Preliminary Study on Effect of NaOH Concentration on Early Age Compressive Strength of Kaolin-Based Green Cement

H. Kamarudin1, A.M. Mustafa Al Bakri1, M. Binhussain2, C.M Ruzaidi1, M. Luqman1, C.Y. Heah1 +,

Y.M. Liew1

1 Green Concrete@UniMAP, School of Materials Engineering, 01000, P.O Box 77, D/A Pejabat Pos besar, Kangar, Perlis, Malaysia

2 King Abdul Aziz City Science and Technology, P.O Box 6086, Riyadh 11442 Kingdom of Saudi Arabia

Abstract. Research works carried out in developing other alkali activated binders such as fly ash and metakaolin show that this new binder based on kaolin is likely to have enormous potential to become an alternative binder to current concrete. Sodium hydroxide was mixed with sodium silicate to prepare liquid alkali activator 24 h prior to use. Kaolin powder was mixed well with alkali activator using mixer. The fresh paste was then rapidly poured into steel mould and put into the oven at suitable temperature. This study aims to analyze the effect of Sodium Hydroxide (NaOH) concentration (6M-14M) on compressive strength of kaolin cement paste. The result shows that the kaolin binder has adequate compressive strength and is able to apply for non-loading construction materials. This paper outlines the potential of kaolin to produce an environmental friendly, energy saving, clean technology to conserve the natural environment and resources. Kaolin binders are still in the early stages of development and; hence, they need further research work in order to become technically and economically viable construction materials.

Keywords: Kaolin; alkali activated binder; green cement

1. Introduction Joseph Davidovits [1] used kaolinite and metakaolin as source of alumino-silicate oxides to synthesize

and produce geopolymers. Other researchers [2, 3] have also focused on the manufacture of geopolymeric products and their industrial applications by using either kaolinite or metakaolinite as the main reactant. Kaolinite is the main structure forming species in the overall geopolymerization process. Xu and Van Deventer [3, 4] proposed that the addition of kaolinite is necessary since the rate of Al dissolution from the raw materials is not enough to produce a gel of the desired composition and due to the low reactivity of kaolinite, it requires sufficient time for interactions among the source materials to form. A weak structure is formed, if only kaolinite is used without the presence of other alumino-silicates.

Geopolymer is synthesized by the polycondensation of silico-aluminate structures. Highly alkaline solutes such as NaOH and KOH are incorporated with source materials rich in SiO2 and Al2O3 [5]. The geopolymer binders show good bonding properties and utilize a material such as fly ash or metakaolin as the source of silicon and aluminium for reaction by an alkali. Geopolymer binders are used together with aggregates to produce geopolymer concrete. There is no Portland cement involved in this cementing material. The basic mechanism of kaolinite was illustrated by Davidvoits [1]. Aluminosilicate kaolinite reacts with NaOH and polycondenses into hydrated sodalite or hydro-sodalite. Synthesis of geopolymer consists of three basic steps. The first is the dissolution of alumino-silicate under strong alkali solution; this is followed by reorientation of free ion clusters, and the last step is polycondensation. The forming rate of geopolymer is ___________________________________ + Corresponding author. Tel.: +60125711154

E-mail address: chengyongheah@hotmail.com

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2011 International Conference on Chemistry and Chemical Process IPCBEE vol.10 (2011) © (2011) IACSIT Press, Singapore

very rapid; as a result, these three steps take place almost at the same time, which makes the kinetics of these three steps inter-dependent.

Wang et al. [3, 6] have proven experimentally that the compressive strength as well as the apparent density and the content of the amorphous phase of metakaolinite-based geopolymers, increase with an increase of NaOH concentration (within the range 4–12 mol/L). This can be attributed to the enhanced dissolution of the metakaolinite particulates and hence the accelerated condensation of the monomer in the presence of the higher NaOH concentration. Granizo et al. [3] supported the idea that the alkali activation of metakaolin using solutions containing sodium silicate and NaOH results in the production of materials exhibiting higher mechanical strength compared to activation with only NaOH. Ubolluk rattanasak and prinya chindaprasirt [7] studied the influence of NaOH solution on the synthesis of fly ash geopolymer, the results revealed that solubility of fly ash depends on concentration of NaOH and duration of mixing with NaOH where the use of 10M and 15M NaOH gave relatively high strength. As expected, 5M NaOH gave low strength due to low leaching of Si and Al ions in NaOH solution.

