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Article

Optimizing Alkali-Activated Mortars with Steel Slag and Eggshell Powder

by
Behailu Zerihun Hailemariam
1,
Mitiku Damtie Yehualaw
1,*,
Woubishet Zewdu Taffese
2,3,* and
Duy-Hai Vo
4
1
Faculty of Civil and Water Resource Engineering, Bahir Dar Institute of Technology, Bahir Dar University, Bahir Dar 6000, Ethiopia
2
School of Research and Graduate Studies, Arcada University of Applied Sciences, Jan-Magnus Jansson Aukio 1, 00560 Helsinki, Finland
3
Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO 65401, USA
4
Department of Civil Engineering, University of Technology and Education—The University of Danang, 48 Cao Thang Street, Hai Chau District, Da Nang 550000, Vietnam
*
Authors to whom correspondence should be addressed.
Buildings 2024, 14(8), 2336; https://doi.org/10.3390/buildings14082336
Submission received: 30 June 2024 / Revised: 21 July 2024 / Accepted: 25 July 2024 / Published: 28 July 2024
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

:
The cement industry is known for being highly energy-intensive and a significant contributor to global CO2 emissions. To address this environmental challenge, this study explores the potential of using the waste materials of steel slag (SS) and eggshell powder (ESP) as partial replacements for cement in alkali-activated mortars (AAMs) production, activated by NaOH and Na2SiO3. Mortar samples are prepared with 50% of ordinary Portland cement (OPC) as part of the total binder, and the remaining 50% is composed of ESP, incrementally replaced by SS at levels of 10%, 20%, 40%, and 50%. The activation process was performed with an 8% NaOH concentration and a silica modulus of 2. Key findings include that the workability of AAMs decreased with increasing SS content, requiring admixtures like superplasticizers or additional water to maintain workability. At 50% SS replacement, the water consistency and slump flow values were 32.56% and 105.73 mm, respectively, with a setting time reduction of approximately 36%, losing plasticity within 2 h. Both absorption capacity and porosity decreased as SS content increased from 10% to 50% of ESP. Additionally, the bulk density, compressive strength, and uniformity of the hardened mortar samples were enhanced with higher SS content, achieving maximum compressive strength (28.53 MPa) at 50% SS replacement after 56 days of curing. Furthermore, OPC-based AAMs incorporating SS and ESP demonstrate good resistance to sulfate attack and thermal heating. Microstructural analysis reveals the presence of C–S–H, C–A–S–H, and N–A–S–H phases, along with minor amounts of unreacted particles, and the microstructure shows a dense, highly compacted, and homogeneous morphology. These findings suggest that replacing eggshell powder with up to 50% steel slag enhances the hardened properties of AAMs. Further research is recommended to explore cement-free alkali-activated granular ground blast furnace slag (GGBFS) with ESP for more sustainable construction solutions.

1. Introduction

Cement plays a vital role in humanity’s progress by facilitating the construction of essential housing and infrastructure worldwide. However, the cement industry stands as one of the most energy-intensive sectors globally, consuming nearly 15% of the energy consumed by the entirety of the manufacturing sector [1]. Over the past decade, annual production has surged to 3.4 billion tons, accompanied by significant energy consumption, primarily thermal and electrical, resulting in significant environment impact [2,3]. Roughly 11% of industrial energy usage is attributed to cement production alone [4]. On average, producing one ton of cement requires 3.4 GJ of thermal energy (in the dry process) and 110 kWh of electrical energy, representing over 6% of global energy consumption. Additionally, cement production contributes to approximately 5–7% of the total worldwide carbon dioxide (CO2) emissions, with each ton of cement emitting around 900 kg of CO2 [5,6,7,8,9,10]. These emissions significantly contribute to global warming, accounting for about 65% of the phenomenon [11,12]. Furthermore, the production of 1.6 billion tons of cement requires a staggering 2.5 billion tons of raw materials, such as limestone and clay, exacerbating resource depletion concerns [13].
To tackle these challenges, researchers have explored substituting cement with various alternative materials, including natural minerals [14], construction and demolition wastes [15], industrial byproducts [16,17,18], and agricultural wastes [19]. One of the most effective, cost-efficient, and sustainable strategies involves adopting a loop system that converts waste resources into valuable products instead of relying solely on natural resources. This approach offers multiple benefits, not only in reducing CO2 emissions from the cement industry but also in preventing natural resource depletion [20,21]. Additionally, excessive waste in the environment can pose critical risks to humans, animals, and vegetation. Repurposing waste materials destined for landfills promotes waste minimization and diversion, thus advancing more sustainable waste management practices. The partial replacement of OPC with agricultural waste typically ranges from 10% to 30% [22,23].
Eggshells, often regarded as agricultural waste, are a significant contributor to pollution [24]. Globally, 8.4 billion kilograms of chicken eggshell waste are generated annually [25], with the majority ending up in landfills, incurring high management costs [22,23,25,26,27]. According to the Environmental Protection Agency statistics, eggshell waste ranks as the 15th largest contributor to environmental pollution when not properly disposed of in designated locations [28,29,30,31]. Eggshells have a composition similar to that of limestone, consisting of 93.70% calcium carbonate (CaCO3). Calcining eggshell ash at around 500 °C converts the CaCO3 into calcium oxide (CaO), also known as quicklime, which acts as a powerful accelerator in cementitious systems. These accelerators enhance the early strength development and reduce the setting time of alkali-activated binders (AABs), materials formed by the reaction of an alkaline activator solution with certain precursor materials.
AABs represent an alternative to traditional cement-based binders and have attracted attention for their potential to reduce CO2 emissions and utilize industrial byproducts. These binders can be categorized based on their cementitious components, particularly the CaO–SiO2–Al2O3 system, into high-calcium and low-calcium cements [32]. Ground granulated blast furnace slag (GGBFS) is one of the AABs in high-calcium binders, [33,34,35]. The high calcium content in these precursors contributes to the formation of calcium silicate hydrate (C–S–H) gel, similar to that found in ordinary Portland cement (OPC). In low-calcium AABs, the main binder formed is typically N–A–S–H gel (sodium alumino-silicate hydrate), which is derived from the reaction between the alkali activator and the precursor materials rich in silica (SiO2) and alumina (Al2O3). The chemical formula for N–A–S–H gel is Na2O–Al2O3–SiO2–H2O. N–A–S–H gel is analogous to the calcium silicate hydrate (C–S–H) gel found in OPC concrete. Fly ash, metakaolin, and natural pozzolans are among the most common binders in low-calcium AABs.
Eggshell powder (ESP), rich in calcium oxide, has shown potential in replacing cement in mortar and concrete while enhancing their engineering properties [8,36,37,38,39,40]. Recent research has examined the effectiveness of ESP in this regard. A series of compressive strength tests conducted by [8,25,31] found that the optimum compressive strength was attained with a dosage of 5% ESP on both the 7th and 28th days of the curing period. However, the addition of ESP to cement-based mortar and concrete may reduce workability [8,36,40,41]. In addition, [42] observed that the rate of water absorption decreased by up to 75% with the addition of ESP into the concrete. Ref. [43] conducted chloride penetration tests of concrete using electrical resistivity and found that the corrosion resistance of concrete samples increased with the inclusion of ESP. However, none of these studies have conclusively proven ESP’s ability to resist some mechanical and environmental actions. Furthermore, previous investigations have shown the engineering properties and performance of cement-free mortar and concrete using alkali-activated GGBFS and fly ash with various agricultural wastes and construction and demolishing wastes such as crushed ceramic waste and brick powders. Nevertheless, there has been insufficient research on the engineering properties of ESP and cement-based alkali-activated binders’ mortars and concrete to fully or partially replace cement. Therefore, the objective of this study is to address this research gap. The novelty of this work lies in its pioneering investigation into the synergistic effects of combining ESP and alkali-activated steel slag as a sustainable alternative to conventional cement. By exploring the comprehensive engineering properties, including the fresh, hardened, and microstructural characteristics, this study provides new insights into the feasibility and performance of innovative binder systems. This research not only fills a significant gap in the existing literature, but also paves the way for more environmentally friendly and cost-effective construction materials.

