This chapter delves into the effects of using blended precursors, specifically fly ash and slag, on the mechanical properties and chloride resistance of geopolymer concrete. It presents experimental findings that demonstrate the positive impact of incorporating ground granulated blast-furnace slag (GGBFS) on compressive and tensile strength, as well as on the reduction of chloride ion penetration. The study employs advanced techniques such as mercury intrusion porosimetry to analyze the pore structure and its relationship to chloride diffusion. The results highlight the potential of optimizing precursor compositions to enhance the performance of geopolymer concrete, making it a valuable resource for professionals seeking to improve the sustainability and durability of concrete structures.
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Abstract
Geopolymer concrete is an environment-friendly material and is presently accepted as an alternative to conventional concrete. It utilizes industrial by-products like fly ash and slag to reduce CO2 emissions associated with cement production. Despite being investigated over the decades, the application of geopolymers in construction is still very limited. Most of the research data refer to geopolymer pastes and mortars and their properties, performances, and durability. Although geopolymer concretes are well-accepted in the research community owing to their comparable or even better performances as a cement substitution.
In this paper, the precursors for geopolymer concrete preparations are blends of fly ash (FA) and ground granulated blast-furnace slag (GGBFS) in three slag proportions: 5%, 20%, and 35% expressed as a percent of FA mass. The concretes were denominated AAC5, AAC20, and AAC35, respectively. Their basic physical and mechanical characteristics were investigated, as were their transport properties of chloride ions. The ASTM C1556 test was applied to determine the chloride ions’ penetration of the geopolymers. The measurements revealed a strong dependence between chloride penetration through the concrete and the precursor composition.
1 Introduction
The work on material that can replace Portland cement in recent decades, has become the target of numerous studies. Cement production alone contributes between 5% to 7% of the anthropogenic CO2 emissions worldwide [1]. Due to successive regulations limiting carbon dioxide emissions, the intensification of research in this area results in a growing base of theoretical [2] and practical [3‐6] knowledge about geopolymer binders. Geopolymer concrete is a type of environmentally friendly concrete made from industrial waste materials such as fly ash, slag, and silica fume. The use of blended precursors, such as a mixture of fly ash and slag, has been shown to improve the mechanical and durability properties of geopolymer concrete.
Several studies have investigated the mechanical properties of geopolymer concretes with blended precursors. According to Kumar et al. [7], the compressive strength of geopolymer concrete is influenced by the type and ratio of the blended precursors used. The study showed that the use of a mixture of fly ash and slag improved the compressive strength compared to using fly ash alone. Another study by Bouziani et al. [8] investigated the effect of different proportions of fly ash and slag on the mechanical properties of geopolymer concrete. The results showed that an optimal ratio of fly ash and slag can lead to an increase in compressive strength, flexural strength, and tensile strength compared to using fly ash or slag alone.
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The durability of geopolymer concrete is also affected by its resistance to chloride ion penetration. According to Raza et al. [9], the use of blended precursors can improve the resistance of geopolymer concrete to chloride ion penetration. The results showed that combining fly ash and slag in the right proportion can significantly slow down the penetration of chloride ions compared to using either one of them individually. Another study by Chen et al. [10] investigated the effect of different curing conditions on the resistance of geopolymer concrete to chloride ion penetration. The results showed that proper curing can improve the resistance of geopolymer concrete to chloride ion penetration, and the use of blended precursors can further enhance this resistance.
The study discussed in this paper examines the relationship between the precursors’ compositions and the durability and mechanical strength of geopolymer concretes, as also the penetration of chloride ions. The analysis of the results was enhanced with the examination of mercury porosity data, leading to credible conclusions.
2 Materials and Methods
The research was carried out on geopolymer concretes, utilizing blends of FA (from the Połaniec power plant in Poland) and GGBFS (from Ekocem in Poland) as the precursors. The oxide compositions of both precursors are provided below (Table 1).
Table 1.
Chemical compositions of FA and GGBFS
wt.%
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
K2O
Na2O
P2O5
TiO2
Mn3O4
Cl−
FA
52.30
28.05
6.32
3.05
1.71
0.28
2.51
0.76
0.69
1.35
0.07
–
GGBFS
39.31
7.61
1.49
43.90
4.15
0.51
0.36
0.47
–
–
–
0.04
The activator for preparing the geopolymer binder was an aqueous solution of sodium silicate Geosil® 34417, with an additional amount of water added. Its chemical composition is presented in Table 2.
Table 2.
