Durability and Radiation Shielding Performance of GGBS- and SF-Modified Mortar under High-Temperature Sulfate Exposure in Arid Environments

FatimaI.AlHamadi1,2

RemilekunA.Shittu1,2

MohsinaSherief2,3

AkramAlFantazi2,3

AhmedK.AlKaabi1,2

Mechanical and Nuclear Engineering DepartmentKhalifa University of Science and TechnologyUAE

2Emirates Nuclear Technology CenterKhalifa University of Science and TechnologyUAE

3Chemical Engineering DepartmentKhalifa University of Science and TechnologyUAE

Fatima I. AlHamadi^1,2, Remilekun A. Shittu^1,2, Mohsina Sherief^2,3, Akram AlFantazi^2,3, Ahmed K. AlKaabi^1,2

1. ^{Mechanical and Nuclear Engineering Department, Khalifa University of Science and Technology, UAE}.

2. ^{Emirates Nuclear Technology Center, Khalifa University of Science and Technology, UAE}.

3. ^{Chemical Engineering Department, Khalifa University of Science and Technology, UAE}.

Abstract

A durable and radiation-resistant concrete is crucial for NPPs and other nuclear facilities where radiation exposure is critical. Concrete in arid regions is subjected to elevated temperatures, external sulfate exposure, and aggressive chemical attacks. The study investigates the impact of elevated temperatures on concrete durability and radiation shielding properties, along with chemical sulfate exposures. To achieve this, experiments were conducted to assess mechanical properties, including compressive and flexural strength. Non-destructive techniques like resistivity and UPV were used as indicators for degradation of mortars. The attenuation coefficient was determined by utilizing Cs-137 and Co-60 radioactive sources along with a HPGe detector. The results showed a correlation between sulfate-induced deterioration and changes in both mechanical performance and shielding efficiency, with apparent variations depending on the sulfate solution type and exposure conditions. At photon energy of 1173 keV, the results indicated that samples with SF and GGBS exhibited increases in attenuation coefficient of approximately 23%, 15%, and 4% after exposure to Na₂SO₄, MgSO₄, and Na₂SO₄ + NaCl, respectively, at 50°C for 120 days. While Ordinary samples showed a reduction of approximately 7%, 10% and 10% under the same conditions. This research supports advanced concrete design for durable nuclear infrastructure applications.

Keywords:

linear attenuation coefficients

ettringite

gypsum

gamma emission

containment buildings

durability

List of Abbreviations

Abbreviation

Full Term

C–A–S–H

Calcium Alumino-Silicate Hydrate

Calcium Hydroxide

Co-60

Cobalt-60

Cs-137

Cesium-137

C–S–H

Calcium Silicate Hydrate

DEF

Delayed Ettringite Formation

HPGe

High Purity Germanium Detector

GGBS

Ground Granulated Blast Slag

HPGe

High-Purity Germanium

HVL

Half-Value Layer

TVL

Tenth-Value Layer

ITZ

Interfacial Transition Zone

MCA

Multichannel Analyzer

MgSO₄

Magnesium Sulfate

M–S–H

Magnesium Silicate Hydrate

NaCl

Sodium Chloride

Na₂SO₄

Sodium Sulfate

NPP

Nuclear Power Plant

Ordinary Mortar Sample

SEM

Scanning Electron Microscopy

Silica Fume

Silica Fume and GGBFS Modified Mortar Sample

UPV

Ultrasonic Pulse Velocity

1. Introduction

Concrete has long been regarded as a cornerstone material in nuclear power plant (NPP) constructions, including radioactive waste repositories and spent fuel storage facilities. Concrete is preferred due to its cost-effectiveness, mechanical strength, and its excellent capacity to attenuate ionizing radiation [1]. Its role is not limited to structural support, but it also serves as a critical radiation barrier that shields and protects both personnel and the environment from radiation exposure over the operation years. Concrete shielding structures should maintain mechanical and radiation attenuation properties over service lives extending beyond 60 years, as per the International Atomic Energy Agency standards [2].

The shielding performance of concrete is primarily affected by its density, atomic composition, thickness, and internal microstructural integrity [3]. However, any degradation of the physical structure of concrete or the material density would compromise the shielding effectiveness, which would result in violating the dose limits established for nuclear facility safety. Thus, maintaining the long-term shielding efficiency of concrete is crucial for the sustainability of nuclear infrastructure, as well as ensuring the concrete's mechanical durability over time.

The radiation attenuation properties of concrete directly depend on its internal microstructure. Where ideal shielding concrete is required, it must be dense, homogeneous, and free of large pores and microcracks. However, that is not the case in reality; instead, constructors aim to maximize shielding efficiency. Any increase in porosity or crack network formation would reduce the mass per unit area available for interaction with gamma photons, which will lower the linear attenuation coefficient [4]. Proceeding, interconnected pores might form a preferable pathway for radiation transmission that will not attenuate photons as required, undermining the uniform shielding barrier. Thus, if microstructural degradation occurs, it will not only reduce the mechanical strength of the concrete structure but also reduce the radiation protection offered by the concrete [4].

A sulfate attack on concrete is a chemical process that degrades concrete, resulting in sulfate ions (

$\:\text{S}\text{O}₄²⁻$

) from the external environment permeating into the concrete matrix. These ions mainly enter through the capillary pores and microcracks in the hardened cement paste. Subsequently, sulfate ions interact with the primary hydration products of cement. The first reaction occurs with Portland cement's tricalcium aluminate (

$\:\text{C}₃\text{A}$

) component, and ettringite (

$\:\text{C}₃\text{A}·3\text{C}\text{a}\text{S}\text{O}₄·32\text{H}₂\text{O}$

) is formed in the presence of calcium ions and water. This reaction can be generalized as follows [5–7]:

$\:\text{C}₃\text{A}\:+\:3\:\:\text{C}\text{a}\text{S}\text{O}₄·2\text{H}₂\text{O}\:+\:26\:\text{H}₂\text{O}\:\to\:\:\:\text{C}₃\text{A}·3\text{C}\text{a}\text{S}\text{O}₄·32\text{H}₂\text{O}$

Besides ettringite formation, sulfate ions react with calcium hydroxide [

$\:\text{C}\text{a}\left(\text{O}\text{H}\right)₂$

], a typical product of cement hydration, to produce gypsum (

$\:\text{C}\text{a}\text{S}\text{O}₄·2\text{H}₂\text{O}\:$

) [5–7]:

$\:\:\text{C}\text{a}\left(\text{O}\text{H}\right)₂\:+\:\text{S}\text{O}₄²⁻\:+\:2\:\text{H}₂\text{O}\:\to\:\:\:\text{C}\text{a}\text{S}\text{O}₄·2\text{H}₂\text{O}\:+\:2\:\text{O}\text{H}⁻$

Both ettringite and gypsum have higher molecular volumes than the initial substances[8] ; therefore, they induce expansive stresses in concrete that may result in cracking, debonding, and spalling.

