applied
sciences
Article
Impact of Rice Husk Ash on the Mechanical Characteristics and
Freeze–Thaw Resistance of Recycled Aggregate Concrete
Wei Zhang, Huawei Liu and Chao Liu *
College of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
* Correspondence: chaoliu@xauat.edu.cn
Citation: Zhang, W.; Liu, H.; Liu, C.
Impact of Rice Husk Ash on the
Mechanical Characteristics and
Abstract: With the accelerating growth of infrastructure construction, carbon emission and environmental pollution problems have become increasingly severe. In order to promote the sustainable
development of the construction industry, using rice husk ash (RHA) in recycled aggregate concrete
has aroused extensive interest. This study aims to investigate the impact of the partial replacement
(0%, 10%, 20%, 30% of binder) of ordinary Portland cement (OPC) with RHA by equal mass on recycled concrete’s mechanical characteristics and freeze–thaw resistance. The workability, compressive
strength, mass loss and dynamic elastic modulus of recycled concrete were tested, and the hydration
products and microstructure were analyzed using scanning electron microscope (SEM) tests. The
mechanism of the freeze–thaw damage deterioration of RHA recycled aggregate concrete was revealed. The results indicate that the incorporation of RHA has an adverse effect on the workability
of fresh concrete. Its high specific surface area will provide a large number of nucleation sites for
the hydration reaction, refining the pore structure in the paste and improving the weak bonding of
the interfacial transition zone (ITZ) by enhancing the matrix’s pozzolanic reaction effect and filling
effect, thus improving the compressive strength of concrete. Furthermore, the porous structure of
the recycled aggregate attached mortar and mesoporous RHA will absorb a lot of water during the
freeze–thaw cycles. With the continuous accumulation of expansion pressure, the interior pores and
cracks will gradually expand and extend, leading to more severe damage to the concrete, and the
degree of freeze–thaw damage deterioration grows as the RHA replacement ratios increase.
Freeze–Thaw Resistance of Recycled
Aggregate Concrete. Appl. Sci. 2022,
12, 12238. https://doi.org/10.3390/
Keywords: rice husk ash; recycled aggregate concrete; mechanical characteristics; freeze–thaw
resistance; deterioration mechanism
app122312238
Academic Editor: Laurent
Daudeville
Received: 9 November 2022
Accepted: 27 November 2022
Published: 29 November 2022
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4.0/).
1. Introduction
With the continuous development of the construction industry, cement production has
grown dramatically, resulting in increasingly aggravated carbon emissions and environmental pollution [1]. The main sources of carbon emissions in cement production are the
chemical reaction of clinker components and the combustion of fossil fuels [2]. Based on the
above two points, it is estimated that the total emissions from the cement industry are about
8% of global carbon emissions [3]. The partial replacement of cement with low-energy
industrial and agricultural solid waste can reverse the rising carbon emissions trend, thus
achieving energy efficiency and environmental protection [4–6]. Although using industrial
solid wastes such as silica fume (SF), fly ash (FA) and ground granulated blast furnace slag
(GGBFS) in concrete has the potential to reduce carbon dioxide emissions and contribute to
the idea of sustainable development in the cement sector, their sources are limited by geographical conditions, technological conditions and economic costs [7,8]. Therefore, using
agricultural solid wastes such as rice husk ash (RHA) in concrete has aroused extensive
interest [9,10].
The rice husk is a broad source of agricultural waste collected during the grinding
process from the surface layer of rice grains. In the field of green technology, rice husks
are mainly used as fuel for processing rice grains, as boiler steam or as fuel for producing
Appl. Sci. 2022, 12, 12238. https://doi.org/10.3390/app122312238
https://www.mdpi.com/journal/applsci
Appl. Sci. 2022, 12, 12238
2 of 15
electricity in biomass power plants [11,12]. RHA is an agricultural residue generated by the
incineration of rice husk [13]. Because of its high amorphous silica concentration, specific
surface area and pozzolanic activity, it is regarded by many researchers as a cementitious
material for increasing the properties of concrete [14–16].
Recycled concrete aggregate is an environmentally friendly material prepared from
construction and demolition waste (CDW) by sorting, crushing, sieving and cleaning,
which can be used in the construction sector to alleviate the gradually decreasing natural
aggregate and solve the problem of the piling and disposal of construction solid waste,
which is conducive to sustainable development [17,18]. Currently, the use of supplementary
cementitious materials and recycled concrete aggregate for concrete preparation in the
construction industry has attracted widespread attention.
Therefore, many researchers have studied the mechanical characteristics and
durability of concrete prepared synergistically from RHA and recycled aggregate.
Qureshi et al. [19] studied the coupling effect of hook-ended steel fibers (HSF) and 15%
RHA on the mechanical properties and durability of RAC. The results revealed that the joint
action of RHA and fibers significantly enhanced the compressive strength, splitting tensile
strength, impermeability and acid attack resistance (AAR) of RAC. Rattanachu et al. [20]
evaluated recycled concrete’s mechanical properties, chloride penetration depth and steel
corrosion using ground RHA. The findings revealed that using RHA had adversely affected
the compressive strength of RAC, while greatly improving steel corrosion and chloride
resistance. Alyousef et al. [5] studied the effect of waste mineral admixtures and glass fibers
on the mechanical and permeability properties of recycled aggregate concrete. The results
showed that the combined action of RHA and glass fibers resulted in recycled aggregate
concrete exhibiting improved mechanical and durability properties compared to natural
aggregate concrete. Nuaklong et al. [21] investigated the influence of RHA replacing silica
nanoparticles on the mechanical and fire resistance properties of recycled concrete. The
results showed that the addition of rice husk ash could lead to enhanced mechanical properties of recycled concrete by improving the microstructure of the paste, but RHA will
adversely affect the post-fire residual strength of recycled concrete. Padhi et al. [22] investigated the engineering application effect of 0–35% RHA and recycled aggregate to prepare
concrete. The findings revealed that incorporating RHA and recycled aggregate greatly
enhanced the performance of concrete. When the replacement ratio of RHA and recycled
coarse aggregate was 10–15% and 100%, respectively, the concrete mix ratio satisfied the
engineering application requirements. Koushkbaghi et al. [23] investigated the impact
of RHA and fibers on the mechanical properties and acid attack performance of recycled
concrete. The results revealed that RHA could enhance the inferior properties of concrete
by enhancing the bond between concrete and fibers, and the improvement effect increased
with the rise in the RHA replacement ratio. Salahuddin et al. [24] studied the influence of
temperature increases on RHA recycled concrete performance. The results showed that
a 20% replacement ratio of RHA and a 100% substitution rate of recycled concrete aggregate resulted in a reduction of the compressive strength of concrete. Alnahhal et al. [25]
investigated the influence of the partial replacement of cement by RHA on the engineering
performance and environmental effects of recycled concrete. The results indicated that
the partial replacement cement with RHA not only improved the performance of recycled
concrete but also reduced carbon dioxide emissions during the cement production.
