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Advances in Materials Science and Engineering
Volume 2019, Article ID 3202794, 10 pages
https://doi.org/10.1155/2019/3202794
Research Article

Effects of Water on Pore Structure and Thermal Conductivity of Fly Ash-Based Foam Geopolymers

1School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, China
2Shandong Provincial Key Laboratory of Eco-Environmental Science for Yellow River Delta, Binzhou University, Binzhou 256603, China

Correspondence should be addressed to Dongmin Wang; nc.ude.btmuc@nimgnodgnaw

Received 7 August 2018; Accepted 25 November 2018; Published 3 January 2019

Academic Editor: Michelina Catauro

Copyright © 2019 Yong Cui and Dongmin Wang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The influence of the water-to-solid ratio (W/S) on the viscosity, pore characteristics, bulk density, compressive strength, and thermal conductivity of foamed fly ash-based geopolymers with thermal conductivity less than 0.065 W/(m·K) was investigated, and their properties and cost analysis were also compared with that of foamed ordinary Portland cement (OPC). When the W/S varied from 0.38 to 0.5, the apparent viscosity of geopolymer paste 15 min after the preparation decreased significantly from 168 Pa·s to 6 Pa·s. The increasing W/S ratio contributed to the rise of the number of microcapillaries (φ < 50 nm) and macrocapillaries (50 nm < φ < 50 μm) but contributed to the decline of artificial air pores (φ > 50 μm). The refinement of pore characteristics lowered the 28 d thermal conductivity of foamed geopolymers from 0.06 W/(m·K) to 0.048 W/(m·K). Although the slight increase of total porosity of foamed geopolymers from 89% to 92% with the increase of the W/S ratio weakened their 28 d compressive strength from 0.75 MPa to 0.45 MPa, this strength still meets the Ordinary Portland Cement (OPC) based Foam Insulation Board standard of JC/T2200-2013 (>0.4 MPa for 0.25 g/cm3). The production cost of foamed geopolymers was slightly higher by 1.1–1.5 times than that of foamed OPC. However, considering the more beneficial effect of environmental load reductions and better mechanical and thermal properties of foamed geopolymers than those of foamed OPC, slightly higher cost would be acceptable for practical application.

1. Introduction

Foamed geopolymer is a kind of alkali-activated aluminosilicate-based porous materials and can be manufactured by a chemical or mechanical foaming technology [1]. Foamed geopolymer is well known not only for its relatively low thermal conductivity, usually 10–50% of that normal concrete [2], but for its less energy consumption and less environmental loads compared to the typical ordinary Portland cement (OPC) foamed concrete [3, 4]. Unlike structural materials, foamed geopolymer is widely applied in, but not limited to building engineering including insulation, partition and voids filling [5]. For thermal other than mechanical purpose, the thermal conductivity of the typically foamed geopolymer being used generally ranges from 0.072 W/(m·K) to 0.48 W/(m·K) with its corresponding density and compressive strength ranging from 300 kg/m3 to 1400 kg/m3 and 0.7 MPa to 48 MPa, respectively [611]. However, few publications on geopolymer based foamed materials with thermal conductivity less than 0.065 W/(m·K) are available. With the improvement of energy efficiency standards from 50% to 65% and even to 75% in Beijing and Tianjin, China, the relatively high thermal conductivity (0.07–0.48 W/(m·K)) of these porous inorganic materials, compared with that of porous organic materials such as polyurethane (PU) board (0.026 W/(m·K)) and extruded polystyrene (XPS) (0.029 W/(m·K)) [12], has limited their use as the thermal insulator. The optimization of pore-size distribution is one of most effective paths to lower the thermal conductivity of foamed materials [13].

