Article

Sustainable Cementitious Materials: Strength and

Microstructural Characteristics of Calcium Carbide
Residue-Activated Ground Granulated Blast Furnace Slag–Fly
Ash Composites
Xing Liu 1,2 , Guiyuan Xiao 1,2, * , Dunhan Yang 1,2 , Lin Dai 1,2 and Aiwei Tang 1,2

1 Guangxi Key Laboratory of Geomechanics and Geotechnical Engineering, Guilin University of Technology,
Guilin 541004, China; liuxingsaturn@163.com (X.L.)
2 Guangxi Key Laboratory of Green Building Materials and Construction Industrialization,
Guilin University of Technology, Guilin 541004, China
* Correspondence: xiaoguiyuangit@163.com

Abstract: This study developed a sustainable low-carbon cementitious material using calcium carbide
residue (CCR) as an alkali activator, combined with ground granulated blast furnace slag (GGBS) and
fly ash (FA) to form a composite. The objective was to optimize the CCR dosage and the GGBS-to-FA
ratio to enhance the unconfined compressive strength (UCS) of the composite, providing a viable
alternative to traditional Portland cement while promoting solid waste recycling. Experiments were
conducted with a water-to-binder ratio of 0.55, using six GGBS-to-FA ratios (0:10, 2:8, 4:6, 6:4, 8:2, and
10:0) and CCR contents ranging from 2% to 12%. Results indicated optimal performance at a GGBS-
to-FA ratio of 8:2 and an 8% CCR dosage, achieving a peak UCS of 18.04 MPa at 28 days, with 79.88%
of this strength reached within just 3 days. pH testing showed that with 8% CCR, pH gradually
decreased over the curing period but increased with higher GGBS content, indicating enhanced
reactivity. Microstructural analyses (XRD and SEM-EDS) confirmed the formation of hydration
Citation: Liu, X.; Xiao, G.; Yang, D.;
products like C-(A)-S-H, significantly improving density and strength. This study shows CCR’s
Dai, L.; Tang, A. Sustainable
Cementitious Materials: Strength and
potential as an effective and environmentally friendly activator, advancing low-carbon building
Microstructural Characteristics of materials and resource recycling in construction.
Calcium Carbide Residue-Activated
Ground Granulated Blast Furnace Keywords: calcium carbide residue; ground granulated blast furnace slag; fly ash; alkali-activated
Slag–Fly Ash Composites. cementitious materials; unconfined compressive strength; microstructural properties; sustainable
Sustainability 2024, 16, 11168. building materials
https://doi.org/10.3390/
su162411168

Academic Editor: Syed Minhaj

Saleem Kazmi 1. Introduction
The rapid development of China’s economy has significantly increased its energy
Received: 14 November 2024
demand. As the world’s largest energy producer, consumer, and greenhouse gas emitter,
Revised: 14 December 2024
China faces tremendous pressure in terms of energy resources and environmental sustain-
Accepted: 17 December 2024
Published: 19 December 2024
ability [1]. By the end of 2019, China consumed 1.91 billion tons of coal, accounting for
51.7% of global consumption. Simultaneously, its CO2 emissions reached 10.04 billion tons,
contributing 30% of the global total. These figures underscore the substantial role of China’s
coal consumption in driving CO2 emissions, with far-reaching implications for global cli-
Copyright: © 2024 by the authors. mate change and environmental challenges [2]. To advance low-carbon transitions and
Licensee MDPI, Basel, Switzerland. achieve sustainable development, China has established ambitious “dual carbon” goals: to
This article is an open access article peak carbon emissions by 2030 and achieve carbon neutrality by 2060 [3]. These objectives
distributed under the terms and pose significant challenges for the civil engineering sector, necessitating the exploration of
conditions of the Creative Commons innovative pathways to enhance energy efficiency and reduce emissions. A pivotal strategy
Attribution (CC BY) license (https://
involves decreasing reliance on traditional high-energy-consumption materials such as ce-
creativecommons.org/licenses/by/
ment. Furthermore, repurposing industrial waste to develop low-carbon, environmentally
4.0/).

Sustainability 2024, 16, 11168. https://doi.org/10.3390/su162411168 https://www.mdpi.com/journal/sustainability

Sustainability 2024, 16, 11168 2 of 19

friendly materials and minimizing energy-intensive processes aligns with the principles of
green, clinker-free, and sustainable construction practices [4].
Alkali-activated binders represent an emerging class of environmentally friendly
materials characterized by high compressive strength, excellent chemical stability, and
outstanding heat resistance, making them highly suitable for applications in civil engi-
neering [5–10]. In comparison to traditional Portland cement, alkali-activated materials
offer distinct advantages. For example, they utilize abundant industrial by-products as
raw materials, thereby reducing the depletion of natural resources, and their production
process eliminates the need for energy-intensive high-temperature calcination, leading to a
substantial reduction in carbon emissions. These benefits position alkali-activated binders
as an ideal alternative to Portland cement, garnering significant attention as a research
focus in both academic and industrial fields [5,11,12].
Ground granulated blast furnace slag (GGBS), a by-product of the ironmaking pro-
cess, demonstrates excellent cementitious properties and is typically activated by alkaline
substances such as lime (CaO or Ca(OH)2 ). With the rapid development of China’s steel
industry, the annual production of slag has reached 240 million tons [13]. Similarly, fly ash,
primarily derived from residual coal combustion in thermal power plants, has seen an an-
nual output approaching 900 million tons by 2020 [14]. The abundant availability of GGBS
and fly ash provides a rich source of raw materials for research on alkali-activated materials.
In the research and application of alkali-activated binders, calcium hydroxide
(Ca(OH)2 ) has been widely used as an alkali activator due to its low cost and good dura-
bility [15,16]. Andal Mudimby suggested that lime improves the workability of alkali-
activated materials (AACs), and a small amount of lime can enhance their compressive
strength [17]. Compared to NaOH, Ca(OH)2 offers the advantage of a more controlled
activation process for GGBS. However, its activation efficiency is relatively low, and it is
typically used in combination with other activators. Additionally, Yang et al. found that
adding Ca(OH)2 in amounts exceeding 10% of the solid mass significantly reduces the
compressive strength of the material [16]. Further investigation is needed to understand
the stability and strength development of binders based on Ca(OH)2 -activated GGBS and
FA. Other commonly used alkali activators, such as NaOH, water glass, and Na2 CO3 ,
have demonstrated remarkable effectiveness in activating GGBS and FA [18,19]. However,
these activators are often associated with significant drawbacks, including environmen-
tal pollution, high cost, and poor transportation safety, which limit their feasibility for
large-scale application [20]. In this context, finding an alkali activator that is low-carbon,
environmentally friendly, cost-effective, and easy to transport has become a critical research
focus. Calcium carbide residue (CCR) has recently attracted attention due to its unique
physicochemical properties and environmental advantages, positioning it as a promising
alternative in this field.
In recent years, calcium carbide residue (CCR) has gained attention as a potential
low-carbon and environmentally friendly alkali activator [21–23]. CCR is an industrial by-
product generated during acetylene production, with an annual output in China exceeding
400 million tons [24,25]. With calcium hydroxide (Ca(OH)2 ) content of up to 85%, CCR
exhibits strong alkalinity (pH > 12) [26]. However, the open-air disposal of CCR not
only occupies valuable land resources but also poses significant environmental risks,
as rainwater leaching can contaminate soil and water systems, severely impacting the
ecosystem [27]. Studies have demonstrated that CCR can serve as an alkali activator for
GGBS-based soil stabilization [28,29]. It effectively stimulates the hydration reactions of
GGBS and FA, forming calcium aluminate hydrate (CAH) and calcium silicate hydrate
(CSH) phases, which significantly enhance material strength [30–32]. Additionally, CCR can
act as a calcium source to further improve the strength of cementitious materials [33–36].
Replacing traditional limestone with CCR could potentially avoid up to 20 million tons of
CO2 emissions annually [25].
Nevertheless, current research on the interactions within the composite systems of
CCR, GGBS, and FA, as well as the underlying micro-mechanisms, remains insufficient.
Sustainability 2024, 16, 11168 3 of 19

