W. HE et al.: EFFECT OF CYCLIC NaCl SOLUTION IMMERSION CURING ON THE MECHANICAL PROPERTIES ...
165–171
EFFECT OF CYCLIC NaCl SOLUTION IMMERSION CURING ON
THE MECHANICAL PROPERTIES OF PLAIN CONCRETE
MEHANSKE LASTNOSTI NEOJA^ANEGA GRADBENEGA BETONA PO
OBDELAVI Z IZMENI^NIM POTAPLJANJEM V SLANICI
Wei He
2,3
, Shoujun Wu
1,2*
, Bo Zhang
2
, Yiming Luo
2
, Yanyu Liu
2
, Guo Fu
2
1
Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A&F University,
Yangling, Shaanxi 712100, China
2
College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling, Shaanxi 712100, China
3
Hunan Provincial Communications Plannning, Survey & Design Insititute Co., Ltd, Changsha 410200, China
Prejem rokopisa – received: 2023-02-13; sprejem za objavo – accepted for publication: 2024-01-29
doi:10.17222/mit.2023.794
The effects of cyclic curing (CC) alternating between NaCl solution immersion and drying for 28 d on the microstructure, phase
composition and mechanical properties of ordinary concrete are studied in comparison with those cured by standard curing us-
ing municipal water. Compared to standard curing, CC enhances the compressive strength, flexural strength and static compres-
sive elastic modulus of concrete by (5.71, 11.14 and 8.06) %, respectively. Ca(OH)2 is significantly reduced, while C-S-H is in-
creased, and a minor amount of 3Ca(OH)2·CaCl2·12H2O is formed. Hydration products become finer, more uniform and denser.
The use of CC is proposed as it improves the strength.
Keywords: concrete, microstructure, phase compositions, mechanical property, curing, NaCl solution
Avtorji v ~lanku opisujejo analizo u~inkov cikli~nega potapljanja obi~ajnega betona v slanico (vodno raztopino s 3,5 % NaCl) in
njegovo alternativno obdelavo s staranjem oziroma su{enjem pri sobni temperaturi za 28 dni (angl.: CCA curing). Analizirali so
mikrostrukturo, fazno sestavo in mehanske lastnosti tako obdelanih vzorcev betonov in jih primerjali s standardno obdelanim
betonom z uporabo mestne (pitne) vode. Primerjava lastnosti CCA betona s standardno obdelanim betonom je pokazala, da ima
CCA beton za 5,71 % vi{jo tla~no trdnost, 11,4 % vi{jo upogibno trdnost in izbolj{an stati~ni tla~ni elasti~ni modul za 8,06 %.
Vsebnost Ca(OH)2 se je pomembno zmanj{ala in pove~al se je dele` C-S-H vezi, tvorila se je zanemarljiva vsebnost
3Ca(OH)2·CaCl2·12H2O. Produkti hidracije so postali finej{i (drobnej{i), bolj enakomerne velikosti in z ve~jo gostoto. V ~lanku
so avtorji razlo`ili mehanizem utrjevanja CCA betona in v zaklju~ku ~lanka priporo~ili uporabo CCA obdelave betona, ~e se
zahteva njegova ve~ja trdnost.
Klju~ne besede: beton, mikrostruktura, fazna sestava, mehanske lastnosti, obdelava s potapljanjem v solno raztopino
1 INTRODUCTION
Concrete is the most widely used building materials
at present.
1
Mixing and curing are essential steps in the
process of concrete preparation and they have a direct
impact on the performance of concrete.
2–7
In most cases,
the mixing and curing of concrete require the use of mu-
nicipal water, usually fresh water, as recommended in
EN 1008-2002,
8
C1602/C1602M–12,
9
ACI 308.1M-11,
10
C31/C31M-21a,
11
JGJ 63-2006 (Chinese standard),
12
etc.
There is a serious shortage of water resources in the
arid saline alkali areas of Northwest China, such as the
Tarim Basin, Turpan Basin, Jungar Basin, Qaidam Basin,
etc. In these areas, it is usually difficult to ensure suffi-
cient fresh water supply, but it is relatively easy to obtain
chlorinated salt (mainly composed of NaCl) water at a
construction site.
13–15
Therefore, it is a promising option
to use chlorinated salt water around a construction site
for concrete curing.
It is generally recognized that chlorine usually results
in a strength degradation of concrete due to the reaction
between chloride ions and the cement aluminate phase
(C3A) as well as the leaching of calcium hydroxide and
C-S-H.
