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OPEN Experimental study and analytical

model for the pore structure
of epoxy latex‑modified mortar
Pengfei Li1, Wei Lu1, Xuehui An2*, Li Zhou2, Xun Han3, Sanlin Du4 & Chengzhi Wang1
pemulihan Concrete repair and rehabilitation prolong the effective service lives of structures and are important
topics in the building field worldwide. Epoxy latex-modified cementitious materials have shown
promise for a number of applications in building and construction, but the mix design processes
remain arbitrary because their pore structures are not well understood. Porosity and pore size
distributions are pore structure parameters that have direct effects on the mechanical properties and
durability of concrete. In this paper, mercury intrusion porosimetry (MIP) was used to analyze the
porosities and pore size distributions of epoxy latex-modified mortars. The effects of the polymer-to-
cement ratio on the pore structures of epoxy latex-modified mortars were investigated. Mortars with
polymer-to-cement ratios of 0%, 5%, 10%, 15%, and 20% were cured for 7, 28, 60, and 90 days in this
study. Images of specimen microstructures were obtained by scanning electron microscopy (SEM),
which showed that increases in the amount of epoxy latex added caused the proportion of micropores
in the mortar to decrease, while the proportion of macropores and gel pores increased. The pore
size distribution of epoxy latex-modified mortar was described with a composite logarithmic model.
Relationships between the pore size distribution and the polymer-to-cement ratio and the curing
age were obtained. The method described herein might be sufficiently accurate and convenient to
evaluate or predict the pore size distribution of an epoxy latex-modified mortar, i.e., by determining
the statistical distribution and analyzing the probability. The process for design of the polymer
concrete mix ratio will be facilitated by methods that accurately describe the structure of the epoxy
latex-modified mortar.

The pore structure is one of the most significant characteristics of cementitious materials, and it has major
impacts on both mechanical p ­ roperties1–4 and d
­ urability5–7. The pore structures of cementitious materials deter-
mine important properties such as s­ trength3,8, ­permeability9,10, and s­ hrinkage11,12. Cement-based materials with
the same total porosity may also exhibit different properties. Therefore, it is important to investigate the porosity
size distributions of cementitious materials to explain their properties.
Pore sizes, arrangements, and connections in cement-based materials are random and include gel pores (the
interlayer pores in cement hydration produce calcium-silicate-hydrate gels), capillary pores, and air v­ oids13.
The pore distributions of cementitious materials are extremely complex because of the wide range and random
distribution of pore s­ izes14. MIP is commonly used to investigate the pore structures of cementitious materi-
als and is also one of the most widely used methods. The total porosity, pore size distribution, and density of a
cementitious material can be obtained by MIP. The pores in cementitious materials are formed during hydra-
tion in the cement, and they mainly comprise two types of p ­ ores1,15: (a) gel pores, with sizes ranging from 0.5
to 10 nm, are mainly related to the cement hydration process and do not have a major impact on the strength
of cementitious materials but have greater effects on the shrinkage and creep of the material; (b) capillary pores
with sizes distributed primarily from 10 to 10,000 nm are strongly associated with the strength of a cementitious
material, which is generally determined by the water-to-cement ratio of the material.
With the rapid developments occurring in the field of construction materials, synthetic polymer latexes, such
as epoxy resin latex in cementitious material systems, have been used in many ­projects16–19. Polymer-modified
cement mortar (PMM) is commonly used to enhance the physical properties and durabilities of structures

1
Department of Harbor, Waterway, and Coastal Engineering, Chongqing Jiaotong University, Chongqing 400074,
China. 2State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084,
China. 3Geotechnical Engineering Department, Nanjing Hydraulic Research Institute, Nanjing 210024,
China. 4Huaneng Tibet Hydropower Safety Engineering Technology Research Center, China Huaneng Group Co.,
Ltd., Beijing 100031, China. *email: anxue@mail.tsinghua.edu.cn

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Figure 1.  Cement particle size distribution curves.

