Effect of Active MgO on Compensated Drying Shrinkage and Mechanical Properties of Alkali-Activated Fly Ash–Slag Materials

Abstract

The influences of MgO activity and its content on the mechanical properties, drying shrinkage compensation, pore structure, and microstructure of alkali-activated fly ash–slag materials were investigated. Active MgO effectively compensated for the alkali-activated materials’ (AAMs’) drying shrinkage. The drying shrinkage increased rapidly with the increase in curing age and stabilized after 28 d. Within a certain range, the material’s drying shrinkage was inversely proportional to the content of active MgO. The higher the activity of MgO, the lower the drying shrinkage of the AAMs under the same MgO content. The drying shrinkage values of the test groups with 9% R-MgO, M-MgO, and S-MgO at 90 d were 2444 με, 2306 με, and 2156 με, respectively. In the early stage of hydration, the addition of S-MgO reduced the compressive strength. As the content of M-MgO increased, the compressive strength first increased and then decreased, reaching a maximum of 72.28 MPa at an M-MgO content of 9%. The experimental group with 9% M-MgO exhibited higher compressive and flexural strengths than those with 9% S-MgO and R-MgO, demonstrating better mechanical properties. The results of this study provide an important theoretical basis and data support for the optimal application of MgO in AAMs. MgO expansion agents have great application potential in low-carbon buildings and durable materials. Further research on their adaptability in complex environments will promote their development for engineering and provide innovative support for green buildings.

Full text

Buildings 2025, 15, 256 https://doi.org/10.3390/buildings15020256
Article
Effect of Active MgO on Compensated Drying Shrinkage
and Mechanical Properties of Alkali-Activated Fly
Ash–Slag Materials
Hongqiang Ma 1,2,3,*, Shiru Li 1, Zelong Lei 1, Jialong Wu 1, Xinhua Yuan 1 and Xiaoyan Niu 1,2,3,*
1 College of Civil Engineering and Architecture, Hebei University, Baoding 071002, China;
lishiru@stumail.hbu.edu.cn (S.L.); leizelong@stumail.hbu.edu.cn (Z.L.);
wujialong@stumail.hbu.edu.cn (J.W.); 18733840611@163.com (X.Y.)
2 Engineering Research Center of Zero-Carbon Energy Buildings and Measurement Techniques,
Ministry of Education, Hebei University, Baoding 071002, China
3 Technology Innovation Center for Testing and Evaluation in Civil Engineering of Hebei Province,
Hebei University, Baoding 071002, China
* Correspondence: mahongqiang@hbu.edu.cn (H.M.); niuxiaoyan@hbu.edu.cn (X.N.)
Abstract: The influences of MgO activity and its content on the mechanical properties,
drying shrinkage compensation, pore structure, and microstructure of alkali-activated fly
ash–slag materials were investigated. Active MgO effectively compensated for the alkali-
activated materials’ (AAMs’) drying shrinkage. The drying shrinkage increased rapidly
with the increase in curing age and stabilized after 28 d. Within a certain range, the mate-
rial’s drying shrinkage was inversely proportional to the content of active MgO. The
higher the activity of MgO, the lower the drying shrinkage of the AAMs under the same
MgO content. The drying shrinkage values of the test groups with 9% R-MgO, M-MgO,
and S-MgO at 90 d were 2444 µε, 2306 µε, and 2156 µε, respectively. In the early stage of
hydration, the addition of S-MgO reduced the compressive strength. As the content of M-
MgO increased, the compressive strength first increased and then decreased, reaching a
maximum of 72.28 MPa at an M-MgO content of 9%. The experimental group with 9% M-
MgO exhibited higher compressive and flexural strengths than those with 9% S-MgO and
R-MgO, demonstrating beer mechanical properties. The results of this study provide an
important theoretical basis and data support for the optimal application of MgO in AAMs.
MgO expansion agents have great application potential in low-carbon buildings and du-
rable materials. Further research on their adaptability in complex environments will pro-
mote their development for engineering and provide innovative support for green build-
ings.
Keywords: alkali-activated materials; MgO activity; drying shrinkage; mechanical
properties; microstructure
1. Introduction
Portland cement is currently the most widely used cementitious material in the
world [1]. The production of cement consumes a significant amount of non-renewable
energy and emits large quantities of greenhouse gases such as CO2 and NOx [2], making
it one of the major contributors to environmental degradation. In the context of green,
low-carbon, and low-emission initiatives, there is a pressing need to develop new eco-
Academic Editor:
Giuseppina Uva
Received: 21 December 2024
Revised: 11 January 2025
Accepted: 13 January 2025
Published:
16 January 2025
Citation:
Ma, H.; Li, S.; Lei, Z.; Wu,
J.; Yuan, X.; Niu, X. Effect of Active
MgO on Compensated Drying
Shrinkage and Mechanical
Properties of Alkali
-Activated Fly
Ash
–Slag Materials. Buildings 2025,
15
, 256. hps://doi.org/10.3390/
buildings15020256
Copyright:
© 2025 by the authors.
Submied for possible open access
publication under the terms and
conditions of the Creative Commons
Aribution (CC BY) license
(hps://creativecommons.org/li-
censes/by/4.0/).

