Effect of Solid Sodium Silicate on Workability, Hydration and Strength of Alkali-Activated GGBS/Fly Ash Paste

Abstract

Based on economic and environmental considerations, the recycling economy of mineral waste has been found to have great potential and economic benefits worldwide, in which alkali-activated cementitious materials are one of the main developing directions. The alkali activators commonly used in alkali-activated cementitious materials are the composite activators of sodium silicate solution and solid sodium hydroxide, which not only need to deal with high viscosity and corrosive chemicals, but also need to be prepared in advance and properly stored. In this paper, ground granulated blast furnace slag (GGBS) and fly ash were used as precursors, while solid sodium silicate powder was applied as the alkali activator. In addition, the precursors were mixed with the activator in advance and activated by adding water to prepare alkali-activated GGBS/fly ash cement. The influence of precursor components, the dosage of the alkali activator and the liquid–solid ratio on the working performance, mechanical strength and hydration process of alkali-activated cement was studied. The results showed that the further incorporation of GGBS accelerated the alkali activation reaction rate and improved the strength of the specimen. However, in the specimen with GGBS as the main component of the precursor, the main hydration product was C-A-S-H gel, which was different in the structural order and quantity. The compressive strength indicated that there was the best amount of activator to match it in terms of the precursor with certain components. A too high or too low amount of activator will hinder the alkali activation reaction. This study can provide some significant reference material for the use of solid alkali activators in alkali-activated cementitious materials.

Full text

Citation: Dong, T.; Sun, T.; Xu, F.;
Ouyang, G.; Wang, H.; Yang, F.;
Wang, Z. Effect of Solid Sodium
Silicate on Workability, Hydration
and Strength of Alkali-Activated
GGBS/Fly Ash Paste. Coatings 2023,
13, 696. https://doi.org/10.3390/
coatings13040696
Academic Editor: Andrea Nobili
Received: 2 March 2023
Revised: 17 March 2023
Accepted: 22 March 2023
Published: 29 March 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
coatings
Article
Effect of Solid Sodium Silicate on Workability, Hydration and
Strength of Alkali-Activated GGBS/Fly Ash Paste
Tingkai Dong 1, Tao Sun 2 ,3 ,* , Fang Xu 4, Gaoshang Ouyang 1, Hongjian Wang 5, Fan Yang 6and Ziyan Wang 1
1School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
2
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China
3Wuhan University of Technology Advanced Engineering Technology Research Institute of Zhongshan City,
Wuhan 528400, China
4Faculty of Engineering, China University of Geosciences, Wuhan 430074, China
5Shandong High Speed Maintenance Group Co., Ltd., Hubei Company, Wuhan 430010, China
6China Construction Advanced Technology Institute, Wuhan 430073, China
*Correspondence: sunt@whut.edu.cn
Abstract:
Based on economic and environmental considerations, the recycling economy of mineral
waste has been found to have great potential and economic benefits worldwide, in which alkali-
activated cementitious materials are one of the main developing directions. The alkali activators
commonly used in alkali-activated cementitious materials are the composite activators of sodium
silicate solution and solid sodium hydroxide, which not only need to deal with high viscosity and
corrosive chemicals, but also need to be prepared in advance and properly stored. In this paper,
ground granulated blast furnace slag (GGBS) and fly ash were used as precursors, while solid sodium
silicate powder was applied as the alkali activator. In addition, the precursors were mixed with
the activator in advance and activated by adding water to prepare alkali-activated GGBS/fly ash
cement. The influence of precursor components, the dosage of the alkali activator and the liquid–solid
ratio on the working performance, mechanical strength and hydration process of alkali-activated
cement was studied. The results showed that the further incorporation of GGBS accelerated the alkali
activation reaction rate and improved the strength of the specimen. However, in the specimen with
GGBS as the main component of the precursor, the main hydration product was C-A-S-H gel, which
was different in the structural order and quantity. The compressive strength indicated that there
was the best amount of activator to match it in terms of the precursor with certain components. A
too high or too low amount of activator will hinder the alkali activation reaction. This study can
provide some significant reference material for the use of solid alkali activators in alkali-activated
cementitious materials.
Keywords:
solid powdered sodium silicate; workability; GGBS–fly ash blends; alkali activation;
compressive strength
1. Introduction
Concrete is an important part of modern architecture, and about 12 billion tons of con-
crete are produced every year [
1
], so we have to face the problems of energy consumption
and CO
2
emissions brought about by cement production, and the continuous reduction
in mineral materials as aggregates. For environmental and economic considerations, it is
urgent to apply recyclable waste to the construction field. Karalar et al. [
2
] used waste
marble powder to replace parts of cement for concrete preparation. Although the com-
pressive strength of concrete has decreased, the replacement rate of 10~20% is feasible
from the perspective of bending and cracking behavior. Qaidi et al. [
3
] used PET plastic
waste to partially replace fine aggregate for the preparation of lightweight concrete with
low conductivity and high toughness. Zeybek et al. [
4
] applied waste glass to concrete
production and found that the compressive strength of concrete decreased when waste
Coatings 2023,13, 696. https://doi.org/10.3390/coatings13040696 https://www.mdpi.com/journal/coatings