Chemical component CaO SiO2 Fe2O3 Al2O3 SO3 MgO K2O TiO2 BaO SrO Na2O
Content (wt.%) 69.42 14.7 4.21 3.67 3.2 1.55 1.42 0.597 0.489 0.265 0.184

Table 1.  Chemical composition of the cement.

because the reticulated film structure in the PMM can improve the pore ­structure20–23. Ordinary mortars have
relatively more p ­ ores24. However, PMM fills the pores created during hydration of the cement by forming a
polymer film in the mortar s­ ystem25. As a result, PMMs based on various synthetic polymeric latexes are already
being applied in the construction industry. Generally, the porosity and pore size distribution of the PMM system
need to be considered when investigating the mechanical properties and durability of the m ­ aterial26. The physical
properties and durability of ordinary cementitious materials are affected by pore structure parameters such as
porosity and pore size distribution. However, the pore structure parameters of PMM materials may affect the
mechanical properties and the durability of the material more than any other ­characteristic27. Therefore, the
strengths, durabilities, and permeabilities of PMMs are strongly impacted by their pore structure parameters,
such as porosities and pore size d­ istributions28,29.
The microstructures of PMMs reported in the literature are mainly described by porosity and pore structure
distribution. Then, the relationships between pore structure parameters and macroscopic properties are estab-
lished to guide the application of PMMs in practical engineering. However, there are few accurate simulations
of the pore structures of PMMs. The use of mathematical models to accurately describe the pore structure dis-
tribution has important implications for studies of PMM microstructures. Additionally, accurate descriptions
of the pore structure and predictions of void structure parameters are important to promote the design of PMM
mixtures. In view of the above, the pore structure parameters of PMMs obtained through MIP experiments, such
as porosity and pore size distribution, can be used to investigate the relationships between polymer addition and
pore structure distributions of cement mortars. This paper investigates the effects of different polymer-cement
ratios and curing ages on the pore size structures of epoxy latex-modified cement mortars. MIP was used to
obtain the pore structure changes of the specimens. The pore size distributions of epoxy latex-modified mortars
were simulated with a compound log-normal distribution model. The model results were also compared with
the test results of this study.

Materials and methods

Materials. Ordinary Portland cement (PO 42.5) was used in this investigation, and it had a relative density
of 3080 kg/m3. The specific surface area was 0.129 ­m2/g, as measured using a Bettersize 2000 (DL) laser particle-
size analyzer. The cement particle-size distribution is shown in Fig. 1. The chemical composition of the cement
determined by X-ray fluorescence spectrometry is shown in Table 1. The fine aggregate in this study was well-
graded manufactured quartz sand. It had a specific gravity of 2700 kg/m3. The particle size distribution of the
quartz sand was in the range 0.075–4.75 mm and is shown in Fig. 2.
Epoxy latex was prepared by emulsifying a bisphenol A-based epoxy resin in water with an emulsifier and
an added hardener. As shown in Fig. 3, the epoxy latex used in this investigation was mainly composed of three
parts: (a) bisphenol A-based epoxy resin, (b) water, and (c) hardener, with a solids content of 50%, which mainly
consisted of a curing agent and emulsifier. The epoxy latex in this investigation had a density of 1.00–1.04 g/cm3
and a total solids content of 50 ± 1%.

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Figure 2.  Particle size distribution curve for the quartz sand used in this investigation.

Figure 3.  Images of epoxy latex materials before and after mixing.

Specimen preparation. Previous studies showed that polymer-to-cement ratios of 5–15% were optimal
for preparation of epoxy latex-modified ­mortars20,27. In our previous ­research30,31, we found that the compressive
strengths of epoxy-modified mortars tended to decrease and the flexural strengths of epoxy-modified mortars
tended to increase with increasing epoxy latex content. Therefore, epoxy resin-to-cement ratios of 0, 5%, 10%,
15%, and 20% were chosen to investigate the effects of the various contents of epoxy latex. The volumetric water-
to-cement ratios of all mortar specimens were kept at 1.10. The cement-to-sand volume ratio was maintained
at 1.00 for all specimens. Referring to the provisions of JC/T 986-201832, all mortars were adjusted to exhibit a
flowability of 200–210 mm by adding superplasticizers. The superplasticizer used has been applied in many pre-
vious ­studies33–38. Table 2 shows the mixing ratios for the materials in the epoxy latex-modified mortar. Further-
more, the P/C of the modified mortar indicates the dry epoxy resin-to-cement ratio of mass. The prepared epoxy
emulsion-modified mortar was cast in square molds with dimensions of 70.7 mm. All specimens were cured at
100% RH and 20 °C for 1 day and then demolded. Then, the specimens were cured in a standard maintenance
(60 ± 5% RH, 20 ± 1 °C) chamber for 7, 14, 28, or 90 days.

