Sulfuric acid corrosion resistance of recycled aggregate concrete containing magnetized water

Abstract

Excessive consumption of natural aggregates in concrete production has led to resource depletion and environmental degradation. Recycled aggregate concrete (RAC) offers a sustainable alternative; however, its mechanical strength and durability are often inferior, especially under aggressive acidic environments such as acid rain. This study aims to enhance the mechanical and durability performance of RAC by combining magnetized water (MW) and nano-silica (NS) as complementary modification techniques. A total of 80 concrete mixtures were prepared with varying recycled concrete aggregate (RCA) contents (0–100%), NS dosages (0–6%), and MW exposure times (0–30 min). The specimens were exposed to simulated acid rain with pH values of 2.5, 4.0, 5.5, and 7.0 for 28, 56, and 90 days. Tests for compressive strength, electrical resistivity, mass loss, and sorptivity coefficient were conducted to evaluate performance. Results showed that the use of RCA reduced compressive strength by up to 25.4%, while increasing the acidity from pH 7 to 2.5 caused an additional 16.2–25.4% decline. However, the synergistic use of 6% NS and 30-min MW improved compressive strength by up to 14.1% compared to control specimens. Similarly, the combination of MW and NS enhanced electrical resistivity by 12–38% and reduced mass loss and compressive strength by 33% and 32%, respectively, demonstrating a denser and more durable microstructure. These findings confirm that MW and NS mitigate the detrimental effects of acid exposure and recycled aggregates by accelerating cement hydration, refining pore structure, and strengthening the interfacial transition zone. The optimal mix was found at 25% RCA, 6% NS, and 30 min MW exposure, which achieved superior performance across all parameters. It is concluded that the combined application of MW and NS offers a feasible and eco-efficient approach to improve the corrosion resistance and sustainability of RAC under acid rain. Future studies are recommended to investigate the long-term field performance of RAC in other aggressive media such as chloride environments.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-38607-3.

Introduction

Concrete is a porous composite material whose mechanical and durability properties are largely governed by its pore structure and connectivity^1–3. Recent studies have shown that optimizing the porosity and refining the microstructure of cementitious systems can significantly improve mechanical performance and long-term durability, especially when sustainable or recycled materials are employed^4–6. These findings highlight that both microstructural improvement and material sustainability are key to advancing concrete technology, linking durability enhancement at the microscopic scale with the global challenge of reducing environmental impacts^7–9.

In this broader sustainability context, concrete production remains a major contributor to global environmental challenges due to the extensive consumption of natural aggregates and cement, leading to high carbon emissions and depletion of natural resources^10–12. The previous investigations showed that the need for natural aggregates will double in the next 20–30 years¹³. Globally, nearly 48.3 billion tons of natural aggregate were used in 2015, and this amount continues to grow by more than 5% every five years^14–16. Excessive extraction of natural aggregates causes significant ecological imbalances along riverbanks, including water and soil pollution^17,18. Additionally, about 50% of construction and demolition waste (CDW) consists of concrete waste. Since CDW makes up about one-third of all solid waste worldwide, concrete waste accounts for roughly 16% of the total solid waste^16,19. The enormous amount of concrete waste requires extensive landfill space, potentially occupying farmlands^10,20, which increases waste disposal costs and raises environmental concerns^21,22.

Although the use of recycled concrete aggregate (RCA) has been proposed to mitigate these impacts^23,24, its application in structural concrete is still limited because RAC generally exhibits lower strength, higher porosity, and inferior durability than those of natural aggregate concrete (NAC), particularly in aggressive acidic environments such as acid rain^25,26. This constitutes the general problem faced by the construction industry, achieving sustainability without compromising structural performance^27–30. The specific problem arises from the combined influence of recycled aggregates and acid exposure, which accelerates degradation and reduces the service life of RAC structures¹⁷. While researchers have attempted to improve RAC using various supplementary cementitious materials (SCMs) and chemical admixtures, these measures have not fully overcome its weaknesses under acid corrosion^31–35.

Several strategies have been proposed to address this technical challenge^36–41. Using magnetized water (MW) to produce concrete is a novel solution to improve its engineering properties^31,36,42. This study hypothesizes that MW modifies the hydrogen bonding and ionic structure of water, leading to enhanced cement hydration and improved interfacial bonding between recycled aggregate and cement paste. Reports indicate that MW enhances mechanical properties by 10% to 23% due to increased cement hydration compared to tap water (TW)³⁶. Due to the limited availability of clean drinking water, using MW in concrete can reduce water use and also improve workability, strength, and durability []. Another way to enhance the durability and strength of RAC is by replacing some cement with nano-materials like nano-silica (NS)^43–45. NS is a supplementary cementitious material that improves RAC’s microstructure by filling gaps, creating nucleation sites, strengthening the interfacial transition zone (ITZ), and participating in the hydration process⁴⁶.

The knowledge gap identified from previous studies is that no research has comprehensively examined the synergistic use of MW and NS to improve the mechanical and durability performance of RAC under acid rain exposure. Existing works typically addressed MW or NS separately, tested only normal concrete, or ignored realistic pH variations and combined effects of recycled aggregate content. Therefore, the novel contribution of this study lies in conducting a systematic experimental program including 80 mixes, simultaneously varying RCA (0–100%), NS (0–6%), and MW exposure time (0–30 min) under simulated acid rain (pH = 2.5–7), as shown in Fig. 1. The results provide new quantitative insights into how MW and NS interact to refine the microstructure, enhance corrosion resistance, and improve long-term mechanical properties of RAC. This approach contributes a practical pathway for developing eco-efficient and durable concrete suitable for sustainable infrastructure in acid-prone environments.

Fig. 1.

A graphical abstract of the performed study.

Literature review

Concrete is the most commonly used material in construction, and improving its properties is a crucial issue in civil engineering that has attracted researchers’ attention over the last few decades^47–49. As aggregate and water are the main part of concrete; using recycled aggregates in concrete can help preserve natural resources and contribute to environmental sustainability^10,19. Studies have shown that using recycled aggregates in concrete can be environmentally and economically beneficial, as it can cut material costs by 10% to 20% and reduce greenhouse gas emissions by 68%^50–52.

Recycled aggregates from construction and demolition waste often have old mortar stuck to their surfaces, causing them to absorb more water than natural aggregates^53–55. Studies have shown that the compressive strength of concrete drops as the amount of recycled aggregate increases⁵⁶. However, when recycled aggregate makes up less than 30%, its effect on compressive strength is minimal⁵⁷. Also, recycled concrete aggregates (RCA) usually create a larger interfacial transition zone (ITZ) in concrete than natural aggregates (NAs), leading to higher porosity, which can help with internal curing and improve durability^58,59. However, this increased porosity affects not only the concrete’s strength but also its permeability^60,61. As a result, RAC tends to have higher levels of water, chloride ions, and carbon dioxide compared to NAC⁶².

Cement-based materials are especially susceptible to damage from acid rain^63,64. Acid rain weakens concrete’s mechanical strength, causing cracks, loss of weight, and eventual structural failure^65,66. Since the impact of acid rain is unavoidable, many researchers have studied how cementitious materials break down and fail when exposed to it^65,67. They found that the combined effects of hydrogen and sulfate ions cause concrete to corrode from the outside inward until it is fully damaged^68–70. Concrete’s porosity initially increases in acidic conditions but decreases over time with longer exposure⁷¹. Because RAC generally has lower strength, carbonation resistance, and chloride resistance than regular concrete under acid rain, using more recycled aggregate can reduce its resistance to acid attack^57,72. To address these issues, researchers have suggested various methods, like using nanoparticles, to improve the microstructure of recycled aggregates and strengthen the ITZ to reduce acid rain damage^73–75.