In particular, systematic studies on the optimization of CCR dosage, the coordination and
enhancement of hydration reactions among different components, and the mechanisms for
improving strength are lacking. Moreover, to further investigate and address the issue of
strength reduction caused by excessive Ca(OH)2 , we incorporated FA into the GGBS-based
system. This study aims to explore the strength characteristics and microstructural features
of GGBS-FA blends under varying CCR activation ratios.
In summary, this study focuses on the following key aspects. (1) The effect of CCR
content on the unconfined compressive strength (UCS) of GGBS-FA composite binders with
varying ratios. Through systematic experiments, the optimal mass ratio of FA to GGBS and
the ideal CCR dosage were identified. (2) Strength development under different curing
durations. The compressive strength evolution of an 8% CCR-activated GGBS-FA (8:2)
composite was analyzed over various curing periods (3, 7, 14, and 28 days), revealing the
material’s strength development trends. (3) Mechanisms of hydration reactions and mi-
crostructural evolution. Advanced characterization techniques, including X-ray diffraction
(XRD) and scanning electron microscopy coupled with energy-dispersive spectroscopy
(SEM-EDS), were employed to investigate the hydration products, providing deeper in-
sights into the activation mechanisms of CCR in GGBS-FA composite binders. This study
not only contributes to the theoretical understanding of the resource utilization of industrial
by-products but also offers practical guidance for developing low-carbon eco-friendly con-
struction materials. By addressing critical challenges in material innovation, this research
holds promise for facilitating the building industry’s progress toward achieving the “dual
carbon” goals.

2. Materials and Methods

2.1. Materials
2.1.1. Ground Granulated Blast Furnace Slag
The ground granulated blast furnace slag (GGBS) used in this study was procured
from Longze Water Purification Materials Co., Ltd., located in Gongyi City, Henan Province.
The material appears white in color. The physical and chemical properties of GGBS
are presented in Table 1, while its particle size distribution curve is shown in Figure 1.
The primary chemical composition of the GGBS is listed in Table 2, and its XRD pattern
is depicted in Figure 2. In an alkaline environment, the glassy structure of GGBS is
prone to degradation, progressively decomposing to form hydration products, such as
Sustainability 2025, 17, x FOR PEER REVIEW
calcium silicate hydrate (C-S-H) and calcium aluminum silicate hydrate (C-(A)-S-H).4These
of 21
hydration products contribute to the enhanced mechanical performance of the material [37].

Figure
Figure 1.
1. Particle
Particle size
size grading
grading curves
curves of
of CCR,
CCR, GGBS,
GGBS, and
and FA.
FA.
Sustainability 2024, 16, 11168 4 of 19
Figure 1. Particle size grading curves of CCR, GGBS, and FA.

Figure
Figure 2.
2. XRD
XRDspectrum
spectrum of
of GGBS.
GGBS.

Table
Table 1.
1. Physical
Physical and
and chemical
chemical properties
properties of
of GGBFS,
GGBFS, FA,
FA, and
and CCR.
CCR.

Material Density/(g·cm−3) Specific surfacearea/(m

Specific ·kg )
2 −1 Activity Index/%
Material Density/(g·cm−3 ) Activity Index/%
GGBS 2.93 3112.67
surfacearea/(m 2 −
·kg )1
115
FA GGBS 2.34 2.93 2090.67
3112.67 85
115
CCR FA 2.24 2.34 966.33
2090.67 85-
CCR 2.24 966.33 -

Table 2. Chemical composition of GGBS, FA, and CCR.

Table 2. Chemical composition of GGBS, FA, and CCR.
Materials GGBS/% FA/% CCR/%
SiO2
Materials 34.50
GGBS/% 53.96
FA/% 1.55
CCR/%
Al
SiO2O2 3 16.70
34.50 31.14
53.96 1.96
1.55
Fe22O33
Al O 1.50
16.70 4.16
31.14 0.48
1.96
Fe2 O3 1.50 4.16 0.48
CaO 35.30 4.01 67.87
CaO 35.30 4.01 67.87
MgO
MgO 5.01
5.01 1.01 1.01 00
SO33
SO 1.24
1.24 0.72 0.72 0.31
0.31
K22O
K O ---- 2.03 2.03 ----
TiO2 -- 1.13 --
TiO2
Na2 O
---- 1.13 0.88
----
Na
P2 O2O
5 ---- 0.88 0.67 ----
P2O5 (LOI)
Loss on ignition --
0.96 0.67-- --
25.85
Moisture content
Loss on ignition (LOI) 0.20
0.96 -- -- 0.50
25.85
Moisture content 0.20 -- 0.50
2.1.2. Fly Ash
2.1.2. The
Fly Ash
fly ash (FA) used in this study was supplied by Longze Water Purification Ma-
terials Co., Ltd., located in Gongyi City, Henan Province. The material is characterized
by a gray-black appearance. The physical and chemical properties of FA are summarized
in Table 1, while its particle size distribution curve is presented in Figure 1. The primary
chemical composition of the FA is detailed in Table 2. As indicated in Table 2, the Al2 O3
content of the FA exceeds 30%, categorizing it as high-alumina FA. Under alkaline activa-
tion, this material is more prone to forming hydration products such as calcium aluminum
silicate hydrate (C-(A)-S-H), which significantly enhances the structural performance of
the material. According to the Chinese national standard “Fly Ash Used in Cement and
Concrete” (GB/T 1596-2017), Grade I FA must have a combined SiO2 + Al2 O3 + Fe2 O3
content of no less than 50% and a SO3 content of no more than 5%. In this study, the FA
has a SiO2 + Al2 O3 + Fe2 O3 content of 89.26%, far exceeding the 50% requirement, and a
SO3 content of 0.72%, well below the 5% limit. Thus, the FA used in this research meets the
criteria for Grade I FA. X-ray diffraction (XRD) analysis revealed that the FA is primarily
composed of three mineral phases, quartz, mullite, and rutile, as illustrated in Figure 3.
 Concrete” (GB/T 1596-2017), Grade I FA must have a combined SiO2 + Al2O3 + Fe2O3 con-
SiO2 + Al2O3 + Fe2O3 content of 89.26%, far exceeding the 50% requirement, and a SO3 con-
tent of no less than 50% and a SO3 content of no more than 5%. In this study, the FA has a
tent of 0.72%, well below the 5% limit. Thus, the FA used in this research meets the criteria
SiO2 + Al2O3 + Fe2O3 content of 89.26%, far exceeding the 50% requirement, and a SO3 con-
for Grade I FA. X-ray diffraction (XRD) analysis revealed that the FA is primarily com-
tent of 0.72%, well below the 5% limit. Thus, the FA used in this research meets the criteria
posed of three mineral phases, quartz, mullite, and rutile, as illustrated in Figure 3.
Sustainability 2024, 16, 11168 for Grade I FA. X-ray diffraction (XRD) analysis revealed that the FA is primarily 5com- of 19
posed of three mineral phases, quartz, mullite, and rutile, as illustrated in Figure 3.

Figure 3. XRD spectrum of FA.