Coatings 2023,13, 696 2 of 14
glass powder was used to replace part of cement and that the appropriate replacement
rate was 20%; however, when waste glass powder is used to replace a part of the cement
and waste glass particles were used to replace a part of the aggregate, the mechanical
strength of concrete was improved by 10% in its substitution rate. Karalar et al. [
5
] used
coal bottom ash to replace fine aggregate to prepare reinforced concrete beams, taking
the bending and fracture behavior of the beam as the performance index, in which a 75%
substitution rate was the most appropriate. Qaidi et al. [
6
] found that the use of waste
glass as an aggregate in concrete can effectively reduce the static load of concrete, although
it will reduce the workability and mechanical properties of concrete. Shcherban et al. [
7
]
used coconut fiber as the fiber reinforcement of concrete, and found that its mechanical
properties and elastic modulus were improved, and the best fiber reinforcement rate was
1.75%.
Beskopylny et al. [8]
used rubber tree seed shell to replace a part of coarse aggre-
gate for concrete manufacturing. When the substitution rate is between 2% and 6%, the
microstructure of concrete achieved the highest density. It is estimated that this method can
reduce the consumption of mineral coarse aggregate by 8%. In addition, alkali-activated
cementitious materials are also an important research direction because they can partially
or completely use solid waste instead of cement.
Alkali-activated cementitious materials are materials that react with alkali activa-
tors using silica–aluminum materials with pozzolanic activity or potential hydraulic
properties [9,10]
. According to the differences in their precursors, alkali-activated cement-
ing material systems can be divided into two categories [
11
,
12
]. One is a calcium-rich and
silica-rich system represented by GGBS, whose main reaction products are C-A-S-H gel
with a low Ca/Si ratio and high Al inclusion. The other is an aluminum-rich and silica-rich
system represented by (low calcium) fly ash, the reaction product of which is N-A-S-H gel
with a three-dimensional network. In recent years, scholars generally believe that it is an
environmentally friendly and sustainable alternative to traditional Portland cement. On
one hand, because aluminum silicate precursor can partially or completely use industrial
wastes, and the production process of alkali-activated cementing materials does not require
‘two grindings and one burning’ [
13
], it solves the three major problems that commonly
exist in the traditional cement industry: high energy consumption, difficulty supplying
raw materials and large carbon dioxide emissions. On the other hand, alkali-activated
cementing materials generally perform better than cement; they have a high early strength,
fast hardening, chemical corrosion resistance and good durability [
14
,
15
]. At present, the
most commonly used alkali activator is the composite activator of sodium silicate solution
and solid sodium hydroxide. The following problems need to be faced during the appli-
cation: (1) in the configuration of sodium silicate solution, it is necessary to deal with the
high viscosity of sodium silicate solution and caustic sodium hydroxide; (2) the sodium
silicate solution needs to be pre-configured and needs a certain standing time after the
configuration is completed; (3) the storage and handling of the chemical solution have to
be faced. In contrast, it is more convenient and safer to mix the solid activator with the
precursor in advance, and add water to activate it during application [
16
,
17
]. Meanwhile,
research on the application of solid base activators is relatively lacking at present.
Common solid activators include strong alkali, silicate, carbonate, sulfate and alumi-
nate, etc. [
18
–
20
]. Zhang et al. [
21
] used sodium hydroxide and sodium carbonate powder
as composite solid activators, and the compressive strength at 28 days exceeded 50 MPa.
Jiang et al. [
22
] used calcined sodium carbonate as the alkali activator, and found that its
activation effect was similar to that of sodium silicate solution. Koloušek et al. [
23
] prepared
alkali-activated cement by calcination of kaolin and solid sodium hydroxide, whose com-
pressive strength was still less than 1 MPa at 7 days. Hajimohammadi et al. [
24
–
26
] mixed
geothermal silica with solid sodium aluminate as a source of silica, alumina and alkali, but
the slurry took 14 days to set and harden until the mold was removed.
Feng et al. [27]
stud-
ied the alkali-activation effect of albite mixed with sodium hydroxide or sodium carbonate
powder at a high temperature, and found that its compressive strength reached 40 MPa at
28 days. Sturm et al. [
28
] used rice husk ash as the precursor and solid sodium aluminate

Coatings 2023,13, 696 3 of 14
as the activator to prepare alkali-activated cement, which exhibited a compressive strength
of 30.1 MPa after 7 days under the curing temperature of 80
◦
C. It can be found that when
using non-silicates as solid activators, a higher compressive strength is usually achieved by
calcination, the application of strong alkalis or higher reaction temperatures. In addition,
the early strength is generally lower because the base activators do not provide additional
[SiO4] monomers. This is contrary to the original intention of developing solid activators.
When silicate is used as an alkali activator, these problems can be effectively avoided;
Ma et al. [
29
] used three different types of solid sodium silicate powders (Na
2
SiO
3
-
anhydrous, Na
2
SiO
3·
5H
2
O, and Na
2
SiO
3·
9H
2
O) individually as activators. Na
2
SiO
3
-
anhydrous had the most significant alkali activation effect in terms of compressive strength.
Liu et al. [
30
] used sodium silicate powder as the alkali activator, and the strength of
the alkali-activated cement after 28 days reached 20.5 MPa when the modulus was 0.5.
Yang et al. [
31
] used the compound alkali activator of sodium silicate powder and sodium
hydroxide powder, and produced a good activation effect on the precursor composed of
GGBS and fly ash, with the compressive strength exceeding 50 MPa at 28 days. In addition,
Temuujin [
32
], Nematollahi [
33
] and Yang [
34
] et al. also used GGBS and fly ash, as precur-
sors with single Na
2
SiO
3
or composite activators consisting of Na
2
SiO
3·
nH
2
O and NaOH
were considered as activators, from which alkali-activated cement with a compressive
strength of 40~70 MPa at 28 days was prepared at room temperature. Their research shows
that sodium silicate powder can be feasibly used as a solid alkali activator, but their research
focuses on the chemical composition and microstructure of hydration products, and lacks
the mechanism of the influence of sodium silicate powder on the working performance
and mechanical strength of alkali-activated cement.
In this paper, low-calcium fly ash and GGBS were used as precursors, and sodium
silicate powder with modulus of 2.0 was used as the solid alkali activator. The mechanisms
of the precursor components, the sodium silicate content and the liquid–solid ratio on
the working and mechanical properties of alkali-activated cement were investigated. The
type and microstructure of the hydration products were characterized by XRD, TG/DTG
and FTIR.
2. Experimental Program
2.1. Materials
The ground granulated blast furnace slag (GGBS) used in this study’s test was obtained
from Wuhan Iron and Steel Group Co., Ltd (Wuhan, China). According to the national
standard GB/T 203, the reactivity of the GGBS is evaluated by the quality factor (K), is
determined by the total amount of oxides (CaO, Al
2
O
3
, and MgO), and is divided by total
amount of oxides (SiO
2
, MnO, and TiO
2
), as shown in Equation (1); in this paper, the quality
factor K value is 1.6. The F grade fly ash was obtained from Zhuhai Power Plant Co., Ltd
(Zhuhai, China). The solid alkali activator was an instant powdered sodium silicate with a
modulus of 2.0, purchased from Henan Platinum New Materials Co., Ltd (Hennan, China),
which is a kind of white powder material that can be rapidly dissolved in water, and whose
aqueous solution is alkaline. The chemical compositions of the cementitious materials are
shown in Table 1, where
ω
means the mass fraction of oxides (wt.%). LOI in the table is
the abbreviation for loss on ignition, which refers to the percentage of the mass loss of
raw materials that have lost external moisture after drying in the temperature range of
105~110 ◦C and burning for a long time under certain high-temperature conditions.
K=ωCaO +ωMgO +ωAl2O3
ωSiO2+ωMnO +ωTiO2
(1)
The cumulative particle size distribution of GGBS and fly ash is shown in Figure 1; the
mean particle sizes (D50) of the GGBS and fly ash are 12.1 and 16.5
µ
m, and their specific
surface areas are 1.13 m2/g and 1.11 m2/g, respectively.

