Effect of superplasticizers on workability retention and initial setting time of cement pastes

Construction and Building Materials 24 (2010) 1700–1707

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Effect of superplasticizers on workability retention and initial setting time of cement pastes Min-Hong Zhang, Kritsada Sisomphon *, Tze Siong Ng, Dao Jun Sun Department of Civil Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore

a r t i c l e

i n f o

Article history: Received 16 June 2008 Received in revised form 8 February 2010 Accepted 8 February 2010 Available online 4 March 2010 Keywords: Heat development Initial setting Lignosulphonate Polynaphthalene Polycarboxylate Rheological parameters Superplasticizer Workability retention

a b s t r a c t This paper presents an experimental study on the effect of a newly developed modified lignosulphonate (PLS) superplasticizer on the loss of workability and initial setting time of cement pastes in comparison to those of polycarboxylate (PCE) and polynaphthalene (PNS) superplasticizers. The workability loss was monitored by yield stress and effective viscosity of the pastes. The initial setting was monitored by heat development, change of rheological parameters with time, and penetration depth in cement pastes. The relations among these methods were discussed. Different dosages of the superplasticizers were used to obtain cement pastes with yield stress <6 Pa at 30 min at given water-to-cement ratios. The results showed that the pastes with PLS lost workability more slowly and had longer initial setting time compared with those with PCE and PNS admixtures. Although the longer workable time is beneficial for hot weather concreting, the longer initial setting time of such material has to be taken into consideration where early strength development is essential. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Properties of hardened cement paste and concrete are affected by their flow behavior which is controlled primarily by the dispersing of cement in mixing water and concrete mixture design. Superplasticizers have been used extensively to disperse the cement particles and to improve the workability of concrete in practice. In hot weather conditions, superplasticizers with retarding effects are commonly used to reduce the rate of cement hydration and to extend workable time at jobsites to reduce pumping problems and to prevent cold joints. Thus, the selection of adequate chemical admixtures and the evaluation of their effect on early age properties such as workability retention and initial setting time are of significant importance. These properties influence the behavior of hardened cement pastes and concretes. Lignosulphonates (LS) have been widely used in concrete as regular water reducing admixtures for many years due to their relatively low prices. Significant advances have been made in the production of LS based admixtures. With the development of new modified LS superplasticizers, it has been reported that it is possible to produce self-compacting concrete with such an admix* Corresponding author. Present address: Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN, Delft, The Netherlands. Tel.: +31 1527 81325; fax: +31 1527 86383. E-mail addresses: [email protected], [email protected] (K. Sisomphon). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.02.021

ture [1]. However, there is not much information available on the effect of the newly developed modified LS superplasticizer on workability retention and initial setting time of cement pastes in comparison to those of polycarboxylate and polynaphthalene based superplasticizers. The research was therefore carried out. This paper compares the results and discusses the effect of these admixtures on workability retention and initial setting time of cement pastes. The workability retention was evaluated based on the change of yield stress and effective viscosity with time. The initial setting time was evaluated based on heat development, rheological parameters change with time, and penetration depth in cement pastes. The test methods for determining the initial setting time was reviewed and discussed. The research on cement pastes will provide information for designing concrete mixtures to achieve workable concrete. 2. Background on determining the initial setting time and workability retention 2.1. Initial setting determined by penetration depth Setting is a term used to describe the stiffening of the cement paste, mortar, or concrete. In principle, the setting of cement is a percolation process in which isolated or weakly bound particles are connected by the formation of hydration products so that solid paths are formed in the hardening pastes [2–5]. The setting of