According to the Knudsen effect [13], the smaller the pore size, the lower the air thermal conductivity in the pores. Generally, the air thermal conductivity is ∼0.002 W/(m·K) within the pore sized <50 nm, ∼0.015 W/(m·K) within the pore sized between 50 nm and 50 μm, and ∼0.026 W/(m·K) within the pore sized >50 μm. Many parameters have been investigated about their effects on the pore structure (porosity, pore-size distribution, and circularity and connectivity of pore) of foamed geopolymer including the type and content of the foaming agent [14, 15], the type and content of composition of binder [16, 17], the type and content of alkali activator and the temperature [18, 19], the route of foaming [20, 21], and the type and concentration of surfactant [22, 23]. However, very rare papers can be obtained about the influence of the water-to-solid ratio on the properties and pore structure of foamed geopolymer. It is well known that water is one of the crucial components in the formation of geopolymers, and it not only plays an important role in the dissolution of aluminosilicate precursors as a medium but also helps the transfer of various ions and polycondensation of Si and Al monomeric and oligomeric species [24]. Moreover, after geopolymers are hardened, water remains in the geopolymers in three different forms, with free water entrapping in the pore, with interstitial water bonding to the developed 3-D geopolymer network, and/or with OH groups possibly relating to silanol and aluminol groups within the structure [25]. After water loss, the pore belonging to the microcapillaries category vanishes, which have a positive effect on thermal resistance of materials. In addition, water has a tremendous influence on the viscosity of geopolymer paste [26]. The viscosity of the initial mixture is a very important parameter for the foaming process, regardless of the foam type, and can influence the foam structure in terms of regularity, porosity, pore distribution, etc. During foaming, a suitable viscosity may increase the stability of the bubble and reduce the irregularity of pores. An unsuitable viscosity may cause a nonuniform distribution of bubbles during the foaming process. Therefore, it is necessary to explore the effects of the mass ratio of water to solid on the properties and pore structure of foamed geopolymers.

In this work, fly ash-based foamed geopolymers with a mass ratio of water to solid from 0.38 to 0.50 were investigated throughout the viscosity of paste, pore structure, compressive strength, bulk density, thermal conductivity, and cost analysis of foamed geopolymers. Understanding the effects of water content and change rules of these properties is helpful for the wide use of this materials.

2. Experimental

2.1. Raw Materials

Fly ash (FA) was supplied from the Pingshuo power station in Shuozhou, China, with true density = 2.17 g/cm3. Table 1 shows the chemical composition of FA as determined by X-ray fluorescence (XRF). The flaky shape of FA is represented in Figure 1. NaOH pellets (96 wt.% of purity) and distilled water were mixed in a water glass (with modulus 2.4, 54.2% of water) to prepare alkali activator (AA) solution (with modulus 1.5, 56.9 wt.% of water) with 20 min stirring and then resting 24 h before use. Hydrogen peroxide (30 wt.% H2O2) as a foam blowing agent and calcium stearate as a foam stabilizer (FS) were employed.

Table 1: Chemical component of fly ash (by mass ratio).
Figure 1: SEM of FA
2.2. Preparation and Synthesis of Specimen

According to the previous experiments by authors [27], the FA-based geopolymers have strong strength at a SiO2/Al2O3 and Na2O/Al2O3 molar ratio of 3.1 and 0.7, respectively. In order to study the influence of the water-to-solid ratio (W/S) on properties and pore structure of foam geopolymers, additional H2O was added to adjust a mass ratio of W/S from 0.38 to 0.50 at a step of 0.03. For clarity, all prepared samples are presented in Table 2.

Table 2: Initial compositional ratio of geopolymers.

The synthesis protocol of geopolymers is illustrated in Figure 2. FA, AA, FS, and additional H2O (if necessary) were mixed using a JJ-5 blender whose rotation speed stands at 140 ± 5 r/min to stir 5 min and get a homogeneous paste. Then, H2O2 was added to the fresh paste and stirred for 30 s to obtain a consistent mixture. The foamed mixture was cast in steel molds of 40  mm × 40  mm × 160 mm slabs for bulk density and strength tests and in 300  mm × 300  mm × 30 mm cuboids for thermal conductivity tests. These molds were sealed with polyethylene film and placed into a standard curing box. All the samples were cured at 60°C for 24 h, demolded, and further cured under ambient conditions (∼65%RH, 23°C).

Figure 2: Synthesis protocol of foamed geopolymers.
2.3. Methods of Analysis
2.3.1. Viscosity Testing

The viscosity of geopolymer paste was measured at ambient temperature using the rheometer (DV3T, USA). The data were recorded at a constant shear rate of 6 s−1 for all the samples.

