Influence of polymer latex on the setting time, mechanical properties and durability of calcium sulfoaluminate cement mortar

Construction and Building Materials 169 (2018) 911–922

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Influence of polymer latex on the setting time, mechanical properties and durability of calcium sulfoaluminate cement mortar Lin Li, Ru Wang ⇑, Qinyuan Lu Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, School of Materials Science and Engineering, Tongji University, 4800 Cao’an Road, Shanghai 201804, China

h i g h l i g h t s
Polymer latexes perform water-reduction effect and delay the setting of CSA cement.
SBR latex modified CSA cement mortar performs the best properties.
Polymer latex modified CSA cement mortars perform good durability.
Shrinkage rate of CSA cement mortar is decreased by addition of polymer latex.
Polymer latex contributes to the flexural strength increasing of CSA cement mortar.

a r t i c l e

i n f o

Article history: Received 23 August 2017 Received in revised form 27 February 2018 Accepted 1 March 2018

Keywords: Calcium sulfoaluminate cement Polymer latex Setting time Mechanical properties Durability

a b s t r a c t In this study, three different types of polymer latexes (styrene butadiene rubber (SBR), styrene acrylic ester (SAE) and polyacrylic ester (PAE)) with three different dosages (0%, 10% and 20%) were employed to prepare polymer latex modified calcium sulfoaluminate (CSA) cement mortar based on the same workability. The setting time, mechanical properties including strength, shrinkage, weight loss, water capillary adsorption, anti-penetration property and durability properties such as resistance to freeze-thaw cycle, carbonization and sulfate attack were measured. The experimental results show that polymer latexes perform good water-reduction effect and delayed setting behavior on CSA cement mortar. Meanwhile, polymer latexes are helpful to enhance the mechanical properties of CSA cement mortar. Generally, SBR latex modified mortar performs the best properties in terms of strength, weight loss, water capillary adsorption and anti-penetration property. Addition of polymer latex decreases strength when subjected to extreme conditions, however, polymer latex modified mortars perform good resistance to carbonization and sulfate attack, although only SBR latex improves the freeze-thaw cycle resistance. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Polymer latexes are widely utilized for modification of cement mortar and concrete [1]. The key properties of polymer latexes are their ability to form flexible and homogeneous polymer films after dehydration [2], which provide good adhesion and cohesion in cement mortar and concrete [3]. Meanwhile, polymer latexes bring many superior properties to overcome some of the shortcomings of conventional cement and concrete [4]. Among these polymer latexes, SBR (styrene butadiene rubber), SAE (styrene acrylic ester), EVA (ethylene–vinyl acetate) and PAE (polyacrylic ester) latex are the most frequently used latex types [3,5].These polymer latexes contribute to improve workability [6], increase ⇑ Corresponding author. E-mail address: [email protected] (R. Wang). https://doi.org/10.1016/j.conbuildmat.2018.03.005 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

mechanical strength [6–8], decrease porosity [5,9], reduce the water absorption and permeability [8,10,11] and improve the durability [8,11]. Because of these superior properties, polymer latexes make possible for a variety of niche applications in terms of repair mortars, tile adhesives, waterproofing membranes and self-leveling floors [12]. Calcium sulfoaluminate (CSA) cement was developed in the 1970 s by China Building Materials Academy [13], it mainly con in vari belite (C2S), and anhydrite (CS) sists of ye’elimite (C4A3S), ous ratios [14], these phases could be formed at 1250 °C, which is 200 °C lower than that of Portland cement (OPC) clinker [15]. Besides, less CO2 is released from the production of CSA clinker due to the lower limestone amount [16]. Meanwhile, CSA cement demonstrates several advantageous properties, e.g. fast drying and setting, high strength at early stage [17], low shrinkage, good resistance to sulfate-rich environments [18] and high frost and

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L. Li et al. / Construction and Building Materials 169 (2018) 911–922

corrosion resistance [19], etc. Thus, CSA cement has been promoted as one alternative to OPC because of above environmental and technical benefits. Contrary to OPC in which alite (C3S) is the dominant phase which accounts for main mechanical properties, CSA cement performs different hydration reactions. Ye’elimite reacts with calcium sulfate immediately to form ettringite (AFt) and aluminum hydroxide (AH3) gel according to Eq. (1) [20]. This reaction is mainly responsible for the early mechanical proprieties of CSA cement matrix, which are generally superior to OPC [21]. When calcium sulfate is completely consumed, ye’elimite will react with only water to generate monosulfate (AFm) according to Eq. (2). The presence of lime which could be from the hydration products of OPC benefits to accelerate hydration kinetics and generate more ettringite according to Eq. (3). When ye’elimite is completely consumed, belite (C2S) begins to react with aluminum hydroxide and water to give rise to strätlingite (C2AH8) formation according to Eq. (4) [21]. Some other hydrates such as calcium silicate hydrate (C-S-H) gel, monocarboaluminate or hydrogarnet could be formed when minor phases present in the clinker formulations [15,22].

