Strength and Microstructural Analysis of Geopolymer Prepared with Recycled Geopolymer Powder

WANG Xinjie, YANG Wei, LIU Hui, ZHU Pinghua＊, ZONG Ningwen, FENG Jincai

(Department of Civil Engineering, Changzhou University, Changzhou, 213164, China)

Abstract: The strength and microstructural analysis of recycled geopolymer are presented in this paper. Five kinds of geopolymers containing 0%, 20%, 50%, 80% and 100 % of recycled geopolymer powder were prepared using metakaolin as the source material. The alkali activator solution was a mixture of sodium silicate (Na2SiO3) and 12 M sodium hydroxide (NaOH). The change laws of compressive and flexural strength of recycled geopolymer specimens were investigated. And the microscopic characteristics were carried out by SEM, XRD and FTIR to observe the internal morphology and analyze changes in components of recycled geopolymers at different substitution rates. The results show that, with the increase of substitution rate of recycled geopolymer powder, the mechanical properties of recycled geopolymers degenerate and the looser structure are formed. When the substitution rate is less than 50%, the recycled geopolymer specimen meets the use requirements of heavy traffic load class. And the specimen with 80% of substitution rate satisfies the requirements of plastering mortar.

Key words: recycled geopolymer powder; geopolymer; substitution rates; mechanical properties;microstructure

1 Introduction

Conventional cement sector acts as the main contributors to climate change globally, emitting 2.2 Gt carbon dioxide (CO2) annually[1-3], which accounts for 5%-7% of global carbon dioxide emissions[4-7]. In order to reduce CO2emissions and protect environment,the green and low-carbon geopolymers have emerged as a cement substitute. Compared with same amount of cement, the CO2produced by geopolymer reduces by nearly 60% throughout the whole production and transportation process[8]. Furthermore, geopolymer forms an amorphous three-dimensional network during the polycondensation reaction[9-11], which endows itself good mechanical properties, chemical properties and fire resistance. Connieet al[12]found that the strength of geopolymer gel after consolidation reached 70 MPa or even 90 MPa in the case of suitable liquid-to-binder ratio, curing temperature and time. Therefore, geopolymer is gradually being promoted and used due to their excellent performance and environmental friendliness.

It is necessary to consider its sustainable utilization, because geopolymer has a wide prospect of engineering application. Many scholars have devoted themselves to the researches on recycling of geopolymer[13-15]. Gharzouniet al[16]reported that 20% of crushed geopolymer affected the mechanical properties of recycled geopolymer. It was possible to reuse 20%of geopolymer wastes to synthesize the new geopolymer materials. But different substitution rate of wastes has distinct effects on the performance of products. For metakaolin which was incrementally replaced (33.3%and 50%) by red ceramic waste in geopolymer formulation, the particle packing was improved and compressive strength exhibited the maximum values[17]. However, systematic research on recycling of geopolymer powder to prepare new geopolymer is still lacking.

Considering that the synthesized geopolymers was a pure model system without crystalline and vitreous phases[18], metakaolin was chosen as raw materials to produce geopolymers in this study. The recycled geopolymer powder was prepared in the laboratory due to the difficulty in obtaining geopolymer wastes.The raw metakaolin was replaced by 0%, 20%, 50%,80%, 100% of recycled geopolymer powder, respectively, to synthesize new geopolymer materials known as recycled geopolymer. The compressive and flexural strength of recycled geopolymer were tested and the morphology and composition changes of recycled geopolymer were characterized by SEM, XRD and FT-IR.

2 Experimental

2.1 Raw materials and sample preparation

The metakaolin was prepared by calcinating kaolinite from Gongyi, Henan Province at 800 ℃ for 5 h,and its chemical composition was measured by X-ray fluorescence (XRF) as shown in Table 1.

