Synergistic Effects of Steel Fibers and Silica Fume on Concrete Exposed to High Temperatures and Gamma Radiation

Abstract

The study explores the resistance of high-strength C40/50 concrete with steel fiber and silica fume admixture to high temperature and gamma radiation. The purpose is to create concrete composites with radiation shielding properties and high temperature resistance for use in nuclear power plants and radioactive waste storage facilities. For that purpose, concrete specimens containing 0.64 wt% industrial steel fiber and different proportions of silica fume (0%, 5%, 10%, 15%) were first subjected to high temperature according to ISO 834 and ASTM E119 after 28 days of curing at a target temperature of 900 °C based on a working fire scenario and then subjected to 94 kGy gamma radiation and analyzed using compressive strength, flexural strength, ultrasonic pulse velocity (UPV), SEM-EDX and XRD tests. It was found that 94 kGy gamma radiation increased the compressive strength of steel fiber concrete by SFC 20.98%, SFC-5 26.36%, SFC-10 26.45%, and SFC-15 25.34%, flexural strength by SFC 24.85%, SFC-5 25.06%, SFC-10 24.11%, and SFC-15 23.65%, and led to microstructure improvement and densification. XRD analysis revealed that samples exposed to 94 kGy gamma radiation accumulated and increased their calcite peak, resulting in decreased porosity and increased compressive and flexural strength. Under high temperature (900 °C) conditions, a significant decrease in the mechanical properties of concrete was observed in the compressive strength of SFC 78.99%, SFC-5 76.71%, SFC-10 76.62% and SFC-15 76.05% and in the flexural strength of SFC 79.44%, SFC-5 78.66%, SFC-10 79.68% and SFC-15 80.11%. In conclusion, results highlight the synergistic role of silica fume in reducing porosity and enhancing radiation-induced cement matrix reactivity, as well as that of steel fibers in improving thermal shock resistance and residual mechanical integrity. The developed composite materials are promising candidates for structural and shielding components in nuclear reactors, radioactive waste storage units, and other critical infrastructures requiring long-term durability under combined thermal and radiological loading.

1. Introduction

Concrete is one of the building materials that comes to mind in the construction sector. Concrete’s resilience to environmental influences is increased by new materials and additions, resulting in stronger and less maintenance-intensive constructions [1]. Research is still being conducted to create concrete materials that are more resilient to environmental pressures, focusing on sustainability and performance forecasting [2,3,4]. As the use of concrete in buildings, reactors, dams, offshore and onshore structures continues to increase, advanced material models are needed to accurately predict the behavior of concrete under different loading conditions. Concrete technology is rapidly evolving, and new types of concrete with stronger and higher performance are being developed [5]. For example, concrete is a brittle material with low shear capacity and flexural strength. As a result, research has shown that strengthening concrete’s mechanical properties requires adding steel fibers to the mix [6].

Steel, glass, polypropylene films, and chemical and pozzolanic admixtures are added to concrete to give it new properties and significantly improve some of its properties. Fibers used in concrete increase tensile and flexural strength and reduce shrinkage cracks [7,8]. Steel fibers used in concrete production are produced in various cross-sections and sizes. The defining elements of fiber are fiber length, fiber diameter, and fiber tensile stress. In all studies conducted to date, it has been found that there is a significant decrease in workability and a decrease in void measurement after adding fiber to concrete [9] In steel fiber concrete, increasing silica fumes instead of increasing steel fibers and using up to 20% and 30% silica fumes instead of cement increases concrete adherence to steel and compressive strength [10,11]. Fly ash and silica fume are used as pozzolanic materials in concrete production. Since silica fume is a finer material than cement, it tightens and strengthens the aggregate–cement paste interfacial zone to minimize voids and increase the compressive strength of concrete. Silica fume prevents segregation in concrete and positively affects homogeneity [12]. Concrete radiation and fire resistance significantly increase with silicon fume (SF) compared to other cementitious materials (SCM). Its properties provide a more balanced microstructure and better mechanical performance thanks to its small particle size and pozzolanic activity [13]. Research suggests fire safety concerns may be more related to concrete’s physical characteristics than ingredients, but high-strength concrete with silica fume maintains structural integrity at high temperatures [14]. Silica fume concrete is more fire resistant because of its decreased permeability, which inhibits the production of steam pressure during fire exposure [15]. Since silica fume concrete can attenuate gamma radiation more efficiently than regular concrete, its thick microstructure also helps explain why it works so well as a radiation shield [16].

In general, 25–100 kGy gamma radiation is the maximum value widely used for sterilization in hospital structures [17,18]. In nuclear research facilities, the gamma radiation dose to reinforced concrete elements is carefully managed and typically does not exceed 100 kGy [19,20]. Concrete around a nuclear reactor’s pressure vessel has the highest radiation dose, with a commercial nuclear power plant’s integrated absorbed dose of gamma rays being around 90 kGy, lower than the critical dose assuming a 60-year plant lifetime [21].

Nuclear reactor outlet temperatures can vary depending on the design and intended use. For example, a next-generation nuclear power plant requires high reactor outlet temperatures in the range of 750 to 950 °C [22]. In nuclear reactors, steam pipe penetrations, shield walls, pedestals of steam-powered equipment, elements adjacent to high-temperature piping, and structures within the primary containment can be exposed to continuous temperatures up to 150 °C due to the significant amount of heat generated by steam generation and utilization and the nuclear fission process. These hot–cold cycles can cause loss of mechanical properties and cracking in these materials [23]. When steam leaks from broken pipes, higher temperatures (up to about 350 °C, [24]) can be expected.

Many studies have observed that in the case of natural or accidental fire, high temperatures can reach up to 1000 °C [25]. Experimental findings show that spall damage occurs at high heating rates between 200 °C and 350 °C. In some tunnel fires in Europe, the peak temperature can reach over 1000 °C in a few minutes, and the heating rate can increase from 100 °C/min to 200 °C/min [26,27].