16–20
On the other hand, it is revealed that chloride
ions can react with the aluminate phase to form Friedel’s
salt, which can block the pores in cement and is there-
fore beneficial for the strength improvement. In addition,
the research of the effect of sea water as the mixing wa-
ter on hydration and mechanical properties of ordinary
Portland cement (OPC) revealed that the chloride in sea
water can obviously accelerate the tricalcium silicate
hydration rate at the early age. And thus, the early com-
pressive strength (7 and 14) d of the OPC mortar using
sea water as the mixing water increased rapidly com-
pared to that using distilled water as the mixing wa-
ter.
21,22
However, it is found that at a later age of 28 d,
C-S-H remains non-hydrated in the OPC paste using sea
water as the mixing water and its compressive strength is
lower than that of the OPC paste using distilled water as
the mixing water. Experimental results demonstrated
that, compared with the curing of OPC with fresh water,
the curing with continuous immersion in a sodium chlo-
ride (NaCl) solution deteriorates the strength of con-
crete.
20,23,24
The research shows that the strength of con-
crete may be different even if the curing medium is the
Materiali in tehnologije / Materials and technology 58 (2024) 2, 165–171 165
UDK 625.821.5:52-334.2:613.45 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(2)165(2024)
*Corresponding author's e-mail:
shoujun_wu@163.com (Shoujun Wu)

same and only the curing process is different.
3,25–27
For
example, the work by Raza et al. demonstrated that for
plain concrete, the ponding method of concrete curing is
more effective than sprinkling or wet cover curing
method.
3
Compared to continuous immersion in water,
the curing by alternating between the wetting and drying
methods, during which concrete samples are subjected to
12-h curing in water, followed by 12-h curing in ambient
air, has an adverse effect on the strength.
27
At a construc-
tion site, the curing of concrete by alternating between
wetting and drying is more economical and feasible.
Therefore, it can be concluded that the effect of chlori-
nated salt water on the concrete cured by alternating be-
tween wetting and drying might be different from that of
curing with continuous immersion in chlorinated salt wa-
ter. Unfortunately, reports on curing concrete with chlo-
rinated salt water by alternating between wetting and
drying is lacking.
In the present work, ordinary-concrete samples are
cured for 28 d with standard curing using municipal wa-
ter and with cyclic curing, alternating between NaCl so-
lution immersion and drying. The effects of the cyclic
curing with NaCl solution immersion and drying on the
phase composition, compressive and flexural strength of
OPC are investigated.
2 MATERIALS AND METHODS
The used cement is P.O. 42.5R ordinary Portland ce-
ment (Jidong Cement Co., Ltd., China). The used-water
reducing agent is SBTJM–II (Sobute New Materials Co.,
Ltd., China). Gravel of (5 and 20) mm is used as the
coarse aggregate. Sand from the banks of the Weihe
River with a fineness modulus of 2.2 and apparent den-
sity of 2.64 g cm
–3
is adopted as the fine aggregate. Ta-
ble 1 lists the physical properties of the coarse and fine
aggregates. Figure 1 shows the grading curve of the ag-
gregates obtained with a screening test.
Municipal water from the Yangling City is used as
the mixing water. The mix proportions of concrete are
shown in Table 2. The tested air content of the mixture
and the slump are 4.4 % and 94 mm, respectively. The
measured concrete density is 2350 kg·m
–3
.
Table 2: Mix proportions of concrete (kg·m
–3
)
W/C
ratio
Cement Water
Coarse
aggregate
Sand
Water reduc-
ing agent
0.50 314 157 1069 840 3.14
For the tests, three types of samples are molded. Cu-
bic samples with nominal dimensions of (100 × 100 ×
100) mm are molded for the compressive strength test.
Cylindrical samples with a diameter of Ø150 mm and
height of 300 mm are molded for the static compressive
modulus test. Prism samples with nominal dimensions of
(100 × 100 × 400) mm are molded for the bending
strength test.
After 24 h, samples are demolded and then cured
with two methods. The first one is standard curing in a
chamber at 20 °C with a relative humidity (R.H.) of
95 % for 28 d. The second one is cyclic curing alternat-
ing between NaCl solution immersion and drying, desig-
nated as CC. During the latter, samples are immersed in
an NaCl solution with a concentration of 3.5 % for 4 d,
then dried for 3 d.
