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Article

Investigating the Properties of Composite Cement-Based Mortar Containing High Volumes of GGBS and CCR

by
Zahraa Jwaida
1,
Awad Jadooe
2,
Anmar Dulaimi
3,4,5,*,
Raid R. A. Almuhanna
2,
Hayder Al Hawesah
6,
Luís Filipe Almeida Bernardo
7,* and
Jorge Miguel de Almeida Andrade
7
1
Industrial Preparatory School of Vocational Education Department, Educational Directorate Babylon, Ministry of Education, Babylon 51001, Iraq
2
Department of Civil Engineering, College of Engineering, University of Kerbala, Karbala 56001, Iraq
3
Department of Civil Engineering, College of Engineering, Kerbala University, Karbala 56001, Iraq
4
Department of Civil Engineering, College of Engineering, Warith Al-Anbiyaa University, Karbala 56001, Iraq
5
School of Civil Engineering and Built Environment, Liverpool John Moores University, Liverpool L3 2ET, UK
6
University Centre, Wigan & Leigh College, Wigan WN1 1RR, UK
7
GeoBioTec, University of Beira Interior, 6201-001 Covilhã, Portugal
*
Authors to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(6), 301; https://doi.org/10.3390/jcs9060301
Submission received: 29 March 2025 / Revised: 6 June 2025 / Accepted: 11 June 2025 / Published: 13 June 2025
(This article belongs to the Special Issue Feature Papers in Journal of Composites Science in 2025)
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Abstract

This study explores the potential of calcium carbide residue (CCR) as an alternative activator for ground granulated blast-furnace slag (GGBS) to reduce reliance on ordinary Portland cement (OPC) in mortar production. A series of OPC-GGBS-CCR ternary binders were prepared and evaluated for their fresh and mechanical properties over various curing periods. The findings showed that mortars’ fresh and mechanical characteristics were significantly improved with longer curing times, suggesting CCR’s potential to efficiently activate GGBS, thereby benefiting the environment and economy. Significant enhancements in compressive strengths were observed after 7 days of curing, with increases of 44%, and 69–144% for OPC and OPC-GGBS-CCR ternary binders, respectively, while the utilization of activated binders led to flexural strength growth compared to three days of curing, with improvements of 70–173% for OPC-GGBS-CCR ternary binders, respectively. Microstructural analyses confirmed accelerated hydration and increased product formation due to CCR’s calcium content. An optimal mix ratio of OPC:GGBS:CCR = 1:1:0.5 demonstrated mechanical properties comparable to OPC mortars after 28 days, highlighting CCR’s potential for sustainable cementitious materials.