In general, high temperatures damage concrete in two ways. One is the deterioration of the mechanical properties of concrete, including aggregate and cement paste thermal incompatibility and physicochemical changes (also known as phase transitions). The other is concrete pouring. When the cross-sectional area is severely reduced due to spalling, the internal concrete is exposed to high temperatures, resulting in further spalling damage, also known as progressive spalling damage [28]. Heat primarily leads to mass loss and spalling of concrete, and the extent of deterioration is significantly affected by temperature levels [29]. Using fibers alone improves thermal properties by creating a more porous structure [30]. Silica fume admixture improves thermal properties and reduces permeability. The mixture of silica fume and fibers leads to synergistic effects in concrete, resulting in superior mechanical properties and greater resistance to environmental influences. The denser microstructure formed by silica fume and fibers can contribute to better radiation protection properties [31]. As a result of these developments, fiber-reinforced and silica fume-modified concrete is becoming a promising material for applications that require high performance under extreme conditions, such as continuous radiation exposure in hospitals, nuclear facilities, space structures, etc. [32,33,34].

It is important to recognize the importance of concrete carbonation for the longevity of concrete structures subject to radiation [35]. The lifespan of concrete buildings, especially those subjected to radiation, is greatly influenced by concrete carbonation. The carbonation process greatly impacts concrete’s durability and service life, which is the interaction of carbon dioxide with cement hydrates. This phenomenon is particularly relevant in the context of nuclear infrastructure, where concrete buildings are exposed to certain environmental conditions [36,37]. The properties of concrete are affected by radiation heat at both structural and microstructural levels. Thermal stress, which can be significant enough to damage concrete, is introduced at the structural level by the thermal gradient caused by radiation heat. Microcracks in cement paste can result from a microstructural mismatch between the paste’s thermal stresses and the aggregate’s response to radiation heat, creating a lot of stress at the interface between the two [38].

The effects of neutron and gamma ray radiation on cement paste in concrete at high temperatures (above 600 °C) remains to be investigated [20,39,40].

The tensile strength of beam specimens and the compressive strength of cube specimens were measured. TS 10514 [41] was followed in the selection of steel fiber content. The samples were sent to the Nuclear Energy Research Institute of Turkey (NUKEN) for gamma irradiation. Using the ISO 834 [42] fire scenario and ASTM E119 standards [43] for building and construction materials, the various effects of 94 kGy gamma radiation and 900 °C high temperature on the strength and characterization of silica-fume-substituted concrete specimens with 0%, 5%, and 10% silica fume substitution were investigated to better understand the abrupt changes in the behavior of silica-fume-substituted waste steel fiber concrete, the beneficial contribution of silica fume to concrete under harsh conditions such as gamma radiation and fire, and its effects on microstructural properties.

2. Materials and Methods

2.1. Materials

2.1.1. Cement

In the experimental investigation, Portland cement of the CEM I 42.5 N [44] type was utilized.

Table 1. Chemical properties of cement and silica fume.

Component	Cement %	Silica Fume %
SiO2	20.63	94
Fe2O3	3.41	0.7
AI2O3	4.71	1.2
CaO	63.64	0.8
SO3	2.98
CI−	0.04
Glow Loss	1.25	0.7
K2O	0.91	0.9
Na2O	0.23	0.3
Free lime CaO	1.1

lists the cement’s chemical and physical characteristics. Portland cement (ASTM C Type I, [45]) with a density of 3150 kg/m³ and silica fume with a density of 380 kg/m³ and a Blaine specific surface area of 23,000 m²/kg were used in the experiment. The specific surface areas of the materials were measured following the ASTM C 204 [46] and EN 196:6 [47] standards.

Figure 1a,b show the morphology of CEM I 42.5 N cement grains in SEM images at different magnifications. Figure 1c shows the Ca, Si, O, Mg, K, Na, Fe, and Al peaks in CEM I 42.5 N cement.

XRD analysis of Portland cement revealed C₃S, C₂S, C₃A and C₄AF as the main components, and the XRD pattern of the cement was found to have a crystal structure suitable for a typical Portland cement (Figure 2a).

The silica peak concentrated at 2θ ≈ 23° exhibits a broad series of peaks instead of sharp crystalline peaks in silica fume, indicating its non-crystalline structure. Therefore, XRD is used to determine its amorphous structure [48] (Figure 2b).

2.1.2. Aggregates

Natural stone aggregates of 0–5 mm, 5–15 mm, and 15–25 mm have been used. The density of sand composed of fine aggregate is 2.62 g/cm³, the density of sand composed of medium aggregate is 2.69 g/cm³, and the density of sand composed of coarse aggregate is 2.72 g/cm³. The TS 802 [49] (concrete mix design calculation principles) standard was used as the basis for creating the concrete mixtures (

Table 2. The physical properties of fine and coarse aggregates.

S/No	Physical Properties	Fine Aggregate	Coarse Aggregate 12.5 mm
1	Percent of particles retained on the 4.75 mm sieve	1	96
2	Percent of particles passing the 4.75 mm sieve	98	4
3	Percent of particles passing the 0.075 mm sieve	2.8	0
4	Fineness modulus	2.73	2.68
5	Coefficient of uniformity (Cu)	3.70	1.97
6	Coefficient of curvature (Cc)	1.12	0.89
7	Bulk density	1653	1652
8	Specific gravity	2.62	2.69
9	Moisture (water) absorption (%)	1.39	0.44
10	Aggregate crushing value (%)	-	19
11	Aggregate impact value (%)	-	14

). The largest aggregate diameter was 12.50 mm, following the fiber content chosen within the parameters of the TS 10514 (mixture proportions and manufacturing of fiber reinforced concrete) standard, because waste steel fiber added to the concrete was produced within the study’s purview. Additionally, the 45/55 fine/coarse aggregate ratio rule has been followed within the parameters of the standard. The fiber content selected within the parameters of the TS 10514 (mixture ratios and production of fiber-reinforced concrete) standard is recommended to be mixed in the range of 30 to 70 kg per 1 m³ of concrete. For this reason, a fixed amount of 50 kg has been added to 1 m³ of concrete. This corresponds to a ratio of 0.64% by weight.