The uniaxial compressive strength and the flexural
strength of the samples are carried out on an Instron
1195 machine at room temperature (25 °C and 65 %
R.H.); every test involves the testing of three samples in
each configuration. The loading rate for the compressive
strength test and flexural strength test is 0.3 MPa·s
–1
and
0.2 mm·min
–1
, respectively. Compressive strength is cal-
culated with the average value of the test values that are
selected with an error range within 10 %. The flexural
strength of the samples is tested using the three-point
bending method with a span of 300 mm. A WE–1000
hydraulic universal testing machine and MT–II elastic
modulus tester are used for static compressive elastic
modulus testing as shown Figure 2. An MT–II elastic
modulus tester is placed in the middle part of a sample
with a distance to the upper end surface and lower end
surface of 75 mm. The samples are continuously loaded
at a rate of 0.3–0.50 MPa·s
–1
until they are damaged. The
secant elastic modulus with the stress ranging from
W. HE et al.: EFFECT OF CYCLIC NaCl SOLUTION IMMERSION CURING ON THE MECHANICAL PROPERTIES ...
166 Materiali in tehnologije / Materials and technology 58 (2024) 2, 165–171
Figure 1: Aggregate grading curve
Table 1: Physical properties of the coarse and fine aggregates
Aggregate
Water absorption
(%)
Apparent density
(kg·m
–3
)
Bulk density
(kg·m
–3
)
Stacking density
(kg·m
–3
)
Clay content
(%)
Crushing index
(%)
Fine 5.3 2640 1570 1940 2.3 -
Coarse 0.22 2720 1522 1650 0.9 11.7

0.5 MPa to 40 % of the failure stress is taken as the static
compressive elastic modulus. The static compressive
elastic modulus testing is conducted on four samples of
each group.
The flexural strength of the samples, f
, is calculated
with the following equation
28
 f
PL
bh
=
3
2
2
max
(1)
where L denotes the span during the bending test and is
equal to 300 mm; b and h are the width and height of the
cross-section of a bending sample, respectively.
X-ray diffraction (XRD; BRUKER, D8 ADVANCE
A25 X) is used for phase composition characterization of
the samples. The samples are washed with water and
dried before conducting the XRD examination. XRD is
operated at 40 KV and 40 mA, in a range of 2 = 10–70°
and with a 0.02 step, respectively. The microstructures of
the samples are checked with a scanning electron micro-
scope (SEM; ZEISS Gemini 300) equipped with EDS
(OXFORD Xplore).
3 RESULTS AND DISCUSSION
Table 3 lists the failure load, and the corresponding
compressive and flexural strengths of the concrete sam-
ples with the two curing methods. The compressive
strength (converted to the equivalent strength of a stan-
dard sample with dimensions of (150 × 150 × 150) mm
and flexural strength of the concrete cured with standard
curing are 44.25 MPa and 5.02 MPa, respectively. The
compressive strength (converted to the equivalent
strength of a standard sample with dimensions of (150 ×
150 × 150) mm and flexural strength of the concrete
cured with cyclic curing including NaCl solution immer-
sion and drying are 46.77 MPa and 5.58 MPa, respec-
tively. Therefore, compared with those samples cured
with standard curing, the compressive strength and flex-
ural strength of the concrete cured with cyclic curing in-
cluding NaCl solution immersion and drying are im-
proved by 5.71 % and 11.14 %.
Figure 3 shows the typical load-displacement curves
of the concrete during bending. The load-displacement
curves of the concrete cured with the two methods dur-
ing bending are similar, suggesting typical brittle dam-
age. Therefore, it seems that cyclic curing alternating be-
tween NaCl solution immersion and drying does not
change the damage model of concrete. However, the
breaking load and corresponding deformation of the con-
crete after cyclic curing including NaCl solution immer-
sion and drying for 28 d are higher than those of the con-
crete after 28 d of standard curing.
Figure 4 shows the representative stress-strain data
and their fitting results for the concrete samples during
W. HE et al.: EFFECT OF CYCLIC NaCl SOLUTION IMMERSION CURING ON THE MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 58 (2024) 2, 165–171 167
Figure 2: Photo of static compressive elastic modulus testing
Table 3: Compressive and flexural strength of the concrete
Curing method
Compressive strength Bending
Maximum
load (kN)
Strength
(MPa)
Average
strength (MPa)
Effective
strength (MPa)
Maximum
load (kN)
Strength
(MPa)
Average
strength (MPa)
CC
516.91 51.69
49.24 46.77
12.49 5.62
5.58 492.41 49.24 12.18 5.48
467.77 46.78 12.54 5.64
Standard
480.91 48.09
46.58 44.25
11.32 5.09
5.02 415.34 41.53 11.29 5.08
501.00 50.10 10.87 4.89
Figure 3: Typical load-displacement curves of the concrete during
bending

the static compressive elastic modulus testing with the
stress ranging from 0.5 MPa to 40 % of the failure stress.
