Effects of superplasticizer type on packing density, water film thickness and flowability of cementitious paste

Construction and Building Materials 86 (2015) 113–119

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Effects of superplasticizer type on packing density, water film thickness and flowability of cementitious paste Leo G. Li a, Albert K.H. Kwan b,⇑ a b

School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou, China Department of Civil Engineering, The University of Hong Kong, Hong Kong, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Different types of superplasticizer

Different types of superplasticizer have different effects on the packing density, water film thickness and flowability of cementitious paste, as illustrated in the diagram below, where SP means superplasticizer, SP1 is a naphthalene-based superplasticizer and SP2 is a polycarboxylate-based superplasticizer. 0.84 0.82 Packing density_

have different dispersion effects.  They all have beneficial effects on packing density and water film thickness.  Even at same water film thickness, they have different effects on flowability.  This may be caused by different effectiveness in cohesiveness reduction.

0.80 0.78 0.76 0.74 0.72 0.70

A-SP1

B-SP1

A-SP2

B-SP2

C-SP1 C-SP2

0.68 0.0

1.0

2.0

3.0

4.0

5.0

SP dosage (%)

Packing density versus SP dosage

a r t i c l e

i n f o

Article history: Received 31 January 2015 Received in revised form 26 March 2015 Accepted 27 March 2015 Available online 9 April 2015 Keywords: Superplasticizer Packing density Water film thickness Flowability

a b s t r a c t Superplasticizer (SP) is nowadays an indispensable ingredient for the production of concrete. With SP added, the cementitious materials would be dispersed to reduce agglomeration, and thus the packing density, water film thickness (WFT) and flowability of the cementitious paste could be improved. However, there have been few studies on the effects of SP type on the packing density and WFT. This study aims to evaluate the roles of SP type in the packing density, WFT and flowability of cementitious paste. In the study, cementitious paste samples with three cementitious material compositions, two SP types (namely, a naphthalene-based SP and a polycarboxylate-based SP) and increasing SP dosages were tested. The test results showed that the addition of either type of SP would significantly increase the packing density and WFT, but the polycarboxylate-based SP is more effective than the naphthalene-based SP. Besides, the combined effects of SP and WFT on the flowability of cementitious paste were analyzed. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Early in 1938, a plasticizer made with naphthalene sulfonate was patented in USA, which may be regarded as the predecessor of modern day superplasticizer (SP). In 1960s, a naphthalene-based SP was ⇑ Corresponding author. E-mail addresses: [email protected] (L.G. Li), [email protected] (A.K.H. Kwan). http://dx.doi.org/10.1016/j.conbuildmat.2015.03.104 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

developed in Japan and a melamine-based SP was developed in Germany [1]. These SPs disperse the cementitious materials mainly through the electrostatic repulsion between particles produced by imparting similar electrostatic charges onto the particle surfaces. More recently, polycarboxylate-based SP made of synthetic molecules has also been developed; such SP disperses the cementitious materials through not only the electrostatic repulsion, but also the steric repulsion between particles produced by wrapping the

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L.G. Li, A.K.H. Kwan / Construction and Building Materials 86 (2015) 113–119