2. Materials and Experimental Methods

2.1. Materials Used

In this study, cement, ESP, and steel slag (SS) were the main binders used in sample preparation. Ordinary Portland cement (OPC) grade 42.5R from Dangote Cement factory was adopted, and its quality was tested in accordance with ASTM C 1084 [44]. The OPC had a specific gravity of 3.15 with a surface area of 318 m2/kg, meeting all ASTM C 150 [45] standards. For the ESP, chicken eggshell waste was collected from different food-related enterprises, including cafeterias, restaurants, hotels, and poultry farms in and around Arba Minch town, Ethiopia. The eggshells were sun-dried for 5 to 7 days, manually crushed into small pieces, and then oven-dried at 100 to 105 °C for 24 h. Steel slag scrapes were collected from Kotebe Metals Factory in Addis Ababa, Ethiopia. The slag was initially sun-dried for 3–5 days and then burned with benzene for 30 min in a large metal pan to remove lubricant oils and grease. Both the oven-dried and calcined eggshells and steel slag were finely ground using a Los Angeles testing machine with 10 balls with a diameter of 48 mm for 30 to 60 min. The resulting powders, passing through 75 µm and 150 µm sieves, were used. The ESP and SS powders appeared dark gray and black, respectively (Figure 1). The summarized physical properties and chemical compositions of the binders are presented in Table 1. Chemical composition analysis was performed using an X-ray fluorescence (XRF) spectrometer. The BET surface area and specific gravity of the ESP were measured as 543.51 m2/g and 2.62, respectively, and that of the steel slag were 544.22 m2/g and 3.48. As shown in Table 1, XRF analysis revealed that ESP was predominantly composed of CaO (70.40%), which is higher than in OPC, with moderate amounts of MgO (6.19%), SiO2 (3.90%), and Al2O3 (1.29%). For SS, the oxide present at the highest level in the composition was Fe2O3 (77.88%), with significant amounts of SiO2, Al2O3, and CaO. The content of SiO2, Al2O3 and Fe2O3 (85.36%) exceeded the minimum oxide content requirement of 70% given in ASTM C 618 [46].
The scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns of the tailing powders of ESP and SS are shown in Figure 2 and Figure 3, respectively. In Figure 2, the SEM images reveal that the ESP particles are mostly angular and irregularly shaped, while SS particles are smooth, sharp, and spherical. The XRD patterns in Figure 3 indicate that both ESP and SS consist of crystalline quartz as the major crystalline phase, with calcite and mullite as minor amorphous phases. The XRD patterns were obtained using the random powder method with a BRUKER diffractometer model D2-PHASER, with Cu Kα radiation and a 2θ of 10–60°.
Washed natural river sand was collected and transported from Konso in South Ethiopia Regional State, Ethiopia, due to its superior quality and absence of harmful impurities. The sand passing through 4.75 mm sieves was used as a fine aggregate. The gradation curve of the fine aggregate, obtained from a sieve analysis test conducted according to ASTM C117 [47], is shown in Figure 4. Additionally, Table 2 presents the test methods and respective results for the physical properties of the fine aggregates, confirming that they meet the ASTM standards.
Additionally, commercially available white-flake sodium hydroxide (NaOH) pellets with 98% purity and sodium silicate (composition: SiO2 = 25.7%, Na2O = 8.26%, H2O = 66.04%, and SiO2/Na2O = 3.11) were used as the alkaline solution, as identified in [52,53,54]. Clear potable water which was obtained from the laboratory was used for mixing.

2.2. Mix Proportions

The mix design for this study was prepared for cement and ESP-based alkali-activated steel slag mortar (AAM) samples with a cement-to-sand ratio of 1:2.75 at a constant water-to-binder ratio of 0.48, in accordance with ASTM C452 [55]. In addition to including OPC in the mixture, the binders (ESP and SS) were activated using sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) to achieve better early- and later-age strength. The NaOH solution was prepared by dissolving pellets in deionized water for 30 min to attain a uniform solution with a concentration of 8% by water weight, whereas the Na2SiO3 solution was prepared with a silica modulus (Na2SiO3/NaOH) of 2 by mass. Both solutions were cooled and stored independently at ambient temperature for 24 h prior to use. The alkali-activated mixtures of OPC, ESP, and SS were prepared with a fixed OPC content of 50% from the total binders and varying the amount of SS in the range of 10–50%, with 10% increments, as a partial replacement of ESP from the remaining 50% of the binder. The mixtures were coded as ESP-90, ESP-80, ESP-60, and ESP-50. For example, ESP-90 means 50% OPC, 45% ESP, and 5% SS, indicating that ESP and SS make up 90% and 10%, respectively, of the remaining 50% of the binder. The detailed mix proportions are shown in Table 3 in terms of the percentage and mass of materials for 1 m3 cubes of mortar samples.