Buildings 2025, 15, 256 2 of 14
friendly cementitious materials. Alkali-activated materials (AAMs), synthesized through
the hydration of fly ash and slag catalyzed by alkali activators, are recognized as im-
portant components of sustainable construction materials [3,4]. Compared to Portland ce-
ment, AAMs offer advantages such as simpler production processes [5], lower energy
consumption [6], reduced CO2 emissions [7], and higher utilization rates of industrial solid
wastes. They also exhibit outstanding characteristics, including high strength [8,9], low
hydration heat [10], corrosion resistance [11], and impermeability [12], making them im-
portant for the development of modern construction materials [13–17].
However, one of the major challenges limiting the practical application of AAMs in
engineering is their propensity for significant shrinkage and cracking, which occur during
the hardening process due to water loss through evaporation [18]. A variety of strategies
have been developed with the objective of reducing or compensating for the shrinkage of
AAMs. These include the use of shrinkage-reducing admixtures (SRAs) [19], internal cur-
ing agents [20], and expansive agents. The addition of expansive agents, particularly
MgO, to AAMs has emerged as an effective technical approach to address this issue owing
to MgO’s excellent expansive properties. Cao et al. [21] analyzed the microstructure of
cement-hardened paste in which varying amounts of MgO expansive agent was incorpo-
rated using scanning electron microscopy (SEM) and mercury intrusion porosimetry
(MIP). It was found that the MgO-expander-induced swelling originated mainly from the
growth in brucite crystals and that the crystals were generated mainly along the subpore
walls of the confined regions, while relatively lile growth occurred on the surface of the
MgO particles. The formation of brucite crystals is controlled by the degree of supersatu-
ration of the Mg2⁺ concentration in the pore solution. Ma et al. [22] investigated the effects
of the slag content as well as alkali activator modulus and content on the coagulation be-
havior, mechanical properties, and drying shrinkage characteristics of AACGS materials.
Furthermore, the intrinsic mechanism of drying shrinkage was analyzed in depth using
MIP and SEM-EDS techniques. It was found that the drying shrinkage of AACGS materi-
als was significantly higher compared with that of Portland cement materials. Wang et al.
[23] found that MgO activity had lile effect on the crystallization of C-S-H gel, whereas
increasing the MgO activity contributed to the degree of hydration of MgO and AAMs.
Crystallinity and hydration analyses revealed that the use of high-activity MgO achieved
higher hydration degrees for both MgO and the AAMs. Gu et al. [24] added MgO to ce-
ment-based steel tube concrete and found that higher MgO activity resulted in a smaller
final restrained expansion ratio and an earlier onset of expansion. An appropriate MgO
content, ranging from 9% to 12%, could refine the pore structure and provide good expan-
sive behavior and volume stability. Cui et al. [25] also observed an earlier onset of expan-
sion with higher MgO activity when MgO was added to ultra-high-performance concrete
(UHPC) to compensate for shrinkage. Mo et al. [26] investigated the influence of curing
temperature on the hydration process and expansion characteristics of expansive MgO
agents with varying reactivities in cement paste. The results showed that at 20 °C, high-
activity MgO exhibited beer expansion within 28 d than low-activity MgO, but the ex-
pansion decreased later on. At temperatures ranging from 20 °C to 80 °C or in the later
stages, low-activity MgO demonstrated beer expansion performance. Sherir et al. [27]
explored self-healing cement-based materials by adding MgO expanders to cement-based
materials to make ECC-MgO. The results showed that the incorporation of small amounts
of cyanless MgO and fly ash produced self-healing ECC-MgO with stable durability. Sam-
ples with higher degree of pre-cracking under autoclave conditions had higher strength,
confirming the potential of the system to self-heal. It was found that increasing the content
of the slag material and the modulus of alkali activator induced stronger drying shrinkage
in AAMs [28]. The collapse and rearrangement of the gels produced by polymerization