Although previous studies have investigated the impact of RHA on various mechanical characteristics as well as the durability of recycled concrete, a mechanism analysis of
the impact of RHA on the freeze–thaw resistance performance of recycled concrete has not
been reported yet. Therefore, it is of great significance to carry out research on the frost
resistance of RHA recycled concrete. In view of this, this research aims to investigate the
impact of partially replacing Portland cement with RHA by equal mass on recycled concrete’s mechanical characteristics and freeze–thaw resistance. The workability, compressive
strength, mass loss and dynamic elastic modulus of recycled concrete were determined. In
addition, the paste’s hydration products and microscopic morphology were analyzed using
Appl. Sci. 2022, 12, 12238
3 of 15
scanning electron microscope (SEM) tests. The deterioration mechanism of the freeze–thaw
damage of RHA recycled concrete was revealed. The outcome of this research is expected
to promote the application of RHA recycled concrete in severe cold regions.
2. Materials and Methods
2.1. Raw Materials
Appl. Sci. 2022, 12, 12238
In this paper, the Shieldstone P.O 42.5 ordinary Portland cement (OPC) and untreated
residual RHA were used as the cementitious materials, and the RHA was sourced from a
biomass power factory in Nanjing. The microstructure of RHA is shown in Figure 1, which
shows a honeycomb structure. The physicochemical characteristics of the cementitious
materials are shown in Table 1, and it can be seen that the specific surface area of RHA is
approximately 5–6 times that of cement, and its internal silica content is as high as 92.1%.
4 of 16
The particle size distributions of the cement and RHA are shown in Figure
2, which are
14.77 µm and 14.81 µm, respectively.
Figure 1.1.Microstructure
of RHA
measured
by scanning
electron electron
microscopy.
Figure
Microstructure
of RHA
measured
by scanning
microscopy.
Table 1. The physicochemical characteristics of the cementitious materials used.
Cementitious Materials
Characteristics
Cement
RHA
Physical properties
Mean particle size (µm)
14.77
14.81
1.72
8.60
Specific surface area (m2 /g)
Setting time (min)
Initial time
202
Final time
258
Chemical composition (%)
Silicon dioxide (SiO2 )
21.07
92.08
Ferric oxide (Fe2 O3 )
3.78
0.05
Aluminium oxide (Al2 O3 )
4.91
0.11
Potassium oxide (K2 O)
0.59
3.50
Magnesium oxide (MgO)
1.43
0.49
Calcium oxide (CaO)
65.36
0.78
Sodium oxide (Na2 O)
0.39
Phosphorous oxide (P2 O5 )
1.35
FigureSulfur
2. The particle
size
distributions
of cement, RHA,
river sand and RCA.
trioxide
(SO
2.45
0.92
3)
Loss on ignition
4.52
1.83
The fine aggregate was river sand from the Shaanxi region, with a water absorption
of 3%, a maximum particle size of 3 mm and an apparent density of 2640 kg/m3. The coarse
aggregate was prepared from crushed and manufactured recycled concrete. The basic performance indexes were measured following GB/T 14685-2011 [26], as shown in Table 2.
The mixing water was tap water, and the superplasticizer was the PCA polycarboxylic
superplasticizer produced by Subot New Materials Co., Ltd. (Nanjing, China).
Appl. Sci. 2022, 12, 12238
4 of 15
Figure 1. Microstructure of RHA measured by scanning electron microscopy.
Figure2.2. The
The particle
particle size
RHA,
river
sand
andand
RCA.
Figure
sizedistributions
distributionsofofcement,
cement,
RHA,
river
sand
RCA.
Thefine
fine aggregate
aggregate was river sand
The
sand from
from the
theShaanxi
Shaanxiregion,
region,with
witha awater
waterabsorption
absorption of
3 . The
3. The
of 3%,
a maximum
particlesize
sizeofof33mm
mm and
and an apparent
2640
kg/m
coarse
3%,
a maximum
particle
apparentdensity
densityofof
2640
kg/m
coarse
aggregate was
crushed
andand
manufactured
recycled
concrete.
The basicThe
per-basic
aggregate
wasprepared
preparedfrom
from
crushed
manufactured
recycled
concrete.
formance indexes
were
measured
following
GB/T
14685-2011
[26], [26],
as shown
in Table
2. 2.
performance
indexes
were
measured
following
GB/T
14685-2011
as shown
in Table
The
mixing
water
was
tap
water,
and
the
superplasticizer
was
the
PCA
polycarboxylic
The mixing water was tap water, and the superplasticizer was the PCA polycarboxylic
superplasticizer produced
produced by
Co.,
Ltd.
(Nanjing,
China).
superplasticizer
bySubot
SubotNew
NewMaterials
Materials
Co.,
Ltd.
(Nanjing,
China).
Table 2. The basic properties indexes of recycled concrete aggregate (RCA).
Aggregate Type
Grading
(mm)
Water Absorption
(%)
Apparent Density
(kg/m3 )
Bulk Density
(kg/m3 )
Crush Index
(%)
RCA
5–25
3.8
2458.0
1430.0
17.0
2.2. Mixture Proportions and Specimen Preparation
The concrete design mix is shown in Table 3, in which RHA replaced cement with
0%, 10%, 20% and 30% replacement ratios, respectively. The designation of the specimens
was sourced from the replacement ratios of RHA. An effective water-to-binder ratio of
0.4 was maintained by adjusting the amount of additional water to the system owing to the
high water absorption of the recycled coarse aggregate. A target slump of 100–150 mm was
kept for the fresh concrete by adjusting the superplasticizer amount due to the RHA’s high
specific surface area [20].
Table 3. The mixture proportion of the RHA recycled concrete mixtures.
Mixture Proportion (kg/m3 )
Specimen
W/B
0% RHA
10% RHA
20% RHA
30% RHA
0.40
0.40
0.40
0.40
Cement
RHA
Sand
RCA
Water
AW
SP
410
369
328
287
41
82
123
825
825
825
825
1085
1085
1085
1085
164
164
164
164
22
22
22
22
8.5
9.5
11.5
12.0
Notes: W/B represents the water-to-binder ratio. AW represents the additional water amount. SP represents the
superplasticizer amount.
carried out. The mass loss and relative dynamic elastic modulus were also used to assess
the degree of the macroscopic performance deterioration of concrete [31], which was calculated as follows:
∆𝑚 =
Appl. Sci. 2022, 12, 12238
× 100%,
(1)
5 of 15
where ∆𝑚 is the mass loss; ∆𝑚 and 𝑚 are the masses before and after n freeze–thaw
cycles, respectively.