Coatings 2023,13, 696 4 of 14
Table 1. Chemical composition of cementitious materials (wt.%).
Material SiO2Al2O3CaO MgO SO3Fe2O3MnO TiO2LOI
GGBS 30.6 15.5 36.7 8.9 2.6 1.1 0.5 1.0 1.1
Fly ash 48.2 33.1 5.1 1.2 1.0 5.0 0.1 0.9 2.8
Coatings 2023, 13, x FOR PEER REVIEW 4 of 15
in the temperature range of 105~110 °C and burning for a long time under certain high-
temperature conditions.
K= 𝜔CaO+𝜔MgO+𝜔Al2O3
𝜔SiO2+𝜔MnO+𝜔TiO2
(1)
Table 1. Chemical composition of cementitious materials (wt.%).
Material SiO2 Al2O3 CaO MgO SO3 Fe2O3 MnO TiO2 LOI
GGBS 30.6 15.5 36.7 8.9 2.6 1.1 0.5 1.0 1.1
Fly ash 48.2 33.1 5.1 1.2 1.0 5.0 0.1 0.9 2.8
The cumulative particle size distribution of GGBS and fly ash is shown in Figure 1;
the mean particle sizes (D50) of the GGBS and fly ash are 12.1 and 16.5 µm, and their
specific surface areas are 1.13 m2/g and 1.11 m2/g, respectively.
Figure 1. Particle size distribution of cementitious material.
The crystal phase diagram of the GGBS and fly ash is shown in Figure 2. It can be
seen that the GGBS has no obvious crystalline phase, and the peak hump between 25°and
35° indicates that the GGBS contains a large number of amorphous phases. Meanwhile,
the main crystalline phases of the fly ash are quar, mullite and hematite, and the fly ash
also contains some amorphous phases, with the peak hump between 17°and 35°, which
means that both the GGBS and the fly ash have partial cementing and pozzolanic
properties. The location difference of the amorphous peak hump represents the structural
difference between the GGBS and fly ash.
Figure 1. Particle size distribution of cementitious material.
The crystal phase diagram of the GGBS and fly ash is shown in Figure 2. It can be
seen that the GGBS has no obvious crystalline phase, and the peak hump between 25
◦
and
35
◦
indicates that the GGBS contains a large number of amorphous phases. Meanwhile,
the main crystalline phases of the fly ash are quartz, mullite and hematite, and the fly ash
also contains some amorphous phases, with the peak hump between 17
◦
and 35
◦
, which
means that both the GGBS and the fly ash have partial cementing and pozzolanic properties.
The location difference of the amorphous peak hump represents the structural difference
between the GGBS and fly ash.
Coatings 2023, 13, x FOR PEER REVIEW 5 of 15
Figure 2. XRD paerns of precursors. Legend: Q = quar; M = mullite; He = hematite.
2.2. Mix Proportion Design and Slurry Preparation
2.2.1. Mixture Proportions
The precursor mass of the group of specimens molded in this study was 350 g. The
three experimental variables were the components of the precursor (P), the dosage of the
solid sodium silicate powder (S), and the liquid–solid ratio (L), in which the liquid–solid
ratio is water/(precursor + powdered sodium silicate). The specific mix ratio is shown in
Table 2.
Table 2. Mix proportions of GGBS/fly ash pastes.
Specimen Fly Ash GGBS Activator Water
P14 280.0 70.0
80.0 98.9
P23 210.0 140.0
P32 140.0 210.0
P41 70.0 280.0
S50
70.0 280.0
50.0 92.0
S60 60.0 94.3
S70 70.0 96.0
S90 90.0 101.2
S100 100.0 103.5
S110 110.0 105.8
L0.20
70.0 280.0 80.0
86.0
L0.26 111.8
L0.30 129.0
L0.32 137.6
L0.35 150.5
L0.38 163.4
Note: The unit is g, the leers P, S and L in the sample number represent the mass ratio of GGBS/fly
ash in the precursor component, the mass of solid sodium silicate powder, and the liquid–solid ratio,
respectively . It should be noted that groups P41, S80 and L0.23 are the same group of test pieces.
When preparing the specimen, we weighed the fly ash, GGBS and powdered sodium
silicate according to Table 2, placed them in the clear slurry mixing pot and stirred them
evenly after adding water of the corresponding quality; we stirred them (according to
GB/T7671-1999) at low speed for 120 s, stopped for 15 s, and then stirred them quickly for
120 s. Finally, the slurry was loaded into the molds with a dimension of 40 mm × 40 mm
× 160 mm with film covered. It was placed in the standard curing chamber with a
temperature of (20 ± 2) °C and relative humidity (≥95%) for 24 h until the specified age
and demolded after 24 h.
Figure 2. XRD patterns of precursors. Legend: Q = quartz; M = mullite; He = hematite.
2.2. Mix Proportion Design and Slurry Preparation
2.2.1. Mixture Proportions
The precursor mass of the group of specimens molded in this study was 350 g. The
three experimental variables were the components of the precursor (P), the dosage of the
solid sodium silicate powder (S), and the liquid–solid ratio (L), in which the liquid–solid
ratio is water/(precursor + powdered sodium silicate). The specific mix ratio is shown
in Table 2.

Coatings 2023,13, 696 5 of 14
Table 2. Mix proportions of GGBS/fly ash pastes.
Specimen Fly Ash GGBS Activator Water
P14 280.0 70.0
80.0 98.9
P23 210.0 140.0
P32 140.0 210.0
P41 70.0 280.0
S50
70.0 280.0
50.0 92.0
S60 60.0 94.3
S70 70.0 96.0
S90 90.0 101.2
S100 100.0 103.5
S110 110.0 105.8
L0.20
70.0 280.0 80.0
86.0
L0.26 111.8
L0.30 129.0
L0.32 137.6
L0.35 150.5
L0.38 163.4
Note: The unit is g, the letters P, S and L in the sample number represent the mass ratio of GGBS/fly ash in the
precursor component, the mass of solid sodium silicate powder, and the liquid–solid ratio, respectively. It should
be noted that groups P41, S80 and L0.23 are the same group of test pieces.
When preparing the specimen, we weighed the fly ash, GGBS and powdered sodium
silicate according to Table 2, placed them in the clear slurry mixing pot and stirred
them evenly after adding water of the corresponding quality; we stirred them (accord-
ing to GB/T7671-1999) at low speed for 120 s, stopped for 15 s, and then stirred them
quickly for 120 s. Finally, the slurry was loaded into the molds with a dimension of
40 mm ×40 mm ×160 mm
with film covered. It was placed in the standard curing cham-
ber with a temperature of (20
±
2)
◦
C and relative humidity (
≥
95%) for 24 h until the
specified age and demolded after 24 h.
2.2.2. Testing Methods
1. Working performance, setting time and mechanical properties
A slump flow test was used to evaluate the working performance of the slurry based
on GB/T 8077-2012; after filling the fluidity mold with the newly mixed slurry to be
measured, the mold was quickly lifted from the glass plate in the vertical direction. After
30 s, the average of the two maximum diameters in the vertical direction of each other was
taken as the fluidity.
The setting time of the slurry was determined according to GB/T 1346-2011.
The 40 mm
×
40 mm
×
160 mm samples were adopted to measure the compressive
strength based on GB/T 17671-1999. The compressive strength is given by Equation (2),
as follows:
fc=F
Ac(2)
where f
c
is the compressive strength (MPa), Fis the maximum load at failure (N), and A
c
is
the cross-sectional area of the specimen on which the compressive force acts (mm2).
2. Mineralogical compositions and microstructure
The mineralogical compositions of the specimens were monitored using X-ray diffrac-
tion (XRD) with a Bruker D8 Advanced X-ray diffractometer (Bruker Corporation, Billerica,
MA, USA); the Fourier Transform infrared spectroscopy (FTIR) measurement was per-
formed in a Nicolet 6700 instrument (Thermo Fisher Scientific, Massachusetts, USA) with
the wave numbers ranging from 4000 to 600 cm−1.
A thermogravimetry (TG) analysis was conducted in an STA449F3 instrument (NET-
ZSCH, Free State of Bavaria, Germany); the grinded powder samples were heated from 40

