Scholarly article on topic 'The effects of the sequential addition of synthesis parameters on the performance of alkali activated fly ash mortar'

The effects of the sequential addition of synthesis parameters on the performance of alkali activated fly ash mortar Academic research paper on "Civil engineering"

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{"Mixing method" / "Sequential addition" / "Synthesis parameters" / "Fly ash-based geopolymer mortar" / "Compressive properties"}

Abstract of research paper on Civil engineering, author of scientific article — Jean-Baptiste Mawulé Dassekpo, Xiaoxiong Zha, Jiapeng Zhan, Jiaqian Ning

Abstract Geopolymer is an energy efficient and sustainable material that is currently used in construction industry as an alternative for Portland cement. As a new material, specific mix design method is essential and efforts have been made to develop a mix design procedure with the main focus on achieving better compressive strength and economy. In this paper, a sequential addition of synthesis parameters such as fly ash-sand, alkaline liquids, plasticizer and additional water at well-defined time intervals was investigated. A total of 4 mix procedures were used to study the compressive performance on fly ash-based geopolymer mortar and the results of each method were analyzed and discussed. Experimental results show that the sequential addition of sodium hydroxide (NaOH), sodium silicate (Na2SiO3), plasticizer (PL), followed by adding water (WA) increases considerably the compressive strengths of the geopolymer-based mortar. These results clearly demonstrate the high significant influence of sequential addition of synthesis parameters on geopolymer materials compressive properties, and also provide a new mixing method for the preparation of geopolymer paste, mortar and concrete.

Academic research paper on topic "The effects of the sequential addition of synthesis parameters on the performance of alkali activated fly ash mortar"

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Results in Physics

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The effects of the sequential addition of synthesis parameters on the performance of alkali activated fly ash mortar

Jean-Baptiste Mawule Dassekpo, Xiaoxiong Zha *, Jiapeng Zhan, Jiaqian Ning

Department of Civil and Environmental Engineering, Shenzhen Graduate School, Harbin Institute of Technology, 518055, China

ABSTRACT

Geopolymer is an energy efficient and sustainable material that is currently used in construction industry as an alternative for Portland cement. As a new material, specific mix design method is essential and efforts have been made to develop a mix design procedure with the main focus on achieving better compressive strength and economy. In this paper, a sequential addition of synthesis parameters such as fly ash-sand, alkaline liquids, plasticizer and additional water at well-defined time intervals was investigated. A total of 4 mix procedures were used to study the compressive performance on fly ash-based geopolymer mortar and the results of each method were analyzed and discussed. Experimental results show that the sequential addition of sodium hydroxide (NaOH), sodium silicate (Na2SiO3), plasticizer (PL), followed by adding water (WA) increases considerably the compressive strengths of the geopolymer-based mortar. These results clearly demonstrate the high significant influence of sequential addition of synthesis parameters on geopolymer materials compressive properties, and also provide a new mixing method for the preparation of geopolymer paste, mortar and concrete.

© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4XI/).

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ARTICLE INFO

Article history:

Received 17 February 2017

Received in revised form 15 April 2017

Accepted 16 April 2017

Available online 21 April 2017

Keywords:

Mixing method

Sequential addition

Synthesis parameters

Fly ash-based geopolymer mortar

Compressive properties

Introduction

As the concept of sustainable development continues to move forward, geopolymer material will be certainly used in the construction industry as an alternative for Portland cement, which enhances an intensive diffusion of carbon dioxide in the environment [1,2]. Geopolymer binders are obtained from the chemical reaction between alumina-silicate raw materials and concentrated solutions of alkali hydroxides [3-6], silicates [7], sulphates [8], carbonates [9] or the combinations those previously cited solutions [10,11]. The choice of the two materials depends essentially on factors such as availability, cost, field of application, and specific demand of the users. Geopolymerization is the chemical reaction that occurs after combining many small molecules known as oligomers into a covalently bonded network. The chemical composition of the derived geopolymer material is similar to natural Zeolitic with amorphous microstructure [12,13]. This chemical process occurs in three steps: dissolution of raw materials into an alkaline solution to form Si and Al gel, transportation or orientation and polycondensation to form networked polymeric oxide structures

* Corresponding author. E-mail addresses: dassekpo.jb@gmail.com (J.-B.M Dassekpo), zhaxx@hit.edu.cn (X. Zha), zhan_jiapeng@126.com (J. Zhan), 809873360@qq.com (J. Ning).