The transport of sulfate ions takes place by diffusion processes, which are usually dependent on the microstructure of the concrete. According to Fick’s law of diffusion sulfate ions are transported as a function of the connectivity, size, and distribution of the pores in the material[9]. The increased porosity and higher permeability result from a high water-cement ratio, inadequate curing, or microcracks caused by shrinkage or thermal stresses[10]. Various environmental factors and material properties affect the severity of a sulfate attack, as indicated by Zhang et al. [10]. The concentration of sulfate ions in the environment – including from marine water or sulfate-bearing soil- determines the severity of the attack. At elevated temperatures, ettringite and gypsum formation rates are increased, and the diffusion rates are enhanced[10]. Furthermore, the pH of the concrete pore solution is an essential factor; fresh concrete has a high pH (above 12), which supports the stability of the hydration products. However, the pH decreases over time through processes such as carbonation, and thus, the concrete becomes more vulnerable to sulfate-induced reactions.

Research results generally show that concrete specimens that come into contact with sulfate-rich environments suffer a loss of compressive as well as tensile and flexural strength that ranges between 20% to 40% or more than non-exposed well-cured control specimens [11–14]. For instance, a study performed by Yu et al. [13] studied the effect of sulfate concentrations of 5% and 15% on the mechanical properties of mortars[13]. The results showed that compressive strength peaked after about 120–150 days of exposure, followed by reductions of roughly one-quarter to one-third and about one-third to nearly one-half compared with the peak values. Overall, Yu et al. [13] characterized the property evolution in two distinct stages:

1. The initial stage where sulfate reacts and fills the pores with ettringite and gypsum increases the mechanical properties: compressive strength by ~ 32–69%, static elastic modulus by ~ 21%, peak stress by ~ 31–37%, and hardened density by ~ 1–2%.

2. In the second stage, the significant expansion of gypsum and ettringite causes cracks, reducing their mechanical properties.

Also, the study by Yu et al. implemented SEM-EDS and showed the effect of sulfate attack of concentration 5% and 10% on the microstructure of mortars [13]. It was noted that gypsum formed from calcium hydroxide crystals, and significant C–S–H crystal formations were noted at the specimen’s surface. Air voids and pores in the mortar were found to fill with clusters of needle-like ettringite crystals and gypsum. This makes the material's properties at this stage better, more compact, and less permeable to moisture transport. However, when the exposure time was increased to 270 days, needle-shaped ettringite crystals grew in abundance, and gypsum showed vertical growth on the surface of the specimen, while micro-cracking throughout the sample was extensive. Consequently, the interparticle bonding within the hardened mortar was weakened, leading to a degradation of its overall properties.

Seawater usually contains elevated levels of sulfate ions (SO₄²⁻) and elevated amounts of chloride ions (Cl⁻). Figure 1 shows the chemical composition of seawater [15]. A realistic exposure condition in arid coastal environments will include chloride-sulfate interactions. Chloride ions and sulfate ions coexisting yield a wide range of secondary products, such as Ettringite, Gypsum, Thenardite, Halite, and Friedel’s salt [16]. The accumulation of these products would cause surface scaling, microcracking, and spalling. These deformations would serve as preferential pathways for radiation, compromising shielding performance.

Fig. 1

Chemical composition of seawater, data retrieved from [15].

Another severe exposure is Magnesium sulfate attack which represents another chemically aggressive form of sulfate deterioration. As Mg²⁺ ions react with calcium hydroxide to form gypsum and react with the principal binding phase, calcium silicate hydrate, which is subsequently transformed into magnesium silicate hydrate (M–S–H) and Brucite (Mg(OH)₂) [17]. M-S-H gel has a weak, non-cementitious structure, which weakens the entire structure, Brucite, on the other hand, lowers the pore solution pH, thus leading to continued C-S-H destabilization and matrix disintegration.

Comparing magnesium sulfate attack to sodium sulfate attack, ettringite formation is less favorable in magnesium sulfate attack, especially at elevated temperatures. However, magnesium sulfate attack cause extensive decalcification of hydrates and softens the cement matrix. The formed degradation causes increased porosity, mass loss, and mechanical instability of the attacked concrete structure [17, 18]. Nevertheless, little is known about how these transformations affect radiation shielding.

Sulfate-induced concrete degradation is a critical issue in structures exposed to high-sulfate-content environments, especially those in nuclear power plants. Sulfate attack affects the mechanical properties of concrete, as well as the radiation shielding properties of concrete [1]. A paper by Akkurt et al. examines the influence of sulfate attack on the linear attenuation characteristics of concrete where experiments expose barite-containing concrete to various photon energies of sodium sulfate for several months. The results indicate a significant decrease in the concrete’s attenuation coefficient[19].

Another study by Khatri et al. confirms that sulfate attack on concrete affects its density [20]. Since the linear attenuation property of concrete is greatly affected by its density, sulfate exposure diminishes the concrete’s radiation-shielding properties. A study by Wu et al. suggests that sulfate attack on concrete leads to the weakening of interfacial transition zones (ITZ) around aggregates, where gypsum is preferentially deposited [21]. Exposure to sulfate for several months resulted in increased degradation and ITZ volume content, leading to a reduction in density and uniformity in concrete structures. This also shows that sulfate exposure reduces the radiation-shielding properties of concrete.

Meanwhile, Mobasher discusses the positive effect of adding sodium sulfate to the concrete mixing process [22]. Sodium sulfate can be used as an activator in blast furnace slag-based concrete, utilizing Ba(OH)₂, which enhances sulfate resistance. Sodium sulfate reacts with blast furnace slag to form stable calcium-aluminosilicate-hydrate and insoluble salts, such as barium sulfate (BaSO₄). BaSO₄ has a high atomic number, thereby strengthening the matrix and limiting sulfate-induced damage over the long term. Also, it immobilizes the sulfate-containing waste from radioactive waste [23]. The use of sodium sulfate for controlled activation increases the density of the matrix, which improves the radiation-shielding properties. Thus, optimizing sodium sulfate resistance not only improves durability but also supports the development of concrete with superior shielding properties [22].

Supplementary cementitious materials (SCMs), such as fly ash, silica fume, and ground granulated blast furnace slag (GGBS), enhance the sulfate resistance of concrete by lowering its permeability and refining its microstructure. This improvement arises from the pozzolanic reaction of these materials with calcium hydroxide during cement hydration, which generates additional calcium silicate hydrate (C–S–H) that densifies the matrix by filling voids within the concrete structure [24]. As a result, the sulfate ions will penetrate less, thereby minimizing their ingress. Optimal cement replacment by fly ash falls between 15% and 40% [25]. While optimal replacement rates of GGBS range from 40% to 70% [26]. However, silica fumes require the addition of superplasticizers to improve workability.

Studies have shown that using fly ash has significantly reduced the expansion and cracking of concrete caused by sulfate attack [27]. Similarly, slag cement demonstrated its effectiveness in reducing sulfate attack behavior due to its lower C₃A content [28]. Concrete matrix densification has been further enhanced due to the pozzolanic activity and the fine particle size of silica fume [29], which provides better protection against sulfate attack [30]. Additionally, the denser microstructure achieved with mineral admixture materials supports better maintenance of the radiation shielding capacity by preserving material density and reducing microstructural discontinuities[31]. Nonetheless, current studies did not quantify how much microstructural preservation translates into shielding performance, leaving a critical research gap in evaluating the dual functional durability of radiation-shielding mortars under aggressive chemical exposure.

