PRODUCTION OF FLY ASH BASED GEOPOLYMERS USING ACTIVATOR SOLUTION WITH DIFFERENT NA 2 O AND NA 2 SIO 3 COMPOSITIONS

In this work, samples of geopolymers were produced by the activation of fly ash with different alkaline solutions (NaOH and NaOH + Na2SiO3). The mechanical strength and microstructural characterization were employed to study the influence of the geopolymerization process in the final proprieties of the binder material. The results showed that the geopolymers have high mechanical strength in the first 24 hours of curing (28 MPa) increasing to 48 MPa after 28 days. The physical proprieties such as water absorption, apparent porosity, and density changed when different solutions were used. The SEM images showed a denser morphology with aluminosilicate gel formation on the surface which is responsible by an increase of mechanical strength of fly ash based geopolymers.


INTRODUCTION
Nowadays the consumption of the ordinary Portland cement (OPC) is increasing year after year.According to Torgal and Jalali (2010), almost 2600 tons of OPC are consumed each year, and a quantity of 10400 tons are expected for 2050 1 .The production of OPC releases high amounts of greenhouse gases and the energy necessary to produce the binder material is very elevated 2 .Therefore researchers developed materials with the same characteristics or higher than OPC, giving rise to the geopolymers.
Geopolymers are materials with high initial mechanical strength, high resistance to acid attack, freeze-thaw cycles, and high-temperature resistance [3][4][5][6][7][8] .Glukhovsky first studied these materials in 1957 and Davidovits in the 70's started to produce researches about the microstructure and properties of geopolymers.Davidovits gave the name "geopolymerization" to the process to the production of geopolymers 9,10 .
The geopolymer structure is a network consisting of SiO4 and AlO4 tetrahedra linked by sharing all oxygen atoms.Due to the presence of the Al 3+ in IV-fold coordination, the presence of positive ions (Na + , K + , Ca 2+ ) is necessary to balance the negative charge provided by aluminium 11  Geopolymers can be made using a wide variety of raw materials, the control of the raw material composition is very important to obtain geopolymers with high mechanical strength.The fly ash (FA), metakaolin (MK) and ground granulated blast furnace slag (GGBFS) are some examples of the raw materials used to produce the inorganic polymers 4,12 -17 .
In the work of Belmokhtar et al. the authors produced the geopolymeric cement using sludge provide for the manufacture of sanitary ceramic in order to decrease the amount of this kind of wastewater.The results showed that the geopolymers produced using this material obtained high mechanical strength (close to 43 MPa).These results demonstrated that geopolymers can be synthesized using byproducts of the industrial process 18 .
In the research promoted by Kastiukas et al., the geopolymer was produced using tungsten-mining waste as start material.The results showed that the geopolymerization reactions occur and the network structure of the geopolymers are formed.The study demonstrated that time and temperature of cure of the specimens modify the reactions between the Si and Al rich phases and contributed to the formation of the binder with high mechanical strength 19 .
There is a great number of works worldwide about the production of the geopolymeric binders.The focus of these studies is the data collection about the use of the different raw materials in the geopolymer synthesis and the influence of it in the mechanical, chemical e physical proprieties of the hardened structure.The geopolymeric cement is commercialized in some countries with different commercial names.Lone Star, Pyrament and Metamax in the United States, Geopolymère in France, Tollit in Germany and MetaMax in New Zeland.The commercialization of this new class of binders demonstrates how they are likely to replace Portland cement in the construction industry because of superior properties 20,21 .
The raw materials used to produce the geopolymer materials must have a high amorphous aluminosilicate content, that, after the contact with the alkaline solutions (NaOH or NaOH + Na2SiO3), promotes the geopolymerization process.
Fly ashes used to produce the geopolymers have different compositions and morphological characteristics, that, consequently, change the proprieties of the hardened fly ash based geopolymers.
The chemical composition of the alkaline solutions is very important to produce geopolymers with high initial mechanical strength.The aim of this work is to investigate how the Brazilian fly ash from the thermoelectric power plant of Jorge Lacerda can be used to produce geopolymers by different alkaline solutions.The time of cure was evaluated to increase the information of how the geopolymerization reactions occur on different days after the synthesis.

STARTING MATERIALS
The FA used to produce the geopolymeric binders was supplied by the thermoelectric power plant of Jorge Lacerda located in Capivari de Baixo, Santa Catarina, Brazil.The chemical composition is presented in Table 1.According to ASTM C 618 the fly ash used in this work can be classified as class F, of low CaO content (lower than 10 %) and the SiO2, Al2O3 e Fe2O3 concentration higher than 70 %.The specific mass of the fly ash is 2.21 g/cm 3 and D50, close to 30.82 µm.The simple alkaline activators used in the inorganic polymerization reaction were sodium hydroxide solutions (10, 12 and 16 mol.L -1 ) prepared by commercial NaOH pellets (98% purity) and deionized water.The compound solutions were prepared by replacing 10, 20 and 30 wt% of NaOH by sodium silicate, Na2SiO3.The Na2SiO3 used to produce the compound solutions has a SiO2/Na2O ratio and total humidity close to 2.4 and 51.4 %, respectively.