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cement pastes, therefore, will depend on factors that affect the connectivity level between particles such as w/c [5,6]. In practice, the initial set indicates the loss of workability and the beginning of the stiffening of the paste or concrete. Initial setting of cement may be determined by ASTM methods C 191 based on the principle of penetration resistance. In this test method, normal consistency of the paste is required. According to ASTM C 191, the initial setting of a cement paste occurs when the Vicat needle penetrates 25 mm into the cement paste. 2.2. Heat development due to cement hydration and initial setting When in contact with water, cement hydrates which generates heat. The workability retention and initial setting of cement paste are related to the rate of cement hydration. Cement hydration at early stage can be monitored from the heat generated using an isothermal calorimeter as shown in Fig. 1. According to Mindess et al. [7], Stage 1 in Fig. 1 corresponds to initial wetting Peak ‘‘A” followed by induction or dormant period (Stage 2). At the end of the dormant period and beginning of the accelerating period (Stage 3), C3S begins to hydrate rapidly and reaches a maximum rate (Peak ‘‘B”) at the end of acceleration period. By this time, final set has been passed and early hardening has begun. Initial setting typically occurs at the beginning of the accelerating period. Taylor et al. [8] investigated various combinations of cements, mineral admixtures, and chemical admixtures using this method and their results indicate fair correlation between the initial set determined by this method and that by penetration resistance, rheology, and ultrasonic p-wave. 2.3. Change of rheological parameters with time and workability retention Workability change with time is also of significance in practice for transportation and casting of concrete before the initial set. The workability retention may be evaluated by the change in rheological parameters with time. With continued cement hydration, cement paste and concrete loss workability, and set. The flow of cement pastes has been reported to fit several different mathematical forms [9], e.g. Bingham s ¼ so þ gc_ and Herschel–Bulkley s ¼ so þ K c_ n models, where s is shear stress applied to material in Pa, so is yield stress in Pa, and c_ is shear rate in s1. In Bingham model, g is plastic viscosity in Pa.s and is assumed to be constant. In Herschel–Bulkley model, K is the consistency index and n is the power-law index. The Herschel–Bulkley model takes into account changes in the effective viscosity with shear rate by assuming the power-law expression, g ¼ K c_ n1 [10]. Depending on the value of ‘‘n”, the material flows as a shear thinning fluid (n < 1) or as a shear thickening fluid (n > 1). The yield stress is the minimum shear stress that must be exceeded in order for the material to flow. Once the flow has started, the effective viscosity determines flow rate of the cement pastes. Amziane and Ferraris [11] examined the possibility of using rheology method to determine initial setting of cement pastes as an

Fig. 1. Heat curve [7].

alternative to the penetration depth method determined by Vicat needle. The yield stresses vs. time for the pastes are shown in Fig. 2. The curves generally consist of two sections: (1) a slow steady increase in the yield stress with time, and (2) a significant increase in the yield stress indicating the change of cement paste from a liquid to a solid state. They define the time to reach the last point (‘‘B” in Fig. 2) before the sharp increase in the curve as the initial setting time. They investigated cement pastes with w/c of 0.30–0.45 without admixtures, and found that the rheology method correlates well with the Vicat setting time but the former provides more information before the initial setting time than does the latter.

3. Experimental details 3.1. Materials used Portland cement of ASTM Type I (Table 1) was used in the study, and deionized water was used for cement pastes. Modified lignosulphonate (PLS), polynaphthalene (PNS), and polycarboxylate (PCE) based superplasticizers were included in this study. Their characteristics are summarized in Table 2. The polycarboxylate based admixture was obtained as solution with a concentration of 39.3%. The other two admixtures were initially in powder form, and solutions were made in laboratory with concentrations of 30% and 28% by mass for PNS and PLS admixtures, respectively, recommended by manufacturer. Approximately 0.5% tributylphosphate by mass of dry admixture was added to the solutions of all three admixtures to control the air that may be entrained.

3.2. Proportion of cement paste mixtures Proportion of mixtures with various superplasticizers was designed so that the yield stress of the cement pastes was less than 6 Pa at 30 min. The mix proportions of the cement pastes used in this research are shown in Table 3. The w/c of the cement pastes was 0.26 and 0.32.

Fig. 2. Yield stress vs. time of cement pastes with different w/c ratios [11].