Buildings 2025, 15, 256 3 of 14
reactions result in volume reduction, refinement of the internal pore structure, and in-
creased capillary tensile stress. A material cannot fully rebound from drying shrinkage.
In summary, research on the effect of active MgO on the hydration reaction mecha-
nisms of AAMs is still limited. Alkali-activated precursor materials are rich in MgO. In
order to apply the concepts of green development and low-carbon energy conservation,
different amounts of active MgO were added to solid waste: alkali-activated fly ash–slag
materials, and NaOH, which is inexpensive, was selected as the alkali activator. The ef-
fects of MgO on the compensation for drying shrinkage, mechanical properties, pore
structure, and microstructure of alkali-activated fly ash–slag materials were studied. The
hydration reaction process of AAMs and MgO is shown in Figure 1. It is of great signifi-
cance to improve the expansion effect of MgO to improve the stability and durability of
AAMs and promote the application of low-carbon and environmentally friendly AAMs
in engineering.
Figure 1. Reaction diagram of AAMs with MgO.
2. Materials and Methods
2.1. Materials
This experiment employed S95-grade granulated blast furnace slag, Class F fly ash,
and Chinese ISO standard sand as raw materials. The active MgO was sourced from Wu-
han Sanyuan Special Building Materials Co., Ltd., Wuhan, China. By controlling the cal-
cination conditions, three different active MgO samples were obtained: rapid-reacting
MgO (R-MgO, reaction time < 100 s), moderately reacting MgO (M-MgO, reaction time
100–200 s), and slowly reacting MgO (S-MgO, reaction time > 200 s). The chemical com-
positions of the fly ash, slag, and active MgO were determined through X-ray fluorescence
analysis and are presented in Table 1. Their XRD paerns are presented in Figure 2.
Table 1. The main chemical composition of raw materials (wt%).
Material
CaO
MgO
Al
2
O
3
SiO
2
Fe
2
O
3
SO
3
LOI
Slag
35.58
7.16
16.32
36.10
0.23
1.71
2.9
Fly ash
2.66
0.24
32.79
55.71
4.43
—
0.97
R-MgO
1.31
92.65
0.7
1
1
0.2
1.6
M-MgO
1.3
90.92
0.5
2.2
0.8
0.1
2.2
S-MgO
2.6
90.04
0.5
2.7
1
0.1
1.9