× dry
100%,
𝐸 , = the
(2)
In order to prepare concrete specimens,
solid components were taken following
the mass in the mixture proportion, which was subsequently dumped into a blender and
where 𝐸 , is the relative dynamic elastic modulus; 𝐸 and 𝐸 are the dynamic elastic
mixed evenly for 2 min. Then, superplasticizer and water were used, and the mixture
moduli of concrete before and after n freeze–thaw cycles, respectively, and the measuring
was cast into cubic specimens for mechanical characteristics testing after mixing uniformly.
instrument is the NELD-DTV dynamic elastic modulus testing machine.
Finally, the fresh concrete mixtures were loaded into the mold and vibrated on a table
for compacting. The demolding and curing of the specimens were conducted following
2.3.4. SEM Test
GB/T 50081-2019 [27].
The small pieces of paste samples with different RHA replacement ratios were immersed
in ethanolMethod
solution to prevent further hydration [32], dried in a GZX-9030MBE
2.3. Experimental
drying oven, coated with conductive tape and pasted on the specimens, and they were
In this paper, the impact of RHA on recycled concrete’s mechanical characteristics and
subsequently subjected to spray gold treatment using a Cressington 108 coater. The specfreeze–thaw resistance was investigated using experiments, and the hydration products
imens were analyzed by a Gemini SEM 500 field emission scanning electron microscope
and microstructure were analyzed in combination with scanning electron microscopy (SEM)
with a resolution of 0.5 nm, an acceleration pressure of 0.02 KV to 30 KV, a probe-current
experiments. The experimental procedure is presented in Figure 3, and the details of the
of 3 pA to 20 nA and a magnification of 50 X to 20,000 KX.
experiments are given below.
Figure3.3.Experimental
Experimentalprocedure.
procedure.
Figure
2.3.1. Workability
3. Results and Discussions
According to GB/T 50080-2016 [28], the slump cone experiment was performed to
3.1. Workability of RHA Recycled Aggregate Concrete
evaluate the workability of fresh concrete. The slump value is measured in mm, the
The workability
of fresh to
concrete
mixtures
be reflected
by the
value
measurements
were accurate
1 mm and
the testcan
results
were revised
to slump
5 mm [29].
[33,34]. The impact of RHA with different replacement ratios on the workability of recycled
concrete
was investigated by means of a slump cone test, and the results
2.3.2.aggregate
Mechanical
Characteristics
are shown in Figure 4. It is noteworthy that 0% RHA and 10% RHA showed the same
According to GB/T 50081-2019 [27], the compressive strength of concrete specimens
slump value, which may be due to the increase in the superplasticizer (Table 3), resulting
with dimensions of 100 mm × 100 mm × 100 mm cubic specimens was tested, and the
in improved concrete workability. However, the improvement degree of the superplastiYAW-3000 universal loading machine was used. The test ages for strength were 7d, 28d,
cizer is similar to the reduction degree caused by the increase in RHA content. At the same
90d and 360d, and the experimental value is the average of three specimens.
2.3.3. Freeze–Thaw Cycling
According to GB/T 50082-2009 [30], the concrete specimens were put into the TDR-28
concrete freeze–thaw cycle testing machine for a rapid freeze–thaw cycle experiment. The
freeze–thaw medium was tap water supplied by the laboratory, and the control system was
suspended after reaching 50, 100, 150, 200, 250 and 300 cycles. The surface water of the
specimens was dried, followed by mass and dynamic elastic modulus tests being carried
out. The mass loss and relative dynamic elastic modulus were also used to assess the
degree of the macroscopic performance deterioration of concrete [31], which was calculated
as follows:
m0 − m n
∆mn =
× 100%
(1)
m0
Appl. Sci. 2022, 12, 12238
6 of 15
where ∆mn is the mass loss; ∆mn and mn are the masses before and after n freeze–thaw
cycles, respectively.
E
Er,n = dn × 100%
(2)
Ed0
where Er,n is the relative dynamic elastic modulus; Ed0 and Edn are the dynamic elastic
moduli of concrete before and after n freeze–thaw cycles, respectively, and the measuring
instrument is the NELD-DTV dynamic elastic modulus testing machine.
2.3.4. SEM Test
The small pieces of paste samples with different RHA replacement ratios were immersed in ethanol solution to prevent further hydration [32], dried in a GZX-9030MBE
drying oven, coated with conductive tape and pasted on the specimens, and they were subsequently subjected to spray gold treatment using a Cressington 108 coater. The specimens
were analyzed by a Gemini SEM 500 field emission scanning electron microscope with a
resolution of 0.5 nm, an acceleration pressure of 0.02 KV to 30 KV, a probe-current of 3 pA
to 20 nA and a magnification of 50 X to 20,000 KX.
3. Results and Discussions
3.1. Workability of RHA Recycled Aggregate Concrete
Appl. Sci. 2022, 12, 12238
The workability of fresh concrete mixtures can be reflected by the slump value [33,34].
The impact of RHA with different replacement ratios on the workability of recycled aggregate concrete was investigated by means of a slump cone test, and the results are shown
7 of 16value,
in Figure 4. It is noteworthy that 0% RHA and 10% RHA showed the same slump
which may be due to the increase in the superplasticizer (Table 3), resulting in improved
concrete workability. However, the improvement degree of the superplasticizer is similar
to
theitreduction
degree
caused by
the slump
increase
in RHA
thesame
sameworkabiltime, it is due
time,
is due to the
modification
of the
value,
thus content.
resulting At
in the
to
the
modification
of
the
slump
value,
thus
resulting
in
the
same
workability
[35].
ity [35].
Figure 4. The impact of RHA replacement ratios on the workability of fresh concrete mixtures.
Figure 4. The impact of RHA replacement ratios on the workability of fresh concrete mixtures.
With
the
RHA
substitution
rate,rate,
the the
slump
value
of 20%
With the
thegradual
gradualincrease
increaseinin
the
RHA
substitution
slump
value
of RHA
20% RHA
and 30% RHA decreased by 15.38% and 23.08%, respectively, compared with 0% RHA.
and 30% RHA decreased by 15.38% and 23.08%, respectively, compared with 0% RHA.
This may be due to the increase in the RHA replacement ratio, leading to an increase in
This may be due to the increase in the RHA replacement ratio, leading to an increase in the
the porosity and total specific surface area of the cementitious material (Table 1). The sysporosity and total specific surface area of the cementitious material (Table 1). The system
tem absorbs more water, thus causing a reduction in the free water within the paste, which
absorbs more water, thus causing a reduction in the free water within the paste, which will
will increase the mixtures’ friction and discourage the flow, which in turn leads to the
increase the mixtures’ friction and discourage the flow, which in turn leads to the decrease
decrease in the concrete slump and reduced workability. This finding is similar to that of
in
the et
concrete
and reduced
This
finding
is similar
to thatwith
of Amin
Amin
al. [15], slump
who reported
that theworkability.
slump of fresh
concrete
mixtures
increased
et
al.