EXPERIMENTAL PROCEDURE
Geopolymer specimens were prepared by mixing the fly ash with the activator solutions.In the mixtures, the ratio of activator solution/fly-ash was kept constant at 0.5.However, several NaOH and Na2SiO3 concentrations of the solutions were employed, as pointed out before.The compositions studied are summarized in Table 2.The nomenclature used to identify the different geopolymers is based on the activator alkaline solution used to produce the specimens and their curing time.For example, sample H10S30-7 represents the geopolymer produced using a 10 mol.L -1 sodium hydroxide solution (H10) with 30 wt% of sodium silicate (S30), cured for 7 days.
The fly ash and the alkaline activator solutions were mixed in a mechanical mixer, appropriate for the preparation of cement pastes, for 5 minutes.After mixing, the pastes were poured into plastic cylindrical molds (50 x 25 mm) and vibrated to ensure compaction.The samples were covered with a plastic film to avoid water loss due to evaporation during the geopolymerization process.After 24 hours at room temperature, the plastic film was removed and the specimens were put in an oven for curing at 65 °C for 24 h.The specimens were further cured at room temperature for 1, 7 and 28 days prior to their mechanical and microstructural analysis.For each composition studied 10 specimens were prepared 22 .The morphology of the samples was observed by scanning electron microscopy, SEM.Further properties such as water absorption, apparent porosity and densities were determined according to the EN ISO 10545-3 standard 23 .

MECHANICAL STRENGTH
The compression strengths of the fly ash based geopolymers after curing for 1, 7 and 28 days are listed in Table 3.The mechanical strength of the geopolymers samples H10S0, H12S0 and H16S0 are lower than samples made using the compound solutions.The highest mechanical strength of about 48 MPa was obtained when the H12S30 alkaline solution composition was used as activator.
In the samples H16S0 the mechanical strength was higher in the firsts 7 days, decreasing after 28 days, which was attributed to excessive alkali (Na2O) in the system leading to the deterioration of the aluminosilicate gel, N-A-S-H (N=Na2O, A=Al2O3, S=SiO2, H=H2O).When sodium silicate was added to the system, the mechanical strength of the samples showed significant improvement.Strength increased from initially 9.77 (H12S0-1) to 38.51, 28.27 and 27.78 MPa, for samples H12S10-1, H12S20-1 and H12S30-1, respectively.The samples made using the 16M solution had the mechanical strength increased from 10.17 MPa to 28.05, 24.60 and 38.87 MPa, for samples with 10, 20 and 30% of Na2SiO3, samples H16S10-1, H16S20-1 and H16S30-1, respectively.Therefore, the use of a combined NaOH + Na2SiO3 alkaline activator solution resulted in a strength increase of roughly 300%.
The highest strength of 47 and 44 MPa were obtained by samples of composition H12S30 and H16S30 after 28 days of curing, respectively.Thus the highest mechanical strength was obtained when the combined NaOH and Na2SiO3 solution was used with a SiO2/Na2O ratio close to 1.0, as can be seen in Fig. 1.

WATER ABSORPTION, APPARENT POROSITY AND DENSITY
The results of the physical properties of the fly ash based geopolymers are presented in Table 4.The results show that apparent porosity and water absorption decrease when the specimens are made using the compound activator solution H16S10, H16S20 and H16S30.The higher density values are related to more effective os the alkaline solution during the geopolymerization reactions.The higher Na2O concentration promotes more effective dissolution of the vitreous phase present in the raw material, and, consequently, the reaction productions of the geopolymerization are formed.The use of compound alkaline activator promoted an increase in the N-A-S-H gel formation due to the high effectiveness to dissolve the amorphous material and the subsequent production of the geopolymeric structure.The reactions are increased by the presence of the extra silica provide by the use of Na2SiO3 solution.This aluminosilicate gel formed in the surface can be seen by the SEM analysis, confirm the formation of denser structure.

SEM ANALYSIS
The SEM images of the original fly ash and the hardened geopolymers after 28 days of curing are presented in the Fig. 2.
The original fly ash is composed of spherical particles of different sizes.The morphology of the hardened geopolymers exhibited a denser microstructure.The morphology of the samples was modified by the use of different alkaline activators.The aluminosilicate gel is observed in all samples, showing that geopolymeric reactions occur after the contact of FA with the NaOH or NaOH + Na2SiO3 solutions.However, the samples produced with the compound solutions have a surface containing a larger amount of aluminosilicate gel (N-A-S-H), corroborating the results of the physical properties.The microstructures of inorganic polymer materials show that most of the spherical particles of the fly ash were dissolved during the polymerization process by the alkaline activator solutions, forming the Si and Al rich inorganic gel.It is possible to observe the presence of fly ash particle without reacting to the surface when the simple solution was used to activate the raw material.This can be related to increases in the dissolution rate when the SiO2 is added by the use of the sodium silicate solution, as observed in the compression strength results.Specimens produced with compound activator solutions i.e.NaOH-Na2SiO3 solutions, exhibited denser microstructures in comparison to samples produced with NaOH solutions only, resulting in improved mechanical strength and formation of structures with less porosity.

CONCLUSIONS
The geopolymers produced with fly ash from the thermoelectric power plant of Jorge Lacerda showed high mechanical strength in the first hours of curing when the compound alkaline solutions were used.The highest mechanical strength was obtained using the mixture containing the SiO2/Na2O close to 1.0, showing that it is important to control the chemical composition of the activator solution.The use of sodium silicate promotes the increase of density of fly ash based geopolymers and decreases the water absorption values due to the higher aluminosilicate gel formed, as can be seen by the SEM analysis.The results show that the development of a denser binder material with mechanical strength close to 48 MPa is possible using the present fly ash as raw material.

Figure 1 .
Figure 1.Influence of the Na2O and SiO2 concentrations of the alkaline activator solutions on the mechanical strength of the geopolymers specimens after 28 days.

Figure 2 .
Figure 2. SEM images of the fly-ash and geopolymers produced with different activator alkaline solutions after 28 days.

Table 2 .
Composition of activator solutions.

Table 3 .
Compressive strength of the fly ash based geopolymers.

Table 4 .
Water absorption, apparent porosity and density of the geopolymer samples.