Recently, utilization of MW has been greatly widespread in the cement concrete industry due to its significant influence on the mechanical and durability properties of concrete^76–78, along with its environment-friendly and economic benefits, particularly in addressing freshwater shortages without adversely affecting concrete properties^79,80. Water plays a vital role in concrete mixture, actively participating in the cement’s chemical reaction, controlling fresh concrete properties, cement hydration, the microscopic structure, strength, and overall durability of the concrete⁸⁰. Research indicates that MW can lower the required cement dosage for achieving certain compressive strength and improve resistance to freeze-thaw cycles^76,77,81,82. Moreover, decreasing cement content while using MW has been associated with up to a 25% rise in concrete compressive strength^83,84.

During the magnetization process, water is exposed to a magnetic field that alters the orientation of ions and the arrangement of water molecule clusters³⁶. Magnetic equipment typically includes one or more permanent magnets that influence both ions and water molecules⁷⁷. This magnetic exposure causes the water molecules to align, reducing their bond angle from the standard 105 degrees to a smaller value (Fig. 2)⁸¹. MW has lower viscosity than TW, affecting permeability pressure, surface tension, pH, and electrical conductivity⁸¹. Experimental findings demonstrated that using magnetized water combined with 20% silica fume enhanced self-compacting concrete properties by improving flowability and reducing viscosity, increased compressive strength by as much as 41%, and decreased water absorption by up to 55%⁸⁵. Studies have reported that incorporating MW in concrete mixtures can boost compressive strength by 10% to 20%^81,86. Additionally, concrete prepared with MW exhibits decreased water absorption and greater density, attributed to improved hydration and fewer pores³¹. SEM observations of concrete containing MW and metakaolin revealed a reduction in porosity, increased formation of calcium silicate hydrate (CSH) gel, and a decline in calcium hydroxide (CH) content⁸⁷. Furthermore, MW has been found to improve workability by up to 3%⁸⁷, increase slump by approximately 14%, and raise compressive strength by as much as 64% in geopolymer concrete mixtures that include silica fume and fly ash⁸⁸.

Fig. 2.

The water arrangement; (a) TW (b) MW.

Several theories have been proposed to explain how MW affects cement hydration and microstructural development. One of the most accepted mechanisms is related to the enhancement of ion mobility in the pore solution and cement–water interface⁸⁹. The magnetic field alters the hydration shell of ions such as Ca²⁺ and OH⁻ and disrupts hydrogen bonding networks, leading to smaller clusters of water molecules and improved ionic diffusivity⁸⁹. As a result, cement grains dissolve more efficiently, accelerating the release of Ca²⁺ and SiO₂ ions, which enhances the precipitation of hydration products^31,90,91.

Another mechanism concerns the nucleation and growth of C–S–H gel. The modification of the ionic environment and reduced size of water clusters under magnetic influence lower the energy barrier for C–S–H nucleation. Consequently, C–S–H forms more uniformly and densely, filling micro-voids and refining the pore structure. According to⁹², the use of MW in cement paste promoted faster formation of C–S–H nuclei and decreased portlandite content, leading to higher long-term compressive strength and reduced permeability.

A third explanation relates to the reduction in surface tension and viscosity of MW. Magnetic treatment reduces the angle between hydrogen bonds from the natural 104.5° to a smaller value (typically 103–101°), thereby lowering the surface tension by approximately 5–8%. This facilitates better wetting of cement grains, improved penetration of water into capillary pores, and more complete hydration⁹³. In addition, a lower surface tension leads to increased workability and decreased water demand at a given consistency, improving particle dispersion and reducing segregation⁹⁴.

Despite these advances, a critical gap remains in the current understanding of how these three mechanisms, ion mobility enhancement, nucleation modification, and surface tension reduction, interact under aggressive environments such as acid rain exposure. Most previous works have been limited to normal curing conditions or focused solely on mechanical properties, without exploring the durability behavior or corrosion resistance of RAC prepared with MW.

The inclusion of nanomaterials in concrete notably improves its characteristics due to their diminutive size, filling capacity, and strong pozzolanic reactions^95–98. Specifically, NS is widely employed to enhance the microstructural integrity and mechanical performance of RAC, achieving improvements in performance reaching up to 130%^99–102. Investigations using mercury intrusion porosimetry and scanning electron microscopy reveal that NS refines the ITZ within RAC, exhibiting lower overall porosity and reduced pore volume in these regions compared to conventional RAC mixtures¹⁰³. NS accelerates cement hydration, forming more CSH gel responsible for increased strength^99,104–106. Research indicates that substituting up to 4% of cement with NS enhances both the mechanical strength and durability of concrete, particularly in demanding environments such as those involving corrosion and high temperatures^107,108. Furthermore, NS improves the performance of cementitious materials in acid rain, enhancing mechanical properties and increasing pH resistance^65,73. However, excessive NS can cause agglomeration, reducing workability due to poor dispersion⁹⁹.

A summary of the previous studies about enhancing the properties of concrete by MW and other SCMs is presented in Table 1. As summarized in Table 1, most previous studies have investigated either (i) the influence of MW on normal concrete or self-compacting concrete, or (ii) the effect of SCMs such as silica fume or nano-silica on RAC under limited conditions. For example, Ghorbani et al.¹⁰⁹ used magnetized water with granite waste but only in natural aggregate concrete, and Devi and Khan¹¹⁰ investigated graphene oxide–reinforced RAC without considering acid rain or magnetized water. However, no comprehensive experimental study has yet examined the combined effect of MW and NS on the mechanical and durability performance of RAC under acid rain exposure. The existing literature demonstrates partial insights: MW enhances hydration and strength but has mostly been tested on natural aggregate concrete; NS improves microstructure and acid resistance but has rarely been coupled with MW or tested across a range of recycled aggregate replacement levels. Moreover, none of the previous works have systematically evaluated performance across multiple pH levels representing real acid-rain severity.

Table 1.

Summary of the previous studies about enhancing the properties of concrete by MW and other SCMs.

Ref.	Year	Application	RA type	RA (%)	Treatment method	Descriptions
88	2023	GPC	–	–	FA, SF, MW	MW increased the flowability and compressive strength of GPC by 14% and up to 64%, respectively; and decreased water absorption up to 38%
111	2022	NC	–	–	VA, MW	VA and MW increased the workability and compressive strength by 15% and 33%, respectively
112	2022	NC	–	–	MW	MW improved workability and compressive strength
113	2022	SCC	–	–	SF, MW	SF and MW increased the compressive strength and density of SCC
109	2021	NC	–	–	MW, GWD	MW and GWD enhanced the durability and mechanical properties of concrete were under aggressive environment
114	2021	RAC	Concrete wastes crushed	0–100	SF & GGBFS	SF improved the compressive strength of the RAC with GGBFS
73	2018	NC	–	–	NS	NS enhanced the mechanical properties and durability of concrete under acidic environment
115	2018	RAC	Demolition waste concrete	50,100	NS	NS increased the compressive strength of RAC
108	2018	RAC	Demolished concrete blocks	100	NS	NS improved the compressive strength of RAC
116	2017	NC	–	–	MW, SF	MW improved the compressive strength of concrete under various curing conditions
117	2017	NC	–	–	MW	MW significantly enhanced the compressive strength, water absorption and porosity of concrete, especially at early ages
118	2017	RAC	Ceramic Waste	20, 40, 60	MW	MW increased the compressive strength up to 15%
74	2014	RAC	Demolished building concrete	100	Colloidal NS	NS enhanced the compressive strength of RAC
81	2008	NC	–		MW	MW increased the compressive strength of NC up to 20%

Therefore, this study fills the identified knowledge gap by designing and testing 80 different concrete mixtures that simultaneously vary RCA content (0–100%), NS dosage (0–6%), MW exposure time (0–30 min), and environmental pH (2.5–7). This multidimensional experimental program provides the first integrated assessment of how MW and NS synergistically improve the acid-corrosion resistance, compressive strength, and durability properties of RAC. Mechanical and durability properties under simulated acid rain, including compressive strength (CS), electrical resistivity (ER), mass loss (ML), and capillary water absorption were systematically assessed. The findings deliver novel insights into the physico-chemical mechanisms responsible for durability enhancement, thereby extending the practical understanding of sustainable RAC under aggressive acid-rain conditions. On the other hand, the substitution of tap water (TW) with MW provided both environmental and economic benefits, notably boosting the acid rain resistance of RAC containing NS, a critical concern in global future scenarios.