[15],
who
reported
that
the
slump
of
fresh
concrete
mixtures
increased
with
increasing
increasing RHA content, and the mixtures with 30% RHA content had the worst workaRHA
bility. content, and the mixtures with 30% RHA content had the worst workability.
3.2. Compressive Strength of RHA Recycled Aggregate Concrete
The impact of RHA with different replacement ratios on the compressive strength of
recycled concrete is shown in Figure 5. As shown in the figure, the compressive strength
of the recycled concrete gradually increased as the hydration reaction continued, and the
Appl. Sci. 2022, 12, 12238
7 of 15
3.2. Compressive Strength of RHA Recycled Aggregate Concrete
Appl. Sci. 2022, 12, 12238
The impact of RHA with different replacement ratios on the compressive strength of
recycled concrete is shown in Figure 5. As shown in the figure, the compressive strength
of the recycled concrete gradually increased as the hydration reaction continued, and the
growth rate shows a trend of first being rapid, then slowing down and finally gradually
stabilizing. For instance, the compressive strength of 30% RHA at 7 days, 28 days, 90 days
and 360 days was 33.8 MPa, 42.9 MPa, 46.4 MPa and 48.3 MPa, which increased by 69.98%,
8 of 16
18.84%, 7.25% and 3.94%, respectively. The variation in recycled concrete’s compressive
strength may be because, in the initial stage of curing, the strength growth of the concrete is
mainly due to the RHA particle’s filling action. The fine RHA particles can fill the pores and
microcracks
of thegrowth
paste and
interface
transition
zone,ages
reducing
theby
porosity
to a certain
[38]. The strength
of concrete
at different
curing
is caused
a combination
extent
and increasing
the
bulk density
of the
concrete,
significantly
improving
of the cement
hydration
reaction,
the filling
effect
and thethereby
pozzolanic
effect of RHA
parti- the
early
strength of concrete.
cles [39,40].
Figure 5. The impact of RHA replacement ratios on the compressive strength of recycled concrete.
Figure
5. The impact of RHA replacement ratios on the compressive strength of recycled concrete.
Thecontrast,
compressive
recycledofconcrete
shows
a tendency
first increase
In
the strength
strengthofgrowth
recycled
concrete
in thetolater
stages ofand
curing
then
decrease
as
the
RHA
replacement
ratio
increases.
For
instance,
the
compressive
is primarily due to the RHA particle’s pozzolanic effect [36]. Although the pozzolanic
strength between
of 10% RHA,
20%
RHA
30% RHA
at 360 days
52.8 MPa
andpaste
reaction
RHA
and
freeand
calcium
hydroxide
(CH)was
led51.9
to aMPa,
reduction
in the
48.3 MPa and 102.17%, 103.94% and 95.08% of 0% RHA, respectively. This is probably
porosity [37], the existence of the porous structure of the recycled aggregate attached
because the high silica content and high specific surface area of RHA particles, which can
mortar led to the absorption of large amounts of water by the system and a reduction in
contact and react with more CH and water, produce extra C-S-H gels to improve the mathe free water content of the matrix, resulting in a weakening of the pozzolanic effect of
trix compactness, thus leading to the improvement of concrete compressive strength.
the RHA particles, thus causing a reduction in the total volume of hydration products,
However, when the strength loss due to the decrease in cement content was more signifiwhich
leads to a slow strength growth of the concrete in the later curing period. This is
cant than the compressive strength increment contributed by the RHA filling effect and
similar
to the
results
Salas
the later
variation
of RHAofnatural
pozzolanic
effect,
thisof
will
leadettoal.a on
reduction
in strength
the compressive
strength
concreteaggregate
at a
concrete
[38].
The
strength
growth
of
concrete
at
different
curing
ages
is caused
high RHA replacement ratio. According to the variation in the compressive strength
of by a
combination
of
the
cement
hydration
reaction,
the
filling
effect
and
the
pozzolanic
effect
concrete with an RHA replacement ratio, it is clear that the optimum replacement ratio of of
RHA
[39,40].
RHA particles
is 20%, which
is similar to the report of RHA natural aggregate concrete. For inThe
compressive
of recycled
shows
a tendency
to first
increase
stance, He et al. [32] andstrength
Chao-Lung
et al. [41]concrete
found that
the concrete
specimens
with
a
and
then
decrease
as
the
RHA
replacement
ratio
increases.
For
instance,
the
compressive
20% RHA replacement ratio resulted in the highest paste compactness due to the joint
strength
RHA,
and 30%
RHA
360 daysinwas
51.9 MPa,
52.8 MPa and
action of of
the10%
filling
effect20%
andRHA
pozzolanic
effect,
thusatresulting
the optimum
mechanical
48.3
MPa and of
102.17%,
characteristics
concrete.103.94% and 95.08% of 0% RHA, respectively. This is probably
because the high silica content and high specific surface area of RHA particles, which can
3.3. Mass
Loss
of RHA
Aggregate
Concrete
contact
and
react
withRecycled
more CH
and water,
produce extra C-S-H gels to improve the matrix
compactness,
to the improvement
of concrete
strength.
However,
The massthus
lossleading
is an essential
indicator representing
thecompressive
damage degree
of surface
when
theofstrength
due to subjected
the decrease
in cement content
wasFigure
more 6significant
spalling
concreteloss
specimens
to freeze–thaw
cycles [42].
shows the than
the
compressive
strength
increment
contributed
by theRHA
RHAreplacement
filling effect
and during
pozzolanic
variation
in recycled
concrete’s
mass loss
with different
ratios
the freeze–thaw
cycle.
shown in in
thethe
figure,
the mass strength
loss tends
decreaseatwith
theRHA
effect,
this will lead
to As
a reduction
compressive
oftoconcrete
a high
increase
in
the
number
of
cycles
at
the
initial
stage
of
the
freeze–thaw
cycle,
and
the
re- with
replacement ratio. According to the variation in the compressive strength of concrete
duction amount increases with the rise in the RHA substitution rate. For instance, when
concrete specimens were subjected to 100 freeze–thaw cycles, the mass losses of concrete
with 0% RHA, 10% RHA, 20% RHA and 30% RHA were −0.48%, −0.71%, −0.73% and
−0.83%, respectively. This could be attributed to the existence of the porous structure of
the recycled aggregate attached mortar, which resulted in more external water transfer to
Appl. Sci. 2022, 12, 12238
8 of 15
an RHA replacement ratio, it is clear that the optimum replacement ratio of RHA is 20%,
which is similar to the report of RHA natural aggregate concrete. For instance, He et al. [32]
and Chao-Lung et al. [41] found that the concrete specimens with a 20% RHA replacement
ratio resulted in the highest paste compactness due to the joint action of the filling effect
and pozzolanic effect, thus resulting in the optimum mechanical characteristics of concrete.
