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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 66, 2018 

A publication of 

 

The Italian Association 
of Chemical Engineering 

Online at www.aidic.it/cet 

Guest Editors: Songying Zhao, Yougang Sun, Ye Zhou 
Copyright © 2018, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-63-1; ISSN 2283-9216 

Study on Structure and Properties of Kaolin Composites-

based Geopolymers 

Jie Wen, Ying Zhou, Xuchu Ye* 

College of Materials Science and Engineering, Nanjing Tech, University, Nanjing 210009, China 

njyexuchu@163.com 

In order to study the pore structure and corrosion resistance of kaolin composite-based geopolymers, in this 

paper, kaolin composite-based geopolymers were prepared by replacing fly ash with 15%, 25%, 35%, 50%, and 

60% kaolin. Three different low-temperature curing methods were used to study the effect of kaolin ratio on the 

pore structure and corrosion resistance of geopolymers. The results show that with the increase of kaolin amount 

added, the corrosion resistance of the composites-based geopolymers increases first and then decreases, and 

the composite-based geopolymers with 35%kaolin content has the smallest total porosity and the pores are 

mostly harmless pores; the different low-temperature curing methods had little effect on the total porosity of 

kaolin composite-based geopolymers, and the corrosion resistance performance was not much different. The 

anti-corrosion performance of the 1d low-temperature standard-curing and 60°C curing method was better. 

1. Introduction 

Geopolymer is an inorganic compound material with pozzolanic activity, and it is widely used as construction 

materials, sealing materials, heat-resisting materials and high-strength materials (Majidi, 2013) for its excellent 

compressive strength, freeze-thaw resistance, and corrosion resistance; its preparation process is relatively 

simple, and it also can make use of industrial waste so as to reduce the burden on the environment. Therefore, 

the study of geopolymers has important significance for reducing environmental pollution, improving the 

performance of building materials and expanding its application range. 

Metakaolin, obtained by firing kaolin (Morgenstern, 1967; Kakali, 2001) at a specific temperature, is an active 

silica-alumina mineral that can be used to improve the mechanical properties of composite-based geopolymers 

(Wild, 1996). Incorporation of kaolin into concrete (Sabir, 2001) or silicate materials can improve its durability. 

There are a large number of domestic and foreign researches on kaolin composite geopolymer reaction 

mechanism (Poon, 2001) and related studies on its anti-permeability and corrosion resistance (Kong, 2007; 

Duxson, 2007). In this paper, kaolin and fly ash are used as raw materials for composite geopolymers to study 

the influence of the content and the preparation process of kaolin on the structure and corrosion resistance of 

composite geopolymers. 

2. Geopolymers 

2.1 Chemical structures of geopolymers 

The excellent properties of geopolymers are related to its stable structure on the one hand, it is a three-

dimensional network zeolite structure composed of alumina-oxygen tetrahedron and silica–oxygen tetrahedron; 

on the other hand, it may be because it can avoid the expansion caused by the alkali-aggregate reaction.  

2.2 Chemical reaction mechanism of geopolymers 

The reaction of geopolymers is a complex process (Rahier, 2007). For the studies of its reaction mechanism, 

what’s currently widely accepted is the theory of disaggregation and polycondensation proposed by the French 

scholar J. Davidovits. He believes that the process of setting and hardening of composite geopolymers is a 

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1866078 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Wen J., Zhou Y., Ye X., 2018, Study on structure and properties of kaolin composites-based geopolymers, Chemical 
Engineering Transactions, 66, 463-468  DOI:10.3303/CET1866078   

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process in which the aluminum-oxygen bonds and silicon-oxygen bonds in raw materials containing aluminum 

and silicon broke and reassembled under the action of alkaline catalysts. 

2.3 Preparation of kaolin composites-based geopolymers 

2.3.1 Chemical composition of kaolin and fly ash 

The main chemical composition of kaolin powder and fly ash are shown in Table (1) and Table (2). 

Table 1: Main chemical composition (by mass) of metakaolin (wt %) 

Compositions SiO2 Al2O3 GaO SO3 Fe2O3 Na2O 

Content 54.68 41.38 0.05 0.30 0.39 0.38 

Table 2: Main chemical composition (by mass) of fly ash 

Compositions SiO2 Al2O3 GaO SO3 Fe2O3 Na2O 

Content/% 58.50 29.88 3.58 0.36 2.97 0.69 

2.3.2 Chemical preparation process 

 

Figure 1: Preparation process of geopolymers 

Figure (1) shows the preparation process of kaolin composite-based geopolymers, the raw materials are 

prepared according to a certain ratio. 

