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CANMET/ACI International Conference on Recent Advances in Concrete Technology, pp. 1-15, 2003-06-01

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Acceleration of the rate of the strength development for concrete mixes containing 30% replacement of portland cement by fly ash

Raki, L.; Chen, M.; Mailvaganam, N. P.

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Acceleration of the rate of the strength development for concrete mixes containing 30% replacement of Portland cement by fly ash

Raki, L.; Chen, M.; Mailvaganam, N.P.

NRCC-46262

A version of this document is published in / Une version de ce document se trouve dans :

6th CANMET/ACI International Conference on Recent Advances in Concrete Technology,

Bucharest, Romania, June 8-11, 2003, pp. 1-15

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Acceleration of the Rate of the Strength Development for

Concrete Mixes Containing 30% Replacement of Portland

Cement by Fly Ash

by L. Raki, M. Chen and N. P. Mailvaganam

Synopsis: The merits of the use of fly ash in concrete mixes have been widely published. A replacement of cement > 20%, however, decreases productivity in construction. The use of an accelerating admixture to offset the delayed hydration resulting from the inclusion of fly ash levels >20 % in normal portland cement concrete mixtures is discussed. The objective of this investigation was to produce a concrete mixture with a significant amount (30%) of fly ash and yet developed early strength similar to 100% normal portland cement mixtures. The mechanism of the acceleration process of the fly ash/normal portland cement hydration will be discussed subsequently in a companion paper.

Concrete mixtures with design strengths of 35 MPa containing a 30% replacement fly ash were accelerated to achieve 1 day-strength comparable to 100% normal portland cement. Mechanical properties such as compressive strength development were determined. The degree and rate of hydration were followed by X-ray diffraction, thermal analysis, and conduction calorimetry. It is shown that the addition of a new experimental accelerator in fly ash mixtures containing 30% fly ash replacement provided a compressive strength close to that obtained from a 100% normal portland cement mixture. The similarity of the hydrates formed in mortar mixtures containing the accelerator to mixtures containing 100% normal portland cement was also confirmed by X-ray diffraction and thermal analysis.

Keywords: accelerator, cement, conduction calorimetry, fly ash,

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L. Raki, a research officer at the Institute for Research in Construction, National Research Council Canada, is the secretary of the “Cements Division” of the American Ceramic Society. She is also an ACI member of the 522 committee on “Pervious Concrete” and an active member of the new RILEM technical committee on “Nanotechnology in Construction Materials”. Dr. Raki’s research focuses on the recycling and utilization of industrial waste materials.

M. Chen was a Master student at the time of this investigation. He is currently employed as a Q.C Engineer for ready mix Co. in Malaysia. N. Mailvaganam is a principal research officer at the Institute for Research in Construction, National Research Council Canada. He is a member of ACI and CSA committees in the fields of Admixtures and Repair and has published books of these subjects.

Introduction

Fly ash, or pulverized fuel ash, is a residue derived from the combustion of pulverized coal in thermal power plant furnaces. Its incorporation - for environmental concerns and cost-effective objectives- as a supplementary cementing material has significantly increased during the last two decades. Because of its fine glassy particles, fly ash can act as filler between cement grains and aggregates.1 Consequently, the microstructure of resultant concrete and its physical properties are modified. Addition of fly ash, however, causes retardation of the hydration process. This effect has been attributed to the presence of the glassy alumino-silicates particles in fly ash and to the formation of a film made of Ca(OH)2 surrounding the glass particles of fly ash, which acts as a barrier to the ingress of water. 2-4

This investigation presents the effects of an accelerating admixture on properties of concrete containing normal portland cement with a 30% replacement of fly ash. These effects were studied by conduction calorimetry, X-ray diffraction analysis (XRD), thermal analysis (DTA and TGA), and compressive strength measurements.

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Experimental

Materials

The materials used in this study were: normal portland cement supplied by Lafarge Corporation, Ottawa Sand ASTM C778, silica fume from SKW, Bécancour, Québec, ASTM C 618 Class F fly ash from Point Tupper from Nova Scotia, and the newly-developed fly ash accelerator. Table 1 shows the chemical composition and physical properties of normal portland cement. The compositions of fly ash and silica fumed used in this study are provided in Table 2. The accelerator consists of a mixture of alkaline salts, naphthalene based superplasticizer, and silica fume. For

comparison, a commercial accelerator was tested concurrently.