3.3. Mass Loss of RHA Recycled Aggregate Concrete
Appl. Sci. 2022, 12, 12238
The mass loss is an essential indicator representing the damage degree of surface
spalling of concrete specimens subjected to freeze–thaw cycles [42]. Figure 6 shows the
variation in recycled concrete’s mass loss with different RHA replacement ratios during
the freeze–thaw cycle. As shown in the figure, the mass loss tends to decrease with the
increase in the number of cycles at the initial stage of the freeze–thaw cycle, and the
reduction amount increases with the rise in the RHA substitution rate. For instance, when
concrete specimens were subjected to 100 freeze–thaw cycles, the mass losses of concrete
with 0% RHA, 10% RHA, 20% RHA and 30% RHA were −0.48%, −0.71%, −0.73% and
−0.83%, respectively. This could be attributed to the existence of the porous structure of
the recycled aggregate attached mortar, which resulted in more external water transfer to
the internal pores and cracks. In addition, the high porosity of RHA particles will enhance
9 of 16
the permeability of the concrete; thus, it will also cause a large amount of water absorption.
The synergistic effect of both will reduce the mass loss of the concrete.
Figure6.6.The
Therelationship
relationshipbetween
betweenthe
themass
massloss
lossof
ofrecycled
recycledconcrete
concrete and
and the
the number
number of
of freeze–thaw
freeze–
Figure
thaw
cycles
with
different
RHA
replacement
ratios.
cycles with different RHA replacement ratios.
However,the
the water
water absorbed
will
lead
to concrete
freeze–thaw
However,
absorbedby
bythe
theRHA
RHAparticles
particles
will
lead
to concrete
freeze–thaw
damage
due
to
icing
pressure
and
expansion
pressure
with
the
increase
in
the
number
of of
damage due to icing pressure and expansion pressure with the increase in the
number
freeze–thaw
cycles.
During
the
freeze–thaw
cycle,
the
original
pore
structure
continuously
freeze–thaw cycles. During the freeze–thaw cycle, the original pore structure continuously
expands into visible pores and sprouts to generate microcracks. It then gradually develops
expands
into visible pores and sprouts to generate microcracks. It then gradually develops
into macrocracks under repeated expansion pressure, which eventually leads to spalling
into macrocracks under repeated expansion pressure, which eventually leads to spalling
damage and aggregate loss on the concrete surface. When the mass loss of concrete caused
damage and aggregate loss on the concrete surface. When the mass loss of concrete caused
by surface damage is greater than the mass increment generated by the absorbed water of
by surface damage is greater than the mass increment generated by the absorbed water of
the concrete specimens, the mass of concrete will decrease, and the mass loss will increase.
the concrete specimens, the mass of concrete will decrease, and the mass loss will increase.
According to GB/T 50082-2009 [30], if the mass loss exceeds 5% as the basis of concrete
According
to GB/T 50082-2009 [30], if the mass loss exceeds 5% as the basis of concrete
freeze–thaw resistance damage, the concrete specimens with 10% RHA and 20% RHA lose
freeze–thaw
resistance
damage,
specimens
with 10%
RHA and
their freeze–thaw
resistance
afterthe
300concrete
cycles, while
the concrete
specimens
with20%
30%RHA
RHA lose
their
resistance
afterafter
300 only
cycles,
the concrete
lose freeze–thaw
their freeze–thaw
resistance
250while
freeze–thaw
cycles.specimens with 30% RHA
lose their freeze–thaw resistance after only 250 freeze–thaw cycles.
3.4. Relative Dynamic Elastic Modulus of RHA Recycled Aggregate Concrete
The stiffness of concrete specimens subjected to the freeze–thaw cycles can be characterized by the variation in the relative dynamic elastic modulus [43]. Figure 7 shows the
variation in the recycled concrete’s relative dynamic elastic modulus with different RHA
replacement ratios during the freeze–thaw cycles. As shown in the figure, the relative dynamic elastic modulus of concrete decreases continuously with an increase in the number
of freeze–thaw cycles. Specifically, the relative dynamic elastic modulus decreases slowly
Appl. Sci. 2022, 12, 12238
9 of 15
3.4. Relative Dynamic Elastic Modulus of RHA Recycled Aggregate Concrete
Appl. Sci. 2022, 12, 12238
The stiffness of concrete specimens subjected to the freeze–thaw cycles can be characterized by the variation in the relative dynamic elastic modulus [43]. Figure 7 shows
the variation in the recycled concrete’s relative dynamic elastic modulus with different
RHA replacement ratios during the freeze–thaw cycles. As shown in the figure, the relative
dynamic elastic modulus of concrete decreases continuously with an increase in the number
of freeze–thaw cycles. Specifically, the relative dynamic elastic modulus decreases slowly
in the initial stage of the cycle, while it decreases rapidly in the later stage of the cycle,
indicating that the internal damage of the concrete gradually increases, subjected to the
freeze–thaw cycle. The variation in the relative dynamic elastic modulus is related to the
number of cracks and pores in the concrete itself, as there are many unfavorable interfacial
transition zones within the recycled concrete aggregate, thus resulting in a lower initial
dynamic modulus of elasticity for the concrete. The higher the initial damage degree, the
easier it is for the specimens to form new cracks [44]. Under the freeze–thaw cycling, the
expansion and icing pressure of pore water inside the specimen will lead to numerous new
pores and new microcracks in the unfavorable interfacial transition zone. The presence of
10 of 16
these discontinuities in the matrix increases the propagation time of the ultrasonic
pulse
velocity, which leads to a decrease in the relative dynamic elastic modulus.
Figure7.7. The
The relationship
relationship between
between the
Figure
the relative
relative dynamic
dynamic elastic
elasticmodulus
modulusofofrecycled
recycledconcrete
concreteand
and the
the number of freeze–thaw cycles with different RHA replacement ratios.
number of freeze–thaw cycles with different RHA replacement ratios.
In addition,
addition, the
relative
dynamic
elastic
modulus
increased
In
the decreasing
decreasingdegree
degreeininthe
the
relative
dynamic
elastic
modulus
increased
with
the
increase
in
the
RHA
replacement
ratio.