1. Introduction

Since the 19th century, ordinary Portland cement (OPC) has represented a crucial and most used inorganic material for different industries such as construction, and composites [1]. Global cement production, including OPC, was estimated at approximately 4.1 billion metric tons in 2022 [2]. The prime components of OPC are gypsum and clinker [3]. The clinker is made from burning limestone (75–85 weight%), and alumina-silica sources from clay and rocks (15–25 wt.%) at significant temperatures (1300–1500 °C) [4]. Such a process poses significant environmental challenges such as the destruction of wildlife habitat, variation in landscape, blast, dust, and reduction in natural resources [5]. Moreover, the significant environmental concern lies in the amount of carbon dioxide (CO2) emissions—each produced ton of OPC releases about 0.87 ton of CO2, accounting for about 10% of the world’s industrial CO2 emissions [6,7]. Therefore, a global sustainability awareness has emerged, urging industries to find solutions that minimize CO2 emissions and usage of natural resources [8]. As such, researchers have been exploring alternative materials for the production of OPC [9,10], rich in lime as the main component of cement from limestone [11]. On the other hand, substantial amounts of waste and by-products are produced from different sectors worldwide, such as urban, agricultural, and industrial sources. The landfilling and inadequate disposal of such wastes can adversely impact the environment due to the burning, accumulation, and decomposition of such materials. Therefore, the key solution relies on recycling waste and by-products into valuable resources. For many years, the use of waste and by-products such as palm oil fuel ash, rice husk ash, ground granulated blast slag, silica fume, and fly ash has been very successful in replacing OPC in mortars and concrete [12,13].
Many research has been performed on the use of ground granulated blast furnace slag (GGBS) to replace OPC [14,15] as an effective sustainable approach, with only 0.07 tons of CO2 released for a produced ton of GGBS [16]. It is a by-product of the iron industry [17]. A review was carried out by Özbay et al. (2016) [17] on the use of GGBS in concrete and reported improvements in long-term strength, durability, abrasion resistance, permeability, and sulfate resistance, though minimal impact on elastic modulus. Additionally, regardless of the content of GGBS, studies have shown a reduction in the porosity of concrete with the inclusion of GGBS [18]. Although GGBS can hydrate, its reaction is slow, depending on the intrinsic material properties [19]. Because of its rapid rate of cooling upon production, it mainly occurs in a glassy phase [20]. Thus, to increase the speed of the reaction, activators have widely been applied such as reactive magnesia (MgO), soluble silicates, alkaline hydroxides, quicklime (CaO), and hydrated lime (Ca(OH)2) [21,22]. The amount and type of activators can highly influence the structure and composition of the hydration products, thereby impacting the performance of activated GGBS [23]. Alkali activated materials represent a promising alternative material that can compete with OPC and be used in various applications [24]. It has been widely studied, providing adequate mechanical properties with proper activators. Zheng et al. (2019) [25] found that MgO activation of GGBS produced superior results compared to lime activation, as evidenced by unconfined compressive strength testing. Ma et al. (2023) [26] studied the use of GGBS and fly ash activated by sodium hydroxide and sodium silicate to prepare a geopolymer. They found that the amount of alkali activation significantly impacts the results as well as the content of GGBS. Microscopic analysis supported the macroscopic experiments, showing that the reaction system led to increased strength as it filled the geopolymer pores. Similarly, Mehta and Siddique (2018) [27] reported a significant strength development after three curing days when rice husk ash and GGBS were utilized to produce geopolymer concrete, making it a viable alternative to conventional cement concrete and effectively reducing carbon dioxide emissions.
On the other hand, calcium carbide residue (CCR) represents a by-product from the production of acetylene, with 64 g of prepared acetylene providing about 74 g of CR. The main component of the CR is calcium hydroxide (Ca(OH)2) [28]. It also possesses significant amounts of heavy metals due to their accumulation during the chemical process or their presence from low-grade raw materials. Li Xin and Huang (2020) [29] and Cao et al. (2021) [30] reported the presence of Sb, Zn, Cu, Mn, Ni, As, Cr, Cd, Pb, and Ni, all exceeding the required disposal limits. The significant production volume and high alkalinity of CCR contribute to environmental concerns, including landfilling, as well as water and soil pollution [31]. Thus, adequate disposal methods of CCR are crucial. Consequently, CCR has been widely incorporated in concrete and mortars in various applications and with the combination of waste and by-products. Chindaprasirt et al. (2020) [32] reported adequate stabilization characteristics of lateritic soil with CR due to pozzolanic reactions. Additionally, Dulaimi et al. (2020) [33] reported the addition of 6 wt.% of CCR as a filler in hot mix asphalt enhanced the stiffness modulus of road pavements. Furthermore, because of its high Ca(OH)2 content, it can be recycled to efficiently substitute limestone to fabricate cement, resulting in an annual decrease of almost 20 million tons of CO2 emissions [34]. It is important to remember, though, that raising the CCR replacement level may result in a reduction in OPC mechanical strength [35]. Adamu et al. (2021) [36] investigated the partial replacement of cement with CCR and reported a decrease in strength and elastic modulus as the CCR content exceeded 15 wt.%. Furthermore, due to its mineral being similar to hydrated lime, CCR can be used as an alkaline activator to GGBS. According to Guo et al. (2021) [37], after 28 days of curing, the fly ash and GGBS composite’s mechanical strength was increased by 10%, yielding a compressive strength of 53 MPa. Similarly, Zhang et al. (2020) [38] found increased compressive strength when 7.5 wt.% of CCR was added to GGBS. Li and Yi (2020) [39] prepared a paste of activated GGBS with CCR and compared the findings with hydrated lime activation of GGBS. They observed similar compressive strength, although CCR content above 10 wt.% led to a reduction in the mechanical properties of the paste.
Although CCR has been explored for various applications, its use in mortars and concrete remains limited [39]. It is, thus, evident that the use of CCR to activate GGBS requires further investigation. This paper aims to prepare and investigate a mortar made with GGBS activated by CCR as a partial replacement for cement. This study is expected to contribute to the knowledge base and offer a sustainable composite by reducing the disposal issues of used materials and reducing the carbon footprint of cement production.