2.3. Sample Preparations and Test Methods

Initially, the NaOH and Na2SiO3 solutions were measured based on the mix design and mixed together to achieve a homogeneous mixture. Subsequently, the OPC, ESP, and SS were manually combined in a mixing pan to maintain the uniformity of the blended powders. Then, the sieved fine aggregates were added to the blended binders and manually mixed for 5 min. Following this, trial mix tests were conducted to achieve the desired slump and consistency as per ASTM C1437 [56] and ASTM C187 [57], respectively. The alkali-activator solutions were then mixed manually with the blended powders and sand for 3 min in the mixing pan. The fresh mortar mix was poured into cubic molds sized 50 × 50 × 50 mm for testing the properties of the hardened mortar samples. After casting the specimens, the molds were covered with plastic and allowed to air dry at a room temperature of 25 ± 2 °C and a relative humidity of 65 ± 5% for 48 h. Subsequently, after curing the specimens for 24 h, all samples were demolded and placed in a concrete curing chamber cabinet maintained at a temperature of 25 ± 2 °C and a humidity level of 95 ± 5% until the testing day. Additionally, the fresh properties were evaluated by conducting setting time and soundness tests in accordance with ASTM C191 [58] and ASTM C151/C151M [59].
The mechanical performance of the alkali-activated mortar was assessed through compressive strength tests for curing periods of 3, 7, 28, and 56 days, following the ASTM C109 standards [60]. To examine the quality and internal homogeneity of the hardened AAMs, ultrasonic pulse velocity (UPV) tests were performed at the same ages as the compressive strength test following ASTM C597 [61]. The UPV results were also utilized to correlate and compare with the compressive strength values. Moreover, the absorption capacity, bulk dry density, and void space (air content) of the AAMs were determined following the ASTM C642 [62] procedures. The samples were oven-dried at a temperature of 100 to 110 °C for a minimum of 24 h and allowed to cool in dry air in a desiccator to a temperature of 20 to 25 °C. After drying and cooling, the specimens were immersed in water at approximately 21 °C for no less than 48 h, and the surface moisture was removed with a towel. Subsequently, the samples were boiled in tap water in a covered receptable for 5 h, and finally the specimens were suspended by a wire in water. Equations (1)–(3) show the calculations for determining the absorption capacity, bulk dry density, and void space, respectively.
Absorption   after   immersion   and   boiling ,   % = B − A / A   ×   100
Bulk   dry   density ,   kg / m 3 = A / B − C   ×   ρ
Volume   of   permeable   pore   or   void   space ,   % = B − A / B − C   ×   100
where:
A = mass of oven-dried sample in air, kg;
B = mass of surface-dry sample in air after immersion and boiling, kg;
C = apparent mass of sample in water after immersion and boiling, kg;
ρ = density of water, 1 mg/m3 = 1 g/cm3 = 1000 kg/m3.
Furthermore, a sulfate attack test was conducted to assess the durability of the alkali-activated mortar samples by evaluating their resistance to acid attack. This involved immersing the hardened AAMs in an acidic solution. Mortar cubes of 50 × 50 × 50 mm were cast from each mix, and then cured for 28, 56 and 90 days. Subsequently, they were soaked in a 5% MgSO4 solution following ASTM C1012 [63] and the procedures outlined in [64,65]. After soaking, the cubes were dried for 24 h at a standard room temperature of 25 ± 2 °C. Sulfate resistance was determined by measuring the loss in compressive strength with respect to the specimens not exposed to sulfate solutions.
The microstructure behavior of the specimens was characterized by SEM, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). Small slices from broken sections of the samples, post-compressive strength testing, were examined. These slices were immersed in ethanol with 97% alcohol purity for 24 h to halt hydration, then oven-dried for 24 h at a temperature of 105 ± 5 °C, as reported in [66]. SEM analysis was performed using post-compressive strength testing (JEOL model JSM-6390LV) on pieces of the soaked and dried samples. The SEM images were used to identify hydration products such as calcium silicate hydrate gel (C–S–H), calcium hydroxide (C–H), ettringite, and other unreacted particles and pore spaces within the AAM specimens.
Regarding the FTIR analysis and TGA, crushed and ground powders of AAMs were obtained by passing them through a 75 µm (#200) sieve. Alongside the SEM images, similar chemical compounds were identified by using FTIR tests, including unreacted particles, C–S–H, C–A–S–H, and N–A–S–H gels, as well as carbon and water within the mortar samples. This was accomplished using a DIGILAB FTS-3500 with a transmittance wavelength range of 400–4000 cm−1. TGA tests were conducted to evaluate the thermal stability of the OPC–ESP-based alkali-activated steel slag mortar samples using a PerkinElmer STA6000 machine, with a temperature range of 40–800 °C and a heating rate of 20 °C per 5 min. Table 4 stipulates the standard test methods used, the curing ages, and the number of mortar cube samples cast to assess the properties of AAM specimens.

3. Results and Discussions

3.1. Fresh Properties

3.1.1. Water Consistency

Water consistency refers to the amount of water required to achieve a cement paste of standard or uniform consistency. Figure 5 illustrates the consistency results of OPC- ESP-based alkali-activated steel slag paste produced with varying contents of ESP and SS. According to this figure, the cement consistency decreased as the percentage of ESP replaced with SS increased. The water demand to achieve consistency increased from ESP-90 to ESP-50. The highest water consistency was observed when 50% ESP was replaced with SS, with an average consistency value of 32.56%. This increase could be attributed to the porous surface of the SS, which absorbs more mixing water [54]. Conversely, the lowest consistency was observed for ESP-90, with a value of 27.56%, due to the inclusion of only a small quantity of SS, which absorbs less water. These results demonstrate that including SS improved the normal consistency of the blended alkali-activated pastes. Compared to the ESP-90 mortar mix, the water demand increased by 18% for the ESP-50. Overall, the water consistency values for all mixes ranges from 26% to 33%, in accordance with the Ethiopian Standard Agency [64].

3.1.2. Slump Flow

The test results for alkali-activated mortar cast from OPC and various compositions of ESP and SS are shown in Figure 6. This figure indicates that the slump flow of the mortar declined with the inclusion of steel slag from 10% to 50% in place of eggshell powder. The slump flow values ranged from 105.73 mm to 114.59 mm, falling within the ASTM requirement range of 110 ± 5 mm. The maximum slump flow was achieved for the mortar containing 50% OPC, 45% ESP, and 5% SS (ESP-90), with a value of 114.59 mm. The workability of the mortar was negatively impacted by the partial replacement of ESP with 50% of SS due to the high surface area and tension, chemical composition, mineralogical behavior, and rough pore surface characteristics of the steel slag. These properties of SS increase the absorption of mixing water, reducing workability. The inclusion of a high amount of ferrous-based pozzolanic materials can lead to high resistance (high viscosity), preventing the mixture from flowing freely [53,67]. Compared to the ESP-90 mix, the mortar with 25% SS (ESP-50) required 8% additional water to achieve the workable consistency. Figure 6 also reveals that the ESP-90 mix mortar achieved better flowability compared to ESP-80 and ESP-60. This indicates that more water was needed to lubricate mixes with higher SS contents, which reduced the fluidity of the fresh mortar.