Coatings 2023,13, 696 6 of 14
to 1000
◦
C, at 10
◦
C/min, with nitrogen as the carrier gas. All the sample analyses were
carried out at the age of 7 days.
The preparation and test process are shown in Figure 3.
Coatings 2023, 13, x FOR PEER REVIEW 6 of 15
2.2.2. Testing Methods
1. Working performance, seing time and mechanical properties
A slump flow test was used to evaluate the working performance of the slurry based
on GB/T 8077-2012; after filling the fluidity mold with the newly mixed slurry to be
measured, the mold was quickly lifted from the glass plate in the vertical direction. After
30 s, the average of the two maximum diameters in the vertical direction of each other was
taken as the fluidity.
The seing time of the slurry was determined according to GB/T 1346-2011.
The 40 mm × 40 mm × 160 mm samples were adopted to measure the compressive
strength based on GB/T 17671-1999. The compressive strength is given by Equation (2), as
follows:
𝑓
=𝐹
𝐴

(2)
where fc is the compressive strength (MPa), F is the maximum load at failure (N), and Ac
is the cross-sectional area of the specimen on which the compressive force acts (mm2).
2. Mineralogical compositions and microstructure
The mineralogical compositions of the specimens were monitored using X-ray
diffraction (XRD) with a Bruker D8 Advanced X-ray diffractometer (Bruker Corporation,
Billerica, MA, USA); the Fourier Transform infrared spectroscopy (FTIR) measurement
was performed in a Nicolet 6700 instrument (Thermo Fisher Scientific, Massachuses,
USA) with the wave numbers ranging from 4000 to 600 cm−1.
A thermogravimetry (TG) analysis was conducted in an STA449F3 instrument
(NETZSCH, Free State of Bavaria, Germany); the grinded powder samples were heated
from 40 to 1000 °C, at 10 °C/min, with nitrogen as the carrier gas. All the sample analyses
were carried out at the age of 7 days.
The preparation and test process are shown in Figure 3.
Figure 3. Flow chart for experiments.
3. Results and Discussion
3.1. Workability
The fluidity of each group of alkali-activated GGBS/fly ash cement is shown in Figure
4, and the seing time is shown in Figure 5. Groups P14~P41 regulated the mass ratio of
GGBS/fly ash in the precursor, and the two numbers after the leer P represent the
Figure 3. Flow chart for experiments.
3. Results and Discussion
3.1. Workability
The fluidity of each group of alkali-activated GGBS/fly ash cement is shown in
Figure 4, and the setting time is shown in Figure 5. Groups P14~P41 regulated the mass
ratio of GGBS/fly ash in the precursor, and the two numbers after the letter P represent the
proportion of GGBS and fly ash in the precursor. On the whole, with the increase in the
proportion of GGBS in the precursor, the loss of slurry fluidity is obvious, and the setting
time is shortened; the reasons for this phenomenon are as follows: (1) compared with the
rough particles and angular structure of GGBS, the smooth spherical particles of fly ash
can play the role of a “ball bearing”, reducing the loss of fluidity [
35
,
36
]; and (2) due to
the high degree of polymerization and low amorphous properties of fly ash, the hydration
activity of fly ash is lower than that of GGBS. With the increase in the proportion of GGBS,
the active CaO in the system increases significantly, which further leads to an increase in
Ca
2+
ions that can participate in an alkali-activated reaction, speeding up the hydration
reaction rate and shortening the setting time [37].
Coatings 2023, 13, x FOR PEER REVIEW 7 of 15
proportion of GGBS and fly ash in the precursor. On the whole, with the increase in the
proportion of GGBS in the precursor, the loss of slurry fluidity is obvious, and the seing
time is shortened; the reasons for this phenomenon are as follows: (1) compared with the
rough particles and angular structure of GGBS, the smooth spherical particles of fly ash
can play the role of a “ball bearing”, reducing the loss of fluidity [35,36]; and (2) due to
the high degree of polymerization and low amorphous properties of fly ash, the hydration
activity of fly ash is lower than that of GGBS. With the increase in the proportion of GGBS,
the active CaO in the system increases significantly, which further leads to an increase in
Ca2+ ions that can participate in an alkali-activated reaction, speeding up the hydration
reaction rate and shortening the seing time [37].
Figure 4. Effects of precursor composition, dosage of solid powdered sodium silicate and liquid–
solid ratio on fluidity of fresh paste.
Figure 5. Effects of precursor composition, dosage of solid powdered sodium silicate and liquid–
solid ratio on seing time.
When using a solid alkali activator to prepare alkali-activated cement, the process is
often faced with the problem that the seing time of slurry is too long [23–26]. Therefore,
subsequent studies were based on group P41, in which groups S50~S110 regulated the
dosage of solid powdered sodium silicate to explore the influence of a solid alkali activator
on the fluidity and seing time of alkali-activated cement; the number after the leer S
represents the mass of solid powder sodium silicate. When the dosage of powdered
sodium silicate in group S50 is low (50 g), the slurry almost loses its fluidity, which is only
75 mm. In group S60, a small increase in the dosage of powdered sodium silicate (10 g)
will greatly improve the working performance of the slurry, reaching 120 mm. When the
dosage of sodium silicate continues to increase, the rising trend seen in the fluidity of the
slurry tends to be gentle. This is because when the dosage of sodium silicate is 50 g, the
concentration of OH− in the system is low, the erosion of precursor is slow, and the slurry
is mainly manifested as the highly viscous sodium silicate aqueous solution and
suspended particle mixture. At this point, if the content of sodium silicate is slightly
Figure 4.
Effects of precursor composition, dosage of solid powdered sodium silicate and liquid–solid
ratio on fluidity of fresh paste.