2. Methodology

2.1. Materials

The ternary mixed mortar binders were prepared using a variety of ingredients. They included water, sand, GGBS, CCR, and OPC. Hanson Heidelberg Cement Group, UK, provided the GGBS; CEMEX Quality Department, UK, provided the OPC; and BOC, UK and Ireland, provided the CCR. CEM-II/A/LL 32.5-N type of OPC was used per BS EN 197-1 [40]. The CCR was supplied in wet and large blocks which were dried in the oven for a day at 110 °C after breaking into small pieces. Then, a mechanical grinder was used to grind the pieces subjected to dry, intensive, and low energy agitation, with the application of a mortar and pestle for 15 min to prevent agglomeration that can adversely affect the quality of CCR. The particle size distribution (PSD) of sand is shown in Figure 1, with 2.62 specific gravity and passing through a #6 IS sieve. The mixing water used in this study was obtained from the local municipal supply and met the requirements outlined in BS EN 1008:2002 [41]. This standard defines the permissible characteristics of water for use in the production of concrete and mortar to ensure that it does not adversely affect setting time, strength, or durability [42].
Calcium carbide residue (CCR) is a highly alkaline industrial by-product primarily composed of calcium hydroxide (Ca(OH)2). In cementitious systems, CCR acts as a chemical activator due to its high pH. When combined with ground granulated blast-furnace slag (GGBS), CCR increases the pH of the pore solution, which enhances the dissolution of the glassy slag structure. This releases calcium, silica, and alumina ions that react to form calcium silicate hydrate (C–S–H) and other strength-giving hydration products, thus contributing significantly to the development of mechanical properties.
A Beckman Coulter laser was used for analyzing the PSD of GGBS, CCR, and OPC materials. The findings are shown in Figure 2. According to the data, 90% of the GGBS particles and the dominating PSD of CCR were both smaller than 40 μm. After being grounded for 15 min, the CCR presented an average grain size (D50) of around 24.6 μm. The impact of PSD is very significant on the strength of mortars and cement pastes, as reported by Celik et al. (2008) [43]. The properties of the mixtures might be significantly improved if the CCR particles were ground to a fineness comparable to that of slag or cement. Faster strength growth and possibly greater long-term strength could result from the improved hydration reaction caused by the larger surface area. But, in order to prevent thermal cracking in big pours, it would be necessary to control the heat of hydration as a result of this increased fineness. Significant temperature gradients within the concrete could result from the greater heat of hydration, which, if not effectively controlled, could cause thermal stress and fracture. Table 1 shows the characteristic particle sizes (D10, D50, and D90) estimated from PSD curves for OPC, CCR, and GGBS.
The specific gravity of GGBS and CCR used in this study were measured as 3.05 g/cm3 and 2.24 g/cm3, respectively, using a gas pycnometer. Although CCR has a lower density than OPC (2.94 g/cm3), binder replacement in this study was conducted by mass, in accordance with prior literature where similar material properties were reported [25,39]. This approach allows for consistency in experimental comparison, though it is recognized that volume-based substitution may yield different performance outcomes and could be explored in future work.
Meanwhile, the binder materials were chemically analyzed using a Shimadzu EDX 720 energy dispersive X-ray fluorescence spectrometer. The findings are displayed in Table 2. The results showed the presence of lime, silica, alumina, sodium oxide, and magnesium oxide GGBS, which are similar to the ones obtained by Chaunsali and Peethamparan (2011) [44]. Whereas, the main oxides in CCR were lime and silica, which are similar to those obtained by Hanjitsuwan et al. (2018) [45]. The results also showed the significant content of lime in CCR relative to GGBS which in turn has a high content of silica and lime. This significant lime content along with the chemical composition makes the CCR a very attractive material for activating GGBS [46].
X-ray diffraction (XRD) was examined using a Rigaku Miniflex diffractometer and the results are shown in Figure 3, Figure 4 and Figure 5. A number of crystalline components that are frequently present in the OPC are revealed by the X-ray diffraction pattern (Figure 3). The presence of alite (PDF# 49-0442), belite (PDF# 33-0302), ferrite (PDF# 33-0664), calcite (PDF# 05-0586), and periclase (PDF# 45-0946) was confirmed by indexing all peaks using the ICDD PDF database. To guarantee total transparency of the phase analysis, each discernible peak has been labeled.
The XRD pattern of GGBS, shown in Figure 4, displays a broad diffuse halo between 25° and 35°, confirming its largely amorphous and glassy nature. This amorphous structure is a key characteristic of GGBS, resulting from rapid cooling during its production. The lack of sharp crystalline peaks suggests high reactivity, which is essential for chemical activation. This property enables GGBS to effectively react with alkaline activators such as CCR, promoting the formation of strength-giving hydration products like calcium silicate hydrate (C–S–H) gels.
On the other hand, calcite and Portlandite alkalis are the main constituents of CCR. The reactivity of GGBS will be accelerated by the presence of such alkalis. This acceleration occurs because the alkalis increase the pH of the system, which enhances the dissolution of the aluminosilicate glass phase in GGBS. This promotes the release of calcium, silicon, and aluminum ions, facilitating the rapid formation of hydration products such as calcium silicate hydrate (C–S–H) and contributing to early strength development.
The pH of the CCR solution was found to be 13.1, higher than that of GGBS, which may help accelerate the reactions. Specifically, the high pH promotes the breakdown of the aluminosilicate glass structure in GGBS, releasing reactive ions such as Ca2+, Si4+, and Al3+ into the solution. These ions subsequently react to form hydration products like calcium silicate hydrate (C–S–H), thereby accelerating the hardening process and early strength development of the binder.
Calcite (CaCO3) is present as a minor phase, while portlandite (Ca(OH)2) is the dominant crystalline phase, as indicated by the XRD analysis of CCR in Figure 5. ICDD cards for calcite (PDF# 05-0586) and portlandite (PDF# 44-1481) were used for peak identification and indexing. To promote the transparency and repeatability of the phase identification, each observed peak was given the appropriate label.
Furthermore, the materials microstructure was examined using scanning electron microscopy (SEM). The micrographs in Figure 6 show the angular and irregular shapes of GGBS and OPC particles, with noticeable agglomeration of the particles. Agglomeration refers to the tendency of fine particles to cluster together due to van der Waals forces or moisture content, forming larger aggregates. This can reduce the effective surface area available for reaction and potentially influence the hydration kinetics and workability of the mixture.