Table 1 Chemical and mineral compositions and physical properties of cement used. Physical properties Initial setting time, min Final setting time, min Blaine fineness, m2/kg

145 210 391

Chemical composition, % Calcium oxide, CaO Silica, SiO2 Aluminum oxide, Al2O3 Iron oxide, Fe2O3 Magnesia, MgO Sodium oxide, Na2O Potassium oxide, K2O Total alkalinity as Na2O + 0.658K2O Sulphuric anhydride as SO3 Loss on ignition (LOI)

65.0 20.3 4.7 3.2 0.9 0.2 0.5 0.53 2.4 1.9

Mineral composition according to Bogue calculation, % Tricalcium silicate, C3S Dicalcium silicate, C2S Tricalcium aluminate, C3A Tetracalcium alumninoferrite, C4AF

67 8 7 10

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Table 2 Characteristics of admixtures used. Type

Notation

Polycarboxylate Polynaphthalene Purified lignosulphonate

F F G/F

Soluble SO4, % of dry admixture

a

n.m.a n.m.a 0.2

n.m. n.m.a 0.5

n.m.: not measured.

Table 3 Mix proportion of the cement pastes used for determining the heat development, rheological parameters, and setting time of the cement pastes.

a

Reducing substances, %

Mix

Water/ cement

Superplasticizer type

Superplasticizer dosage, % sbmca

1 2 3 4 5 6 7 8

0.26

No admixture PCE PNS PLS No admixture PCE PNS PLS

– 0.13 0.39 0.35 – 0.10 0.30 0.28

0.32

sbmc: solid by mass of cement, %.

3.3. Test methods 3.3.1. Determine the effect of admixtures on the rate of heat development of cement pastes The rate of cement hydration was monitored based on the heat development from the cement pastes by an isothermal calorimeter at a temperature of 30 °C. This temperature was selected with the consideration of weather conditions in tropical countries such as Singapore. The calorimeter was calibrated and conditioned at 30 °C before experiments. All the ingredient materials, mixing utensils, and sample ampoules were pre-conditioned to 30 °C as well. The cement was added into the solution of deionised water and admixture, and hand mixed for about 1 min. The cement paste sample of 10 ± 2 g was then transferred into a sample ampoule with the sample mass recorded. After capping the ampoule, the sample and reference ampoules were inserted into the calorimeter. The heat generated from the cement hydration was monitored continuously for about 48 h. The calorimeter started to record heat 10 min after the cement was in contact with water. Because of this procedure, the heat generated initially during mixing of the cement and water was not captured. 3.3.2. Determine the initial setting time of cement pastes by penetration depth method The initial setting time of the cement pastes was determined according to ASTM C 191 Method B with modification using an automatic Vicat apparatus. ASTM Method specifies that pastes used for the test should be proportioned and mixed to normal consistency, and the initial setting time was the elapsed time required to achieve a penetration of 25 mm. In this research, however, mix proportions shown in Table 3 were used for the pastes. Therefore, the initial setting determined is affected by the w/c and chemical admixtures used in the pastes. 3.3.3. Determine the rheological parameters of cement pastes The rheological parameters were determined at various intervals from the time water and admixture was in contact with the cement until the yield stress of the cement pastes increased sharply compared with that at initial stage. The admixture was added into mixing water and then mixed with the cement in a Hobart mixer for about 4 min (1 min at speed 1, 2 min at speed 2, and another 1 min at speed 2 after scrapping the bottom of the bowl). Before each test, the paste was remixed in the mixer for about 10 s. A Hakka RV1 rheometer with coaxial cylinder configuration was used to determine the rheological parameters of the cement pastes. The torque resulting from the material resistance was measured at the inner rotating cylinder, which had a length of 55 mm and a diameter of 38 mm. Gap between the inner and the outer cylinders was 2.7 mm. The surfaces of the inner and outer cylinders were serrated so that the slippage between the cement paste samples and the surfaces of the cylinders was minimized. The shear rate first increased from 0.5 to 50 s1in 50 s and then decreased to 0.5 s1 also in 50 s. A total of 100 data points were collected at various shear rates from 50 to 0.5 s1. The down curve (from 50 to 5 s1) was used to calculate the yield stress of the cement paste based on Herschel–Bulkley model. Due to the shape of the curves, using Bingham model fitting resulted in negative yield stresses as observed by De Larrard et al. [12] as well. The data, therefore, were fitted by Herschel–Bulkley model to determine the yield stress and effective viscosity.