Experimental program

Materials

In this section, materials and constituents were rigorously specified to ensure reproducibility and practical relevance. In all concrete specimens, Type II Portland cement conforming to ASTM C150-07¹¹⁹ was used, with its chemical and physical composition detailed in Table 2. The NS colloidal solutions applied had a concentration of 30% and a purity level of 99.9%. Potable MW was utilized during all mixing procedures. To achieve the target workability corresponding to a W/C ratio of 0.5, a polycarboxylate superplasticizer with a specific gravity of 1.20 kg/l was added. Fine and coarse aggregates included partially replaced RCAs substituted by rounded washed sand and crushed stone. The RCA used in this study was obtained from crushed concrete debris collected from a local demolition site of 25–30 MPa compressive strength concrete. Prior to grading, the aggregates were air-dried for 24 h to remove moisture content. Grading was conducted using a shaking table following ASTM C136-01 standard¹²⁰, and grading curves for both coarse and fine aggregates are presented in Fig. 3. Additionally, the physical properties of the aggregates were determined in accordance with ASTM C127-01¹²¹ and ASTM C128-01¹²² standards, with results summarized in Table 3. The aggregate crushing value (ACV) was measured as 21.4%, confirming a moderate mechanical strength suitable for use in structural grade RAC¹²³.

Table 2.

Chemical and physical properties of the used cement.

Chemical components
CaO (%)	62.94
SiO2 (%)	21.15
MgO (%)	2.75
Al2O3 (%)	4.52
Fe2O3 (%)	3.83
SO3 (%)	1.92
K2O (%)	0.68
Na2O (%)	0.30
LOI (%)	1.35
Mineralogical composition
C3S (%)	54.13
C2S (%)	19.83
C3A (%)	5.51
C4AF (%)	11.65
Physical properties
Blain (m2/kg)	320
Specific gravity (g/cm3)	3.21
Initial setting time (min)	186
Final setting time (min)	276

Fig. 3.

Grading curve of fine and coarse aggregates.

Table 3.

Physical properties of natural and recycled aggregates.

Type	Water absorption (%)	Specific gravity (kg/m3)	Bulk density (kg/m3)
Fine NA	2.6	2525	1550
Coarse NA	1.4	2550	1500
Fine RCA	5.3	2495	1400
Coarse RCA	4.9	2450	1320

Mixtures design

In this research, a comprehensive factorial program was implemented to isolate and quantify the effects of MW, RCA, and NS, individually and in combination, on concrete performance under controlled acid rain exposure. In this research, 80 concrete mixes were prepared to investigate the effects of various combinations of MW, RCA, and NS under different acidic environments. The RCA substitution levels were set at 0%, 25%, 50%, 75%, and 100%, while NS was incorporated at 0%, 2%, 4%, and 6%. All mixes were prepared using MW magnetized for durations of 0, 10, 20, and 30 min and exposed to solutions with pH values of 2.5, 4, 5.5, and 7. The replacements were based on mass to ensure uniformity. The W/C ratio was maintained at 0.5 across all samples, and slump values were controlled to 9 ± 1 cm according to ASTM C143/C143M-03 standard¹²⁴. For acid rain simulation, an acid rain simulator (ARAS), adapted from the design used by Mahdikhani et al.⁷³, was employed. The ARAS applied simulated acid rain as a fine spray onto the specimen surfaces for 1 h per day, followed by 23 h in laboratory conditions. This cycle was repeated daily for the entire exposure period (28, 56 or 90 days). During the daily cycle, specimens remained on supports that prevented pooling and allowed runoff, simulating real rainfall rather than full immersion. Acidic rainwater was synthesized by incremental addition of concentrated H₂SO₄ to deionized water under stirring while monitoring pH with a calibrated pH meter (± 0.01 pH). Only sulfuric acid was selected as the acidifying agent to isolate its specific effect, because in most industrial and urban regions, H₂SO₄ is the dominant component of acid precipitation, typically accounting for more than 60–70% of total acidity compared to nitric acid and other species. Four target pH levels were used to simulate different acid rain severities: pH = 7.0 (control), pH = 5.5, pH = 4.0 and pH = 2.5. To maintain consistent chemical aggressiveness, the bulk acid solution in the ARAS reservoir was refreshed weekly, and replaced with freshly prepared solution of the same target pH. Also, pH of the reservoir was measured daily prior to the spray; if pH drifted by more than ± 0.05 units from the target, it was adjusted by careful addition of H₂SO₄ (to decrease pH) or deionized water (to increase pH). For every specimen group at the predefined ages (28, 56 and 90 days), mechanical and durability properties including compressive strength, mass loss, capillary water absorption and electrical resistivity were evaluated. The details of each test are provided in the next section.

It should be noted that only sulfuric acid was used in this experimental phase to maintain controlled and comparable chemical conditions across all samples. Future studies are planned to include mixed acid systems (H₂SO₄–HNO₃) to simulate realistic atmospheric precipitation chemistry and to assess potential synergistic corrosion mechanisms.

Magnetization equipment was specifically developed to generate MW with controllable magnetic flux density, fixed at 1.6 Tesla, as illustrated in Fig. 4. This system magnetized water flowing through a 0.5-inch diameter PVC pipe at 20 L per minute, with an overall water storage capacity of 0.01 m³. The magnetization duration varied and represents the primary control variable, with tap water exposed to magnetic fields for 10, 20, and 30 min¹²⁵. Water velocity was 39.5 m/min, corresponding to 100, 200, and 300 cycles of magnetization for the respective exposure times. In this context, one cycle refers to a single complete pass of the water through the magnetic field between the north and south poles of the magnetizing unit. As the water continuously circulated through the closed-loop pipe system, the total number of cycles increased proportionally with the exposure duration. Therefore, 100, 200, and 300 cycles correspond to approximately 10, 20, and 30 min of total exposure to magnetic field, respectively. The specimens were categorized into four groups based on their constant pH levels:

Fig. 4.

The magnetizing device; (a) Magnetizing equipment (b) Schematic of magnetic circuit.

• Group A: pH = 2.5.

• Group B: pH = 4.

• Group C: pH = 5.5.

• Group D: pH = 7.

Each sample was given an ID code in the format of Xa_b_c, where X represents the pH group, “a’’ denotes the NS ratio, “b” indicates the RCA ratio, and “c” signifies the MW exposure time. Conventional concrete without RCA, NS, and MW was used as a reference concrete in each group, denoted as X_ref. An example of this naming convention is B2_50_10, representing a specimen in group B (pH = 4) with 2% NS, 50% RCA, and 10 minutes of MW exposure. The specific designs of these concrete mixtures are detailed in Table 4.

Table 4.

Concrete mixture designs of specimens.

