In this study, two types of acrylic latexes, PA (polyacrylate) and PU/PA (polyurethane modified PA), are investigated in their influences on mechanical properties of mortar under different interaction mechanisms. In light of a previous study, the polymer–cement hydrates interaction mechanisms in PA and PU/PA modified mortars are illustrated respectively, and the microstructures are simulated using a computer model. Through mechanical experiments, it is revealed that the incorporation of polymer tends to reduce the compressive strength and elastic modulus except PA at low P/C ratio, while improve the flexural strength and toughness. As compared with PA, PU/PA is more effective in these influences. All of the influences of PA and PU/PA on mechanical properties can be explained successfully based on the interaction mechanisms and microstructures. In addition, it’s also found that the compressive strength of polymer modified mortar can be roughly estimated based on a modified gel/space ratio, and the incorporation of polymers does not change the relationship between elastic modulus and compressive strength. A hightemperature curing procedure is concluded to be suitable for preparation of high-performance cement composites in short period.
Trang 1Microstructures and mechanical properties of polymer modified mortars
under distinct mechanisms
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China
h i g h l i g h t s
Microstructures of polymer modified pastes are illustrated by a computer model
Effects of different polymer latexes on mechanical properties of mortar are revealed
Gel/space ratio is modified for polymer modified cement composites
a r t i c l e i n f o
Article history:
Received 25 January 2013
Received in revised form 17 April 2013
Accepted 4 May 2013
Available online 8 June 2013
Keywords:
Polymer modified mortar
PA
PU/PA
Interaction mechanism
Mechanical properties
a b s t r a c t
In this study, two types of acrylic latexes, PA (polyacrylate) and PU/PA (polyurethane modified PA), are investigated in their influences on mechanical properties of mortar under different interaction mecha-nisms In light of a previous study, the polymer–cement hydrates interaction mechanisms in PA and PU/PA modified mortars are illustrated respectively, and the microstructures are simulated using a com-puter model Through mechanical experiments, it is revealed that the incorporation of polymer tends to reduce the compressive strength and elastic modulus except PA at low P/C ratio, while improve the flex-ural strength and toughness As compared with PA, PU/PA is more effective in these influences All of the influences of PA and PU/PA on mechanical properties can be explained successfully based on the interac-tion mechanisms and microstructures In addiinterac-tion, it’s also found that the compressive strength of poly-mer modified mortar can be roughly estimated based on a modified gel/space ratio, and the incorporation
of polymers does not change the relationship between elastic modulus and compressive strength A high-temperature curing procedure is concluded to be suitable for preparation of high-performance cement composites in short period
Ó 2013 Elsevier Ltd All rights reserved
1 Introduction
In the last half century, polymer modified mortar and concrete
have been widely utilized in construction practice, as polymer
modification can improve the workability, adhesive strength,
waterproofness and many other properties of cement based
mate-rials[1–4] Although polymer modified mortar and concrete are
mainly used as finishing or repair materials in their history[3],
fol-lowing the development of polymerization and composition
tech-niques, they have been sometimes used massively as the major
construction material in some projects, e.g pavement[5] In such
applications, polymer modification may be expected to increase
the tensile or flexural strength and toughness of the cement-based
materials, without inducing severe decrease in compressive
strength
In polymers that are commonly used for modification of ce-ment-based materials, in forms of latex, emulsion or re-dispersible powder, SBR (styrene butadiene rubber), EVA (ethylene–vinyl ace-tate copolymer) and acrylics have been deeply studied and broadly utilized in practice[2,3,6–8] In hardened state, the phenomena of noticeable increase in flexural strength and no improvement or even reduction of compressive strength of polymer modified cement-based composites, as compared with unmodified ones, have been commonly reported[3,4,9] Of course, there are a lot
of different and even contrary reports, e.g Pei et al.[10]found that the incorporation of polymer latex negatively influence both compressive and flexural strength; Mohammed et al.[11]reported that waste latex paint modification with low P/C (polymer to ce-ment mass ratio) might be positive in improving compressive strength of concrete Actually, results from the literature cannot
be compared with each other directly, if one does not recognize that polymers are generally added into cement composites in two different ways, say, keeping constant W/C (water to cement ratio) to obtain similar hydration of cement and keeping constant
0950-0618/$ - see front matter Ó 2013 Elsevier Ltd All rights reserved.