4. Results and discussion 4.1. Effect of the admixtures on heat development due to cement hydration and initial set The rates of cement hydration for the pastes with the superplasticizers was monitored by a calorimeter and compared with those of the corresponding control cement pastes (OPCs) as showed in Figs. 3 and 4. The time from the cement first in contact with water and admixtures to the end of the dormant period and the beginning of the acceleration period is summarized in Table 4. Comparing the time of the control Portland cement pastes to those with the admixtures, it seems that all the admixtures investigated retarded the cement hydration. However, the degree of retardation varied with the type and dosage of the admixtures. For the control cement pastes, the acceleration period started at approximately 1 h, whereas the beginning of the acceleration period delayed about 2–3 h for the pastes with PNS and PCE admixtures, and about 16 h for the pastes with PLS admixtures. From the occurrence time and intensity of the Peak ‘‘B” after the acceleration period, it seems that PLS admixture had much stronger retarding effect compared with the PCE and PNS admixtures. The time when the acceleration period started showed the same trend (Table 4). Experimental results showed that the w/c ratio also played a role on the heat development of the cement pastes. For all the admixtures, the pastes with higher w/c had lower Peak ‘‘B” compared with those with lower w/c. This may be attributed to the lower unit cement content in the pastes with higher w/c. However, the heat curves of the pastes with w/c of 0.32 showed slightly broader peaks than those with w/c 0.26 which may be attributed to the higher free water content in the formers which prolong the acceleration period of cement hydration. It was noted that the w/c had no significant effect on the occurrence time of Peak ‘‘B” and the time when the acceleration period started which were controlled primarily by the properties of the cements and the retarding effect of the admixtures used. Based on the above remarks, it can be concluded that this method cannot be used for determining initial setting time of pastes since the setting is affected by the w/c. However, the method may provide useful infor-

Rate of Heat Evolution (mW/g)

a

PCE PNS PLS

ASTM type

6.0 OPC

5.0

PCE PNS

4.0

PLS

3.0 2.0 1.0 0.0 0

500

1000

1500

2000

2500

3000

Time (minutes) Fig. 3. Rate of heat evolution of cement pastes with w/c of 0.26.

1703

5.0

OPC

Shear stress (Pa)

6.0 PNS

4.0 PCE

3.0

PLS

2.0 1.0 0.0 0

500

1000

1500

2000

2500

3000

200 180 160 140 120 100 80 60 40 20 0

PLS PNS

PCE

0

Time (min)

20

40

60

Shear rate (1/s)

Fig. 4. Rate of heat evolution of cement pastes with w/c of 0.32.

mation for estimating initial setting time of cement pastes at given w/c due to the effect of admixtures. The mechanism of retardation in cement paste depends on the type of admixtures. For lignosulphonate based admixtures, it has long been recognized that residual matters such as sugars and related salts are only partly responsible for their retarding effect [13,14]. Various other mechanisms of retardation such as adsorption, complexation, precipitation, and nucleation also contribute to retardation effects on cement hydration [15–17]. The retardation of lignosulphonates has been believed to be caused partly by adsorption of their molecules onto the surface of anhydrous cement compounds, which creates a barrier to the cement hydration [18,19]. The inhibition of nucleation and growth of crystalline hydroxide due to the adsorption of organic compounds was also suggested as a retarding feature of C3S hydration [18]. The nucleation model considers the adsorption of admixtures on hydration products and not on anhydrous cement compounds [17]. The creation of a layer of precipitated calcium salts on anhydrous alkaline cement compounds was also proposed to be partly responsible for hydration retardation [20]. For polynaphthalene based superplasticizers, Jolicoeur and Simard [21] suggested that the retardation was mainly due to the adsorption of admixtures on nucleating hydrate particles and intercalation into hydrate phases already formed such as ettringite which inhibit the development of hydration products. For polycarboxylate based superplasticizers, Uchikawa et al. [22] showed that a chelate formed in pastes as a result of interaction between Ca2+ ions and the admixture molecules would lower the Ca2+ concentration in the system, thus hinder solid phase nucleation and hydration products growth, and retard cement hydration. 4.2. Effect of the admixtures on the change of rheological parameters with time Shear stress vs. shear rate graphs of the cement pastes with different admixtures at w/c ratios of 0.26 and 0.32 at 30 min are

Fig. 5. Shear stress vs. shear rate for cement pastes with w/c = 0.26 at 30 min (descending curves).