particles with co-polymer side chains [2–4]. Generally, the polycarboxylate-based SP is more effective as a dispersing agent [5–8]. With SP added, the cementitious materials would be dispersed to avoid the formation of agglomerates. Hence, the SP can be added without changing the mix proportions to increase the flowability of the concrete. Alternatively, at the same flowability requirement, the water/cementitious materials (W/CM) ratio can be decreased to improve the strength and durability of the concrete, or the cementitious paste volume can be decreased to improve the dimensional stability and reduce the carbon footprint of the concrete. Because of these advantages, SP has become so popular that it is nowadays an indispensable ingredient for the production of almost all kinds of concrete, especially high-performance concrete [9–16]. It is only that with SP added, the concrete mix design would become more complicated. In simple terms, a SP works by dispersing the solid particles in water. If no SP is added, the solid particles would tend to form agglomerates, thus causing the solid particles to be loosely packed. When a SP is added to disperse the solid particles, agglomeration would be reduced, thus allowing the solid particles to be more closely packed. Therefore, in theory, the dispersion effect of the SP should improve the packing density of the solid particles. Somehow, in practice, the dispersion effect may vary from one SP to another SP, indicating that the effect of SP on packing density improvement is dependent on the SP type. However, there have been few studies on such effect of SP on the packing density of cementitious materials. This is probably due to the lack of an appropriate test method for packing density measurement that is capable of incorporating the effect of SP. The conventional methods of packing density measurement measure the packing density of the solid particles under dry condition [17–20]. These methods, which may be classified as the dry packing methods, are not applicable to materials containing fine particles because under dry condition, the fine particles tend to form agglomerates and the packing density so measured is very sensitive to the compaction applied [21]. More importantly, the effects of water and SP in the concrete mix cannot be included. To resolve these problems, the authors’ research group has recently developed a new method, called the wet packing method, for measuring the packing densities of cementitious materials, fine aggregate, cementitious materials plus fine aggregate, blended fine and coarse aggregates and concrete mixes [22–26] with the effects of water and SP included. However, in the previous studies, only one type of SP was used and the SP dosage was often fixed at a certain percentage by mass of cementitious materials. So far, there has been little research on how the SP type and dosage would affect the packing density of cementitious materials. By adding a SP, the solid particles in concrete would be dispersed to reduce the degree of agglomeration and increase the packing density. The increased packing density would then decrease the volume of voids between the solid particles, which are to be filled with water when the solid particles are mixed to produce concrete. As a result, at the same water content, there would be more excess water (water in excess of that needed to fill the voids between the solid particles) available for forming water films coating the solid particles to provide lubrication and increase the flowability of the concrete. Meanwhile, the SP would also reduce the cohesiveness of the concrete to increase the flowability of the concrete. Hence, the addition of a SP increases the flowability of a concrete mix through two additive effects: first, by increasing the packing density and thus the amount of excess water available to form water films; and second, by reducing the cohesiveness. However, to separately study these two effects, it is necessary first of all to evaluate the effect of SP addition on the amount of excess water and the water film thickness (WFT) in the cementitious paste.

In recent studies, with the packing density measured using the wet packing method, the WFT has been determined as the excess water to solid surface area ratio [27]. The authors’ research group has demonstrated that the WFT is the single most important factor governing the rheology of cementitious paste [28], mortar [29] and concrete [30]. As any SP added would disperse the solid particles to increase the packing density and thus the amount of excess water, the SP should have significant effect on the WFT. However, it is believed that even at the same WFT, the SP type should have significant effect on the rheology. In other words, both the WFT and SP type should be playing certain roles in the rheology of superplasticized paste, mortar and concrete. Previous studies by the authors’ research group have revealed that a polycarboxylate-based SP can have great effects on the rheology and cohesiveness of cementitious paste containing condensed silica fume [31] and cement-sand mortar [32]. However, the combined effects of WFT and SP type on the rheology of paste, mortar and concrete have not been studied yet. In order to address the above issues, this research project was launched to study the effects of the SP type and SP dosage on the packing density, WFT and flowability of cementitious paste. For such research, a number of cementitious paste samples made with three cementitious material compositions, two SP types (a naphthalene-based SP and a polycarboxylate-based SP) and various SP dosages were tested by a wet packing method to measure their packing densities, and by a mini slump flow test to measure their slump and flow spread values. 2. Experimental details 2.1. Experimental program To investigate the roles of SP type and SP dosage in the packing density, WFT and flowability of cementitious pastes, an experimental program was launched, in which three series of cementitious materials mixes, namely Series A, Series B and Series C, were tested, as summarized in Table 1. In Series A, ordinary Portland cement (OPC) was the only cementitious material used. In Series B, pulverized fuel ash (PFA) was added to replace part of the OPC and the PFA content was 20% by mass of the total cementitious materials. In Series C, condensed silica fume (CSF) was also added to replace part of the OPC, and the PFA and CSF contents were both 20% by mass of the total cementitious materials. In all the three series, the packing density was measured by the wet packing method under three different testing conditions, namely, SP0, SP1 and SP2, as depicted in Table 2. For testing under condition SP0, no SP was added. For testing under conditions SP1 and SP2, a naphthalene-based SP (named as SP1) and a polycarboxylate-based SP (named as SP2) were added, respectively. In Series A, the SP1 or SP2 dosages were 0.5%, 1.0%, 2.0% and 3.0% by mass of cementitious materials whereas in Series B and C, the SP1 or SP2 dosages were 1.0%, 2.0% 3.0% and 4.0% by mass of cementitious materials, as depicted in the second column of Table 3. Moreover, to study the effects of SP on WFT and flowability of cementitious paste, a number of cementitious paste samples with a W/CM ratio by volume of 0.6 were produced to measure their WFT, slump and flow spread. For easy identification, each paste sample was assigned a sample number. The sample number is in the form of X-Y-Z, where X (=A, B or C) denotes the series number, Y (=SP0, SP1 or SP2) denotes the testing condition and Z denotes the SP dosage (as a percentage by mass), as listed in the first column of Table 3. 2.2. Materials Three types of cementitious materials, namely, ordinary Portland cement (OPC), pulverized fuel ash (PFA) and condensed silica fume (CSF), were used in the experiments. The OPC was a commonly used cement of strength class 52.5 N, which had been tested to comply with BS EN 197-1: 2011 [33]. The PFA was a classified ash, Table 1 Proportions of cementitious materials by mass. Series No.