The harsh environment surrounding the Barakah NPP in the UAE represents a critical case study. Where high ambient temperatures of up to 50°C in summer, elevated sulfate concentrations in groundwater and soil, and low annual rainfall cause salt accumulation and cyclic wetting and drying effects on surfaces [32–34]. These conditions accelerate the kinetics of sulfate attack and microstructural degradation. Such alterations may influence both the mechanical performance and radiation shielding capacity of concrete structures. Preserving the integrity of concrete, particularly with respect to its shielding efficiency, is essential to guarantee the safe and sustainable operation of the UAE’s first commercial nuclear power plant throughout its service life.

Previous studies have examined the impact of sulfate attack on the mechanical properties of concrete, as well as other studies that have optimized radiation shielding. However, critical gaps remain, as no studies have evaluated the impact of sulfate-induced chemical degradation on gamma-ray shielding efficiency or the combined effects of realistic environmental exposure conditions, such as those presented at Barakah NPP. This gap requires integrated experimental studies that simultaneously assess both mechanical deterioration and radiation performance.

To the best of the authors' knowledge, this study presents the first systematic experimental investigation correlating sulfate-induced degradation with reductions in gamma-ray shielding performance in cementitious materials. Unlike previous works focusing exclusively on mechanical properties or shielding optimization, this research integrates chemical durability and radiation attenuation assessments under field-representative sulfate exposure conditions, providing novel insights for nuclear infrastructure durability management. To achieve this aim, the study seeks to evaluate the mechanical (compressive and flexural strength) and non-destructive (UPV, resistivity, density) properties of mortar exposed to sulfate attack and sulfate combination with magnisum and chloride at elevated temperature, quantify changes in gamma-ray shielding efficiency (linear attenuation coefficient) post degradation,characterize microstructural evolution using SEM-EDS and investigate the effect on geopolymer mortars on enhancing the dual durability and shielding in sulfate-aggressive environments.

2. Materials and Methods

2.1. Mortar and Concrete Preparation

2.1.1. Characterization of raw materials

The binder materials used in this study were Type I Ordinary Portland Cement (OPC), silica fume (SF), and ground granulated blast furnace slag (GGBFS). The SF consisted of ultrafine particles with sizes in the range of 0.2–0.3 µm.To investigate the phase composition of these binders, X-ray diffraction (XRD) analysis was conducted. Where each sample was scanned for diffraction angles (2θ) and the resulting patterns are presented in Fig. 2.

The cement sample displayed several well-defined sharp peaks. The most prominent peak is located at around 32°, indicative of its crystalline nature and complex phase composition. The cement was identified to contain crystalline compounds such as Alite (Ca₃SiO₅, JCPDS 49–0442), Belite (Ca₂SiO₄, JCPDS 33–0302), Tricalcium Aluminate (C₃A, JCPDS 38-1429), and Tetra-calcium Aluminoferrite (C₄AF, JCPDS 40–0218), after comparing these peaks to standard reference cards. During cement hydration, these phases govern the reactions that contribute significantly to strength gain.

In contrast, the XRD patterns of GGBS and SF were characterized by broad humps centered around 30° and 22°, respectively. That reflects their predominantly amorphous nature. The broad halo in the GGBS spectrum corresponds to JCPDS 35–0610, confirming its glassy structure typical of slag-based materials. Similarly, the dominant hump in the SF profile was consistent with the presence of amorphous silica (SiO₂), matching JCPDS 46-1045.

Fig. 2

the XRD profile of the three cementitious binders used in mortar preparation.

The aggregates employed in casting the mortar samples consisted of dune sand (DS) and crushed sand (CS), with water absorption capacities of 0.7% and 1.2%, respectively. The specific gravities of DS and CS were 2.69 and 2.63, respectively. The physical and chemical characteristics of Type I cement, silica fume (SF), and ground granulated blast furnace slag (GGBS) are provided in Table 1, while Table 2 summarizes the physical properties of the aggregate.

Table 1
Summary of chemical and physical characteristics of the cementitious materials.
			SF		GGBS
Chemical Composition	C₃S	70.1	SiO₂	86	SiO₂	35
	C₃A	7.42	S	0.76	CaO	40
	SO₃	2.1	Total alkalis	2.4	Al₂O₃	15
	MgO	1.35	Slag activity (7 days)	124	SO₃	0.3
	Na₂O + K₂O	0.62	LOI	3.7	Total alkalis	0.44
	Insoluble residual	0.62	Moisture	0.3	Slag activity (7 days)	86
	LOI	1.5	Moisture	0.3	LOI	0.8
Physical properties	Density (g/cm³)	3.12	Density (g/cm³)	2.16	Density (g/cm³)	2.86
Physical properties	Grain size (cm²/g)	3600	Specific surface (m²/g)	23.9	Grain size (cm²/g)	4800

Table 2
Summary of the physical characteristics of the aggregates.
	Crushed Sand	Dune Sand	Coarse Aggregates
Specific gravity	2.69	2.63	2.69
Absorption (%)	1.2	0.7	0.6

2.1.2. Casting of mortar and concrete samples

The mix design for the mortar and concrete is displayed in Table 3. The mortar samples were cast according to the specifications of EN 196-1 [35]. After casting, specimens were vibrated to remove entrapped air, demolded after 24 hours, and water-cured at 25°C for 28 days. Specimens were cast into different molds based on the proposed testing procedure:

50 × 50 × 50 mm cubes for compressive strength testing and density measurments,

40 × 40 × 160 mm prisms for flexural strength, and non-destructive testing,

100 × 100 × 100 mm cubes for gamma radiation attenuation tests.

For the radiation experiment, the 100 mm cube samples were sliced into approximately 10 mm sizes after curing.

Table 3
Mix design for mortar and concrete samples.
	Mass (kg/m³)
Mix code	Water	Cement	GGBS	SF	Dune sand	Crushed Sand	HWRA
O	146	376	-	-	230	706	4.578
SG	146	113	19	244	230	706	4.578
All samples have same water-binder ratio of 0.40. Notes: GGBS: ground granulated blast furnace slag; SF: silica fume; HWRA: High water reducing admixture; O: Ordinary mortar cube/beam; SG: SF & GGBS mortar.

2.2. Exposure Conditions

Following the initial 28-day curing period, the mortar specimens were subjected to sulfate attack through full immersion in sulfate solutions under controlled conditions. Four exposure regimes were employed:

S2F: Specimen were fully immersed in a 10% sodium sulfate (Na₂SO₄) solution maintained at 25°C,

S5F: Specimen were fully immersed in a 10% sodium sulfate (Na₂SO₄) solution maintained at 50°C,

M5F: Specimen were fully immersed in a 10% magnesium sulfate (MgSO₄) solution maintained at 50°C,

CS5F: Specimen were fully immersed in a mixed solution of equal ratios of 10% sodium sulfate and 10% sodium chloride (NaCl) maintained at 50°C.