3. Effect of kaolin ratio on the structure and chemical properties of geopolymers 

3.1 Experiment design 

The composite-based geopolymers were prepared by mixing kaolin, fly ash and ordinary Portland cement. This 

study examined the effect of kaolin ratio on the structure and chemical properties of geopolymers. The design 

scheme is shown in the following table (3). The addition amount of kaolin is 15wt%, 25wt%, 35wt%, 50wt% and 

60wt%, respectively. To ensure that the sample has good fluidity, with the increase in the amount of kaolin 

added, the corresponding water-cement ratio should also increase. 

Table 3: Mix ratio of geopolymers 

No. Metakaolin Percentage (%) Water-cement Ratio (wt%) 

C15 15% 0.45 

C25 25% 0.48 

C35 35% 0.53 

C50 50% 0.57 

C60 60% 0.62 

3.2 Effect of kaolin ratio on the pore structure of geopolymers 

The pore size distribution of geopolymers with different kaolin ratio after 28 days of curing. As can be seen from 

the figure that, the pores in sample C15 are about 20 nm at the most, the pores of C35 are mainly distributed at 

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less than 10 nm, and the pores of C60 are about 100 nm. The experimental results show that with the increase 

of the content of kaolin, the pore size decreases first and then increases. When the content of kaolin is 35wt%, 

the pore size at this time is the smallest and all pores are gel pores. 

Table 4: Porosity of the geopolymer 

No. Total Porosity 

(%) 

Porosity of Pores at all levels (%) 

r>200nm 50nm<r<200nm 20nm<r<50nm r<20nm 

C15 39.14 2.88 0.50 26.08 9.68 

C35 18.38 2.31 0.58 0.45 15.04 

C60 45.95 2.06 37.20 4.08 2.61 

 

Some studies have shown that the size of pores in geopolymers has a direct relationship with the strength of 

the material. The porosity of this experiment is shown in Table (4). It can be seen from the table that the sample 

with the smallest total porosity is C35, which is a composite geopolymer containing 35wt% kaolin, the porosity 

for pores less than 20 nm is about 15%, indicating that a large amount of aluminosilicate gel is formed inside 

the geopolymer, and it has excellent chemical properties; in sample C15, more than 35% of the pores are 

harmless and less harmful, so there is little effect on the chemical properties; for sample C60, about 40% of the 

pores are harmful pores, which may be due to a higher water-cement ratio. 

3.3 Effect of kaolin ratio on microscopic morphology of geopolymers 

According to the experimental results of the pores, the SEM scan results of the samples C15 and C35 are shown 

in the following figure (2). Comparing the cross-section structure diagrams C and A of samples C15 and C35, 

we can see that the cross-section of sample C15 is significantly looser than that of sample C35, and there are 

significant cracks; this result may be due to the cracks caused by moisture evaporation and shrinkage during 

the curing drying process. Comparing figures B and D of the fly ash particles of two samples shows that the 

particles of sample C15 are more complete and the surface is covered by gel; the particles of sample C35 have 

partially broken and deformed, and there are tiny gel particles, more complete dissolution polymerization has 

occurred. 

  

  

Figure 2: SEM diagrams of geopolymers (A: Sample C35X500, B: Sample C35X10K, C: Sample C15X500, D: 

Sample C15X10K) 

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3.4 Effect of kaolin ratio on corrosion resistance of geopolymers 

0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

1.2

C
o

rr
o

s
io

n
 R

e
s
is

ta
n

c
e

 C
o

e
ff
ic

ie
n

t

Time (d)

 C15

 C35

 C60

 

Figure 3: Relationship of corrosion resistance coefficient of samples with metakaolin dosage 

The durability of geopolymers is related to the size and distribution of the pores inside the geopolymers, some 

corrosive media will enter the inside of the geopolymers through the pores and react with the geopolymers so 

as to cause damage. Through studying the corrosion resistance of kaolin with different ratios, we can research 

the relationship between corrosion resistance and the pore structure of geopolymers. Geopolymers with different 

ratios are subjected to standard curing for 28 days and then are immersed in a corrosive medium for corrosion 

resistance testing. The test results are shown in the following figure (3). 

The experimental results find that the corrosion resistance and porosity of kaolin composite-based geopolymers 

are basically the same, and the pores in C15 and C35 are essentially harmless. After 100 days of soaking, the 

corrosion resistance coefficients can maintain above 0.90; however, due to the large pores of C60, corrosive 

substances enter the matrix, therefore, the corrosion resistance coefficient continues to decline, reaching about 

0.75 after about 100 days. 

4. Effect of curing conditions on the structure and chemical properties of composite-based 
geopolymers 

The preparation process has a great influence on the structure and function of geopolymers. In this experiment, 

the following three curing methods in Table (5) are used to study the influence of processing conditions on the 

pore structure, microstructure, and corrosion resistance of geopolymers. 