⇑ Corresponding author Tel.: +852 2358 8751; fax: +852 2358 1534.
E-mail addresses: mhy1103@gmail.com (H Ma), zongjin@ust.hk (Z Li).
Contents lists available atSciVerse ScienceDirect Construction and Building Materials
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c o n b u i l d m a t
Trang 2consistency by adjusting the W/C or the inclusion of plasticizer
[12,13] The former is a typical laboratory procedure, while the
la-ter is a trial-and-error procedure with its results directly applicable
in practice Generally, significant improvements of flexural
strength were reported in studies using the later method
Barlu-enga and Hernández-Olivares[12]noted that with SBR latex
mod-ification, keeping constant W/C, resulted in constant flexural
strength and noticeably decreased compressive strength, while
keeping constant consistency led to constant compressive strength
and significantly increased flexural strength
In a recent study, the advantages and disadvantages of different
types of polymers, as modifiers of cement composites, were
intro-duced[14], through which it can be seen that acrylics have perfect
mechanical properties and durability They are indeed frequently
employed in flooring compounds and mortars where the highest
level of physical performance (adhesion, abrasion resistance,
flex-ural strength, and impermeability) is required [2,6] Compared
with SBR, acrylics as modifier can improve flexural strength more
significantly at the same P/C, and may not reduce the compressive
strength at low P/C[15] The purpose of the present study is right
to modify cement mortar using acrylic latexes, and improve the
flexural strength and toughness without reducing compressive
strength obviously
Mechanisms of the interaction between organic and inorganic
phases in acrylics modified mortars have been discussed in the
previous study [14] Two types of acrylic latexes were used as
modifiers One was polymerized by emulsion polymerization with
monomers of MMA (methyl methacrylate), AA (acrylic acid), HEMA
(2-hydroxyethyl methacrylate) and cross-linking agent It was
la-beled as PA (polyacrylate) The other one was PU/PA (polyurethane
modified PA) In the present study, the interaction mechanisms are
introduced firstly Then, the microstructure evolution of polymer
modified cement pastes are simulated using a status-oriented
computer model based on the interaction mechanisms Keeping
constant W/C, influences of PA and PU/PA latexes on mechanical
properties of mortars are investigated experimentally, and
ex-plained based on the interaction mechanisms and simulated
microstructures
2 Materials and experiments
PA and PU/PA latexes were used as polymer modifiers The details of the
synthe-ses and physical properties of them can be found in the previous work [14] Cement
that satisfies the requirements of BS EN197-1:2000 for CEM I Portland cement of
strength class 52.5N (roughly equivalent to the requirements of ASTM C150 for
Type I Portland cement) was used as binder The chemical compositions and
phys-ical properties of the cement are listed in Tables 1 and 2 , respectively Siliceous sand
was used for preparing mortars with and without polymer The fineness modulus of
the sand is 1.73, while the average and maximum grain sizes of the sand are
0.33 mm and 2.36 mm respectively Deionized water was used for mixing various
mixtures Besides, in the mixing process of polymer modified mixtures, a type of
organosilicon defoamer in proper amount was added to suppress the foaming effect
of surfactants in the latexes.