 þ 2CS  þ 38H ! C3 A  3CS   32H þ 2AH3 C4 A 3 S

ð1Þ

  12H þ 2AH3  þ 18H ! C3 A  CS C4 A 3 S

ð2Þ

bridge and pavement, residential construction and renovation mainly for waterproofing, tile adhesion, and the fast construction for time as well as cost saving, there will be booming for the development of polymer modified calcium sulfoaluminate cementbased materials. In this study, the influence of polymer latex on the setting behavior, mechanical properties and durability of CSA cement mortar is investigated. Three widely used polymer latexes including SBR, SAE and PAE latex and one CSA cement are taken to analyze. Mortars with three different ratios (0%, 10%, and 20%) by solid of polymer latex to CSA cement at a constant workability were investigated. 2. Experimental 2.1. Materials The cement used in this investigation was one rapid-hardening CSA cement, its chemical composition was shown in Table 1 and the mineral composition was displayed in Table 2. It has a Blaine fineness of 5350 cm2/g. The median diameter of the particle size distribution (D50) of the CSA cement is 9.0 mm. Three kinds of polymer latexes including SBR, SAE, and PAE latex were separately added to modify the properties of CSA cement mortar. The parameters of these three polymers were shown in Table 3. Tap water and standard sand according to ISO 679:2009 [32] were used in all experiments.

2.2. Mix proportions, specimen preparation and test methods

  32H  þ 8CS  þ 6CH þ 74H ! 3C3 A  3CS C4 A 3 S

ð3Þ

C2 S þ AH3 þ 5H ! C2 ASH8

ð4Þ

While there are an extremely large number of studies on the performance of polymer-modified cement mortar and concrete based on OPC, there is a few of literatures reporting the effect of polymers on the properties of CSA cement. Chang et al. [23] synthesized two kinds of SBR latexes with and without emulsifier respectively and investigated these two latexes on the performance of their modified rapid hardening calcium sulphoaluminate cement mortars. The results show that water to cement ratio of polymer modified CSA cement mortar is lower than that of control mortar, the more polymer is added, the less water is required when the same level of flow table value is achieved. The compressive and flexural strengths of latex modified CSA mortar are also lower than that of control mortar. Ye et al. [24] and Meng et al. [25] investigated the mechanical properties of CSA cement mortar modified with VAE polymer powders. The addition of VAE polymer in CSA cement mortar benefits to improve flexural and adhesion strengths while decreases compressive strength. Zhang et al. [26] investigated SBR and VAE latexes modified CSA cement mortar and revealed both of these two latexes would decrease flexural strength which conflicts with literatures [24,25]. Brien et al. [27] assessed adhesion characteristics for polymer modified CSA cement mortar by the use of four different kinds of redispersible polymer powders at a consistent polymer/cement ratio of 0.15, and the results show that SBR polymer demonstrates the poorest adhesion performance. Brien et al. [28] also found that the split tensile strength of CSA cement mortar gradually decreases when with more VAE polymer powders, which is not well consistent with literature [24]. The durability of polymer modified CSA cement mortar was also evaluated to some extent. Li et al. [29,30] and Lu et al. [31] analyzed the sulfate resistance of SAE latex modified CSA cement mortar, the results indicate that SAE latex contributes to increase impermeability as well as sulfate resistance to CSA cement mortar. It can be seen from above analysis that the study on the influence of polymer latex on CSA cement mortar has not been well reported. Considering the increasing interest on the application of polymer modified cement mortar in infrastructures such as

Three different types of polymer latexes (SBR, SAE and PAE) have been employed to prepare polymer modified mortars. The sand to cement ratio was kept constant at 3 for all mixtures. Polymer latex was mixed with water firstly and then a certain amount of CSA cement was weighted into the liquid for the fresh mortar preparation in accordance to ISO 679:2009 [32]. The water to cement ratio (mw/ mc) was adjusted by keeping the same flow table value. The constant flow table value was achieved referring to EN 459-2:2010 [33], aiming at a similar state of workability for the CSA cement mortar matrix. The flow table values of fresh mortars were determined by the cone-shaped metal ring, which was filled with fresh mortar on a shaking table and vibrated 25 times within 25 s, and then the final diameter of the mortar mix in two vertical directions was measured. The average value of the measured diameters is the so-called flow table value. The water to cement ratio of the polymer latex modified CSA cement mortars was determined by fixing the flow table value at a constant value of (170 ± 5) mm, as listed in Table 4. Meanwhile, the water-reduction rate of polymer latex was calculated according to Eq. (5).