Mixture proportions

All mortars and cement pastes were made up by weight to the following compositions:

Normal portland cement/Fly ash: 70/30 100% normal portland cement: 100/0

The W/C was maintained at 0.4 and mortars were prepared with sand/cementitious proportion of 2.75/1. The admixtures were added at the appropriate dosages based on the weight of cementitious material as shown in Table 3.

Techniques

The XRD analyses were carried out using Cu Kα radiation at 40KV and 30 mA on a Rikagu diffractometer. The XRD scan was chosen at 10°/min steps from 2θ = 2 to 75°. Samples were prepared by dispersing a little amount of the powder in acetone and then applying the mixture on a glass slide and allowing to air dry.

TGA and DTA measurements were performed on a Thermal Analysts 2100 System (TA Instruments Inc) with alumina reference and sample pans under flowing air (100 ml/min) and a heating rate of 20°C per minute from room temperature to 1000°C.

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Calorimetric studies were conducted on a NESLAB Instruments Inc instrument to determine rates of heat evolution for different mixes. For every run, five samples were prepared and the heat released was monitored for a 48-hour period

Compressive strength was done according to the ASTM C109 M-95 “Standard Test Method for Compressive Strength of Hydraulic Cement Mortars using 2-in Cube Specimens.” The specimen preparation was performed at room temperature with a relative humidity of about 50%. Prior to mixing, water containing accelerator (if any), cement and/or fly ash and sand were weighed according to W/C of 0.4 (water/cementitious) and 2.75 (sand/cementitious). After mixing for 2.5 minutes, the mortar was poured and tamped in lubricated molds for easy de-molding. Compressive strength was determined after the specimens were cured for 24 hour.

Results and Discussion

Degree of hydration

The XRD analyses of fly ash/cement paste samples with different compositions are presented in Figs. 1a-1c. Fig. 1a represents the hydration products of a 100% normal portland cement mixture. Upon reaction with water, C3S and C2S produce Portlandite [Ca(OH)2] and amorphous calcium-silica-hydrate [(C-S-H)] (Fig.1a). Portlandite is represented by peaks appearing at 2θ = 18.1, 34.1 and 47.1°. Peaks located at 9.2 and 15.9° show Ettringite formation. The ettringite is formed by the reaction of C3A and gypsum present in cement. The relatively low intensity of clinker phases in Fig. 1a indicates that most of them have reacted to form CSH.

As regards the fly ash/cement pastes, hydrates are presented in Figs. 1b, 1c. Fig. 1b represents the hydration products of a 70/30 normal portland cement /fly ash mixture. This sample shows poorly resolved peaks with the highest diffraction maxima occurring at 2 θ = 18.5° and 34.6° corresponding to the diffraction of the planes (001) and (101) of Ca(OH)2, respectively. This is indicative of a limited formation of Ca(OH)2 caused by a slow hydration process. Fig. 1c shows the phases resulting from a mixture of 70/30 normal portland cement/fly ash in the presence of an accelerator.

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The profile of the hydrates is basically similar to those of 100% normal portland cement sample (Fig. 1a). The larger intensity of Portlandite peaks indicates a higher degree of hydration of the clinker phases, whose peaks are still present but less intense in this sample (Fig. 1c). The amount of CH seems to be slightly increased in the case of Fig. 1c but the strength measurement will show (Table 5) a lower strength for this sample compared to the 100% normal portland cement sample (Fig. 1a). The CH content is not the only contributor in strength development for concrete, as it has been reported.1 The ettringite diffraction peaks are more intense compared to previous samples. The product also contains amorphous compounds around 2θ=30°, which could be due to the poorly crystalline C-S-H phase. Some quartz peaks are also present in the hydrated products.

Fig. 2 represents the curves corresponding to the thermal analysis of pastes hydrated for 2 days. All samples show similar effect. Fig. 2a represents the decomposition profile of a 100% normal portland cement mixture. The TG profile indicates a weight loss between 40 and 200°C primarily due to the loss of water from ettringite, which formed during the hydration process as confirmed by XRD analysis. The endothermic peak at 448°C associated with the weight loss between 400 and 500°C is caused by Ca(OH)2 decomposition.5 Finally the third endothermic peak at 647°C is due to the loss of CO2 from calcium carbonate. DTA pattern of the sample with fly ash and accelerator addition (Fig. 2c) shows higher intensities of the CH endotherm, confirming a greater Ca (OH)2 formation, which is in line with the results of XRD.

Heat of hydration of pastes

Conduction calorimetry studies were performed to test the reactivities of fly ash by determining heat evolution rates in different mixes. The heat evolution profile of different cement preparations with a cement/fly ash proportion of 70/30 are presented in Figure 3, which shows the evolution of the hydration heat over 2 days (48 hours). The first 15 hours are the most crucial as the curves clearly delineate the effect of 30% fly ash substitution and roles played by the accelerator in accelerating the hydration process.