Figure
Figure 3.
2.1.3. 3. XRD spectrum of FA.
Calcium Carbide Residue
2.1.3.The calcium
Calcium carbide
Carbide residue (CCR) used in this study was sourced from Shandong
Residue
2.1.3. Calcium Carbide Residue
WanhuaTheChemical Chlor-Alkali
calcium carbide residueThermal
(CCR) Power
used inCo.,
thisLtd. Thewas
study material is gray
sourced frominShandong
color and
The Chemical
calcium carbide residue (CCR) Power
used inCo.,
thisLtd.
study was sourced frominShandong
was sieved through a 0.075 mm mesh prior to use. The physical and chemical properties
Wanhua Chlor-Alkali Thermal The material is gray color and
Wanhua Chemical Chlor-Alkali Thermal Power Co., Ltd. The material is gray in color and
of
wasCCR are presented
sieved in Table
through a 0.075 mm1,mesh
and its particle
prior size
to use. Thedistribution curve
physical and is shown
chemical in Figure
properties of
was sieved through a 0.075 mm mesh prior to use. The physical and chemical properties
1. The main chemical components of CCR are listed in Table 2, while its XRD pattern is1.
CCR are presented in Table 1, and its particle size distribution curve is shown in Figure
of
The CCR
mainarechemical
presentedcomponents
in Table 1, and its particle
of CCR size distribution
are listed curve its
in Table 2, while is shown in Figure
XRD pattern is
illustrated in Figure 4. The primary mineral phases identified in CCR are calcium hydrox-
1. The main
illustrated in chemical components
Figure 4. The of CCR phases
primary mineral are listed in Table
identified in 2,
CCRwhile its XRD hydroxide
are calcium pattern is
ide and calcite. Upon dissolution in water, CCR exhibits strong alkalinity, with a pH
illustrated
and calcite.inUponFigure 4. The primary
dissolution mineral
in water, CCRphases identified
exhibits in CCR arewith
strong alkalinity, calcium
a pHhydrox-
greater
greater
than 12 than 12 (solid-to-liquid
(solid-to-liquid ratio of ratio
1:10 of 1:10on
based based
mass).on mass).
ide and calcite. Upon dissolution in water, CCR exhibits strong alkalinity, with a pH
greater than 12 (solid-to-liquid ratio of 1:10 based on mass).

Figure 4. XRD spectrum of CCR.

2.2. Methods
2.2.1. Test Procedure
This study aimed to examine the effects of varying CCR dosages on the mechanical
properties of composite binders with different GGBS-to-FA ratios. GGBS and FA were
used as the primary materials, with CCR serving as an external alkali activator. Prior to
the commencement of this study, preliminary experiments were conducted to evaluate
the workability and strength of materials under different water-to-binder (w/b) ratios.
The results indicated that a w/b ratio of 0.55 offered the best balance between workability
and strength. Consequently, this ratio was maintained throughout the study to ensure
consistency and comparability. Drawing on relevant studies by other researchers [37,38],
the CCR dosages were set at 2%, 4%, 6%, 8%, 10%, and 12% of the total mass of GGBS
and FA. The mass ratios of GGBS to FA were adjusted to 0:10, 2:8, 4:6, 6:4, 8:2, and 10:0,
respectively. The specimens were cured for 3, 7, 14, and 28 days to evaluate the effects of
curing time on mechanical performance. A total of 39 mix designs were prepared, and the
detailed mix proportions are listed in Table 3.
 The mass ratios of GGBS to FA were adjusted to 0:10, 2:8, 4:6, 6:4, 8:2, and 10:0, respec-
tively. The specimens were cured for 3, 7, 14, and 28 days to evaluate the effects of curing
time on mechanical performance. A total of 39 mix designs were prepared, and the de-
Sustainability 2024, 16, 11168 tailed mix proportions are listed in Table 3. 6 of 19

Table 3. Mix proportion for the experiment.

Table 3. Mix proportion for the experiment.
Groups CCR/% GGBS/% FA/% Water-Cement Ratio Curing Time/d
0 100 Curing
Groups CCR/% GGBS/% FA/% Water-Cement Ratio
20 80 Time/d
2, 4, 6, 8, 40 0 60 100
A 0.55 3
10, 12 60 20 40 80
40 60
A 2, 4, 6, 8, 10, 1280 20 0.55 3
60 40
100 80 0 20
B 8 80 100 20 0 0.55 7, 14, 28
B 8 80 20 0.55 7, 14, 28
2.2.2. Sample Preparation and Test Methods
2.2.2.Following the proportions
Sample Preparation outlined
and Test Methodsin Table 3, the required amounts of CCR, GGBS,
and FA were accurately
Following weighedoutlined
the proportions and thoroughly
in Tablemixed before adding
3, the required water.
amounts The mixture
of CCR, GGBS,
was then placed into a mixer, with the speed set to 30 rpm, and stirred for 3 minutes. After
and FA were accurately weighed and thoroughly mixed before adding water. The mixture
mixing,
was thenthe blendinto
placed wasa poured into standard
mixer, with the speedmoldsset to with dimensions
30 rpm, of 40
and stirred for×340 × 40After
min. mm
mixing, the blend was poured into standard molds with dimensions
and compacted using vibration. Once the specimens were molded, they were coveredof 40 × 40 × 40 mm
and compacted using vibration. Once the specimens were molded, they were covered
with plastic film and left to cure at room temperature for 24 hours before demolding. After
with plastic film and left to cure at room temperature for 24 h before demolding. After
demolding,
demolding, thethe specimens
specimens were
were further
further cured
cured at at room
room temperature
temperature for
for periods
periods ofof 3,
3, 7,
7, 14,
14,
and 28 days. The experimental procedure is illustrated in Figure 5, and the detailed
and 28 days. The experimental procedure is illustrated in Figure 5, and the detailed testing testing
process is described
process is described as
as follows.
follows.

Figure
Figure 5.
5. The
The whole
whole process
process of
of the
the test.
test.

1. The unconfined compressive strength (UCS) test was performed on specimens cured
for 3, 7, 14, and 28 days, with a loading rate of 1 mm/min. Each group comprised
three parallel specimens, and each specimen was tested three times under consistent
experimental conditions. The UCS values from all tests were recorded, and the
average of these values was taken as the representative UCS for the group. All tests
were conducted using a universal testing machine manufactured by Shenzhen Suns
Technology Stock Co., Ltd., Shenzhen, China.
2. pH measurement. After the UCS tests, representative sample fragments were im-
mersed in anhydrous ethanol for 24 h to halt the hydration process. The fragments
were then dried at 60 ◦ C for 24 h and subsequently ground into powder. According to
the Chinese standard “Geotechnical Testing Method” (GB/T 50123—2019), 5 g of the
powder was mixed with 50 g of deionized water and stirred at 180 rpm for 30 min.
After standing for 30 min, the pH of the suspension was measured using a pH meter
with an accuracy of 0.01. The pH meter used was the PH-100A model, produced by
Zhejiang Lichen Instrument Technology Co., Ltd., Shaoxing, China.
3. X-ray diffraction (XRD) analysis: The powder obtained in step (2) was passed through
a 0.075 mm (200 mesh) sieve for X-ray diffraction (XRD) analysis. The analysis
was carried out using an Empyrean X-ray diffractometer (PANalytical, Almelo, The
Netherlands). The operating conditions were as follows: a tube voltage of 40 kV, a
Sustainability 2024, 16, 11168 7 of 19

tube current of 40 mA, a wavelength λ = 0.15406 nm, a 2θ scanning range from 5◦ to

80◦ , a step size of 0.01◦ per step, and a scanning speed of 10◦ /min.
4. SEM-EDS analysis: Dried fragments from step (2) were processed into approximately
5 mm flake-shaped particles for analysis. Scanning electron microscopy (SEM), com-
bined with energy-dispersive X-ray spectroscopy (EDS), was used to examine the
microstructure and determine the elemental composition. The analysis was conducted
using a Tescan Mira 4 field-emission scanning electron microscope (Brno, Czech Re-
public), operating at an accelerating voltage of 15 kV and a magnification of 2000×.
The energy-dispersive spectrometer (EDS) used in this study was an Ultim Max 65
manufactured by Oxford Instruments, Oxford, UK.

3. Results and Discussion

3.1. Effect of CCR Doping and GGBS-FA Ratio on the Strength of Materials
Figure 6 presents the unconfined compressive strength (UCS) results of the composite
cementitious material cured for 3 days. The results indicate that the ratio of GGBS to FA
significantly affects the strength of the composite material. As shown in Figure 6a, when
the CCR content is below 8%, the strength of the composite cementitious material increases
significantly with the increase in GGBS content (and the corresponding decrease in FA
content) [39]. This is because GGBS, compared to FA, exhibits higher reactivity under
the activation of CCR, providing abundant calcium elements and additional nucleation
sites [40–42], which further promote the formation of C-(A)-S-H gel [43], thereby enhancing
the hardening properties of the material. Notably, when the GGBS-FA ratio is 10:0 and
Sustainability 2025, 17, x FOR PEER REVIEW the
8 of 21
CCR content is 2%, 4%, and 6%, the compressive strengths of the samples reach maximum
values of 8.31 MPa, 11.58 MPa, and 14.31 MPa, respectively.