This
is
probably
because
the
addition
of of
with the increase in the RHA replacement ratio. This is probably because the addition
RHA can enhance the pore structure within the concrete, reducing the freezing point and
RHA can enhance the pore structure within the concrete, reducing the freezing point and
pore connectivity, retarding the crack generation and water migration between the pores
pore connectivity, retarding the crack generation and water migration between the pores
inside the concrete. However, the introduction of the RHA mesoporous structure will lead
inside the concrete. However, the introduction of the RHA mesoporous structure will lead
to the expansion and extension of the pore structure under the action of expansion presto the expansion and extension of the pore structure under the action of expansion pressure,
sure, resulting in the deterioration of the pore structure, which will adversely affect the
resulting in the deterioration of the pore structure, which will adversely affect the concrete’s
concrete’s freeze–thaw resistance, thus resulting in the freeze–thaw damage of concrete
freeze–thaw
resistance,
thus
resulting
in the freeze–thaw
damagethe
of reduced
concretecement
specimens
specimens with
20% RHA
after
300 freeze–thaw
cycles. In addition,
with
20%
RHA
after
300
freeze–thaw
cycles.
In
addition,
the
reduced
cement
content
content at a high RHA replacement rate resulted in the poor filling of paste pores, thus at a
high
RHA
resulted
in the poor
of paste
thus
leading to the
leading
to replacement
the damage ofrate
concrete
specimens
withfilling
30% RHA
afterpores,
only 250
freeze–thaw
damage
of
concrete
specimens
with
30%
RHA
after
only
250
freeze–thaw
cycles.
cycles.
In
introducingRHA
RHAwill
will
reduce
concrete
freeze–thaw
resistance
In conclusion,
conclusion, introducing
reduce
thethe
concrete
freeze–thaw
resistance
com-compared
to
ordinary
concrete
(without
RHA),
and
the
decreasing
degree
increases
pared to ordinary concrete (without RHA), and the decreasing degree increases withwith
the the
increase
replacementrate.
rate.The
The
influence
mechanism
of the
RHA
substitution
increase in
in the
the RHA
RHA replacement
influence
mechanism
of the
RHA
substitution
rate on the freeze–thaw damage deterioration of recycled concrete will be analyzed in detail in Section 3.6.
3.5. Mechanistic Analysis of the Influence of RHA on the Microstructure of Recycled Aggregate
Concrete
Figure 8 shows the influence of different RHA replacement ratios on paste samples’
Appl. Sci. 2022, 12, 12238
10 of 15
rate on the freeze–thaw damage deterioration of recycled concrete will be analyzed in detail
in Section 3.6.
3.5. Mechanistic Analysis of the Influence of RHA on the Microstructure of Recycled
Aggregate Concrete
Appl. Sci. 2022, 12, 12238
Figure 8 shows the influence of different RHA replacement ratios on paste samples’ microstructure and hydration products before the freeze–thaw cycles. As shown in
Figure 8a, the 0% RHA paste is characterized by significant amounts of rod-shaped CH
crystals and numerous pores in the matrix, and the surface microstructure is relatively loose.
This is probably because the insufficient hydration of cement particles leads to a decrease
in the degree of paste compactness, which agrees with previous research results [45,46].
The 10% RHA paste is characterized by a dense paste without obvious pores in the matrix,
and cracks of significant width were found in the interfacial transition zone. The 20% RHA
paste is characterized by dense paste at different locations in the matrix, small microcracks
between the interfacial transition zone of the aggregate and mortar and no significant
11 of 16 CH
crystals in the paste. This may be due to the smaller particle size of RHA particles, which
can promote cement hydration due to their high specific surface, resulting in a pozzolanic
reaction
CH generated
by the cement
hydrationcompacting
and the amorphous
silica
the RHAbetween
to reach the
a threshold
value, leading
to the optimum
effect of the
gen-of the
RHA
to
reach
a
threshold
value,
leading
to
the
optimum
compacting
effect
of
the
generated
erated C-S-H gels on the paste pores and enhancing the bonding of the interfacial transiC-S-H
gelsreducing
on the paste
pores andofenhancing
the will
bonding
of thetointerfacial
transition
tion zone,
the formation
cracks, which
contribute
the mechanical
char-zone,
reducing
the
formation
of
cracks,
which
will
contribute
to
the
mechanical
characteristics
acteristics of concrete enhancement [47,48]. As shown in Figure 8d, the 30% RHA paste is of
concrete
enhancement
[47,48].
Asa shown
in Figure
8d, the 30%
RHA paste
is characterized
characterized
by large pores
and
large number
of randomly
distributed
needle-shaped
by
large
pores
and
a
large
number
of
randomly
distributed
needle-shaped
AFTs in the
AFTs in the matrix. This may be due to the lower cement content, which leads to a reduced
matrix.
This
may
be
due
to
the
lower
cement
content,
which
leads
to
a
reduced
CH content
CH content generated by cement hydration, which in turn leads to a weaker degree
of
generated
cement
hydration,
which intoturn
to a weaker
degree of
pozzolanic
pozzolanic by
reaction,
which
is not conducive
poreleads
compactness.
In conclusion,
the
compactness degree
samples
an increase followed
by a decrease
with an
reaction,
which of
is the
notpaste
conducive
toshowed
pore compactness.
In conclusion,
the compactness
increasing
RHA
replacement
ratio, which
corresponds
to its compressive
degree
of the
paste
samples showed
an increase
followed
by a decreasestrength
with anmagniincreasing
tude. replacement ratio, which corresponds to its compressive strength magnitude.
RHA
Figure 8.
8. Microscopic
Microscopic morphology
ratios
be-before
Figure
morphologyof
ofthe
thepaste
pastesamples
sampleswith
withdifferent
differentRHA
RHAreplacement
replacement
ratios
fore
the
freeze–thaw
cycles.
the freeze–thaw cycles.
The microscopic
microscopic morphology
ra-ratios
The
morphologyof
ofpaste
pastesamples
sampleswith
withdifferent
differentRHA
RHAreplacement
replacement
tios after
undergoing
freeze–thaw
cycles
presentedininFigure
Figure9.9. As
As shown
shown in
after
undergoing
freeze–thaw
cycles
is is
presented
in Figure
Figure9a,
9a, the
the RHA
0% RHA
paste
shows
a relativelydense
densemicrostructure
microstructure with
ofof
pores.
0%
paste
shows
a relatively
withaasmall
smallnumber
number
pores. It
It can
seenfrom
fromFigure
Figure9b
9b that
that aa penetrating
was
found
on on
the the
surface
of the
can
bebe
seen
penetratingcrack
crack
was
found
surface
of10%
the 10%
RHA
paste
sample,
which
may
be
caused
by
human
factors
during
the
sample
preparaRHA paste sample, which may be caused by human factors during the sample preparation
tion process
[40,49].
Figure
showsthat
that 20%
20% RHA
characterized
by aby
noticeable
process
[40,49].