All solutions were prepared using analytical-grade salts and deionized water. To maintain consistent chemical concentrations, the immersion solutions were renewed every 30 days. The immersion period was maintained for 4 months, providing sufficient time under accelerated conditions to assess both mechanical and radiation shielding performance degradation.The current experimental work was conducted using an accelerated test with a high concentration of sulfate and chloride solution (10% w/v), which exceeds real-life concentration levels. Although the mechanism of ESA remains largely unchanged, the kinetics of the reaction is increased. Due to time constraints in exposing the samples at actual concentrations obtainable in the environment, accelerated testing remains a viable option. The laboratory experiment will take years before any significant effect of sulfate attack is noticed if the actual concentration of the salt in the real environment is simulated.

2.3. Mechanical Testing

Mechanical properties were evaluated before and after sulfate exposure. Compressive strength was determined in accordance with ASTM C109/C109M [36] using 50 mm cube specimens. The tests were conducted on a universal testing machine at a loading rate of 1 mm/min until failure, and the reported results represent the average of three specimens for each condition. Flexural strength was evaluated in accordance with ASTM C348 [37] using 40 × 40 × 160 mm prism specimens subjected to three-point bending. The tests were performed on a universal testing machine under displacement-controlled loading at a rate of 0.05 mm/min. During testing, load-displacement data were continuously recorded, and flexural stress-strain curves were derived. Compressive and flexural strength tests were performed at 28 days (prior to exposure) and subsequently at monthly intervals for up to four months of sulfate exposure.

Fig. 4

Setup for the (a) compressive testing and (b) flexural testing

2.4. Non-Destructive Testing

Non-destructive testing was employed to evaluate the internal microstructural integrity of the specimens without causing damage. Ultrasonic Pulse Velocity (UPV) measurements were carried out in accordance with ASTM C597 [38] using a portable UPV tester. Measurements were taken along the longitudinal axis of each specimen to identify internal defects and track the progression of microcrack development. Electrical resistivity measurements were performed using a four-probe Wenner array resistivity meter by AASHTO T358 standards [39]. Electrical resistivity served as an indicator of pore structure, microcrack connectivity, and changes in permeability resulting from sulfate attack. Bulk density was measured in accordance with ASTM C642 [40] by recording the mass and geometric dimensions of the specimens. Variations in density provided indirect insights into internal deterioration and the evolution of porosity. Each non-destructive test was repeated 3 times and mean values were reported.

Fig. 5

Setup illustration for (a) Resistivity and (b) UPV

2.5. Radiation Shielding Testing

To determine the gamma attenuation coefficient of different concrete samples, an experiment was designed using the HPGe detector, a holder, and two radioactive sources, Cs-137 and Co-60; details are summarized in Table 4.These sources were used due to their well-characterized emission spectra and covering a good enough wide range of gamma energies. The HPGe detector is interfaced with a multichannel analyzer (MCA) for spectral data acquisition and cooled by liquid nitrogen to reduce the thermal noises which enhances high resolution. The background radiation was recorded prior to each measurement to correct for ambient gamma radiation effects.

Table 4
Radioactive sources and their properties
Radioactive source	Energy peak (keV)	Activity	$\:{\mathbf{T}}_{1/2}$ (Half life)	Sources
Cs-137	662	5 $\:{\upmu\:}\text{C}\text{i}$ since Aug 2011	30.07 years
Co-60	1173, 1330	1 $\:{\upmu\:}\text{C}\text{i}$ since Aug 2011	5.27 years

The intensity of transmitted gamma radiation (

$\:\text{I}$

) through the samples was measured at different thicknesses (

$\:\text{x}$

) to get the attenuation coefficient. Also, the detector was calibrated using a standard source before the measurements. First, the initial gamma-ray intensity (

$\:{\text{I}}_{\text{o}}$

) is recorded without samples facing the beam path. Second, concrete samples were placed in the collimated gamma-ray path, and the transmitted intensity (

$\:\text{I}$

) was measured by the HPGe detector, as shown in Fig. 6. This was repeated for three slabs of the same concrete sample, a total of 3 cm thick. Each of the measurements was set for 5 minutes each to record the counts in the photopeak region; enough to see the full peak of each 662 keV, 1173 keV, and 1330 keV. Each measurement was repeated 3 times to ensure consistency, and the average value was used for further analysis.

The linear attenuation coefficient was then determined using the beer-lambert law as shown in Equation x:

$\:\text{I}\:=\:{\text{I}}_{\text{o}}\:{\text{e}}^{-{\upmu\:}\text{x}}$

where,

$\:\text{I}\:$

is the transmitted intensity,

$\:{\text{I}}_{\text{o}}$

is the initial intensity,

$\:{\upmu\:}\:$

is the linear attenuation coefficient (

$\:{\text{c}\text{m}}^{-1}$

$\:\text{x}\:$

is the sample thickness (

$\:\text{c}\text{m}$

). Then, by plotting

$\:\text{l}\text{n}(\text{I}/{\text{I}}_{\text{o}}\:)$

versus thickness, the attenuation coefficient

$\:{\upmu\:}$

will be obtained as the slope of the resulting linear regression.

Fig. 6

Setup for the radiation shielding testing

2.6. Microstructural Analysis

Microstructural investigations were conducted to characterize the internal changes induced by sulfate attack using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). Gold-coated powdered samples were examined using SEM-EDS to visualize crack networks, observe sulfate reaction products, and map elemental distributions. These microstructural analyses provided essential insights into the chemical and physical deterioration mechanisms affecting both mechanical and radiation shielding performance.

3. Results and Discussions

This work results will study the effects of exposure conditions ,elevated temperature and chemical attacks on mechanical properties (section 3.1), non-destructive parameters (section 3.2), microstrucural defects (section 3.3) and radiation shielding properties (section 3.4); while comparing geopolymer mortars enhanced with SF and GGBS (symboled: SG) to ordianry sample (symboled: O).

3.1. Effect of Enhanced Mortars by SF and GGBS on Mechanical Properties

The compressive strength results are shown in Fig. 7 to illustrate the impact of replacing part of the cement in the ordinary mortars ( O sample) by Silica Fume and GGBS (SG sample). Before exposure to sulfate environment(i.e., 0 days in Fig. 7), SG mortars showed higher compressive strength than ordinary mortars. The improvement in SG mortars is due to continued pozzolanic reactions, which cause further C-S-H gel formation, which refines the microstructure and improves strength over time [41]. Overall, both ordinary and SG mortars showed an increase in compressive strength over 4 months under all conditions.

Fig. 7

Compressive strength results of SG samples compared to ordinary samples that study (a) temperaure effect on Sodium Sulfate attack, (b) ion effect on sulfate attack at elevated temperature 50^oC.

Meanwhile Fig. 7a demonstrate the impact of elevated temperature on sulfate exposure over 30, and 120 days, compared to room temperature condtions. Sulfate attack at elevated temperature have minor impact on compressive strength of SG mortars, while bit more on ordinary mortars. Notably, compressive strength is maximum after 30 days of sulfate exposure regardless of temperaure impact. This is due to the developed ettringite and gypsum filling the pores, making the microstructure denser, but not enough to generate cracks and weaken the structure as the drop in compressive strength after 120 days of exposure.