It seems that the stress and strain during the static com-
pressive elastic modulus testing of the concrete samples
cured with the two methods can be well fitted by linear
fitting.
The static compressive elastic modulus of the con-
crete cured with standard curing is 35.51 GPa, while that
of the concrete cured with cyclic curing alternating be-
tween NaCl solution immersion and drying is 38.37 GPa,
enhanced by 8.05 %. The above mechanical-property re-
sults indicate that cyclic curing including NaCl solution
immersion and drying has a positive effect on the me-
chanical properties of concrete. And these results are dif-
ferent from those for the samples cured with NaCl solu-
tion immersion.
20,23,24
Figure 5 shows XRD patterns of the concrete after
standard curing and cyclic curing alternating between
NaCl solution immersion and drying for 28 d under the
same diffraction conditions. It can be found that the main
products of both cured samples are CaCO
3
, Ca(OH)
2
and
C-S-H. In addition, from the magnified view at a 2 range of 32–62°, a weak signal of 3Ca(OH)
2
·CaCl
2
·
12H
2
O can be detected in the concrete cured with cyclic
curing including NaCl solution immersion and drying.
However, it should be noted that the content of Ca(OH)
2
is significantly reduced, while the content of C–S–H is
significantly increased in the sample cured with cyclic
curing including NaCl solution immersion and drying,
compared with the sample cured with standard curing.
Figure 6 shows the microstructures of the concrete
after standard curing and cyclic curing alternating be-
tween NaCl solution immersion and drying for 28 d. It
seems that the sample cured with cyclic curing including
NaCl solution immersion and drying is denser than that
cured with standard curing. Moreover, from a magnified
view, it can be seen that the sample cured with cyclic
curing including NaCl solution immersion and drying
has fine and uniform spherical crystals as shown in Fig-
ure 6c, while the sample cured with standard curing has
relatively large, short, columnar or platelet-shaped crys-
tals, shown in Figure 6d. Finer and more uniform crys-
tals as well as a denser morphology suggest that a
hydration process enhances nucleus formation and is
thus beneficial for the strength.
Due to the porous structure of silicate concrete, chlo-
ride ion can diffuse inward easily. During immersion in
the NaCl solution, free Cl
–
inside the concrete might re-
act with hydration-formed Ca
2+
as shown with Equa-
tion (2)
29
and thus forms CaCl
2
as shown with Equation
(3).
25
Furthermore, CaCl
2
reacts with Ca(OH)
2
to form
insoluble solid phase 3Ca(OH)
2
·CaCl
2
·12H
2
O as illus-
trated with Equation (4).
21,30
The formation of 3Ca(OH)
2
·
CaCl
2
·12H
2
O leads to an accelerated hydration of con-
crete,
21
accompanied with an increased hydration tem-
perature. This in return results in an accelerated forma-
tion of C–S–H
29–33
as illustrated with Equation (5). As a
result, the cement shows finer and more uniform crystals
as well as a denser morphology. Moreover, ionic ex-
change between calcium silicate hydrates and NaCl,
shown with Equation (6), also leads to Ca(OH)
2
con-
sumption.
34,35
Therefore, a minor amount of 3Ca(OH)
2
·
CaCl
2
·12H
2
O can be detected; the content of Ca(OH)
2
is
significantly reduced, while that of C-S-H is signifi-
cantly increased in the sample cured with cyclic curing
including NaCl solution immersion and drying, com-
W. HE et al.: EFFECT OF CYCLIC NaCl SOLUTION IMMERSION CURING ON THE MECHANICAL PROPERTIES ...
168 Materiali in tehnologije / Materials and technology 58 (2024) 2, 165–171
Figure 4: Linear fitting of the representative stress-strain data for the
concrete samples during static compressive elastic modulus testing
Figure 5: XRD patterns of the concrete after 28 d curing: 1 – standard
curing; 2 – CCA curing

pared with the sample cured with standard curing. Ac-
cording to possible reactions (3) and (4), the NaOH and
Na
2
SiO
3
·H
2
O products are easily dissolved in water and
their contents are very small. The concerned samples are
washed with water and dried before the XRD examina-
tion. Therefore, the signal of Na
+
cannot be detected by
XRD for the CC-cured sample.