2.2. Experimental Work

2.2.1. Samples Preparation

In order to prepare the binders, a ratio of 2 between GGBS and CCR was used to increase the amount of CCR in the binders, thereby enhancing the economic and environmental impacts of the developed binders. A 150 g binder of OPC was used with a w/c ratio of 0.4 and cement to sand ratio of 2.5, and this cement was replaced by a constant amount of GGBS/CCR. Initially, the GGBS and CCR were mixed to obtain binary binders which in turn were used to partially replace OPC and obtain ternary binders. The mixing proportions are shown in Table 3. A fixed amount of sand and water was utilized. It was also observed that the equivalent alkalinity of binders was higher than the specified value of OPC in international standards for preventing alkali-silica reactivity damage. Nevertheless, it was stated by Hester et al. (2005) [47] that the replacement of 50% OPC with GGBS provided a very low level of expansion with more than 5 kg/m3 alkalinity equivalent loads. This was because of the suppression of OH- ions transportation with high replacement of OPC to a level that limits the negative alkalinity impacts. Thus, to overcome such a problem, the ratio of 2 GGBS/CCR was adopted in this study.
The selected replacement levels (OGC1–OGC4) were designed to cover a systematic range of OPC substitution with increasing GGBS and CCR content. This range allowed us to evaluate the interaction effects of CCR activation on GGBS at both moderate and high replacement ratios. While the substitution percentages are informed by ranges commonly used in GGBS research [48,49], the specific ratios were tailored for this study to explore the performance of a novel ternary system incorporating CCR.
It should be noted that the high CCR content used in some mixes, particularly OGC4, was intentionally selected to explore the upper performance limits of CCR as an activator for GGBS. This approach aimed to assess not only the feasibility of CCR in ternary binders but also to define practical dosage limits by observing behavior across a broad compositional range. By systematically increasing CCR content, we aimed to capture its influence on early-age activation and hydration potential within the mix design.

2.2.2. Testing

Characteristics of the Fresh Mix
The consistency and setting time of the ternary binders were measured using the Vicat equipment in accordance with BS EN 196-3 [50].
Compressive Strength
The compressive strength of the binders was assessed using a compression machine in compliance with BS EN 196-1 [51]. Type Control Automax 5 of the machine was used, providing efficient implementation of loads and downloading of stress-loads data through a connected computer. Several curing periods were conducted prior to testing, including 3, 7, 14, and 28 days. Testing specimens were cast for each binder and tested under uniaxial compression. Then, compressive strength was determined as the average of four parts. Three specimens were tested for each age group, and the test result was determined by taking the average value.
Flexural Strengths
Testing for flexural strength is a popular technique for determining a material’s resistance to bending. The test was performed according to BS EN 12390-5 [52]. The prism measuring 160 mm by 40 mm by 40 mm was subjected to flexural tests after 3, 7, 14, and 28 days. The flexural response was assessed using three-point bending tests at a constant loading rate of 0.3 mm/min until the failure of specimens. Three specimens were tested for each age group, and the test result was determined by taking the average value.
SEM and EDX
These tests are commonly used for analyzing the hydration degree, morphology, and elemental compositions of hardened products [33,53]. The Quanta 200 and EDX Oxford Inca x-act sensor of a 5–20 kV accelerating voltage, along with the FEI SEM model Inspect S, were used in the tests. The specimens underwent curing for 1, 7, and 28 days. The samples were coated with a gold layer using a sputter coater to improve visibility before testing. The tests were carried out on raw materials and optimum binders.

3. Results and Discussion

3.1. Fresh Characteristics

Figure 7 displays the results of the prepared mortars’ fresh characteristics. There are several factors that can impact the standard consistency test including the fineness of binder materials, hydration rate, and water to binder ratio [54]. The consistency results, i.e., requirement for water, increased with the increase in the replacement level of OPC. The lowest consistency value was obtained for OPC mortars and the highest consistency was obtained for the OGC4 mortar with the highest replacement level of OPC. The reason for this behavior could be the fineness of the GGBS and CCR, providing a greater surface area than OPC and thereby requiring more water. The high lime content and alkalinity of CCR also contribute to the results [55]. Meanwhile, as the OPC replacement amount increased, the results of the initial and final setting times increased. For both, the increase in GGBS and CCR gradually increased the times, with the highest results obtained for OGC4 mortar of about 7.5% longer results than OPC mortar. The increase in consistency and setting times with higher CCR-GGBS content can be attributed to a combination of physical and chemical effects. Although CCR raises the pH and promotes the activation of GGBS, the increase in setting times can be attributed to the lower intrinsic reactivity of GGBS compared to OPC. Also, the fine particle size and high surface area of CCR increase the water demand to achieve standard consistency. Meanwhile, the relatively slow early-age reactivity of GGBS delays the formation of initial hydration products, thereby extending the setting time. These factors slow the initial formation of a rigid structure despite the enhanced alkalinity, resulting in delayed setting times Thus, both materials contribute synergistically to the observed behavior, with CCR influencing water demand and GGBS influencing reaction kinetics. A similar behavior was observed in [37].