In the preparation of all mortars, with and without polymer, the W/C ratio was
kept 0.5, while the sand to cement weight ratio (S/C) was kept 2 Various P/C ratios
were selected to investigate its influence on mechanical properties The mix
propor-tions of these mixtures are listed in Table 3 After mixing, the mixtures were cast in
steel moulds with different sizes for different test purpose The size of specimen is
50 mm 50 mm 50 mm for compression test, 40 mm 40 mm 160 mm for
flexural test, and 25 mm 25 mm 160 mm for fracture energy test After casting,
the mixtures were covered with plastic sheets to avoid evaporation After 24 h, all
specimens were demoulded and cured according to three different procedures
Pro-cedure 1 was a simple wet curing in a moisture room where the temperature and
relative humidity were approximately 23 °C and 95% respectively Procedure 2 con-sisted of a 2-day wet curing and a following dry curing in ambient environment (22 ± 2 °C and 40–70% RH) In procedure 3, a 2-day steam curing at 60 °C was fol-lowed by a 4-day oven-drying at 60 °C and subsequently a dry curing in ambient environment Procedure 1 is the most beneficial to cement hydration, and thus to the evolution of mechanical properties of unmodified mortars Procedure 2 is a commonly adopted method for polymer modified mortars as the dry curing is be-lieved to be necessary for polymer film formation [3] Procedure 3 is similar to the one that was used to prepare a high flexural strength cement paste, in which the steam curing and oven drying were used to promote cement hydration and film formation respectively in short stages [16]
Mechanical tests were conducted at ages of 3, 7, 28 and 60 days Compressive strengths of mortars were measured according to ASTM C109, on a MTS 815 ROCK Mechanics machine Flexural strengths were tested using three-point bending on a MTS 858 Mini machine, and calculated as
ff¼3PpL
where f f represents flexural strength, P p the peak load, L (140 mm) the span of imen, b (40 mm) and t (40 mm) the width and height of the cross-section of the spec-imen respectively Elastic moduli of mortars were estimated from the load– displacement curves of three-point bending according to the following equation,
E ¼ L
3
4bt3dP
where dP/dd is the gradient of the load–displacement curve corrected for the small amount of distortion in the three-point loading system As this was not a standard method, the results could not be compared with the data in the literature Thus, only the relative values of elastic moduli E r , i.e the values after being normalized by the elastic modulus of the reference mortar MPC at 60 days, would be shown in the pres-ent paper The fracture energy G F of mortars were roughly determined using a sim-plified method This method imitated the draft recommendation proposed by RILEM Committee on Fracture Mechanics of Concrete-Test Methods [17] based on the ficti-tious crack model Small-size specimens that could not fulfill the recommendation were used, and the notch length was equal to half depth of the beam It must be noted that G F measured in this way may not be the true values, thus they can only
be used to compare with each other, rather than with other data in the literature.
3 Latex-hydrates interaction mechanisms and microstructure evolution
3.1 Interaction mechnisms of different types of polymer latices and cement hydrates
The interaction mechanisms of the two different types of la-texes (PA and PU/PA) and cement hydrates have been studied in
a previous work[14] It has been found that after being incorpo-rated into cement paste, PA latex is destabilized and demulsified
PA molecules react with cement hydrates chemically to form a compound rather than form high purity film On the other hand, PU/PA latex is just slightly demulsified, and still forms film with high purity Behaviors and properties of PU/PA latex modified paste
or mortar can be explained by Ohama’s multi-step model[3]to a large extent, as PU/PA latex is sterically stabilized and PU/PA mol-ecules are relatively passive Behaviors of PA latex modified ce-ment composites have to be explained by the newly developed 4-step model [14] In step 1, immediately after mixing, a large amount of polymer particles adsorb on the surface of cement par-ticles or coalesce because of the demulsification The rest are still dispersed in the aqueous phase Adsorption happens in mixing process and the first several minutes after mixing In step 2, the hydration of cement is successively governed by dissolving and migration of ions Some chemical reactions take place between la-tex and cement hydrates Polymer particles adsorbed on the sur-face of cement particles are partially or totally embedded in hydrates This stage lasts for tens of minutes until flocculation occurs In step 3, cement hydration further progresses With the reaction between calcium hydroxide and surfactant, the dispersion
is severely destabilized Polymer particles with cement hydrates settle down together, and flocculation happens Due to the porous nature of the flocs, the growth or sedimentation of hydrates and the chemical reactions can continue in it, so that a complex
Table 1
Chemical composition (%) of cement.
SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 Loss on ignition
Trang 3compound of cement hydrates and flocculated PA particles are
formed Roughly speaking, this stage ends after wet curing ends
At last in step 4, with the drainage of water, in a few spaces,
poly-mer film forms, but rarely and not continuously More general
sit-uation is the formation of an organic–inorganic co-matrix
3.2 Simulation of the microstructural evolution of latex modified
cement pastes
Based on the latex-hydrates interaction mechanisms introduced
above, in light of a status-oriented computer model for cement
hydration (details can be found in Refs.[18,19]), the
microstruc-ture evolution of polymer latex modified cement paste can be
sim-ulated According to the status-oriented computer model, once W/
C and particle size distribution of cement are fixed, the
microstruc-ture of cement paste can be simulated as a function of degree of
hydration The simulated microstructure is composed of
anhy-drous cement grains, inner hydrates layers, outer hydrates layers
and large capillary pores The initial microstructure of polymer
modified cement paste can be simulated through randomly
replac-ing capillary water voxels by polymer voxels until the polymer
vol-ume fraction calculated from P/C is achieved One polymer voxel is
used to simulate one polymer particle InFig 1, the initial states of
pure cement paste (W/C = 0.5) and polymer modified cement paste
(W/C = 0.5, P/C = 0.1) are compared In the figures, white, dark and magenta represent cement particle, capillary water and polymer particles respectively The term ‘initial state’ means an assumed special state occurs immediately after mixing and before any chemical reaction happens, thus PA and PU/PA modified pastes need not to be distinguished from each other At this state, polymer particles are still uniformly dispersed in aqueous, just like in the polymer latex Keeping the same W/C, in the polymer modified ce-ment paste, the volume fraction of cece-ment is lower than that in pure cement paste due to the incorporation of polymer Note that all two-dimensional images shown below are cross-sectional views cut from the simulated three-dimensional microstructure The evolution of the microstructure of pure cement paste is shown inFig 2, in which white, blue, green and dark represent anhydrous cement grain, inner hydrates layer, outer hydrates layer and large capillary pore respectively.Fig 2a shows the state at the age of 3 days, which is the end of wet curing.Fig 2b shows the state at 28 days when the paste is under air-dry curing and has been relatively mature
It is assumed that the incorporation of polymer latex in cement paste does not influence the microstructure of inner hydrates layer, but only significantly influence the microstructure of outer hy-drates layer To simulate the microstructure evolution of polymer latex modified cement paste, irrespective of the type of latex, the outer hydrates layer is divided into two layers, i.e the compound layer and the composite layer formed in two individual processes The compound layer is formed in the wet curing period under the reaction between cement hydrates and the adsorbed polymer par-ticles The volume fraction of the compound layer can be easily cal-culated based on the degree of hydration of cement and the adsorption ratio of polymer, and the formation of this layer con-tacting with the original surfaces of cement particles can be then simulated according to the algorithm as described in Refs [18,19] Polymer voxels in this layer are dispersed as they cannot form any polymer film due to the chemical reactions The composite layer is formed following the withdraw of water and the formation of polymer films It is a composite of cement hydrates and polymer films The volume fraction calculation and
Table 2
Physical properties of cement.
Specific gravity Blain specific surface area (cm 2
/g) Setting time (min) Compressive strength (MPa)
Table 3
Mix proportions of mortars in weigh ratio.
