrographs of S0.0, S3.0, and S5.0 showed a complete cement paste reaction at 50,000× magnification, as it has formed a well-defined C–S–H framework [8]. Despite the good performance of mix S0.0 in developing the C–S–H gel, there were visible micropores, as shown in Figure 13(d).

Regarding the mix S3.0, as shown in Figure 13(e), it showed a uniform and impervious C–S–H gel network with few micropores when compared to S0.0. It explains the efficiency of an optimum NSS content in improving the rate of cement reaction, which leads to the development of additional C–S–H gel and forming an impervious C–S–H gel network. Even though the NSS at 3.0% filled the remaining nano-voids, it did not affect the cement reaction [84]. Besides, the increase in NSS up to 5.0% shows a pessimistic result as shown in Figure 13(f), and this agrees with the results obtained by Khalaf et al. [13]. It could be because of the inadequate space required to form hydration products. Besides, the excessive amount of nanoparticles would also lead to uniform dispersion difficulties.

Based on the SEM micrographs (Figure 13g–i), the magnitude of the crack width in S0.0 and S5.0 was found to be significantly larger when compared to the mix S3.0. The smaller size of the crack in S3.0 could be because of additional C–S–H gel formation, which led to a denser paste matrix when compared to the other mixtures [84]. The SEM observation confirmed a denser matrix of the S3.0 mix that resulted in better mechanical performance and resistance to the transport of fluids compared to S0.0 and S5.0.

The outcome of this test agrees with the results of the study by Li [85]. It indicated that the nano-silica would strengthen the concrete microstructure and help in filling in the blanks. The amount of calcium hydroxide crystals is formed because of the hydration reaction between cement and water. The Ca(OH)₂ crystal is hexagonal; it facilitates the binding of the aggregates with the paste matrix in the ITZ so that it affects the concrete’s performance. The nano-silica has a high specific surface area as compared with micro-silica; it shows high activity. It can react quickly with Ca(OH)₂ to form C–S–H gels; the outcome of this reaction is reducing the crystal size and the quantity of Ca(OH)₂. As the C–S–H gel occupies the voids, the ITZ increases in density, thereby increasing the volume of the binding paste matrix. The C–S–H gel is composed of 70% of the hydration products; its relative diameter is around 10 nm. The filling of the voids in the structure of C–S–H gel by nano-silica particles increases the paste matrix density. The nano-silica particles can act as a nucleus in the structure of C–S–H gel, which facilitate the formation of a strong bond with the C–S–H gel particles. It improves the reliability of the hydration product structure. In addition, it improves the long-term mechanical properties of the concrete.