WRA ¼

Wc  Wp Wc

ð5Þ

where: WRA: water-reduction rate; Wc: water requirement of control mortar; Wp: water requirement of polymer latex modified mortar. The setting time of polymer latex modified CSA cement mortar was determined according to ISO 9597:2008 [34] by the use of Vicat apparatus. The hardened specimens for the measurement of mechanical strength, dry shrinkage and water capillary adsorption were prepared by casting fresh mortar mixtures into 40 mm  40 mm  160 mm prismatic molds and compacted with an external vibrator, the mortar specimens with molds were cured initially under 20 °C ± 1 °C and 95% ± 1%RH, then all specimens were demolded one day after casting and then kept in 20 °C ± 2 °C and 50% ± 5%RH for subsequent curing. The compressive and flexural strength values were determined according to standard ISO 679:2009 [32]. The shrinkage rate of the hardened mortars was determined referring to EN 13454-2:2003 [35], which was calculated according to the length of the mortar at different curing ages and the initial length which was measured after demolding at 1 day. The water capillary adsorption of polymer latex modified CSA cement mortar was measured according to standard ISO 15148-2002 [36]. The mortar specimens were cured for 6 days at 20 °C ± 1 °C and 50% ± 5%RH after demolding and then taken out to dry at 40 °C for 2 days. The four around surfaces were sealed with paraffin before the upside of the specimens was dipped into water. The water capillary adsorption was calculated based on the adsorbed water at different times. The anti-penetration capacity was determined by using Chinese standard DL/T 51262001 [37]. Conical mortar specimens with upside and underside surface diameters of 70 mm and 80 mm, height of 30 mm were prepared with the same curing process as above mentioned. When the total curing age reached 7 days, the around surface of the specimens were sealed with paraffin wax and the underside surface was contacted with water. The water pressure was increased step by step to 2.0 MPa to

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L. Li et al. / Construction and Building Materials 169 (2018) 911–922 Table 1 Chemical composition of CSA cement (w %). Al2O3

CaO

SO3

SiO2

Fe2O3

K2O

MgO

Mn2O3

Na2O

P2O5

SrO

TiO2

LOI

23.84

44.20

15.04

9.82

1.95

0.26

2.46

0.02

0.08

0.11

0.10

0.95

1.17

Table 2 Mineral composition of CSA cement (w %).  C4A3S

C2S

 CS

C4AF

C12A7

CT

CaMg(CO3)2

MgO

Amorphous

35.4

26.4

14.3

4.6

3.1

1.0

1.5

1.9

11.7

Table 3 Characteristics of polymer latex.

Solid content/% pH value Viscosity/(mPas, 23 °C) Density/(g/cm3) Glass transition temp./°C D50/lm

3. Results and discussions SBR latex

SAE latex

PAE latex

51 ± 1 7.0–9.0 50–300 1.04 +14 0.11

57 ± 1 7.0–8.5 300–750 1.04 8 0.14

47 ± 1 8.5–10.5 100–200 1.04 +12 0.11

3.1. Effect of polymer latex on the properties of fresh mortar 3.1.1. Water-reduction rate The water-reduction rate of polymer modified CSA cement mortar is given in Fig. 3. As seen in Fig. 3, the water-reduction rate of polymer latex on the CSA cement mortar at a given flow table value varies from polymer types as well as their additions. SAE latex represents lowest water-reduction rate (18.8%) at the dosage of 10% while PAE latex shows the highest water-reduction rate (29.2%). It is also found from Fig. 3 that the water-reduction rate is markedly increased when the polymer to cement ratio is increased from 10% to 20%, however, SAE latex continues to demonstrate the lowest water-reduction ratio (33.3%). The results are consistent with previous studies in OPC mortar that the use of polymer latex benefits to reduce water requirement of cement mortar for a specific consistency [9]. For example, Eren et al. [3] tested the spread diameters of fresh mortar by keeping the same mw/mc as well as the mw/mc by keeping the same consistency, the results both indicated that the water reducing rate of SBR is better than SAE; Hwang et al. [38] reported that PAE latex was the more effective polymer to reduce mw/mc than SBR, which is consistent with our results. The reason of water reducing effect of polymer latex is attributed to the presence of fine polymer particles in latex emulsions, these polymer particles act as ‘‘ball bearings” to ease the relative movement of cement and hydration particles. Meanwhile, the surfactants added in the latexes to reduce segregation of the suspension act like water reducing admixtures and significantly reduce water demand [39].

observe whether the water leaks on the surface of specimens. If water penetrates through three of six specimens under the same pressure, then the antipenetration pressure was determined as the pressure that the mortar can survive. The durability performance of polymer latex modified CSA cement mortar was considered by calculating the durability coefficient (C), which was determined by the ratio of the flexural and compressive strength with and without attack according to Eq. (6). The process for durability evaluation was displayed in Fig. 1.

C¼

Fwa Fwoa

ð6Þ

where: U: the durability coefficient; Fwa: strength with attack; Fwoa: strength without attack. The specimens prepared for the measurement of freeze-thaw cycle were prestored in water for 6 h until the test of freeze-thaw cycle. One group of specimens as reference sample was cured in the air under 20 °C ± 1 °C and 50% ± 5%RH, and the other one was transferred into the environmental chamber for freeze-thaw measurement. The detail procedure of freeze-thaw cycle was displayed in Fig. 2, totally 56 freeze-thaw cycles were conducted. Two groups of specimens were casted to determine anti-carbonization property, the one as reference was cured in the air under 20 °C ± 1 °C and 50% ± 5%RH, the other one was cured in the carbonization chamber, the concentration of CO2 gas in the chamber was stabilized in 20% ± 3%. For the measurement of anti-sulfate attack, three groups of specimens were prepared, the reference group was cured in the deionized water for 28 days, and the other two groups of specimens were moved into the solutions with different sodium sulfate concentration (3%, 5%). The slices for microstructure and morphology observations were collected from the specimens after compressive and flexural strength measurement at 7 days. The samples were etched with 5% hydrochloric acid (HCl) for one hour, and cleaned with isopropanol afterwards. The slices were dried and gold coated before the measurement using Quanta 200 FEGSEM.