Coatings 2023,13, 696 7 of 14
Coatings 2023, 13, x FOR PEER REVIEW 7 of 15
proportion of GGBS and fly ash in the precursor. On the whole, with the increase in the
proportion of GGBS in the precursor, the loss of slurry fluidity is obvious, and the seing
time is shortened; the reasons for this phenomenon are as follows: (1) compared with the
rough particles and angular structure of GGBS, the smooth spherical particles of fly ash
can play the role of a “ball bearing”, reducing the loss of fluidity [35,36]; and (2) due to
the high degree of polymerization and low amorphous properties of fly ash, the hydration
activity of fly ash is lower than that of GGBS. With the increase in the proportion of GGBS,
the active CaO in the system increases significantly, which further leads to an increase in
Ca2+ ions that can participate in an alkali-activated reaction, speeding up the hydration
reaction rate and shortening the seing time [37].
Figure 4. Effects of precursor composition, dosage of solid powdered sodium silicate and liquid–
solid ratio on fluidity of fresh paste.
Figure 5. Effects of precursor composition, dosage of solid powdered sodium silicate and liquid–
solid ratio on seing time.
When using a solid alkali activator to prepare alkali-activated cement, the process is
often faced with the problem that the seing time of slurry is too long [23–26]. Therefore,
subsequent studies were based on group P41, in which groups S50~S110 regulated the
dosage of solid powdered sodium silicate to explore the influence of a solid alkali activator
on the fluidity and seing time of alkali-activated cement; the number after the leer S
represents the mass of solid powder sodium silicate. When the dosage of powdered
sodium silicate in group S50 is low (50 g), the slurry almost loses its fluidity, which is only
75 mm. In group S60, a small increase in the dosage of powdered sodium silicate (10 g)
will greatly improve the working performance of the slurry, reaching 120 mm. When the
dosage of sodium silicate continues to increase, the rising trend seen in the fluidity of the
slurry tends to be gentle. This is because when the dosage of sodium silicate is 50 g, the
concentration of OH− in the system is low, the erosion of precursor is slow, and the slurry
is mainly manifested as the highly viscous sodium silicate aqueous solution and
suspended particle mixture. At this point, if the content of sodium silicate is slightly
Figure 5.
Effects of precursor composition, dosage of solid powdered sodium silicate and liquid–solid
ratio on setting time.
When using a solid alkali activator to prepare alkali-activated cement, the process is
often faced with the problem that the setting time of slurry is too long [
23
–
26
]. Therefore,
subsequent studies were based on group P41, in which groups S50~S110 regulated the
dosage of solid powdered sodium silicate to explore the influence of a solid alkali activator
on the fluidity and setting time of alkali-activated cement; the number after the letter
S represents the mass of solid powder sodium silicate. When the dosage of powdered
sodium silicate in group S50 is low (50 g), the slurry almost loses its fluidity, which is only
75 mm. In group S60, a small increase in the dosage of powdered sodium silicate (10 g)
will greatly improve the working performance of the slurry, reaching 120 mm. When the
dosage of sodium silicate continues to increase, the rising trend seen in the fluidity of the
slurry tends to be gentle. This is because when the dosage of sodium silicate is 50 g, the
concentration of OH
−
in the system is low, the erosion of precursor is slow, and the slurry
is mainly manifested as the highly viscous sodium silicate aqueous solution and suspended
particle mixture. At this point, if the content of sodium silicate is slightly increased, the
concentration of OH
−
is enough to erode the precursor, while if the content of sodium
silicate continues to increase, excessive OH
−
will form Ca (OH)
2
with Ca
2+
, inhibiting an
increase in the fluidity [38].
The influence of the dosage of solid powdered sodium silicate in group S on the
setting time and fluidity of the slurry is not completely the same. The dosage of powdered
sodium silicate in group S50 is 50 g, and the initial and final setting times are 52 and
64 min
,
respectively. At this time, the setting time of the slurry will be shortened with the increase
in the dosage of powdered sodium silicate. The shortest setting time occurs when the
dosage of powdered sodium silicate is 80 g, and when the initial and final setting times
are 37 and 42 min, respectively; however, when the dosage of powdered sodium silicate
continues to increase, the setting time will be prolonged. For example, when the dosage
is 110 g, the initial and final setting times are 48 and 60 min, respectively. It can be seen
that when the dosage of powdered sodium silicate continues to increase, the setting time of
the slurry presents a trend of first shortening and then increasing; this is mainly because
the alkali concentration of group S50 is too low, and the corrosion dissolution process of
the precursor is slow, showing a long setting time. When the dosage of sodium silicate is
increased appropriately, such as in group P41, the increase in the alkali concentration can
not only improve the dissolution rate of the precursor [
39
], but also increase the solubility
of silica and alumina in solution, which is conducive to the alkali-activated reaction [
40
,
41
].
Therefore, the setting time has been shortened to a certain extent, but the increase in the
sodium silicate content makes the reaction product quickly generated and attached to
the precursor particle surface so that it will hinder the dissolution process and prolong
the setting time [
42
]. This also shows that for the precursor with certain components, the
optimal dosage of the activator with fixed modulus is matched, and an activator dosage
that is too high or too low will have a certain negative effect on the alkali-activated reaction.

Coatings 2023,13, 696 8 of 14
When the mass ratio of GGBS/fly ash is 280/70 and the dosage of powdered sodium
silicate is 80 g, groups L0.20~L0.38 regulate the amount of water added in order to explore
the effect of the liquid–solid ratio on the setting time and fluidity of the slurry; the number
after the letter L represents the specific value of the liquid–solid ratio. As a whole, the
increase in the liquid–solid ratio can greatly improve the fluidity of the slurry and extend the
setting time. Although water does not participate in the alkali excitation reaction, the liquid–
solid ratio can affect the fluidity and setting time; because water plays a neutralization role,
the increase in the amount of water will change the alkalinity of the activator (decrease the
pH value) and the viscosity of the system solution (improve the fluidity), thus reducing the
reaction rate and extending the setting time [43].
3.2. Mechanical Performance
The mechanical properties of groups P14~P41, S50~S110 and L0.20~L0.38 were tested,
as shown in Figure 6. Among the variables discussed in this paper, the mass ratio of
GGBS/fly ash has the greatest influence on the mechanical properties of the specimen.
In addition, it can be observed that except for groups L0.30~L0.38, which have a high
liquid–solid ratio, the strength of other groups of specimens develops rapidly, and the
compressive strength at 7 days is very close to the strength at 28 days.
Coatings 2023, 13, x FOR PEER REVIEW 9 of 15
(a)
(b) (c)
Figure 6. Effects of precursor composition, dosage of solid powdered sodium silicate and liquid–
solid ratio on compressive strength: (a) Precursor composition, (b) Dosage of solid powdered
sodium silicate, (c) Liquid–solid ratio.
It can be seen from groups P14~P41 that with the increase in the proportion of GGBS
in the precursor, the compressive strength of the specimen significantly improves. The
mass ratio of GGBS/fly ash in group P14 is 70/280, and the strength after 28 days is only
34.1 MPa, while the mass ratio of GGBS/fly ash in group P41 is 280/70, and the strength
after 28 days reaches 76.5 MPa; this is mainly because the activity of GGBS is much higher
than fly ash, and more Si and Ca are released and C-A-S-H gel is formed [44,45].
It can be seen from groups S50~S110 that when the dosage of powdered sodium
silicate is increased from 50 to 80 g, the strength after 28 days increases slightly, which is
concentrated between 70~80 MPa. When the dosage of sodium silicate is continuously
increased to 110 g, the strength shows an obvious downward trend, and the 28 days
strength is only 41.8 MPa. The reason for this is similar to the effect of sodium silicate
content on the seing time, so it will not be repeated here.
From groups L0.20~L0.38, it can be seen that when the liquid–solid ratio increases
from 0.2 to 0.3, it has no obvious effect on the compressive strength, and the strength of
the test piece after 28 days is maintained above 70 MPa. When the liquid–solid ratio
continues to increase to 0.38, the compressive strength decreases significantly, and the
strength after 28 days is only 38.6 MPa.
3.3. X-ray Diffraction (XRD)
The XRD paerns of precursors, considering group P14, P41, S110 and L0.38 at the
age of 7 days, are shown in Figure 7. It can be seen that the main product of all samples is
C-A-S-H gel with poor crystallinity at 29.5°. The presence of the second main product,
hydrotalcite, can be observed in groups P41, S110 and L0.38, where the precursor
component is mainly GGBS; meanwhile, in group P14, where the precursor component is
mainly fly ash, there is a lack of a hydrotalcite phase. The reason for this phenomenon is
Figure 6.
Effects of precursor composition, dosage of solid powdered sodium silicate and liquid–solid
ratio on compressive strength: (
a
) Precursor composition, (
b
) Dosage of solid powdered sodium
silicate, (c) Liquid–solid ratio.
It can be seen from groups P14~P41 that with the increase in the proportion of GGBS
in the precursor, the compressive strength of the specimen significantly improves. The
mass ratio of GGBS/fly ash in group P14 is 70/280, and the strength after 28 days is only
34.1 MPa, while the mass ratio of GGBS/fly ash in group P41 is 280/70, and the strength
after 28 days reaches 76.5 MPa; this is mainly because the activity of GGBS is much higher
than fly ash, and more Si and Ca are released and C-A-S-H gel is formed [44,45].