Experimental tests. Mercury intrusion porosimetry (MIP). The MIP test is one of the most widespread
methods used to investigate pore size distributions of cementitious materials. It provides accessible information,
such as total pore volume, density, and pore size distribution for the cementitious material. MIP is very simple
in principle. It is based on the physical principle that the applied pressure determines the extent of mercury
intrusion into a porous medium. In the MIP experiment, the prepared sample is put into a chamber filled with
mercury, and then the pressure applied to the mercury is steadily raised. With increasing pressure applied to
the mercury, the volume of mercury pressed into the pores of the specimen also increases. To fill a pore with a

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Mix label Vw/Vc vs./Vc P/C (%) C S W ER H

Control 0 0.00 0
Latex 1 5 41.60 31.20
Latex 2 1.10 1.00 10 832.70 1199.80 281.50 83.20 62.40
curing ages
Latex 3 15 124.80 96.60
Latex 4 20 166.40 124.80

Table 2.  Mix proportions for epoxy latex-modified mortars (kg/m3). C, S, W, ER, and H are the quantities of
cement, quartz sand, water, dry epoxy resin, and dry hardener, respectively.

diameter d with a nonwetting fluid, a pressure P that is inversely proportional to the diameter of that pore must
be ­applied39. The Washburn equation, as shown below, can be utilized to express the association between applied
pressure and pore ­size40:
−4γ cos ϕ
D= (1)
P
where P = the absolute pressure applied; φ = the contact angle of the sample and mercury (132° used here); γ = the
surface tension of mercury; and D = the diameter of the intruded cylinder.
MIP tests were performed herein on a Micromeritics Poresizer AutoPore IV 9500 instrument with a maximum
intrusion pressure of 61,000 psi (420 MPa). The pores of two different specimens were tested, and average values
were calculated for the test results. If the results of the two tests differed by more than 5%, it may have been due
to experimental error, and a third specimen was tested.

Scanning electron microscopy (SEM). SEM was conducted to obtain a clearer perspective on the microscopic
effects of epoxy latex on cement mortar. The samples were immersed in absolute ethanol for 24 h to terminate
hydration of cement and when the mortar specimens reached the ages of 7, 14, 28, or 90 days of maintenance.
After drying the samples, SEM was conducted directly.

Results and discussion

Pore size distribution. The pore structures of epoxy latex-modified mortars were obtained by MIP after 7,
14, 28, and 90 days of curing. Figure 4 shows cumulative pore diameter distribution curves for modified mortar
specimens with different P/C ratios. As the P/C ratio increased, the pore size distribution curves of the speci-
mens gradually shifted upward and to the right, and all of them were higher than that of the control mortar. This
indicated that admixture of the epoxy latex led to an increase in the total pore space of the mortar. Similar results
were reported in the l­ iterature30,41; polymer latexes introduced large pores into cement mortars, which increased
the total porosities of PMMs. To better show the relationship between the P/C ratio and pore size distribu-
tion, the pore size distributions of the specimens were separated into three size ranges: gel pores (D < 10 nm),
micropores (10–100 nm), and macropores (D > 100 nm)39. Figure 5 shows the classification of pore sizes. The
pore sizes shown in the figure were mainly micropores and macropores. For epoxy latex-modified mortar, as
the P/C ratio increased, the pore size increased substantially at all curing ages. For macropores, all samples
had higher pore percentages with increasing P/C ratios, which is consistent with the results reported for epoxy
cement ­mortars27, and the proportion of pores with sizes greater than 75 nm increased as the polymer-cement
ratio increased. The micropore range has always had the largest proportion for all cases. Gel pore sizes of all
epoxy latex-modified mortars increased with a smaller trend. A mixture containing epoxy latex has been shown
to have a remarkable impact on the pore structure of epoxy latex-modified mortar during the curing p ­ rocess26.