3.1.3. Setting Time

Figure 7 illustrates the setting time of five mixtures of the alkali-activated SS slag paste incorporating OPC and ESP. The initial and final setting times ranged from 50.27 to 86.22 min and from 206.40 to 327.38 min, respectively. It is evident that increasing the percentage of steel slag from 10% to 50% in the mix significantly accelerates the setting time of the paste. Substituting ESP with SS from 10% to 50% reduced the initial setting time by 41.7% (35.95 min). However, the cement paste completely loses its plasticity and becomes hard and stiff in just over 2 h (37%). Similar findings have been reported elsewhere [68], suggesting that factors such as NaOH concentrations and using a silica modulus beyond 1.3 can reduce the setting time. This may be attributed to the particle size and high water absorption tendency of steel slag through its pores along with the relatively high silica level in steel slag compared to eggshell powder [69,70]. The presence of alkalic cation metal elements (Na+ and K+) in the alkaline materials could also play a significant role, as they have a catalytic effect [71,72,73]. Consequently, additional plasticizers or a higher water demand is necessary to achieve a paste with better workability. The setting time results align with those obtained from consistency and slump tests. Importantly, the setting times remained within the required range stated by both the ASTM and Ethiopian standards.

3.2. Hardened Properties

3.2.1. Bulk Dry Density

Bulk dry density measurements of the hardened AAM cube samples were conducted according to the ASTM C642 [62] test standard at curing periods of 3, 7, 28, and 56 days. The findings, as shown in Table 5, indicate that the bulk dry density of all mix types containing OPC, ESP, and SS complied with the BS EN 988–2 standard [74] and exceeded the minimum density requirement for masonry mortar (1300 kg/m3) set by the standard. The lowest density was recorded for the mix composition of 50% OPC, 45% ESP, and 5% SS (ESP-90), with a value of 1519.18 kg/m3 at the 3rd day of curing. However, the maximum density of 1660.31 kg/m3 was observed on the 56th day for the ESP-90 mortar mix. The density of all mortars cube sample improved with the inclusion of SS content from 10% to 50% instead of ESP, regardless of the curing period. Additionally, denser samples were developed over the curing periods of the AAMs. Comparing the ESP-50 mortar mix to ESP-90 cubes, ESP-50 demonstrated superior density results, with differences of 86.10 kg/m3, 96.54 kg/m3, 54.79 kg/m3, and 73.93 kg/m3 at days 3, 7, 28, and 56, respectively. The higher specific gravity of steel slag relative to the other binders contributed to achieving better density for the mortar.

3.2.2. Absorption Capacity and Porosity

Figure 8a,b illustrate the combined water absorption capacity and porosity (pore/air space) of mortar mix samples composed of OPC and ESP and SS binders and activated by NaOH and Na2SiO3 solutions. The AAM cubes were prepared with a constant amount of OPC and varying replacement levels of ESP with SS from 10% to 50% and were investigated after 3, 7, 28, and 56 days. The trend of the results (Figure 8a) indicates that the ability of the samples to absorb water through the pore spaces significantly decreased with increasing SS content and curing period. At the early stage of curing (3 and 7 days), the maximum absorption of mortar cubes was observed for ESP-90, with absorption rates of 10.41% and 8.58% at 3 and 7 days, respectively. Similarly, the best results were 5.86% and 5.10% for ESP-50 mixes. Ref. [75] suggested that the addition of eggshell powder improves the microstructure of concrete, thereby reducing absorption capacity. The difference in absorption capacity between ESP-90 and ESP-50 mortar blends was 4.55% at 3 days and 2.30% at 56 days. By the end of the 16-week curing period (56 days), the water absorption capacity decreased to a minimum of 2.91% and 0.61% for the ESP-90 and ESP-50 mix types, respectively. Previous research by [76] suggested that the SS content in alkali-activated mortar strongly affects its water absorption properties.
Additionally, as shown in Figure 8b, the accumulated void or pore space in AAMs gradually decreased with the partial replacement of ESP with different amounts of SS. Ref. [77] observed that larger voids diminished and transformed into smaller pores with the inclusion of SS and as the curing age increased. At the 3rd day of curing, the air space content per percentage sample of mortar from ESP and SS substitution levels of 10%, 20%, 40%, and 50% showed porosities of 21.45%, 16.42%, 14.39%, and 12.97%, respectively. However, for similar replacement contents, the void volumes began to decrease during the 56th day, with reductions of 60%, 73%, 82%, and 90%. A denser mortar structure was observed compared to the early-age specimens, as indicated by bulk dry density. This reduction in void volume is attributed to the pozzolanic reaction, the growth of chemical compounds from cement hydration, and the presence of large CaO compositions from the cement [78]. These factors fill the pores, decreasing their volumes and numbers over time.

3.2.3. Compressive Strength

The compressive strength of OPC-based alkali-activated steel slag mortar containing eggshell powder at curing periods of 3, 7, 28 and 56 is illustrated in Figure 9. Higher compressive strength was observed in ESP-50 compared to other mixes, and the compressive strength increased sharply with increasing ESP content and age for all mixes. It is normally estimated that AAMs using a relatively higher content of steel slag exhibit higher compressive strength, and this result is consistent with the findings of [79]. Figure 9 clearly shows that the mortar cubes exhibited early compressive strength gains of 7.87 MPa, 10.68 MPa, 11.71 MPa, and 12.22 MPa for ESP-90, ESP-80, ESP-60, and ESP-50, respectively. Compared to the ESP-90 samples, ESP-50 samples showed a 47% and 37% improvement in compressive strength at 28 and 56 days, respectively. This strength development of alkali-activated slag-based materials was attributed to the formation of calcium silicate hydrate gel (C–S–H) with compositional and structural differences [80].
The maximum compressive strength was achieved by all AAM cubes when the steel slag replacement ratio rose to 50% of the ESP content. At days 7 and 28, the compressive strength of alkali-activated mortar composition of 50% OPC, 40% ESP, and 10% SS (ESP-80) reached about 93% of that of ESP-50, while ESP-90 (50% OPC, 45% ESP, 5% SS) attained 95% of the strength of ESP-50. Interestingly, increasing SS beyond 40% of the ESP or 20% of the total binder content did not show any significant effect on the compressive strength. According to another study [81], including a high amount of SS (50% of the total binders) and the coarse pore structure resulting from localized hydration products near the steel slag grains contribute to the slower strength development of mortar cubes beyond ESP-50.