Coatings 2023,13, 696 9 of 14
It can be seen from groups S50~S110 that when the dosage of powdered sodium
silicate is increased from 50 to 80 g, the strength after 28 days increases slightly, which is
concentrated between 70~80 MPa. When the dosage of sodium silicate is continuously
increased to 110 g, the strength shows an obvious downward trend, and the 28 days
strength is only 41.8 MPa. The reason for this is similar to the effect of sodium silicate
content on the setting time, so it will not be repeated here.
From groups L0.20~L0.38, it can be seen that when the liquid–solid ratio increases
from 0.2 to 0.3, it has no obvious effect on the compressive strength, and the strength of the
test piece after 28 days is maintained above 70 MPa. When the liquid–solid ratio continues
to increase to 0.38, the compressive strength decreases significantly, and the strength after
28 days is only 38.6 MPa.
3.3. X-ray Diffraction (XRD)
The XRD patterns of precursors, considering group P14, P41, S110 and L0.38 at the age
of 7 days, are shown in Figure 7. It can be seen that the main product of all samples is C-A-S-
H gel with poor crystallinity at 29.5
◦
. The presence of the second main product, hydrotalcite,
can be observed in groups P41, S110 and L0.38, where the precursor component is mainly
GGBS; meanwhile, in group P14, where the precursor component is mainly fly ash, there is
a lack of a hydrotalcite phase. The reason for this phenomenon is that the main components
of hydrotalcite are Mg and Al in GGBS. In group P14, a large number of residual crystalline
phases, including quartz, mullite and hematite, can be observed in the fly ash [46].
Coatings 2023, 13, x FOR PEER REVIEW 10 of 15
that the main components of hydrotalcite are Mg and Al in GGBS. In group P14, a large
number of residual crystalline phases, including quar, mullite and hematite, can be
observed in the fly ash [46].
Figure 7. XRD paerns of precursors, group P14, P41, S110 and L0.38 at the age of 7 days. Legend:
Q = quar; M = mullite; He = hematite; Ht = hydrotalcite; CS = C-A-S-H.
By comparing the peak strength and position of C-A-S-H gel at 29.5° for each group,
it can be found that although there are differences in the dosage of the alkali activator and
the liquid–solid ratio among group P41, S110 and L0.38, their main product types are
basically the same. The main reason for this is that their precursor components are the
same, which is the most important factor affecting the gel structure [47]. Due to the
different precursor components of group P14, the types of reaction products are also
different. When comparing the peak of C-A-S-H gel in groups P41 and L0.38, group L0.38
is higher and the width is narrower, which indicates that the liquid–solid ratio improves
the order degree and reaction degree of the C-A-S-H structure. However, since water does
not participate in alkali-activated reactions, this effect is more caused by the addition of
water and how it affects the alkalinity in the system [48]. When comparing groups P41
and S110 to study the influence of the alkali activator dosage on alkali-activated reactions,
it can be found that increasing the alkali activator dosage has lile influence on the type
and structure of the reaction products. The reasons for this are as follows [47,48]: (1) the
liquid–solid ratio adopted in this paper is water/(precursor + solid alkali activator), and
the addition of water is increased at the same time that the dosage of activator is increased.
The results show that the concentration of Na2O has lile difference and the effect on the
reaction degree is not great. (2) Since the precursor components of groups P41 and S110
are mainly GGBS, and there is sufficient Si in the system, the addition of the alkali
activator-activated SiO2 also has a very limited effect on the Al-doped C-A-S-H gels, and
there is no change in the structure of the band.
In general, the product type and structure of alkali-activated reactions mainly
depend on the mass ratio of GGBS/fly ash in the precursor; in addition, the dosage of the
alkali activator and the liquid–solid ratio affect the reaction degree of the alkali-activated
reaction and the number of gel products by changing the alkalinity of the system and the
amount of active SiO2 [49].
Figure 7.
XRD patterns of precursors, group P14, P41, S110 and L0.38 at the age of 7 days. Legend:
Q = quartz; M = mullite; He = hematite; Ht = hydrotalcite; CS = C-A-S-H.
By comparing the peak strength and position of C-A-S-H gel at 29.5
◦
for each group,
it can be found that although there are differences in the dosage of the alkali activator
and the liquid–solid ratio among group P41, S110 and L0.38, their main product types are
basically the same. The main reason for this is that their precursor components are the same,
which is the most important factor affecting the gel structure [
47
]. Due to the different
precursor components of group P14, the types of reaction products are also different. When
comparing the peak of C-A-S-H gel in groups P41 and L0.38, group L0.38 is higher and the
width is narrower, which indicates that the liquid–solid ratio improves the order degree
and reaction degree of the C-A-S-H structure. However, since water does not participate in
alkali-activated reactions, this effect is more caused by the addition of water and how it

