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

*e 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 0.065W/(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 15min after the preparation decreased significantly from 168 Pa·s to 6 Pa·s. *e 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). *e refinement of pore characteristics lowered the 28 d thermal conductivity of foamed geopolymers from 0.06W/(m·K) to 0.048W/(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 0.75MPa to 0.45MPa, this strength still meets the Ordinary Portland Cement (OPC) based Foam Insulation Board standard of JC/T2200-2013 (>0.4MPa for 0.25 g/cm). *e 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.


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/m 3 to 1400 kg/m 3 and 0.7 MPa to 48 MPa, respectively [6][7][8][9][10][11].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.e 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 ∼0.002W/(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].e 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.erefore, 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.

Experimental
2.1.Raw Materials.Fly ash (FA) was supplied from the Pingshuo power station in Shuozhou, China, with true density � 2.17 g/cm 3 .Table 1 shows the chemical composition of FA as determined by X-ray fluorescence (XRF).e 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.% H 2 O 2 ) as a foam blowing agent and calcium stearate as a foam stabilizer (FS) were employed.[27], the FA-based geopolymers have strong strength at a SiO 2 /Al 2 O 3 and Na 2 O/ Al 2 O 3 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 H 2 O 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.

Preparation and Synthesis of Specimen. According to the previous experiments by authors
e synthesis protocol of geopolymers is illustrated in Figure 2. FA, AA, FS, and additional H 2 O (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.
en, H 2 O 2 was added to the fresh paste and stirred for 30 s to obtain a consistent mixture.e 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.
ese 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).

Viscosity Testing.
e viscosity of geopolymer paste was measured at ambient temperature using the rheometer (DV3T, USA).e data were recorded at a constant shear rate of 6 s −1 for all the samples.Advances in Materials Science and Engineering reported results were the average of three independent measurements.

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

Quantitative Analysis of Pore Structure.
e total porosity of samples was obtained by comparing the difference of true density and bulk density of foam geopolymers as (1).e 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).
e 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 ρ true was the true density of the geopolymer, 2.17 g/cm 3  was determined by a 250 mL Li bottle according to the Archimedes method and ρ bulk was the bulk density of the foam geopolymer.P total means the total porosity of the foam geopolymer; P MIP means the porosity of the geopolymer sized <360 μm.P IA means the porosity of the geopolymer sized >360 μm.  which indicates that an increase of water can e ectively 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.e nonlinear decline of the viscosity showed that there exists a relationship between viscosity and W/S as described by the following equation [28]:

Results and Discussion
where α is the viscosity of paste; α L is the viscosity of the liquid phase (water), 0.9 × 10 −3 Pa•s; φ is the volume fraction of the solid phase; φ m is the maximum volume fraction of the solid phase when viscosity is in nite; 1 was selected for simple calculation; and B is a constant standing for intrinsic viscosity.e volume fraction of the solid phase, φ, 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 α was proposed based on the regression analysis of test data and is expressed as follows (Figure 5): where the experimental constants of A and B were determined to be 0.026 and 11.54 for y ash-(FA-) based geopolymer pastes and 0.016 and 3.20 for metakaolin-(MK-) based geopolymer pastes [29], respectively.

Pore Structure of Products.
Considering the limitations of MIP and the advantages of image analysis, the pore system of the foamed geopolymer can be e ectively 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 classi ed into microcapillaries (φ < 50 nm), macrocapillaries (50 nm < φ < 50 μm), and arti cial air pores (φ > 50 μm) [30].e microcapillaries are caused by the evaporation of water in the gel pores.e macrocapillaries and arti cial air pores are caused by the decomposition of H 2 O 2 and insu cient compaction [4,31].In Figure 6, the  Advances in Materials Science and Engineering 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.e porosity of microcapillaries also displayed an upward trend from 0.15% to 0.52% with an addition of water (Table 3).e 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 FAbased 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.e 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.

SEM Analysis.
e 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.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/cm 3 ).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.e relationship between 28 d bulk density (ρ d ) and 28 d compressive strength (f d ) of foamed geopolymers is represented in Figure 9. e existing data of foamed OPC [33,34] within ρ d < 0.3 g/cm 3 are also given for comparison.For dimensional analysis, ρ d and f d were normalized with reference values of ρ o (�1 g/cm 3 ) and f o (�1 MPa), respectively.At constant values of ρ, f d obtained from the FA-based foamed geopolymers here was typically higher than that obtained from foamed OPC.In addition, the increase rate of f d as a function of ρ d was greater in the prior investigation than that in the existing foamed OPC data. is 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 f d was proposed based on a regression analysis of the test data.Although the pore distribution and pore shape have a slight  Advances in Materials Science and Engineering effect on f d , only ρ d was considered for a simple calculation.