3.2.4. Homogeneity

The homogeneity or uniformity of the alkali-activated mortar samples was assessed using the UPV test method, which measures the time taken for a pulse to transfer from one end of the mortar cube to the other end in meters per second (m/s). This test method evaluates the closeness and compactness of the hydration products within the mortar samples. According to IS 13311 (Part 1) [82], samples with higher UPV values indicate better uniformity and quality. The UPV results of all of the alkali-activated mortar mixes are presented in Figure 10, as a function of ESP and SS content and curing ages. The experimental results reveal that the UPV values ranged from 2804.83 m/s to 3496.33 m/s.
Based on this figure, it can be observed that the quality and homogeneity of AAM mixes were enhanced with the partial replacement of ESP with SS and increasing curing ages. ESP-90 samples cured for 3 days were classified as doubtful or poor-quality according to the IS 13311 (Part 1) concrete quality grading and classifications. This could be due to most of the hydration products staying close to the slag grains, leaving the interstitial space comparatively free, which may result in an uneven microstructure. A similar phenomenon was noted in another study [83]. A barrier to ion diffusion could be formed by more dense precipitates deposited in ESP-90 and ESP-80 mortar mixes, which would result in an uneven microstructure. The other mix types were categorized as medium grading, but the UPV values of the ESP-60 and ESP-50 cubes cured for 28 and 56 days approached good quality. Referring to Figure 10, at the curing period of 56 days, the time taken to travel the 5 cm width of the mortar mixes prepared from ESP-60 and ESP-50 was as fast as 154.60 m/s and 201.36 m/s when compared to ESP-90, respectively. Substituting ESP with 50% SS constantly upgraded the quality, homogeneity, and uniformity of the mortar by 6% at ages of 7, 28, and 56 days compared to when the SS amount was 5% of the ESP content. This trend aligns with results found in the bulk dry density, pore space, and compressive strength of the AAMs. The uniformity properties of the alkali-activated mortars measured closely matched those reported earlier by [84]. Additionally, [76] observed and argued that the fast hydration rate at later ages and a dense, compacted, and homogeneous interface transition zone (ITZ) are attributed to the better quality in alkali-activated concretes and mortar. Moreover, Figure 11 illustrates the relationships between absorption capacity and the variables of compressive strength and UPV. This figure reveals that both compressive strength and UPV have an indirect correlation with the absorption capacity, with an R2 value of 0.90287 and 0.91552, respectively.

3.2.5. Sulfate Resistance

Sulfate attack can occur when cement is exposed to solutions containing sulfates, such as some natural or polluted groundwaters. In OPC, this type of attack can lead to strength loss, expansion, spalling of surface layers, and ultimately disintegration. However, alkaline inorganic polymer cements, including alkali-activated metakaolin and fly ash, have been found to exhibit excellent resistance to conventional sulfate attack and seawater due to the absence of high-calcium phases [83]. The resistance to sulfate attack of the alkali-activated steel slag mortar incorporating eggshell powder was assessed by measuring the loss in compressive strength compared to cubes immersed in sulfate-free solutions. Table 6 displays the results of the mortar samples. As shown in this table, the addition of steel slag into the mortar mixes constantly improved the sulfate resistance of the samples over the curing time. However, all samples exposed to magnesium sulfate solutions showed a notable reduction or loss in compressive strength.
The analysis findings demonstrate that mortar made with 25% SS content (ESP-50) and preserved in a sulfate solution exhibited very good stability when immersed in sulfate solutions, achieving mechanical strengths comparable to those attained using a sulfate-free solution. Only a 0.20 MPa and 0.69% loss in compressive strength were observed when the mortar samples were prepared from ESP-50 and cured for 56 days. This result aligns with the previous findings provided by [85]. Nevertheless, this table reveals that ESP-90 mixes experienced the maximum loss in compressive strength at all curing stages, with reductions of 2.09 MPa (26.54%) and 1.45 MPa (6.95%) at the 3rd and 56th days, respectively. The breakdown of calcium–alumino–silicate–hydrates (C–A–S–H) and sodium–alumino–silicate–hydrates (N–A–S–H), as well as the migration of alkalis from the specimens into the solution, superimposed by the acid attack, may have contributed to the notable strength reduction observed in the ESP-90 mixes [83]. Another factor mentioned by [78] is the chemical reactions between monosulfate and the large amount of CaO (Table 1) presented in OPC and ESP, forming calcium hydroxide with C–A–S–H, N–A–S–H, and sulfate ions, leading to the production of crystal needles (ettringite) and gypsum in the mortar-free spaces.