Coatings 2023,13, 696 10 of 14
affects the alkalinity in the system [
48
]. When comparing groups P41 and S110 to study
the influence of the alkali activator dosage on alkali-activated reactions, it can be found
that increasing the alkali activator dosage has little influence on the type and structure
of the reaction products. The reasons for this are as follows [
47
,
48
]: (1) the liquid–solid
ratio adopted in this paper is water/(precursor + solid alkali activator), and the addition of
water is increased at the same time that the dosage of activator is increased. The results
show that the concentration of Na
2
O has little difference and the effect on the reaction
degree is not great. (2) Since the precursor components of groups P41 and S110 are mainly
GGBS, and there is sufficient Si in the system, the addition of the alkali activator-activated
SiO
2
also has a very limited effect on the Al-doped C-A-S-H gels, and there is no change in
the structure of the band.
In general, the product type and structure of alkali-activated reactions mainly depend
on the mass ratio of GGBS/fly ash in the precursor; in addition, the dosage of the alkali
activator and the liquid–solid ratio affect the reaction degree of the alkali-activated reaction
and the number of gel products by changing the alkalinity of the system and the amount of
active SiO2[49].
3.4. Fourier Transform Infrared Spectroscopy (FTIR)
The FTIR results of the procedures, regarding group P14, P41, S110 and L0.38, are
presented in Figure 8. As can be seen from the figure, the absorption band of 691 cm
−1
in
the GGBS is related to the asymmetric tensile vibration of the tetrahedral T-O group, and its
absorption is mainly at 943 cm
−1
, which comes from the asymmetric tensile vibration of the
terminal Si-O bond. The absorption band of 599 cm
−1
in fly ash is related to the octahedral
coordination aluminum in mullites, and its main absorption band is located at 1099 cm
−1
,
which corresponds to the asymmetric tensile vibration of the bridge Si-O-T bond [50].
Coatings 2023, 13, x FOR PEER REVIEW 11 of 15
3.4. Fourier Transform Infrared Spectroscopy (FTIR)
The FTIR results of the procedures, regarding group P14, P41, S110 and L0.38, are
presented in Figure 8. As can be seen from the figure, the absorption band of 691 cm−1 in
the GGBS is related to the asymmetric tensile vibration of the tetrahedral T-O group, and
its absorption is mainly at 943 cm−1, which comes from the asymmetric tensile vibration of
the terminal Si-O bond. The absorption band of 599 cm−1 in fly ash is related to the
octahedral coordination aluminum in mullites, and its main absorption band is located at
1099 cm−1, which corresponds to the asymmetric tensile vibration of the bridge Si-O-T
bond [50].
Figure 8. FTIR spectra of group P14, P41, S110 and L0.38.
In the infrared absorption spectra of all samples, absorption bands around 3400 and
1650 cm−1 (3400 cm−1 is not shown in the figure) were observed, respectively,
corresponding to the bending vibration of water molecules and the a-/ symmetric
stretching of O-H, indicating the presence of chemically bound water in the reaction
product [48]. In addition, the absorption band near 1000 cm−1 in each sample is the
asymmetric tensile vibration of the Si-O terminal bond (non-bridging) in the reaction
product, which is the representative absorption segment of the chain C-A-S-H gel of the
product of alkali-activated GGBS, which is consistent with the diffraction peak at 29.5° in
the XRD test. Compared with the absorption band of GGBS located at 943cm−1, the
absorption band of the Si-O terminal bond in C-A-S-H gel is higher, because the
polymerization degree of the Si-O network in the gel is higher; meanwhile, the absorption
band of the Si-O terminal bond in the gel is lower than that of the asymmetric stretching
vibration of the bridge Si-O-T bond in the fly ash [49,50]. This is because the high
crosslinking bridging Si-O bond in fly ash is decomposed. It is worth noting that the gel
absorption band of group P14, which is dominated by fly ash among the precursor
components, is significantly higher than that of P41, S110 and L0.38, which are dominated
by GGBS. This is due to the effect of Ca2+. At the initial stage of the reaction, GGBS releases
a large amount of Ca2+ in an alkali environment, which consumes the limited Si and Al
units in the solution. The remaining Si and Al units are not enough to be polymerized into
a high crosslinked network that is dominated by silicate and alumina (bridge Si-O bond
with a high wave number), and the content of GGBS in P14 is low; therefore, the
absorption band here is slightly higher than the other three groups [51]. However, when
comparing P41, S110 and L0.38, it can be found that although there are differences in the
dosage of the alkali activator and the liquid–solid ratio, the difference in the absorption
wave number is not particularly obvious, mainly because the product C-A-S-H gel is
Figure 8. FTIR spectra of group P14, P41, S110 and L0.38.
In the infrared absorption spectra of all samples, absorption bands around 3400 and
1650 cm
−1
(3400 cm
−1
is not shown in the figure) were observed, respectively, correspond-
ing to the bending vibration of water molecules and the a-/symmetric stretching of O-H,
indicating the presence of chemically bound water in the reaction product [
48
]. In addition,
the absorption band near 1000 cm
−1
in each sample is the asymmetric tensile vibration
of the Si-O terminal bond (non-bridging) in the reaction product, which is the representa-
tive absorption segment of the chain C-A-S-H gel of the product of alkali-activated GGBS,
which is consistent with the diffraction peak at 29.5
◦
in the XRD test. Compared with the
absorption band of GGBS located at 943cm
−1
, the absorption band of the Si-O terminal
bond in C-A-S-H gel is higher, because the polymerization degree of the Si-O network in