[14,15]. This gel can coexist in binders based on high-calcium and low-calcium blends precursors [16,17].

The type of alkali activator, the molar ratio concentration of Si/ Al in the solution during dissolution, water content, and the curing temperature and time has significant effect on the resulting material compressive strength. It was indicated by [18-23] that the curing temperature plays an important role in the exothermic reaction of geopolymer. It should be noted that the presence of water in the mixture, therefore does not play any role in the chemical reaction, it merely provides the workability to the mixture during mixing. This chemical reaction of water in geopolymer material is the same with Portland cement concrete mixture during hydration process.

However, for geopolymer to become a dominant construction material, efforts must be made to form design methods and mix design processes. According to the literature, research [24,25] has shown that the mix design proportions are available for both geopolymer concrete and mortar. Several mixing methods were suggested by different researchers. Rangan et al. [26] recommended the mixing of alkaline liquids for 24hours prior to casting, and proposed that the aggregates were dry mixed with fly ash for 2 min. The alkaline liquids along with an additional water and plasticizer were then added and mixed for 3-5 min. Diaz-Loya et al. [27] adopted the mixing of fly ash with sodium hydroxide solution for 3 min and the addition of sodium silicate for another 2 min. Aggregates were then added to the paste and mixed for

http://dx.doi.org/10.1016/j.rinp.2017.04.019 2211-3797/® 2017 The Authors. Published by Elsevier B.V.

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another 3 min. Additional water along with any plasticizer were then added and mixed for 2 more minutes. Moreover, another method proposed the dry mixing of saturated and surface dried (SSD) aggregates with fly ash for about 2 min. The silicate solution was then added to the dry mix and mixed for 3 min. Hydroxide solution was then added and mixed for 2 more minutes. Additional water along with any admixtures was then added and mixed for an additional 2 min period. This last method was recently used by Talha Junaid et al. [28] by developing a systematic approach for selecting mix proportions.

Apart from these researches, there has not been any research on the effects of the synthesis constituents or parameter addition order on the mixture performance during the production of geopolymer materials; furthermore, no research works have yet been published on the comparison between these previous proposed mixing methods. The aim of this paper is to investigate on the sequential addition of synthesis parameters such as fly ash-sand, alkaline activator, plasticizer and additional water at well-defined time intervals on class F fly ash-based geopolymer mortar.

Materials and synthesis experimental program

Fine aggregate and fly ash

Fine aggregate is a key material in the mix design of geopolymer mortar. The fine aggregates can be found in natural or artificial environments but its grading and sources must be uniform throughout the mixing of the geopolymer mortar. In this study, locally available low calcium Class F fly ash from a thermal power station in Shenzhen that conforms to ASTM C-618 specification, and locally available river sand were used throughout the research. The particle size distribution of fly ash and sand used are provided in Fig. 1. The composition of fly ash as determined by XRF is also presented in Table 1. It can be observed that the content of calcium oxide is very low; therefore it can be classified as Class-F fly ash according to ASTM C618-08 [29]. It can also be seen that, the content of oxides of silicon and aluminum is relatively high with 60.70% and 24.72% respectively. Fig. 2 shows the fly ash and the river sand used in the mix preparation.

Alkaline liquids

The activator solution used for the geopolymerization process is a combination of sodium hydroxide (NaOH) and sodium silicate

Particle-size (microns)

Fig. 1. Particle size distribution curve of fly ash and fine sand used in this study.

(Na2SiO3). The solutions were prepared by mixing sodium hydroxide and sodium silicate according to the proposed mix preparation methods in this study. Fig. 3 shows the alkaline activator used in the experimental test.

Mix design proportion

This study adopted the mix proportion displayed in Table 2. Note that the concentration of sodium hydroxide solution can be measured in terms of molarity and the value of (14 M) concentration of NaOH was assumed to reach a higher value of compressive strength [30]. On the other hand, the alkaline activator and fly ash (AL/FA) ratio adopted in this study follows the research work of [31] and serves as the basis of the mixture components calculation.