3.3. Microstructural Properties

3.3.1. Fourier Transform Infrared Spectroscopy

The FTIR test aims to identify the mineralogical composition within paste, mortar, and concrete specimens at different wavenumbers. Figure 12, Figure 13 and Figure 14 show the experimental FTIR results of the mortar sample composed of OPC, ESP, and SS cured for 7 days (Figure 12), 28 days (Figure 13), and 56 days (Figure 14) for mortar mixes of ESP-90 and ESP-50. The FTIR spectra at the 7th day, as shown in Figure 12, indicate that the major peaks were approximately similar for all selected samples. The main peaks observed for ESP-90 specimens were 3915–3590, 1630, 1565, 1415, 1260, 1030, and 815–515 cm−1, while for ESP50, they were 3910–3570, 1620, 1560, 1410, 1230, 970, and 815–515 cm−1. This reveals that the inclusion of SS instead of ESP within the mortar mixes at varying contents had an insignificant effect on the formation of hydration products. Ref. [68] suggested that a broad band at 980 cm−1 was associated with a highly depolymerized silica network, mainly contributed by the unreacted slag at earlier ages. Based on Figure 12, the presence of unreacted particles, particularly ferrous (Fe2O3) from ESP and SS, was detected within the bandwidth between 815 and 515 cm−1 for ESP-90 and ESP-50. These particles may retard the hydration reaction at an early age and impede strength evolution [83]. The production of C–A–S–H and N–A–S–H gels was observed at peak and sharp wavenumbers of 970 and 1030 cm−1 for the mortar mixes of ESP-50 and ESP-90, respectively. This implies that the formation of gels in the ESP-50 mixes occurred at a higher level than in the ESP-90 specimens. The band between 960 and 1030 cm−1 is assigned to the asymmetric stretching mode of the Si–O–T (T = tetrahedral Si or Al) bonds within the reaction products. Additionally, water in the form of O–H–O and crystalline carbon were shown at peak spectra wavenumber ranges of 1620–1030 cm−1 in ESP-90 and 1620–1230 cm−1 in ESP-50.
At the curing age of 28 days, similar wavenumber ranges were identified for all mixes compared to the samples at the 3rd day (Figure 13), with slight changes observed in the bands. The highest peaks were examined in the bands of 3910–3600, 1720–1000, and 800–500 cm−1. For the alkali-activated mix type of ESP-90, major peaks were observed at 1700, 1555, 1415, 1230, 1000, and 780–505 cm−1. The presence of amorphous unreacted particles was observed in the bandwidth of 780–505 cm−1, while the formation of C–A–S–H and N–A–S–H gels was found around 1000 cm−1. Additionally, the spectra were slightly shifted to 1230–1555 cm−1 to examine the occurrence of carbon and water compared to the samples at the 3rd day. Notably, the wavenumber stretched to 800–500 cm−1 for ESP-50; hence, the number of unreacted particles reduced and the formation of gels increased. The growth of C–A–S–H and N–A–S–H gels was observed at stretched bands of 1630–1245 cm−1 in ESP-50. In an OPC system, [76] noted that the C–A–S–H gel includes layers of tetrahedrally coordinated silicate chains with a dreierketten structure. In ESP-50, the presence of water and carbon was detected at 1630–1415 cm−1.
Figure 14 displays the curve pattern trends of FTIR of the 56-day cured AAM specimens cast with the ESP-90 and ESP-50 mix types. It was observed that after 56 days, there was a progressive shift in bands to 3840–3530 cm−1 and then 855–490 cm−1. Unreacted particles were noticed at a wavenumber range of 855-500 cm−1 in ESP-90, while this range decreased to 725–510 cm−1 for ESP-50. The presence of C–A–S–H and N–A–S–H gels was shown around the 1000 cm−1 band for ESP-90. At a wavenumber of around 970 cm−1, an increased development degree of C–A–S–H and N–A–S–H gel products was found in ESP-50 specimens. The highest intensity of bands after 56 days of curing also indicates a greater extent of reaction product formation at these ages (56 days) of curing. It was suggested by [86] that the interlayer region of the gels contains Ca2+ cations, alkalis, and hydration water that are chemically combined into the gel structure, and some alkali cations also balance the net negative charge produced when Al3+ substitutes Si4+ in the tetrahedral chain locations.

3.3.2. Thermal Stability

The mass loss due to the temperature change or thermal stability of the OPC-based alkali-activated steel slag mortar incorporating eggshell powder was investigated using TGA. The results for specimens cured for 3, 28, and 56 days prepared from ESP-90 and ESP-50 mixes are depicted in Figure 15, Figure 16, and Figure 17, respectively. According to studies by [14,78], the dehydration of calcium hydroxide (C–H) occurs within the temperature range of 400–550 °C, while temperatures between 550 and 750 °C lead to the carbonation of CaCO3 and the dihydroxylation of C–S–H, C–A–S–H, and N–A–S–H gels.
In Figure 15, representing the 7th-day cured alkali-activated mortar, the breakdown of Ca(OH)2, CaCO3, and the C–A–S–H and N–A–S–H gels was observed between 400 and 500 °C and between 530 and 790 °C. For ESP-90 mixes, C–H decomposition into CaO and H2O resulted in an average weight loss of 5% at approximately 400 °C, reaching 7% near 600 °C, with no decomposition until 640 °C. Beyond 640 °C, a mass loss of 38% occurred due to the dehydration and dehydroxylation process. ESP-50 mixes decomposed at 400 °C, losing an equivalent amount of CaO and water (35%). ESP-50 lost 35% of its weight due to the dehydration of water from C–H, and 31% of the mass vanished due to the carbonation of CO2 from CaCO3 at 400 °C and 700 °C, respectively. As the temperature continued to rise to 790 °C, H2O burned out from the gels, resulting in a linear thermal stability at 41%.
Figure 16 illustrates the TGA results of mortar samples with ESP-90 and ESP-50 mixes after 28 days. This figure clearly shows that increasing the partial replacement of ESP with SS, while keeping the OPC level constant, significantly enhances the thermal stability of alkali-activated mortar specimens at around 700 °C. Additionally, these mortar samples exhibit better temperature change resistance compared to those cured for 7 days. This figure also reveals that dehydrations C–H occurred in ESP-90 and ESP-50 at 550 °C, with respective mass losses of 38%, and 2%. The TGA results (Figure 16) indicate that the decomposition of CaCO3 into CaO and CO2 (carbonation) occurred between 550 and 700 °C, with negligible mass losses of 2% in ESP-90; however, the mortar samples with 50% SS content (ESP-50) displayed enhanced thermal stability, potentially leading to carbonation. Furthermore, between 720 and 800 °C, the ESP-90 mortar samples lost approximately 77% of their mass due to the presence of significant zeolites structures, as discussed in a previous study [86]; at the same temperature, the ESP-50 samples lost 27% of their mass. According to [78], the removal of CaO, CO2, and H2O molecules from CaCO3, C–A–S–H, and N–A–S–H gels at 550 °C might not be complete, or the temperature might not be high enough to eliminate these molecules.
As shown in Figure 17, at 56 days, the AAMs containing 5% (ESP-90) and 25% (ESP-50) SS content lost 27% and 10% of their wight, respectively, in the temperature range of 200 °C to 550 °C due to the release of H2O or water from Ca(OH)2. For the same curing period, the results indicate that the carbonation of CO2 from CaCO3 and evaporation of water from the gels occurred when the temperature exceeded 550 °C. It was observed that the removal of CO2 from CaCO3 and the loss of water from the gels were attributed to average mass instabilities of 35% and 39% for ESP-90 and ESP-50, respectively, at temperatures approaching 800 °C. The significant mass loss observed in ESP-50 was most likely due to the substitution of ESP with SS, which reduced the CaO content in the gels and resulted in lower thermal resistance. According to a previous study by [14], this substantial mass loss at later ages was caused by the inclusion of more SS, which required extra mixing water to make the mortar workable. This increased the heat of hydration, causing the water within the gels and the CO2 in the CaCO3 to evaporate rapidly, leading to significant dehydroxylation and decarbonation of mortar samples.