Coatings 2023,13, 696 11 of 14
the gel is higher; meanwhile, the absorption band of the Si-O terminal bond in the gel
is lower than that of the asymmetric stretching vibration of the bridge Si-O-T bond in
the fly ash [
49
,
50
]. This is because the high crosslinking bridging Si-O bond in fly ash
is decomposed. It is worth noting that the gel absorption band of group P14, which is
dominated by fly ash among the precursor components, is significantly higher than that of
P41, S110 and L0.38, which are dominated by GGBS. This is due to the effect of Ca
2+
. At the
initial stage of the reaction, GGBS releases a large amount of Ca
2+
in an alkali environment,
which consumes the limited Si and Al units in the solution. The remaining Si and Al units
are not enough to be polymerized into a high crosslinked network that is dominated by
silicate and alumina (bridge Si-O bond with a high wave number), and the content of GGBS
in P14 is low; therefore, the absorption band here is slightly higher than the other three
groups [
51
]. However, when comparing P41, S110 and L0.38, it can be found that although
there are differences in the dosage of the alkali activator and the liquid–solid ratio, the
difference in the absorption wave number is not particularly obvious, mainly because the
product C-A-S-H gel is limited by the chain structure, and the difference in the degree of
polymerization is not large.
In addition, the absorption bands of 710, 875, and 1430 cm
−1
were derived from O-C-O
vibrations in the carbonate, possibly due to improper sample preservation and a certain
degree of carbonization.
3.5. Thermogravimetry (TG/DTG)
The thermogravimetry results for group P14, P41, S110 and L0.38 are presented in
Figure 9. As can be seen from the figure, all samples exhibit significant mass loss before
110 ◦C
due to the loss of free and loosely bound water in the product. The slow mass loss at
105
◦
C to 300
◦
C is due to the dehydration decomposition of C-A-S-H gel [
48
,
51
]. Small DTG
peaks at 300~400
◦
C due to the dehydration of the hydrotalcite phase, including interlayer
water loss at 270
◦
C and main-layer dehydration at 400
◦
C, were not observed in P14. This
is due to the low GGBS content in P14, which is consistent with the absence of the hydrous
talc phase in the XRD analysis of P14 [
52
]. Mass loss at 600~800
◦
C was associated with the
decomposition of calcium carbonate, and likewise, the O-C-O bond in calcium carbonate
was observed in FTIR.
Coatings 2023, 13, x FOR PEER REVIEW 12 of 15
limited by the chain structure, and the difference in the degree of polymerization is not
large.
In addition, the absorption bands of 710, 875, and 1430 cm−1 were derived from O-C-
O vibrations in the carbonate, possibly due to improper sample preservation and a certain
degree of carbonization.
3.5. Thermogravimetry (TG/DTG)
The thermogravimetry results for group P14, P41, S110 and L0.38 are presented in
Figure 9. As can be seen from the figure, all samples exhibit significant mass loss before
110 °C due to the loss of free and loosely bound water in the product. The slow mass loss
at 105 °C to 300 °C is due to the dehydration decomposition of C-A-S-H gel [48,51]. Small
DTG peaks at 300~400 °C due to the dehydration of the hydrotalcite phase, including
interlayer water loss at 270 °C and main-layer dehydration at 400 °C, were not observed
in P14. This is due to the low GGBS content in P14, which is consistent with the absence
of the hydrous talc phase in the XRD analysis of P14 [52]. Mass loss at 600~800 °C was
associated with the decomposition of calcium carbonate, and likewise, the O-C-O bond in
calcium carbonate was observed in FTIR.
Figure 9. TG analysis of group P14, P41, S110 and L0.38.
The mass loss of each sample between 105~300 °C was observed, and the dehydration
decomposition of C-A-S-H gel occurred mainly in this temperature range. It can be found
that the mass loss of P14 was significantly lower than that of the other three groups, mainly
because the content of C-A-S-H gel products in group P14 was less, which was explained
by the lack of Ca2+ [47]. The mass loss of L0.38 is more than that of P41 and S110, which is
consistent with the XRD analysis, demonstrating that the peak strength of group L0.38 at
29.5° is higher than that of P41 and S110. Although group L0.38 has the largest amount of
C-A-S-H gel, its high liquid–solid ratio also leads to the matrix loosening, which leads to
the poor performance of L0.38 in terms of compressive strength.
4. Conclusions
In this paper, alkali-activated GGBS/fly ash cement was prepared by mixing the
precursor with solid powder sodium silicate, followed by wet mixing. The experimental
results are similar to those of Nematollahi et al. [33]. When the mass ratio of GGBS/fly ash
in the precursor is 1/3, the cement prepared with sodium silicate powder (Na2SiO3-
Anhydrou activator in powder form) as the activator is about 35 MPa at 28 days. However,
as the main purpose of this study was to explore the advantages and disadvantages of
different types of solid sodium silicate powder, there is a lack of research on the
mechanisms involved in influencing the cement performance. This paper aimed to study
Figure 9. TG analysis of group P14, P41, S110 and L0.38.
The mass loss of each sample between 105~300
◦
C was observed, and the dehydration
decomposition of C-A-S-H gel occurred mainly in this temperature range. It can be found
that the mass loss of P14 was significantly lower than that of the other three groups, mainly
because the content of C-A-S-H gel products in group P14 was less, which was explained by
the lack of Ca
2+
[
47
]. The mass loss of L0.38 is more than that of P41 and S110, which is
consistent with the XRD analysis, demonstrating that the peak strength of group L0.38 at

Coatings 2023,13, 696 12 of 14
29.5
◦
is higher than that of P41 and S110. Although group L0.38 has the largest amount of
C-A-S-H gel, its high liquid–solid ratio also leads to the matrix loosening, which leads to
the poor performance of L0.38 in terms of compressive strength.
4. Conclusions
In this paper, alkali-activated GGBS/fly ash cement was prepared by mixing the
precursor with solid powder sodium silicate, followed by wet mixing. The experimental
results are similar to those of Nematollahi et al. [
33
]. When the mass ratio of GGBS/fly
ash in the precursor is 1/3, the cement prepared with sodium silicate powder (Na
2
SiO
3
-
Anhydrou activator in powder form) as the activator is about 35 MPa at 28 days. However,
as the main purpose of this study was to explore the advantages and disadvantages of
different types of solid sodium silicate powder, there is a lack of research on the mechanisms
involved in influencing the cement performance. This paper aimed to study the influence of
the mass ratio of GGBS/fly ash in the precursor, and the dosage of sodium silicate and the
liquid–solid ratio on the working performance and mechanical strength of alkali-activated
cement. In addition, the type, the structure and the quantity of hydration products were
analyzed using XRD, TG/DTG and FTIR. The following conclusions were obtained:
1.
A higher proportion of GGBS in the precursor contributes to a shorter setting time
and a higher compressive strength, which also induces a decrease in the fluidity. It is
recommended that the proportion of GGBS in the precursor is higher than 60%;
2.
A dosage of sodium silicate within the range of 50~110 g leads to the setting time
shortening, which is followed by the promotion and extension of the compressive
strength promotion, and then a decline. Excessive or insufficient sodium silicate
leads to the inhibition of alkali-activated reactions. On one hand, the dissolution
of the precursor will be hindered; on the other hand, the formation of hydration
products will be limited and there will be different optimal amounts for different
precursor components. We should consider both factors in order to obtain the required
cement properties;
3.
The performance of alkali-activated cement can be adjusted by changing the liquid–
solid ratio. Within the range of 0.20~0.38, a higher liquid–solid ratio extends the setting
time and improves the fluidity of the slurry, but decreases the compressive strength;
4.
The microscopic test shows that the main hydration product is C-A-S-H gel and that the
secondary product is hydrotalcite for the mix whose precursor is mainly GGBS. The
difference between them is the amount of hydration product, which can be reflected
in the mass loss in TG/DTG.
Author Contributions:
Methodology, Data curation, Writing—original draft, Writing—review and
editing, T.D.; Funding acquisition, Supervision, T.S.; Project administration, Resources, F.X.; Data
curation, G.O.; Data curation, H.W.; Data curation, F.Y.; Visualization, Z.W. All authors have read and
agreed to the published version of the manuscript.
Funding:
The authors gratefully acknowledge the financial support from the National Key R&D
Program of China (No. 2021YFC3100805), the Third Batch of Special Fund for Science and Tech-
nology Development of Zhongshan City in 2020 (2020-18), the Sanya Yazhou Bay Science and
Technology City Administration (No. SKJC-KJ-2019KY02), Shandong Province Enterprise Tech-
nology Innovation Project (202160101791), Fundamental Research Funds for the Central Univer-
sities (Nos. 2020III042 and 205259003), major science and technology project in Zhongshan city
(
No. 200825093739256
), and China Scholarship Council, Key Research and Development Program of
Hubei Province (2020BAB065), Hubei Key Laboratory of Roadway Bridge and Structure Engineer-
ing (No. DQJJ201902), the financial support from the CECSC Key Laboratory of Civil Engineering
Materials-Industrial solid waste utilization (CSCEC-PT-002), Study on design and preparation of high
performance concrete using Guangdong granite-based manufacture sand (CSCEC4B-2021-KTA-11),
and China Scholarship Council.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Coatings 2023,13, 696 13 of 14
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
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