Mark Water Cement Sand Polymer solid weight
Fig 1 Initial states of pure and polymer latex modified cement pastes (100lm 100lm): (a) pure cement paste (W/C = 0.5); (b) polymer latex modified cement paste (W/
Trang 4overall formation simulation of this layer are similar with that of
the compound layer Continuous polymer film in this layer can
be simulated by adding polymer voxels adhering to randomly
determined seed polymer voxels or existed polymer film voxels
until the volume fraction is achieved
The microstructure evolution of PA modified cement paste is
shown inFig 3, while that of PU/PA modified cement paste in
Fig 4 In these two figures, green and yellow are used to indicate
the cement hydrates in the compound layer and composite layer,
respectively, while magenta still the polymer phase In the
simula-tions, it is assumed that the degree of hydration of cement in
poly-mer modified pastes are the same as that in pure cement paste at
the same age, as it has been proved experimentally that the
incor-poration of polymer latex only retards early hydration (especially
in the first day), but influences little on long-term hydration[20]
It can be seen that, regardless of the type of paste, the
micro-structure consists of several layers Hydrated pure cement paste
contains four layers, i.e anhydrous cement grain, inner hydrates
layer, outer hydrates layer, and large capillary pore layer Under
the effects of polymers, the outer hydrates layer of polymer
mod-ified cement paste can be divided into two sub-layers, i.e
com-pound layer and composite layer The comcom-pound layer is mainly
formed by the organic–inorganic compound in the wet curing
per-iod as described above, while the composite layer is a cement
hy-drates-polymer film interpenetrated structure formed following
the withdraw of water in the dry curing period It is obvious that
PA modified paste has relatively large volume of compound layer
due to the high adsorption ratio of PA, higher than 70%, compared
with lower than 20% of PU/PA[14], while PU/PA modified paste has
large volume of composite layer at comparable degree of
hydra-tion Besides, the much higher volume fraction of polymer in com-posite layer of PU/PA modified paste guarantees that the formed polymer network is continuous and interpenetrate with cement hydrates The microstructural differences between these two dif-ferent types of polymer modified pastes are shown more clearly
inFig 5, in whichFigs 3b and 4b are locally zoomed in and com-pared Incorporation of polymer latex can improve the flexural strength of cement composites because of the formation of contin-uous polymer network which is interpenetrated with cement hy-drates, as well as the constrain of polymer on skin and inner micro-cracks[21] Both PA and PU/PA are effective for the later mechanism As a continuous polymer network can form and inter-penetrate with cement hydrates in the composite layer of PU/PA modified paste, the PU/PA latex should be more effective in increasing the flexural strength of mortar Generally, incorporation
of polymer reduces the compressive strength, as polymer can be considered as pore phase in the cement composite to a certain de-gree[9], while to what a degree depends on the type of polymer
4 Experimental results 4.1 Compressive strength 4.1.1 Influence of curing method on cement hydration and compressive strength
It has been mentioned in Section 2 that three different curing procedures were adopted for different purpose Cement degree of hydration and compressive strength evolution of un-modified mortar cured under different procedures are shown inFig 6 The Fig 2 Microstructure evolution of pure cement paste (W/C = 0.5, 100lm 100lm): (a) 3 days; (b) 28 days.
Trang 5evolutions of hydration degree of the cement (at W/C of 0.5) under
20 °C and 60 °C (isothermal) are from simulations using Hymostruc
developed in TU Delft Note that these are rough simulations, as
only temperature is considered while the influence of the relative
humidity under different curing conditions cannot, due to the
lim-itations of the software However, the accuracy of the simulation
result is enough for comparison purpose rather than accurate
cal-culation It can be seen fromFig 6that the development of
com-pressive strength is directly related to degree of hydration Due
to the high temperature, degree of hydration of cement at 3 days under 60 °C reaches the level of that at 28 days under 20 °C Thus, the compressive strength of mortar cured by procedure 3 reaches a mature level at very early age That is why Bracknbury et al.[16] used such a method to get high maturity in a short stage The dif-ference of compressive strength developments following proce-dure 1 and proceproce-dure 2 can be attributed to the availability of water for continuous hydration
4.1.2 Influence of polymer on compressive strength Although the wet curing is beneficial to cement hydration, and the high temperature treating can promote hydration in a short period and get high maturity fast, the wet plus air dry curing fol-lowing procedure 2 is the most similar with that in practice Thus, more attention focuses on the comparison of unmodified mortar MPC and polymer modified mortars cured by procedure 2 Follow-ing curFollow-ing procedure 2, the influence of polymers on the evolution
of compressive strength is shown inFig 7.Fig 7a shows that the incorporation of PA at the P/C of 0.05 (MA05) increases the com-pressive strength at each age, but higher incorporation ratio leads
to the reduction of compressive strength InFig 7b, it can be seen that the incorporation of PU/PA tends to reduce the compressive strength of mortar, and the higher the P/C, the larger the reduction, except MUA08 at early ages
According to Neville [22], regardless of age and aggregate volume fraction, the compressive strength of concrete is directly
Fig 4 Microstructure evolution of PU/PA cement paste (W/C = 0.5, P/C = 0.1, 100lm 100lm): (a) 3 days; (b) 28 days.