Buildings 2025, 15, 256 4 of 14
Figure 2. XRD paerns of raw materials.
2.2. Preparation Process
We dissolved sodium hydroxide (NaOH) solid in deionized water to prepare a
NaOH solution. After stirring well, we let it stand for 24 h to allow the solution tempera-
ture to drop to room temperature and to compensate for any water loss due to evapora-
tion. We added sodium silicate, which we ultrasonically stirred for 1 h to prepare the alkali
activator. We placed fly ash, slag, standard sand, and active MgO materials in a mixer and
stirred them at low speed for 30 min. Then, we added the alkali activator and stirred rap-
idly for 5 min to form a uniform paste. We quickly poured the freshly mixed paste into
cube molds of 50 mm × 50 mm × 50 mm, rectangular molds of 40 mm × 40 mm × 160 mm,
and special molds for testing drying shrinkage of 40 mm × 40 mm × 160 mm. After vibrat-
ing to ensure compaction, we sealed the molds with polyethylene film to prevent water
evaporation [29]. The samples were cured under standard curing conditions (temperature
20 ± 2 °C, humidity 95 ± 1%) for 24 h before demolding and then sealed with polyethylene
film and continuously cured under standard curing conditions until the experiment was
finished. The active MgO content was designed to be 0%, 6%, 9%, 12%, and 15% (as a
percentage of the total mass of fly ash, slag and active MgO). The Na2O concentration was
4%, the alkali activator modulus was 1.0, the liquid-to-solid ratio was 0.36, and the sand-
to-binder ratio was 2. A total of 13 experimental groups were set up, with specific mix
proportions shown in Table 2. The experimental flowchart is shown in Figure 3.
Table 2. Mix proportion of alkali-activated fly ash–slag materials.
Sample No.
Fly Ash
Slag
MgO Type
MgO Content
AAFS
50%
50%
—
0
AAFS-S6
47%
47%
S
6%
AAFS-S9
45.5%
45.5%
S
9%
AAFS-S12
44%
44%
S
12%
AAFS-S15
42.5%
42.5%
S
15%
AAFS-M6
47%
47%
M
6%
0
2000
4000
6000
8000
10,000
12,000
S-MgO ▲
▲
▲
▲
Periclase(MgO)
0
2000
4000
6000
8000
M-Mgo ▲
▲
▲
▲
Periclase(MgO)
0
2000
4000
6000
Intensity/cps
R-MgO ▲
▲
▲
▲
Periclase(MgO)
0
50
100
150
200
250
Slag
★◆
◆
★
◆
★
★Ca2AlSiO7
◆Ca2MgSi2O7
10 20 30 40 50 60 70 80
0
400
800
1200
1600
2θ
●
Fly ash
■●
■●SiO2
■Al2O3

Buildings 2025, 15, 256 5 of 14
AAFS-M9
45.5%
45.5%
M
9%
AAFS-M12
44%
44%
M
12%
AAFS-M15
42.5%
42.5%
M
15%
AAFS-R6
47%
47%
R
6%
AAFS-R9
45.5%
45.5%
R
9%
AAFS-R12
44%
44%
R
12%
AAFS-R15
42.5%
42.5%
R
15%
Figure 3. Flow chart of experiment.
2.3. Testing Methods
2.3.1. Compressive Strength Tests
The tests of compressive strength and flexural strength were conducted at 3 d, 28 d
and 90 d of the specimen’s curing. The tests were carried out using a WDW3300 100 KN
universal testing machine with a loading speed of 0.5 mm/min. Three samples were tested
in each group, and their average value was taken as the value of compressive strength, as
shown in Figure 4a.
(a)
(b)
Figure 4. AAMs tests. (a) Compressive strength. (b) drying shrinkage.