Figure 6. UCS of specimens with different GGBS/FA ratios at different CCR doping levels. (a) CCR = 2,
Figure 6. UCS of specimens with different GGBS/FA ratios at different CCR doping levels. (a) CCR
4, and 6%; (b) CCR = 8, 10, and 12%.
= 2, 4, and 6%; (b) CCR = 8, 10, and 12%.
Moreover, it is evident from the figure that as the FA content increases, the UCS of the
Moreover,
composite it isgradually
material evident from the figure
decreases, that as the
particularly whenFAthecontent
GGBS-FAincreases,
ratio isthe UCS
0:10, and of
thecompressive
the composite material
strength gradually decreases,
reaches its particularly
lowest values of 0.25when
MPa,the
0.26GGBS-FA
MPa, and ratio
0.29isMPa,
0:10,
and the compressive
respectively. strength
This reduction reaches is
in strength itsattributed
lowest valuesto theoflower
0.25 reactivity
MPa, 0.26of MPa, and 0.29
the material
MPa, the
when respectively.
GGBS contentThis reduction
is reduced,inresulting
strength in is aattributed
decrease to in the
the lower
amount reactivity of the
of C-(A)-S-H
gel produced during the hydration process, which in turn lowers
material when the GGBS content is reduced, resulting in a decrease in the amount the overall strength of of
C-
the material.
(A)-S-H gel produced during the hydration process, which in turn lowers the overall
As shown in Figure 6b, with the increasing dosage of CCR (8%, 10%, and 12%), the
strength of the material.
unconfined compressive strength (UCS) of the composite cementitious materials increases
with aAs shown
higher in Figure
GGBS 6b, with
content. the increasing
The UCS reaches itsdosage
maximum of CCR (8%,
values 10%,the
when andGGBS/FA
12%), the
ratio is 8:2, with values of 14.41, 14.98, and 15.20 MPa, respectively, which areincreases
unconfined compressive strength (UCS) of the composite cementitious materials higher
with a higher
compared GGBS
to other content.ratios.
GGBS-FA The UCSThisreaches
behavior its can
maximum valuesas
be explained when the GGBS/FA
follows: at lower
ratio is 8:2, with values of 14.41, 14.98, and 15.20 MPa, respectively, which are higher com-
pared to other GGBS-FA ratios. This behavior can be explained as follows: at lower CCR
dosages (CCR < 8%), the limited OH− provided by CCR results in a relatively slow hydra-
tion reaction, leading to fewer hydration products. In contrast, when the CCR dosage is ≥
8%, the abundant OH− enhances the alkalinity of the G10F0 system, effectively activating
Sustainability 2024, 16, 11168 8 of 19

CCR dosages (CCR < 8%), the limited OH− provided by CCR results in a relatively slow
hydration reaction, leading to fewer hydration products. In contrast, when the CCR
dosage is ≥ 8%, the abundant OH− enhances the alkalinity of the G10F0 system, effectively
activating the hydration of GGBS. Additionally, the high calcium content in GGBS (as
shown in Table 2) promotes a rapid hydration reaction within the composite material. This
leads to the formation of significant amounts of hydration products, such as C-(A)-S-H gel,
with an ideal Ca/Al molar ratio of 1.5–2.5. However, in the G10F0 system, the calculated
Ca/Al molar ratio is 4.42, which is much higher than the ideal range. This excess hydration
Sustainability 2025, 17, x FOR PEER REVIEW 9 of 21
of CaO generates excessive heat, causing the formation of microcracks during the setting
process (as shown in Figure 7), which damages the structural integrity and ultimately
reduces both the UCS and overall stability of the composite material [44].

Figure 7. Specimens with 8% CCR, with a GGBS/FA ratio of 10:0.

Figure 7. Specimens with 8% CCR, with a GGBS/FA ratio of 10:0.
In contrast, the inclusion of FA moderates the hydration reaction rate, thereby enhanc-
In summary,
ing the toughness of low
theCCR contentmaterial.
composite is insufficient
Samples in containing
activating FA
the exhibit
performance of the
no significant
blended material, while an excessively high CCR content overly activates the
cracking during the curing process. As demonstrated in Figure 6b, for CCR content values 100% GGBS
system, resulting
of 8%, 10%, in material
and 12%, the UCS defects. To fully
of samples harness
with the material’s
a GGBS-to-FA potential
ratio of while en-
8:2 is consistently
hancing
higher thanthe toughness of thewith
that of samples cementitious
a GGBS-to-FAsystem,
ratiothe
of addition
10:0. of FA is necessary when
CCR In content is ≥8%,
summary, as indicated
low CCR content by the above analysis.
is insufficient Thus, the optimal
in activating CCR content
the performance is
of the
blended material, while an excessively high CCR content overly activates
determined to be ≥8%, with the ideal GGBS-to-FA ratio being 8:2. This ensures an adequatethe 100% GGBS
system, resulting in material defects. To fully harness the material’s potential while en-
supply of OH− from the alkali activator while minimizing defects that could compromise
hancing the toughness of the cementitious system, the addition of FA is necessary when
mechanical properties. The FA content is fixed at 20% of the blended system, as higher FA
CCR content is ≥8%, as indicated by the above analysis. Thus, the optimal CCR content is
levels reducetohydration
determined efficiency
be ≥8%, with andGGBS-to-FA
the ideal adversely affect
ratioearly-age
being 8:2.performance.
This ensures anThis obser-
adequate
vation is consistent
− with the findings of Luo [45].
supply of OH from the alkali activator while minimizing defects that could compromise
mechanical properties. The FA content is fixed at 20% of the blended system, as higher
3.2.
FA Optimal Ratio of
levels reduce CCR
hydration efficiency and adversely affect early-age performance. This
Figure 8 illustrates the effect
observation is consistent with theoffindings
varyingofCCR
Luodosages
[45]. on the unconfined compressive
strength (UCS) of the composite cementitious material with a GGBS-FA ratio of 8:2. The
3.2. Optimal Ratio of CCR
test results show that as the CCR dosage increases, the UCS of the composite material
Figure 8 illustrates the effect of varying CCR dosages on the unconfined compressive
gradually rises, eventually reaching a plateau. This indicates that while the increase in
strength (UCS) of the composite cementitious material with a GGBS-FA ratio of 8:2. The
CCR dosageshow
test results providesthatabundant
as the CCROHdosage
− ions to activate the composite material, the reactive
increases, the UCS of the composite material
components in GGBS
gradually rises, and FAreaching
eventually that can abeplateau.
activatedThis
by the alkali are
indicates thatlimited. Therefore,
while the increasethe
in
continued
CCR dosage increase
providesin CCR dosage
abundant OH doesions
− not to
result in a proportional
activate the compositeand unlimited
material, growth
the reactive
in the strength
components in of the composite
GGBS and FA thatmaterial. Once the by
can be activated CCRthedosage exceeds
alkali are a certain
limited. thresh-
Therefore, the
old, the rateincrease
continued of strength increase
in CCR gradually
dosage does notdecreases,
result in aas shown in Table
proportional 4.
and unlimited growth
in the strength of the composite material. Once the CCR dosage exceeds a certain threshold,
the rate of strength increase gradually decreases, as shown in Table 4.
 gradually rises, eventually reaching a plateau. This indicates that while the increase in
CCR dosage provides abundant OH− ions to activate the composite material, the reactive
components in GGBS and FA that can be activated by the alkali are limited. Therefore, the
continued increase in CCR dosage does not result in a proportional and unlimited growth
Sustainability 2024, 16, 11168 in the strength of the composite material. Once the CCR dosage exceeds a certain thresh-
9 of 19
old, the rate of strength increase gradually decreases, as shown in Table 4.