Table 1 Chemical composition of metakaolin

Table 2 Mix proportions of recycled geopolymer/(kg/m3)

The metakaolin and alkali activator solution were mixed according to the mixing proportion given in Table 2. The alkali activator solution consists of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH 12 M,ACS reagent grade). The preparation process of geopolymer was shown in Fig.1. Firstly, NaOH solution and metakaolin was mixed and stirred for 1 min. Then,Na2SiO3solution was added and stirred for 4.5 min.After that, the geopolymers were poured into the molds of 50 mm×50 mm×50 mm and 40 mm×40 mm×160 mm respectively, and vibrated for 1 min to remove the air bubbles. The specimens were cured at 60 ℃ for 6 h and then preserved at standard curing room (20 ± 2 ℃,RH ≥ 95%) for 7 days. The pristine geopolymer specimens was crushed and ground into powders (sieved at 75 μm) to prepare the recycled geopolymer powder.The geopolymer powder of different mass substitution rates (20% 50%, 80%, 100%) was mixed with metakaolin and alkali activator solution to produce the recycled geopolymer specimens. At least three specimens were prepared for each mechanical property test.

2.2 Measurement

2.2.1 Mechanical properties

Compressive strength and flexural strength of recycled geopolymer specimens with different substitution rates were tested by electro-hydraulic servo universal testing machine (YNS 300) according to Chinese Standard GB/T 50081-2019[19].

2.2.2 Microscopic analysis

The morphologies of recycled geopolymer with different substitution rates were observed using scanning electron microscope (SEM, SUPRA55, Zeiss) at the accelerating voltage of 20 kV.

X-ray diffractometer (XRD, D/MAX2500, Rigaku) was used to analyze the component and crystalline phase variations of recycled geopolymer. The parameters were set as a voltage of 40 kV, a current of 30 mA and Cu Kα radiation (k= 0.154 18 nm).

Fig.1 Preparation process of geopolymer

Fourier transform infrared spectroscopy (FT-IR)was adopted to characterize the phase compositions of recycled geopolymer, which was performed on A Bio-Rad FTS 6000 FT-IR analyzer by using KBr pellet techniques. The resolution and scanning times were 2.0 cm-1and 16, respectively.

3 Results and discussion

3.1 Mechanical properties

The content of recycled geopolymer powder affecting the mechanical properties of recycled geopolymer was explored by compressive and flexural strength analysis. As can be seen from Fig.2, the compressive strength of geopolymer (G0) without recycled geopolymer powder reaches about 80 MPa, revealing the excellent mechanical properties of geopolymer. The compressive strength ofG20decreases by 7% than that ofG0specimen. When the content of recycled geopolymer powder is 50 wt%, the compressive strength ofG50still reaches 65 MPa, meeting the requirements for medium or heavy traffic load class in a road engineering[20]. With the increase of substitution rate, the compressive strength of recycled geopolymer exhibits a sharp downward trend. At substitution rate of 80%, the compressive strength drops to 23 MPa, constraining the application of recycled geopolymer in pavement engineering, but it is as strong as M20 plaster mortar. When 100% metakaolin is replaced by recycled geopolymer powder, the compressive strength ofG100is only about 1 MPa, which fails to meet the strength requirements for pavement engineering or even plastering engineering. The compressive strength of recycled geopolymer decreases exponentially with the increase of recycled geopolymer powder contents as shown in Fig.2(b), presenting a good correlation.

Fig.2 Compressive strength of geopolymer at different substitution rates (a) and the fit curve of compressive strength (b)

Fig.3 Flexural strength of geopolymer at different substitution rates (a) and the fit curve of flexural strength (b)

Similar to the compressive strength, the flexural strength of recycled geopolymer decreases with the increase of recycled powder contents. Fig.3(a) indicates the flexural strength of geopolymers with different recycled geopolymer powder contents. The flexural strength of G0is 8.3 MPa. When the metakaolin is replaced by 20 wt% of recycled geopolymer powder, the flexural strength of G20is 7.6 MPa. As the substitution rate increases to 50%, the flexural strength decreases by 35%, which still meets the strength requirement of road for heavy traffic load (≥5.0 MPa)[20]. But when the substitution rate is 80%, the flexural strength is only 1.6 MPa, failing to meet the strength requirements for pavement engineering. When the substitution rate is 100%, the flexural strength is failed due to the loose structure of recycled geopolymer specimens. Fig.3(b)shows the relationship between flexural strength and substitution rate, indicating a good correlation.