No.	RCA (%)	Cement (kg)	NS (%)	MW (min)	No.	RCA (%)	Cement (kg)	NS (%)	MW (min)	No.	RCA (%)	Cement (kg)	NS (%)	MW (min)	No.	RCA (%)	Cement (kg)	NS (%)	MW (min)
1	0	350	0	0	21	25	343	2	0	41	50	336	4	0	61	75	329	6	0
2	0	350	0	10	22	25	343	2	10	42	50	336	4	10	62	75	329	6	10
3	0	350	0	20	23	25	343	2	20	43	50	336	4	20	63	75	329	6	20
4	0	350	0	30	24	25	343	2	30	44	50	336	4	30	64	75	329	6	30
5	0	343	2	0	25	25	336	4	0	45	50	329	6	0	65	100	350	0	0
6	0	343	2	10	26	25	336	4	10	46	50	329	6	10	66	100	350	0	10
7	0	343	2	20	27	25	336	4	20	47	50	329	6	20	67	100	350	0	20
8	0	343	2	30	28	25	336	4	30	48	50	329	6	30	68	100	350	0	30
9	0	336	4	0	29	25	329	6	0	49	75	350	0	0	69	100	343	2	0
10	0	336	4	10	30	25	329	6	10	50	75	350	0	10	70	100	343	2	10
11	0	336	4	20	31	25	329	6	20	51	75	350	0	20	71	100	343	2	20
12	0	336	4	30	32	25	329	6	30	52	75	350	0	30	72	100	343	2	30
13	0	329	6	0	33	50	350	0	0	53	75	343	2	0	73	100	336	4	0
14	0	329	6	10	34	50	350	0	10	54	75	343	2	10	74	100	336	4	10
15	0	329	6	20	35	50	350	0	20	55	75	343	2	20	75	100	336	4	20
16	0	329	6	30	36	50	350	0	30	56	75	343	2	30	76	100	336	4	30
17	25	350	0	0	37	50	343	2	0	57	75	336	4	0	77	100	329	6	0
18	25	350	0	10	38	50	343	2	10	58	75	336	4	10	78	100	329	6	10
19	25	350	0	20	39	50	343	2	20	59	75	336	4	20	79	100	329	6	20
20	2	350	0	30	40	50	343	2	30	60	75	336	4	30	80	100	329	6	30

Testing procedure

A standardized preparation and exposure protocol was adopted to evaluate mechanical and durability responses of concrete under simulated acid rain. All concrete specimens underwent vibration prior to casting to ensure proper compaction. After casting, specimens were cured under laboratory conditions by covering them with wet towels for 24 h. Subsequently, all specimens were removed from the molds and directly exposed to the simulated acid rain condition until the designated testing ages. Therefore, no additional curing was performed beyond the initial 24-hour period. This approach ensured that the entire curing and exposure process realistically simulated the continuous environmental conditions of acid rain throughout the test duration⁷³. Exposure to acidic environments with pH levels of 2.5, 4, 5.5, and 7 was maintained for periods of 28, 56, and 90 days. Following exposure, tests were conducted to evaluate compressive strength and durability under acidic conditions. Compressive strength measurements were performed on 100 mm cubic samples following the BS 1881 standard¹²⁶, with three specimens tested per mixture and condition, and the average value reported.

Electrical resistivity testing conformed to ASTM C1202-97¹²⁷, also using 100 mm cubic samples. The test involved applying DC voltage and measuring electrical resistivity along two perpendicular directions of each specimen, from which an average resistivity was calculated. For mass loss assessments, the 100 mm cubes were removed from the acid rain simulator at specified ages (28, 56, and 90 days), then equilibrated at laboratory conditions for 24 h to reach constant mass before mass loss measurement.

Capillary water absorption tests were performed on the same-sized specimens in accordance with ASTM C1585-04¹²⁸. Samples were dried at 50 °C and cured with wet towels for 24 h. To ensure uniaxial water absorption, lateral surfaces were coated with epoxy resin. Specimens rested on rods to allow water contact solely from the bottom. Initial mass (M₁) was recorded before immersion in acidic rainwater. Specimens were exposed for 28, 56, and 90 days, then returned to lab conditions for 24 h, reaching constant mass. Subsequent masses (M₂) were recorded after 3, 6, 24, and 72 h of water contact. Water level was maintained at 4 cm throughout testing. The sorptivity coefficient (kg/s^0.5.m²) was calculated using Eq. 1.

1

Where M is the specimens’ mass (kg); A is the surface area of specimens that is in contact with water (m²); t is the time (s); C is the constant coefficient.

Results and discussion

In the result section, the performance of the experimental mixes under simulated acid rain is presented and interpreted. Results are organized by response variable so that compressive strength, electrical resistivity, mass loss, and capillary water absorption are evaluated in sequence. Comparisons are made across recycled aggregate replacement levels, nano-silica dosages, and magnetized water treatment times at pH values of 7.0, 5.5, 4.0, and 2.5 and at ages of 28, 56, and 90 days. Emphasis is placed on trends that remain consistent across ages and acidities, with numerical values used to support the observed patterns. Particular attention is given to combined effects that are relevant for recycled aggregate concrete in practice.

Compressive strength

This section evaluates compressive strength to assess the influence of RCA content, nano-silica dosage, magnetized water treatment, and pH at 28, 56, and 90 days. The results of the compressive strength (CS) test at 28, 56, and 90 days are presented in Figs. 5, 6 and 7 and Table A1. The higher porosity of RCA compared to NA results in a decrease in the CS of RAC, particularly in corrosive environments like acid rain^53,72. The findings indicated that replacing NA with RCA in conventional concrete consistently reduced CS across all ages and groups. For example, the 28-day CS decreased in average by 3.2%, 6.4%, 15.9%, and 25.4% for RCA replacement ratios of 25%, 50%, 75%, and 100% respectively across all pH levels and exposure times to MW.

Fig. 5.

The CS of 28-day specimens.

Fig. 6.

The CS of 56-day specimens.

Fig. 7.

The CS of 90-day specimens.

The reduction rates at 56 and 90 days remained consistent in all acidic environments (pH groups A, B, C), while in group D, the CS decreased from 5.1% to 28.2%. Notably, as acidity increased, the CS of concrete specimens declined further. The 28-day CS for groups A, B, and C decreased by 16.2%, 11.7%, and 8%, respectively, compared to group D (pH = 7), irrespective of whether RCA or NA was used. For 56-day specimens, the CS of C_ref was reduced by 15.8%, while RAC specimens showed a reduction of 14.2%. Additionally, B_ref and A_ref experienced greater reductions at 19.7% and 25.4%, respectively, compared to their counterparts in the non-acidic environment. For instance, 56-days CS of RAC in group A and B reduced by 24% and 18.2%, respectively, compared to the one in group D. For 90-day CS, variations existed among specimens with different RCA replacement ratios; for instance, the 90-day CS for group A0_25 was 43.6% lower than D0_25. The reductions are similarly significant, with group B0_25 and C0_25 showing decreases of 29.2% and 18.6%, respectively, compared to group D. Overall, the CS improved by 2.2% to 6.8% with increased exposure time to MW (10, 20, and 30 min), compared to counterparts with identical RCA ratios using TW. Research on cementitious composites made with MW indicated that both fresh and hardened state properties were considerably enhanced due to improved water molecule distribution resulting from magnetization¹²⁹. This facilitated better water penetration into cement particles, stimulating hydration and reducing capillary pores¹¹².

The inclusion of NS in the cement mixture enhanced concrete performance by filling pores and densifying the structure⁷³. In this study, the CS of group D concrete samples with NS was improved by 2.4% to 6.8% at 28 days and by 0.6% to 5.4% at 56 days. However, at 90 days, CS decreased by up to 1.6% with NS ratios up to 4%, but increased by 2.3% with 6% NS. In groups A, B, and C, the 28-day CS increased from 2.4% to 6.8% with higher NS content, while this improvement ranged from 3.2% to 20.3% for 56 and 90 days’ CS.

The improvement in CS mainly results from the pozzolanic activity of NS, which reacts with calcium hydroxide produced during cement hydration to form additional CSH⁷³. Previous studies have reported that cement mortars and concrete containing NS exhibit superior mechanical properties compared to plain concrete, especially during early curing stages¹³⁰. This enhanced early-age strength is attributed to the acceleration of cement hydration and formation of denser CSH gel facilitated by the fine NS particles acting as nucleation sites for hydration products¹³⁰.