Figure
Figure 8.
8. The
The effect
effect of
of CCR
CCR dosage
dosage on
on the
the UCS
UCS of
of G8F2.
G8F2.

TableIn practicalgrowth
4. Strength engineering
rate and applications, it is necessary to balance material dosage,
average strength.
strength, and strength growth rate to enhance material utilization efficiency. Based on the
CCR/% λ/% S/Mpa
above analysis, this study proposes a comprehensive optimization method to maximize
2 380.50
the activation of
4 the GGBS/FA = 8:2 composite.
148.35The specific steps are as follows:
6 97.50
12.55
8 93.95
10 28.55
12 11.00

In practical engineering applications, it is necessary to balance material dosage,

strength, and strength growth rate to enhance material utilization efficiency. Based on the
above analysis, this study proposes a comprehensive optimization method to maximize the
activation of the GGBS/FA = 8:2 composite. The specific steps are as follows:
(1) The strength growth rate for each dosage point is calculated based on the experi-
mental data. Based on Equation (1) and the data from Figure 8, the strength growth rate (λ)
for different CCR dosages is determined, with the results shown in Table 4.

Si − Si − 1
λ= (1)
Ci − Ci−1

Si represents the strength value at the i−th dosage point, in MPa; Ci denotes the
corresponding dosage, in %.
(2) The average strength S of all the test samples is determined. The calculation is
performed using Equation (2), and the results are presented in Table 4.

1 n
n i∑
S= Si (2)
=1

In this equation, n represents the total number of dosage levels, with n = 6.

(3) Dosage levels with strength values exceeding the average are selected to establish
a candidate range. As shown in Table 4, the dosages where the strength values surpassed
the average are 8%, 10%, and 12%.
(4) From the candidate range identified in Step (3), the dosage with the highest growth
rate is selected as the optimal dosage. According to Table 4, the growth rate at 8% is 93.95%,
which is higher than the rates observed at 10% and 12%. Therefore, 8% is selected as the
optimal CCR dosage for the GGBS-FA ratio of 8:2.
Sustainability 2024, 16, 11168 10 of 19

3.3. Effect of Maintenance Time on the Strength of Composite Cementitious Materials

Figure 9 illustrates the unconfined compressive strength (UCS) of composite cemen-
titious materials activated with 8% CCR, containing varying ratios of GGBS and FA, at
curing periods of 3, 7, 14, and 28 days. As shown in Figure 9, the compressive strength of
the composite materials increases progressively with longer curing times. Taking G8F2 as
an example, the UCS values at 3, 7, 14, and 28 days are 14.41, 16.76, 17.61, and 18.0411MPa,
Sustainability 2025, 17, x FOR PEER REVIEW of 21
respectively. Compared to the samples cured for 3 days, the strength values of the samples
cured for 7, 14, and 28 days increase by 16.31%, 22.21%, and 25.19%, respectively.

Figure9.
Figure 9. Effect
Effect of
of maintenance
maintenance time
time on
onUCS
UCSof
ofcomposite
compositecementitious
cementitiousmaterials.
materials.

From these
From these data,
data, itit can
can be
beobserved
observed that
that the
thecompressive
compressive strength
strength of
of the
the specimens
specimens
does
does not show a significant increase with extended curing time. Additionally, the
not show a significant increase with extended curing time. Additionally, the com-
com-
pressive strength of the composite cementitious material at 3 days reaches 79.88% of its
pressive strength of the composite cementitious material at 3 days reaches 79.88% of its
28-day strength, indicating excellent early strength performance and suggesting promising
28-day strength, indicating excellent early strength performance and suggesting promis-
potential for engineering applications. This early strength characteristic implies that the
ing potential
composite for engineering
material can reduce applications. Thisand
construction time early strength
costs characteristic
in practical engineeringimplies that
projects.
the composite
Moreover, it is material
noteworthy can that
reduce construction
throughout the time
entireand costsperiod,
curing in practical engineering
the compressive
projects. of
strength Moreover,
the G8F2 itsamples
is noteworthy that exceeds
consistently throughout
thatthe entire curing
of samples period,
with other the com-
ratios. This
pressivedemonstrates
further strength of the that G8F2
the samples
combined consistently
use of GGBSexceeds thatcan
and FA of samples
optimizewith other ra-
the material
tios. This further
properties, achievingdemonstrates
an optimalthat the combined
performance state.use of GGBS and FA can optimize the
material properties, achieving an optimal performance state.
3.4. Effect of GGBS-FA Ratio and Curing Time on pH
Figure
3.4. Effect of 10 shows the
GGBS-FA pHand
Ratio variation
Curing in cementitious
Time on pH materials with different GGBS-FA
ratios activated by 8% CCR over different curing periods. As illustrated in the figure,
Figurecuring
for a fixed 10 shows the the
period, pH pHvariation
value in
of cementitious
the compositematerials with material
cementitious different gradually
GGBS-FA
ratios activated
increases by 8% GGBS
with a higher CCR over different
content curing
(and lower FAperiods. AsUsing
content). illustrated in time
a curing the figure, for
of 3 days
a fixed curing period, the pH value of the composite cementitious material
as an example, as the GGBS content in the system increases from 0% to 100%, the pH of the gradually in-
creases with
composite a higher
system GGBS
rises fromcontent
12.34 to(and lower
12.70, FA content).
representing Usingincrease.
a 2.91% a curing This
time suggests
of 3 days
as an
that example,
the hydration as reaction
the GGBS of content in the system
GGBS, activated increases
by CCR, from
releases 0% to amount
a greater 100%, the OH−
of pH of,
the composite system rises from 12.34 to 12.70, representing a 2.91% increase. This sug-
thereby elevating the system’s pH. The GGBS used in this study contains 35.30% CaO,
which provides
gests that an abundant
the hydration source
reaction of of CaO activated
GGBS, for the alkali activation
by CCR, reaction,
releases leading
a greater to the
amount of
formation of more C-(A)-S-H gel and the release of additional OH − . Moreover, the experi-
OH , thereby elevating the system’s pH. The GGBS used in this study contains 35.30%
−
mental results indirectly confirm the synergistic effect between CCR and GGBS during the
CaO, which provides an abundant source of CaO for the alkali activation reaction, leading
hydration process, which enhances the system’s alkalinity by generating more OH− .
to the formation of more C-(A)-S-H gel and the release of additional OH−. Moreover, the
experimental results indirectly confirm the synergistic effect between CCR and GGBS dur-
ing the hydration process, which enhances the system’s alkalinity by generating more
OH−.
Sustainability 2024,
Sustainability 17, 11168
2025, 16, x FOR PEER REVIEW 1211of
of 21
19

Figure 10. Effect

Effect of FA-GGBS ratio and curing time on pH value.