A B C

Proportions of cementitious materials by mass (%) OPC

PFA

CSF

100 80 60

0 20 20

0 0 20

L.G. Li, A.K.H. Kwan / Construction and Building Materials 86 (2015) 113–119

and SP2, measured in terms of liquid mass, should be 0.5% to 2.0% and 0.5% to 3.0% by mass of the cementitious materials, respectively, but higher dosages were used in this study to evaluate their full range effects.

Table 2 Testing conditions. Testing condition

Dry/wet

Superplasticizer

SP0 SP1 SP2

Wet

No SP added Naphthalene-based SP Polycarboxylate-based SP

Table 3 Packing density results. Mix No.

SP dosage (%)

Packing density

A-SP0-0.0 A-SP1-0.5 A-SP1-1.0 A-SP1-2.0 A-SP1-3.0 A-SP2-0.5 A-SP2-1.0 A-SP2-2.0 A-SP2-3.0

0.0 0.5 1.0 2.0 3.0 0.5 1.0 2.0 3.0

0.701 0.706 0.713 0.718 0.722 0.709 0.714 0.723 0.725

– 0.7 1.7 2.4 3.0 1.2 1.9 3.1 3.4

B-SP0-0.0 B-SP1-1.0 B-SP1-2.0 B-SP1-3.0 B-SP1-4.0 B-SP2-1.0 B-SP2-2.0 B-SP2-3.0 B-SP2-4.0

0.0 1.0 2.0 3.0 4.0 1.0 2.0 3.0 4.0

0.721 0.736 0.750 0.754 0.756 0.749 0.760 0.762 0.764

– 2.1 4.0 4.6 4.9 3.9 5.4 5.6 6.0

C-SP0-0.0 C-SP1-1.0 C-SP1-2.0 C-SP1-3.0 C-SP1-4.0 C-SP2-1.0 C-SP2-2.0 C-SP2-3.0 C-SP2-4.0

0.0 1.0 2.0 3.0 4.0 1.0 2.0 3.0 4.0

0.728 0.753 0.791 0.812 0.813 0.763 0.807 0.823 0.827

– 3.4 8.6 11.5 11.6 4.7 10.8 13.0 13.5

Increase in packing density due to SP (%)

Cumulative percentage passing (%) _

The packing density was measured using the wet packing method developed by the authors’ research group [22]. Basically, this method determines the packing density of the cementitious materials as the maximum solid concentration achieved by the cementitious materials when they are mixed with water at different W/CM ratios by volume. To measure the packing density, six to eight samples of cement paste were formed of the cementitious materials, starting at a relatively low W/CM ratio and successively increasing the W/CM ratio until the solid concentration had reached a maximum value and then decreased. From the test results obtained, the solid concentration of the cementitious materials in the paste can be determined as follows. Let the mass and volume of the paste in the mould be M and V, respectively. The wet bulk density of the paste is equal to M/V. If the cementitious materials consist of several different materials denoted by a, b, c and so forth, the solid concentration / can be worked out as:

M=V

qw uw þ qa Ra þ qb Rb þ qc Rc

ð1Þ

where qw is the density of water, qa, qb and qc are the solid densities of OPC, PFA and CSF, uw is the W/CM ratio by volume, and Ra, Rb and Rc are the volumetric ratios of OPC, PFA and CSF to the total cementitious materials. The maximum value of / so obtained is taken as the packing density (/max) of the cementitious materials. From the measured packing density, the minimum voids ratio (umin), which is defined as the ratio of the minimum voids volume to the solid volume of the particles, may be evaluated as:

umin ¼

1  /max /max

ð2Þ

2.4. Measurement of flowability The mini slump cone test was used to measure the slump and flow spread of the paste samples. The mini slump cone test for paste may be regarded as a reduced scale version of the slump test for concrete. There are several different versions of mini slump cone. The version adopted in this study was the same as that used by Okamura and Ouchi [37]. Basically, the mini-slump cone has a base diameter of 100 mm, a top diameter of 70 mm and a height of 60 mm. To perform this test, the paste was filled into the slump cone until the slump cone was full, the top surface of the paste in the slump cone was trowelled flat and smooth, and the slump cone was lifted vertically upwards to allow the paste to slump downwards and flow outwards under its own weight to form a patty. When the paste had stopped deforming and flowing, the slump was determined as the drop in height of paste patty whereas the flow spread was determined as the average diameter of paste patty minus the base diameter of the slump cone. Details of the test procedures have been given before [38].

80 CSF PFA

2.3. Measurement of packing density

/¼

100

60

115

OPC

2.5. Determination of water film thickness

40 Having obtained the minimum voids ratio umin by the wet packing test, the excess water ratio uw0 (defined as the ratio of the volume of excess water to the solid volume of the cementitious materials) can be calculated as:

20

u0w ¼ uw  umin

0 0.1

1

10

100

Particle size (μm) Fig. 1. Particle size distributions of OPC, PFA and CSF.

which had been tested to comply with BS EN 450-1: 2012 [34]. The CSF was imported from Europe and had been tested to comply with ASTM C 1240-03 [35]. Their solid densities had been measured in accordance with BS 4550-3: 1978 [36] as 3112, 2329 and 2196 kg/m3, respectively. Their particle size distributions were measured by a laser diffraction particle size analyzer and the results obtained are plotted in Fig. 1. Based on these particle size distribution results, the specific surface areas of the OPC, PFA and CSF were calculated as 1.55  106, 1.75  106 and 1.33  107 m2/m3, respectively. Two types of SP were used in the study, namely, SP1 and SP2, respectively. SP1 was a second-generation naphthalene-based SP, which disperses the cementitious materials by electrostatic repulsion. SP2 was a third-generation polycarboxylatebased SP, which disperses the cementitious materials by both electrostatic repulsion and steric repulsion. SP1 has a solid content of 35% whereas SP2 has a solid content of 20%. The relative densities of SP1 and SP2 have been measured to be 1.18 and 1.03, respectively. According to the suppliers, the normal dosages of SP1

ð3Þ

where uw is the water ratio (defined as the ratio of the volume of water to the solid volume of the cementitious materials), which is the same as the W/CM ratio by volume. On the other hand, the specific surface area of the solid particles AS (defined as the solid surface area per unit solid volume) is given by:

AS ¼ Aa  Ra þ Ab  Rb þ Ac  Rc

ð4Þ

in which Aa, Ab, and Ac are respectively the specific surface areas of OPC, PFA and CSF, and Ra, Rb and Rc are respectively the volumetric ratios of OPC, PFA and CSF to the total cementitious materials. With the values of uw0 and AS so determined, the WFT, which has the physical meaning of being the average thickness of the water films coating the solid particles, may be obtained as:

WFT ¼

u0w AS

ð5Þ

3. Experimental results and discussions 3.1. Effects of SP on packing density The packing density results of the paste samples in Series A, B and C under the three testing conditions with increasing SP

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materials are different. As the SP1 dosage increased from 0% to 3.0%, the packing density of paste samples in Series A and B can only be increased by up to 3.0% and 4.6%, respectively, but the packing density of paste samples in Series C can be increased by up to 11.5%. Similarly, as the SP2 dosage increased from 0% to 3.0%, the packing density of paste samples in Series A and B can only be increased by up to 3.4% and 5.6%, respectively, but the packing density of paste samples in Series C can be increased by up to 13.0%. These results reveal that the effect of SP on packing density improvement is generally larger when the cementitious materials contain CSF and that the filling effect of CSF could be more fully utilized by adding a more effective SP to better disperse the CSF particles.