80 70

Shear stress (Pa)

Rate of Heat Evolution (mW/g)

M.-H. Zhang et al. / Construction and Building Materials 24 (2010) 1700–1707

PLS

60 50 40

PNS

30 PCE

20 10 0 0

20

40

60

Shear rate (1/s) Fig. 6. Shear stress vs. shear rate for cement pastes with w/c = 0.32 at 30 min (descending curves).

shown in Figs. 5 and 6, respectively. Based on Herschel–Bulkley model (s ¼ so þ K c_ n ), yield stress so and parameters K and n of the cement pastes are summarized in Table 5. The R2 of data fitting to the model was P0.95. The yield stress of the cement pastes at 30 min was <6 Pa. For all the pastes samples, the power-law index ‘‘n” was >1 which indicates that these cement pastes behave as shear thickening materials, consistent with the observations for cement pastes with superplasticizers by Martin et al. [23]. The yield stress of the cement pastes increased with time due to cement hydration (Figs. 7 and 8). The time when the yield stress of the cement pastes increased sharply is summarized in Table 4 to be compared with other data. Although the yield stresses of the cement pastes with PCE and PNS admixtures were lower than that with PLS admixture at 30 min (Table 5), the yield stress of the formers increased with time in faster rates compared with that of the latter (Figs. 7 and 8). The yield stress of the pastes with PCE and PNS increased significantly with time after about 1.3–

Table 4 Initial setting time determined by various methods.

a

Type of admixture

w/c

Time to the beginning of acceleration period from heat curve, min

Time when yield stress increase significantly, min

Initial setting time by Vicat test, min

No admixture PCE PNS PLS No admixture PCE PNS PLS

0.26

60 260 175 1025 60 220 170 1030

n.m.a 110 75 475 n.m.a 150 120 680

90 300 230 990 140 415 290 1105

0.32

n.m. – not measured due to high yield stress.

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Table 5 Yield stress and parameters ‘‘K” and ‘‘n” based on Herschel–Bulkley model for various cement pastes at 30 min. Type of admixture

w/c

Yield stress, Pa

Consistency index, ‘‘K”

Power-law index, ‘‘n”

PCE PNS PLS PCE PNS PLS

0.26

1.83 5.27 5.71 1.00 1.01 2.72

0.14 0.34 1.88 0.04 0.17 0.52

1.59 1.46 1.15 1.62 1.38 1.23

Yield Stress, Pa

50 PCE PNS PLS

40 30 20 10 0 0

200

400

600

800

Time (min) Fig. 7. Yield stress change with time, w/c = 0.26.

50

Yield Stress, Pa

2.5 h, whereas that with PLS admixture increased significantly with time after about 7.9–11.3 h depending on the w/c. From Table 5, it was observed that the pastes with PLS admixture had higher ‘‘K” value, but lower ‘‘n” value compared with those of the corresponding ones with PCE and PNS admixtures. The ‘‘K” and ‘‘n” values did not remain constant. The former generally trended up with time, whereas the latter generally trended slightly lower with time. Among the three admixtures, PLS had the lowest index ‘‘n” values ranged between 1.0 and 1.3. Values of ‘‘n” higher than 1.5 were observed for the pastes with PCE and PNS admixtures at the first 15 min, which indicate higher shear thickening behavior for these pastes at early age. From the parameters of ‘‘K” and ‘‘n” in Herschel–Bulkley model, effective viscosity of the cement pastes was calculated at c_ = 25 s1, and plotted against time (Figs. 9 and 10). The effective viscosity of the pastes with the PLS admixture was higher than the corresponding ones with PCE and PNS admixtures at 30 min. At w/c of 0.32, the effective viscosity of the cement pastes with PCE and PNS admixtures increased more quickly with time than that of the paste with PLS admixture. At w/c of 0.26, no data on effective viscosity for pastes with PNS and PCE admixtures were obtained after about 2 and 3 h, respectively, due to the increase in the yield stress so that the measurement became difficult. However, the effective viscosity of the paste with PLS admixture remained relatively low until about 8 h. Comparing the cement pastes with w/c of 0.26 and 0.32, it seems that the former has higher effective viscosity than the latter. The viscosity of the cement pastes without admixtures generally increases with time due to cement hydration. However, it was observed from Figs. 9 and 10 that the viscosity of the pastes with PLS decreased slightly during the first hour, whereas the viscosity of the pastes with PCE and PNS increased with time. This may be explained by the effect of admixture adsorption and mechanism of dispersion. Published research indicates that different admixtures have different mechanisms for dispersing cement particles in water. For polycarboxylate based superplasticizers, the main mechanism of the dispersion is due to steric hindrance effect that results from the extension of their graft chains away from the surface of cement particles [24–27]. For polynaphthalene based superplasticizers, the