Table 4 shows that without the accelerator, the hydration of cement paste with fly ash was retarded severely. Although the commercial product

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provided acceleration, the results were not similar to that of 100% OPC mix. On the other hand, the experimental accelerator counteracted the retardation as shown by the curve.

Compressive strength

To establish the development of mechanical property, mortar samples with different compositions were tested. The replacement of 30% fly ash in the mortar reduced the 1-day compressive strength by 28%. But with the accelerator, the normal portland cement/fly ash mortar achieved 18.7 MPa, an improvement of about 22%. On the other hand, the commercial product that was added to the normal portland cement/fly ash mortar did not show any significant effect on the early strength development. The results are presented in Table 5.

As indicated by XRD and TG/DTA analyses, the main hydration product, calcium-silicate-hydrate (C-S-H), of the reactions of fly ash/cement systems is essentially the same as that of the hydration of normal Portland cement. When fly ash is in contact with water, limited amounts of calcium and aluminum ions are released in the solution. For this reason, the hydration of mixtures with fly ash is severely retarded at early ages until additional activators such as alkali, calcium hydroxides, or sulfates are present in the medium. Fly ash consists mainly of glassy spherical particles and the morphology of fly ash reaction product has been described to be a dense gel-like material.1 Therefore, the reaction of fly ash depends upon breakdown and dissolution of the glassy phase by the alkali ions. Like any other pozzolanic material, the hydration process of fly ash has been described as interplay of three chemical processes: hydrolysis, ion exchange and precipitation6. The addition of alkali ion salts to such mixes helped in “etching” the surface of the glassy particles of fly ash. This produced a large number of active groups and led to more hydration products, which resulted in a high early strength development as proved by the XRD analysis and strength measurement. On the other hand, the amount of heat released during the hydration of cement is usually reduced when fly ash is used as a partial replacement of cement. In this present investigation, mixtures with 30% replacement of cement by fly ash in the presence of experimental admixture have produced comparable heat release at early ages (2 days).

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Conclusion

Previous work has shown that initial (1 day) strength of concrete mixes containing cement replacement by fly ash exceeding 20% are significantly lower than those obtained with 100% portland cement mixtures. From the results of the investigation presented here, we concluded the following:

– Fly ash/normal portland cement mixtures are retarded by the inclusion of fly ash. This is evidenced by the slow initial formation of Ca(OH)2 as shown by XRD and TGA.

– The alkaline accelerator increases the rate of formation of Ca(OH)2 and rate of strength development.

This work aimed to reduce energy and CO2 production by replacing up to 30% of cement by fly ash. The addition of an experimental admixture to different mixes with fly ash, showed similar kinetic and same reaction products to the mix made of 100% normal portland cement. The mechanical properties of mixes with 70/30 normal portland cement/fly ash in the presence of our accelerator were comparable to mixes made of 100% normal portland cement and performed better than mixes with commercial accelerator.

Acknowledgments

This work was funded by grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada. Its financial support is gratefully acknowledged

.

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References

1. Xu, A., Sakkar, S. L., and Nilson. L. O., Effect of Fly Ash on the Microstructure of Cement Mortar. Materials and Structures, 26, pp. 414-424, 1993.

2. Fraay A. L. A., Bijen J. M., and de Hann Y. M. The Reaction of Fly Ash in Concrete - A Critical Examination. Cement and Concrete Research, Pergamon Press Ltd., USA, v. 19, p 235-246, 1998.

3. Diamond, S., Ravina, D., and Lovell, J. The Occurrence of Duplex Films on Fly Ash Surfaces. Cement and Concrete Research, 10, pp. 297-300, 1980.

4. Raask, E., and Bhasker, M. C. Pozollanic Activity of Pulverized Fuel Ash. Cement and Concrete Research, 5, pp. 363-376, 1975. 5. Ramachandran, V. S., Paroli, R. M., Beaudoin, J. J., and Delgado,

A. H. Handbook of Thermal Analysis of Construction Materials. Noyes Publications, Ed. V. S. Ramachandran et al, 2003.

6. ACI Committee 226, Use of Fly Ash in Concrete. ACI Materials Journal, September-October 1987, pp. 381-409.

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List of Tables

Table 1: Chemical and physical properties of normal Portland cement

Table 2: Chemical and physical properties of fly ash and silica fume

Table 3: Dosages of the admixtures

Table 4: Heat peak generated by various cement pastes

Table 5: One-day mortar compressive strength for pastes with various accelerators