2.1.3. Fiber Steel

Steel fiber coded KMX 6560 was also used in the production of steel-wire-reinforced concrete, for which concrete mix proportions were calculated according to Turkish Standard 802 [49]. Crimped and hooked steel fiber with a diameter of 0.90 mm and a length/diameter ratio (slenderness) of 65 was used (Figure 3a,b). Its density is 7800 kg/m³, and its tensile strength is 1100 MPa.

2.1.4. Silica Fume

Silica fume is a pozzolanic material with a fine amorphous structure and a high content of SiO₂. It can be used instead of cement in concrete production and contributes positively to durability [50,51,52,53]. It also comprises pozzolanic elements such as fly ash, slag, silica fume, and Portland cement. The primary ingredients of these “low pH” cement pastes are ettringite and calcium silicate hydrate (C-S-H). The pH is between 0.7 and 1.2, and the Ca/Si ratio in C-S-H decreases when silica-rich materials are present [54,55]. Low pH cement pastes refer to reducing clinker content by adding supplementary cementitious materials (SCMs) that reduce the calcium hydroxide (CH) content and therefore the pH. However, this reduction can be compensated for over time by additional CH formation [56].

The morphology of silica fume was studied via scanning electron microscopy (SEM), and the silica fume particles appear as fine, irregular aggregates with a porous texture (Figure 4a,b). The average particle diameter of silica fume is 0.37 µm, and the surface area of silica fume particles is 20 m²/g [57].

The energy dispersive X-ray analysis (EDX) results confirm the presence of silica (SiO₂) in the Si/O ratio, and it is also seen in the peaks in Figure 4c that the silicon ratio is high and the calcium ratio is low.

The XRD powder patterns of SF are displayed in Figure 2b. The strong, broad peaks of SF were aligned with the strong, broad peak of amorphous SiO₂, with corresponding centers at 23° and 22° (2θ) [58,59]. The findings imply that SF is now in an ambiguous situation.

2.2. Specimen Preparation

The tests for the mechanical strength and the specimen dimensions are provided in TS EN 12390 [60]. For each design group, three concrete examples measuring 10 × 10 × 10 cm, 15 × 15 × 15 cm, 15 × 15 × 60 cm, and 4 × 4 × 16 cm were made in accordance with TS 802 [49]. A total of 36 samples were produced: four samples of 4 × 4 × 16 cm, nine samples of 10 × 10 × 10 cm cubes, and nine samples of 15 × 15 × 15 cm cubes, totaling nine samples for each of the four separate mixtures. Additionally, three large samples of 15 × 15 × 60 cm were produced from each mixture, totaling 12 samples, which were not subjected to fire and gamma radiation. They were then allowed to cure for 28 days at 20 ± 5 °C and 97 ± 2% humidity in a standard controlled atmosphere. The following procedures were used to prepare the concrete mix: steel fibers, cement, silica fume, sand, and coarse aggregate were added to the mixer. After combining the ingredients for three minutes, we added the superplasticizer and tap water. The mixer was then run for an additional two minutes to create new, uniformly dispersed concrete with steel fibers.

2.3. Mix Proportioning

For the study, six concrete mixes (15 × 15 × 15 cm and 4 × 4 × 16 cm) and three concrete mixes (15 × 15 × 15 cm cubic) were created. After being exposed to high temperatures (900 °C) and gamma radiation (94 kGy), the impact of steel fiber content on compressive strength, flexural strength, and other parameters was examined. At 28 days of age, the mechanical characteristics and characterization of both plain concrete (PLC) and composite concrete with steel fibers were examined. The ratio of cement to water was fixed at 0.45. To improve consistency, superplasticizer chemicals were utilized.

Table 3. Mixing quantities for 1 m³ of new concrete (kg/m³).

Material	Mix Proportions
	PLC	SFC	SFC-5	SFC-10	SFC-15
Portland Cement	420	420	399	378	357
Silica Fume	0	0	21	42	63
Water	189	189	189	189	189
Steel Fiber	0	50	50	50	50
Fine Aggregate (0–6 mm)	1200	1200	1200	1200	1200
Coarse Aggregate (6–12.5 mm)	600	600	600	600	600
Superplasticizer	4	4	4	4	4
Total	2413	2463	2463	2463	2463

provides the mix ratios for the concrete designs. A W/C ratio 0.45 makes the concrete mix sufficiently fluid to simplify compaction and placing [61]. Studies show that this ratio has a good impact on early and late strength development and is also linked to increased compressive strength [62]. This ratio of superplasticizers can successfully lower the water content of concrete without sacrificing its structural integrity [63]. In addition, the Flowaid SCC brand superplasticizer was used.

2.4. Test Method

In this research, a 4 × 4 × 16 cm bar beam, 15 × 15 × 60 cm beam and 15 × 15 × 15 cm and 10 × 10 × 10 cube concretes with high-performance silica fume substitution of class C40/50 as per ASTM C1240 [64] were produced using 0.64 wt.% industrial steel fiber.

Depending on the fire situation, building and construction materials can reach temperatures of over 1000 °C in 90 min, as per the ISO 834 [42] and ASTM E119 [43] standards (Figure 5). In a fire room, the prepared plain and composite concrete were subjected to fire.

They were burnt in three batches, each lasting 90 min, at temperatures of 900 °C in an electric muffle furnace (MT-1200-100-B2, Tetra Isı Sistemleri, Istanbul, Turkey), simulating the fire situation depicted in the standards (Figure 5). The maximum temperature of 900 °C was chosen because, despite its high temperature, it allows for the testing of specimens’ mechanical strength.

Additionally, 94 kGy gamma radiation was used to test the changes in the microstructure and mechanical strength of the concrete without affecting the structural integrity of the research centers (Turkish Nuclear Research Institute).