With aa constant
With constantGGBS/FA
GGBS/FA ratio, ratio, the
thepH pHvalues
valuesof ofthe
thecomposite
compositecementitious
cementitious materials
materi-
gradually decrease as the curing time is extended. For instance, in the G8F2 system, the pH
als gradually decrease as the curing time is extended. For instance, in the G8F2 system,
values at 3, 7, 14, and 28 days of curing are 12.60, 12.46, 12.31, and 12.29, respectively. The
the
pH pHat 3 values
days isatnotably
3, 7, 14,higher
and 28 days
than at of curing
7, 14, andare 12.60, This
28 days. 12.46, 12.31, occurs
decline and 12.29, respec-
because, as
tively.
curing The pH at 3OH
progresses, days is notably
− ions higher than
are gradually consumedat 7, 14, and 28 days.
in activating the This decline
reactions occurs
of FA and
because,
GGBS, which as curing
promotes progresses, OH− ions
the formation of the areC-(A)-S-H
graduallygel. consumed
While this inprocess
activating the reac-
enhances the
tions of FAmechanical
material’s and GGBS, which promotes
strength, it also resultsthe information
a steady of the C-(A)-S-H
decrease gel. While
in the system’s pH. this
process enhances
It should the material’s
be noted that althoughmechanical
the resultsstrength, it also
in Section 3.1results in that
indicate a steady decrease in
the compressive
the system’s pH.
strength of G8F2 is greater than that of G10F0, this does not contradict the finding in this
section that the pH value of G8F2 is lower than that of G10F0.
It should be noted that although the results in Section 3.1 indicate that the compres- This is because a high
−
GGBS content results in the release of more OH ions, thereby increasing the pH of the
sive strength of G8F2 is greater than that of G10F0, this does not contradict the finding in
composite material. However, compressive strength is not solely dependent on pH; it is
this section that the pH value of G8F2 is lower than that of G10F0. This is because a high
also affected by factors such as the formation of hydration products, reaction activity, and
GGBS content results
the densification of thein the release
internal of more
structure. OH− ions,
Although the pHthereby
of G10F0increasing
is higher, the
thepH of the
addition
composite material. However, compressive strength is not
of FA in G8F2 improves the toughness and internal structure of the material, ultimatelysolely dependent on pH; it is
also affected
resulting in aby factors
higher such as thestrength
compressive formation of hydration
compared products, reaction activity, and
to G10F0.
the densification of the internal structure. Although the pH of G10F0 is higher, the addi-
3.5. XRD
tion of FAand in SEM-EDS
G8F2 improves Analysis the toughness and internal structure of the material, ulti-
mately resulting
Scanning in a higher
electron compressive
microscopy (SEM), strength comparedspectroscopy
energy-dispersive to G10F0. (EDS), and X-ray
diffraction (XRD) analyses were conducted on samples with GGBS-FA ratios of 0:10, 4:6, 8:2,
and XRD
3.5. 10:0 after a curing period
and SEM-EDS Analysis of 28 days. These analyses aim to reveal the microstructure,
elemental distribution, and crystalline phase composition of GGBS-FA composites activated
Scanning
by CCR, electron
providing microscopy
a deeper (SEM), energy-dispersive
understanding spectroscopy (EDS), and X-
of their reaction mechanisms.
ray diffraction
As shown (XRD) in Figure analyses
11a, thewereSEMconducted
image of the on G0F10
samples with reveals
sample GGBS-FA ratios of loose
a relatively 0:10,
4:6, 8:2, and 10:0 after a curing period of 28 days. These analyses
particle arrangement with poor structural integrity, a limited amount of gel products, and a aim to reveal the micro-
structure,
significantelemental
presence distribution,
of unreacted and crystalline
FA particles. Inphase composition
conjunction with theof GGBS-FA
UCS strength compo-test
sites activated
results from Sectionby CCR, 3.1,providing
this indirectlya deeper understanding
confirms of theiramount
that an excessive reactionofmechanisms.
FA adversely
Asthe
affects shown
UCSin ofFigure 11a, the SEM
the composite image of the
cementitious G0F10[46,47].
material sampleThisreveals a relatively
deficiency loose
primarily
arises from two factors: first, the inherently low reactivity
particle arrangement with poor structural integrity, a limited amount of gel products, of FA, and, second, the lack
and
of sufficient calcium sources in the system, which leads to a slow hydration reaction
a significant presence of unreacted FA particles. In conjunction with the UCS strength test
and hinders the formation of C-(A)-S-H and other flocculent or clustered gel products,
results from Section 3.1, this indirectly confirms that an excessive amount of FA adversely
ultimately resulting in insufficient strength development in the sample. Additionally, an
affects the UCS of the
energy-dispersive composite (EDS)
spectroscopy cementitious
mapping material
analysis [46,47]. This deficiency
was performed primarily
on region A in
arises from two factors: first, the inherently low reactivity
Figure 11a, with the elemental composition shown in Figure 11b. The data indicate of FA, and, second, the lack of
that
sufficient
the calcium calcium
content sources
in the in the system,
system whichlow,
is relatively leadsat to a slow
only 1.96%,hydration
while the reaction
aluminum and
hinders
and silicon thecontents
formation areof C-(A)-S-H
13.74% and other
and 14.96%, flocculentMoreover,
respectively. or clustered gelamounts
small products, of ulti-
iron
mately
(0.46%) resulting
and titanium in insufficient
(0.38%) arestrength development in the sample. Additionally, an en-
also detected.
ergy-dispersive spectroscopy (EDS) mapping analysis was performed on region A in Fig-
ure 11a, with the elemental composition shown in Figure 11b. The data indicate that the
Sustainability 2025, 17, x FOR PEER REVIEW 13 of 21

calcium content in the system is relatively low, at only 1.96%, while the aluminum and
Sustainability 2024, 16, 11168
silicon contents are 13.74% and 14.96%, respectively. Moreover, small amounts of12iron
of 19

(0.46%) and titanium (0.38%) are also detected.

Figure 11. Microscopic test results of G0F10: (a) morphology, (b) energy spectrum of region A; and
Figure 11. Microscopic test results of G0F10: (a) morphology, (b) energy spectrum of region A; and
(c) XRD.
(c) XRD.
As illustrated in Figure 11c, the XRD analysis of the G0F10 sample identifies six
As illustrated
minerals: in Figure
quartz, mullite, 11c, the
calcite, XRD analysis
calcium aluminate,of the G0F10 sample
hematite, identifies
and rutile. six min-
As illustrated
erals: quartz,
in Figure 11c,mullite,
the XRD calcite, calcium
analysis of thealuminate,
G0F10 sample hematite, and rutile.
identifies As illustrated
six mineral in Fig-
phases: quartz,
ure 11c, the XRD analysis of the G0F10 sample identifies six mineral phases: quartz, mul-
mullite, calcite, calcium aluminate, hematite, and rutile. Quartz, mullite, hematite, and
lite, calcite, calcium aluminate, hematite, and rutile. Quartz, mullite, hematite, and rutile
rutile originate from the mineral phases of FA, with hematite and rutile being non-reactive
in the geopolymerization
originate from ◦the mineral process
phases[27]. Mullite
of FA, withishematite
primarilyand observed at 2θ angles
rutile being of 16.39in◦ ,
non-reactive
35.2 , and 40.8 , while quartz is predominantly detected at 2θ angles of 20.82 and 26.22◦ .
◦ ◦
the geopolymerization process [27]. Mullite is primarily observed at 2θ angles of 16.39°,
Calcite mainly originates from the carbonation reaction of CCR, and its low peak intensity,
35.2°,
as seen andin 40.8°, whilealigns
the figure, quartzwith
is predominantly
the low calcium detected
contentat 2θ angles of
observed in 20.82°
the EDS andanalysis.
26.22°.
Calcite mainly originates from the carbonation reaction of CCR,
A small amount of calcium aluminate is formed following the reaction between calcium and its low peak intensity,
as seen infrom
elements the figure,
CCR and aligns with theelements
aluminum low calcium fromcontent
FA [48].observed in the EDS analysis. A
smallAs amount
illustratedof calcium aluminate
in Figure 12, the is formed following
microstructural the reaction
analysis between
of the G4F6 samplecalcium el-
reveals
ements from CCR and aluminum elements from FA [48].
significant improvements. Figure 11a shows that gel-like substances fill the voids between
As illustrated
particles, resulting in in aFigure
marked 12,increase
the microstructural analysiscompared
in material density of the G4F6 sampleThe
to G0F10. reveals
EDS
data in Figure 12b indicate that with the increased GGBS content,
significant improvements. Figure 11a shows that gel-like substances fill the voids between the calcium concentration
in the G4F6 system reaches 8.62%, representing a substantial increase relative to G0F10.
particles, resulting in a marked increase in material density compared to G0F10. The EDS
In contrast, the concentrations of aluminum and silicon decrease to 8.88% and 9.93%,
data in Figure 12b indicate that with the increased GGBS content, the calcium concentra-
respectively. Additionally, 0.17% sodium was detected. The XRD analysis in Figure 12c
tion in thethe
identifies G4F6 systemofreaches
formation C-(A)-S-H 8.62%,
gel, representing
with diffraction a substantial
peaks observed increase
at 2θrelative
angles to
of
G0F10. In contrast,
6.66◦ , 20.80 ◦ , 36.94◦ ,the
andconcentrations of aluminum
49.43◦ . The formation and silicon
of C-(A)-S-H decrease
gel suggests an to 8.88% and
enhancement
9.93%, respectively.
in the system’s Additionally,
hydration reaction,0.17%
which sodium was related
is closely detected.to The XRD analysis in
the incorporation ofFigure
GGBS.
12c
Theidentifies
additional the formation
calcium of C-(A)-S-H
provided by GGBS gel, with diffraction
significantly boostspeaks observed at
the hydration 2θ angles
activity and
of 6.66°, 20.80°,
promotes 36.94°, and
the formation of 49.43°.
hydration The products.
formation Lloyd
of C-(A)-S-H gel suggests
et al. [49], through an enhance-
microscopic
ment in the
analysis, system’s hydration
demonstrated that thereaction,
calcium which
sourceisinclosely related to FA-GGBS
alkali-activated the incorporation
systems of is
critical for the development of early strength.
GGBS. The additional calcium provided by GGBS significantly boosts the hydration
 Sustainability 2025, 17, x FOR PEER REVIEW 14 of 21

