“Study of the Relastionship of Alkali-activated Fly Ashes/Metakaolin Based Geopolymers Curing at Room Temperature”

Alexandre Silva de Vargas1, a, Ruby M. de Gutierrez2,b and João P. Castro-Gomes3,c

1University Feevale RS 239, 2755, Novo Hamburgo/RS, Brazil 2University Del Valle, Cidad Universitaria Meléndez, Cali, Colombia

3University of the Beira Interior, Street Marquês d'Ávila e Bolama, Covilhã, Portugal aalexandrev@feevale.br, brudeguti@hotmail.com, ccastro.gomes@ubi.pt

Keywords: alkali-activation, Fly ash, Metakaolin. Abstract. Alkali activation uses components that are rich in aluminosilicate as raw materials to obtain binders that have a low environmental impact. Fly ash (FA) has been used as a precursor for these binders. However, thermal curing accelerates the polycondensation of aluminosilicate, which limits the application of this new binder in the construction industry. The objective of this study was to evaluate special binders with good mechanical properties cured at room temperature. The different compositions evaluated were FA and metakaolin (MK) that were activated using combined NaOH and Na2SiO3 alkaline solutions. Strengths of about 40 MPa were achieved at 91 days for the samples containing 70% FA and 30% MK and combined NaOH (50%) and Na2SiO3 (50%) activators. SEM images showed that the compressive strength of the alkali-activated samples was directly associated with the composition of the alkali activator, which may promote the dissolution of FA and MK particles that form the aluminosilicate gel. XRD pattern showed the formation of Natrite in alkali-activated samples, as well as the presence of crystalline compounds, such as quartz, mullite and hematite, in FA.

Introduction Alkali-activated (AA) cements, or geopolymers, classified as the third generation of cements, are an alternative to ordinary Portland cement due to their superior durability and environmental performance.

Alkali-activation is the transformation of glass structures into compact and cementing compounds [1]. Metakaolin (MK), one of the main raw materials to obtain alkali-activated cements, is composed of SiO2 and Al2O3 and has pozzolanic properties [2]. The MK get their pozzolanic properties during kaolin calcination at 650 °C to 800 °C, depending on the purity and crystallinity of the kaolin used [2, 3]. Alkali activation is used, in this case, to promote the reactivity of this material, which is generated during calcinations by means of loss of hydroxyl groups to SiO2 and to Al2O3.

To obtain AA cements, the medium should be strongly alkaline to dissolve SiO2 and Al2O3 and to produce the hydrolysis of the raw material surface, which thus generates polymerization. For that purpose, some substances, such as NaOH, Na2SiO4, KOH, and others, may be used [5].

Fly ashes (FA), resulting from coal combustion in coal-fired power plants, are another type of raw material that may be used in alkali activation [6]. The author found that alkali-activated fly-ash cements had very low compressive strength 30 days after curing at room temperature. However, the same cements, when cured at 60 °C, reached a compressive strength of 25 MPa at 24 hours [6]. Alkali-activated fly-ash samples prepared using combined solutions of Na2SiO4 and NaOH the compressive strength was 35 MPa after curing at 85 °C for 2 hours [1].

The objective of this study is to evaluate MK and FA matrices cured at room temperature using combined NaOH and Na2SiO4 as alkali activators.

Experimental

MK and FA were used as polymeric precursors. MK was obtained from kaolin calcination at 850 °C for 2 h. kaolin and MK XRD pattern are shown in Figure 1.

Fig. 1 XRD pattern: a) kaolin and b) metakaolin (MK). q – quartz; k - kaolinite

Figure 1 shows a significant difference in XRD pattern between kaolin (Figure 1A) and metakaolin (Figure 1B). This change indicates that the crystalline phases of kaolinite in the original material underwent amorphization due to the dehydroxylation produced by calcination (850 °C). In the table 1 is the chemical composition of MK and FA.