3.3.3. SEM Analysis (Hydration Products)

The morphological and microstructural features of the alkali-activated mortars were investigated on selected 56-day cured samples using secondary SEM images taken from sliced portions of the specimen samples. Figure 18a,b present the final examined images of the ESP-90 and ESP-50 mix types. The SEM images illustrate that the morphologies and appearances of the alkali-activated specimens were completely different from those of the starting materials in powder form shown in Figure 2. The SEM image of ESP-90 in Figure 18a indicates that the alkali-activation of the OPC-based mortar incorporating eggshell powder and steel slag resulted in a less dense, fragmented, and porous structure with a large amount of unreacted steel slag particles. This led to heterogeneous, low-quality, and high absorption capacity mortars. This finding is aligned with the findings of a previous study [54]. As the SS content increased in ESP-50, a high alkali-activation geopolymerization process resulted in more compact and homogeneous gel formation within the AAMs (Figure 18b) compared to ESP-90. In Figure 18a, the higher content of CaO in ESP-90 enhanced the production of C–S–H and C–A–S–H gels, although some pore spaces and particles remained unreacted and were not completely converted to C–A–S–H and N–A–S–H gels, as discussed in a previous study [87]. The initial mass loss peak in Figure 17 indicates that the microstructure of these binders is dominated by a C–S–H type gel under these activation conditions when slag quantities of up to 10% are included in the ESP contents. A fully compact, uniform, dense, and non-porous morphology was identified in the micrographs of the ESP-50 mortar samples cured for 56 days (Figure 18b). The primary reaction product in sodium silicate-activated steel slags was calcium silicate hydrate. The SEM image shows that the ESP-50 specimen had a visibly different structure compared to ESP-90. N–A–S–H and C–A–S–H gels were the major hydration products created in ESP-50 AAMs. According to a study by [88], increasing the SS content in alkali-activation systems produced a higher amount of N–A–S–H with an Si–O–Al–O structure; the Ca2+ ions from the steel slag facilitated the formation of C–S–H and C–A–S–H, in addition to hydrated N–A–S–H gel.

4. Conclusions

This experimental research explored the feasibility of using SS and ESP to partially replace OPC, activated with NaOH and Na2SiO3 solutions. This study involved replacing ESP and SS at a proportion of up to 50% each, while keeping the OPC content at 50% of the total binder. The key findings from this investigation are as follows:
  • Chemical composition: SS is primarily composed of Fe2O3 (77%), with notable quantities of SiO2, Al2O3, and CaO. In contrast, ESP is rich in CaO (about 70%), with significant amounts of MgO, SiO2, Al2O3, and Fe2O3. Both SS and ESP contained crystalline quartz.
  • Workability: The fine and porous nature of SS reduces the workability of the alkali-activated paste and mortar, necessitating additional water as the SS content increases.
  • Mechanical Properties: Increasing the SS content from 10% to 50% in place of eggshell powder significantly reduced absorption capacity and pore space while enhancing the dry density, compressive strength, and quality of the AAMs. The best mechanical performance was achieved with a 50% SS replacement, and the mortar exhibited no significant deterioration when exposed to MgSO4 solutions.
  • Thermal Stability: AAM mixes with ESP-50 cured for 56 days exhibited good thermal stability, attributed to the high CaO and Fe2O3 content in OPC and ESP.
  • Microstructural Analysis: Extended curing revealed a dense, compact, and homogeneous microstructure, with major mineralogical phases including C–S–H, C–A–S–H and N–A–S–H. The presence of unreacted samples was minimal, contributing to improved mechanical performance and microstructural integrity.
  • Recommendations: This study recommends that a combination of eggshell powder and steel slag can replace up to 50% of cement with an alkali activator, without compromising the engineering properties of the mortar.
  • Future Research Directions: Future studies could explore the use of alkali-activated granular ground blast-furnace slag (GGBFS) mortar or concrete without OPC to further enhance sustainability in construction.

Author Contributions

Conceptualization, B.Z.H.; methodology, B.Z.H.; software, B.Z.H.; validation, B.Z.H. and M.D.Y.; formal analysis, B.Z.H.; investigation, B.Z.H.; resources, M.D.Y., W.Z.T. and D.-H.V.; data curation, B.Z.H.; writing—original draft preparation, B.Z.H., writing—review and editing, M.D.Y., W.Z.T. and D.-H.V.; visualization, B.Z.H.; supervision, M.D.Y., W.Z.T. and D.-H.V.; project administration, M.D.Y., W.Z.T. and D.-H.V.; funding acquisition, W.Z.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be made available upon request.