Table 5: Different curing condition 

No. Method 

1# Standard 

2# Standard 1Day + 60°C Curing Condition 

3# 60°C Curing Condition 

4.1 Effect of curing conditions on the pore structure of geopolymers 

The pore size distribution of kaolin composite-based geopolymers after 28 days of curing under different curing 

conditions is shown in the following figure (4) and the pore size characteristics are shown in Table (6). The 

experimental results show that the pore diameters obtained under the three curing conditions are mainly 

harmless pores with diameter less than 20 nm, and the pores obtained by the curing method 2# are even 

smaller, this is because low temperature conditions are favorable for maintaining the structural order of 

geopolymers, in the process of 60°C curing at later period, there are more reaction products filled into the pores, 

and finally the porosity is greatly reduced. For the curing method 3# which adopted higher temperature in the 

initial stage, the structure of geopolymers is destroyed in the early stage, forming larger pores, which is also the 

direct cause of the increase of pores with diameters from 20 nm to 50 nm in the table. Overall, the effect of the 

change of three curing temperatures on the total porosity is not significant, all remain at about 36%. 

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1 10 100

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

d
V

/d
lo

g
D

 (
m

L
/g
）

Pore Diameter (um)

 1#

 2#

 3#

 

Figure 4: Pore size distribution of samples at different synthesis temperature 

Table 6: Porosity of geopolymers cured 28d at different synthesis temperatures 

No. Total Porosity 

(%) 

Porosity of Pores at all levels (%) 

r>200nm 50nm<r<200nm 20nm<r<50nm r<20nm 

1# 35.39 1.27 0.18 0.32 33.62 

2# 35.46 1.55 0.23 0.26 33.42 

3# 36.86 1.67 0.14 6.40 28.65 

4.2 Effect of curing conditions on microscopic morphology of geopolymers 

 

  

  

   

Figure 5: SEM diagrams of geopolymers prepared at different curing condition (a: 1#-28d, b: 2#-28d, c: 3#-28d, 

A: 1#-7d, B:2#-7d, C:3#-7d) 

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The cross-section topography of the three curing methods is shown in Figure (5) below, in which figures (A), 

(B), (C) and (a), (b), (c) are photos of the samples that are cured for 28d and 7d respectively. By observing the 

photos, we can see that, the air pores obtained under the three curing methods are relatively small and 

distributed uniformly, which is in agreement with the experimental results of porosity. 

From Figure (A), it can be seen that there are still particles that didn’t occur reaction in the sample under standard 

curing conditions, but there are no large pores, the polymerization reaction proceeds at a lower rate, undissolved 

particles can easily crack under the action of external forces, the force between the particles and the gel is 

relatively weak. In figure (B), under the second curing condition, it can be observed that the particles have fully 

dissolved and combined with the gel to form dense tissues, and the binding force between the tissues is also 

strong. Figure (C) shows unevenly distributed punching holes, and the structure of the gel phase is also loose, 

it might because the temperature is too high at early stage, the dissolution reaction occurs too fast and the 

silicon and aluminum in the particles are difficult to precipitate. 

4.3 Effect of curing conditions on corrosion resistance of geopolymers 

The experimental results show that the corrosion resistance of the three curing methods is similar, which is 

consistent with the results of the distribution of the internal pore size and structure of the three curing methods. 

Among them, the performance of the second curing method is better. It’s because when adopting low-

temperature curing in the early stage, the water content is sufficient, the alkali activator can fully contact and 

react with the powder material to produce precursor, and then the curing temperature is turned up to help 

accelerate the diffusion of particles within the geopolymers, so that the dissolution reaction continues and the 

particles inside the structure can form a new structural integrity, therefore it can improve the overall corrosion 

resistance of geopolymers. 

5. Conclusion 

This paper uses kaolin and fly ash as the main raw materials to prepare kaolin composite-based geopolymers, 

the structure and chemical properties of geopolymers were studied by the ratio of kaolin and the preparation 

process, the following conclusions are obtained: 

(1) With the increase of kaolin amount added, the corrosion resistance of composite-based geopolymers 

increases first and then decreases, which is related to the large porosity within the geopolymers, the composite-

based geopolymers with 35% kaolin content have the smallest total porosity, and most of the pores are harmless 

pores. 

(2) Different low-temperature curing methods have little effect on the total porosity of the kaolin composite-based 

geopolymers, and the effect of corrosion resistance performance is similar. The anti-corrosion effect of 1d low-

temperature standard curing and 60°C curing methods is better. 

References 

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Engineering Aspects, 292(1), 8-20, DOI: 10.1016/j.colsurfa.2006.05.044 

Kakali G., Perraki T., Tsivilis S., Badogiannis E., 2001, Thermal Treatment of Kaolin: The Effect of Mineralogy 

on the Pozzolanic Activity, Applied Clay Science, 20(1), 73-80, DOI: 10.1016/S0169-1317(01)00040-0 

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