3.2. Compressive Strength Results

The produced compressive strength data at different curing durations are displayed in Figure 8. The compressive strength of OPC mortar was significantly decreased by the introduction of activated GGBS, particularly during the initial curing stages. This effect was more noticeable at greater OPC replacement levels. This early-age strength reduction is mainly due to the slower reaction kinetics of GGBS, which requires a high-pH environment and longer curing to fully react. Additionally, the replacement of highly reactive OPC with GGBS and CCR introduces a dilution effect, lowering the initial availability of hydration products. The higher water demand also increases porosity in the early matrix, further limiting strength development during initial curing stages. [56]. Then, significant improvements in the compressive strengths were obtained at 7 days of curing, increasing by 44%, 69%, 109%, 144%, and 144% for OPC, OGC1, OGC2, OGC3, and OGC4, respectively, in comparison with the 3 days of curing. This showed the progress and development of compressive strength with the increase in the replacement of OPC. Such results could occur due to the activation of GGBS by CCR, providing a more compacted and denser matrix, hence substantially improving the compressive strength. The strength development was higher for the activated binder than the OPC binder which could be a result of the additional C–S–H gel of the reaction between GGBS and CCR in addition to the high CCR alkalinity that improved the dissolving of the glassy structure of GGBS [57]. The gel provides higher strength, a denser and impermeable matrix by growing into the capillary voids and filling up the pores [58]. Furthermore, after 28 days of curing, the use of OGC1 and OGC2 binders resulted in a compressive strength similar to that of OPC mortar. This could be attributed to the improvement in the pozzolanic reaction at later ages of GGBS by the use of an alkaline CCR activator. Nevertheless, the use of OGC4 mortar substantially reduced the compressive strength after 28 curing days. This may be explained by the rising calcium hydroxide resulting from an increase in GGBS to CCR content, which can negatively affect the development of strength. Comparable findings were reported by Li and Yi (2020) [39]. The observed reduction in 28-day compressive strength for OGC4 suggests that excessive CCR may lead to diminished mechanical performance, highlighting the importance of identifying an optimal dosage rather than assuming that higher activator content yields better results.
The minimum compressive strength requirement for standard curing conditions after 28 days for OPC mortar is 10 N/mm2 (MPa) according to BS EN 196-1 [51]. By comparison to the standard value, OPC, OGC1, OGC2, OGC3, and OGC4 provided higher strengths by about 280%, 285%, 260%, 230%, and 160%, respectively. Nevertheless, it can be asserted that replacing OPC with GGBS and CCR in ternary mortars can produce promising mortars for construction, especially when cured at later ages, offering strength comparable to that of traditional OPC mortar for a range of applications.
Standard deviations (SD) were considered to evaluate the consistency of strength results. Most mixes showed low SD values (typically <1.5 MPa), indicating good repeatability of the tests. Notably, OGC3 exhibited both high strength and low variability, suggesting stable performance. OGC4 showed slightly higher SD in early-age tests, which may be attributed to increased porosity or uneven hydration due to excessive CCR content.

3.3. Flexural Strength Results

Figure 9 displays the flexural strength for each specimen at 3, 7, 14, and 28 days. The overall patterns of the results indicate that the flexural strength of OPC mortar was significantly decreased by the addition of activated GGBS, particularly during the early curing stages. This trend is consistent with the compressive strength results and can be attributed to the same factors: the slower early hydration of GGBS, the dilution effect from OPC replacement, and the initial porosity induced by higher water demand due to CCR’s fine particle structure. Then, the control mix flexural strength increased by 44% from 3 days to 7 days of curing, or roughly 8 MPa. On the other hand, compared to 3 days of curing, the usage of activated binders highly increases the strength growth by 70%, 111%, 124%, and 173% for OGC1, OGC2, OGC3, and OGC4, respectively. The increase in flexural strength could be attributed to the creation of the C-S-H matrix from active binders, which is a necessary structure for solids to percolate into the microstructure during the early curing age [59]. On the prolonged duration of normal curing up to 28 days, the flexural strength of OGC1, OGC2, and OGC3 specimens developed significantly, with comparable performance to the OPC specimen. The 28-day strength increased initially and then decreased as the CCR substitution rate increased. The minimum flexural strength requirement for standard curing conditions after 28 days for OPC mortar is 3.5 N/mm2 (MPa) according to BS EN 12390-5 [52]. By comparison to the standard value, OPC, OGC1, OGC2, OGC3, and OGC4 provided higher strengths by about 166%, 160%, 151%, 140%, 80%, respectively. This trend could have its origins in the “concentration effect,” which implies the following. An increase in CCR causes the concentration of OH− to rise, the activating impact on the GGBS to intensify, and the rate of hydration product production to accelerate. When the CCR is higher than the optimal value, the OH− concentration is abnormally high, and the hydrates created by rapid activation will erect a barrier to prevent further hydration of the partial GGBS particles. A high concentration of OH− permeates the GGBS and reshapes the gel substance as the aging period extends, leading to an increase in internal volume and internal pressure, the formation of microcracks, and a 28-day decrease in compressive and flexural strength [37].
As for compressive strength, SD values for flexural strength were generally low, demonstrating acceptable consistency across replicates. OGC3, again, presented strong and stable results. In contrast, the OGC4 mix showed greater dispersion in early-age tests, suggesting early-age variability potentially due to high OH− concentrations from excessive CCR.