Also, Fig. 7 (b), illustrate how the sulfate attack with Sodium, Magnisum and Chloride ions at elevated temperaure of 50^oC affected the compressive strength after 4 months of exposure. SG samples under MgSO₄ exposure after 120 days had slight reduction in compressive strength; due to C-S-H degradation and brucite formation [17]. Where Mg²⁺ ions react with calcium hydroxide (CH) and C-S-H, leading to the formation of non-cementitious phases (brucite Mg(OH)₂) and gypsum, that weakening the cement matrix and decreases the compressive strength. Obviously, this scenario is less dominant in ordinary moratrs. The maximum compressive strength was observed after 120 days of NaCl + Na₂SO₄ exposure for both ordinary and SG mortars, which is due to the chloride ions that may have stabilized some sulfate reactions, preventing excessive ettringite formation and reducing expansive damage [42–44].

Fig. 8

Flexural results of SG samples compared to ordinary samples that study temperaure effect on Sodium Sulfate attack over 30 and 120 exposure days. (a) Ordianry sample, (b) SG sample

Fig. 9

Flexural results of SG samples compared to ordinary samples that study ion effect on sulfate attack at elevated temperature 50^oC over 120 days. (a) Ordianry sample, (b) SG sample

The flexural performance are presented in Figs. 8 and 9, which show the load versus extension curves for each exposure condition, comparing the behavior of the ordinary and SG samples. Before exposure, SG and ordinary mortars showed similar peak loads. As illustrated in Fig. 8, after 30 days, both ordinary and SG samples exhibit an increase in flexural strength at both temperature regime, indicating early pore-filling or ettringite formation that temporarily strengthens the structure, which supports the compressive strength results. Another indication is that ordinary mortars better tolerate early-age high-temperature exposure with less mechanical degradation compared to SG samples over 120 days of exposure.

Figure 9 discusses the effect of different aggressive environments on flexural strength over 120 days for ordinary mortars (Fig. 9a) and SG mortars (Fig. 9b). Surprisingly, MgSO₄ gave the best flexural peak for the ordinary mortars, which could be due to brucite formation and internal pore-filling. Then, the mortars were exposed to NaCl + Na₂SO₄, and the samples with Na₂SO₄ had less flexural strength, respectively, but still higher than the non-exposed samples. Figure 9b suggests that all ion-exposed SG samples performed equal to or better than the unexposed sample, proving the durability-enhancing effect of silica fume and GGBFS. At 50°C, SG samples exposed to combined NaCl + Na₂SO₄ ions exhibited the highest flexural strength (~ 5.1 kN), although the failure was abrupt and brittle, suggesting internal microcracking. MgSO₄ exposure produced the most stable and ductile behavior with a slightly enhanced peak load (~ 3.9 kN), while Na₂SO₄ showed only mild degradation.

3.2. Influence of Silica Fume and GGBS on the Durability Performance of Mortars

Figure 10 presents resistivity results, a non-destructive indicator of concrete durability that particularly reflects pore structure, moisture content, and ion penetration resistance. SG mortars exhibited high resistivity compared to ordinary samples, regardless of the exposure conditions, due to their denser microstructure, resulting from pozzolanic activity and lower porosity.

Fig. 10

Resistivity results of SG samples compared to ordinary samples that study (a) temperaure effect on Sodium Sulfate attack, (b) ion effect on sulfate attack at elevated temperature 50^oC.

As indicated by Fig. 10a, at low temperatures, ordinary mortars exhibit low resistivity throughout aging, with minor reductions over time, which suggests high porosity and severe ion ingress. At the same time, SG mortars start with relatively high resistivity and decrease moderately over 120 days, still excellent durability, despite sulfate exposure. At elevated temperatures, ordinary mortars exhibit further reduced resistivity, confirming the acceleration of sulfate ingress at elevated temperatures. SG mortars begin high and increase even more at 120 days, indicating better pore structure retention even at 50°C. Thus, the SG matrix remains durable and relatively impermeable, confirming the effectiveness of silica fume and slag in resisting ion penetration and microstructural degradation.

Figure 10b shows that SG mortars consistently outperform ordinary mortars across all ionic conditions at 50°C over 120 days of exposure. The Na₂SO₄ is the only environment that is least damaging. MgSO₄ and NaCl + Na₂SO₄ combinations are more aggressive, especially for ordinary mortars, which show extremely low resistivity. Even under harsh chloride–sulfate synergy, SG still demonstrates strong ion-blocking capability. The data highlight the importance of SCMs in preserving microstructural integrity against combined thermal and chemical attack.

Fig. 11

UPV results of SG samples compared to ordinary samples that study (a) temperaure effect on Sodium Sulfate attack, (b) ion effect on sulfate attack at elevated temperature 50^oC.

Ultrasonic Pulse Velocity (UPV) test results are summarized in Fig. 11, that’s another non-destructive technique used to evaluate internal integrity, and microcracking of mortars. The UPV results indicate that ordinary samples consistently maintain higher and more stable ultrasonic pulse velocities than SG mortar under sulfate exposure, regardless of temperature or ion type. The chemical exposure have decresed the UPV values in general, due to the cracks formation by the expansive products. Especially, for SG mortars exposed to MgSO₄ which agrees with the previuos results of brucite and gypsum formation that weaken the structure internally.

Density measurements revealed that SG samples consistently possessed higher bulk densities than ordinary mortar, both initially and after sulfate exposure as shown in Fig. 12. Figure 12a indicated that temperature increases have limited negative effect on density. SG mortars showed more pronounced changes, particularly under magnesium sulfate exposure, which may reflect less stable internal structures, as indicated by Fig. 12b.

Fig. 12

Density results of SG samples compared to ordinary samples that study (a) temperaure effect on Sodium Sulfate attack, (b) ion effect on sulfate attack at elevated temperature 50^oC.

3.3. Effect of Silica Fume and GGBS on Gamma Radiation Shielding Performance

In this section, the results presented evaluate the radiation shielding performance of ordinary and SG mortar samples in terms of their linear attenuation coefficients (µ, cm⁻¹), Half Value Layer (HVL) and Tenth Value Layer (TVL) before and after exposure to aggressive environments for 4 months. The results were analyzed at different photon energies (662 keV, 1173 keV, 1333 keV).

Fig. 13

Linear attenuation coefficnet results of SG samples compared to ordinary samples that study temperaure effect on Na₂SO₄ Sodium Sulfate attack

The attenuation coefficient of SG and ordinary mortars as a function of the photon energy is illustrated in Fig. 13. It is noted that before exposure, ordinary mortars exhibit a slightly higher attenuation coefficient across all energy levels compared to SG mortars. Exposing the ordinary mortars to Na₂SO₄ at 25^oC, have reduced the attenuation coefficent significantly over time. While higher attenuation was noticed by elevated temperaure exposure over time compared to lower temperature, still less than non-exposed ordinary mortars; suggesting microstructural deterioration. SG mortars at 25^oC and 50^oC have outperformed the ordinary samples of same exposure conditions. SG samples retain superior attenuation coefficients, especially at high temperatures, even after prolonged exposure. highlighting the stability and densification benefits provided by silica fume and GGBFS under aggressive chemical and thermal conditions. The enhanced matrix of SG mortars minimizes porosity and microcrack formation, preserving their radiation shielding capabilities over time[45].