The compressive strength of concrete specimens subjected to high temperatures and 94 kGy gamma radiation was measured in a press machine. A three-point bending press was also used to measure flexural strengths. The outcomes of the ambient (20 °C) designs are contrasted with the designs that were exposed to the fire scenario.

Following that, the mechanical testing was transformed into the proper formats for the specimens of fractured concrete, and the mechanical outcomes were contrasted with studies using XRD and SEM-EDX.

2.4.1. Radiation (Cobalt 60) Resistance

The box-style Class IV NUKEN Gamma Irradiation Facility, built in 1993, uses the Co-60 radioisotope as its irradiation source.

The amount of radiation affecting concrete was examined in the literature. Notably, 40 × 40 × 160 mm specimens were sent to the Nuclear Research Institute of Turkey (NUKEN) and exposed to gamma, Cobalt 60 rays from all directions, and the bar specimens on the front and back of the specimens were replaced. During this time, they were exposed to gamma radiation for 34 days at 2.75 kGy daily for a total of 93.5 kGy (±10%).

Irradiation boxes measuring 45 × 45 × 90 cm are filled with the products to be irradiated (maximum 50 kg) (Figure 5d). Due to this, the product boxes’ dimensions need to match those of the irradiation boxes. The carrier car travels on the train to transport the product-filled boxes to the irradiation room, where the radiation source is situated (Figure 5a). The products are exposed to radiation after the irradiation boxes are moved around the radiation source for a specified time based on the desired dose (Figure 5c). By absorbing all or a portion of the energy of the gamma rays released by the Co-60 radioisotope as they travel through the product, the products are exposed to radiation. Stated differently, the quantity of radiation energy absorbed by the products is known as the radiation dose (Gray = Joule/kg), which is determined by employing dosimeters (Figure 5b). To determine the greatest and lowest dosage values that the product received, the dosimeters are irradiated alongside the products and assessed after the procedure. Therefore, it is established that the product receives the prescribed dosage.

2.4.2. High Temperature Resistance

The ISO 834 [42] and ASTM E119 [43] standards, as well as the TS 1263 Regulation on Fire Protection of Buildings [65] and Republic of Turkey Official Gazette 19.12.2007, Issue: 26735 [66] are examined, and the resistance of the materials against fire from temperatures up to 500 °C and 1000 °C, for between 60 min and 120 min, and especially the critical 90 min mark for human life is emphasized. All new concrete mix samples of 40 × 40 × 160 mm size produced were kept in the ash furnace at 900 °C for 90 min in the laboratory. The time it takes for structural components to reach critical failure temperatures, which might differ depending on building materials and fire development rates, is frequently associated with the 90-min mark. Fire safety engineering standards frequently use this timescale to ensure that structures are structurally stable for as long as inhabitants can safely leave [67]. Although 90 min is a common benchmark, some contend that increasing this time to 120 min might offer more safety margins, especially in high-rise structures where structural complexity and human behavior may cause evacuation to take longer [68].

After 28 days of water curing, three different SFC concrete specimens of 40 × 40 × 160 mm of each mix were kept at room temperature at 20° C for 48 h before exposure to high temperature.

In the muffle furnace set to 25 °C/min, it was increased to 900 °C in 36 min and exposed to high temperature for a total of 90 min, then the heat source switch of the furnace was turned off and the ventilation hole of the furnace was opened and the samples were slowly cooled to the furnace ambient temperature (Figure 6).

2.4.3. Ultrasonic Pulse Velocity Test

Before the specimens were subjected to flexural and compressive tests, a Pundit+ Lab Ultrasonic Wave Velocity Tester (Yüksel Laboratory Equipment, Ankara, Türkiye) was used on all specimens according to ASTM C-597-02 standards [69] (Figure 7).

An ultrasonic testing instrument can be used for quality control and to find cracks, cavities, or other potential flaws in rock and concrete specimens. Guidelines for testing fresh concrete, hardened concrete, and concrete in structures are provided by TS EN 12504-4 [70].

2.4.4. Flexural Strength Test

During the flexural test, the gap between the carriers was 100 mm, and the ring was placed from the ½ point on the prism. The loading rate was 50 N/s. The loading mechanism is a three-point mechanism (Figure 8a); a YKM-CM106 model machine (Yüksel machinery manufacturing construction export import trade Co., Ltd., Ankara, Turkey) was used, calibrated by Turkak (Turkish accreditation Agency, Ankara, Turkey) in the construction laboratory of Siirt University (Siirt, Turkey).

These specimens were first subjected to flexural strength at three points according to the TS EN 12372 standard [71] (Figure 8b).

2.4.5. Compressive Strength Test

The average compressive strength of three specimens from each design group was calculated after testing. According to ASTM C39 [72] guidelines, the compressive strength was measured using a YKM-CM106 concrete press machine with a load of 0.6 N/mm²/s (Figure 8c).

2.4.6. Microstructure Analysis

The concrete pieces taken separately from each sample were cut into pieces no larger than 5 mm and made suitable for working in the SEM device. After the samples were put into the SEM device’s chamber, the procedure began. Scanning electron microscopy characterized the surface morphologies of the samples at various scales (Zeiss Sigma 300, ZEISS Microscopy, Cambridge, MA, USA). In order to take images of the prepared samples, the surfaces to be imaged were made conductive by coating the non-conductive samples with very thin (approximately 3 Å/second) conductive (Au) material. For XRD analysis, the concrete pieces were ground and pulverized to pass through a 0.20 mm sieve. At a wavelength of 1.5406 (λ), between 10 and 90°, XRD examination was carried out at a step rate of 0.02° and a scan rate of 2° per minute. An Empyrean XRD PANalytical equipment (Malvern Panalytical B.V., Almelo, The Netherlands) was employed.

3. Experimental Outcomes and Discussion

3.1. Compressive Strength

In Figure 8, the concrete was designed as C40/50 in the material section. It was observed that the compressive strength of C40 concrete specimens without a steel fiber and silica fume admixture was (PLC) 50.65 MPa, and the presence of 0.64% steel fiber in the concrete mix increased the compressive strength by 11.15% to 56.30 MPa. Steel fiber admixture increased the compressive strength [73,74,75].