3.4. SEM and EDX Results

Figure 10 provides the SEM images of the OGC3 selected paste at 1, 7, and 28 curing days. From the images at 1 and 7 days, it can be seen the presence of portlandite with a fatty shape in addition to the ettringite with needle shape. According to Sadique et al. (2013) [60], this shows the binder’s reactivity capacity to enhance the mechanical properties of the mortars. Along with the binding properties of the mixtures, the images in Figure 10 also demonstrate the creation of Calcium Silicate Hydrate (C–S–H) gel, which is the substance responsible for strength growth [61]. There were no intact particles and their surfaces were covered by CH, ettringite, and C–S–H gel with crystalline structure and apparent voids. Shi and Day (1995) [62] stated that the pH of the activating material is the main factor impacting the dissolving of slag and the production of hydration products. In this study, the activation of GGBS by CCR led to the precipitation of ettringite, which acts with CH and C–S–H gel to provide the stiffness and mechanical characteristics of the mortars at early ages [63]. The activation of GGBS accelerated the synthesis of C–S–H gels at later curing ages, leading to a denser and more compact structure with crystalline products. Ettringite generation was decreased, and the majority of the binding properties and strength development were brought about by the CH and C–S–H gel production. This suggests that some early strength was attributed to ettringite, but C–S–H gel and CH were mostly responsible for later-age strength growth. The use of GGBS and CCR, along with their reaction products, reduced both the size and continuity of voids, thereby limiting water movement and improving the material’s volumetric characteristics. The results are in a good agreement with Nassar et al. (2016) [64] and Dulaimi et al. (2017) [65].
While ettringite formation at early stages contributes to matrix stiffening and may aid initial strength development, its long-term presence must be controlled. If excessive or not well-distributed, ettringite can potentially interfere with the formation of more stable hydration products or even cause expansion. However, in this study, the later dominance of calcium silicate hydrate (C–S–H) and calcium hydroxide (CH) likely played the primary role in strength development beyond 7 days. The observed microstructural densification at 28 days suggests that ettringite formation will not negatively impact long-term strength in this case.
Figure 11, Figure 12 and Figure 13 present the EDX spectrum of the OGC3 selected paste mixture from 1 to 28 days of curing. It can be seen the presence of C, O, Ca, Si, and Al and traces of K, S, Mg, and Fe. At early ages and because of the small quantity of CCR alkali and the uneven reaction between the CCR and the GGBS in the reaction system, there was enough dissolution of the raw materials to generate the high Ca content of the gelling products. When water comes into contact with the Ca–O, Si–O, and Al–O bonds present in GGBS and CCR, these bonds break and disintegrate. The extensive breaking of Ca–O bonds in the system led to a higher distribution of Ca, while the limited breaking of Al–O bonds and their slower dissolution rate resulted in a lower distribution of Al [39]. A very coarse agglomerate can be seen, presenting dark areas identified by the analysis (Figure 11 and Figure 12) as carbon including traces of other elements. Since the prismatic morphology showed elevated Ca, C, and O levels, this region was interpreted as possible calcium carbonate. However, as EDX analysis only provides elemental composition, it cannot definitively identify the specific phase. Other calcium-rich compounds, such as calcium hydroxide or calcium silicate hydrate (C–S–H), may also present similar elemental signals. Therefore, this phase identification should be considered tentative and based on indirect evidence.
The agglomerate light-gray substance is thought to be calcium hydroxide clusters. Actually, during SEM investigations, these clusters, which are made up of an aggregation of calcium hydroxide crystals, were the most common. Early on, the gelled material exhibited a granular morphology; later on, it developed into a denser product—a compact microstructure composed primarily of C–S–H and CH, with reduced porosity and enhanced binding characteristics. With increasing curing time (Figure 13), the amount of Ca in the mixture reduced while the levels of alumina and silica increased, indicating progressive activation of GGBS and enhanced development of strength.
The high calcium content observed in early EDX spectra reflects the dominant presence of Ca(OH)2 introduced through CCR. While beneficial for early activation of GGBS, such elevated Ca(OH)2 concentrations may also present long-term durability concerns, particularly in relation to carbonation. A highly alkaline matrix can facilitate the initial formation of C–S–H and ettringite; however, excessive free lime (Ca(OH)2) can also make the system more vulnerable to carbonation-induced strength degradation over time. This risk is especially relevant when CCR is used at high dosages, as in OGC4. Therefore, while CCR acts as an effective activator, its optimal dosage must balance early activation benefits with long-term chemical stability. Based on the microstructural findings and strength development patterns, it appears that CCR may be more suitable as a lower-dosage additive, similar to Type II mineral admixtures, rather than a primary cement substitute.
The microstructure appears denser, possibly due to a higher Si/Al ratio compared to earlier samples, which may influence the formation of aluminosilicate gels. Although some ettringite may be present and could contribute to early strength through pore filling, its concentration does not directly determine mechanical strength. Excessive ettringite formation can lead to expansion or internal stress in some systems. Furthermore, the Si/Al ratio primarily governs the formation of C–S–H and other aluminosilicate phases, rather than ettringite itself. Therefore, while ettringite may play a role in early-stage densification, the overall strength gain is more closely tied to C–S–H development and matrix refinement. At later curing stages (e.g., 28 days), the microstructure appears denser and more compact, with increased packing of hydration products such as calcium silicate hydrate (C–S–H) and calcium hydroxide (CH), and reduced visible porosity. Although the microstructure appears denser, with regions likely influenced by hydration products, the identification of ettringite (AFt) remains inconclusive due to the low magnification and the absence of clearly visible needle-like structures typically associated with AFt. Moreover, given the relatively large EDX analysis areas compared to the fine particle sizes, phase identification should be interpreted cautiously. Nevertheless, the overall microstructural densification may suggest a positive contribution to strength development.