The declined trend in mechanical properties of recycled metakaolin geopolymer is consistent with the report of recycled cement concrete[21]. With the increase of substitution rate, the mechanical properties of geopolymer degenerate correspondingly. When the substitution rate is more than 50%, the flexural strength degenerates seriously. This is mainly due to the fact that,the active ingredients in geopolymer reduce with the increase of substitution rate, leading to a greatly reduce in polymerization degree of recycled geopolymer[16,22].Therefore, to ensure the mechanical properties of recycled geopolymer meeting pavement engineering requirements, it is suggested that the substitution rate of recycled geopolymer powder should not exceed 50%.For the geopolymer prepared with 80% of recycled geopolymer powder, it is proposed to be used as the plastering mortar[23].

3.2 SEM analysis

Fig.4 shows the micro-morphologies of recycled geopolymers at different substitution rates. As shown in Fig.4(a), the internal structure ofG0is compact,suggesting that geopolymerization occurs completely.But because the test process does not stir the geopolymer thoroughly, some metakaolin fail to participate in the geopolymerization. When 20% of metakaolin is replaced by recycled geopolymer powder, the morphology ofG20shows almost no difference from theG0’s.Only a small amount of metakaolin powder circled with white does not participate in the geopolymerization[24],which is due to the presence of recycled geopolymer powder. The incorporation of recycled geopolymer powder may hinder the polycondensation rate[16]and reduce the reaction degree, resulting in more cracks.These cracks extend through the unreacted powder(Fig.4(b)), which can be inferred that the bond strength between unreacted powder and new geopolymer material is lower. Hence, the mechanical properties ofG20are slightly lower than that ofG0. With the increase of substitution rate, much inactive recycled geopolymer powder incorporates in the recycled geopolymer thus reducing the compressive and flexural strength. Fig.4(c)-4(e)show that the internal structure of recycled geopolymer blocks gradually turns to be unconsolidated when the recycled geopolymer powder contents increase. At the substitution rate of 50%, microstructure of geopolymer is still visually dense. Once the substitution rate exceeds 50%, the structure of recycled geopolymer becomes looser and a large amount of particulate matter with poor activity produces. When 100% metakaolin is replaced, the recycled geopolymer powder appears agglomeration, implying its poor activity. Thus, it can be inferred that the recycled geopolymer powder with poor activity is the main reason for the decrease in compressive and flexural strength of recycled geopolymer.

Fig.4 SEM of recycled geopolymer at different substitution rates: (a) 0%; (b) 20%; (c) 50%; (d) 80%; (e) 100%

3.3 XRD analysis

The XRD patterns of recycled geopolymers are revealed in Fig.5. As shown in the left figure, the geopolymer is dominated by an amorphous structure. After the alkali-activated reaction, some peaks changed,indicating that some crystals partially dissolved in the reaction[25]. The broad diffuse humps of metakaolin and geopolymer are at around 2θ=20° and 2θ=25°-35°,respectively. This is due to the formation of amorphous aluminosilicate which is the primary binder phase in geopolymer matrix, thus endowing the geopolymer excellent mechanical properties[26,27]. The right figure is a detailed view of the block diagram in the left figure.When metakaolin reacts with alkali activator to form geopolymer, the SiO2content is reduced, indicating that the SiO2participated in the reaction to form the aluminosilicate[6,28]. And it is found that the phase compositions of G20is alike to the G0ones, which is another reason for their similar mechanical properties. But as the substitution rate increases, the content of SiO2increases correspondingly, suggesting that the geopolymerization in recycled geopolymer weakens.