0.84

Packing density_

0.82 0.80 0.78 0.76 0.74 0.72 0.70

A-SP1

B-SP1

C-SP1

A-SP2

B-SP2

C-SP2

0.68 0.0

1.0

2.0

3.0

4.0

5.0

3.2. Effects of SP on WFT

SP dosage (%) Fig. 2. Packing density versus SP dosage.

dosages are tabulated in the third column of Table 3 and plotted against the SP dosage in Fig. 2. From Fig. 2, it can be seen that the packing density curves of Series B are always higher than those of Series A whereas the packing density curves of Series C are always higher than those of Series B. This indicates that the packing density always increased in the order of Series A to Series B to Series C. This observation is reasonable and may be explained in terms of the higher fineness and spherical shape of PFA and CSF. Both the PFA and CSF have higher fineness than the OPC and therefore the PFA and CSF particles would fill into the voids between the OPC particles to decrease the voids volume and increase the packing density. This is called the filling effect. On the other hand, the spherical shape of PFA and CSF would allow the PFA and CSF particles to act as ball bearings so that the OPC particles in contact with the PFA and CSF particles could adjust their relative positions to achieve a higher packing density. This is called the ball bearing effect. Comparing the packing density curves for the two types of SP in the same series, it is noted that in all the three series, the packing density curve for SP2 is always higher than that for SP1. For instances, for Series A, at a SP dosage of 3.0%, the packing density of paste sample with SP1 is 0.722 whereas the packing density of paste sample with SP2 is 0.725. For Series B, at a SP dosage of 4.0%, the packing density of paste sample with SP1 is 0.756 whereas the packing density of paste sample with SP2 is 0.764. For Series C, at a SP dosage of 4.0%, the packing density of paste sample with SP1 is 0.813 whereas the packing density of paste sample with SP2 is 0.827. These results reveal that SP2 (the polycarboxylate-based SP) is more effective than SP1 (the naphthalene-based SP) in packing density improvement. Besides, it is also found that in all the three series, as the SP dosage increased, the packing density of the paste sample always increased. For instance, when SP2 was used, in Series A, the packing density increased with the SP dosage from 0.701 to 0.725, in Series B, the packing density increased with the SP dosage from 0.721 to 0.764, and in Series C, the packing density increased with the SP dosage from 0.728 to 0.827. Hence, the SP dosage has significant beneficial effect on the packing density of cementitious materials. However, it should also be noted that when the SP dosage was increased to beyond 3.0%, there was little further improvement in packing density. So, the SP dosage of 3.0% by mass of cementitious materials may be taken as the saturation SP dosage (the dosage beyond which further addition has little effect) for the particular types of SP and the cementitious materials mixes tested. Last but not the least, it can be seen that the effects of SP on packing density improvement for the different cementitious