Acknowledgments

The authors gratefully acknowledge the invaluable support and resources provided by the Faculty of Civil and Water Resource Engineering at Bahir Dar Institute of Technology (BiT), Bahir Dar University, the Faculty of Civil Engineering at Arba Minch Institute of Technology (AMiT), Arba Minch University, Adama Science and Technology University, and the Ethiopian Geological Survey Institute, Addis Ababa, all located in Ethiopia.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Visual comparison between (a) eggshell powder and (b) steel slag.
Figure 1. Visual comparison between (a) eggshell powder and (b) steel slag.
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Figure 2. SEM images (×1500 magnification) of (a) eggshell powder and (b) steel slag.
Figure 2. SEM images (×1500 magnification) of (a) eggshell powder and (b) steel slag.
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Figure 3. XRD patterns of eggshell powder and steel slag.
Figure 3. XRD patterns of eggshell powder and steel slag.
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Figure 4. Gradation curve of fine aggregates.
Figure 4. Gradation curve of fine aggregates.
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Figure 5. Water consistency of alkali-activated binder blended paste.
Figure 5. Water consistency of alkali-activated binder blended paste.
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Figure 6. Workability (slump flow) of the alkali-activated mortar.
Figure 6. Workability (slump flow) of the alkali-activated mortar.
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Figure 7. Initial and final setting time results of the blended paste.
Figure 7. Initial and final setting time results of the blended paste.
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Figure 8. (a) AAMs’ water absorption capacity and (b) porosity.
Figure 8. (a) AAMs’ water absorption capacity and (b) porosity.
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Figure 9. Strength development of AAMs cubes at different curing ages.
Figure 9. Strength development of AAMs cubes at different curing ages.
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Figure 10. UPV results of all mortar mixes with different curing periods.
Figure 10. UPV results of all mortar mixes with different curing periods.
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Figure 11. Relationship between pore space, compressive strength, and UPV results.
Figure 11. Relationship between pore space, compressive strength, and UPV results.
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Figure 12. FTIR results of ESP-90 and ESP-50 at the 7th day.
Figure 12. FTIR results of ESP-90 and ESP-50 at the 7th day.
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Figure 13. FTIR results of ESP-90 and ESP-50 at the 28th day.
Figure 13. FTIR results of ESP-90 and ESP-50 at the 28th day.
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Figure 14. FTIR results of ESP-90 and ESP-50 at the 56th day.
Figure 14. FTIR results of ESP-90 and ESP-50 at the 56th day.
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Figure 15. Seventh-day TGA values of ESP-90 and ESP-50 mortar mixes.
Figure 15. Seventh-day TGA values of ESP-90 and ESP-50 mortar mixes.
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Figure 16. Twenty-eighth-day TGA values of ESP-90 and ESP-50 mortar mixes.
Figure 16. Twenty-eighth-day TGA values of ESP-90 and ESP-50 mortar mixes.
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Figure 17. Fifty-sixth-day TGA values of ESP-90 and ESP-50 mortar mixes.
Figure 17. Fifty-sixth-day TGA values of ESP-90 and ESP-50 mortar mixes.
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Figure 18. SEM images of AAMs cured for 56 days: (a) ESP-90 and (b) ESP-50.
Figure 18. SEM images of AAMs cured for 56 days: (a) ESP-90 and (b) ESP-50.
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Table 1. Physical properties and chemical compositions of binder samples.
Table 1. Physical properties and chemical compositions of binder samples.
Binder Compositions and Properties OPCESPSS
Chemical Compositions, [%]SiO222.003.965.42
Al2O35.61.292.06
Fe2O35.20.6477.88
CaO6470.402.17
MgO1.986.190.02
Na2O-0.150.00
K2O-0.350.63
TiO2-0.610.20
MnO-0.000.56
P2O5-0.170.52
LOI-7.549.93
Physical PropertiesSpecific gravity, [–]3.152.623.48
Surface area, [m2/g]318543.51544.22
ColorGrayDark grayBlack
Table 2. Test methods and results for fine aggregates properties.
Table 2. Test methods and results for fine aggregates properties.
S. No.PropertyTest MethodUnitTest ResultAcceptable Limit
1Silt ContentASTM C40 [48] [%]4.17Max., 8
2Absorption or Bulking of SandASTM C128 [49][%]2.54Max., 3
3Fineness ModulusASTM C136 [50][–]2.382.2–2.6
4Moisture ContentASTM C566 [51][–]5.382–6
5Unit WeightASTM C128 [49][kg/m3]15971520–1680
6Specific GravityASTM C128 [49][–]2.642.50–2.68
Table 3. Mix proportions of materials in 1 m3 alkali-activated mortar samples.
Table 3. Mix proportions of materials in 1 m3 alkali-activated mortar samples.
Mix CodeCement ContentESP ContentSlag ContentNaOHNa2SiO3WaterSand
[%][kg][%][kg][%][kg][kg][kg][lit][kg]
ESP-9050283.3390255.001028.3310.8821.76239.361558.33
ESP-8050283.3380226.672056.6710.8821.76239.361558.33
ESP-6050283.3360170.0040113.3310.8821.76239.361558.33
ESP-5050283.3350141.6750141.6710.8821.76239.361558.33
Table 4. Types of conducted tests and standards used for the mortar properties.
Table 4. Types of conducted tests and standards used for the mortar properties.
Test SamplesTest PropertiesTest StandardsEvaluated SamplesCuring PeriodsNumber of Cubes Cast
Fresh PropertiesConsistency ASTM C1437 [56]All mixes
Slump ASTM C187 [57] –
Setting timeASTM C191 [58]
Hardened PropertiesBulk densityASTM C642 [62] All mixes3, 7, 28, 56 days5 × 4 × 3 = 60 *
Absorption capacityASTM C642 [62]
Pore spaceASTM C642 [62]
Compressive strengthASTM C109 [60]
Uniformity (UPV)ASTM C597 [61]
Sulfate resistanceASTM C1012 [63]
Microstructure Mineralogical composition (FTIR) ESP-90, ESP-507, 28, 56 days
Thermal stability (TGA) –
SEM analysis 56 days
* 5 = number of mixes, 4 = number of curing periods, and 3 = number of cube samples for each test.
Table 5. Average bulk dry density of all AAM mixes.
Table 5. Average bulk dry density of all AAM mixes.
Mix CodeBulk Dry Density, kg/m3
3rd Day7th Day28th Day56th Day
ESP-901519.181528.801570.501586.38
ESP-801570.421580.361610.951626.47
ESP-601588.151608.851617.151647.76
ESP-501605.281625.341625.291660.31
Table 6. Effect of magnesium sulfate on the compressive strength of mortar cubes.
Table 6. Effect of magnesium sulfate on the compressive strength of mortar cubes.
Mix CodeChange in Compressive Strength, MPaPercentage Loss in Compressive Strength, %
3rd Day 7th Day28th Day56th Day3rd Day 7th Day28th Day56th Day
ESP-902.091.991.621.4526.54%13.52%9.05%6.95%
ESP-801.791.551.371.2816.74%9.19%6.49%5.64%
ESP-601.671.481.170.9014.24%7.28%4.83%3.38%
ESP-500.810.640.390.206.65%2.91%1.49%0.69%
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Hailemariam, B.Z.; Yehualaw, M.D.; Taffese, W.Z.; Vo, D.-H. Optimizing Alkali-Activated Mortars with Steel Slag and Eggshell Powder. Buildings 2024, 14, 2336. https://doi.org/10.3390/buildings14082336

AMA Style

Hailemariam BZ, Yehualaw MD, Taffese WZ, Vo D-H. Optimizing Alkali-Activated Mortars with Steel Slag and Eggshell Powder. Buildings. 2024; 14(8):2336. https://doi.org/10.3390/buildings14082336

Chicago/Turabian Style

Hailemariam, Behailu Zerihun, Mitiku Damtie Yehualaw, Woubishet Zewdu Taffese, and Duy-Hai Vo. 2024. "Optimizing Alkali-Activated Mortars with Steel Slag and Eggshell Powder" Buildings 14, no. 8: 2336. https://doi.org/10.3390/buildings14082336

APA Style

Hailemariam, B. Z., Yehualaw, M. D., Taffese, W. Z., & Vo, D. -H. (2024). Optimizing Alkali-Activated Mortars with Steel Slag and Eggshell Powder. Buildings, 14(8), 2336. https://doi.org/10.3390/buildings14082336

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