Similar to RAC specimens, NAC specimens also exhibited increases in CS of 2.2% to 6.8% with the utilization of MW¹³¹. For example, A2_0_30 showed a 6.8% improvement in all ages compared to A2_0_0. Under acidic conditions, NAC concretes experienced reductions in CS ranging from 8% to 16.2% at 28 days, 15.8% to 25.4% at 56 days, and 20.1% to 44.7% at 90 days; however, specimens with NS experienced varying degrees of CS reduction based on age and NS content, ranging from 8% to 41.4%. An increased NS ratio contributed to improve 90-day CS in groups A and B. For example, the CS of A6_0 in 90 days decreased by 34.9% compared to reference one, whereas this ratio was 41.4% and 38.2% for A2_0 and A4_0, respectively. On the other hand, increasing the NS in concrete mixture resulted in greater reduction in 56-days and 90-days CS in group C. For example, the CS of C2_0, C4_0 and C6_0 in 56 days reduced by 4.9%, 5.1%, and 8.4%, respectively. Recent studies indicated that using NS and silica fume, along with MW, improved the mechanical properties of concrete¹³². The addition of NS up to 10% as a replacement of cement, along with 1% steel fiber and MW with 1 Tesla magnetic flux density resulted in a 152.26% increase in compressive strength for 7-days specimens and a 114.03% increase for 28-days specimens¹³³.

The simultaneous use of RCA and NS in concrete mixtures yielded varying outcomes as environmental acidity escalated. While RCA generally reduced CS, NS inclusion mitigated this reduction, sometimes bringing CS in line with reference concrete, particularly at a 6% NS and 50% RCA combination. For instance, the 28-day CS of specimens with 6% NS and 50% RCA in all groups aligned closely with their respective reference concrete. At 56 days, specimen A2_25_0 exhibited matching CS with A_ref, indicating that NS supplementation could counteract RCA drawbacks. Additionally, the specimen with 25% RCA and 2% NS content forms an optimal combination in group A. In this scenario, the usage of MW enhanced compressive strength by maximum 6.8% for 30 min of subjecting to magnetic field compared to using TW. Notably, CS increments vary across groups and ages, with changes from 0.3% to 16.4% in group A, 0.6% to 11.5% in group B, 0.3% to 11% in group C, and 0.3% to 3.3% in group D. Alternatively, reductions ranged from 3.2% to 24.9% in acidic environments and from 0.9% to 23.6% in non-acidic environments. Previous studies have shown that MW combined with silica fume improved not only flowability and viscosity of self-compacting concrete, but also increased its CS up to 41% while reducing water absorption by 55%^69,134.

The results of this study show that the C6_50_30 exhibited improvements in CS at 28 days, with a 6.8% increase compared to C6_50_0 and a 6.7% increase compared to the reference one (C_ref). For specimens with the same ratios of RCA and NS exposed to identical pH conditions, increasing the exposure time of MW consistently resulted in proportional increases in compressive strength at all tested ages. In group B, the CS of the specimens with identical RCA and NS content improved by 2.2% when the water was subjected to the magnetic field for 10 min in all ages. Furthermore, by increasing the exposure time of water to the magnetic field for 20 and 30 min, the improvement reached 5.2% and 6.8%, respectively¹³⁵.

It can be concluded that replacing TW with MW in RAC significantly enhances the mechanical performance due to a more complete cement hydration process¹³⁵. Specifically, doubling the exposure duration of water to the magnetic field from 10 to 20 min led to a 2.9% increase in SC. This improvement is attributed to the split of water clusters into smaller molecules, facilitating better penetration of water into cement particles and promoting efficient hydration that strengthens the concrete matrix¹³⁵. For example, in C0-50-0 under acidic conditions with a pH of 5.5 and using TW, the CS decreased by about 6.4% compared to the C_ref. By incorporating 2%, 4%, and 6% NS, the CS increased by 2.4%, 3.7%, and 6.8%, respectively. This enhancement ratio remained consistent when MW was used instead of TW, with the same proportion of RCA and NS and under the same pH conditions. For instance, the CS of C2_50_20 and C2_50_30 improved by 2.4% compared to C0_50_20 and C0_50_30.

Overall, the study demonstrated that MW improved concrete’s mechanical properties by 2.2% to 6.8%. The addition of NS enhanced this effect further, resulted in gains of 2.4% to 6.8%. The best improvement in compressive strength, reaching a 14.1% increase, occurred when NS and MW were used simultaneously, regardless of the RCA replacement ratio. This suggested a potent and eco-friendly strategy for enhancing mechanical properties in RAC.

From another perspective, acidic environments resulted in long-term CS reductions, while non-acidic conditions tended to show improvements over time. Specifically, the 90-day CS of reference concretes in groups A, B, and C decreased by 26.8%, 12.8%, and 3.8%, respectively, compared to their 28-day values, while group D showed a positive increase of 10.9%. The addition of NS benefited performance in severe acid conditions, yielding CS reductions of 26.8% in A0_75_10 compared to 28 days, contrasted with reductions of 24.9%, 22.5%, and 17.6% at 2%, 4%, and 6% NS inclusions, respectively⁷³.

Electrical resistivity

In this section, electrical resistivity (ER) was examined as a proxy for pore connectivity and transport resistance across RCA contents, nano-silica dosages, magnetized-water durations, pH levels, and ages. The electrical resistivity (ER) of specimens after 28, 56, and 90 days of exposure under acidic conditions is presented in Figs. 8, 9 and 10 and Table A2. The findings indicated that the ER of RAC specimens decreased by 2%, 4.8%, 10.7%, and 24.2% with 25%, 50%, 75%, and 100% RCA replacement, respectively, compared to the reference concrete, regardless of pH group, MW exposure time, and specimen age. As the pH level of the solution decreased, indicating increasingly acidic conditions, the ER of the concrete specimens declined at varying rates depending on both the specific pH level and the duration of specimen exposure. Lower pH environments accelerate deterioration processes, weakening the microstructure and reducing resistivity. This trend aligns with findings that acidified solutions chemically degrade the concrete matrix and induce corrosion in RAC, thereby lowering ER as the acidity intensifies and exposure time increases. For instance, in pH group A, the ER of specimens reduced by 53.8%, 58.9%, and 69.5% at 28, 56, and 90 days, respectively, irrespective of RCA or NA usage. The 90-day ER reduction was 31.1% and 51.3% when the pH level dropped to 5.5 and 4, respectively. It is evident that the ER of specimens decreased with lowering pH levels. Additionally, in groups A, B, and C, under acidic conditions, the ER diminished as the mixture aged. In contrast, in non-acidic environments, the ER increased with specimen age. This was attributed to increased porosity at later ages; as the microstructure became denser and porosity decreased, the lowest ER was observed during the early stages⁷³.

Fig. 8.

The ER of 28-day specimens.

Fig. 9.

The ER of 56-day specimens.

Fig. 10.

The ER of 90-day specimens.

Adding NS to conventional concrete resulted in substantial improvements in ER across all ages, regardless of RCA content, MW exposure time, and solution pH. For example, the 90-day ER in group D increased by 12.4%, 17.7%, and 32.6% for 2%, 4%, and 6% NS, respectively, compared to the D_ref. Similarly, the increments were 16.1%, 24%, and 38.4% for 28 days, and 14.1%, 21.1%, and 36.6% after 56 days. By decreasing the solution pH, the ER enhancement rate varied for 56-days and 90-days tests, while remained consistent for 28 days across all groups. The specimen A0_6 exhibited a 55.9% improvement in its 90-day ER, while B0_6 and C0_6 showed 36.1% and 35.1% improvements, respectively. Previous studies have confirmed that specimens containing NS had higher ER at 28 and 90 days^136,137.

Moreover, extending MW exposure from 0 to 30 min improved the ER of concrete specimens, ranging from a 1.5% increase for 10 min to a 4.6% increase for 30 min, with consistent RCA and NS in pH groups A, B, and C at all ages. However, in group D, the ER enhancement for all ages and NS contents was between 1% and 3%. The results demonstrated that decreasing solution pH correlated with reduced ER, with the rate of decline varying based on acidity levels. For example, in NAC specimens, the ER of C_ref, B_ref, and A_ref after 28 days decreased by 20.6%, 38.1%, and 53.8%, respectively. The reduction rate varied for 56-day and 90-day tests for each group. Adding NS to the concrete mixture enhanced the long-term ER of specimens. This means that while the 28-day ER declined similarly to NAC, the behavior at 56 and 90 days varied based on MW exposure time. The ER of A6_0_0 at 56 and 90 days was 54.5% and 64.1% less than that of D6_0_0, respectively; similarly, under 30-minute MW exposure, this reduction was 63.6% for A6_0_30 compared to D6_0_30 at 90 days.