By using Eqs. (2) and (4), the minimum voids ratio and specific surface area of the solid particles were calculated and tabulated in the second and fourth columns respectively of Table 4. Comparing the packing density results and minimum voids ratio results, it can be seen that as the packing density increased with the SP dosage, the minimum voids ratio decreased dramatically. On the other hand, the specific surface area of the solid particles of paste samples in Series A (pure OPC) was 1.55  106 m2/m3. With PFA and CSF added to replace part of OPC, the specific surface areas of the solid particles of paste samples in Series B and C were increased to 1.60  106 and 4.48  106 m2/m3, respectively. To study the effects of SP on WFT, the excess water ratio and WFT of paste samples at W/CM ratio by volume of 0.6 were calculated. First, the excess water ratios of the paste samples were calculated using Eq. (3). The values of excess water ratio so obtained are tabulated in the third column of Table 4. From the table, it is seen that as the packing density increased and the minimum voids ratio decreased, the excess water ratio increased. And, in general, under the same testing condition and at the same SP dosage, the excess water ratio of paste samples in Series C is always higher than that in Series B, whereas the excess water ratio of paste samples in Series B is always higher than that in Series A. Then, by using Eq. (5), the WFT of the paste samples were determined from the excess water ratio and the specific surface area of solid particles. The values of WFT so determined are tabulated in the fifth column of Table 4 and plotted against the SP dosage in Fig. 3. From Fig. 3, it can be seen that the WFT curves of Series B are always higher than those of Series A whereas the WFT curves of Series A are always higher than those of Series C. This indicates that the WFT always increased in the order of Series C to Series A to Series B. This observation may be explained in terms of the combined effect of packing density and specific surface area of solid particles. Although PFA and CSF additions would increase the packing density, leading to decrease of minimum voids ratio and increase of excess water ratio at a given W/CM ratio, their additions would also increase the specific surface area. The increase in packing density and the increase in surface area have opposite effects on the WFT and therefore, depending on their relative magnitudes, their net effect may be positive or negative. For Series B, the addition of PFA increased the packing density significantly but increased the specific surface area only marginally, and thus the net effect was positive, whereas for Series C, the addition of CSF further increased the packing density only slightly but increased the specific surface area dramatically, and thus the net effect was negative. More importantly, by comparing the two WFT curves in the same series, it is noted that the WFT curve for SP2 is always higher than that for SP1. For Series A, with the SP1 dosage increased from 0% to 3.0%, the WFT increased by 24.1%, whereas with the SP2 dosage increased from 0% to 3.0%, the WFT increased by 27.4%.

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L.G. Li, A.K.H. Kwan / Construction and Building Materials 86 (2015) 113–119 Table 4 Excess water ratio and water film thickness at W/CM ratio by volume = 0.6. Mix No.

Minimum voids ratio

Excess water ratio

A-SP0-0.0 A-SP1-0.5 A-SP1-1.0 A-SP1-2.0 A-SP1-3.0 A-SP2-0.5 A-SP2-1.0 A-SP2-2.0 A-SP2-3.0

0.427 0.416 0.403 0.393 0.385 0.410 0.400 0.384 0.379

0.173 0.184 0.197 0.207 0.215 0.190 0.200 0.216 0.221

B-SP0-0.0 B-SP1-1.0 B-SP1-2.0 B-SP1-3.0 B-SP1-4.0 B-SP2-1.0 B-SP2-2.0 B-SP2-3.0 B-SP2-4.0

0.387 0.359 0.333 0.326 0.323 0.334 0.316 0.313 0.309

0.213 0.241 0.267 0.274 0.277 0.266 0.284 0.287 0.291

C-SP0-0.0 C-SP1-1.0 C-SP1-2.0 C-SP1-3.0 C-SP1-4.0 C-SP2-1.0 C-SP2-2.0 C-SP2-3.0 C-SP2-4.0

0.373 0.328 0.264 0.232 0.230 0.311 0.239 0.215 0.209

0.227 0.272 0.336 0.368 0.370 0.289 0.361 0.385 0.391

Specific surface area (m2/m3)

WFT (lm)

1,550,030

0.112 0.118 0.127 0.134 0.139 0.122 0.129 0.140 0.142

– 6.0 14.0 19.6 24.1 9.4 15.4 24.9 27.4

1,599,278

0.133 0.151 0.167 0.171 0.173 0.166 0.178 0.180 0.182

– 13.3 25.2 28.5 30.1 24.6 33.4 34.8 36.6

4,480,307

0.051 0.061 0.075 0.082 0.083 0.065 0.080 0.086 0.087

– 19.7 47.8 62.2 62.9 27.4 58.7 69.6 72.0

0.12

the WFT of cementitious paste. Such increases in WFT are caused by the dispersion effect of SP, which increases the packing density without changing the specific surface area (in contrast, the additions of PFA and CSF also increase the packing density but do not always increase the WFT because of the corresponding changes in specific surface area).