Fig. 14

Linear attenuation coefficnet results of SG samples compared to ordinary samples that study ion effect on sulfate attack at elevated temperature 50^oC, at different energy levels (a) 1333 keV (b) 1173 keV (c) 663 keV.. (a) 1333 keV, (b) 1173 keV, (c) 663 keV

The results in Fig. 14a show the high energy of 1333 keV, which exhibits a similar trend to 1173 keV results in Fig. 14b. SG samples maintain or increase their attenuation coefficient, while OA samples slightly decrease after exposure. The percentage change in attenuation coefficent after 120 days at elevated temperature is summarized in Table 5. However, in both energies, 1173 and 1333 keV, attenuation results have increased after MgSO₄ exposure. MgSO₄ effect porosity, leading to pore refinement over time, or due to the secondary hydration products formed in the presence of Mg²⁺ which could have led to a denser structure, contributing to higher gamma-ray absorption. Moreover, the combined NaCl + Na₂SO₄ exposure does not show a significant change. Figure 14c show the attenuation coefficients at lower photon energy of 662 keV, that showed higher values for both SG and ordinary mortars. This is due to increased interaction probability.

Table 5
Percentage change in linear attenuation coefficient (µ) for Ordinary (O) and SG mortar samples after 120 days of sulfate and mixed-ion exposure at 50°C, across three photon energy levels (1333 keV, 1173 keV, and 663 keV). Positive values indicate improvement; negative values indicate reduction.
Exposure Conditions	1333keV		1173keV		663keV
Exposure Conditions	O sample	SG sample	O sample	SG sample	O sample	SG sample
Na₂SO₄ at 50^oC	-0.5%	2.6%	-7.3%	23.9%	0.6%	4.6%
MgSO₄ at 50^oC	-7.3%	15.6%	-9.3%	14.8%	-9.4%	7.5%
Na₂SO₄+NaCl at 50^oC	-14.2%	3.6%	-9.9%	4.0%	-13.6%	-3.8%

These results show that SG samples after exposure exhibit consistently higher attenuation coefficients across all conditions at 120 days, which was not the case in the initial measurements before exposure. While ordianry mortars show a minor decline in attenuation coefficient, further supporting that aggressive conditions affected their microstructure more significantly. Moreover, although the tested samples were mortars, their attenuation coefficient values still fall within the expected range for ordinary concrete used in NPP, 0.05 cm⁻¹ to 0.20 cm⁻¹[46]. This indicates that mortar based mixtures, particularly those enhanced with mineral admixtures, can achieve attenuation performance comparable to conventional concrete in nuclear applications.

Table 6
HVL (cm) before and after exposure conditions for SG and OA sample for 30 and 120 days.
Exposure Conditions	Exposure Days	Ordinary samples			SG samples
	Exposure Days	1333keV	1173keV	663keV	1333keV	1173keV	663keV
	0 Days	4.827	4.661	3.558	4.878	5.200	3.597
Na₂SO₄ at 50^oC	30 Days	5.097	4.584	3.567	5.142	4.404	3.620
Na₂SO₄ at 50^oC	120 Days	4.851	5.026	3.538	4.754	4.196	3.438
Na₂SO₄ at 20^oC	30 Days	7.429	5.608	4.438	4.851	4.690	3.485
Na₂SO₄ at 20^oC	120 Days	5.112	5.700	4.384	5.386	5.138	3.520
MgSO₄ at 50^oC	30 Days	5.287	4.696	3.667	4.593	4.390	3.399
MgSO₄ at 50^oC	120 Days	5.208	5.138	3.927	4.221	4.530	3.347
Na₂SO₄+NaCl at 50^oC	30 Days	5.085	4.757	3.786	4.902	4.466	3.447
Na₂SO₄+NaCl at 50^oC	120 Days	5.626	5.173	4.116	4.709	5.001	3.741

Table 7
TVL (cm) before and after exposure conditions for SG and OA sample for 30 and 120 days.
Exposure Conditions	Exposure Days	Ordinary samples			SG samples
	Exposure Days	1333keV	1173keV	663keV	1333keV	1173keV	663keV
	0 Days	16.035	15.485	11.820	16.204	17.274	11.949
Na₂SO₄ at 50^oC	30 Days	16.931	15.229	11.851	17.081	14.629	12.024
Na₂SO₄ at 50^oC	120 Days	16.113	16.697	11.754	15.793	13.938	11.422
Na₂SO₄ at 20^oC	30 Days	24.679	18.629	14.741	16.113	15.579	11.577
Na₂SO₄ at 20^oC	120 Days	16.981	18.936	14.564	17.891	17.069	11.694
MgSO₄ at 50^oC	30 Days	17.564	15.600	12.183	15.259	14.583	11.293
MgSO₄ at 50^oC	120 Days	17.300	17.069	13.046	14.023	15.050	11.118
Na₂SO₄+NaCl at 50^oC	30 Days	16.894	15.804	12.576	16.284	14.836	11.450
Na₂SO₄+NaCl at 50^oC	120 Days	18.690	17.183	13.673	15.643	16.613	12.426

HVL and TVL were calculated and summarized in Table 6 and Table 7. The results highlight the changes in gamma-ray shielding performance of both Ordinary and SG mortar samples under different sulfate and mixed-ion exposure conditions over time. In general, SG samples consistently demonstrated lower HVL and TVL values compared to Ordinary samples across all energy levels (1333 keV, 1173 keV, and 663 keV), indicating superior gamma attenuation performance. After 120 days of exposure to Na₂SO₄ at 50°C, the HVL values of Ordinary samples slightly increased, while SG samples showed minimal change or slight improvement, particularly at 1333 keV and 1173 keV, where values remained below their Ordinary counterparts.

Ordinary samples under Na₂SO₄ exposure at 20°C showed a notable increase in HVL and TVL. This suggests significant microstructural degradation. In contrast, SG mortars retained relatively stable values. MgSO₄ exposure led to elevated HVL and TVL in Ordinary samples, especially at 663 keV, while SG samples again showed better stability. However, in the combined Na₂SO₄ + NaCl exposure case, the increase in HVL and TVL was moderate for both mixes, though SG samples maintained lower values across all conditions. The findings confirm that the denser matrix of SG mortars and their enhanced durability offer better long-term shielding capacity under aggressive chemical and thermal environments compared to Ordinary mortars.

3.4. Visual and Microstructural Examination of Ordinary Mortars versus SF- and GGBS-Modified Mortars

Mortar samples were visually inspected before and after exposure to see the effect of aggressive chemical exposures under elevated temperature (50°C). This revealed apparent differences in surface degradation patterns between the ordinary and SG mixtures. As shown in Fig. 15, surface deterioration was progressively noticed when ordinary samples were exposed to MgSO₄. Early signs included mild discoloration and surface roughening. However, with prolonged exposure of 4 months, the damage intensified, and cement paste leaching, surface spalling, and the accumulation of white crystalline deposits were noticed. These residues are attributed to brucite and gypsum formation. Also, decalcification of C–S–H and efflorescence due to ion migration contributed to surface softening. Exposed SG samples exhibited interfacial transition zone (ITZ) disruption, localized microcracking, and matrix swelling, which indicate expansive phase formation. The later stages showed shiny salt crystals on the surface, likely the result of capillary-driven salt transport and precipitation.