3.1.2. Setting time In general, the setting time of polymer modified cement mortar and concrete would be prolonged to some extent when in comparison with original cement mortar and concrete, and its trend is dependent on the polymer types and polymer to cement ratio [1,40]. The setting time of polymer latex modified CSA cement mortar is illustrated in Table 5, in which we can find that SAE latex modified CSA cement mortar causes the most delay in setting, while PAE latex does not show significant delay effect. The setting

Table 4 Mix proportions of polymer modified mortar. CSA

SBR latex (solid dosage)

SAE latex (solid dosage)

PAE latex (solid dosage)

Sand

mw/mc

Flow/mm

100 100 100 100 100 100 100

0 10 20 0 0 0 0

0 0 0 10 20 0 0

0 0 0 0 0 10 20

300 300 300 300 300 300 300

0.48 0.35 0.28 0.39 0.32 0.34 0.28

175 175 173 170 170 170 172

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L. Li et al. / Construction and Building Materials 169 (2018) 911–922

Fig. 1. Process for durability evaluation.

modified OPC mortar. Regardless of the polymer types, the mechanism of delayed setting time can be assumed by retardation effect of polymers on cement hydration. Generally, the retardation mechanism of polymer latexes on the cement hydration could be explained by two aspects: 1) chemical retardation, which is mostly related to the complexation of R-COO- group with calcium ions, thus the induction period is prolonged [42] and 2) physical retardation, that is mainly due to the adsorption of polymer particles on cement surface [41]. Fig. 2. Procedure of freeze-thaw cycle.

Fig. 3. Water-reduction rate of polymer latex modified CSA cement mortar.

Table 5 Setting time of polymer latex modified CSA cement mortar.

Control SBR10% SBR20% SAE10% SAE20% PAE10% PAE20%

Initial setting time/min

Final setting time/min

42 60 63 68 71 45 43

52 68 70 83 78 52 50

time of SBR and SAE latex is prolonged with increasing of polymer latex from 10% to 20%, however, when the dosage of PAE latex increases, the setting time is a little bit shortened. The above results are well consistent with literature [41] but for polymer

3.2. Effect of polymer latex on the properties of hardened mortar 3.2.1. Strength The flexural and compressive strength of polymer latex modified CSA cement mortar are plotted in Fig. 4(a) and (b), respectively. Generally, polymer latex could be employed to decrease mortar brittleness [43]. However, different types of polymer latexes perform different effect on the strength. As seen from Fig. 4(a), the flexural strength of control mortar at 1 day, 7 days and 28 days is 6.7 MPa, 7.6 MPa and 8.1 MPa, respectively. Addition of 10% SBR latex decreases the flexural strength to 5.3 MPa at 1 day. However, the flexural strength at 7 days catches up to 9.2 MPa, and that is much higher than that of control at 28 days. The flexural strength of CSA cement mortar when in addition of 20% SBR latex is higher than that when in addition of 10%. Different trends of flexural strength development could be found from CSA cement mortar with SAE latex and PAE latex. It could be noted from Fig. 4(a) that the flexural strength (6.8 MPa) of CSA cement mortar with addition of 10% SAE latex at 1 day is similar as that of control mortar, whereas it is lower than that with addition of 20% SAE latex. When the curing time reaches 7 days and after, the flexural strength of CSA cement mortar with more SAE latex tends to decrease. With addition of 10% PAE latex in CSA cement mortar, the flexural strength is obviously higher than that with SBR latex and SAE latex, addition of 20% PAE latex contributes to higher flexural strength at 1 day, however, the strength development tends to be slower than others. Generally, PAE latex tends to be more effective to improve flexural strength of CSA cement mortar when the polymer latex addition is 10%, and the mortar with SBR latex shows the highest flexural strength among these three polymer latexes when the dosage is 20%. The flexural strength improvement of polymer modified cement mortar could be attributed to the possible film formation or interactions between the cement hydrates and the polymer particles, thus to increase the flexural strength of the binder matrix between the aggregate and the binder [44]. The result is consistent with literature [45] that SBR latex performs better flexural strength than SAE latex in OPC mortar. Ramli et al.

L. Li et al. / Construction and Building Materials 169 (2018) 911–922

915

Fig. 4. Strength of polymer latex modified CSA cement mortar: (a) flexural strength and (b) compressive strength.