The compressive strength of all heated concrete cubes is shown in Figure 8c at ambient temperature; the average compressive strength of steel-fiber-reinforced composite specimens is higher than that of heated SFC. The compressive strength of the cube specimens decreased with heating. The range of variation in the data did not change by more than ±0.6 MPa (Figure 9a). Compared to SFC, SFC-5 increased by 2.02%, SFC-10 by 14%, and SFC-15 by 10.95%, i.e., the decrease in compressive strength decreased with silica fume substitution at 900 °C.

When the pressure resistance of samples tested at 20 °C without any special treatment is compared with the pressure resistance of samples kept in a furnace at 900 °C for 90 min, SFC containing 0% silica fume substitute, SFC-5 containing 5% silica fume substitute, SFC-10 containing 10% silica fume substitute, and SFC-15 containing 15% silica fume substitute decreased by 78%, 99%, 76.71%, 76.62%, and 76.05%, respectively (Figure 9a).

The compressive strength of high-performance C40/50 concrete with silica-fume-doped steel fibers, gamma irradiated at a dose of 94 kGy (Cobalt 60), and silica fume doped at 20 °C natural radiation is shown in Figure 9b. The compressive strength values of SFC, SFC-5, SFC-10, and SFC-15 specimens were 49.18, 52.43, 59.77, and 57.05 MPa, respectively, with SFC increasing by 20.98%, SFC-5 by 26.36%, SFC-10 by 26.45%, and SFC-15 by 25.34%. Comparisons were made on 28-day specimens [76]. Gamma irradiation modifies the microstructure of polycrystalline silicon beams in MEMS, hence affecting their mechanical characteristics and resistance to pressure [77]. The primary hydration product in slag-blended cement matrices, calcium aluminum silicate hydrate (C–A–S–H) gel, does not change much after gamma irradiation and maintains its strength and integrity. Because of its resilience, the material can tolerate pressure without suffering appreciable deterioration [78].

The standard deviation of the compressive strength of steel-fiber-reinforced concrete containing silica fume varies between 1.08 and 1.39. This range indicates that, despite the overall consistency of compressive strength, variations may occur due to a number of factors, including mixture design, the ratio of steel fibers to silica fume, curing conditions, and a series of fire and gamma radiation variables, as also shown in

Table 4. The effect of silica fume substitution on the compressive strength values of concrete. The effect of gamma radiation and fire on the compressive strength values of industrial steel-fiber-reinforced silica fume concrete.

DAY 28
	20 °C Natural Radiation		20 °C 94 kGy Radiation		900 °C Natural Radiation
	Measured	σ 1	Measured	σ 1	Measured	σ 1
	(MPa)		(MPa)		(MPa)
SFC	56.3	1.21	65.48	1.08	11.87	1.26
SFC-5	57.79	1.27	68.73	1.14	13.46	1.30
SFC-10	63.57	1.23	76.67	1.21	14.87	1.27
SFC-15	61.82	1.37	73.35	1.34	14.80	1.39

¹ standard deviation.

.

3.2. Flexural Strength

For 900 °C heat and 94 kGy gamma radiation, three specimens of 4 × 4 × 16 cm were prepared for each group. The 15 × 15 × 60 cm specimens were prepared for specimens but not subjected to heat and gamma radiation. It was observed that the tensile force was concentrated in the central parts of the bar prisms, in the regions of maximum bending stress, and cracking occurred in these regions. Cracking occurred in the concrete specimens with steel fibers (0.64%); however, the specimens did not break instantly into two pieces. However, they curved with some degree of elastic behavior, deflection occurred, and continued to carry load, albeit at low levels.

As the temperature of the specimens subjected to 900 °C heat treatment increased, the flexural strength of SFC decreased by 79.44%, SFC-5 by 78.66%, SFC-10 by 79.68%, and SFC-15 by 80.11%, respectively [79,80] (Figure 10a).

The flexural strength of SFC increased by 24.85%, SFC-5 by 25.06%, SFC-10 by 24.11%, and SFC-15 by 23.65% of the composite specimens exposed to gamma rays (94 kGy Cobalt 60) with radiation [76] (Figure 10b).

It was also observed that the flexural strength of pure PLC C40/50 high-strength concrete was 4.25 MPa. Notably, 80% of the flexural strength decreased at 900 °C. When a 94 kGy gamma radiation dose was received, the flexural strength increased by 21%.

The flexural strength values of steel-fiber-reinforced concrete with silica fume added show substantial variability, with a standard deviation ranging from 1.07 to 1.34. This range shows that there are notable performance variations across the tested samples, even though the flexural strengths are generally consistent. The ratios of steel fiber and silica fume employed, the special characteristics of the concrete mixture, and the many impacts (fire, gamma radiation) shown in

Table 5. The effect of gamma radiation and fire on the flexural strength values of industrial steel-fiber-reinforced silica fume concrete.

DAY 28
	20 °C Natural Radiation		20 °C 94 kGy Gamma Radiation		900 °C Natural Radiation
	Measured	σ 1	Measured	σ 1	Measured	σ 1
	(MPa)		(MPa)		(MPa)
SFC	7.16	1.22	8.94	1.07	1.47	1.29
SFC-5	8.88	1.25	11.11	1.12	1.89	1.34
SFC-10	10.04	1.28	12.46	1.20	2.04	1.33
SFC-15	10.01	1.35	12.38	1.32	1.99	1.36

¹ standard deviation.

are some of the causes of this diversity.