Fig. 15

Surface appearance of ordinary (O sample) and silica fume–slag modified (SG) mortar samples after full immersion in 10% MgSO₄ solution at 50°C for (a) 0 month, (b) 1 month, (c) 4 months.

Exposing samples to Na₂SO₄ for 4 months at 50 ^oC caused noticeable degradation, as inspected in Fig. 16. Both ordinary and SG samples suffered from surface discoloration, white staining, and localized spalling that emerged over time. Where white deposits are possibly from the precipitation of ettringite and gypsum, efflorescence of sodium and calcium sulfates likely contributed to the whitening, particularly under thermal-driven evaporation conditions. Under prolonged exposure, ordinary samples showed surface softening and edge deterioration. SG samples exhibited a delayed onset of visible distress and maintained a denser appearance in early stages, but still developed spalling and surface dullness by the fourth month.

Fig. 16

Surface appearance of ordinary (O sample) and silica fume–slag modified (SG) mortar samples after full immersion in 10% Na₂SO₄ solution at 50°C for (a) 0 month, (b) 1 month, (c) 4 months.

Following Na₂SO₄ + NaCl exposure at 50°C, Fig. 17 shows surface changes. Both mortar types exhibited gradual surface whitening and chalky discoloration, with intensity increasing over time. From the surface inspection, the deterioration appears less aggressive than that caused by MgSO₄ or Na₂SO₄ alone. However, salt crystallization, residue formation, and mild efflorescence were apparent. The surface precipitates are likely composed of Friedel’s salt, gypsum, and potentially ettringite.

Interestingly, Cl⁻ ions may have partially suppressed expansive ettringite formation, which may explain the relatively lower incidence of cracking or spalling. SG samples showed a slower degradation and better surface retention in the early stages. This reflects some protective effect of supplementary cementitious materials. However, both ordinary and SG mixes exhibited comparable levels of efflorescence and salt accumulation by the end of the exposure period. These results highlight the complex interaction between sulfate and chloride ions that demonstrate the need for multi-ion durability assessments in coastal or nuclear infrastructure scenarios.

Fig. 17

Surface appearance of ordinary (O sample) and silica fume–slag modified (SG) mortar samples after full immersion in 10% Na₂SO₄ + NaCl solution at 50°C for (a) 0 month, (b) 1 month, (c) 4 months.

Figures 18–26 reveal the differences in microstructural integrity and degradation mechanism by comparing ordinary and SG mortar samples exposed to different conditions. Before exposure, the unexposed samples after 28 days of curingdemonstrated that the SG mix had a denser, more compact matrix with finer C–S–H (calcium silicate hydrate) and fewer microcracks, see Fig. 18. This is due to pozzolanic and latent hydraulic reactions from silica fume and GGBS. In contrast, ordinary samples had a more porous matrix with visable CH (calcium hydroxide) crystals, unhydrated clinker, and microcracking. This suggest greater openness to ion ingress and phase transformation.

Fig. 18

SEM micrographs of mortar samples after 28 days of curing and before exposure:(a) Ordinary sample showing C–S–H, CH crystals, unhydrated clinker, pores, and microcracks; (b) SG sample containing silica fume and GGBS, displaying a denser C–S–H matrix with fewer microcracks and lower Ca/Si ratio.

Another aggressive condition was the combined chloride and sulfate exposure at 50°C for 4 months. Friedel’s salt and gypsum were observed in SG samples (see Fig. 19), but compared to ordinary samples in Fig. 20, SG maintained structural compactness and a lower level of visible micro-cracks. Ordinary samples exhibited more deterioration with delayed ettringite formation (DEF), Friedel’s salt, and gypsum accumulation. In general, the presence of Cl⁻ and SO₄²⁻ was confirmed in both samples. However, SG has a lower Ca/Si ratio and higher alumina content, and a C-A-S-H matrix. These contributed to stronger ionic binding, reducing expansion and preserving microstructural integrity in SG samples more than ordinary samples.

Fig. 19

SEM-EDS micrographs of the SG mortar sample after 4 months exposure to combined NaCl and Na₂SO₄ solution at 50°C: Left: Presence of Friedel’s salt, gypsum, C–S–H, and microstructural pores; Right: Identification of Friedel’s salt, gypsum, ettringite, and C–A–S–H, with visible cracking. EDS spectra confirm Ca-rich hydration products and the presence of Cl, S, and Mg, indicating ion binding and secondary phase formation.

Fig. 20

SEM-EDS analysis of the ordinary mortar sample after 4 months of exposure to combined NaCl and Na₂SO₄ solution at 50°C:Top row: Formation of Friedel’s salt, gypsum, and ettringite within a porous and cracked matrix;Bottom row: Abundant ettringite and delayed ettringite formations (DEF), gypsum accumulation, and evidence of physical degradation. EDS spectra confirm elevated calcium and oxygen levels, with detectable Cl, S, Fe, and trace rare-earth elements (e.g., Tb, Bk), suggesting aggressive ion incorporation and phase transformation.

Under magnesium sulfate exposure at 50°C for 4 months, distinct chemical deterioration was observed (see Figs. 21&22). Ordinray samples suffered from extensive decalcification of C–S–H and high levels of brucite (Mg(OH)₂) along with the formation of M–S–H (magnesium silicate hydrate). Where M-S-H is weak gel forms when Mg²⁺ replaces Ca²⁺ in C–S–H, leading to loss of mechanical strength and disintegration of the matrix. On the other hand, SG samples were less affected and preserved more of their original structure. This was obvious from the brucite coexisting alongside unreacted clinker and slag-related hydration products, which suggests that SG system partially shields the Mg²⁺ exchange and limits M–S–H formation through its latent hydraulic reactivity and the formation of more stable hydrates.

Fig. 21

SEM-EDS images of the SG mortar sample after 4 months exposure to MgSO₄ solution at 50°C (M5F): Top: Formation of brucite, gypsum, and needle-like ettringite and delayed ettringite in the outer matrix; Bottom: Coexistence of brucite, unreacted clinker, and sulfate-related phases within a deteriorated zone. EDS analysis confirms high Mg and O, with the presence of Ca, S, Al, and minor K, consistent with the formation of brucite and sulfate attack products.

Fig. 22

SEM-EDS micrographs of the ordinary mortar sample after 4 months of exposure to MgSO₄ solution at 50°C (M5F): Top: Extensive formation of brucite and magnesium silicate hydrate (M–S–H), with areas showing C–S–H conversion and delayed ettringite crystallization; Bottom: Microcracks, ettringite, gypsum, and unhydrated clinker in a heavily disrupted matrix.EDS spectra show high Mg, confirming brucite and M–S–H formation. Moderate levels of S, Si, and Al also indicate sulfate-related degradation and phase alteration.