Ca
3
SiO
5
+3H
2
O 3Ca
2+
+ 4OH
–
+H
2
SiO
4
2–
(2)
3Ca
2+
+ 4OH
–
+H
2
SiO
4
2–
+2NaCl 2Ca(OH)
2
+ CaCl
2
+Na
2
SiO
3
·H
2
O (3)
3Ca(OH)
2
+ CaCl
2
+ 12H
2
O 3Ca(OH)
2
·CaCl
2
·12H
2
O (4)
2Ca
2+
+ 2OH
–
+H
2
SiO
4
2–
 C–S–H (5)
C–S–H–OH
–
+Na
+
+Cl
–
 C–S–H–Cl
–
+Na
+
+OH
–
(6)
During curing with continuous immersion in the
NaCl solution, the rapid formation of cementitious
Ca(OH)
2
and C–S–H due to accelerated hydration is ben-
eficial to the strength. However, the hydrated cementi-
tious layer rapidly forms on the periphery of the sample,
filling in the pores by forming expansive 3Ca(OH)
2
·
CaCl
2
·12H
2
O that prevents the diffusion of chloride ions
into the interior,
19,36
thus reducing internal hydration.
Moreover, the expansion of 3Ca(OH)
2
·CaCl
2
· 12H
2
O
may deteriorate the rapidly formed cementitious layer on
the periphery. As a result, curing with continuous immer-
sion in the NaCl solution deteriorates the strength of
concrete as reported by others.
20,23,24
In the case of cyclic
curing alternating between NaCl solution immersion and
drying, the accelerated formation of cementitious
Ca(OH)
2
and C–S–H, filling in the pores by forming ex-
pansive 3Ca(OH)
2
·CaCl
2
·12H
2
O during the first cycle of
immersion in the NaCl solution is limited. Therefore,
this curing cycle is favorable for the strength enhance-
ment. The drying cycle makes the concrete more evenly
hydrated inside and outside so that the periphery of the
sample can maintain a certain degree of pore opening.
During the subsequent NaCl solution immersion, chlo-
ride ions can still diffuse inside along the pores and thus
accelerate the inner hydration. The previously formed
3Ca(OH)
2
·CaCl
2
·12H
2
O and C–S–H on the pore walls
prevent chloride ions to diffuse through the pore walls
and thus impede the plugging of the pores. As a result,
the internal strength can be gradually improved during
the subsequent curing cycles. Consequently, unlike the
W. HE et al.: EFFECT OF CYCLIC NaCl SOLUTION IMMERSION CURING ON THE MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 58 (2024) 2, 165–171 169
Figure 6: Micromorphologies of the concrete after 28-d curing: a), c) standard curing; b), d) CC

curing with continuous immersion in the NaCl solution,
the cyclic curing including NaCl solution immersion and
drying is beneficial for the mechanical properties of
OPC.
It should be noted that Friedel’s salt is not detected
by XRD in the present work. It is supposed that the for-
mation of 3Ca(OH)
2
·CaCl
2
·12H
2
O and accelerated for-
mation of hydrated calcium silicate due to the accelera-
tion of hydration lead to a reduction of Ca(OH)
2
. On the
other hand, the ionic exchange between calcium silicate
hydrates and NaCl also leads to the Ca(OH)
2
consump-
tion. Thus, the reduction of Ca(OH)
2
, which can partici-
pate in the reaction producing Friedel’s salt, limits the
formation of Friedel’s salt.
4 CONCLUSIONS
Compared to standard curing in water, cyclic curing
alternating between 3.5 % NaCl solution immersion and
drying (CC) has a positive effect on the mechanical prop-
erties of concrete. The compressive strength, flexural
strength and static compressive elastic modulus of the
concrete cured by CC for 28 d are enhanced by (5.71,
11.14 and 8.06) %, respectively, compared to those ob-
tained with standard curing using municipal water.
The main products for both cured samples are
CaCO
3
, Ca(OH)
2
and C–S–H. Compared to standard cur-
ing using municipal water, CC results in significantly re-
duced Ca(OH)
2
, while C–S–H is significantly increased;
in addition, a minor amount of 3Ca(OH)
2
·CaCl
2
·12H
2
O
is formed. The hydration accelerated due to CC makes
hydration products finer, more uniform and denser than
those formed during standard curing.
The strength improvement due to CC can be ex-
plained with the following mechanisms. During the alter-
nating curing process, immersion in NaCl accelerates
hydration, while the subsequent drying helps to limit
possible damage caused by expansive 3Ca(OH)
2
·CaCl
2
·
12H
2
O. Multiple curing cycles gradually accelerate
hydration from the outside to the inside, thus improving
the overall strength of the samples.
Acknowledgment
This work was supported by the fund of the Natural
Science Foundation of the Shaanxi Province
(2017JQ5113).
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