4. Conclusions

In this study, the potential of utilizing calcium carbide residue (CCR) as an activator for ground granulated blast-furnace slag (GGBS) in ternary binders was investigated. The research focused on assessing the effects of varying OPC, CCR, and GGBS contents on the consistency, setting time, and mechanical properties of mortars. Additionally, the hydration characteristics of CCR-GGBS pastes were analyzed using energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM). After analysis and discussion of the results, the following conclusions can be drawn:
-
The increase in the replacement level of OPC in ternary binders by CCR-GGBS increased consistency, initial and final setting times;
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Although the replacement of OPC by CCR-GGBS significantly reduced the early compressive and flexural strengths at 3 days, the strength development at 7 days was significantly greater than OPC mortars. The developed strength was comparable to OPC at 28 days of curing for all mixtures except for OGC4;
-
SEM images revealed the presence of ettringite, which contributes to early strength development, along with C–S–H and CH. At later curing stages, the formation of C–S–H and CH dominated, resulting in a denser and more compact microstructure;
-
EDX analysis of the optimum paste showed a significant presence of Ca at early ages, while later ages exhibited lower Ca content and higher concentrations of Si and Al. This indicated the activation of GGBS and the formation of C–S–H and ettringite;
-
It can thus be concluded that using CCR to activate GGBS holds significant potential for producing construction materials suitable for various applications.
-
While CCR effectively activates GGBS and enhances early strength, its high calcium hydroxide content—especially at substitution levels up to 26.5%—raises concerns about increased carbonation risk and long-term durability. The EDX results confirm elevated Ca levels. CCR may be better suited as a lower-dosage additive rather than a major cement replacement. Further studies on carbonation resistance, sulfate attack, and pore structure stability are needed to validate its long-term performance.
-
Among the tested binders, the OGC3 mixture with a 1:1:0.5 ratio of OPC:GGBS:CCR (40% OPC, 40% GGBS, 20% CCR) was identified as the optimal formulation. This blend provided the best balance of early and late-age mechanical performance, closely approaching the strength of OPC mortar at 28 days. It also avoided the reduction in strength observed in OGC4, where excessive CCR content likely led to microstructural inefficiencies. Moreover, OGC3 offers improved sustainability due to a 60% reduction in OPC content and the beneficial reuse of industrial waste materials, making it attractive for environmentally responsible construction applications.
While CCR proved to be effective in activating GGBS and enhancing strength development, the high calcium hydroxide content associated with CC, particularly in the OGC4 mix, raises potential concerns about durability, such as increased carbonation susceptibility or pore instability. This was reflected in the reduced 28-day strength of OGC4, suggesting that excessive CCR may compromise long-term performance. Therefore, OGC3 (20% CCR) was identified as the most balanced and promising formulation.
Although the mechanical performance of CCR-GGBS mortars is promising, especially for OGC3, further studies are needed to evaluate long-term durability characteristics, particularly for binders with high CCR content, including resistance to carbonation, sulfate attack, and pore structure stability under different environmental exposures.

Author Contributions

Z.J.: Investigation, conceptualization, writing—review and editing. A.J.: Visualization; validation, formal analysis. A.D.: Project administration, conceptualization, writing—review and editing, supervision, funding acquisition. R.R.A.A.: Conceptualization, formal analysis. H.A.H.: Conceptualization, validation, data curation. L.F.A.B.: Writing—review and editing, visualization. J.M.d.A.A.: Writing—review and editing, visualization. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors declare that the data supporting the findings of this study are available within the paper.