0.08

3.3. Effects of SP on slump and flow spread

0.20

0.16 WFT (μm)

Increase in WFT due to SP (%)

0.04 A-SP1

B-SP1

C-SP1

A-SP2

B-SP2

C-SP2

0.00 0.0

1.0

2.0

3.0

4.0

5.0

SP dosage (%) Fig. 3. WFT versus SP dosage.

For Series B, with the SP1 dosage increased from 0% to 4.0%, the WFT increased by 30.1%, whereas with the SP2 dosage increased from 0% to 4.0%, the WFT increased by 36.6%. For Series C, with the SP1 dosage increased from 0% to 4.0%, the WFT increased by 62.9%, whereas with the SP2 dosage increased from 0% to 4.0%, the WFT increased by 72.0%. These results reveal that comparatively, the polycarboxylate-based SP2 is more effective than the naphthalene-based SP1 in increasing the WFT. This is because SP2 is more effective than SP1 in packing density improvement. Lastly, it is found that in all the three series, as the SP dosage increased, the WFT always increased. For Series A, the WFT increased with the SP1 dosage from 0.112 to 0.139 lm, and increased with the SP2 dosage from 0.112 to 0.142 lm. For Series B, the WFT increased with the SP1 dosage from 0.133 to 0.173 lm, and increased with the SP2 dosage from 0.133 to 0.182 lm. For Series C, the WFT increased with the SP1 dosage from 0.051 to 0.083 lm, and increased with the SP2 dosage from 0.051 to 0.087 lm. Hence, the SP dosage has significant effect on

To study the effects of SP on flowability, the slump and flow spread of paste samples at W/CM ratio by volume of 0.6 were measured. The slump and flow spread results are respectively tabulated in the second and third columns of Table 5, and plotted against the SP dosage in Figs. 4 and 5. From Figs. 4 and 5, it can be seen that the slump and flow spread curves of Series B are always higher than those of Series A whereas the slump and flow spread curves of Series A are always higher than those of Series C. This indicates that the slump and flow spread always increased in the order of Series C to Series A to Series B. Such increasing order of slump and flow spread for Series A, B and C fully matches the increasing order of WFT for Series A, B and C, probably because the increases in flowability were to some extent caused by the corresponding increases in WFT due to the addition of SP. More importantly, by comparing the slump and flow spread curves in the same series, it is noted that the slump and flow spread curves for SP2 are always higher than that for SP1. For instances, for Series A, at a SP dosage of 3.0%, the slump and flow spread of paste sample with SP1 are 18 mm and 22 mm, respectively, whereas the slump and flow spread of paste sample with SP2 are 52 mm and 286 mm, respectively. For Series B, at a SP dosage of 3.0%, the slump and flow spread of paste sample with SP1 are 22 mm and 40 mm, respectively, whereas the slump and flow spread of paste sample with SP2 are 57 mm and 318 mm, respectively. For Series C, at a SP dosage of 3.0%, the slump and flow spread of paste sample with SP1 are 6 mm and 0 mm, respectively, whereas the slump and flow spread of paste sample with

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Table 5 Slump and flow spread results at W/CM ratio by volume = 0.6.

400

Slump (mm)

Flow spread (mm)

A-SP0-0.0 A-SP1-0.5 A-SP1-1.0 A-SP1-2.0 A-SP1-3.0 A-SP2-0.5 A-SP2-1.0 A-SP2-2.0 A-SP2-3.0

0 0 0 6 18 10 25 42 52

0 0 0 12 22 8 16 136 286

B-SP0-0.0 B-SP1-1.0 B-SP1-2.0 B-SP1-3.0 B-SP1-4.0 B-SP2-1.0 B-SP2-2.0 B-SP2-3.0 B-SP2-4.0

0 0 10 22 38 32 48 57 58

0 0 18 40 88 33 200 318 354

C-SP0-0.0 C-SP1-1.0 C-SP1-2.0 C-SP1-3.0 C-SP1-4.0 C-SP2-1.0 C-SP2-2.0 C-SP2-3.0 C-SP2-4.0

0 0 0 6 20 15 36 52 56

0 0 0 0 25 18 102 262 308

350 Flow spread (mm)

Mix No.