PCE PNS PLS

40 30 20 10 0 0

200

400

600

800

1000

Time (min) Fig. 8. Yield stress change with time, w/c = 0.32.

6 Effective viscosity

0.32

4

2 PCE

0

0

200

400 Time (min)

PNS

600

PLS

800

Fig. 9. Effective viscosity change with time, w/c = 0.26 (shear rate = 25 s1).

dispersion is dominated by electrostatic repulsion [28,29]. For modified lignosulphonate based superplasticizers, the dispersion may be attributed to a combination of electrostatic repulsion and steric hindrance effect [30]. According to Ramachandran et al. [31], although lignosulphonate based admixtures have linear molecular structures, the steric effect is caused by the cross-linked molecules taking up a relatively large volume on the surface of cement particles. Fig. 11 [32] shows the adsorption of the three admixtures on cement particles and hydration products in fresh cement pastes with w/c ratio of 0.34 within the first 60 min. The adsorption of superplasticizers was measured by solution depletion method by comparing the amount of superplasticizers remained in the solutions extracted from fresh cement pastes and the amount originally added. The results were expressed as percentages of the superplasticizers adsorbed relative to the total amounts of the superplasticizers added. The concentrations of PNS and PLS superplasticizers in the solutions were measured by a UV–visible spectrometer. The concentrations of the PCE superplasticizer in the solutions were determined by a total organic carbon content (TOC) analyzer. Details of the adsorption test procedure can be found in [32]. In the pastes with PCE admixture, almost all the admixture was immediately adsorbed on cement particles, and remained

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4.3. Effect of the admixtures on initial setting determined by penetration depth

1.5

Penetration depth of the cement pastes with various admixtures determined by the Vicat needle test is shown in Figs. 12 and 13 in comparison to that of the corresponding control pastes.

1.0 0.5 PCE

0.0

0

200

400

PNS

600

PLS

800

40

1000

Penetration (mm)

Time (min) Fig. 10. Effective viscosity change with time, w/c = 0.32 (shear rate = 25 s1).

30 20

PCE PNS PLS Control

10

100%

0 0

Adsorption (%)

80%

300

600

900

1200

1500

Time (min) 60% Fig. 13. Penetration depth from Vicat needle test, w/c = 0.32.

40%

PCE PNS

20%

40

40

PLS 10

20 30 40 Time (minutes)

50

60

Fig. 11. Adsorption of superplasticizers in cement pastes with a w/c of 0.34 [32].

Yield stress (Pa)

0% 0

Yield stress

PCE

Penetration

30

30

20

20

10

10

Penetration (mm)

Effective viscosity

2.0

40 0 200

0 600

400

30

Time (min) 40

20

40

PCE PNS PLS Control

10

0 0

300

600

900

1200

1500

Time (min)

PNS

30

Penetration

30

20

20

10

10

Penetration (mm)

Yield stress

Yield stress (Pa)

Penetration (mm)

0

Fig. 12. Penetration depth from Vicat needle test, w/c = 0.26.