Figure
9c9c
shows
RHApaste
pasteisis
characterized
a noticeable
increase in the number of pores as well as a large number of staggered cracks in the cement
paste matrix, which may be because, although the low content of RHA can improve the
paste compactness and the bonding of the interfacial transition zone, it cannot compensate
for the lack of its porous properties, which will cause a reduction in the freeze–thaw resistance of the concrete.
Appl. Sci. 2022, 12, 12238
Appl. Sci. 2022, 12, 12238
11 of 15
increase in the number of pores as well as a large number of staggered cracks in the cement
paste matrix, which may be because, although the low content of RHA can improve the
paste compactness and the bonding of the interfacial transition zone, it cannot compensate
12 of 16
for the lack of its porous properties, which will cause a reduction in the freeze–thaw
resistance of the concrete.
Figure 9.
ofof
thethe
paste
samples
with
different
RHARHA
replacement
ratiosratios
after after
Figure
9. Microscopic
Microscopicmorphology
morphology
paste
samples
with
different
replacement
the freeze–thaw
freeze–thaw cycles.
the
cycles.
3.6. Mechanistic
the the
Influence
of RHA
the Mechanical
and significantly
Freeze–
As shown Analysis
in Figureof9d,
surface
of theon30%
RHA pasteCharacteristics
sample showed
Thaw Resistance
of Recycled
Concrete
porous
characteristics,
andAggregate
the mortar
matrix appeared brittle, indicating that the addition
of a large
number
of RHA process
would further
deteriorate
compactness
degree
of the
cement
During
the hydration
of cement
paste, thethe
RHA
particle’s high
specific
surpaste.
This
is probably
because
of the
recycled
structure,
face area
will
promote the
hydration
reaction
of aggregate
cement. Atattached
the samemortar’s
time, theporous
amorphous
which
increases
the water
absorption
the the
concrete.
In addition,
reductionof
inthe
cement
silica within
the RHA
particles
will reactofwith
CH generated
by thea hydration
cement to
produce
gel. The
C-S-Hdensification.
gels and the incompletely
content
leads
to a C-S-H
weakening
ofextra
the paste
This leads hydrated
to a morecement
significant
and RHA
particles
will refine
thethe
pore
structure
in the paste
improve the
bonding
of
icing
pressure
generated
inside
concrete
subjected
to theand
freeze–thaw
cycles,
an increase
thethe
interfacial
which in
leads
a densified
consequently
in
numbertransition
of pores zone,
and cracks
thetopaste
due tomicrostructure
the expansionand
pressure
and further
has a positive
contribution
to the development
of the compressive
strength ofby
thethe
concrete.
damage
to the
internal structure,
which is macroscopically
manifested
increase in
However,
under
the
condition
of
a
high
RHA
replacement
rate,
the
reduced
free
water
mass loss and the decrease in dynamic elastic modulus.
content leads to a weaker hydration reaction, less CH generation and a reduced poz-
zolanic
reaction Analysis
between of
CH
amorphous
silica,
leading
to a reduction
in C-S-H
3.6.
Mechanistic
theand
Influence
of RHA
on the
Mechanical
Characteristics
andgel
Freeze–Thaw
Resistance
of conducive
Recycled Aggregate
Concretecompactness of the paste pore and
production, which
is not
to the improved
the interfacial
transition
zone,
thus leading
to apaste,
loose the
microstructure
of the
cement
paste
During the
hydration
process
of cement
RHA particle’s
high
specific
surface
and
a
weaker
bonding
of
the
interfacial
transition
zone,
which
will
have
a
negative
impact
area will promote the hydration reaction of cement. At the same time, the amorphous silica
on the load-bearing
capacity
the concrete.
within
the RHA particles
willofreact
with the CH generated by the hydration of the cement to
When
recycled
coarse
aggregate
is added
to incompletely
concrete, two hydrated
types of interfaces
areRHA
produce C-S-H gel. The extra C-S-H gels
and the
cement and
formed inside the concrete: the attached mortar–old aggregate interface and the new morparticles will refine the pore structure in the paste and improve the bonding of the interfacial
tar–attached mortar interface, as shown in Figure 10a. The recycled aggregate attached
transition zone, which leads to a densified microstructure and consequently has a positive
mortar’s porous structure will make the concrete absorb a large amount of water during
contribution to the development of the compressive strength of the concrete. However,
the process of the freeze–thaw cycle. The RHA particle’s high specific surface area will
under the condition of a high RHA replacement rate, the reduced free water content leads
cause water enrichment to occur on its surface and enter the interface transition zone of
to a weaker hydration reaction, less CH generation and a reduced pozzolanic reaction
the new mortar–attached mortar interface of the recycled aggregate through cracks and
between
CH and amorphous silica, leading to a reduction in C-S-H gel production, which is
pores. When the pore water inside the concrete is in the process of repeated freeze–thaw,
not
conducive
to the improved
compactness
of the paste
pore and
thepores
interfacial
it causes the continuous
accumulation
of the expansion
pressure
of the
withintransition
the
zone,
thus
leading
to
a
loose
microstructure
of
the
cement
paste
and
a
weaker
concrete, which consequently leads to the gradual transformation of the tiny pores inbonding
the
of
the interfacial
zone, pores
whichunder
will have
a negative
impact
interface
transitiontransition
zone into larger
the action
of tensile
stress on
andthe
theload-bearing
gradual
capacity
of the
development
ofconcrete.
microcracks into macrocracks, which will provide a channel for the miWhen
recycled
coarse
aggregate
to concrete,
types
of interfaces
gration
of water.
The more
water
retainedisinadded
the pores
and crackstwo
inside
the concrete,
the are
formed
inside the
the attached
mortar–old
aggregate
and the new
more significant
theconcrete:
volume increase
when freezing,
which
leads to ainterface
higher expansion
pressure inside the concrete and accelerates the formation of cracks [50]. The interface
transition zone, as the strength-limiting phase of concrete, makes the concrete reach an
Appl. Sci. 2022, 12, 12238
Appl. Sci. 2022, 12, 12238
12 of 15
mortar–attached mortar interface, as shown in Figure 10a. The recycled aggregate attached
mortar’s porous structure will make the concrete absorb a large amount of water during the
process of the freeze–thaw cycle. The RHA particle’s high specific surface area will cause
water enrichment to occur on its surface and enter the interface transition zone of the new
mortar–attached mortar interface of the recycled aggregate through cracks and pores. When
the pore water inside the concrete is in the process of repeated freeze–thaw, it causes the
continuous accumulation of the expansion pressure of the pores within the concrete, which
consequently leads to the gradual transformation of the tiny pores in the interface transition
zone into larger pores under the action of tensile stress and the gradual development of
microcracks into macrocracks, which will provide a channel for the migration of water.