Acknowledgments

The authors express sincere gratitude for the support received from Kerbala University in Iraq.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Particle size distribution of sand.
Figure 1. Particle size distribution of sand.
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Figure 2. PSD of OPC, GGBS, and CCR.
Figure 2. PSD of OPC, GGBS, and CCR.
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Figure 3. XRD pattern of the OPC indicating the main crystalline phases: alite (A), belite (B), calcite (C), ferrite (F), and periclase (Pc). The peaks were indexed using standard ICDD PDF cards.
Figure 3. XRD pattern of the OPC indicating the main crystalline phases: alite (A), belite (B), calcite (C), ferrite (F), and periclase (Pc). The peaks were indexed using standard ICDD PDF cards.
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Figure 4. Powder XRD of GGBS.
Figure 4. Powder XRD of GGBS.
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Figure 5. XRD pattern of the CCR showing the main crystalline phases: portlandite (P) and calcite (C). The peaks were indexed using standard ICDD PDF cards.
Figure 5. XRD pattern of the CCR showing the main crystalline phases: portlandite (P) and calcite (C). The peaks were indexed using standard ICDD PDF cards.
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Figure 6. SEM images: (a) OPC powder, (b) GGBS, and (c) CCR.
Figure 6. SEM images: (a) OPC powder, (b) GGBS, and (c) CCR.
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Figure 7. Standard consistency and setting times.
Figure 7. Standard consistency and setting times.
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Figure 8. Compressive strength of mortars at various curing ages.
Figure 8. Compressive strength of mortars at various curing ages.
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Figure 9. Flexural strength of mortars at various curing ages.
Figure 9. Flexural strength of mortars at various curing ages.
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Figure 10. Morphology details of the microstructure: (a) morphology at 1 day, (b) morphology at 7 days, and (c) morphology at 28 days.
Figure 10. Morphology details of the microstructure: (a) morphology at 1 day, (b) morphology at 7 days, and (c) morphology at 28 days.
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Figure 11. EDX analysis results for OGC3 selected paste at 1 day.
Figure 11. EDX analysis results for OGC3 selected paste at 1 day.
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Figure 12. EDX analysis results for OGC3 selected paste at 7 days.
Figure 12. EDX analysis results for OGC3 selected paste at 7 days.
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Figure 13. EDX analysis results for OGC3 selected paste at 28 days.
Figure 13. EDX analysis results for OGC3 selected paste at 28 days.
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Table 1. Estimated characteristic particle sizes (D10, D50, D90) for OPC, CCR, and GGBS from the PSD curves.
Table 1. Estimated characteristic particle sizes (D10, D50, D90) for OPC, CCR, and GGBS from the PSD curves.
MaterialD10 (µm)D50 (µm)D90 (µm)
OPC1.01040
CCR2.02080
GGBS1.51250
Table 2. XRF analysis (%).
Table 2. XRF analysis (%).
MaterialCaOSiO2Al2O3MgOFe2O3SO3K2OTiO2Na2O
OPC62.37926.6392.4351.5721.7452.5880.7240.3851.533
GGBS41.56238.2655.4263.9420.1210.000.5360.6242.721
CCR81.8414.080.900.770.000.770.200.121.32
Table 3. Mix ID for the mortar made with various ratios (weight%) of OPC, GGBS, and CCR.
Table 3. Mix ID for the mortar made with various ratios (weight%) of OPC, GGBS, and CCR.
Mix IDOPC (%)GGBS (%)CCR (%)Binder (g)Water (g)Sand (g)Water/Binder RatioBinder/Sand Ratio
OPC10000150603750.402.5
OGC18013.36.6150603750.402.5
OGC26026.613.3150603750.402.5
OGC3404020150603750.402.5
OGC42053.326.6150603750.402.5
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Jwaida, Z.; Jadooe, A.; Dulaimi, A.; Almuhanna, R.R.A.; Hawesah, H.A.; Bernardo, L.F.A.; Andrade, J.M.d.A. Investigating the Properties of Composite Cement-Based Mortar Containing High Volumes of GGBS and CCR. J. Compos. Sci. 2025, 9, 301. https://doi.org/10.3390/jcs9060301

AMA Style

Jwaida Z, Jadooe A, Dulaimi A, Almuhanna RRA, Hawesah HA, Bernardo LFA, Andrade JMdA. Investigating the Properties of Composite Cement-Based Mortar Containing High Volumes of GGBS and CCR. Journal of Composites Science. 2025; 9(6):301. https://doi.org/10.3390/jcs9060301

Chicago/Turabian Style

Jwaida, Zahraa, Awad Jadooe, Anmar Dulaimi, Raid R. A. Almuhanna, Hayder Al Hawesah, Luís Filipe Almeida Bernardo, and Jorge Miguel de Almeida Andrade. 2025. "Investigating the Properties of Composite Cement-Based Mortar Containing High Volumes of GGBS and CCR" Journal of Composites Science 9, no. 6: 301. https://doi.org/10.3390/jcs9060301

APA Style

Jwaida, Z., Jadooe, A., Dulaimi, A., Almuhanna, R. R. A., Hawesah, H. A., Bernardo, L. F. A., & Andrade, J. M. d. A. (2025). Investigating the Properties of Composite Cement-Based Mortar Containing High Volumes of GGBS and CCR. Journal of Composites Science, 9(6), 301. https://doi.org/10.3390/jcs9060301

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