3.3. Ultrasonic Pulse Velocity (UPV) Test

UPV analysis also showed that the sound transmission velocities decreased significantly at 900 °C due to the effect of temperature and an increase in crack formations and widths due to void formation. Concrete homogeneity, the existence of voids and fissures in the material, the concrete’s time-varying characteristics, and its dynamic physical qualities can all be ascertained through the use of wave velocity measurement [81]. Ultrasonic testing equipment can be used for quality control purposes and to detect cracks, voids, or other possible defects in concrete and rock specimens. It was performed on composite specimens in accordance with TS EN 12504-4 and ASTM C-597-02 [69,70]

The probes in the direct approach receive ultrasonic waves along the transmitted length at right angles to the surface. The shortest distance between the probes is covered by the ultrasonic waves during transmission in the direct technique. As a result, measurements have an accuracy of ±1 [82,83]. The direct method was chosen for the study due to its greater precision over alternative approaches as well as its ability to reach the test specimens’ opposing surfaces.

As seen in

Table 6. UPV analysis results (m/s) and concrete grading.

Effects on 28-Day Samples	SFC	SFC-5	SFC-10	SFC-15	Concrete Quality Classification According to BIS 13311-92-Part-I [86]
Natural Radiation 20 (°C)	4690	4758	4879	4780	Ultrasonic pulse velocity (UPV), m/s	Concrete quality classification
Natural Radiation 900 (°C)	257	361	505	412
					Over 4500	Excellent
94 kGy Gamma Radiation 20 (°C)	4848	4580	5303	5124	3500–4500	good
					3000–3500	Average
					Less than 3000	doubtful

, ultrasound wave velocities are shown for 20 °C and 900 °C temperatures. The wave velocities of the composite samples with silica fume added are higher at 20 °C. At 900 °C, the sound transmission rate decreased due to the voids formed by the crack formation of the composite samples that were left in the muffle furnace for 90 min [84,85].

As seen in Table 6, the crystallization in the internal structure of composite samples exposed to gamma rays (Cobalt 60) has become denser due to ionization [87,88]. Therefore, it was observed that the sound transmission rate increased. Therefore, Table 6 of the UPV test also supported that the mechanical strengths increased with the gamma irradiation in Figure 9b and Figure 10b, and the crystallization in the internal structure of silica-fume-doped steel fiber concrete showed that the internal structure became denser with ionization, and the strength increased as the porosity decreased [89].

3.4. SEM-EDX Analysis

3.4.1. SEM Analysis

SEM observations at 20 °C and natural radiation level (Figure 11a) with (Figure 11b) 900 °C high-temperature-exposed high-strength silica-fume-doped steel fiber concrete show how the cracks widen and how the crack widths increase on the concrete surfaces. Via SEM images of the concrete exposed to 20 °C room temperature and 900 °C high temperature, the cracks between 2.4 and 10.9 µm in Figure 10a increase to 29.7–2.1 µm, and the increase in crack sizes seen in Figure 11b supports the strength reductions.

Although 0.93 µm–10.43 µm cracks were observed in SFC, SFC-5, and SFC-10 concretes as a result of 94 kGy gamma radiation, it is thought that there is an increase in the compressive and flexural strengths of silica-fume-substituted steel fiber concretes due to ionization, that is, an improvement in the internal structure and this improvement is thought to continue (Figure 11c) [76]. In addition, the steel fibers holding the concrete together are thought to prevent the growth and further propagation of cracks in the concrete caused by temperature and gamma radiation [90].

At 94 kGy, while the mechanical properties of concrete continued to improve, cracks were observed in the fibers added to increase the tensile strength of concrete (Figure 12c) [91], this may be an indication that the steel fibers became brittle [92,93]. SEM images of 94 kGy gamma radiation show the formation of superficial cracks with widths of 0.49, 0.42, 0.72 µm in the steel fiber (Figure 12c) [93,94].

In Figure 12b, it is seen in the SEM Analysis that CH and CSH, and even the crystal needle structures forming ettringite, are disrupted at the high temperature of 900 °C. The decomposition of ettringite is related to water loss [95,96]. In addition to quartz (SiO₂), it indicates the existence of portlandite (CH), calcium silicate hydrate (C S-H), ettringite, etc., in concrete. At 900 °C, these phases became less as the temperature changed (Figure 12b). Concrete with a lower CH concentration has lost strength [80,97,98].

In addition, the concrete matrix itself can also develop cracks due to radiation-induced changes in its microstructure. These cracks compromise the overall strength and durability of the material [99].

In summary, depending on the dose, gamma radiation can heal and damage concrete. It can be said that there is a delicate balance between repair and damage [100]. In the SEM images, 94 kGy gamma radiation started to form cracks in the concrete. In the cracks formed in unadmixed steel fiber concrete (SFC), ettringite formations were seen along the crack (Figure 12c). Although the precise effect of gamma radiation on ettringite production is unknown, it may theoretically change the microstructure of cementitious materials or the hydration environment, which might have an impact on the crystallization of ettringite [101]. However, radiation may not significantly affect the fundamental chemical processes that lead to ettringite formation. This suggests that environmental conditions and chemical composition remain the most important determining factors, rather than radiation exposure [102,103]. It is known that ettringite formations cause volume expansion in concrete [104,105,106]. Ettringite formation (Figure 13c) shows an increase in the Al peak of SFC in the EDX graphs, indicating the presence of ettringite [107,108]. In the SEM images in Figure 12c, it can be seen that ettringite is dense along the crack inside the cracks seen in SFC. In addition, in the SEM images in Figure 12c, in contrast to SFC, no ettringite formation is observed in the cracks of SFC-5 and SFC-10 silica-fume-substituted steel fiber concrete, indicating that silica fume substitution, which has higher pozzolanic properties than fly ash, reduces crack formation [109], which demonstrates and supports previous findings [110].

3.4.2. EDX Analysis

The EDX results (Figure 13a) show that silicon and calcium have strong peaks. The SEM image shows hexagonal calcium hydroxide, needle-like ettringite, and sheet-like nebular CSH (Figure 12a). Calcium and silicon both showed strong peaks (Figure 12a), but the presence of calcium indicates a relatively low concentration of calcium hydroxide compared to SFC [59]. When EDX analyses of SFC, SFC-5, and SFC-10 samples cured for 28 days are compared (Figure 13a), it is seen that silica fume substitution decreased the calcium and sodium amounts, the Ca/Si ratio by 4.16, 3.54, and 0.6, and increased the aluminum amount. The Si/Al ratio was determined as 8.47, 5.20, and 4.83, respectively. In this case, it can be said that volumetric expansion and related cracks are reduced with silica fume substitution [109,111].