13 of 16
The more water retained in the pores and cracks inside the concrete, the more significant
the volume increase when freezing, which leads to a higher expansion pressure inside the
concrete and accelerates the formation of cracks [50]. The interface transition zone, as the
strength-limiting
phase ofunder
concrete,
makes
the concrete
reachwhich
an ultimate
ultimate tensile strength
a slight
expansion
pressure,
resultstensile
in the strength
freeze–
under
a slight of
expansion
which
results in the
thaw damage
concrete pressure,
due to stress
concentration
[51].freeze–thaw damage of concrete
due to stress concentration [51].
Figure10.
10. (a)
(a)Schematic
Schematicdiagram
diagramofofthe
thecomponent
componentphases
phasesofofRCA;
RCA;(b)
(b)Schematic
Schematicofofthe
the
deterioraFigure
deterioration
tion
mechanism
of
RHA
on
recycled
aggregate
concrete
freeze–thaw
damage.
mechanism of RHA on recycled aggregate concrete freeze–thaw damage.
Meanwhile, as
as the
the bond
bond is
is the
the weakest
weakest at
Meanwhile,
at the
the interface
interface transition
transitionzone
zonebetween
betweenthe
the
new
mortar
and
the
attached
mortar,
the
transverse
tensile
stress
continues
new mortar and the attached mortar, the transverse tensile stress continuestotoincrease,
increase,
whichwill
willcause
cause the
the rapid
rapid expansion
expansion and
which
and extension
extension of
of cracks
cracksin
inthe
thenew
newmortar–attached
mortar–attached
mortarinterface
interface transition
transition zone,
zone, and
mortar
and the
the attached
attached mortar–old
mortar–oldaggregate
aggregateinterface
interfacewill
willalso
also
generatecracks
cracks until
until the
the concrete
concrete specimen
generate
specimen is
is destroyed.
destroyed. The
Theeffect
effectmechanism
mechanismofofrecycled
recycled
concretefreeze–thaw
freeze–thawdamage
damage deterioration
deterioration by
by RHA
concrete
RHA is
is shown
shown in
inFigure
Figure10b.
10b.In
Inthe
theprocess
process
of
freeze–thaw
cycles,
with
the
increase
in
the
RHA
replacement
ratio,
the
degree
of ceof freeze–thaw cycles, with the increase in the RHA replacement ratio, the degree of cement
ment
paste
compactness
decreases.
In
addition,
the
introduction
of
a
large
amount
of
the
paste compactness decreases. In addition, the introduction of a large amount of the RHA
RHA
mesoporous
structure
causes
a
significant
increase
in
the
porosity
and
the
number
mesoporous structure causes a significant increase in the porosity and the number of cracks
of cracks
the concrete,
and the increase
pore connectivity
to the enhance-of
inside
theinside
concrete,
and the increase
in pore in
connectivity
leads toleads
the enhancement
ment of specimen
permeability,
whichthe
makes
the concrete
more sensitive
to freeze–thaw
specimen
permeability,
which makes
concrete
more sensitive
to freeze–thaw
cycles.
cycles. Overall,
the freeze–thaw
resistance
of recycled
a decreasing
trend
Overall,
the freeze–thaw
resistance
of recycled
concreteconcrete
shows ashows
decreasing
trend with
the
with
the
increase
in
the
RHA
replacement
ratio.
increase in the RHA replacement ratio.
Conclusions
4.4. Conclusions
In this
this paper,
paper, the
the impact
impact of RHA on the mechanical
In
mechanical characteristics
characteristicsand
andfreeze–thaw
freeze–thaw
resistance of
of recycled
recycled concrete was investigated,
resistance
investigated, and
and the
the mechanism
mechanismofofthe
thefreeze–thaw
freeze–thaw
damage deterioration
deterioration of
RHA
was
revealed
by combining
SEMSEM
exdamage
ofrecycled
recycledconcrete
concretebyby
RHA
was
revealed
by combining
periments. The
main
conclusions
areare
as as
follows:
experiments.
The
main
conclusions
follows:
(1) The gradual increase in the RHA replacement ratio increased the cementitious
material’s total specific surface area and porosity, which will increase the mixtures’ friction resistance and discourage the flow, which in turn leads to the decrease in the concrete
slump and the reduced workability.
(2) With the continuous hydration reaction process, the compressive strength of recycled concrete increased with the growth of the curing age, and the growth rate shows a
Appl. Sci. 2022, 12, 12238
13 of 15
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The gradual increase in the RHA replacement ratio increased the cementitious material’s total specific surface area and porosity, which will increase the mixtures’ friction
resistance and discourage the flow, which in turn leads to the decrease in the concrete
slump and the reduced workability.
With the continuous hydration reaction process, the compressive strength of recycled
concrete increased with the growth of the curing age, and the growth rate shows a
trend of first being rapid, then slowing down and finally gradually stabilizing. In
addition, the optimum compressive strength of recycled concrete is observed with a
20% RHA replacement ratio.
With the gradual increase in the number of freeze–thaw cycles, the mass loss of
recycled concrete showed a tendency to first decrease and then increase, while the
relative dynamic elastic modulus showed a continuous decreasing trend.
Compared with ordinary concrete (without RHA), the RHA particle’s high specific
surface area will promote the cement hydration and the formation of C-S-H gel by
providing nucleation positions before the freeze–thaw cycle. During the process
of the freeze–thaw cycle, the existence of the recycled aggregate attached mortar’s
porous structure will cause a large amount of water absorption. With the continuous
accumulation of expansion pressure, the internal pores and microcracks will gradually
expand and extend, which will provide a channel for water migration, thus resulting
in recycled concrete undergoing more severe freeze–thaw damage, and the degree of
freeze–thaw damage deterioration grows as the RHA replacement ratio increases.
In addition, more exploration and experimentation in other durability aspects are
needed to expand the application of RHA in recycled aggregate concrete.
Author Contributions: Conceptualization, W.Z. and H.L.; methodology, W.Z. and H.L.; software,
W.Z.; validation, W.Z.; investigation, W.Z.; resources, W.Z.; writing—original draft preparation, W.Z.;
writing—review and editing, C.L.; visualization, W.Z.; supervision, W.Z.; project administration, W.Z.
and C.L. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the National Natural Science Foundation of China (Grant NO.
51878546, Grant NO. 52178251), the Science Foundation Project for Outstanding Youth of Shaanxi
Province (Grant NO. 2020JC-46), the Key Research and Development Projects of Shaanxi Province
(Grant NO. 2020SF-367) and the Xi’an Science and Technology Plan Key Industrial Chain Core
Technology Project (Grant NO. 2022JH-ZCZC-0026).
Data Availability Statement: Not applicable.
Acknowledgments: The authors thank the editor and reviewers very much for their comments and
helpful suggestions.
Conflicts of Interest: The authors declare that they have no conflict of interest to report regarding
the present study.
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