Compound log‑normal distribution for pore size distributions. According to previous ­studies42,
the pore size distribution determined by MIP can be simulated with a mixture of log-normal or composite log-
normal distributions. It was found that the pore distributions of cement paste and mortar were well simulated by
the composite log-normal distribution.
When a variable x (0 < x < ∞) meets the log-normal distribution, the function y = lnx is normally distributed.
The probability density function is shown as follows:
  
1 ln x − µ 2

1
p(x) = √ exp − (2)
2πσ 2 x 2 2

where x is a variation, μ is defined as the location parameter, and σ is the shape parameter. In the log-normal
distribution model, there are several relevant characteristics, as follows:

mean = exp µ + 0.5σ 2 (3)

 

median = exp (µ) (4)

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Figure 4.  Relationship between the cumulative pore size distribution and P/C ratio of epoxy latex-modified
mortar at different ages, (a) 7, (b) 14, (c) 28, and (d) 90 days.

mode = exp µ − σ 2 (5)

 

coefficient of variation = exp σ 2 − 1 (6)

 

variance = mean2 exp σ 2 − 1 (7)

   

Generally, the hydration process for cement can be considered a process of dividing the voids of the cement
system or the spaces between particles in the cement system. In the early stages of hydration, there are large
voids in the cement paste system, i.e., voids between cement particles. As curing progresses, the cement particles
are connected by cement hydration products. Therefore, the large voids in the cement paste system are divided
into smaller pores. This suggests a physical basis for the log-normal model of pore size distribution in cement
­systems42.
The probability density function of the composite log-normal distributions model is shown as follows:
p(x) = f1 p(x, µ1 , σ1 ) + f2 p(x, µ2 , σ2 ) + f3 p(x, µ3 , σ3 ) (8)

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Figure 5.  Pore volume distribution of epoxy latex-modified mortar with pore classifications at (a) 7, (b) 14, (c)
28, and (d) 90 days.

3

fi = 1 (9)
i=1

where μi = the location parameter, σi = the shape parameter, fi = the weight coefficient, and p(x, μi, σi) = the ith
log-normal subdistribution. Research showed that if two or more log-normal distributions explained an attribute
of a system, it is likely that there were multiple distinct response processes occurring in that ­system43. This may
indicate that the different size ranges of pores in cementitious materials had different sources and generation
mechanisms.
The method for modeling pore size distributions in this study was obtained from a group of cumulative prob-
ability data. The relationship between P(x) and x is shown in Fig. 6. In this investigation, x = the pore diameter
and P(x) = the ratio of the volume of the cumulative pore to the total pore determined by MIP tests. In general,
the distribution model and the initial values of the parameters are frequently determined by graphical analysis.
This graphical approach first requires the logarithmic transformation of x to obtain lnx. Then, the standard
normal distribution table is used to determine the quantiles of N(0,1) determined by P(x). For instance, if the
cumulative percentage is 80%, the corresponding quantile is 0.86. With lnx as the vertical axis and the quantiles
of N(0,1) as the horizontal axis, if the curve of lnx versus quantiles of N(0,1) is linear in shape, then x can be
considered to obey a single log-normal distribution.

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Figure 6.  Cumulative pore volumes in epoxy latex-modified mortars hydrated in P/C = 5% for 90 days.

Figure 7.  Plot of lnx versus the quartiles of N(0,1) obtained based on the data in Fig. 6.

The curves for plots of lnx versus the quantiles calculated from the MIP data for epoxy latex-modified mortar
specimens are shown in Fig. 7. Notably, the curves comprised three straight line segments. As shown in Fig. 8,
the slope and intercept of the straight line represented the location (μi) and shape (σi) parameters of the corre-
sponding subdistribution, respectively. The horizontal axis at the intersection points of adjacent line segments
determined the weighting factor. As shown in Fig. 8, the first segment of the line intersected the second segment
at quantile − 0.91, and the corresponding cumulative probability was identified as 0.18 by consulting the standard
normal distribution table. Thus, f1 = 0.18.
The test results for epoxy latex-modified mortars of different ages were compared with the results from the
distribution fitted using the χ2 fitting technique. It is well known that the χ2 test is one of the important indica-
tors used to evaluate whether the fitted data are good or not. This method for evaluating the goodness of fit is
as follows:
k
 (Oi − Ei )2
χ02 = (10)
Ei
i=1

where χ2 = the test statistic, k = the number of bins, Oi = the observed frequencies, and Ei = the expected frequen-
cies. The most appropriate distribution is obtained by adjusting the initially predicted distribution parameters of
the model to obtain the smallest possible test statistic. Subsequently, the critical values of χα,k−β−1
2 with k − β − 1

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Figure 8.  Initial prediction parameter acquisition method for the composite log-normal distribution.