The increase in calcite density is one of the effects of carbonation on the properties of concrete (Figure 14c). It is especially high in the specimen exposed to 94 kGy gamma irradiation in SFC-10, and it is thought that the consumption of portlandite during the reaction increases porosity while the accumulation of calcite decreases porosity [112,113].

At doses up to 94 kGy, concrete showed a healing effect. This means that some of the damage caused by radiation is repaired. The precise mechanism behind this healing process involves combining free radicals and repairing broken chemical bonds within the concrete matrix [114]. Although 0.93–10.43 µm cracks were observed in SFC, SFC-5, and SFC-10 concretes as a result of 94 kGy gamma radiation, it is thought that there is an increase in compressive and flexural strengths, i.e., improvement in the internal structure of silica-fume-substituted steel fiber concrete due to ionization (Figure 12c). The concrete continues to improve at 94 kGy gamma radiation dose [76].

3.5. XRD Analysis

X-ray diffraction and energy dispersive spectrometry tests (XRD) were used to calculate compositions and reveal the effect of silica fume in concrete. Samples from hardened concrete prisms exposed to 20 °C, 900 °C, and 94 kGy gamma radiation (Co60) at 200 °C were subjected to XRD phase analysis, which revealed the presence of quartz (SiO₂), portlandite (CH), and calcium silicate hydrate (C-S-H). The temperature had an impact on the phases, which reduced. Concrete with a lower CH concentration lost strength [80]. Concrete samples gradually lost their CH (2θ ≈ 18° and 2θ ≈ 34°) and CaCO₃ (2θ ≈ 29°) phases due to the high temperature of 900 °C [115]. However, samples exposed to 94 kGy gamma radiation showed an increase [116].

The results of the mechanical and UPV analyses indicated that gamma radiation increased the strength values (Figure 12). This increase is expected to be supported by microstructural analyses. It can therefore be expected that strength peaks based on C-S-H will be strong in samples exposed to radioactivity. However, an examination of the SiO₂ (quartz) peaks in the XRD analysis at 2θ ≈ 26.5° reveals no increase in SFC, SFC-5, and SFC-10 following exposure to gamma radiation (Figure 14a–c).

It is assumed that γ-radiation causes enhanced carbonation in concrete. This radiation carbonation occurs simultaneously with “natural” carbonation. While radiation-induced carbonation can occur in the whole sample, natural diffusion-controlled carbonation can only occur in the surface layer. During carbonation processes, CO₂ reacts with calcite and portlandite as products. The resulting calcite crystals grow through the pores, reducing their diameter and hardening the material [117]. According to the Mohs scale, the hardness of calcite is 3, while the hardness of portlandite is 2 [118]. CO₂ reacts with calcium hydroxide (CH) to produce calcite (CaCO₃), but γ-radiation carbonation in concrete, particularly when contaminants like SO₂ and O₂ are present, can cause notable dissolution and precipitation processes [119]. Following accepted diffusion laws, CO₂ diffuses into calcite-rich materials like cement pastes, resulting in carbonation and the synthesis of CaCO₃, which can influence material properties [120].

Hydration products contain portlandite, calcite, ettringite and quartz peaks. XRD analysis of cement-based mortars typically have a halo at 2θ ≈ 26.5° and a maximum at 2θ ≈ 29.5°. For silica SiO₂ [121], the highest reflection at 2θ = 26.5° is the typical reflection found in silicates between 24° and 31° of the sample [109]. The strongest reflection occurs at 2θ ≈ 26.5° (Figure 14b). Increasing temperature in concrete samples after 400° C caused a decrease in CH (2θ ≈ 18° and 2θ ≈ 34°) and CaCO₃ (2θ ≈ 29°) phases at 900° C (Figure 14b) [115].

XRD analysis showed small proportions of monosulfoaluminate with hemicarboaluminate indicated as primary hydration products and ettringite with 2θ = 23 in Figure 14. Ettringite formation is favorable at young ages due to the initial high sulfate concentrations [109]. However, as sulfate depletion occurs and dissolved aluminum concentrations increase as hydration continues, monosulfoaluminate increases at the expense of ettringite [122,123].

Sulfate concentrations can be lowered by raising the concentrations of carbonate and aluminum in the pore solution, while the hemicarboaluminate breaks down [124,125]. Over time, the SO₃/AL₂O₃ ratio may drop as a result of silica fume hydration. As a result, the silica fume may hydrate and produce increasing levels of monosulfoaluminate; this has been shown in samples made using ground-up fuel ash [126].

Portlandite content was generally similar in irradiated and control samples. There was a slight increase in the amount of portlandite in the irradiated samples [127].

The thermodynamically favorable pathways for carbonation in cements, C-S-H, are reduced when clinker ratios are decreased, and calcium concentration is reduced when there is less carbonate and less portlandite [128].

The peak intensities of calcite appear to be slightly affected by radiation exposure [129,130,131]. There was an increase in calcium carbonate concentration following gamma irradiation, which is consistent with higher carbonation. Additionally, the concentration of the portlandite phase was found to decrease following gamma irradiation, which is consistent with the findings of increased calcite (Figure 14c) [132].