Chemical composition of Geosil® 34417
Characteristic
Unit
Woellner Geosil® 34417
Na2O content
wt.%
16.74
SiO2 content
wt.%
27.5
Density
g/cm3
1.552
Viscosity
mPa*s
470
Weight ratio (WR = wt.% SiO2/wt. Na2O)
–
1.64
Molar ratio (MR = mol SiO2/mol Na2O)
–
1.70
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Three precursor blends were made by substituting FA with GGBFS in the ratios of 5%, 20%, and 35%, expressed as a percentage of the FA mass. These blends were used to prepare three concretes, referred to as AAC5, AAC20, and AAC35, respectively.
The design specifications for the mixtures were based on practical experiences [4, 5] and literature references [11]. Trial batches of concretes with varying slag content were prepared and subjected to preliminary testing. Considering the consistency, physical and mechanical properties of the mixtures, the final compositions of Alkali activated concretes were established. Each of the recipes, outlined in Table 3, can be defined by the following attributes:
Water/binder (w/b) ratio = 0,37
Alkaline Solution/binder ratio = 0,53
Amount of paste in concrete = 300 dm3/m3
Where:
Water - a mass of water added and contained in Geosil® 34417.
Binder - a sum of FA and GGBFS (by mass).
Alkaline Solution - a mass of diluted Geosil® 34417.
Table 3.
Composition of Alkali Activated concretes.
[kg/m3]
AAC5B
AAC20B
AAC35B
FA
336.9
292.3
244.1
GGBFS
17.7
73.1
131.4
Alkaline Solution + water
189.4
195.1
200.5
Sand 0/2
662.4
662.4
662.4
Basalt 2/8
708.9
708.9
708.9
Basalt 8/16
648.4
648.4
648.4
The samples were made following the guidelines of EN 206 + A2 [12]. They were left in molds for 24 h and then taken out and stored in a laboratory environment (18 ± 2 ℃) under plastic wrap to prevent water evaporation. Compressive and splitting tensile strength were performed according to PN-EN 12390-3 [13] and PN-EN 12390–6 [14] standards. The experiments were conducted on cubic specimens with sides measuring 100 mm, at 28 and 180 days.
The study of the chloride diffusion coefficient utilized the ASTM C1556 test method. 360-day-old cylindrical specimens, with dimensions of Ø11 x 5 cm, were cut from larger Ø11 x 22 cm cylinders. The samples were prepared by coating them with epoxy resin and soaking them in distilled water before being placed in a test cell filled with a 16.5% sodium chloride solution for 35 days.
After 35 days of immersion in the 16.5% aqueous NaCl solution, the test specimens were taken out of the solution and allowed to air-dry for 24 h in laboratory conditions. Then, the specimens were ground with the ‘Germann Instruments’ Profile Grinder to produce obtain samples. The grinding process was performed in parallel layers to the exposed surface, with a maximum depth of 5 mm for each layer, ensuring that the weight of the powder sample was at least 10 g for each layer.
Investigations of the acid-soluble chloride contents of the powder samples were performed according to ASTM C1152 [15] method with the use of 5 g instead of 10 g recommended. The process of measuring the chloride content involved taking two 50 ml samples from a solution that was prepared from each powder sample. Each of the solutions had a volume of 250 ml. The average value of chloride content was calculated using a potentiometric titration machine, with a silver nitrate solution as recommended in the ASTM C114 standard [16]. The apparent chloride diffusion coefficient (Da) and surface concentration of chloride ions (Cs) were calculated by applying Eq. 1 to the chloride profile data obtained through non-linear regression analysis using the method of least squares, as per the ASTM C1556 standard.
C(x, t) chloride concentration, measured at depth x and exposure time t, mass %,
Cs projected chloride concentration at the interface between the exposure liquid and test specimen that is determined by the regression analysis, mass %,
Ci initial chloride-ion concentration of the cementitious mixture prior to submersion in the exposure solution, mass %,
x depth below the exposed surface (to the middle of a layer), m,
Da apparent chloride diffusion coefficient, m2/s,
T the exposure time, s,
erf the error function.
To fit Eq. (1) to experimentally obtained points MATLAB® code was used.
Tested materials were also subjected to a porosity analysis using Mercury Intrusion Porosimetry (MIP). It is a method used to measure the pore size distribution of materials. The measurement is performed by injecting mercury under high pressure into the sample and measuring the pressure required to intrude the mercury into the pores. The pressure at which the mercury begins to penetrate the sample is recorded, and this value is used to determine the pore size of the sample. Specimens’ sizes were about 1 cm3. The results were used to interpret the chloride ion penetration results and as the supplementary characteristics of investigated materials.
3 Results and Discussion
3.1 Mechanical Characteristics
The findings of the compressive and tensile strength evaluations indicate that replacing FA with GGBFS had a positive impact on the mechanical properties of AA concretes.
The test outcomes demonstrate a nearly linear correlation between the content of GGBFS and the compressive and splitting tensile strength after 28 days.