[46] reported that SBR provided better strength than PAE mixes in OPC mortar even under various curing conditions. The flexural strength increasing can be also explained by the refinement of interface transition zone (ITZ) of specimens with the addition of polymer, which could effectively fill the internal macro and micro defects of cement matrix, thus improving bond between the aggregate and the matrix [47–49]. It is interesting to note that the compressive strength values were much more influenced from latex modification at constant consistency when compared to flexural strength. The compressive strength of CSA cement mortar modified with addition of 10% polymer latex is lower than that of control at 1 day, 7 days and 28 days, however, the value is different when in addition of different types of polymer latexes. PAE latex modified mortar demonstrates close value to the control while SBR latex modified mortar displays the lowest compressive strength among three polymer latex modified mortars. When the addition of polymer latex in CSA cement mortar increases to 20%, the SBR latex modified mortar shows increased compressive strength, while that of the rest two decreased. SAE latex modified mortar has the lowest compressive strength. The trend of compressive strength values from polymer latex modified mortar is the same as that of flexural strength, which indicates that with addition of 10% polymer latex in CSA cement mortar, the strength of mortar modified with PAE latex > the strength of mortar modified with SAE latex > the strength of mortar modified with SBR latex. Meanwhile, with addition of 20% polymer latex in CSA cement mortar, the strength of mortar modified with SBR latex > the strength of mortar modified with PAE latex > the strength of mortar modified with SAE latex. Generally, inclusion of polymer latex in cement mortar and concrete decreases compressive strength, due to a lower mechanical capacity of polymer film with regard to cement paste [50–52]. However, addition of 20% SBR latex in CSA cement mortar performing increased compressive strength can be attributed to the lower water requirement and relative high mechanical capacity of SBR polymer. 3.2.2. Shrinkage The shrinkage of polymer latex modified CSA cement mortar with various curing ages is presented in Fig. 5. It is found that the shrinkage of CSA cement mortar is quite low (<0.03%), even that the shrinkage could be decreased by the use of polymer latex. However, the extent of shrinkage decreasing is dependent on the types and addition amount of polymer latex. When the addition of polymer is 10%, PAE latex modified mortar performs the lowest shrinkage, although CSA cement mortar modified with SBR latex has the highest shrinkage rate in the modified mortars but it is lower than the control mortar. In this case, the trend of shrinkage of polymer modified CSA cement mortar is quite similar as that of

Fig. 5. Shrinkage of polymer latex modified CSA cement mortar.

strength development. PAE latex demonstrates the best improvement on strength and shrinkage of CSA cement mortar while SBR latex shows the lowest. The shrinkage is sharply decreased with increasing addition of polymer latex. Compared to SAE and PAE latex, the shrinkage of mortar with 20% addition of SBR latex is lower (0.021%) than that with 10% (0.026%) at 28 days. Different from SBR latex, SAE latex modified mortar performs near-zero shrinkage when PAE latex modified mortar demonstrates somewhat expansion property even the expansion is a little bit decreased after 7 days. It can be concluded from above analysis that the addition of polymer in CSA cement can help to reduce shrinkage, which is consistent with the results in polymer modified OPC composites [1], and PAE latex has the best effect on the shrinkage decreasing while SBR latex performs the lowest effect. The formation of polymer film in the latexmodified mortars might act as ‘‘micro-fiber” in the matrix that restricts the shrinkage development [53], meanwhile, these formed polymer films block the pores and improve the pore structures inside of cement matrix [54], which benefits to decrease water evaporation and finally to decrease shrinkage [53,55]. 3.2.3. Weight loss The weight loss of polymer latex modified CSA cement mortar under standard condition was measured and shown in Fig. 6. From the figure, it could be easy found that the weight loss is gradually increased with time prolongation. Addition of polymer latex benefits to decrease the weight loss of CSA cement mortar. When the addition of polymer is 10%, the weight loss of CSA cement mortar modified with PAE latex is the lowest among that of these three polymer latexes, SAE latex modified mortar performs the biggest value even it is still lower than the control. It can be seen that the water loss of CSA cement mortar modified with 10% of polymer

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is observed and the reasons for low water capillary adsorption may be due to the sealing effect of the polymer films formed in the mortar matrix [3], which provided a considerable increase in the resistance to water adsorption.

Fig. 6. Weight loss of polymer latex modified CSA cement mortar.

latex is positive correlation with strength and shrinkage. The weight loss is further declined by increasing polymer amount from 10% to 20%. For SBR latex modified mortar, the weight loss at 42 days will be decreased from 1.0% to 0.23% when the addition of SBR latex is increased from 10% to 20%. Generally, CSA cement mortar modified with SBR latex performs the lowest weight loss while that of SAE latex has the biggest weight loss. The water loss is mainly dependent on the microstructure of cement matrix which is highly influenced by polymer latex, that will be further described in later section. 3.2.4. Water capillary adsorption The water capillary adsorption of polymer latex modified mortar is given in Fig. 7. It can be seen that the water capillary adsorption is significantly decreased with addition of polymer latex in CSA cement matrix, which is consistent with results of polymer modified OPC mortar and concrete [6,55–57]. When with 10% addition of polymer latex, the effect of SBR latex on the water capillary adsorption of CSA cement mortars is more evident than that of SAE and PAE latex, which means SBR latex modified mortar demonstrates the lowest water capillary adsorption, while SAE latex modified mortar shows the highest value among these three polymer modified mortars. The same trend is also found at addition of 20%. Generally, the water capillary adsorption of SBR latex modified mortars < the water capillary adsorption of PAE latex modified mortars < the water capillary adsorption of SAE latex modified mortars. Generally, the trend of water capillary adsorption of polymer modified CSA cement mortar is similar as that of water loss, which is mainly controlled by the microstructures of polymer modified mortar matrix. Regardless of polymer types, a decreasing trend of water capillary adsorption with addition of polymer latex

Fig. 7. Water capillary adsorption of polymer latex modified CSA cement mortar.