Buildings 2025, 15, 256 6 of 14
2.3.2. Drying Shrinkage Tests
After removing the mold, rectangular 40 mm × 40 mm × 160 mm samples were ob-
tained. The straight line distance between the two ends was measured as the initial length
(L0) using a dial comparator (accurate to 0.001 mm). The testing operation is shown in
Figure 4b. The samples were transferred to a drying shrinkage humidity control box for
cement materials for curing (temperature 20 ± 2 °C and relative humidity 60 ± 5 °C), and
the length values were measured for curing ages of 2 d, 3 d, 7 d, 14 d, 21 d, 28 d, 42 d, 60
d and 90 d, recorded as Lt. The dying shrinkage rate of the alkali-activated materials was
calculated based on the L0 and Lt, with Equation (1):
εs = (L0 − Lt)/160 × 100% (1
)
2.3.3. Microscopic Tests
Samples of the alkali-activated material were crushed and cored, and, after terminat-
ing hydration with isopropanol and low-temperature vacuum drying, they were sub-
jected to MIP and SEM tests. The MIP test was conducted using an Autopore II 9220 mer-
cury injection apparatus, with a test pore size range of 3 nm to 275 µm and a maximum
mercury injection pressure of 430 MPa. The pore size distribution of the samples was con-
tinuously tested under pressure. The scanning electron microscope test (SEM test) was
performed using a JSM-7800F field-emission scanning electron microscope, with magnifi-
cations of 2000× and 20,000×. By mapping and scanning micro areas of the samples, the
amounts of elements Ca, Si, Al, and Mg were obtained. Before testing, the samples were
coated with a layer of gold.
3. Results and Analysis
3.1. Mechanical Properties’ Analysis
The graph in Figure 5 illustrates the compressive strength of AAMS cured for 3 d, 28
d, and 90 d with reactive MgO. During the initial curing age, the incorporation of M-MgO
and R-MgO did not significantly affect the compressive strength. However, the addition
of R-MgO reduced the compressive strength, which decreased continuously with increas-
ing content. Specifically, samples containing 6%, 9%, 12%, and 15% S-MgO exhibited de-
creases of 5.3%, 11.1%, 22.9%, and 27%, respectively. This is related to the lower reactivity
of S-MgO, which resulted in a slower hydration rate and lower compressive strength in
the early stages. As the curing progressed, with an increase in the content of reactive MgO,
the 28-day samples all showed a trend of increasing compressive strength, followed by a
decrease, with the highest values observed in the samples with a 9% content. This may
have been due to MgO providing a large amount of Mg2+ for the formation of hydrotalcite.
The formation of an appropriate amount of hydrotalcite enhances the compactness of the
matrix, thereby increasing the compressive strength of the paste, while excessive hy-
drotalcite formation may reduce it [29]. The differences in the compressive strength of the
90-day samples were relatively small, which may have been related to the hydration de-
gree of periclase [30]. The compressive strength of the 9% M-MgO samples at 3 d, 28 d,
and 90 d were 33.7 MPa, 64.5 MPa, and 72.3 MPa, respectively, all higher than those of the
S-MgO and R-MgO samples at the same curing age [28].

Buildings 2025, 15, 256 7 of 14
(a)
(b)
(c)
(d)
Figure 5. Compressive strength of MgO-activated AAMs containing: (a) S-MgO, (b) M-MgO, (c) R-
MgO and (d) 9% active MgO.
Figure 6 displays the flexural strength of the AAMs cured for 3 d, 28 d, and 90 d with
active MgO. During the initial curing age, with an increase in the content of active MgO,
the flexural strength of all samples showed a gradual decline, consistent with previous
reports [31]. Specifically, samples with 15% contents of S-MgO, M-MgO, and R-MgO ex-
hibited decreases of 29.9%, 27.1% and 24.4%, respectively. However, at a content of 9%,
all reactive MgO samples showed a sudden increase in flexural strength. As the curing
age increased, the flexural strengths of S-MgO, M-MgO, and R-MgO samples were all
lower than that of the control group, but the samples with 9% reactive MgO still had a
significant advantage. The flexural strengths of the 9% M-MgO samples at 3 d, 28 d, and
90 d were 6.6 MPa, 10.2 MPa, and 11.8 MPa, respectively, all higher than those of the S-
MgO and R-MgO samples. The 9% M-MgO AAMs exhibited superior mechanical prop-
erties.
AAFS AAFS-S6 AAFS-S9 AAFS-S12 AAFS-S15
0
10
20
30
40
50
60
70
80
Compressive Strength/MPa
3d
28d
90d
AAFS AAFS-M6 AAFS-M9 AAFS-M12AAFS-M15
0
10
20
30
40
50
60
70
80
90
Compressive Strength/MPa
3d
28d
90d
AAFS AAFS-R6 AAFS-R9 AAFS-R12 AAFS-R15
0
10
20
30
40
50
60
70
80
Compressive Strength/MPa
3d
28d
90d
AAFS-S9 AAFS-M9 AAFS-R9
0
10
20
30
40
50
60
70
80
90
Compressive Strength/MPa
3d
28d
90d