B-SP1 B-SP2

C-SP1 C-SP2

300 250 200 150 100 50 0 0.0

1.0

2.0

3.0

4.0

5.0

SP dosage (%) Fig. 5. Flow spread versus SP dosage.

70 A-SP1

60

B-SP1 C-SP1

Slump (mm)_

50

70

A-SP2 B-SP2

40

C-SP2

30 20

60 Slump (mm) __

A-SP1 A-SP2

A-SP1

B-SP1

C-SP1

A-SP2

B-SP2

C-SP2

10

50

0 0.00

40

0.05

0.10

0.15

0.20

0.15

0.20

WFT (μm)

30 Fig. 6. Slump versus WFT.

20 10 400

0

A-SP1

0.0

1.0

2.0

3.0

4.0

350

5.0

Fig. 4. Slump versus SP dosage.

SP2 are 52 mm and 262 mm, respectively. These results reveal that SP2 has greater effectiveness in increasing the flowability of cementitious paste than SP1. To study the combined effects of WFT and SP, the slump and flow spread results are respectively plotted against the WFT in Figs. 6 and 7. It is seen that as the WFT increased, both the slump and flow spread increased. This is reasonable because a larger WFT should provide better lubrication to increase the flowability of the paste. However, by comparing the slump and flow spread curves for the same series of paste samples, it is noted that even at the same WFT, the slump and flow spread values of paste samples with SP2 added are always higher than those of paste samples with SP1 added. The SP increased the flowability not just by increasing the WFT because even at the same WFT, the flowability changed with the SP type. Hence, the SP should be increasing the flowability not only by increasing the WFT but also by certain other effect, which varies with the SP type and dosage. This other effect is speculated

Flow spread (mm)_

SP dosage (%)

300 250

B-SP1 C-SP1 A-SP2 B-SP2 C-SP2

200 150 100 50 0 0.00

0.05

0.10 WFT (μm)

Fig. 7. Flow spread versus WFT.

to be the reduction in cohesiveness due to the dispersive action of the SP. In this regard, the SP2 has greater effectiveness as a dispersing agent and that is why it has greater effectiveness in increasing the flowability of cement paste, even at the same WFT. Further researches on such cohesiveness reduction effect of SP and on the combined effects of WFT and SP are recommended.

L.G. Li, A.K.H. Kwan / Construction and Building Materials 86 (2015) 113–119

4. Conclusions To study the effects of the SP type and dosage on the packing density, WFT and flowability of cementitious paste, a number of cementitious paste samples with three cementitious material compositions, two SP types (a naphthalene-based SP and a polycarboxylate-based SP) and increasing SP dosages were tested. The packing density was measured using the wet packing test, the water film thickness was calculated from the measured packing density and specific surface area, whereas the slump and flow spread were measured using the mini slump cone test. Based on the test results, the following conclusions are drawn: (1) The addition of a SP to disperse the fine solid particles can significantly improve the packing density of cementitious materials, but there exists a saturation SP dosage beyond which further addition of SP returns little further packing density improvement. (2) The effect of SP on packing density improvement is different for different cementitious materials. Generally, the effect of SP is greatest for cementitious materials containing CSF because the filling effect of CSF can be more fully utilized when the CSF particles are dispersed by the addition of SP. (3) PFA and CSF additions have significant beneficial effect on the packing density of cementitious materials due to their filling and ball bearing effects arising from their higher fineness and spherical shape, respectively. (4) Due to simultaneous increase in specific surface area, PFA and CSF additions may or may not increase the WFT, dependent on whether the increase in packing density has greater beneficial effect or the increase in specific surface area has greater adverse effect. (5) Nevertheless, SP addition always increases the WFT of cementitious paste because it can improve the packing density without changing the specific surface area. (6) Both WFT and SP have important effects on the flowability of cementitious paste. A larger WFT provides better lubrication to increase the flowability. But, at the same WFT, the flowability changes with the SP type, indicating that apart from increasing the WFT, the SP has certain other effect, which varies with the SP type and dosage. Quite possible, this is caused by the reduction in cohesiveness of the cementitious paste due to the dispersive action of SP. (7) The effects of the polycarboxylate-based SP on the packing density, WFT and flowability of cementitious paste are generally larger than those of the naphthalene-based SP.

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