Inorganic polymer concretes have emerged as novel engineering materials with the potential to form a substantial element of an environmentally sustainable construction and building products industry. Although extensive research has already been carried out, the development of kaolin as a new binders concrete has not yet been explored. Parameter such as the NaOH concentration is important, that it must be taken into consideration when designing a kaolin-based green cement product for a specific application [8].

2. Experimental Methods

2.1. Materials Kaolin was supplied by Associated Kaolin Industries Sdn. Bhd., Malaysia. The general chemical

composition is tabulated in Table 1 which obtained from supplier’s data sheet. The physical form of kaolin used was of powder type and has minimum 40% of particle size less than 2µm and maximum 2% of moisture content. It was used as Si-Al cementitious materials.

A technical grade sodium silicate solution (waterglass) was supplied by South Pacific Chemicals Industries Sdn. Bhd. (SPCI), Malaysia. The chemical compositions comprised of 30.1% SiO2, 9.4% Na2O and 60.5% H2O with modulus SiO2/Na2O of 3.2, specific gravity at 20°C = 1.4mg/cc and viscosity at 20°C = 400cP. The sodium hydroxide pellet used was classification of caustic soda micropearls, 99% purity with brand name of Formosoda-P, made in Taiwan. Distilled water was used throughout.

Table 1: Chemical Composition of Kaolin

Chemical wt (%) SiO2 50.00 - 52.00 Al2O3 33.00 - 35.00 Fe2O3 0.60 - 1.00 TiO2 0.50 - 0.90 CaO <0.05 K2O 1.50 - 2.00 Na2O 0.01 - 0.05 MgO 0.30 - 0.70

2.2. Sample Preparation Sodium hydroxide and distilled water were firstly mixed in a volumetric flask with molar concentration

ranging from 6-14M and cooled up to room temperature. Then, sodium hydroxide with different concentration was mixed with sodium silicate solution to prepare liquid alkali activator 24 hours prior to use. Kaolin powder was mixed well with alkali activator for 5 minutes by using cement mixer. The fresh cement paste was then rapidly poured into 50x50x50 mm steel mould and the samples were compacted approximately one half of the depth (about 1 in. or [25 mm]) of the mould in the entire cube compartments and the mortar was tamped in each cube compartment at each layer as described in ASTM C109 [9]. Finally,

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the samples were put into the oven at temperature 80°C up to 3 days for curing purpose. The samples were sealed with thin plastic layer during the curing stage.

2.3. Compressive Tests Compressive strength tests of all specimens were evaluated according to the ASTM C 109/C 109M – 08

[9] by using the Instron machine series 5569 Mechanical Tester. A minimum of 3 specimens of different concentration were taken out from oven at day 1, day 2 and day 3 and were examined by compression test to evaluate the early strength gain for the specimens.

2.4. Scanning Electron microscope JSM-6460LA model Scanning Electron Microscope (JEOL) was performed to reveal the microstructure

of kaolin cement and to observe the different degree of reaction at different concentration of NaOH. The specimens were cut into small piece and coated by using Auto Fine Coater, model JEOL JFC 1600 before the examination.

2.5. X-ray Diffraction Samples were prepared in powder form and undergone XRD examination. XRD – 6000, Shimadzu x-ray

diffractometer equipped with auto-search / match software as standard to aid qualitative analysis was used to make a diffraction pattern of the crystalline solid.

2.6. FTIR Spectroscopy Small amount of potassium bromide (KBr) and geopolymer powder were put into a mould. By using

cold press machine, mould which contains powder and KBr was pressed at 4 ton for 2 minutes to produce specimens for examination. Perkin Elmer FTIR Spectrum RX1 Spectrometer was used to evaluate the functional group of the sample.

3. Results and Discussions

3.1. Compressive Strength Compressive strength measurements are used as a tool to assess the success of geopolymerisation. This

is due to the low cost and simplicity of compressive strength testing, as well as due to the fact that strength development is a primary measure of the utility of materials used in different applications of the construction industry [2]. In geopolymerization process, strong alkali are required to activate the Si and Al in kaolin, allowing the structure to totally or partially dissolve and transform into a very compacted composite. The concentration of NaOH solutions has significant effects on the mechanical strength of kaolin-based geopolymers [10]. Geopolymers synthesized from kaolinite mixtures show differences in mechanical strength when different concentration of NaOH are applied [2]. Fig. 1 shows the development of compressive strength of geopolymer from NaOH of different concentration (6, 8, 10, 12 and 14M) at age of 1, 2 and 3 days curing in oven at temperature 80°C.