Buildings 2025, 15, 256 8 of 14
(a)
(b)
(c)
(d)
Figure 6. Flexural strength of MgO-activated AAMs containing: (a) S-MgO, (b) M-MgO, (c) R-MgO
and (d) 9% active MgO.
3.2. Analysis of the Change in Active-Mg-Compensated Drying Shrinkage
The addition of an alkali activator and the inherent characteristics of AAMs contrib-
ute to significant drying shrinkage [32,33]. MgO has been extensively studied in cement-
based materials due to its excellent expansive properties [30,31,34–37]. Previous studies
[18,38–40] have shown that the drying shrinkage of cement is approximately 800–1000 µε,
while that of AAMs is around 2000–6000 µε. This is because the water in AAMs does not
directly participate in the formation of aluminosilicate gels and exists as free water, which
is prone to evaporation and loss [41]. Figure 7 presents the drying shrinkage of AAMs
with reactive MgO at various curing ages. As the curing age increased, the drying shrink-
age of each sample rapidly increased, and, after 28 days, it tended to stabilize and then
slightly decreased, remaining relatively unchanged, which is consistent with previous re-
ports [22]. Furthermore, with an increase in the content of reactive MgO, the degree of
shrinkage of the samples continuously decreased, indicating that MgO is an excellent ex-
pansive agent. This is because the hydration product of MgO is brucite (Mg(OH)2), a very
simple and stable crystal, whose solid-phase volume increases by 126%, effectively reduc-
ing the internal hydration heat and shrinkage of AAMs [21]. At a content of 15% and a
curing age of 28 days, the shrinkage rates of the S-MgO, M-MgO, and R-MgO samples
decreased by 15.8%, 19.1%, and 32.2%, respectively, with the R-MgO sample exhibiting
excellent resistance to shrinkage.
AAFS AAFS-S6 AAFS-S9 AAFS-S12 AAFS-S15
0
2
4
6
8
10
12
14
Flexural Strength/MPa
3d
28d
90d
AAFS AAFS-M6 AAFS-M9 AAFS-M12AAFS-M15
0
2
4
6
8
10
12
14
Flexural Strength/MPa
3d
28d
90d
AAFS AAFS-R6 AAFS-R9 AAFS-R12 AAFS-R15
0
2
4
6
8
10
12
14
Flexural Strength/MPa
3d
28d
90d
AAFS-R9 AAFS-M9 AAFS-S9
0
2
4
6
8
10
12
14
Flexural Strength/MPa
3d
28d
90d

Buildings 2025, 15, 256 9 of 14
(a)
(b)
(c)
(d)
Figure 7. Drying shrinkage of MgO-activated AAMs containing: (a) S-MgO, (b) M-MgO, (c) R-MgO
and (d) active MgO.
3.3. MIP Analysis
The pore structure is a crucial structural characteristic of hardened cementitious ma-
terial paste, influencing the mechanical properties and the supplementary shrinkage per-
formance of the binding materials. It also reflects the hydration reaction degree of compo-
site cementitious materials. In this study, mercury intrusion porosimetry (MIP) was uti-
lized to analyze the evolution of the pore structure and pore size distribution in the AAMs
under changing variables with curing age. To identify the changes in pores, the AAM
pores were classified based on size into micropores (<0.01 µm), transition pores (0.01–0.1
µm), mesopores (0.1–1 µm), macropores (1–100 µm), and harmful pores (>100 µm) [42].
Figure 8a illustrates the pore volume of the M-MgO-activated AAMs. As the content
of reactive M-MgO increased, the volume of macropores gradually decreased, while the
volumes of the micropores and transition pores first increased and then decreased. The
total pore volume showed an overall decreasing trend. The incorporation of active M-
MgO reduced the AAMs’ pore volume, made the material more dense, and enhanced the
mechanical properties.
Figure 8c depicts the influence of different types of active MgO on the AAMs’ pore
volume. The cumulative pore volume of the materials in the R-MgO experimental group
with a 9% content was markedly higher compared to that of both the control group and
the S-MgO and M-MgO experimental groups. The impact of the different types of active
MgO on the pore structure of the AAMs varied significantly. R-MgO effectively increased
the AAMs’ pore volume, which explains the mechanism through which MgO mitigates
the drying shrinkage of materials.
0 20 40 60 80 100
0
500
1000
1500
2000
2500
3000
Drying shirnkage/με
Curing age/day
AAFS
AAFS-S6
AAFS-S9
AAFS-S12
AAFS-S15
0 20 40 60 80 100
0
500
1000
1500
2000
2500
3000
Drying shirnkage/με
Curing age/day
AAFS
AAFS-M6
AAFS-M9
AAFS-M12
AAFS-M15
0 20 40 60 80 100
0
500
1000
1500
2000
2500
3000
Drying shirnkage/με
Curing age/day
AAFS
AAFS-R6
AAFS-R9
AAFS-R12
AAFS-R15
0 20 40 60 80 100
500
1000
1500
2000
2500
Drying shirnkage/με
Curing age/day
AAFS-R9
AAFS-M9
AAFS-S9