As previously discussed, adding RCA to the concrete mixture reduced ER properties, while the simultaneous application of NS and MW in RAC mixtures positively affected ER properties. In group D, for instance, the ER of D6_25_30 was increased by 39.8%, 38%, and 33.9% at 28, 56, and 90 days, respectively, compared to D_ref. Conversely, specimens D4_100 and D2_100 experienced reductions in their ER at all ages, ranging from 3.2% to 12.3%. Notably, the specimen with 100% RCA replacement, 6% NS content, and maximum MW exposure exhibited ER enhancements of 8.1%, 6.6%, and 3.5% at 28, 56, and 90 days, respectively.

Furthermore, aging affected ER properties based on the pH group, regardless of RCA replacement ratio and MW exposure time. In group D (non-acidic environments), ER improved after 90 days, with increments of 10.9%, 7.4%, 5.2%, and 6.1% for 0%, 2%, 4%, and 6% NS content, respectively. In contrast, the application of NS in group D diminished ER improvement in aged samples. In pH groups A, B, and C, the ER of specimens decreased by aging based on NS content, and the reduction intensified as the pH value of solution decreased. For example, in group A, the 90-day ER of specimens with 2% NS was 24.9% less than the 28-day ER, while in group B, this reduction was 14.3%.

Mass loss

This section quantify mass loss (ML) to indicate material degradation under acidic exposure across the tested variables and ages. The results of mass loss (ML) percentage over 28, 56, and 90 days are presented in Figs. 11, 12 and 13, and Table A3. According to the results, the ML percentage of specimens in reference concretes increased by 33.3%, 20.4%, and 66.7% in groups C, B, and A, respectively, compared to group D. When NA was replaced with RCA, the ML percentage increased in relation to the RCA ratio across all pH groups. A concrete mixture with 25% RCA experienced a 10% increase in its ML percentage compared to the reference concrete, regardless of age and pH group. With 50%, 75%, and 100% RCA replacement, the ML percentage increased by 26.5%, 51.8%, and 89.8%, respectively.

Fig. 11.

The ML percentage of 28-day specimens.

Fig. 12.

The ML percentage of 56-day specimens.

Fig. 13.

The ML percentage of 90-day specimens.

As the acidic condition intensified the ML percentage increases across a range of variations, irrespective of RCA usage. The ML percentage in all group C specimens increased by 20.4% over 28 days compared to group D, while specimens from groups B and A showed increase of 33.3% and 66.7%, respectively. In severe acidic environments, such as group A with pH = 2.5, the ML percentage increased by over 1.5 times compared to group D, reaching approximately 161%. The use of MW instead of TW improved this property of specimens. For example, in specimens group B, increasing MW exposure led to a lower ML percentage compared to TW under the same conditions.

Incorporating NS into the concrete mixture decreased the ML percentage; the greater the NS content, the more significant the reduction⁷³. This is due to NS’s ability to reduce porosity compared to plain concrete¹³⁸. Reduction rates were 13.8%, 28.2%, and 35.7% for 2%, 4%, and 6% NS content across all ages and groups, respectively.

In addition, MW positively impacted ML percentages in all group mixtures with different ratios; for instance, by use of MW in group D and C, the ML percentage decreased 1.5%, 3.4% and 5.3% for 10, 20, and 30 min of MW exposure. For example, C6_0_30 showed a 5.3% decrease in ML percentage compared to C6_0_0. It is noteworthy to say that these ratios remain unchanged as the samples aged. In acidic environments, such as group A with pH = 2.5, the ML percentage of reference specimens increased by over 1.6 times compared to group D after 90 days, reaching approximately 161%. The use of MW instead of TW improved this property of specimens. For example, ML percentages in A0, A2, A4, and A6 decreased by 68.3% over 28 days compared to D0, D2, D4, and D6 with 30-minute MW exposure.

The combined use of RCA, NS, and MW influenced ML properties differently. For instance, the ML percentage of D6_25_30 decreased 33% compared to D_ref at all ages, while the D2_100_30 had a 54.8% increase. In group B, B6_25_30 indicated a 33.6% reduction, whereas B2_100_30 showed a 53.3% increase at all ages. Additionally, MW-NS treating RCA specimens could not mitigate their ML under sever acidity condition (group A). For instance, the ML percentage of the A2_100_0 was 63.5% greater than A_ref, and with 30 min exposure of MW, this ratio was 56.4% (in A2_100_30). Lowering the RCA content led to improve in ML percentage. For example, A6_25_30 exhibited a 32.3% lower ML percentage than A_ref across all ages.

Capillary water absorption

Capillary water absorption was assessed to characterize sorptivity and capillary-driven ingress under the specified conditions. Water absorption indicated by the sorptivity coefficient (SC). The SC of specimens tested at 28, 56, and 90 days, and is summarized in Figs. 14, 15 and 16, and Table A4. These findings indicated that the SC of RAC specimens increased compared to reference concrete. Specifically, the SC increased by 10%, 26.5%, 51.8%, and 89.8% with RCA replacement ratios of 25%, 50%, 75%, and 100%, respectively, irrespective of the pH group and the age of the specimens. This increase in SC was associated with a corresponding rise in capillary porosity of the specimens⁷³.

Fig. 14.

The SC of 28-day specimens.

Fig. 15.

The SC of 56-day specimens.

Fig. 16.

The SC of 90-day specimens.

The use of MW resulted in overall improvements in water absorption across all groups, evidenced by reduced SC values. For instance, specimens in group D exhibited reductions in SC of 1%, 2.5%, and 3.9% for MW exposure times of 10, 20, and 30 min, respectively, regardless of the RCA ratio. The improvement is credited to intensified cement hydration triggered by magnetic influence, which leads to a denser concrete matrix with decreased porosity³¹. The reduction rate of SC is divergently by intensifying the acidity of solutions. For example, the use of MW caused reductions in the SC of group A specimens by 1%, 2%, and 2.9% compared to those without MW at all ages, irrespective of RCA content. While these rates are 1%, 2.5%, and 3.9% for group B specimens. Intensifying acidity led to significant SC increases in A_ref, B_ref, and C_ref of 68.2%, 96.9%, and 118.6%, respectively, compared to D_ref at 28 days. Replacing NA with RCA did not alter this trend. This trend intensified as the concrete aged; for instance, B_ref showed a 139.3% SC increase at 90 days. Moreover, the 90-day SC of A_ref more than doubled (243.1%) compared to group D. Notably, MW had no effect on SC in group B specimens while increasing SC in groups A and C compared to TW.

Adding NS to reference concrete reduced SC at all ages and pH groups, with reductions of 13.8%, 28.2%, and 35.7% observed in D2, D4, and D6, respectively^130,138. The concurrent use of MW in this mixture further reduced SC by 1% to 3.9%, irrespective of NS content and age. Maximum levels of combined NS and MW led to a 38.2% improvement in SC compared to the reference one in group D. However, increasing the acidity level affects these improvements differently. For example, A6_0_30 showed a 37.6% reduction in SC, while B6_0_30 and C6_0_30 exhibited reductions of 38.2% and 37.9%, respectively, compared to the reference concrete of each group. Despite increased acidity, SC increased at varying rates influenced by specimen age, MW use, and exposure duration. For example, C_ref, B_ref, and A_ref demonstrated SC increases of 68.2%, 96.9%, and 118.6% at 28 days, respectively. In the case of utilizing MW, SC vaied across different ratios; C0_0_30, for instance, showed a 69% increase in 28-day SC compared to D0_0_30. This trend remained consistent across all NS ratios within identical MW and pH groups.