Fig 5 Microstructural comparison of different types of polymer modified pastes (W/C = 0.5, P/C = 0.1, 50lm 50lm): (a) PA modified paste, magnified from Fig 3 b; (b) PU/
PA modified paste, magnified from Fig 4 b.
Fig 6 Influence of curing method on hydration and compressive strength
Trang 6proportional to the cube of the gel/space ratio, r, and can be
calcu-lated as
where fc is the compressive strength in MPa, the 234 MPa is the
intrinsic strength of the hydrates gel of cement, and the gel/space
ratio, r, is defined as the ratio of the volume of cement hydrates
to the sum of the volumes of hydrated cement and of the original
capillary pores, i.e
r ¼ jhvca
wherejhis the hydrates volume expansion factor of ordinary
Port-land cement, which indicates the volume of hydrates generated
when 1 unit volume of cement is completely hydrated.jh equals
to 2.13 according to Sanahuja et al.[23].vcis specific volume of
ce-ment with the value 0.317 cm3/g.ais the degree of hydration of
ce-ment Generally, the validity of this model is good in predicting
compressive strength of cement composite without polymer, as
shown inFig 8, giving that the cement degree of hydration equals
to 0.48, 0.54, 0.58 and 0.60 at the ages of 3, 7, 28 and 60 days,
respectively, as determined using the thermogravimetric method
described in Ref.[19] The poor evolution of degree of hydration
at late ages should be attributed to the dry curing condition in
the corresponding period Due to the soft nature of polymers
compared with cement hydrates, rather than aggregate which is
much more rigid and strong, polymer phase should be considered
as space to a certain degree Thus, the gel/space ratio in polymer modified cement composite should be modified to
vcaþ W=C þgP=Cq
P
ð5Þ
whereqPis the density of polymer, andgis the effectiveness coef-ficient, which indicates to what a degree the polymer can be consid-ered as space
Experimentally determined compressive strengths of mortars,
as a function of polymer type and P/C, have been plotted in Fig 9 Assumingg= 0, which means treating polymer as aggregate with much higher rigidity, the predicted results according to Eqs (3) and (5)keep constant rather than a function of P/C Assuming
g= 1, which means totally treating polymer as space and makes the predicted results much lower than experimental values By fit-ting experimental results to Eqs (3) and (5), it is found that
g= 0.502 for PU/PA and g= 0.257 for PA give the best fitting respectively PU/PA has higher effectiveness coefficient because its rigidity is lower than PA, thus can be treated as space, or pore
to a higher degree To sum up the above analysis, when studying compressive strength of polymer modified cement composite, polymer phase can be treated as space to a certain degree, while
to what a degree depends on the type, or rigidity of the polymer
It must be noted that this theory has a presupposition, i.e only physical interactions occur between the polymer phase and ce-ment hydrates, e.g the effect of SBR latex on compressive strength
of cementitious materials[24] According to this theory, the com-pressive strength decreases with the increasing of P/C undoubt-edly However, the mortar MA05 gives an obvious exception, and this may be attributed to the chemical reactions, as described in the previous study[14] In a low dosage of polymer addition, the chemical reactions may make cement hydrates bond to each other
to form denser and stronger structure, which enhances the com-pressive strength of polymer modified mortars While in a high dosage of incorporation, the polymer phase tends to form larger particles which act as defects or partial spaces as described by
Eq.(5) Thus, there should be a threshold P/C, below which PA en-hances compressive strength mainly under the chemical mecha-nism, while above which the big grouping effect of PA will exceed its positive effect so that the compressive strength can be roughly estimated according to Eqs.(3) and (5) By a simple obser-vation on the experimental results, this threshold P/C for PA should