Buildings 2025, 15, 256 10 of 14
Figure 8e presents the pore distribution of the M-MgO experimental group at differ-
ent ages. It can be observed that the pore volumes of various sizes at 3 d and 28 d were
similar. However, as the curing age increased, significant changes occurred in the AAMs’
pore structure, with the pore volumes at 90 d being much greater than those at 3 d and 28
d, consistent with previous research results [26].
(a)
(b)
(c)
(d)
(e)
(f)
Figure 8. Pore size distribution of MgO-activated AAMs containing: (a) M-MgO and (b) MgO for
different (c) M-MgO and (d) MgO types; (e) M-MgO curing age, (f) MgO curing age.
3.4. SEM Analysis
The microstructure morphology of the AAMs was observed using scanning electron
microscopy (SEM) testing. Figure 9 shows the SEM photos of the active M-MgO AAMs.
The degree of hydration of fly ash and slag, as well as the accumulation of hydration
10−3 10−2 10−1 100101102103
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Cumulative pore volume (mL/g)
Pore size diameter /μm
AAFS
AAFS-M6
AAFS-M9
AAFS-M12
AAFS-M15
Micropores
Transition
pores Mesopores
Macropores
Harmful
pores
10−3 10−2 10−1 100101102103
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Incremental pore volume (mL/g)
Pore size diameter /μm
AAFS
AAFS-M6
AAFS-M9
AAFS-M12
AAFS-M15
Micropores
Transition
pores
Mesopores
Macropores Harmful
pores
10−3 10−2 10−1 100101102103
0.00
0.02
0.04
0.06
0.08
0.10
Cumulative pore volume (mL/g)
Pore size diameter /μm
AAFS-M9-3d
AAFS-M9-28d
AAFS-M9-90d
Micropores
Transition
pores Mesopores
Macropores
Harmful
pores
10−3 10−2 10−1 100101102103
0.000
0.005
0.010
0.015
0.020
0.025
Incremental pore volume (mL/g)
Pore size diameter /μm
AAFS-M9-3d
AAFS-M9-28d
AAFS-M9-90d
Micropores
Transition
pores
Mesopores
Macropores Harmful
pores
10
−3
10
−2
10
−1
10
0
10
1
10
2
10
3
0.00
0.04
0.08
0.12
0.16
Cumulative pore volume (mL/g)
Pore size diameter /μm
AAFS-M9
AAFS-S9
AAFS-R9
Micropores Transition
pores Mesopores Macropores
Harmful
pores
10
−3
10
−2
10
−1
10
0
10
1
10
2
10
3
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
Incremental pore volume (mL/g)
Pore size diameter /μm
AAFS-M9
AAFS-S9
AAFS-R9
Micropores Transition
pores
Mesopores
Macropores
Harmful
pores