Mixing methods and synthesis parameters sequence

This research work remains in contrast to previous geopolymer materials mix procedure but adopts a typical orderly or sequential addition of the mix constituents in well-defined time intervals. The number of variables was kept the same for all mix design methods by using the same mixture proportions. Note that the fine aggregates were dried mixed and the total mixing time was 12 min. Fig. 4 describes all the manufacturing procedures and synthesis parameter orders adopted in this experiment.

Mixing method - Synthesis parameters sequence 1

The first step was the mixing of sodium silicate and sodium hydroxide (NaOH and Na2SiO3) for 24 h. This method was suggested by Rangan et al. [26] for the mix preparation. The pre-mix alkaline solution was then mixed with fly ash and sand for 6 min, followed by the plasticizer for 2 min and then additional water for another 2 min; afterwards, the whole mixture was mixed for 2 min in order to obtain a homogeneous mortar mixture. The parameters order is described in Fig. 8, with the underlining symbol showing that the solution was mixed 24 h prior.

Mixing method - Synthesis parameters sequence 2

This mix procedure is the same as the one described previously, but the sodium silicate and sodium hydroxide were mixed and used automatically by mixing with fly ash and sand for 6 min, followed by the plasticizer for 2 min and additional water for another 2 min. Afterwards, the whole mixture was mixed for 2 min in order to obtain a homogeneous mortar mixture.

Mixing method - synthesis parameters sequence 3

The first step in this method was the mixing of sodium hydroxide with fly ash and sand for 3 min, followed by the mixing of sodium silicate solution for another 3 min; afterwards, the plasti-cizer was mixed for 2 min and then additional water was added to the mixture and mixed for 2 min. The whole mixture was then mixed for 2 min in order to obtain a homogeneous mortar mixture. This method is similar to that proposed by Diaz-Loya et al. [27].

Mixing method - synthesis parameters sequence 4

The first step in this method was the mixing of sodium silicate with fly ash and sand for 3 min, followed by the mixing of sodium hydroxide for another 3 min; afterwards, the plasticizer was mixed for 2 min and the additional water was added to the mixture and mixed for 2 min. The whole mixture was then mixed for 2 min in order to obtain a homogeneous mortar mixture.

Table 1

Chemical composition of the fly ash based on XRD analysis.

Composition SiO2 AI2O3 Fe2O3 CaO MgO TiO2 MnO K2O P2O5 SO3 LOI

Content (%) 60.70 24.72 6.90 0.70 1.13 - - - - 1.50 2.35

Fig. 3. NaOH in pellets and Na2SiO3 solution used in the experimental test.

Table 2

Fly ash-based geopolymer mortar mix proportion.

Specimen label Fly ash (g) Sand (g) NaOH (g) Na2SiO3 (g) Water (g) Plasticizer (g)

GM-SP1 4200 2100 450 1590 260 60

GM-SP2 4200 2100 450 1590 260 60

GM-SP3 4200 2100 450 1590 260 60

GM-SP4 4200 2100 450 1590 260 60

Specimens manufacturing and curing

The pan mixer with rotating blades was used for manufacturing the geopolymer mortar. Casting of plastic cube molds with 70.7 x 70.7 x 70.7 mm size was followed and the specimens were placed on a vibration table for 3 min to remove entrapped air. The lapsed time between the end of casting of the geopolymer mixture and the start of the temperature curing is referred to as the ''rest period". This study follows the research work of Vora and Dave [32] so a rest period of 24 h was adopted. After this period, the specimens were released from the molds and left in an environment chamber at 60 °C, 50%RH. This temperature value was chosen because [33,34] found that the curing temperature and the time

have a great influence on the geopolymer compressive strength. All the specimens were left at controlled temperatures for 7, 14 and 28 days until the compressive strength test was performed.

Compressive strengths test

All the mortar specimens were load tested with a compression machine as shown in Fig. 5. The test was performed respectively after 7, 14 and 28 days curing time. This is a common practice in Portland cement mortars to characterize the material performance under compressive load. Table 3 summarizes the results obtained from the compressive testing of fly ash-based geopolymer mortar.

Fig. 4. Mixing methods and synthesis parameters sequences description.

100000 90000 80000 70000 60000 50000 40000 30000 20000 10000 0

110000 100000 90000

iz 80000

H 70000

g 60000 J

g 50000

o 40000

13 30000 U

20000 10000 0

" _ GM-SP1-7 GM-SP1-1 days 4 days

GM-SP1-2 8 days

0.5 1.0 1.5 2.0 2.5 Displacement (mm)

a) SP1 deformation curves

...............