3.2.5. Anti-penetration property The effect of polymer latex on the anti-penetration property of CSA cement mortar is also investigated. The result indicates that the water cannot penetrate through the specimens when the water pressure was increased step by step to 2.0 MPa. After the test, the mortar specimens were split to observe the penetration depth of water into the mortar specimens, and the results are listed in Table 6. It can be seen that the water penetration depth decreases sharply with addition of polymer latex. SAE latex modified mortar shows the lowest effect among these three polymer latexes when the addition is 10%. When the addition of polymer latex increases to 20%, no water penetrating into the mortar specimens was observed. Similar results could be found in literatures [54,55,58] that the impermeability of OPC mortar could be significantly enhanced when polymer latex is added. Meanwhile, the pores inside of polymer modified cement matrix are filled with polymer particles, which would destroy the connection between connected pores [54,59]. The additional reason is that the films formed by polymer latex surround the cement particles and aggregates, and further improve the impermeability of cement mortar [54]. Fig. 8 displays the fractured section of polymer modified mortar after anti-penetration test, which can be taken to clearly explain that the polymer latex can enhance the anti-penetration capacity of the mortar remarkably. 3.3. Effect of polymer latex on the durability of CSA cement mortar 3.3.1. Resistance to freeze-thaw cycle The flexural and compressive strength of polymer modified mortar with and without freeze-thaw attack is given in Fig. 9. It is found from Fig. 9(a) that the trend of flexural strength of polymer modified mortar without attack from freeze-thaw cycle is the same as that cured under standard condition. The flexural strength of PAE latex modified mortar is the biggest while that of SAE latex modified mortar is the lowest among these three polymer latex modified mortars when the addition is 10%. However, when in addition of 20% polymer latex, SBR latex modified mortar performs the biggest flexural strength while SAE latex modified mortar has the lowest flexural strength. The similar trend of flexural strength development after free-thaw cycle is also displayed in Fig. 9(a), it can be seen that the flexural strength of all mortars decreases in some extent, however, SBR latex modified mortar performs the lowest decreasing of flexural strength while PAE latex modified mortar exhibits biggest decreasing of flexural strength among polymer latex modified mortars. Fig. 9(b) illustrates the compressive strength with and without attack from freeze-thaw cycle. Surprisingly, the compressive strength of SBR latex modified mortar increases while that of the other two polymer latex modified mortars decrease after attack from freeze-thaw cycle. The trend of the compressive strength of PAE latex modified mortar is the same as the flexural strength, which means it has the biggest strength decreasing. However, PAE latex modified mortar performs higher strength than SAE latex modified mortar. The coefficient of freeze-thaw cycle resistance (UF-T) is calculated to define the influence of polymer latex on the freeze-thaw cycle resistance of CSA cement mortar. The bigger the coefficient is, the better the freeze-thaw cycle resistance is. It can be found from Fig. 10 that the coefficient of freeze-thaw cycle resistance of SBR latex modified mortar is bigger than that of control mortar

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L. Li et al. / Construction and Building Materials 169 (2018) 911–922 Table 6 Penetration depth of water into polymer latex modified mortars with the water pressure increasing to 2.0 MPa.

Average penetration depth/mm

Control

SBR10%

SBR20%

SAE10%

SAE20%

PAE10%

PAE20%

19

1.5

0

2.5

0

1.5

0

Fig. 8. Section of polymer modified mortars after anti-penetration experiment.

while that of SAE and PAE latex modified mortar are lower, which indicates that SBR latex modified mortar performs the best performance after attack of freeze-thaw cycle. The UF-T of PAE latex modified mortar is the lowest among that of polymer modified mortars, which is mainly due to the biggest strength loss when it is subjected to the attack of freeze-thaw cycle. It can be also found that the compressive strength coefficient of freeze-thaw cycle resistance is bigger than the flexural strength coefficient except CSA cement modified mortar with addition of 20% PAE. It can be concluded from above analysis that CSA cement mortar modified with SBR latex performs the best resistance to freeze-thaw cycle while with PAE latex it shows the lowest freeze-thaw cycle resistance even it can gain the comparable strength to that with SAE latex. 3.3.2. Resistance to carbonization The flexural and compressive strength of polymer modified mortar with and without carbonization attack is illustrated in Fig. 11. It is found from Fig. 11(a) that the flexural strength of polymer latex modified mortar is much bigger than that of control mortar. The same trend when with and without carbonization attack could be seen that the flexural strength of PAE latex modified mortar is the biggest while that of SAE latex modified mortar is the lowest among these three polymer latex modified mortars when the addition is 10%. However, SBR latex modified mortar performs the biggest flexural strength while SAE latex modified mortar has the lowest flexural strength when with addition of 20% polymer latex. Meanwhile, the flexural strength of polymer latex modified