Age (days) P/C (%) f1 σ1 μ1 f2 σ2 μ2 f3 σ3 μ3 R2 χ02 2

χα,k−β−1
0 0.13 4.27 − 0.08 0.64 0.72 2.88 0.23 1.40 3.91 0.99 0.45 72.15
5 0.14 4.20 0.11 0.59 0.70 3.03 0.27 1.45 4.01 0.99 0.60 72.15
7 10 0.18 4.18 0.23 0.54 0.68 3.12 0.28 1.42 4.14 0.99 0.75 72.15
15 0.21 4.24 0.33 0.49 0.75 3.26 0.30 1.50 4.22 0.99 1.00 72.15
20 0.24 4.26 0.50 0.45 0.76 3.37 0.31 1.45 4.30 0.99 0.75 72.15
0 0.17 4.21 0.28 0.50 0.76 3.15 0.33 1.42 4.17 0.99 3.71 72.15
5 0.18 4.14 0.68 0.46 0.79 3.29 0.36 1.46 4.29 0.99 1.13 72.15
14 10 0.19 4.20 0.90 0.44 0.85 3.40 0.37 1.43 4.40 0.99 2.00 72.15
15 0.22 4.24 1.12 0.39 0.81 3.50 0.39 1.45 4.49 0.99 1.15 72.15
20 0.24 4.29 1.37 0.35 0.83 3.73 0.41 1.47 4.66 0.99 1.58 72.15
0 0.15 3.84 0.64 0.64 0.70 3.33 0.21 1.60 4.33 0.99 1.30 72.15
5 0.18 3.85 0.97 0.61 0.75 3.53 0.21 1.62 4.44 0.99 1.26 72.15
28 10 0.20 3.90 1.31 0.57 0.76 3.62 0.23 1.60 4.60 0.99 0.33 72.15
15 0.21 3.84 1.82 0.52 0.73 3.79 0.27 1.65 4.75 0.99 0.27 72.15
20 0.22 3.91 2.03 0.43 0.76 3.96 0.35 1.63 4.83 0.99 0.30 72.15
0 0.14 3.53 1.18 0.59 0.96 3.55 0.27 1.76 4.65 0.99 2.81 72.15
5 0.18 3.50 1.80 0.51 0.91 3.82 0.31 1.69 4.74 0.99 0.59 72.15
90 10 0.23 3.58 2.51 0.43 0.94 3.96 0.34 1.77 4.96 0.99 1.78 72.15
15 0.29 3.53 3.12 0.34 0.95 4.09 0.37 1.74 5.06 0.99 1.73 72.15
20 0.33 3.55 3.67 0.27 0.93 4.29 0.40 1.73 5.27 0.99 0.80 72.15

Table 3.  Compound log-normal distribution parameters for the pore size distributions of epoxy latex-
modified mortars.

degrees of freedom are calculated. The α-error is considered to be 0.05. If χ02 > χα,k−β−1
2 , the hypothesis that
the pore size data for the epoxy latex-modified mortar followed a compound log-normal distribution should
be rejected.
Table 3 and Fig. 9 indicate the differences in pore size distribution between the model and experimental
results. For all mortars, the inequalities χ02 < χα,k−β−1
2 were satisfied, and their correlation coefficients were
close to 1. As shown in Fig. 9, the experimental data for the pore size distribution of epoxy latex modified mortar
were in excellent agreement with the model curves obtained. Therefore, the composite log-normal distribution
used in this study predicted the cumulative pore size distribution curves of epoxy latex-modified mortars with
different P/C ratios effectively.