Table 1. Chemical analysis of metakaolin (MK) and fly ash (FA) (% weight)

SiO2 Al2O3 CaO Fe2O3 Na2O TiO2 MgO K2O SO4 MK 58,39 38,20 0,11 1,70 0,27 1,24 0,24 0,18 0,17 CV 70,79 14,65 2,65 5,90 0,13 2,19 0,26 2,35 0,49

MK and FA (class F [7]) have a low calcium content and are essentially composed by SiO2 and

Al2O3. The alkaline activators used were 98% purity, technical-grade sodium hydroxide (NaOH) flakes

and technical grade Na2SiO4 (32.55% SiO2 and 15.10% Na2O). Quartz sand (2.6 g/cm3 density) standardized according to 4 granulometries (1.2, 0.60, 0.30 and

0.15 mm) was used; each granulometry accounted for 25% of the total amount of sand. The ratio of cement (FA+MK) and sand was 1:3 (in weight) by to prepare the mortars.

The pastes and mortars were prepared using 30% FA and 70% MK, 50% FA and 50% MK, and 70% FA and 30% MK. Each sample was mixed with combined solutions of Na2SiO4 and NaOH at different percentages, as seen in Table 2.

Table 2 Alkali-activated fly-ash and metakaolin matrix composition using a combined NaOH

and Na2SiO4 activator solution

Alkaline activator (%): Na2SiO4 (S) and NaOH (N)

% raw material used as geopolymeric precursor 70% FA; 30% MK 50% FA; 50% MK 30% FA; 70% MK

50% S and 50% N E B 60% S and 40% N A 70% S and 30% N C D FA – Fly ash; MK - metakaolin

k

k

k q

q

q k

k

X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the alkali-activated pastes at 28 days of age.

The tests to determine compressive strength were conducted using alkali-activated mortars at 7, 28 and 91 days. All samples were cured at room temperature until the day of the tests. Results and discussion

Compressive strength Table 3 shows the results of compressive strength tests of alkali-activated mortars at 7, 28 and 91 days.

Table 3 Compressive strength of alkali-activated fly-ash and metakaolin-based mortars in

different ages

Compressive Strength (MPa) Age (Days)

Sample 7 28 91 A 3.2 10.3 18.1 B 1.3 6.7 13.4 C 2.6 5.1 5.8 D 0.2 0.7 5.4 E 4.7 25.3 40.9

Results showed that replacing FA with MK, depending on the composition of the activator

solution, contributed to an elevation in compressive strength of alkali-activated matrices. In addition, samples prepared with 100% FA, regardless of activator, had no compressive strength, not even at 28 days. FA-based samples cured at 20 °C did not have any compressive strength at the same age [8]. These findings demonstrate that the alkali activation of samples prepared with FA is technically feasible only when thermal curing is used, which limits the application of this material in construction.

This study found that the combined use of MK and FA promoted the polycondensation of aluminosilicate in the samples that had 70% FA and, therefore, led to compressive strength gains along time. The combined use of raw materials may improve the mechanical performance of alkali-activated samples, as confirmed by [9], who used MK-based samples and blast furnace slags. Those authors described that alkali-activated samples reached strengths of up to 24 MPa at 4 h after curing at room temperature only.

Considering only the concentration of the alkaline activator, the increase in Na2SiO4 concentration (Table 2) did not contribute to the elevation of the compressive strength of the alkali-activated samples (Table 3), differently from findings reported by [10], in whose study FA-based samples that underwent alkali-activation using 14 M NaOH solution (48 MPa) had compressive strengths higher than those found for samples prepared with 8 M solutions (17 MPa). On the other hand, the elevation of the NaOH concentration from 6% to 8% resulted in an increase from 15 MPa to 45 MPa in FA-based samples [11]. However, the higher activator concentrations do not lead to increases in compressive strength, but, rather, that samples activated with 12 M NaOH had higher compressive strength values (34.6 MPa) than those found for samples prepared using 1 M solutions (23.3 MPa) under the same curing conditions (85 °C, 24 h) [1]. The presence of free OH- in the alkali-activated matrix may change its geopolymeric structure and, consequently, produce mechanical losses.

Thus, in addition to the effect of raw materials and curing conditions, the concentration of the alkaline solution is important to optimize the mechanical performance of the alkali-activated samples. High contents may produce oversaturation and, consequently, alkali crystallization in the hardened matrix, which reduces compressive strength. At the same time, when concentrations are below the ideal level, there will be no alkaline environment to promote the efficient polymerization of aluminosilicate and, consequently, compressive strength will be low, as demonstrated by [12].