be between 0.05 and 0.1 In another research which kept constant W/C, the incorporation of a polyacrylate at low P/C was also reported to improve the compressive strength[25]
Fig 7 Influence of polymer modifications on the evolution of compressive Fig 8 Experimentally determined compressive strength of MPC cured by
Trang 7proce-4.2 Flexural strength
The evolutions of flexural strength of mortars listed inTable 3,
cured following procedure 2 are plotted inFig 10 An initial
anal-ysis of the data leads to the following remarks regarding the
influ-ence of polymer First, at the age as early as 3 days, the flexural
strength of almost all polymer modified mortars, irrespective of
the type of polymer and P/C ratio, are lower than that of MPC This
is because the incorporation of polymer retards the early hydration
of cement on the one hand, and the polymer cannot form film in
the wet curing environment on the other Second, at late ages,
e.g 28 days and 60 days, following the withdraw of water and
the gradual formation of polymer film, polymers tend to increase
the flexural strength, especially for the cases with P/C from 0.05
to 0.1 PU/PA modifications with P/C > 0.1 seem to be not so
effec-tive in improving flexural strength, perhaps because the
over-per-colation of polymer film is harmful to the continuity of cement
hydrates Actually this is also true for PA modified mortars
accord-ing to limited data involvaccord-ing high polymer content
Although no continuous film network can form in PA modified
mortars as shown in Fig 3, PA can also improve the flexural
strength, as the incorporation of PA latex can effectively limit the
formation of cracks and delay the propagation of
micro-cracks when loaded Beside this mechanism, continuous polymer
film network can form in PU/PA modified mortars, as shown in
Fig 4, which helps to improve the flexural strength more
signifi-cantly E.g at the age of 60 days, MA05 has a flexural strength of
8.51 MPa, which is 7.59% higher than the 7.91 MPa of MPC, while
the value of PU/PA modified mortar (MUA05) at the same P/C
and the same age is 9.68 MPa, which is 22.38% higher than MPC
The data inFig 10clearly highlight an optimum of polymer
con-tent, corresponding to P/C between 0.05 and 0.1 irrespective of
polymer type, with regard to improving flexural strength A similar
conclusion has also been drawn by Bureau et al.[13]in their study
on mechanical properties of SBR modified mortars
As compared with the mortars cured by procedure 2, the
high-temperature treated mortars following procedure 3 seem to have
higher flexural strength at the age of 28 days, as shown in
Fig 11 The high-temperature steam curing not only promotes
the early hydration and generates more hydrates fast as proved
byFig 6, but also makes sure the high continuity of hydrates phase
at the end of 3 days Under the help of the followed oven curing,
polymer film can form with high quality and without disturbing
the continuity of the hydrates phase With these mechanisms,
PU/PA modifications even with high P/C ratio (0.2) can also
im-prove the flexural strength
4.3 Elastic modulus Due to the reasons described in Section 2, only relative elastic moduli of mortars are plotted against age inFig 12 Simple obser-vation can lead to a conclusion that the incorporation of polymer tends to decrease the elastic modulus of mortar, and the higher the P/C, the larger degree of the decreasing at late ages This is almost a general feature of polymer modified cement composites, especially in the cases of keeping constant W/C [3,20,26]
Fig 9 Influence of polymer on compressive strength at 28 days.
Fig 10 Influence of polymer modifications on the evolution of flexural strength: (a) PA modified mortars vs MPC; (b) PU/PA modified mortars vs MPC.
Fig 11 Flexural strength comparison of PU/PA modified mortars cured by different methods at the age of 28 days.