The carbonation reaction accelerates in the presence of radiation [117,133]. Concrete’s characteristics are affected by carbonation in a variety of intricate ways. Porosity is increased during the reaction when portlandite is consumed, and it is decreased when calcite is deposited (Figure 14) [112,113]. Thus, factors other than radiation-induced carbonation determine concrete’s strength and bearing capacities. Along with natural carbonation, radiation-induced carbonation influences the whole irradiated material rather than only its surface. The conversion of portlandite to calcite is sped up under gamma irradiation, suggesting a quicker carbonation process [117]. The microstructure of concrete is altered by carbonation, which can negatively impact water transport properties—an essential aspect of durability [134]. Due to carbonation, the pH level around reinforcement decreases, which can trigger corrosion and jeopardize the durability of concrete structures [135,136]. SCMs can affect the carbonation process, with materials such as silica fume promoting carbonation under UV radiation [137]. The pozzolanic response of low-calcium SCMs impacts the hydration of clinker by dramatically reducing the pH. This reaction depletes the pore solution and changes the morphology of the calcium-silicate-hydrate (C-S-H) gel, resulting in denser structures with less water availability for hydration [138]. According to studies, low pH concrete’s mechanical qualities are nonetheless on par with those of normal concrete, indicating that the long-term advantages of CH production may outweigh any initial hydration delays [56].

Space group, crystal cell type, and crystal hardness have a complex relationship that includes mechanical, structural, and symmetry characteristics. A crystal’s structural type and hardness are influenced by its space groups, which specify the symmetry and arrangement of its atoms. Crystals are categorized by space groups according to their symmetry operations, such as rotations and translations [139]. Different space groups correlate to different crystal structures with different mechanical properties, including layered materials and perovskites [140]. The type of crystal cell (e.g., cubic, hexagonal) impacts the arrangement of atoms and the bonding interactions, which are crucial for defining hardness [141]. The nature of atomic connections and the coordination number are frequently related to hardness; generally speaking, higher coordination results in harder materials [142]. To summarize briefly, space groups and crystal cell types strongly influence crystal hardness, but atomic bonds and environmental effects are also very important. This complexity suggests that a comprehensive knowledge of crystal properties requires the study of a large number of related elements.

In this research, 94 kGy radiation was limited to gamma, because concrete deteriorates more when the radiation exceeds about 105–107 kGy. This deterioration could damage the research institute’s structures [143].

In summary, regarding the sensitivity of the concrete to the 94 kGy gamma dose, although portlandite is consumed as a result of 94 kGy irradiation, the insignificant increase in portlandite density in XRD analysis shows that the improvement of the concrete continues [127] (Figure 14a–c). Under normal conditions, carbonation causes a decrease in the strength of concrete [144]. However, it was observed that although carbonation occurred in concrete exposed to gamma rays up to 94 kGy, an increase in hardness occurred [117].

In the XRD graphs, in the fire scenario at 900 °C, especially in Figure 14c, with the increase in silica smoke, the decrease in the intensity of quartz peaks is less compared to the sample with no smoke contribution in Figure 14a, and this is also reflected in the bending strength; bending strength, as seen in Figure 9a and Figure 10a, decreased by approximately 80%, but in SFC-10, it decreased by only 2% more [145].

In the XRD graphs, in the radiation scenario, samples subjected to 94 kGy gamma radiation showed an increase in quartz peak intensity and a decrease in calcite intensity; especially in Figure 14c, an increase in quartz peak intensity and an increase in calcite intensity were observed when compared to the sample with no silica fume contribution in Figure 14a. Additionally, the compressive strength and flexural strength increased by approximately 25%, as seen in Figure 9b and Figure 10b. The observed variations in the peak intensities of calcite and quartz in XRD graphs after exposure to gamma radiation demonstrate the minerals’ differential reactivity. In particular, at 94 kGy gamma radiation, the peak intensity of quartz increased while that of calcite decreased, indicating that quartz is more vulnerable to radiation-induced changes that result in increased reactivity and structural disordering [146].

Here, the increase in quartz density is related to the increase in silica fume contribution as well as gamma radiation. Calcite accumulation may be related to gamma radiation, and calcite formed as a result of gamma radiation differs from normal calcite formation in that it fills the voids in the concrete and pores in the concrete, which normally do not enhance the strength of the concrete and remain superficial. However, the ionization of calcite accumulation caused by gamma radiation may have altered the molecular lattice structure, resulting in overall hardening and increased strength [146,147].

4. Conclusions and Recommendations

Based on the current research, the summary of the results is as follows.

The compressive strength of silica-fume-substituted steel fiber concrete exposed to high temperature at 900 °C for 90 min SFC decreased by 78.99%, SFC-5 by 76.71%, SFC-10 by 76.72%, and SFC-15 by 76.05%, compared to 20 °C. In terms of flexural strength, SFC decreased by 79.44%, SFC-5 by 78.66%, SFC-10 by 79.68%, and SFC-15 by 80.11%. A dispersion occurred with the decrease in strength, but steel fibers prevented dispersion.

The flexural strengths of composite specimens exposed to gamma rays (94 kGy Cobalt 60) increased by 24.85% in SFC, 25.06% in SFC-5, 24.11% in SFC-10, and 23.65% in SFC-15.

The increase seen in all concrete samples with gamma radiation is thought to be due to the fact that the gamma rays penetrating the entire concrete accelerate carbonation and fill all the voids in the concrete with the calcite agglomeration seen in XRD analyses, and that the carbonation product formed hardens contrary to normal, and as a result, mechanical strengths have increased.

Considering the mechanical strengths, it can be said that the silica fume additive provides a small resistance of about 2% against the strength reduction at high temperatures, thus reducing crack formation.

XRD analyses show a small increase in silica density at 2θ ≈ 23° in the gamma-irradiated specimens, and a 5% difference in strength increase in the silica-fume-doped specimens compared to the undoped specimens.

The results demonstrate that silica fume is beneficial in reducing the porosity caused by 94 kGy gamma radiation and increasing the reactivity of the cement matrix, thereby improving compressive and flexural strength. Additionally, it has been observed that steel fibers play a synergistic, combined, and greater role in enhancing the thermal shock resistance of concrete, preventing dispersion, and improving mechanical integrity.

The study’s findings open the door for the use of mineral admixtures, including silica fume and fibers, in reinforced concrete applications, including hospitals, nuclear reactors, shelters made of reinforced concrete, and facilities for storing radioactive waste and radiation devices.