0

1

2

3

4

5

6

7

6 M 8 M 1 0 M 1 2 M 1 4 M

Compressive Strength(Mpa)

Na O H Co ncentra tio n

Da y 1

Da y 2

Da y 3

Fig. 1: Compressive strength of geopolymer using NaOH of different concentration (6 - 14M) over time (1, 2 and 3

days)

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From the result, initially at day-1 and day-2 of curing, they are no constant strength gain between different concentrations of NaOH solution. Compressive strength is maximized at NaOH concentration of 12M for 3-day curing implied that there is an optimum alkalinity for activating kaolin. The strength increases with an increase in Na concentration in the activating solutions [11] and as the concentration of NaOH solution increases, the activation of kaolin become quicker and stronger [12]. Solubility of aluminosilicate increases with increasing NaOH concentration [10, 13]. However, the compressive strength decreases with an increase in NaOH concentration from 12M to 14M. This is probably due to excess of Na+ ions [13]. On the other hand, 6M of NaOH solution shows the highest strength gain among other concentration on day-2 of curing. This is most probably because 6M has excess water due to lower concentration and ease of ions transportation during the process of geopolymerization; thus, when it reaches on 3-day curing, it shows slightly highest strength just below 12M and 14M.

All the geopolymer samples set and form hard structure within 24 hours after curing in oven. Curing time plays an important roles in both the acceleration of chemical reaction and the determination of extent of reaction [14]. Curing time generally shows positive effect on the performance of geopolymers [15]. Prolonged curing time of geopolymer mixture improves the geopolymerization process; yielding higher compressive strength [13]. From results, geopolymers show compressive strength in an increasing trend after 1, 2, and 3 days of curing. However, due to low reactivity of kaolin, a weak structure is formed, thus contributes to slow compressive strength development of geopolymers [3, 4].

3.2. Microstructure of Geopolymer Scanning Electron Microscope (SEM) depicts morphological features of geopolymers of different degree

of reaction at different concentrations of activation medium. Fig. 2 shows the morphological features of pure kaolin and changes in morphologies of geopolymer paste for different concentrations at day-3 of curing. Difference in microstructures could be distinguished at different concentration. It can be clearly seen that the morphology of pure kaolin crystals is plate-like as in Fig. 2(a) [16].

Fig. 2: SEM micrograph of kaolin (a) and geopolymer synthesized using different NaOH concentration of 6M (b), 8M

(c), 10M (d), 12M (e) and 14M (f) at day 3 of curing

Kaolin has been activated by the different NaOH concentrations of alkali activator solution. Sponge-like gel formed, indicating that the structure experiences “growth”. This shows that geopolymerization reaction has taken place. As shown in SEM micrograph, sample with 12M of NaOH solution appeared to have more sponge-like amorphous gel than other samples, which contributes to highest compressive strength at day-3, that is 5.752MPa. This shows that the alkaline activation is more effective. Such observation agrees well with the compressive strength measured, which achieves the highest strength among others. The higher the

(c) (b) (a)

(f) (e) (d)

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degree of reaction, the higher the compressive strength [15]. However, large part of unreacted kaolin can still be observed in all samples, which can also be observed through XRD (Fig. 3) and FTIR (Fig. 4) analysis. Strength will be increase if the unreacted part reacted to form a more dense structure.

When 8M of alkaline solution is used, the degree of reaction is the lowest. Surface shows slight activation of particles with few partially reacted particles and large amount of unreacted particles. Degree of reaction for 6M of NaOH solution is slightly higher strength compare to 8M of NaOH solution. This is because 6M of NaOH solution has higher water content. The water eases the geopolymerization process, leading to higher transportation of ion and hence, the micrograph shows denser structure. Conversely, when 14M of NaOH solution is employed, sponge-like amorphous gel is slightly lesser. This might probably because of the excess of Na+ ion as stated above. This observation is compromised with the compressive strength measured.