Compressive Strength of Products.
A basic equation for f d of the FA-based foamed geopolymer was empirically described as follows (Figure 9): In equation ( 5), the value of experimental constants (M 1 and N 1 ) 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.
ermal Conductivity and Bulk Density of Products.6 Advances in Materials Science and Engineering (0.026 W/(m•K)) and density (0.00129 g/cm 3 ) of air compared with y 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.e thermal conductivities were about 0.06 W/(m•K) for ρ d 0.24 g/cm 3 and 0.048 W/(m•K) for ρ d 0.175 g/cm 3 , respectively, which are around one-tenth of that for the normal-weight geopolymer [11].Moreover, the values of λ d of the current FA-based foamed geopolymer were slightly smaller than those of foamed OPC [33,34], as given in Figure 11. is 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 (λ d ) of the FA-based foamed geopolymer can be typically described as follows: where λ o ( 1 W/(m•K)) is the reference value for thermal conductivity.Regression analysis was performed to determine the values of M 2 and N 2 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.

e Relationship between Pore Structure and ermal
Conductivity.e total porosity and experimental thermal conductivity of foamed geopolymers with di erent W/S are displayed in Table 4. e 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%.e phenomenon can be explained by the variation of pore size as shown in Table 3.According to the Knudsen e ect [13], the thermal conductivity of air is ∼0.026W/(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.is 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 arti cial air pores.In order to get a better understanding the e ects 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.en, 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 signi cant downward, which can be justi ed by the re nement of pore-size distribution.At the Advances in Materials Science and Engineering same total porosity, the more the number of pore size < 50 μm, the smaller the thermal conductivity of the foamed geopolymer: where k i is the calculated thermal conductivity of foamed geopolymers with W/S i and i 0.41, 0.44, 0.47, or 0.50.k 1 is the thermal conductivity of foamed geopolymers with the W/S 0.38 and 0.0594 W/(m•K).k 2 is the thermal conductivity of air, 0.026 W/(m•K).ΔP is the variation of total porosity of foamed geopolymers.p t−i is the total porosity of foamed geopolymers with W/S i. p t−0.38 is the total porosity of foamed geopolymer with W/S 0.38.

Economic Analysis Compared to OPC-Based Foamed
Concrete.e 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%.e commercial unit cost (USD/ton) of each constituent materials provided in China Price Information (2017) is also listed in Table 5.
e 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 m 3 foamed geopolymer production is 0.15 USD.Due to the use of H 2 O 2 and calcite stearate as a foam stabilizer both in foamed geopolymer and foamed OPC, the price of H 2 O 2 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.
e production cost of the foamed geopolymer was slightly higher by 1.1-1.5 times than that of  8 Advances in Materials Science and Engineering foamed OPC.It is worthwhile to note that the less the unit binder content used, the smaller the di erence of production cost between foamed geopolymer and OPC.However, considering the bene cial e ect of environmental load reductions including low CO 2 emission, small water pollution, and land occupancy, the slight raise in cost would be acceptable for practical application.

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 signi cantly from 168 Pa•s to 6 Pa•s.(2) e 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 arti cial air pores (φ > 50 μm).e renement 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/cm 3 ).
(4) e production cost of foamed geopolymers was slightly higher by 1.1-1.5 times than that of foamed OPC.However, considering the more bene cial e ect 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.
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
Figure 4 plots viscosity of FA-based geopolymer paste with di erent W/S ratios.Water has a great e ect 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 signi cantly from 168 Pa•s to 6 Pa•s.

Figure 8
presents the compressive strength and specific 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.

Figure 8 :Figure 9 : 3 )Figure 10 :
Figure 8: Compressive strength and speci c strength of FA-based foam geopolymer at 1 d and 28 d with di erent W/S.

Figure 12 :
Figure 12: Experimental and calculated thermal conductivity of foamed geopolymers with di erent W/S.

Table 2 :
Initial compositional ratio of geopolymers.Water to solid by mass ratio.Water including H 2 O in AA, H 2 O 2 , and additional H 2 O. Solid including FA, FS, and solid in AA.

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.

Table 4 :
e total porosity and experimental and calculated thermal conductivity of foamed geopolymers.Figure 11: Regression analysis of thermal conductivity of foamed geopolymer and foamed OPC.

Table 5 :
Commercial unit cost of each constituent material (China Price Information).
* Modulus 2.4, 54.2% of water.Advances in Materials Science and Engineering