Fig. 10. Coefficient of freeze-thaw cycle resistance.

mortar tends to decrease after carbonization attack except that of PAE modified mortar with addition of 20%. Fig. 11(b) displays the compressive strength with and without attack from carbonization. With addition of 10% polymer latex, the compressive strength of SBR latex modified mortar after carbonization increases, which demonstrates the same trend as that of control mortar while the compressive strength of SAE and PAE latex modified mortars decreases. However, when the addition of polymer latex is 20%, the compressive strength of SBR latex modified mortar decreases while that of SAE and PAE latex modified mortars increase. Generally, there is no significant change in compressive strength of polymer modified mortar with and without carbonation attack. The coefficient of carbonization resistance (UCarbonization) is shown in Fig. 12, in which we can see that the flexural strength coefficient of carbonization resistance of polymer latex modified mortar is bigger than that of control mortar, which is consistent with [60] for polymer modified OPC mortar. When with addition of 10% polymer latex, SAE latex modified mortar shows the biggest flexural strength coefficient after attack of carbonization, while SBR latex modified mortar has the lowest flexural strength coefficient even it is still higher than that of control sample. When the addition of polymer latex increases to 20%, the flexural strength

Fig. 9. Strength before and after freeze-thaw cycle: (a) flexural strength and (b) compressive strength.

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Fig. 11. Strength before and after carbonization attack: (a) flexural strength and (b) compressive strength.

Fig. 12. Coefficient of carbonization resistance.

coefficient tends to increase. PAE latex modified mortar shows the biggest flexural strength coefficient among these three samples. For the compressive strength coefficient, it could be found that the coefficient of CSA cement mortar is bigger than 1.0 which indicates CSA cement mortar itself performs good resistance to carbonization attack. With addition of polymer latex in CSA cement mortar, the coefficient of compressive strength of the mortar is more or less influenced although it is still close to 1.0. Addition of 10% SBR latex in CSA cement mortar shows bigger coefficient than control sample while PAE latex modified mortar has the low-

est coefficient. The compressive strength coefficient of polymer modified mortar is quite different when with addition of 20% polymer latex, SBR latex modified CSA cement mortar owns the lowest coefficient than others. Previous studies present conflicting results on the carbonization rate between CSA cement and OPC. Hargis et al. [61] reported that CSA cement mortars carbonate much faster than Portland cement mortars; Zhang et al. [62] demonstrated the equivalent carbonization rate to Portland cement concrete. It is noticed that carbonization would bring higher compressive strength to CSA cement while sacrifice its toughness. The addition of polymer latexes contributes toughness to CSA cement mortar and wins higher coefficient of carbonization resistance in terms of flexural strength. Ohama et al. [63] compared the effect of polymer latexes at addition of 20% on the carbonization depth of OPC mortar exposed under indoor and outdoor environmental conditions and found that the carbonization depth of SBR latex modified mortar is much less than that of PAE latex modified mortar. Generally, the carbonization would promote the formation of calcium carbonate which makes surface structure become much more dense than before, and finally increase compressive strength. In the study, it can be seen from Fig. 12 that when the addition of polymer latex is 20%, the compressive strength of SBR latex modified CSA cement mortar is decreased while that of other two increased after carbonization, which is not well consistent with [63], it can be explained that the main hydration products of CSA cement are ettringite and aluminum hydroxide which can not be easily transformed to calcium carbonate.

Fig. 13. Strength before and after sulfate attack: (a) flexural strength and (b) compressive strength.

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Fig. 14. Coefficient of sulfate attack resistance: (a) flexural strength and (b) compressive strength.

3.3.3. Resistance to sulfate attack The flexural and compressive strength of polymer modified mortar with and without sulfate attack is exhibited in Fig. 13. It can be seen from Fig. 13(a) that the flexural strength of CSA cement mortar is not significantly influenced when subjected to sulfate attack, and similar results could be found from polymer latex modified mortars. However, sulfate attack shows big impact on the compressive strength. The compressive strength decreases with sulfate attack, the bigger concentration of sulfate solution, the less compressive strength of cement mortars. Generally, when with addition of 10% polymer latex, PAE latex modified mortar shows the highest compressive strength although it is lower than control sample, however, when the addition of polymer latex reaches 20%, SBR latex modified mortar has the highest compressive strength. The coefficient of sulfate resistance (UNa2SO4) is given in Fig. 14. It is found From Fig. 14(a) that the flexural strength coefficient of CSA cement mortar when subjected to sulfate attack is nearly close to 1.0 which indicates the promising resistance ability to sulfate attack. When with 10% addition of polymer latex, the flexural strength coefficient of polymer modified mortar tends to decrease, and SAE latex modified mortar shows the lowest coefficient. Addition of 20% polymer seems to increase the flexural strength coefficient of SAE and PAE modified mortar while decrease that of SBR modified mortar especially when subjected to 5% sodium sulfate solution attack. It can be seen from Fig. 14(b) that the compressive strength coefficient of CSA cement mortar when subjected to sulfate attack is declined, the higher concentration of sulfate solution makes lower coefficient of mortar. Addition of polymer latex helps to increase the resistance ability to sulfate attack. SAE latex modified mortar performs the best performance when compared to others. Generally, CSA cement mortar performs well resistance to sulfate attack, and the addition of polymer latex could help to enhance its sulfate attack resistance further. 3.4. Effect of polymer latex on the microstructure of CSA cement mortar The microstructure of fracture surface of polymer modified CSA cement mortar after being treated by 5% HCl solution was exhibited in Fig. 15. It is shown from Fig. 15(a) that there are many erosive cement particles as well as needle-like products co-existed together in the control mortar. These needle-like products named as ettringite are the main hydration products of CSA cement, the ettringite is intertwined together to form dense structure, and finally to contribute to cement composites high strength and durability [14]. However, some micro-cracks are observed in the control mortar, which leads to high water capillary adsorption and low