Pore parameters of epoxy latex‑modified mortars. The composite model with three log-normal dis-
tributions may provide considerable information on the origins and formation methods for pores of various

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Figure 9.  Relationships between predicted and tested cumulative pore size distributions of epoxy latex-
modified mortars at (a) 7, (b) 14, (c) 28, and (d) 90 days.

sizes. Physically, the size distribution of macropore pores (D > 100 nm) can be described by the first subdistribu-
tion. The size distribution of gel pores (D < 10 nm) can be described by the third subdistribution. The middle
subdistribution represents micropore pores (10 nm ≤ D < 100 nm). Table 3 showed that f2 decreased continu-
ously with increasing P/C ratio. In this case, the micropores in the modified mortar system decreased as the P/C
ratio increased, which is consistent with the results of previous ­studies30 showing that the proportion of pores
with sizes ranging from 50 to 100 nm tended to increase with increasing polymer dosage. As the P/C ratios
increased from 5 to 20%, the proportion of macropore pores increased, and f3 increased to a small degree; this
can be explained by the fact that some of the micropores were filled by the hardened epoxy. A mass of aggregated
epoxy resin particles adsorbed cement particles, which led to an increase in macropores. This result is consistent
with ­literature44 indicating that the number of pores (with size ranges 0.1 μm to 1 μm and 10–200 μm) increased
with increasing incorporation of polymer. Adsorption of the epoxy emulsion on cement particles hindered the
hydration process in the mortar system, which resulted in an increased number of gel p ­ ores30,44.
Figure 10 shows Q-ln plots for epoxy latex-modified mortar samples cured for 90 days. Initial estimates for
the slopes of the three fairly linear segments were designated σ1, σ2, and σ3. Initial estimates for the intercepts of
the three segments were designated μ1, μ2, and μ3, respectively. The two deflection points were the initial estimates
for f1 and f2. It is obvious what the weighing factors f1, f2, and (1- f1- f2) mean. Therefore, it is necessary to investi-
gate the relationships among location parameters, shape parameters and P/C ratios of the epoxy latex-modified
mortar pore structures. As shown in Fig. 10, the linear segments at the ends of the curves were almost parallel to
each other. This indicates that the shape parameters σ1, σ2, and σ3 of the log-normal pore size distributions in the

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Figure 10.  Q-ln plots for epoxy latex-modified mortar samples cured for 90 days.

modified mortar systems remained essentially constant as the P/C rate increased. Therefore, in the epoxy latex-
modified mortar system, the shape parameter in the composite log-normal distribution may be related to other
factors. Thus, it is necessary to investigate the relationship between the position parameter and the P/C ratio.

Prediction of the location parameter of epoxy latex‑modified mortar. The location parameter
(μi) relative to the polymer-to-cement ratio (P/C) was studied first. It was found that μ1, μ2, and μ3 may be related
to the epoxy latex. For the same curing age, it was shown that variations in the location parameters with respect
to the P/C ratio were best represented by the relationship µi = a(P/C) + b. A plot of the location parameter
against the P/C ratio is shown in Fig. 11. The regression equation and the corresponding correlation coefficients
are shown in Fig. 11. As shown, the location parameter increased with increases in the P/C ratio. This indicates
that the pore structure of epoxy latex-modified mortar evolved toward large pores as the P/C ratio increased,
which further supports previous fi ­ ndings45 that increases in the polymer latex percentage increased the total
porosity for a constant water-to-cement ratio. The relationships between the location parameters and the P/C
ratios were linear with correlation coefficients as high as R2 > 0.960.
Furthermore, the values a and b for the linear plots of location versus the curing age are plotted in Fig. 12. It
has been observed that the variations of a and b with age t are best expressed as a relationship of the following
form; a, b = c ln (t) + d , where t is the curing age. The regression equation and the corresponding correlation
coefficients are also shown in Fig. 12. It is interesting to note that the plots of values a and b of the linear func-
tion versus the curing age showed logarithmic relationships with correlation coefficients as high as R2 > 0.980.
As concluded above, Equations a and b were substituted into the equation µi = a(P/C) + b. Accordingly,
the relationships between location parameters (μi) and polymer-to-cement ratios (P/C) and curing age (t) can
be expressed as follows:
 