Using RCA, NS, and MW in combination helped identify advantageous SC outcomes across pH groups. The SC of D2_25_0 decreased by 5.2% compared to D_ref, with higher NS ratios resulting in up to a 29.2% reduction in D6_25_0. The utilization of MW enhanced SC, and increased MW exposure time positively affected concrete mixtures, reducing water absorption^31,130,138. Based on the obtained results, the D6_25_30 was the optimal mix, with a 32% reduction in SC relative to D_ref. Specimens with higher RCA ratios showed less improvement or even SC increases; for instance, the SC of D6_100_30, increased by 17.3%, whereas in D6_50_30 and D6_75_30 it decreased by 21.8% and 6.1%, respectively.

Despite SC variations with decreasing pH, the optimal combination of NS, RCA, and MW remained consistent. In group “A,” characterized by the most severe acidity, A6_25_30 showed a 31.3% SC reduction, increasing to 32% for B6_25_30 and 31.6% for C6_25_30. Conversely, A6_100_30 with MW exhibited a SC increase of 18.5%. This trend persisted across all specimen ages. Aging had varied SC effects regardless of RCA, NS, and MW use. In non-acidic condition, SC decreased by 5.1% and 12.3% at 56 and 90 days, respectively, whereas, in acidic groups A, B, and C, 90-day SC increased by 37.6%, 33%, and 24.8%, respectively.

Statistical analysis

In this section, an integrated assessment was performed to relate all indicators and to highlight consistent trends and potential synergies among the variables within the tested domain. To examine the statistical significance and relative contribution of each independent variable on the measured concrete properties, a one-way multivariate statistical analysis was conducted. An analysis of variance (ANOVA) was performed using ordinary least squares (OLS) models that included the main effects of the predictor variables, namely specimen age (Age), acidity of the acidic rain (pH), recycled concrete aggregate content (RCA), nano-silica dosage (NS), and magnetic water ratio (MW). Interaction terms were intentionally excluded to isolate the independent effects of each parameter and to avoid potential overfitting given the limited experimental dataset.

Robust covariance estimation (HC3) was employed to ensure reliability of the parameter significance tests under potential heteroscedasticity. For each model, the following statistical indicators were reported: The F-statistic and p-value for the ANOVA test, the individual regression coefficients with their corresponding robust standard errors, and the variance inflation factor (VIF) to evaluate multicollinearity. The statistical computations were carried out using Minitab software¹³⁹.

The ANOVA results confirmed that each of the five independent factors exerted a measurable and statistically distinct influence on the mechanical and durability-related performance of the concrete. Among the predictors, RCA and NS exhibited the highest F-values and contribution percentages across most responses, underscoring their dominant roles in controlling microstructural densification and ion transport resistance. The Age of the specimens was the next most influential factor, particularly affecting compressive strength (CS) and mass loss (ML) under acidic exposure.

As presented in Table 5, the regression analysis yielded strong coefficients of determination (R² values ranging from 0.89 to 0.94), demonstrating that the selected variables adequately captured the variation in all experimental results. The p-values associated with RCA, NS, and pH were below 0.05 in all models, indicating statistically significant effects. The robust HC3 estimation confirmed the consistency of coefficient signs and magnitudes even when potential heteroscedasticity was accounted for. The VIF values for all predictors were below 5, confirming that multicollinearity was not a concern. Overall, the ANOVA and regression results highlight that:

Table 5.

The results of ANOVA for CS, ER, SC, and ML of NS-MW treated RAC.

Test		Intercept	Age	pH	RCA	NS	MW	Residual
Compressive strength (MPa)	ANOVA
	DoF	–	1	1	1	1	1	954
	Sum of square	–	1999.1	21694.9	17549.9	1684.4	1118.2	6926.1
	F-value	–	275.35	2988.28	2417.36	232.01	154.02
	PR (> F)	–	1.6E−54	3.5E−296	9.2E−264	4.6E−47	6.8E−33
	Contribution (%)	–	3.9	42.5	34.4	3.3	2.2	13.6
	Regression (R2 = 0.8941, adj R2 = 0.8934)
	Coefficients	35.362	− 0.0569	2.8346	− 0.1209	0.5924	0.0965	–
	Robust SE	0.3938	0.0037	0.0598	0.0024	0.0401	0.0078	–
	t	89.79	− 15.27	47.34	− 48.82	14.78	12.36	–
	p-value	0.0E + 00	2.9E−47	1.2E−252	1.0E−261	1.0E−44	1.1E−32	–
Electrical resistivity (kΩ.cm)	ANOVA
	DoF	–	1	1	1	1	1	954
	Sum of square	–	57.7	11157.7	853.7	1551.9	23.5	704.1
	F-value	–	78.19	15117.5	1156.68	2102.75	31.79
	PR (> F)	–	4.4E−18	0.0E + 00	1.0E−166	1.8E−243	2.3E−08
	Contribution (%)	–	0.4	77.8	5.9	10.8	0.2	4.9
	Regression (R2 = 0. 9509, adj R2 = 0.9507)
	Coefficients	1.0015	− 0.0096	2.0328	− 0.0266	0.5686	0.0139	–
	Robust SE	0.1373	0.0011	0.0187	0.0008	0.0133	0.0025	–
	t	7.29	− 8.38	108.63	− 31.81	42.59	5.61	–
	p-value	6.5E−13	1.9E−16	0.0E + 00	5.6E−152	6.0E−223	2.6E−08	–
Sorptivity coefficient (kg/Sect. 0.5 .m2)	ANOVA
	DoF	–	1	1	1	1	1	954
	Sum of square	–	0.6	6.1	3.5	1.9	0.01	1.9
	F-value	–	298.93	2980.67	1704.89	907.47	5.53
	PR (> F)	–	1.8E−58	8.8E−296	1.4E−214	1.2E−140	1.8E−02
	Contribution (%)	–	4.4	43.5	24.9	13.2	0.1	13.9
	Regression (R2 = 0. 8908, adj R2 = 0. 8900)
	Coefficients	0.4095	0.0010	− 0.0477	0.0017	− 0.0197	− 0.0003	–
	Robust SE	0.0069	0.0001	0.0009	0.0001	0.0007	0.0001	–
	t	59.18	15.81	− 47.82	36.32	− 28.30	− 2.34	–
	p-value	1.4e−321	3.4E−50	1.3E−255	4.7E−182	2.0E−128	1.9E−02	–
Mass loss (%)	ANOVA
	DoF	–	1	1	1	1	1	954
	Sum of square	–	5.2	39.7	34.5	18.3	0.2	13.4
	F-value	–	372.29	2819.06	2448.41	1303.23	18.73
	PR (> F)	–	2.7E−70	4.3E−287	1.1E−265	1.3E−180	1.6E−05
	Contribution (%)	–	4.7	35.6	30.9	16.4	0.2	12.1
	Regression (R2 = 0. 8995, adj R2 = 0. 8988)
	Coefficients	1.1689	0.0029	− 0.1214	0.0053	− 0.0618	− 0.0014	–
	Robust SE	0.0159	0.0002	0.0027	0.0001	0.0018	0.0003	–
	t	73.27	17.38	− 44.83	43.25	− 33.77	− 4.30	–
	p-value	0.0E + 00	5.6E−59	4.6E−237	4.4E−227	4.4E−165	1.8E−05	–

• The RCA content negatively affects compressive strength and electrical resistivity, but increases capillary suction and mass loss.

• The incorporation of NS mitigates these adverse effects by refining the pore structure and enhancing durability.

• Lower pH levels (stronger acidity) increase degradation, while the use of MW slightly improves both strength and resistance parameters.

• The Age parameter consistently improves all mechanical and durability indicators due to continued hydration and pore refinement.

These findings confirm that the simplified, main-effect models can effectively predict concrete performance and can be used to estimate intermediate values of RCA, NS, MW, and pH beyond the tested range.

Mechanistic interpretation and central hypothesis

The central hypothesis of this research is that the exposure of mixing water to a magnetic field alters its molecular and ionic structure, which in turn accelerates cement hydration reactions and strengthens the ITZ between recycled aggregate and cement paste.