2.3.2. Strength and Bulk Density

Compressive strength and bulk density tests were operated immediately after curing 1 d, 3 d, and 28 d according to GB/T 5486-2008. All the reported results were the average of three independent measurements.

2.3.3. Thermal Conductivity

Thermal conductivity was recorded by the standard test method for determining the steady-state thermal transmission properties at 25°C using a heat flow meter apparatus (DD300F-D15from Foreda, China) in accordance with GB/T 10294-2008.

2.3.4. Quantitative Analysis of Pore Structure

The total porosity of samples was obtained by comparing the difference of true density and bulk density of foam geopolymers as (1). The porosity and pore-size distribution of foam geopolymer ranging from 5 nm to 360 μm were measured by an autopore IV 9500, 60000 psi mercury intrusion porosimetry (MIP). The pore sized >360 μm was detected using SEM (MLA250, USA) and analyzed by using Image-Pro Plus 6.0 software to investigate its size distribution (Figure 3). So, the total porosity of the foam geopolymer can be expressed as shown in Equation (2):where was the true density of the geopolymer, 2.17 g/cm3 was determined by a 250 mL Li bottle according to the Archimedes method and was the bulk density of the foam geopolymer. means the total porosity of the foam geopolymer; means the porosity of the geopolymer sized <360 μm. means the porosity of the geopolymer sized >360 μm.

Figure 3: Pore-size analysis (>360 μm) of the selected foamed geopolymer by using Image-Pro Plus (W/S = 0.38).

3. Results and Discussion

3.1. Viscosity Analysis of the Fresh Paste

Figure 4 plots viscosity of FA-based geopolymer paste with different W/S ratios. Water has a great effect on viscosity of geopolymer pastes corresponding to workability. When the W/S varied from 0.38 to 0.5, the apparent viscosity of paste 15 min after preparation decreased significantly from 168 Pa·s to 6 Pa·s. which indicates that an increase of water can effectively reduce resistance friction between solid phases. Moreover, due to the formation of amorphous geopolymerization products, an upward trend was observed in the viscosity of FA-based geopolymer pastes with the time increasing to 30 min after preparation. The nonlinear decline of the viscosity showed that there exists a relationship between viscosity and W/S as described by the following equation [28]:where is the viscosity of paste; is the viscosity of the liquid phase (water), 0.9 × 10−3 Pa·s; is the volume fraction of the solid phase; is the maximum volume fraction of the solid phase when viscosity is infinite; 1 was selected for simple calculation; and is a constant standing for intrinsic viscosity. The volume fraction of the solid phase, , in this study could be obtained through the mass ratio of W/S. For further development of workability of geopolymers, a basic equation to predict was proposed based on the regression analysis of test data and is expressed as follows (Figure 5):where the experimental constants of and were determined to be 0.026 and 11.54 for fly ash- (FA-) based geopolymer pastes and 0.016 and 3.20 for metakaolin- (MK-) based geopolymer pastes [29], respectively.

Figure 4: Viscosity of FA-based geopolymer pastes with different W/S.
Figure 5: Relationship between viscosity and W/S of geopolymer paste.
3.2. Pore Structure of Products