3.3. X-Ray Diffraction (XRD) XRD pattern in Fig. 3 shows that kaolin contains kaolinite (K) as major minerals and some dickite (D)

and quartz (Q). Illite (I) can be found in trace amount. XRD pattern of geopolymer samples shows that large part of unreacted materials remains in the system. A number of characteristic kaolinite peaks can be seen in spectra of geopolymer samples. The characteristic kaolinite peaks are at 2θ values of 12.3°, 19.8°, 24.9°, 45.4°, 55.1° and 62.2° [17]. Small intensities in XRD pattern of geopolymer products between 18° and 25° indicate that the geopolymer products have amorphous structure. According to previous research [18], the higher concentration of NaOH solution, the higher the amorphous content of the reaction products. However,it can be observed that NaOH solution of 12M shows higher amorphous content of geopolymer products than NaOH solution of 14M, which comply with its higher compressive strength than that of 14M of NaOH solution.

Fig. 3: XRD pattern of kaolin and geopolymer products

3.4. Fourier Transform Infrared Spectroscopy (FTIR) Fig. 4 shows the IR spectra of kaolin and geopolymer products synthesized using sodium silicate

solution and different NaOH concentration (6M – 14M). Transformation taken place during the synthesis is indicated by the different absorption frequencies of kaolin and the synthesized geopolymers [17]. This is also showed in XRD pattern (Fig. 3).

Fig. 4: FT-IR spectra of kaolin and geopolymer products at day-3

14M of NaOH solution

12M of NaOH solution

10M of NaOH solution

8M of NaOH solution

6M of NaOH solution

Kaolin

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According to Davidovits [19], network of silico-aluminate based geopolymers consists of SiO4 and AlO4 tetrahedral linked alternately by sharing all the oxygens. In IR spectrum of kaolin, the peak around 1113 cm-1 is attributed to Si-O vibration in SiO4 molecules, which vanished after geopolymerisation reaction. Also, a weak band of Si-O symmetrically stretching vibration is observed at 640 cm-1. Absorption at 995 cm-1 and 790 cm-1 are assigned as Al (IV)-OH (6 fold coordinated) and Al (IV)-O (6 fold coordinated), respectively. A shift of the asymmetric bending of the bonds O-Si-O and O-Al-O to lower frequencies can be observed, which is accordance to previous research [20].

The main band analyzed in IR spectrum of geopolymer is in the region of 900-1300 cm-1, corresponding to the Si-O-T linkages. Other major bands are broad band at 3000 – 3500 cm-1 and 1650 – 1655 cm-1 which are the stretching and deformation vibration of OH and H-O-H groups from water molecules. Bands at around 1400 cm-1 are assigned to the Si-O-Si stretching. Bands at around 700 cm-1 and 660 cm-1 show the characteristic of amorphous polymer formed, which is the Si-O-Si and Si-O-Al symmetric stretching. The peaks 537 cm-1 originate from Si-O-Al bonds, where Al is present in octahedral coordinate [17]. These wavenumber shifted from kaolin suggesting that there are changes in chemical bonding taken place in the system. Only little difference between IR spectrum of kaolin and geopolymer synthesized within 500 -1000 cm-1, suggesting that most part of unreacted kaolin still retain in the geopolymer synthesized [18].

4. Conclusion The strength gain for kaolin based geopolymer materials at day-1 and day-2 of curing exhibits different

rate of strength development. According to early strength study based on day-3 curing, when NaOH concentration increases from 8M to 12M, the strength increases but drops at 14M of NaOH solution due to higher concentration. SEM micrograph reveals that kaolin has been activated by the different NaOH concentration of alkali activator solution, showing that 12M of NaOH solution activate the system the most. XRD pattern indicates 12M of NaOH solution presents higher amorphous content of geopolymer products, which comply with compressive strength measured. FTIR spectra of kaolin and geopolymer products synthesized show wavenumber shifted suggesting that there are changes in chemical bonding taken place in the system and most part of unreacted kaolin still retain in the geopolymer at day-3 curing. The experiment has proved that it is possible to produce geopolymer-based green construction materials through alkali-activation of kaolinitic minerals exits in kaolin. The NaOH concentration has significant effect on the compressive strength of geopolymer samples.

5. Acknowledgements This work is supported by Green Concrete@UniMAP. Also, the authors of the present work wish to

thanks to the KACST for funding this study through collaboration between KACST – UniMAP.

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