// GM-SP GM-SP 3-7 days 3-14day

1 s

_ GM-SP 3-28 day s

5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Displacement (mm)

c) SP3 deformation curves

100000

g 70000

q 60000

o 11 50000

& 30000

U 20000

' _ GM-SP2-7 GM-SP2-1 days 4 days .....

GM-SP2-2 8 days >v 1

100000 90000 80000 70000 60000 50000 40000 30000

Displacement (mm)

b) SP2 deformation curves

— GM-SP 4-7 days 4-14 day

— GM-SP

— GM-SP t-28 day

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Displacement (mm)

d) SP4 deformation curves

Fig. 6. Compressive load-deformation curves of the specimens at 7,14 and 28 days curing period.

^ n J w 12 -

^ 4 -I Q

OO , <N 2 -

SP1 SP2 SP3

Synthesis Parameters Sequence

Fig. 7. Effect of synthesis parameters sequences on the compressive strengths.

Results and discussion

Compressive loadings and deformation curves

Fig. 6 shows the curves related to the compressive load-displacement relationship of the tested fly ash-based

geopolymer mortar specimens at different curing times of 7, 14 and 28 days. It can be seen that the deformation curve shown by the tested specimens remain linearly elastic throughout the test until failure and the specimens manufactured using the synthesis sequence parameters, SP3, presents the most resistance during compression; unlike the specimen prepared with SP2 which presents the weakest compressive loading throughout the curing period. In the case of synthesis sequence parameters 1 and 2, the specimens' deformation curves displayed a sudden drop in the compressive loading which is really contrary to the curves shape for the specimens in the synthesis sequence parameters 3 and 4. These behaviours clearly demonstrate the high significant influence of sequential addition of synthesis parameters on the fly ash-based geopolymer compressive properties.

The average peak compressive loads for the specimens at 7 days curing period are in the range of 55.91 kN, 52.60 kN, 57.89 kN and 54.67 kN for SP1, SP2, SP3 and SP4 respectively. Similarly, the specimens at 14 days curing time present an average peak load values of 79.10 kN, 77.18 kN, 78.74 kN and 74.18 kN respectively for SP1, SP2, SP3 and SP4. The highest peak load average was obtained at 28 days curing time, and the values are in the range of 91.60 kN, 83.54 kN, 105.64 kN and 100.75 kN respectively for the sequence parameters SP1, SP2, SP3 and SP4.

It can also be seen that, the peak loads for all specimens at 28 days curing present higher values compared to those of 14 and 7 days.

110000

■W 14

'и 8-1

s^ -& 6420

SP3-28 SP4-28

SP1-28

SP2-28 SP3-14 SP1-14

ISP4-14 ISP2-14 SP3-7

SP4-7 |SP2-7

Curing Time (days)

Synthesis parameters sequences

SP1 = (Fly Ash + Sand) + (NaOH + Na? SiO3) + PL + WA SP2 = (Fly Ash + Sand) + (NaOH + Na? SiOa) + PL + WA SP3 = (Fly Ash + Sand) + NaOH + Na2 SiO3 + PL + WA SP4 = (Fly Ash + Sand) + Na? SiOa + NaOH + PL + WA

Materials and curing conditions

Low-calcium Class F Fly Ash and river sand

Sodium hydroxide NaOH

Sodium silicate Na2 SiO3

Plasticizer PL

Water WA

Rest period: 24h before curing Curing temperature: 60oC Curing period: 7-14-28 days

Fig. 8. Comparison of different mixing methods at different curing time.

Table 3

Compressive loads and compressive strengths of fly ash-based geopolymer mortar.