Considering the limitations of MIP and the advantages of image analysis, the pore system of the foamed geopolymer can be effectively described by a combination of the two methods. According to Kamseu’s research [16], the pore sized less than 360 was measured by MIP, while the pore sized larger than 360 was recorded by image analysis. Figure 6 represents the pore-size distribution of the FA-based foamed geopolymer. Depending on the pore diameter, φ, the pore system of foamed inorganic materials can be typically classified into microcapillaries (φ < 50 nm), macrocapillaries (50 nm < φ < 50 μm), and artificial air pores (φ > 50 μm) [30]. The microcapillaries are caused by the evaporation of water in the gel pores. The macrocapillaries and artificial air pores are caused by the decomposition of H2O2 and insufficient compaction [4, 31]. In Figure 6, the band of the pores representing microcapillaries increased in intensity and width with W/S increasing, showing a shift of peak of these bands from 10 nm to 25 nm. The porosity of microcapillaries also displayed an upward trend from 0.15% to 0.52% with an addition of water (Table 3). The increase of pore size and porosity of microcapillaries is mainly because a higher W/S will make more free water entrapping in the pore and interstitial water bonding to the developed 3-D geopolymer network filled with geopolymeric gels [25]. After the water lost, the pore appears. Moreover, more water will hinder the polymerization of Si and Al monomer and dimer according to equations (1) and (2) [32], which will further increase the pore volume of microcapillaries. As for the macrocapillaries, a shift of the peak of these bands between 50 nm and 50 μm from 1 μm to 15 μm was observed for FA-based foam geopolymers with varying W/S. A higher W/S mass ratio led to a stronger intensity of these bands, indicating that the porosity of macrocapillaries rose considerably from 11.35% to 41.17%, as described in Table 3 although the number of artificial air pores including pore sized 50 μm–360 μm and >360 μm declined with an increase in W/S, mainly reflecting a reduction of porosity of pores with size >360 μm from 20% to 2.86%; the porosity of these artificial pores is still the largest among the three types of pore, 1.3–7 times more than that of microcapillaries and 10–50 times more than that of macrocapillaries. The reasons for the increase of pore volume of macrocapillaries and a decrease of artificial air pore are related to the viscosity of paste and the concentration of the foaming agent. With increasing W/S, the decline of viscosity and concentration contribute to the dispersion of bubbles, avoiding the accumulation and coalescence of bubbles.

Figure 6: Pore-size distribution with φ < 360 μm (a) and φ > 360 μm (b) of FA-based foam geopolymers with different W/S.
Table 3: Total porosity and porosity with φ < 360 μm and φ > 360 μm and average pore size of FA-based foam geopolymers with different W/S.
3.3. SEM Analysis

The detailed pore structure of FA-based foam geopolymers with different W/S as observed with high-magnification SEM is given in Figure 7. As shown in Figure 7, the geometric separation of a single pore is effective, meaning that image analysis is reasonable and efficient method for such porous materials. Most pores are closed, and change in water content has hardly effect on the pore’s connectivity. However, it will affect its pore-size distribution. When W/S = 0.38–0.41, the higher viscosity of paste resulted in the poor dispersion and of bubbles, and the pore with size >360 μm appeared because of the coalescence of bubbles. When W/S = 0.44–0.50, the porosity of artificial air pore decreased as we can see in Figure 7, which are in agreement with analysis in Table 3.

Figure 7: SEM photographs of FA-based foam geopolymers with different W/S. (a) 0.38. (b) 0.41. (c) 0.44. (d) 0.47. (e) 0.50.
3.4. Compressive Strength of Products

Figure 8 presents the compressive strength and specific strength of the FA-based foam geopolymer at 1 d and 28 d with different W/S. At constant W/S, all compressive strengths of FA-based foam geopolymers rose by 1-2 MPa with 27 d curing time. However, it declined by 40% with W/S increasing from 0.38 to 0.50. It is well known that the compressive strength of foamed geopolymer is supported by the pore wall, which depends on the pore structure (including pore number and pore distribution) and the number and maturity of geopolymerization products. An increase in age contributes to the improvement of number and maturity of geopolymeric gels, justifying the increase of compressive strength. On the contrary, a higher W/S makes the pore volume of foam geopolymer become larger, resulting in the decrease of compressive strength. Although the FA-based foam geopolymer at W/S = 0.5 had the lowest compressive strength at 0.45 MPa for 7 d, this strength still meets the Ordinary Portland Cement (OPC) based Foam Insulation Board standard of JC/T2200-2013 (>0.4 MPa for 0.25 g/cm3). Moreover, the specific strength of FA-based foam geopolymers and the ratio of compressive strength to bulk sensitivity for a material showed little changes for all ages with an increase of W/S, indicating that the relationship between the density and compressive strength of foam geopolymers obeys a positive correlation function.

Figure 8: Compressive strength and specific strength of FA-based foam geopolymer at 1 d and 28 d with different W/S.