The degradation is intensified even more when temperature is increased to 50°C (Figs. 23 and 24). The ordinary samples experienced aggressive ettringite and gypsum formation, leading to severe expansion and cracking. The SG sample also formed delayed ettringite but retained matrix continuity, confirming that higher temperatures accelerate sulfate attack. However, the SG binder mitigates its impact due to the durability of C–A–S–H.

Fig. 23

SEM-EDS micrographs of the SG mortar sample after 4 months of exposure to Na₂SO₄ solution at 50°C: Left: Formation of ettringite within a cracked region and surrounding C–A–S–H gel; Right: Crystallization of delayed ettringite embedded in a C–A–S–H-rich matrix. EDS analysis reveals high Ca and O content, with trace levels of S, Al, and Tm, confirming the presence of sulfate reaction products and rare-earth elemental traces in the hydration matrix.

Fig. 24

SEM-EDS images of the ordinary mortar sample after 4 months exposure to Na₂SO₄ solution at 50°C:

Top and middle: Formation of secondary ettringite, gypsum, and poorly packed C–S–H phases, with visible unhydrated clinker and pores; EDS data reveal high Ca and O levels, along with moderate Si, Al, and S, indicating the formation of expansive sulfate-related products within a porous matrix.

Figures 25 and 26 compare the SG and ordinary samples’ microstructure under sodium sulfate exposure at 25°C for 4 months, which resulted in moderate formation of ettringite and gypsum in both binders. Ordinary samples have more spread porosity, microcracks, and loosely packed hydration products, compared to SG samples. Where SG samples remained more cohesive, forming compact C–A–S–H (calcium alumino-silicate hydrate) that is is more chemically stable than C–S–H, and delayed ettringite with limited structural damage.

Fig. 25

SEM-EDS micrographs of the SG mortar sample after 4 months exposure to Na₂SO₄ solution at 25°C: Left: Formation of gypsum, delayed ettringite, and compact C–S–H and C–A–S–H gels indicating sulfate interaction with pozzolanic products; Right: Needle-like ettringite crystals densely embedded within the hydration matrix. EDS spectra show dominant peaks of Ca, O, and S, alongside Si and Al, confirming sulfate attack products within a calcium-aluminosilicate-rich environment.

Fig. 26

SEM-EDS micrographs of the ordinary mortar sample after 4 months of exposure to Na₂SO₄ solution at 25°C (S2F):Top: Microstructural degradation marked by gypsum, ettringite, microcracks, and physical attack zones with visible porosity;Bottom: Abundant formation of secondary ettringite crystals within a porous and partially disintegrated C–S–H matrix. EDS analysis shows elevated levels of S, Ca, and O, along with measurable Al, Si, and trace K, indicating extensive sulfate interaction and expansive phase formation.

Thus, the degradation mechanism strongly depends on the ion type, also the following observations can be inferred:

Mg²⁺ promotes chemical disintegration via brucite and M–S–H formation

SO₄²⁻ induces expansive damage through ettringite and gypsum

Cl⁻, when combined with sulfates, amplifies both expansion and chemical destabilization.

Across all conditions, SG mortars outperformed ordinary mortars, showing enhanced resistance due to matrix densification, reduced CH content, and the formation of C–A–S–H, which is both chemically and structurally stronger and more durable than C–S–H.

4. Conclustions and Future work

This work investigated the influence of elevated temperature and sulfate-based chemical attack on the mechanical performance and gamma-ray shielding efficiency of the ordinary and SG mortar mixtures. The combined analysis of mechanical, non-destructive, microstructural, and radiation attenuation tests resulted in several conclusions,

Sulfate exposure at elevated temperatures of 50°C led to notable microstructural degradation in both mixtures. This is evident from the formation of expansive ettringite and gypsum, the reduced density, and increased porosity. As a result, both mechanical strength and radiation attenuation capacity were compromised.

Magnesium sulfate exposure caused the most chemically aggressive deterioration, particularly in ordinary mortars. The transformation of C–S–H to M–S–H and the formation of brucite resulted in matrix destabilization and loss of mechanical strength.

Combined sulfate–chloride exposure introduced dual degradation mechanisms. This formed Friedel’s salt and gypsum while promoting surface crystallization. Although the presence of chloride slightly reduced expansion-related cracking, it did not prevent internal deterioration.

Incorporating Silica Fume and GGBS in mortar mixtures consistently outperformed ordinary mixes across all exposure conditions. SG mortars maintained higher compressive and flexural strengths, showed superior resistivity and UPV results, and showed higher linear attenuation coefficients post-exposure. This resilience is caused by their denser microstructure and stable C–A–S–H gel formation.

The gamma-ray shielding performance was measured via the linear attenuation coefficient and half-value layer, where it declined in ordinary mortars after sulfate exposure, while SG mortars preserved and improved attenuation values, even after 4 months of aggressive exposure.

Together, these findings express the role of binder composition in enhancing the durability of concrete in environments that suffer from thermal and chemical stress, similar to those in arid coastal regions surrounding the nuclear facilities. Moreover, these findings hold significant implications for NPPs and radioactive waste storage units in aggressive coastal environments, specifically for the design and maintenance of the radiation shielding concrete. The expected impacts include recommending the use of blended binders such as GGBS and silica fume in regions subjected to sulfate-rich and high-temperature conditions to ensure long-term structural and shielding integrity. Another aspect is the nuclear infrastructure safety, where it demonstrates how sulfate-induced degradation reduces mechanical strength as well as the radiation shielding, which risks the safety margins if not mitigated. Finally, it helps in the material qualification by providing a framework for testing and qualifying cementitious materials under dual-performance criteria, the mechanical durability and radiation shielding, that is tailored for the Middle East and similar arid coastal environments.

To build on the results of this study, the following areas are proposed for future research:

A long-term field study is planned to see the effect of real-time environmental exposure near operational NPPs (Barakah) to validate laboratory findings of accelerated tests under natural wet–dry cycles and multi-ion conditions.

Examine the neutron shielding performance by investigating how sulfate-induced degradation affects neutron shielding, particularly for materials incorporating heavy elements or boron compounds.

Develop predictive models linking microstructural deterioration to changes in attenuation coefficients and dose rates.

Evaluate the effectiveness of nano-silica, graphene, or CNT-reinforced concrete in resisting combined chemical attack and enhancing gamma and neutron shielding.

Incorporate X-ray Computed Tomography (CT) imaging to map internal cracking and porosity evolution in 3D, supporting more precise correlations with gamma attenuation data.

Acknowledgement

The authors will like to appreciate Kevin Halique and Fatima Eisa for helping with the laboratory works. Also, we will like to thank ReadyMix Abu Dhabi for granting us access to their concrete laboratory.

Data Availability

Data will be available upon reasonable request from the corresponding author.

Author Contribution

FA: Methodology, conceptualization, writing original draft, revision, and editing final draft. RAS: Conceptualization, Methodology, writing original draft, revision, and editing final draft. MS: Methodology, writing original draft, revision. AA: Funding, supervision, and conceptualization. AKA: Funding, supervision, and revision.

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