water penetration property. Contrary to control mortar, the polymer films could be observed clearly from those polymer modified CSA cement mortar, and these films benefit to enhance the properties of CSA cement mortar in terms of flexural strength, water adsorption rate, and also durability. However, the morphologies of the films formed by various polymers are different. It is seen from Fig. 15(b) that when with 10% of SBR latex in CSA cement mortar, the film covers the surface of cement grains, meanwhile, a few of needle-like products which are survived from HCl erosion are wrapped by the films, however, some pores can be observed. Similar morphologies of the films could be seen in Fig. 15(d), in which 10% of SAE latex is formulated in CSA cement mortar, although more amount of ettringite is also wrapped by the polymer films. The pores seem to be less than that of SBR latex modified CSA cement mortar. Fig. 15(f) lays out the microstructure of CSA cement mortar modified with 10% of PAE latex. It shows that polymer films formed by PAE latex completely cover the cement grains and there is few of pores can be seen. When the addition of polymer latex increases to 20%, the formed films become much denser and the pores seems less than that with 10% of polymers. It is obvious that the films formed from SBR and PAE latex look strong and cover all the fracture samples (Fig. 15(c) and (g)), while that formed from SAE latex looks loose and cannot shroud all the surfaces (Fig. 15(e)), which gives good explanations that SAE latex modified CSA cement mortar performs the highest weight loss as well as the highest water capillary adsorption. 4. Conclusions In this study an attempt is made to compare the effect of three different polymer latexes on the setting, mechanical and durability properties of CSA cement mortar. The main conclusions are summarized in below: (1) Addition of polymer latex tends to reduce water demand of CSA cement mortar, the water-reduction rate is markedly increased when the polymer to cement ratio is increased from 10% to 20%. Among these three polymer latexes, SAE latex has the lowest water-reducing rate while SBR latex shows similar water-reducing rate as PAE latex. (2) SAE latex performs the most obvious delaying effect on the setting of CSA cement mortar. Such effect is more obvious when with more polymer latex. PAE latex modified mortar shows the shortest setting time among these three polymer latex modified mortars which is comparable with control sample.

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Fig. 15. Secondary electron image of fractured sections of CSA cement mortar modified with various polymer latexes at 7 days: (a) 0%; (b) SBR:10%; (c) SBR:20%; (d) SAE:10%; (e) SAE:20%; (f) PAE: 10%; (g) PAE:20%

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(3) SBR latex modified mortar shows increased compressive strength while SAE and PAE latex modified mortar have decreased strength when the polymer dosage increases from 10% to 20%. When the dosage is 10%, PAE latex modified mortar performs the highest strength at various ages, when the dosage increases to 20%, SBR latex modified mortar gains the highest compressive strength. (4) Polymer latex contributes to the flexural strength increasing of CSA cement mortar. SBR latex modified mortar gains the highest flexural strength while SAE latex modified mortar shows the lowest flexural strength. (5) The shrinkage rate of CSA cement mortar is decreased by addition of polymer latex. Generally, SBR latex shows the lowest shrinkage reduction efficiency among these three polymer latexes, SAE latex modified mortar shows expansion behavior when the dosage is 20%. The shrinkage rate of polymer modified mortar is not directly linked to the weight decreasing. The less weight loss is found when more polymer latex is added for all polymer types. (6) Polymer latexes provide a considerable increase in the resistance to water capillary adsorption. Such an effect could be enhanced by increasing the polymer dosage regardless of polymer types. SBR latex modified mortar demonstrates the best performance on water capillary adsorption decreasing and SAE latex modified mortar shows the lowest effect. (7) CSA cement mortar demonstrates well resistance to freezethaw cycle, carbonization and sulfate attack. Addition of polymer latex will decrease strength when subjected to several extreme conditions. However, these polymer latexes help to enhance the resistance to carbonization and sulfate attack, although only SBR latex could improve the freezethaw cycle resistance. (8) The addition of polymer latex is helpful to improve microstructures by forming the continuous films in CSA cement mortar, which finally contributes to CSA cement mortar several superior properties. Generally, the films formed from PAE and SBR latexes looks strong while that formed from SAE is loose.

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