Sustainability 2024, 16, 11168 activity and promotes the formation of hydration products. Lloyd et al. [49], through mi-
13 of 19
croscopic analysis, demonstrated that the calcium source in alkali-activated FA-GGBS sys-
tems is critical for the development of early strength.

Figure12.
Figure Microscopic
12.Microscopic test results
test results of (a)
of G4F6: G4F6:
SEM (a) SEM morphology,
morphology, (b) energy
(b) energy spectrum spectrum
of region B, of region B,
and(c)
and (c)XRD.
XRD.

Theanalysis
The analysis of Figure
of Figure 13a,c13a,c
revealsreveals
that thethat
G8F2the G8F2compared
sample, sample, to compared
G0F10 andto G0F10 and
G4F6,
G4F6,exhibits
exhibits distinct
distinctformations of plate-like
formations calciumcalcium
of plate-like hydroxide, needle-shaped
hydroxide, ettring-
needle-shaped ettringite
ite (Ca
(Ca Al
6 26 Al
(SO
2(SO 4))
4 33 (OH)
(OH) 12·26H
12 · 26H
2 O),
2 and
O), C-(A)-S-H
and gel.
C-(A)-S-H The formation
gel. The of these
formation mineral
of phases
these mineral phases
significantly
significantly enhances
enhances the material’s compactness,
the material’s reducing the
compactness, number ofthe
reducing pores and de- of pores and
number
fects. Furthermore,
defects. the gel substances
Furthermore, fill the surface
the gel substances and
fill most
the of the pores
surface in the compo-
and most of the pores in the
site binder, creating a continuous and uniform microstructure.
composite binder, creating a continuous and uniform microstructure. This This is consistent with the is consistent
findings
Sustainability 2025, 17, x FOR PEER REVIEW of Kumar et al. [10], who suggested that this may be attributed
with the findings of Kumar et al. [10], who suggested that this may be attributed to the to the
15 formation
of 21
of gel phases,ofwhich
formation result inwhich
gel phases, a denser microstructure.
result in a denser microstructure.
Ettringite is formed through the reaction of Al2O3, calcium sources, and sulfate ions
(SO42−, hydrated from SO3), derived from the GGBS-FA system, under alkaline conditions,
as described in Equation (3). The diffraction peaks corresponding to ettringite are ob-
served at 2θ angles of 6.66°, 20.80°, 36.94°, and 49.43°. Additionally, the EDS data from
Figure 13b indicate the presence of 1.32% sulfur (S) in the G8F2 system, further confirming
the formation of ettringite. This finding underscores the critical role of sulfur, specifically
in the form of sulfate ions, in the ettringite formation process, highlighting its essential
contribution to the material’s microstructural development.

FigureFigure
13. Microscopic test resultstest
13. Microscopic of G8F2: (a) of
results SEM morphology,
G8F2: (a) SEM(b)morphology,
energy spectrum
(b)ofenergy
region spectrum
C, of region C,
and (c) XRD.
and (c) XRD.

6CaO⋅ Al2 O3 ⋅ 3SO3 +26H 2 O → Ca 6 Al2( SO 4 ) 3( OH )12⋅ 26H 2 O (3)

As shown in Figure 14a, compared to G0F10, G4F6, and G8F2, the pores in the G10F0
sample are largely filled with gel-like substances, significantly enhancing the density of
the material, with almost no visible pores or defects on the sample surface. Relevant stud-
Sustainability 2024, 16, 11168 14 of 19

Ettringite is formed through the reaction of Al2 O3 , calcium sources, and sulfate ions
(SO4 2− , hydrated from SO3 ), derived from the GGBS-FA system, under alkaline conditions,
as described in Equation (3). The diffraction peaks corresponding to ettringite are observed
at 2θ angles of 6.66◦ , 20.80◦ , 36.94◦ , and 49.43◦ . Additionally, the EDS data from Figure 13b
indicate the presence of 1.32% sulfur (S) in the G8F2 system, further confirming the forma-
tion of ettringite. This finding underscores the critical role of sulfur, specifically in the form
of sulfate ions, in the ettringite formation process, highlighting its essential contribution to
the material’s microstructural development.

6CaO·Al2 O3 ·3SO3 +26H2 O → Ca6 Al2 (SO4 )3(OH)12 ·26H2 O (3)

As shown in Figure 14a, compared to G0F10, G4F6, and G8F2, the pores in the G10F0
sample are largely filled with gel-like substances, significantly enhancing the density of
the material, with almost no visible pores or defects on the sample surface. Relevant
studies [50,51] have shown that an increased calcium source enhances the solubility of
Sustainability 2025, 17, x FOR PEER REVIEW 16 of 21
silicon and aluminum, leading to the formation of C-(A)-S-H products and contributing to
a denser and more compact microstructure.

Figure 14. Microscopic test results of G10F0: (a) SEM morphology, (b) energy spectrum of region D,
Figure 14. Microscopic test results of G10F0: (a) SEM morphology, (b) energy spectrum of region D,
and (c) XRD.
and (c) XRD.
It can be seen from Figure 14b that the Si element in the material is significantly
3.6. The Mechanism
reduced, of Action of
which is reflected in CCR-FA-GGBS
the XRD patternBinder
by a notable decrease in the quartz peak,
as shown in Figure 14b. Calcite and calcium hydroxide
Based on the microstructural analysis and experimental become the main
results, components
the internal of
interac-
the material, accompanied by a small amount of C-(A)-S-H gel and ettringite formation.
tions within the composite binder system can be divided into three stages, as illustrated
Furthermore, with the increase in GGBS content, the concentrations of sulfur (S) and sodium
in the chemical reaction schematic (Figure 15).
(Na) also increase, reaching 1.77% and 0.80%, respectively. This suggests that the S and Na
(1) Alkaline
elements activation
primarily stage.from
originate The GGBS.
primary component of
Additionally, theCCR, calcium
presence of aoxide
small (CaO),
amountun-
of
Al2 Odergoes
3 in GGBS hydration
provides to
theproduce
necessarycalcium hydroxide
conditions (Ca(OH)2),ofwhich
for the formation significantly
C-(A)-S-H gel.
raises the pH EDS
Additionally, of thedata
system. This G8F2,
for G4F6, high alkalinity
and G10F0 is samples
critical for activating
indicate GGBS and
the presence of
traceFA, facilitating the dissolution of their components and creating favorable conditions
amounts of sodium (Na). However, no significant Na-(A)-S-H gel is observed in the
for the formation of calcium silicate hydrate (C-S-H) and calcium aluminum silicate
hydrate (C-(A)-S-H) gels. These hydration products fill voids and increase density,
thereby improving the overall strength and stability of the composite material (Fig-
ure 14). Previous studies have demonstrated that such alkaline conditions are essen-
tial for optimizing the reactivity of industrial by-products, such as slag and FA, in
Sustainability 2024, 16, 11168 15 of 19