Buildings 2025, 15, 256 11 of 14
products, depended on the content of M-MgO. No significant unreacted fly ash micro-
spheres were observed in the images, indicating that hydration products had largely
formed. When the reactive M-MgO content was 6%, fewer flocculent and platelet-like
products were observed, suggesting the presence of a large amount of unhydrated peri-
clase and slag particles. As the M-MgO content increased, more hydration products such
as brucite and hydrotalcite-like phases were formed in the reactive M-MgO AAMs.
(a)
(b)
(c)
(d)
(e)
Figure 9. SEM photos of AAMs with different contents of active M-MgO. (a) AAFS. (b) AAFS-M6.
(c) AAFS-M9. (d) AAFS-M12. (e) AAFS-M15.
Figure 10 shows SEM photos of the AAMs containing different types of active MgO.
Numerous fine rods and flakes of brucite, numerous small spinel-like hydrotalcite-like
products, and flocculent C-(A)-S-H gel were clearly observed. The AAMs containing re-
active R-MgO exhibited multiple microcracks. R-MgO had high reactivity, a fast hydra-
tion rate, and a high degree of hydration, resulting in the production of numerous hydro-
magnesite and hydrotalcite-like phases, and the accumulation of these products tended
to induce the formation of microcracks, thus adversely affecting the development of the
long-term strength (>90 d) and durability of the materials.
(a)
(b)
(c)
Figure 10. SEM photos of AAMs with different types of active MgO. (a) AAFS-S9. (b) AAFS-M9. (c)
AAFS-R9.
To observe the changes in the microstructural morphology of the AAMs after the
incorporation of M-MgO, scanning electron microscopy (SEM) tests were conducted on
the AAFS-M9 experimental group materials with a 9% M-MgO content at different curing
ages. Figure 11 shows the SEM photos of the reactive M-MgO AAMs at curing ages of 3
Brucite
Hydrotalcite
Brucite
Hydrotalcite
Hydrotalcite
Brucite
Brucite
Brucite
Hydrotalcite
Crack
Hydrotalcite
Brucite
Hydrotalcite

Buildings 2025, 15, 256 12 of 14
d, 28 d, and 90 d. With the increase in the standard curing age, significant changes oc-
curred in the surface microstructure of the AAMs, as shown in the SEM images. At 3 d,
the microstructure of the materials surface was relatively loose with fewer hydration
products, and more fly ash, slag, and MgO particles were undergoing hydration. As the
hydration reaction progressed further, at 28 d, the hydration products became more abun-
dant, and the microstructure became denser. After 90 d, it can be seen that the microstruc-
ture exhibited a high degree of crystallinity, indicating a high degree of hydration reac-
tion. The hydration products were more abundant and had accumulated to form a well-
structured overall structure.
(a)
(b)
(c)
Figure 11. SEM photos of M-MgO AAMs at different curing ages: (a) 3 d, (b) 28 d and (c) 90 d.
4. Conclusions
The effects of three types of active MgO on compensating for the drying shrinkage
and mechanical properties of AAMs containing different amounts of MgO were investi-
gated. The findings and prospects are summarized as follows:
(1) The addition of active MgO can enhance the compressive strength of AAMs to some
extent, but it decreases when the content is too high. The flexural strength exhibited
a decreasing trend, except for a noticeable increase at a content of 9%. This study
found that AAMs with 9% M-MgO exhibited superior mechanical properties.
(2) The incorporation of active MgO induces significant expansion behavior in AAMs,
which increases with content. The shrinkage rates of R-M-S MgO samples decreased
to 15.8%, 19.1%, and 32.2%, indicating a positive correlation between the expansion
behavior of AAMs and the activity of MgO.
(3) High-temperature drying, high-altitude terrain, and wind can accelerate moisture
evaporation within AAMs, promoting drying shrinkage and crack formation. Com-
pared to methods such as using steel reinforcement to inhibit drying shrinkage, ex-
pansive MgO agents offer advantages such as lower cost, easier application, and sig-
nificant effectiveness. As green materials, AAMs contribute to lowering carbon emis-
sions, reducing energy consumption, saving energy, and reducing emissions.
Author Contributions: H.M.: Investigation, writing—review and editing; S.L.: data curation, writ-
ing—original draft; Z.L.: data curation; J.W.: data curation; X.Y.: data curation; X.N.: methodology,
supervision. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Natural Science Foundation of Hebei Province
(E2022201011).
Data Availability Statement: The original contributions presented in this study are included in the
article. Further inquiries can be directed to the corresponding authors.
Conflicts of Interest: The authors declare no conflicts of interest.
Crack
Brucite
Hydrotalcite
Brucite
Hydrotalcite
Brucite
Hydrotalcite

Buildings 2025, 15, 256 13 of 14
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