However, results from tests performed after 180 days show a similar trend, although with slightly reduced precision. The results for both compressive strength and splitting tensile strength tests reveal an increase in values over time.
The results showed a remarkable increase in values for the AA concretes that had a low GGBFS content (in particular, the AAC5 average compressive strength rose from 22.5 to 41.0 MPa and the average splitting tensile strength increased from 3.10 to 4.85 MPa). The results are presented in Figs. 1 and 2.
Fig. 1.
AACs’ compressive strength after 28 and 180 days
Fig. 2.
AACs’ splitting tensile strength after 28 and 180 days
×
×
3.2 Chloride Diffusion Coefficient (Da)
The tests performed provided the profiles of chloride content in the materials. Figures 3–5 display the chloride profiles of the AACs and the curves fitted, as per Eq. (1), to the experimental data points.
Fig. 3.
Chloride profile of AAC5 and curve fitted to the experimental results
Fig. 4.
Chloride profile of AAC20 and curve fitted to the experimental results
Fig. 5.
Chloride profile of AAC35 and curves fitted to the experimental results
×
×
×
Figures 3, 4, and 5 demonstrate the positive impact of adding GGBFS on the depth of chloride penetration. The specimen containing 5% slag has the highest concentration of chloride ions. The curve fitting determined that the surface chloride concentration was 0.61% (the percentage of chloride ion mass in the mass of the powder sample), whereas, for AAC20 and AAC35, the values were 0.49% and 0.54% respectively. The concretes with the greatest amount of GGBFS showed a rapid decrease in chloride concentration, reaching 0.13% in the fifth layer at a depth of 17.5 mm. In contrast, the decrease in chloride ions was slower in concretes with low slag content. For AAC20 and AAC5, the chloride concentration reached 0.13% only at depths of 32 mm and 43.5 mm respectively.
The characteristics of porous materials, such as cement concrete and AAC, are largely determined by the volume and size distribution of their pores. The values of total porosity, compressive and splitting strength, as also chloride diffusion coefficients are presented in Table 4.
Table 4.
Values of fc, ft, Da, and total porosity for AACs
Parameter
AAC5
AAC20
AAC35
fc(180 days) [MPa]
43.16
56.62
79.18
ft(180 days) [MPa]
5.11
5.29
6.60
Da [m2/s]
90.67
44.38
8.55
Porosity [%]
14.28
9.18
7.30
The total porosity of hardened AA concretes is strongly associated with precursors’ compositions. When it comes to the overall pore content of the materials, elevating the level of GGBFS substitution for FA reduces porosity by nearly 50%. Total porosity of AAC5, AAC20 and AAC35 are 14.28%, 9.18% and 7.30%, respectively.
Resistance to chloride ions’ aggression is a complex phenomenon strongly associated with total porosity and pore size distribution but also tortuosity and chloride binding capacity. The chemical properties of the material, which are the subject of further studies, may also affect the penetration of diseased ions.
Noushini A. [11] defined the superior value of chloride diffusion coefficient for geopolymer concretes applied in chloride environments as 14 x 10–12 m2/s. That recommendation was based on experimental results according to standard ASTM C1556 [17]. Values of diffusion coefficient for AAC35 seem to be promising and encourage further research on the material.
Certainly, the variable content of GGBFS in precursors’ compositions triggered changes in the structure's development process. Except for the slag addition, AACs’ components were matching as well as hardening and storage conditions. Therefore the results presented in this paper are closely related to the precursor used. In the case of such assumptions, it can be concluded that the addition of slag affects the pore structure which contributes to the features dependent on porosity.
4 Conclusions
The study results suggest that the inclusion of GGBFS has a substantial impact on the mechanical characteristics and chloride resistance of geopolymer concrete. The tests conducted demonstrate that it is feasible to achieve desirable properties for geopolymer concrete without the need for heating the material.
The conducted research allows for drawing general conclusions:
The results of the study demonstrate that the use of GGBFS in Alkali Activated concrete reduces the porosity significantly.
The use of higher levels of GGBFS in the mix with FA of geopolymer concrete’s binder significantly enhances its mechanical strengths, such as compressive and splitting tensile strength
An increase in the amount of blast furnace slag enhances the ability of geopolymer concrete to withstand attacks from chloride ions.
It has been observed that there is a nearly linear correlation between the amount of GGBFS in the composition of the material and its compressive strength and splitting tensile strength.
The trend of improvement in the mechanical properties of the materials, particularly for concretes with a low amount of GGBFS, becomes more apparent over time.
The outcomes of the study suggest the need for continued exploration to comprehend the chloride penetration process in geopolymer concrete and the effect of incorporating blast furnace slag on the material's physical and mechanical characteristics.
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