XRD analysis

Figure 2 shows the XRD pattern of alkali-activated samples at 28 days, as well as the XRD pattern of their precursors (MK and FA).

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Fig. 2 XRD pattern of alkali-activated samples at 28 days and XRD pattern of their precursors –metakaolin (MK) and fly ash (FA). M – mullite; Q – quartz; H – hematite; N - natrite

The crystalline peaks of quartz (SiO2), identified with the letter Q, mullite (Al6Si2O13), letter M,

and hematite (Fe2O3), letter H, found in FA and MK, were the same in alkali-activated samples before and after alkali-activation, regardless of age or combined solutions used for alkali-activation. This shows that the crystalline phases remain unaffected in a highly alkaline environment. In addition to these crystalline phases, natrite (Na2CO3), identified with the letter N, was formed due to the carbonation of the sodium (Na) that was free in the alkali-activated matrix.

SEM Observation

Figure 3 is a SEM micrograph of the structural changes in A and D samples at 7, 28 and 91 days.

Q Q M M H

M N

Sample E Sample D Sample C Sample B Sample A MK (850°C) FA

Q

Fig. 3 SEM micrograph of A and D samples, at 2000X magnification (a), (b) and (c): sample A at 7, 28 and 91 days; (d), (e) and (f): sample D at 7, 28 and 91 days

. Figures 3a and 3d show that, in the structure of samples A and D at 7 days, several FA grains

did not dissolve. This indicates that, at 7 days, the aluminosilicate gel had not formed efficiently, which is in agreement with the mechanical performance of samples at this age (3.2 MPa and 0.2 MPa, table 3).

The structure changed substantially for sample A at 28 and 91 days in comparison with findings at 7 days. The dissolution of aluminosilicate in the FA and MK samples increased along time, and aluminosilicate gel, responsible for the material’s mechanical properties, was formed efficiently. This can be confirmed by the increase in compressive strength of this sample at 28 (10.3 MPa) and 91 (18.1 MPa) days. In contrast, Figure 3e shows that there was no substantial change in sample D between 7 and 28 days. This finding is in agreement with the compressive strength of this sample at 7 (0.2 MPa) and 28 (0.7 MPa) days. However, the comparison of the structure of sample D at 28 (Figure 3e) and 91 (3f) days shows that there was a substantial change and that the structure was

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denser at 91 days than at 28 days. For sample D, aluminosilicate dissolution was greater at more advanced ages, which shows that time has an important effect on the polycondensation of aluminosilicate. This also affected the mechanical performance of sample D, as compressive strength increased substantially between the 28th (0.7 MPa) and the 91st (5.4 MPa) days.

Figures 4a and 4b show the structure of sample E at 7 and 91 days, at 5000x magnification.                                

Fig. 4 SEM micrograph of sample E, at 5000X magnification (a) 7 days; (b) 91 days

  In 4a, the structure of sample E was partially dissolved at 7 days, and its structure was similar to

that of sample A (Figure 3a). Table 3 shows that compressive strength is 3.2 MPa and 4.7 MPa for samples A and E.

Figure 4b shows that the structure of sample E changed substantially between the 7th and the 91st days. At 91 days, the matrix had a compact aspect, which indicates that the alkali activation of FA and MK was efficient. No FA particles are seen. That finding is confirmed by the result of the compressive strength test of this sample (Table 3) at 91 days (40.9 MPa). Conclusions

The matrix containing 70% FA and 30% MK and that underwent alkali-activation using a combined solution of 50% NaOH and 50% Na2SiO4 reached the greatest compressive strength at 91 days.

That matrix had substantial gains at 28 and 91 days. This increase was associated with FA dissolution, which occurred at older ages and was similar to that found for the pozzolanic reaction of Portland cement.

The increase of Na2SiO4 in the activating solution did not result in greater compressive strength, regardless of the amounts of FA and MK used and the age evaluated.

XRD pattern showed the formation of crystalline phases of natrite in the alkali-activated samples as a result of the carbonation of excessive sodium in the matrix. However, its formation did not affect the mechanical stability of the matrix along time. References

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