List of Figures

Fig. 1: XRD patterns of (a) hydrated portland cement (b) 70/30

portland cement/fly ash (c) 70/30 portland cement/fly ash with accelerator

Fig. 2: Thermal analysis curves of (a) hydrated portland cement (b) 70/30 portland cement/fly ash (c) 70/30 portland cement/fly ash with accelerator

Fig. 3: Heat of hydration generated by cement pastes of pure

portland cement, 70/30 portland cement /fly ash, 70/30 portland cement/fly ash with experimental accelerator, 70/30 portland cement /fly ash with commercial accelerator

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Tables

Table 1: Chemical and physical properties of normal Portland cement

Properties OPC A (%) C3S 61.5 C2S 9.9 C4AF 6.7 C3A 9.4 SO3 3.1 CaO 62.8 MgO 3.0 Al2O3 4.9 Fe2O3 2.2 SiO2 19.6 Free lime 0.9

Total alkali (as Na2O) 0.46

LOI 1.9

Insoluble residue 0.6

Fineness 45µm sieve 92.7 pass Blaine (spec. Surf.) 423 m2/kg Vicat setting time 130 min Autoclave expansion 0.08 Compressive strength:

3 days 28.0 MPa

7 days 33.7 MPa

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Table 2: Chemical and physical properties of fly ash and silica fume

Properties Class F FA(%) Silica fume

K2O 2.6 1.19

Na2O 0.9 0.19

Equivalent alkali (Na2O+0.658 K2O) 2.6 SO3 1.6 0.29 CaO 4.2 0.35 MgO 1.2 0.47 Al2O3 20.3 0.21 Fe2O3 23.7 0.31 SiO2 42.7 93.71 P2O5 0.7 0.14 TiO2 0.9 0.01 LOI 2.4 2.72 Specific gravity 2.58 2.16

Fineness 45µm sieve 75.2 pass 98.9 pass

Blaine (spec. surf.) 227 m2/kg

Median grain size 14.0 µm

Water requirement 95.6

Pozzolanic activity index*:

7 days 79.6

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Table 3: Dosages of the admixtures

Admixtures Experimental accelerator

Commercial accelerator

Specific gravity (x/y) 1.26 1.43

Dosage rate (%) 3.2 2.86

Table 4: Heat peak generated by various cement pastes

Cement Pastes Heat Evolved (cal/g) Time (hr)

100% OPC 3.70 10.0

70/30 OPC/FA 2.68 13.3

70/30 OPC/FA + Acc 3.83 10.0

Table 5: One-day mortar compressive strength for pastes with various accelerators

Mixes Compressive Strength (MPa)

OPC 20.3 OPC/FA 14.6

OPC/FA + Accelerator 18.7

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(a) 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 A ng le (2 Th eta) Count per second ( c ps) 0 (b) 0 1 00 2 00 3 00 4 00 5 00 6 00 7 00 0 1 0 2 0 30 4 0 5 0 6 0 7 0 8 A ng le (2 Th eta) C ount per second ( c ps) 0 © 0 1 00 2 00 3 00 4 00 5 00 6 00 7 00 0 1 0 2 0 30 4 0 5 0 6 0 7 0 8 A ng le (2 Th eta) C ount pe r s e c ond ( c ps ) 0

Fig. 1. XRD patterns of (a) hydrated portland cement (b) 70/30 portland cement/fly ash (c) 70/30 portland cement/fly ash with accelerator

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(a)

(b)

©

Fig. 2: Thermal analysis curves of (a) hydrated portland cement (b) 70/30

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Heat Evolved vs Tim e 0 0.5 1 1.5 2 2.5 3 3.5 4 0 10 20 30 40 50 60 T i m e ( h o u r ) OPC 70/30 70/30+OWN accel2.86%

Fig. 3. Heat of hydration generated by cement pastes of pure portland

cement, 70/30 portland cement /fly ash, 70/30 portland cement/fly ash with new accelerator, and 70/30 portland cement/fly ash with commercial accelerator.

Figure

Table 1: Chemical and physical properties of normal Portland cement
Table 2: Chemical and physical properties of fly ash  and silica fume  Properties  Class F FA(%)  Silica fume
Table 5: One-day mortar compressive strength for pastes with various  accelerators
Fig. 1.  XRD patterns of (a) hydrated portland cement (b) 70/30 portland   cement/fly ash (c) 70/30 portland cement/fly ash with accelerator
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