In addition, when the substitution rate is 50% or more, the crystalline peak intensity of calcite gradually decreases and the diffraction peak of calcium silicate crystals at around 33° appears[29]. The calcite is dissolved to form the calcium silicate crystals due to the presence of sodium silicate[30]. This indicates that geopolymerization degree of recycled geopolymer reduces resulting in less geopolymer gel formed and more alkali-activator remained. The low content of geopolymer gel causes loose structure of recycled geopolymer and deterioration of mechanical properties. Therefore, it is concluded that the decrease of polymerization degree and reduction of geopolymer gel are the important reasons for the degradation of mechanical properties of recycled geopolymer.

Fig.5 XRD patterns of metakaolin (a) and the recycled geopolymer at different substitution rates: (b) 0%; (c) 20%; (d) 50%; (e) 80%; (f)100% (A: anatase; Q: quartz; R: rutile; C: calcite; C2S: calcium silicate)

3.4 FT-IR analysis

Fig.6 shows the FT-IR spectrum of metakaolin geopolymer prepared with different substitution rate of recycled geopolymer powder. The characteristic bands at 3 450 cm-1and 1 650 cm-1are attributed to the O-H asymmetric stretching vibration and the H-O-H bending vibrations in bound water[31]. It illustrates that the free water is turned into bound water by the hydration during the process of geopolymerization[32]. The strong wide bands at around 1 010 cm-1is an asymmetrical stretching vibration peak for Si-O bond of SiO4tetrahedra in the aluminosilicates[33]. When the recycled geopolymer powder contents in geopolymer increase, the vibration of Si-O weakens suggesting the decrease of geopolymer gel contents. In addition, the peak at 710 cm-1belongs to the bending vibration of Al(IV)-O-Si bond of tetra-coordinated Al(IV) in the ring structure.Sitarz[34,35]stated that the conversion from hexa-coordinated Al(VI) to tetra-coordinated Al(IV) marked the formation of an aluminosilicate network. As the substitution rate increase, the transmittance of such peak decreases, indicating that hexa-coordinated Al(VI) is not completely converted to tetra-coordinated Al(IV),thereby obtaining an incomplete geopolymerization.

Fig.6 FT-IR spectra of geopolymer at different substitution rates:(a) 0%; (b) 20%; (c) 50%; (d) 80%; (e) 100%

The bands detected at 1 400 cm-1are attributed to the asymmetric stretching vibrations of Al-O/Si-O bonds[36]and tensile vibration of O-C-O bond due to the carbonation reaction[37]. This is because the geopolymers produced by NaOH activator readily absorb CO2from the atmosphere to form carbonates[32,38].With the increase of substitution rate, the geopolymerization is hindered, resulting in an excessive amount of alkali, which is more favorable for the production of carbonates. This causes loose structure of recycled geopolymer, thus reducing the mechanical properties of recycled geopolymer[39]. Therefore, incomplete geopolymerization and loose internal structure are the critical factors to affect the mechanical properties of recycled geopolymer.

4 Conclusions

a) The compressive and flexural strength of recycled geopolymer exponentially decreases with the increase of recycled geopolymer powder contents. The substitution rate of 50% is a turning point of mechanical properties, where compressive and flexural strength reaches 65 MPa and 5.5 MPa respectively. But when the substitution rate attains 80%, the compressive and flexural strength of recycled geopolymer drops to 23 MPa and 1.6 MPa respectively.

b) The degradation of mechanical properties of recycled geopolymer is mainly attributed to the incorporation of low-active recycled geopolymer powder leading to incomplete geopolymerization and loose structure according to the microscopic analysis.

c) The recycled geopolymer prepared with less than or equal to 50% of recycled geopolymer powder is proposed to be used for the road engineering under a heavy traffic load. While recycled geopolymer at 80%of substitution rate can meet the requirements of plastering mortar used in plastering engineering.