Specimen label

7 days curing

14 days curing

28 days curing

Load (kN)

Compr. Str. (MPa)

Load (kN)

Compr. Str. (MPa)

Load (kN)

Compr. Str. (MPa)

GM-SP1 GM-SP2 GM-SP3 GM-SP4

55.91 52.60 57.89 54.67

11.18 10.52 11.58 10.93

79.10 77.18 78.74 74.18

15.82 15.44 15.75 14.84

91.60 83.54 105.64 100.75

18.32 16.71 21.13 20.15

Effect of synthesis parameters sequences on the compressive strengths

The effect of synthesis parameters addition on the compressive strengths of fly ash-based geopolymer mortar at 28 days curing was investigated and the results were analyzed (Fig. 7). From the figure, it can be seen that the compressive strength value is in the range of 16.71 MPa and 21.13 MPa when cured at 28 days period. The compressive strength of the sequential parameters 1 is in the range of 18.32 MPa, whiles the sequential parameters 2, where the sodium silicate and sodium hydroxide were mixed and used automatically, presents the lowest compressive strength value of 16.71 MPa. This difference of strength proved that the mixing of sodium silicate and sodium hydroxide (NaOH and Na2SiO3) prior for 24 h suggested by Rangan et al. [26] has a great impact on the geopolymer compressive strengths.

It is also remarkable that the mix with sequential parameters 3 at 28 days curing period presents the higher value in compressive strength with an average value of 21.13 MPa followed by the sequential parameter 4 specimen with a compressive strength of 20.15 MPa. Comparing these two values, it can be seen that the mix method using the sequential addition of sodium hydroxide (NaOH), sodium silicate (Na2SiO3), plasticizer (PL), followed by the addition of water (WA) increases considerably the compressive strength of the geopolymer-based mortar. The difference in value in compression compared with the mix method SP4, where sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were inverted, was approximatively 1 MPa. From this result, one can concluded that the method proposed by Diaz-Loya et al. [27] is the most influential geopolymer mixing method. But it is urgent to emphasize that compressive strength value of the specimen mixed using the sequential parameters SP4 is not negligible.

When considering the best compressive strength as the main parameter in geopolymer production, we can suggest the sequential addition of synthesis parameters SP3 presented in (Eq. (1)) as the best for the preparation of geopolymer paste, mortar and concrete.

PS3 = (FlyAsh + Sand) + NaOH + Na2SiO3 + PL + WA (1)

Comparison of different mixing methods at different curing time

The comparison of the compressive strengths of the specimens at different curing times is presented in Fig. 8. As indicated in the figure, the average compressive strengths of SP1 specimen are 18.32 MPa, 15.82 MPa, and 11.18 MPa respectively at 28, 14 and 7 days curing period. The synthesis parameters SP2 specimens also show compressive strength values of 16.71 MPa, 15.44 MPa, and 10.52 MPa respectively at 28, 14 and 7 days curing period. Similarly, the compressive strengths of SP3 specimens are 21.13 MPa, 15.75 MPa, and 11.58 MPa respectively at 28, 14 and 7 days curing period, while the strength values of SP4 specimens are in the range of 20.15 MPa, 14.84 MPa and 10.93 MPa at 28,14 and 7 days curing period respectively.

It can be observed that the compressive strength at 7 and 28 days curing of SP3 and the compressive strength of SP1 at 14 days curing represent the highest strengths values obtained from this experimental tests when compared to others. This does not mean that the other compressive strengths values are negligible. They must be taken into account as all the specimens were manufactured following diverse synthesis parameter addition sequences. It can also be seen that the compressive strengths of all the specimens increase over time.

On the other hand, the compressive strength values of all the specimens at different curing times are significant and are marginally higher as the specimens were manufactured as mortar and cured at a controlled temperature of 60 °C.

Conclusion

The experimental study of the effects of sequential addition of synthesis parameters to perform a better mixing preparation of alkali activated fly ash mortar are presented along with useful mixing methods in this paper. In this approach, the synthesis parameters SP3 presented in Eq. (1), where sequential addition of sodium hydroxide, sodium silicate, plasticizer, followed by the addition of water was identified as the most significant synthesis parameter addition order, determines the compressive properties and the performance of the hardened geopolymer product with confidence. The experimental results show the application of the proposed mixing methods and its ability to produce consistent results in the preparation of geopolymer paste, mortar, and concrete.

More researches are needed to well identify the effects of all synthesis parameters used in the mixing process during the preparation of geopolymer materials for its better application as an alternative to Portland cement by reducing carbon dioxide emission rate.

Acknowledgements

This study is funded by the National Nature Science foundation of China (Grant number: 51578181) and Shenzhen Science and Technology Plan Project (No JCYJ20150327155221857). Acknowledgement is also given to Shenzhen Carbon Storage Cement-based Materials Engineering Laboratory.

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