The relationship between 28 d bulk density (ρd) and 28 d compressive strength (fd) of foamed geopolymers is represented in Figure 9. The existing data of foamed OPC [33, 34] within ρd < 0.3 g/cm3 are also given for comparison. For dimensional analysis, ρd and fd were normalized with reference values of ρo (=1 g/cm3) and fo (=1 MPa), respectively. At constant values of ρ, fd obtained from the FA-based foamed geopolymers here was typically higher than that obtained from foamed OPC. In addition, the increase rate of fd as a function of ρd was greater in the prior investigation than that in the existing foamed OPC data. This advantage of foamed geopolymer is mainly attributed to the stronger strength of geopolymeric gel (N-A-S-H) than that of calcium silicate hydrates (C-S-H). For the further development of foamed geopolymer, a simple equation to predict fd was proposed based on a regression analysis of the test data. Although the pore distribution and pore shape have a slight effect on fd, only ρd was considered for a simple calculation. A basic equation for fd of the FA-based foamed geopolymer was empirically described as follows (Figure 9):

Figure 9: Regression analysis of compressive strength of foamed geopolymers and foamed OPC.

In equation (5), the value of experimental constants ( and ) was determined by regression analysis to be 21.03 and 2.18, respectively, for the FA-based geopolymer, 22.7 and 3.3 for slag-based foamed geopolymer [4], and 5.74 and 1.63, respectively, for the foamed OPC.

3.5. Thermal Conductivity and Bulk Density of Products

Figure 10 plots the 28 d thermal conductivity (λd) and 28 d bulk density of FA-based foamed geopolymer with different W/S. Due to the considerably lower thermal conductivity (0.026 W/(m·K)) and density (0.00129 g/cm3) of air compared with fly ash, an increase of W/S improved the number of pore and optimized pore-size distribution (Table 3), conducting to a lowering of thermal conductivity and bulk density. The thermal conductivities were about 0.06 W/(m·K) for ρd = 0.24 g/cm3 and 0.048 W/(m·K) for ρd = 0.175 g/cm3, respectively, which are around one-tenth of that for the normal-weight geopolymer [11]. Moreover, the values of λd of the current FA-based foamed geopolymer were slightly smaller than those of foamed OPC [33, 34], as given in Figure 11. This may be because the geopolymeric gel (N-A-S-H) possesses a smaller content of chemical bonding water than that of calcium silicate hydrates (C-S-H) [35]. Based on the current experimental data, the thermal conductivity (λd) of the FA-based foamed geopolymer can be typically described as follows:where (=1 W/(m·K)) is the reference value for thermal conductivity. Regression analysis was performed to determine the values of and to be 0.135 and 0.579, respectively, for FA-based foamed geopolymer, 0.26 and 1, respectively, for slag-based foamed geopolymer [4], and 0.129 and 0.522, respectively, for OPC-based foamed concrete.

Figure 10: Thermal conductivity and bulk density of FA-based foamed geopolymers at 28 d with different W/S.
Figure 11: Regression analysis of thermal conductivity of foamed geopolymer and foamed OPC.
3.6. The Relationship between Pore Structure and Thermal Conductivity

The total porosity and experimental thermal conductivity of foamed geopolymers with different W/S are displayed in Table 4. The total porosity of foamed geopolymers only grew slightly by 3% when the W/S ratio increased from 0.38 to 0.5. However, their experimental thermal conductivity was lowered obviously by 18.4%. The phenomenon can be explained by the variation of pore size as shown in Table 3. According to the Knudsen effect [13], the thermal conductivity of air is ∼0.026 W/(m·K) with the pore size larger than 50 μm, ∼0.015 W/(m·K) with pore size between 50 nm and 50 μm, and ∼0.002 W/(m·K) with pore size less than 50 nm. This means that, for the same total porosity, the thermal conductivity of foamed geopolymers with the more micro- and macrocapillaries is smaller than that with more artificial air pores. In order to get a better understanding the effects of variation of pore structure on the thermal conductivity of foamed geopolymers, the foamed geopolymer with the W/S = 0.38 is assumed as a continuous phase and the variation of total porosity induced by the increase of the W/S is assumed as a dispersed phase. Then, these two phases were introduced into Maxwell–Eucken 1 model as shown in equations (7) and (8). As shown in Figure 12, the calculated thermal conductivity of foamed geopolymer declined unnoticeably based on the assumption of considering the variation of total porosity only. However, the experimental thermal conductivity showed a significant downward, which can be justified by the refinement of pore-size distribution. At the same total porosity, the more the number of pore size < 50 μm, the smaller the thermal conductivity of the foamed geopolymer:where is the calculated thermal conductivity of foamed geopolymers with W/S = i and i = 0.41, 0.44, 0.47, or 0.50. is the thermal conductivity of foamed geopolymers with the W/S = 0.38 and 0.0594 W/(m·K). is the thermal conductivity of air, 0.026 W/(m·K). is the variation of total porosity of foamed geopolymers. is the total porosity of foamed geopolymers with W/S = i. is the total porosity of foamed geopolymer with W/S = 0.38.