Sustainability 2025, 17, x FOR PEER REVIEW 17 of 21

mineral phases of these samples. This phenomenon may be attributed to the following
reason: in alkaline-activated systems, when sodium-based activators (such as NaOH or
Na2 SiO3 ) are used to activate low-calcium materials, Na+ ions replace Ca2+ ions as the
replaced by aluminum (Al), resulting in the formation of more stable and complex
primary cation and combine with dissolved Si4+ and Al3+ to form a three-dimensional
C-(A)-S-H gels. This process fills internal voids within the material, increasing its
network structure, resulting in the generation of Na-(A)-S-H gel. However, in the material
density and strength. Additionally, the carbonation of residual Ca(OH)2 further en-
system of this study, primarily high-calcium materials (such as CCR and GGBS) are utilized,
hances the material’s density and stability through reactions (e.g., Equation (10)),
with GGBS containing only trace amounts of Na+ . Furthermore, the high calcium content
leading to the formation of calcite (CaCO3). This secondary consolidation process sig-
in the system hinders the dominance of Na+ in the gel formation process. Consequently,
nificantly improves the material’s long-term durability and resistance to chemical
the predominant gel phase generated is C-(A)-S-H rather than Na-(A)-S-H.
degradation, as reported in previous studies [54,55].
3.6. The MechanismThese stagesofcollectively
of Action CCR-FA-GGBS explain the mechanisms behind the strength and durability
Binder
Based onofthe
the composite binder. The high alkalinity provided by CCR, combined with the syner-
microstructural analysis and experimental results, the internal interac-
gistic activation of GGBS and FA, leads to the formation of stable gel phases that contrib-
tions within the composite binder system can be divided into three stages, as illustrated in
ute to theschematic
the chemical reaction enhanced mechanical performance of the material.
(Figure 15).

Figurereaction
Figure 15. Chemical 15. Chemical reaction
process process
of CCR of CCR to GGBS-FA
to stimulate stimulate GGBS-FA cementitious
cementitious material.
material.

(1) Alkaline activation stage. The primary component of CCR, calcium oxide (CaO),
CaO+H 2 O → Ca(OH) 2 → Ca 2+ +OH - (4)
undergoes hydration to produce calcium hydroxide (Ca(OH)2 ), which significantly
Ca 2 Al2SiO high- +H
This+OH 2+ - 2-
2 O → 2Ca +2[Al(OH) 4 ] +(SiO 3 )
raises the pH of the system. alkalinity is critical for activating GGBS and (5)
7
FA, facilitating the dissolution of their components and creating favorable condi-
- 2+
tions for the formation Ca MgSi 2 Osilicate
of 2calcium and2+calcium aluminum
7 +OH + H 2 O → 2Ca +Mg +2(SiO 3 )
hydrate (C-S-H) 2-
(6)
silicate hydrate (C-(A)-S-H) gels. These hydration products fill voids and increase
- 2-
density, thereby improving the overallSiOstrength
2 +2OH and → (SiO 3 ) +H
stability O composite material
of 2the (7)
(Figure 14). Previous studies have demonstrated -
that such alkaline -
conditions are
Al2 O3of
essential for optimizing the reactivity +2OH +3H 2 O
industrial → 2[Al(OH)such
by-products, 4] as slag and FA, (8)
in cement-based systems [52].
Sustainability 2024, 16, 11168 16 of 19

(2) Hydrolysis and dissolution stage. At this stage, the high concentration of hydroxide
ions (OH− ) disrupts the Si-O-Si and Al-O-Al bonds in slag and FA. This disruption
promotes the dissolution of silicate and aluminate phases, releasing calcium (Ca2+ ),
magnesium (Mg2+ ), silicate (SiO3 2− ), and aluminate ([Al(OH)]4 − ) ions, as shown
in the equations provided in the manuscript. These dissolved ions are critical for
the subsequent formation of hydration products. This process is consistent with
findings reported by An et al. [53], who observed similar dissolution phenomena in
alkali-activated slag systems, which enhanced gel formation
(3) Gel reorganization stage. Calcium ions (Ca2+ ) react with active silicate (SiO3 2− ) and
aluminate ([Al(OH)]4 − ) ions to form amorphous CSH and C-(A)-S-H gels. Initially,
these gels exhibit a loose structure but provide strong adhesion and stability to the
material. As hydration progresses, silicon (Si) sites in the CSH structure are gradually
replaced by aluminum (Al), resulting in the formation of more stable and complex C-
(A)-S-H gels. This process fills internal voids within the material, increasing its density
and strength. Additionally, the carbonation of residual Ca(OH)2 further enhances
the material’s density and stability through reactions (e.g., Equation (10)), leading to
the formation of calcite (CaCO3 ). This secondary consolidation process significantly
improves the material’s long-term durability and resistance to chemical degradation,
as reported in previous studies [54,55].
These stages collectively explain the mechanisms behind the strength and durability of
the composite binder. The high alkalinity provided by CCR, combined with the synergistic
activation of GGBS and FA, leads to the formation of stable gel phases that contribute to
the enhanced mechanical performance of the material.

CaO + H2 O → Ca(OH)2 → Ca2+ +OH− (4)

Ca2 Al2 SiO7 +OH− +H2 O → 2Ca2+ +2[Al(OH) 4 ]− +(SiO 3 )2− (5)

Ca2 MgSi2 O7 +OH− + H2 O → 2Ca2+ +Mg2+ +2(SiO 3 )2− (6)

SiO2 +2OH− → (SiO 3 )2− +H2 O (7)

Al2 O3 +2OH− +3H2 O → 2[Al(OH) 4 ]− (8)

Ca2+ +(SiO 3 )2− +[Al(OH) 4 ]− +H2 O → C-(A)-S-H (9)

Ca(OH)2 +CO2 → CaCO3 +H2 O (10)

4. Conclusions
This study utilized CCR as an alkali activator to develop a low-carbon binder based
on GGBS and FA through systematic experiments and optimization methods. The research
aims to explore the potential of resource utilization for industrial by-products and provide
an alternative to traditional cement. The main conclusions are as follows:
(1) Through systematic experiments and optimization analysis, this study determined that
the optimal mass ratio of GGBS to FA was 8:2, with the optimal CCR dosage being 8%.
These parameters provide reliable design guidelines for improving material performance.
(2) Under the conditions of 8% CCR dosage and a GGBS/FA ratio of 8:2, the high GGBS
content in the alkaline environment produced abundant calcium aluminum silicate
hydrate (C-(A)-S-H) gel, which significantly enhanced the structural stability and
durability of the material. SEM-EDS and XRD analyses revealed that these hydra-
Sustainability 2024, 16, 11168 17 of 19

tion products effectively filled the micropores, reduced porosity, and significantly
improved compressive strength.
(3) The SO3 content in GGBS provided an ample source of sulfate ions for ettringite
formation. Through reactions with the calcium source from CCR and the aluminum
source from FA, the resulting ettringite not only improved the early strength of the
material but also further optimized the microstructure.
(4) This study innovatively utilized CCR as an alkali activator, achieving efficient uti-
lization of industrial by-products. Compared to traditional chemical activators, CCR
offers a new pathway for developing low-carbon, efficient, and cost-effective materials,
demonstrating broad application prospects for sustainable construction materials.

Author Contributions: Investigation, methodology, data curation, writing—original draft, formal

analysis, and visualization: X.L.; conceptualization, validation, resources, writing—review and
editing, and funding acquisition: G.X.; investigation, data curation, and writing—review and editing:
D.Y.; investigation, supervision, and writing—review and editing: L.D.; investigation and writing—
review and editing: A.T. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the National Natural Science Foundation of China (grant
number: 52169022).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data supporting the findings of this study are available upon request.
Conflicts of Interest: The authors declare that they have no relevant conflicts of interest that could
influence the outcomes of this study.

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