0

0 0

400

600

Time (min) 40

40 Penetration

30

30

20

20

10

10

0

Penetration (mm)

PLS

Yield stress

Yield stress (Pa)

adsorbed during the 60 min. For the PNS and PLS admixtures, the adsorption increased slightly with time. As the cement in the pastes with PNS is dispersed mainly by electrostatic repulsion, the more adsorption would reduce the admixture in solution and thus increase the paste viscosity which would reduce the workability. In the pastes with PLS, however, the cement is dispersed through a combination of electrostatic repulsion and steric hindrance effects. Although the increase in the adsorption of PLS with time at early age reduced the admixture in solution, it increased the amount adsorbed. This suggests that the steric hindrance effect will increase with time, whereas the electrostatic repulsion effect will decrease with time for the PLS admixture. This may explain the reduction of effective viscosity at early age observed. The data on the yield stress and effective viscosity indicate that the paste with PLS admixture had longer workable time than that with PCE and PNS admixtures. This is particularly beneficial for concrete cast in hot weather conditions.

200

0 0

300

600

900

1200

1500

Time (min) Fig. 14. Comparison of changes on yield stress and penetration depth with time for pastes with w/c of 0.26.

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The initial setting time of the pastes with the penetration depth of 25 mm was summarized in Table 4. The results show that the initial setting times of the pastes with superplasticizers were longer than those of the control pastes. This indicates retardation of the superplasticizers, consistent with the results on heat development discussed before. The results show that the initial setting of the pastes with w/c of 0.26 was slightly shorter than that with w/c of 0.32. In both cases, the setting was affected not only by w/c, but also by the type and dosage of the superplasticizers used. The initial setting times of the pastes with PCE and PNS admixtures were more than 10 h shorter than that with PLS admixture. Although the pastes with PLS admixture had longer workable time, the longer setting time has to be taken into consideration in practice when early strength development is essential.

4.4. Discussion Table 4 summarizes the initial setting time determined by various methods. From the data shown in Table 4, it seems that the initial setting time determined by the penetration depth from Vicat needle test is much longer than that defined by the yield stress

Penetration depth (mm)

40

40

Yield stress (Pa)

PCE

30

30

20

20

10

10 Yield stress Penetration

0 600

0 0

200

400

Time (min)

Yield stress (Pa)

PNS 30

30

20

20

10

10 Yield stress

Penetration depth (mm)

40

40

Penetration

0

200

400

Time (min) PLS

Yield stress (Pa)

Yield stress Penetration

30

30

20

20

10

10

0 0

300

600

900

1200

Penetration depth (mm)

40

40

5. Summary and conclusions In this research cement pastes were prepared with given waterto-cement ratios of 0.26 and 0.32 using different types and dosages of the superplasticizers. The cement pastes had yield stress of <6 Pa at 30 min. Based on the experimental results, the following conclusions may be drawn:

0 600

0

measurement suggested by Amziane and Ferraris [11]. As aforementioned, the setting time determined by the penetration depth is used to describe an arbitrarily chosen stage of setting. To find out the changes in workability before the setting defined by the penetration depth, the yield stress and penetration depth of each paste is plotted on the same graph as shown in Figs. 14 and 15. The figures show that the yield stress of the pastes increased sharply and the pastes were changing from liquids to solids long before the initial setting times by the Vicat needle test were reached. Since the initial setting time, per definition, indicates a significant loss of workability and the beginning of the stiffening of the paste, the initial setting time determined by the change of yield stress provides more precise information than the penetration depth test of the pastes. This is consistent with the results by Amziane and Ferraris [11]. Although the data obtained with the three different tests in Table 4 show similar trends when comparing cement pastes with different admixtures, it seems that the actual time related to the initial setting varied according to the tests. It is recognized that the heat curves assesses the extent of cement hydration whereas the initial setting is a physical phenomenon; different extents of hydration are required to achieve setting depending on mixture proportions and w/c of mixtures [6]. Nevertheless, the calorimetry and heat curves may provide useful information for estimating initial setting time of cement pastes incorporating superplasticizers with retarding effects at a given w/c, particularly for relative comparison. It is recognized that the hydration reactions starts in the first few minutes after the cement is in contact with water and that it consumes part of the admixture added. The admixture consumed may no longer be available for the dispersion of cement particles. Because significant part of the consumption takes place during the first few minutes, adding the admixtures later will result in better dispersion if the same amount of the admixtures is used. However, due to the different compositions of admixtures and mechanisms on dispersion of these admixtures, delayed addition of the admixtures may not lead to the same extent of the improvement for the dispersion. Composition and specific surface of cement also affect the rate of hydration reactions during the first few minutes and thus consumption of the admixtures. Therefore, the relative amount of the admixtures required to achieve similar workability may vary depending on the type of admixture and on the time of addition of the admixtures.