P
µ1 = (3.815 ln t − 4.881) + 0.483 ln t − 0.973,
C
 
P
µ2 = (0.408 ln t + 1.642) + 0.273 ln t + 2.894,
C
 
P
µ3 = (0.438 ln t + 1.167) + 0.272 ln t + 3.412.
C

Effect of epoxy latex on the microstructure of mortar. The microstructures of an ordinary cement
mortar and an epoxy latex-modified mortar are shown in Figs. 13 and 14, respectively. Figure 13 shows that a
large number of plate-like and needle-like products were generated in the normal mortar. These needle-like
ettringite products and plate-like Ca(OH)2 crystals were the main products of cement hydration, and these prod-
ucts interwove to form a dense structure, which contributed to improvements in the pore structures of cementi-
tious ­materials46. Figure 14 shows that, as with the control mortar, Ca(OH)2 crystals and ettringite products were
formed in the epoxy latex-modified mortar. However, polymer films were observed for the epoxy latex-modified
cement mortar, and these films were beneficial to improving the pore structure of the cement m ­ ortar21. As seen
from Fig. 14a, when 5% epoxy latex was added to the cement mortar, pores were observed on the surfaces of
the plate-like Ca(OH)2 crystals. As shown in Fig. 14b, at a P/C ratio of 10%, round lumps were formed in the
modified mortar. This may be due to adsorption of cement particles by epoxy aggregates. Figure 14c shows the

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Figure 11.  Relationships between location parameters (μi) and polymer-to-cement ratios (P/C); (a) 7, (b) 14,
(c) 28, and (d) 90 days.

microstructure of an epoxy latex-modified mortar with P/C = 15%. This showed that the round lumps formed by
the epoxy aggregates completely covered the hydration products, which led to generation of more macropores.
As shown in Fig. 14d, when the addition of epoxy latex was increased to 20%, a large number of macropores and
round lumps were formed in the mortar system. More macropores were formed in the modified mortar system
as the amount of epoxy latex added was increased, which was consistent with previous fi­ ndings27.

Conclusions
This paper investigated the effects of different polymer-to-cement ratios (P/C) and curing ages on the pore size
structures of epoxy latex-modified cement mortars. The following conclusions can be drawn from this research:

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Figure 12.  Relationships between the values a and b of a linear function and curing ages, (a) a and (b) b.

Figure 13.  SEM images of ordinary cement mortar specimens cured for 28 days.

(1) With increasing P/C ratios, the proportion of micropores in epoxy latex-modified mortars tended to
decrease, while the proportion of macropores tended to increase. The addition of epoxy latex hindered the
hydration process of cement in the mortar system, which led to an increase in the proportion of gel pores.
(2) The addition of epoxy latex was only associated with the location parameters. The shape parameter in the
composite log-normal distribution was independent of epoxy doping.
(3) With increases in the P/C ratio, the epoxy latex-modified mortar developed larger pores.
(4) A composite logarithmic model was used to describe the pore size distribution of epoxy latex-modified
mortar. The relationships between pore size distribution location parameters and the polymer-to-cement
ratio and curing age were also obtained. The analytical methods proposed in this study can be applied in
other studies of pore size distributions.
(5) With increases in the amount of epoxy latex added, larger numbers of epoxy groups were formed in the
modified mortar system, which led to the generation of more macropores.
(6) From the above test results, polymer to cement ratios ranging from 10 to 15% were considered optimal for
preparation of such epoxy latex cement mortars.

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Figure 14.  SEM images of modified mortar specimens containing epoxy latex cured for 28 days: (a) P/C = 5%,
(b) P/C = 10%, (c) P/C = 15%, and (d) P/C = 20%.

Received: 8 December 2021; Accepted: 8 March 2022

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Acknowledgements
This research was supported by the Foundation of China Huaneng Research Project [Grant Number HNKJ19-
H13] and Major projects of Chongqing Education Committee [Grant Number KJZD-M201900702].

Author contributions
Conceptualization, P.L. and X.A.; Data curation, L.Z. and S.D.; Funding acquisition, P.L. and C.W.; Investigation,
C.W. and X.H.; Methodology, X.A. and X.H.; Validation, W.L. and L.Z.; Visualization, W.L. and S.D.; Writing–
original draft, P.L. and W.L.; Writing–review &amp; editing, P.L. and X.A.

Competing interests
The authors declare no competing interests.

Additional information
Correspondence and requests for materials should be addressed to X.A.
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