Trang 8Compared with others, the MA05 only has limited decreasing in
elastic modulus Again, this may be attributed to the chemical
interactions between PA latex and cement hydrates
According to the ACI building code (ACI 318), the elastic
modu-lus of concrete can be estimated using a binary function of its unit
weight and compressive strength Assuming a constant density for
normal weight concrete, the elastic modulus will be directly
pro-portional to the square root of compressive strength[27] Other
standards may use different equations to link elastic modulus to
compressive strength, e.g in Eurocode 2 and CEB-FIP Model Code
the elastic modulus of concrete is directly proportional to the cubic
root of its compressive strength[28] Although only relative elastic
moduli are given in the present study, it should also read
or
Er¼ k2 f1=3
where k1and k2are constants InFig 13, the elastic moduli of all
modified and unmodified mortars at different ages are plotted
against the corresponding compressive strength, and then fitted
using Eq.(6) It is obvious that the relationship between relative
modulus and compressive strength can be fitted by a unique
func-tion, irrespective of the type of mortar In other words, the empirical
equations used to estimate the elastic modulus of concrete based on
compressive strength can also be used for polymer modified
con-crete Based on the digitized three-dimensional microstructure as
described in Section 3, in light of multi-scale micromechanics[29]
or finite element methods[30], the elastic properties of polymer modified cement composite may be predicted more accurately
4.4 Toughness The ratio of flexural strength to compressive strength (ff/fc) of a cement composite is an important indicator of its toughness [24,31] Higher ff/fcindicates higher toughness The toughness indi-cators of all mortars at late ages are shown inFig 14 It can be seen that the incorporation of polymer latexes of both types can im-prove the toughness (indicator) The effect of PA is very limited
at low incorporation ratio (P/C = 0.05), but considerable somewhat
at higher P/C ratio (0.1) PU/PA seems to be more effective in increasing the toughness indicator, due to the formation of contin-uous polymer film network as shown inFig 4
The fracture energy GF of PU/PA mortars have been roughly measured, and the results are shown inFig 15 Cured following procedure 2, the incorporation of PU/PA latex helps to improve the fracture energy When P/C is not larger than 0.1, the degree
of improvement increase with the increasing P/C However, when P/C > 0.1, the improvement becomes not so significant, due to the disturbed hydrates continuity induced by the large volume of poly-mer This trend is consistent with that of toughness indicator, as shown inFig 14 MUA10 (P/C = 0.1) gives the best performance
at 28 days, with an increasing of around 40% in fracture energy,
as compared with MPC (P/C = 0) High-temperature treatment fol-lowing procedure 3 further improves fracture energy, as clearly shown inFig 15 Under the help of high-temperature steam, large amount of hydrates form in the first 3 days with high continuity,
Fig 12 Influence of polymer modifications on the evolution of elastic modulus: (a)
Fig 13 The relation between relative elastic modulus and compressive strength.
Trang 9and in the following oven-curing for facilitating polymer film
for-mation, higher P/C results in higher fracture energy
5 Conclusions
The following conclusions can be drawn from the present
study:
(1) PA and PU/PA modified mortars have different
microstruc-ture, due to their differences in modification mechanisms
This has been clearly illustrated in the present study under
the help of a status-oriented computer model for
micro-structure simulation
(2) Incorporation of polymer tends to reduce the compressive
strength of mortar The compressive strength of polymer
modified mortars can be roughly estimated based on a
mod-ified gel/space ratio
(3) Polymer modifications reduce the elastic modulus of mortar,
but do not influence the elastic modulus-compressive
strength relationship Irrespective of the type of mortar, its
elastic modulus can be estimated based on a unique function
describing the relationship between elastic modulus and
compressive strength
(4) Cured under procedure 2, a curing method similar with the
environment in practice or in situ curing, polymer
modifica-tions can improve the flextural strength and toughness, and
PU/PA performs better than PA due to the formation of
high-quality polymer film network In PU/PA modified mortars,
the P/C range between 0.05 and 0.1 seems to be the most
reasonable considering both function and cost
(5) Cured under the high-temperature procedure 3, which
con-sists of a cement hydration promotion period and a polymer
film formation facilitation period, the flexural strength and
fracture energy of PU/PA modified mortars can be further
improved Thus, this method has the potential to be used
to prepare high-performance cement composites in short
period
Acknowledgements
Financial supports from a China Basic Research Grant, Basic
Re-search on Environmentally Friendly Contemporary Concrete
(2009CB623200) and from Hong Kong RGC, Systematic studies on magnesium phosphate cement-based concrete (615810) are grate-fully acknowledged
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Fig 15 Fracture energy of PU/PA modified mortars at 28 days cured by different
methods.