0 1500

Time (min) Fig. 15. Comparison of changes on yield stress and penetration depth with time for pastes with w/c of 0.32.

1. The modified lignosulphonate (PLS), polycarboxylate (PCE), and polynaphthalene (PNS) based superplasticizers investigated retarded cement hydration. However, the degree of retardation varied with the type and dosage of the admixtures. The PLS admixture had much stronger retarding effect compared with the PCE and PNS admixtures. 2. The shapes of the shear stress – shear rate curves indicated that the cement pastes were shear thickening materials. The pastes with PCE and PNS admixtures showed higher shear thickening behavior than that with PLS admixture. Although the yield stresses and effective viscosity of the cement pastes with the PCE and PNS admixtures were lower than those with the PLS admixture at 30 min, the yield stress and effective viscosity of

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the formers increased more rapidly with time compared with the latter. This indicated that the paste with the PLS admixture had longer workable time than that with the PCE and PNS admixtures. 3. The initial setting times of the pastes with the PCE and PNS admixtures determined by the penetration depth method were much shorter than that with the PLS admixture. Although the pastes with PLS admixture had longer workable time, the longer setting time has to be taken into consideration in practice whenever early strength development is essential. 4. Since the initial setting time indicates the loss of workability to such an extent that the paste is no longer workable, the initial setting time determined by the change of yield stress provides more precise information than the penetration depth test of the cement pastes. The heat development curves may provide useful information for estimating initial setting time of cement pastes incorporating superplasticizers with retarding effects at a given w/c.

Acknowledgements The authors greatly appreciate Borregaard Industry Ltd. for funding this research project, and Dr. Kare Reknes and Mr. Philip Chuah for constructive comments and discussion. The authors would like to thank Ang Beng Oon of Structure laboratory, National University of Singapore (NUS) for his assistance in the various experiments. Dr. Tam Chat Tim’s review was appreciated. References [1] Reknes K, Peterson BG. Self-compacting concrete with lignosulphonate based superplasticizer. In: 3rd International symposium on self-compacting concrete, Reykjavik; 2003. p. 184–9. [2] Jiang SP, Mutin JC, Nonat A. Studies on mechanism, physicochemical parameters at the origin of the cement setting I. The fundamental process involved during cement setting. Cem Concr Res 1995;25(4):779–89. [3] D’Angelo R, Plona T, Schwartz L, Coveney P. Ultrasonic measurements on hydrating cement slurries: the onset of shear wave propagation. Adv CementBased Mater 1995;2(1):8–14. [4] Bentz D. A review of early-age properties of cement-based materials. In: Proceedings 12th international congress on the chemistry of cement, Montreal; 2007. [5] Garcia A, Castro-Fresno D, Polanco JA. Evolution of penetration resistance in fresh concrete. Cem Concr Res 2008;38(5):649–59. [6] Bentz D. Cement hydration: building bridges and dams at the microstructure level. Mater Struct 2007;40(4):397–404. [7] Mindess S, Young JF, Darwin D. Concrete. 2nd ed. Prentice Hall; 2003. [8] Taylor PC, Graf LA, Zemajtis JZ, Johansen VC, Kozikowski RL, Ferraris CF. Identifying incompatible combinations of concrete materials. Technical report FHWA-HRT-06-079: US Department of Transportation, Federal Highway Administration, USA; 2006. [9] Banfill PFG. The rheology of fresh cement and concrete – a review. In: Proceedings 11th international congress on the chemistry of cement, Durban; 2003.

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