MW differs from tap water due to the rearrangement of hydrogen bonds and partial reorientation of water dipoles under the influence of the magnetic field. The Lorentz force acting on the charged species in water modifies their mobility and reduces the clustering of water molecules. This reduction in cluster size enhances the diffusion and reactivity of ions involved in cement hydration. Consequently, the dissolution of C₃S and C₂S is accelerated, and the rate of formation of CSH gel increases.

Additionally, the magnetic field slightly reduces the surface tension and viscosity of water, improving its ability to penetrate fine pores and wet cement particles uniformly. This leads to a more homogeneous distribution of hydration products within the paste and around the aggregate surfaces.

In RAC, the ITZ is typically weaker and more porous due to the presence of old mortar and microcracks in recycled aggregates. When MW is used for mixing, enhanced ion mobility and finer hydration products help fill micro-voids within this zone, resulting in a denser ITZ and improved mechanical bonding between the paste and recycled aggregate. Scanning electron micrographs from previous studies have shown that the CSH gel produced with MW is more continuous and compact, reducing microcracks and permeability pathways^140,141.

Therefore, the physical and chemical mechanisms proposed can be summarized as follows:

1. Hydrogen bond reorientation under magnetic field → smaller water clusters → increased ion mobility.

2. Accelerated hydration → faster C–S–H nucleation and lower CH content.

3. Reduced surface tension → enhanced wetting of cement grains and aggregate surfaces.

4. Densification of ITZ → stronger bond between recycled aggregate and paste, leading to improved strength and durability of RAC.

These mechanisms collectively explain the observed improvements in compressive strength, electrical resistivity, and reduced water absorption in MW-treated RAC, demonstrating that MW acts as a physical catalyst for hydration and microstructural refinement.

The results presented in this study clearly indicated that both MW and NS significantly enhanced the mechanical and durability performance of RAC under acidic exposure. When used individually, each factor improved selected properties; however, when used in combination, their effects were synergistic and consistently superior across all parameters and exposure conditions.

The use of MW alone increased the compressive strength by approximately 2.2–6.8%, depending on magnetization duration (10–30 min), whereas NS addition (2–6%) improved compressive strength by 2.4–6.8% relative to reference mixtures. When MW (30 min) and NS (6%) were used together, the enhancement reached up to 30%. Similarly, electrical resistivity, an indicator of microstructural densification and reduced ionic conductivity, increased by 12–65% under combined MW–NS conditions. The same mixtures exhibited reductions of 40% in mass loss and sorptivity coefficient compared with reference concrete (TW without NS). These improvements were observed even at the most aggressive pH = 2.5 (Group A), confirming the robustness of the synergy under strong acid corrosion.

MW physically alters water’s molecular structure, decreasing the hydrogen bond angle from 104.5° to approximately 101–103° and thereby lowering its surface tension by 5–8%. This modification produces smaller water clusters and enhances the diffusion of Ca²⁺ and OH⁻ ions during cement hydration. As a result, hydration kinetics accelerate, yielding more uniform and denser CSH gels with reduced capillary porosity. In parallel, NS chemically reacts with CH to form additional CSH gel via pozzolanic reactions, filling nano-voids and refining the ITZ between the recycled aggregates and the cement matrix. This dual process (physical activation by MW and chemical refinement by NS) yields a denser, less permeable concrete with higher resistance to acid ingress and leaching.

These findings align with prior works, but extend them in scope and depth. For instance, Reddy et al.³¹ and Ramalingam et al.³⁶ reported 10–23% increases in compressive strength for MW-treated conventional concrete, while Li et al.²⁵ and Kazmi et al.⁴⁶ observed 20–35% durability improvements with NS-modified RAC. The present results fall within and reinforce these ranges, demonstrating that the combined MW–NS approach yields an even greater overall enhancement (up to 65% in ER and 30% in CS), accompanied by marked reductions in acid-induced degradation indices (40% in ML and SC). The results also support the mechanistic model suggested by¹⁴², which links MW-induced cluster size reduction to increased CSH nucleation density. The simultaneous addition of NS amplifies this effect by supplying reactive silica for secondary CSH formation, effectively densifying both the bulk matrix and ITZ.

Therefore, it can be concluded that the synergy between MW and NS provides a twofold mechanism of protection for RAC under acidic environments: (i) physical enhancement through accelerated hydration and reduced pore connectivity induced by MW, and (ii) chemical strengthening via pozzolanic activity of NS and ITZ densification. Together, these effects explain the observed macroscopic improvements in compressive strength, resistivity, and reduced mass loss, providing a unified mechanistic framework consistent with the literature and supported by the quantitative results of this study.

Conclusion

This study investigated the combined influence of MW and NS on the mechanical and durability properties of RAC exposed to simulated acid rain with pH levels of 2.5, 4.0, 5.5, and 7. A total of 80 concrete mixes were tested by varying RCA content (0–100%), NS dosage (0–6%), and MW exposure time (0–30 min). Based on the experimental results, the following key conclusions can be drawn:

1. Acid rain significantly reduced the mechanical performance of concrete. Increasing the acidity from pH 7 to 2.5 caused the compressive strength of reference concrete to decrease by 16.2–25.4%, confirming the severe corrosive effect of low-pH exposure. Also, replacing natural aggregate with RCA reduced compressive strength by an average of 3.2%, 6.4%, 15.9%, and 25.4% for 25%, 50%, 75%, and 100% replacement levels, respectively, due to the higher porosity and weaker ITZ.

2. Adding NS improved the compressive strength by 2.4–6.8%, the electrical resistivity by 12–32%, and reduced mass loss and sorptivity coefficient by 28–36% compared to control specimens without NS. The optimal NS dosage was found to be 6%, which provided the most stable performance across all pH conditions. Also, substituting tap water with MW improved compressive strength by 2.2–6.8% and electrical resistivity by up to 4.6% at 30 min exposure time, mainly due to enhanced cement hydration and reduced porosity.

3. The combined use of MW (30 min) and 6% NS provided the most significant improvement, yielding a 14.1% increase in compressive strength, up to 38.4% rise in electrical resistivity, and reductions of 33% in mass loss and 32% in sorptivity coefficient compared to the reference mix. The best overall performance for all RAC specimens was achieved with 6% NS + 30 min MW, which optimized both mechanical and durability properties under all acidity levels.

4. After 90 days, non-acidic specimens (pH 7) exhibited a 10.9% increase in compressive strength and up to 11% reduction in sorptivity coefficient, while specimens under pH 2.5 lost approximately 27% of compressive strength and showed higher mass loss and sorptivity coefficient, indicating that acidity intensifies degradation over time.

Overall, MW and NS act synergistically to refine the RAC microstructure, densify the ITZ, and mitigate the strength and durability losses caused by recycled aggregates and acid exposure. This strategy provides a cost-effective and eco-efficient pathway for improving the sustainability of concrete in acid rain prone regions.

Limitation and recommendation

The study used simulated acid rain at specific pH levels. Real-world acid rain can vary in composition and intensity, which may affect the long-term performance of the RAC differently. Future studies should investigate the performance of RAC under different acid rain compositions (varying concentrations of sulfuric acid, nitric acid, etc.) and intensities to simulate real-world conditions more accurately. Also, the study primarily focused on compressive strength. Other important mechanical properties like flexural strength, tensile strength, and modulus of elasticity did not evaluate. Thus, assessing other important mechanical properties can provide a more comprehensive understanding of the structural performance of RAC. On the other hand, future research will focus on conducting detailed microstructural analyses to better elucidate the mechanisms underlying the improved performance of microwave-treated recycled aggregates.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Omid Bamshad and Sheyda Salehi conceived and designed the study. Omid Bamshad, Alireza Habibi, and Parvin Montazeri performed the experiments and collected the data. Navid Manouchehri responded to the comments. Iman Hakamian analyzed and interpreted the data. Omid Bamshad and Mahdi Mahdikhani wrote the main manuscript text. Sheyda Salehi prepared the figures and tables. All authors critically reviewed the manuscript and approved the final version for submission.

Data availability

The datasets generated and/or analysed during the current study are available in the appendix repository.

Declarations

Competing interests

The authors declare no competing interests.