Table 4: The total porosity and experimental and calculated thermal conductivity of foamed geopolymers.
Figure 12: Experimental and calculated thermal conductivity of foamed geopolymers with different W/S.
3.7. Economic Analysis Compared to OPC-Based Foamed Concrete

The economic analysis of foamed geopolymer compared to typical foamed OPC is displayed in Figure 13. Based on the case investigation [4], foamed OPC was typically assumed to have a water/binder (W/B) ratio of 50% and designed foamed volume ratio of 65%. The commercial unit cost (USD/ton) of each constituent materials provided in China Price Information (2017) is also listed in Table 5. The steaming temperature could be supplied by the waste steam of the power station. For simple calculation, it is assumed that the stream price for 1 m3 foamed geopolymer production is 0.15 USD. Due to the use of H2O2 and calcite stearate as a foam stabilizer both in foamed geopolymer and foamed OPC, the price of H2O2 and calcite stearate was not considered. As seen in Figure 13, the production cost of foamed geopolymer depending on the type and content of alkali activators used rose rapidly with an increase in unit binder content. The production cost of the foamed geopolymer was slightly higher by 1.1–1.5 times than that of foamed OPC. It is worthwhile to note that the less the unit binder content used, the smaller the difference of production cost between foamed geopolymer and OPC. However, considering the beneficial effect of environmental load reductions including low CO2 emission, small water pollution, and land occupancy, the slight raise in cost would be acceptable for practical application.

Figure 13: Production cost of foamed geopolymers compared to typical foamed OPC (W/B = 0.5).
Table 5: Commercial unit cost of each constituent material (China Price Information).

4. Conclusions

From this study, we can make the following conclusions:(1)When the W/S varied from 0.38 to 0.5, the apparent viscosity of paste 15 min after preparation decreased significantly from 168 Pa·s to 6 Pa·s.(2)The increasing W/S ratio contributed to the rise of the number of microcapillaries (φ < 50 nm) and macrocapillaries (50 nm < φ < 50 μm) but to the decline of artificial air pores (φ > 50 μm). The refinement of pore characteristics lowered the 28 d thermal conductivity of foamed geopolymers from 0.06 W/(m·K) to 0.048 W/(m·K).(3)Although the slight increase of total porosity of foamed geopolymers with the increase of the W/S ratio weakened their 28 d compressive strength from 0.75 MPa to 0.45 MPa, this strength still meets the Ordinary Portland Cement (OPC) based Foam Insulation Board standard of JC/T2200-2013 (>0.4 MPa for 0.25 g/cm3).(4)The production cost of foamed geopolymers was slightly higher by 1.1–1.5 times than that of foamed OPC. However, considering the more beneficial effect of environmental load reductions and better mechanical thermal properties of foamed geopolymers than that of foamed OPC, this slightly higher cost would be acceptable for practical application.

Data Availability

The data used to support the findings of this study are included within the article.

Additional Points

Highlights. Foamed geopolymers with thermal conductivity <0.065 W/(m·K) and bulk density < 300 kg/m3 were investigated with different W/S. Foamed geopolymers have better thermal and mechanical properties than foamed OPC. For the sake of less environmental load and better performance, the slightly higher price of foamed geopolymers is acceptable compared with that of foamed OPC.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was financially supported by the National Key Research and Development Program of China (2017YFC0505904 and SQ2017YFSF050163). The authors would like to thank Dr. Sun Rui for polishing the language of this manuscript.

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