Thermal phenomena of alkali-activated metakaolin studied with a negative temperature coefficient system
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The properties of alkali-activated materials (AAMs) depend on both the type of raw material used and their production procedure. This article presents an inexpensive and easily accessible method, based on using thermistors with a negative temperature coefficient, to analyse phenomena during the geopolymerisation process of AAMs. The described method enables prediction of the final physical and mechanical properties of tested materials and allows unambiguous determination of the quality of raw metakaolin materials in terms of their suitability for geopolymerisation processes and AAM production. This statement was proved by comparing AAMs formed based on metakaolin from three different sources. This article also describes the results of the mineralogical analysis, density, particle size distribution and morphology of the three metakaolins. In addition, the compression strength and FT-Raman spectroscopy of the AAM produced are described. Even though all materials were referred to as metakaolin, the results of this study showed that calcined materials can significantly differentiate the geopolymerisation process and final physical and mechanical properties of AAM.
KeywordsAlkali-activated materials Compressive strength Metakaolin Negative temperature coefficient thermistor Geopolymerisation process
Alkali-activated materials and geopolymers now play an increasing role in industry, due to the ecology and economics of their production as well as a wide spectrum of possible applications. In addition, these materials could be designed to have superior properties compared to materials produced based on Portland cement, namely better resistance to acids and sulphate, better heat resistance, lower drying shrinkage and creep and higher strength .
The starting materials used for the synthesis of AAM and geopolymers are natural silicates (natural pozzolans) from kaolin or volcanic tuffs. The first step is metakaolin (Al2Si2O7) production as a result of kaolin (Al2Si2O5(OH)4) dehydroxylation. The main phenomena during this process are the transformation of octahedral Al into tetrahedral Al, structure amorphisation and retaining a 1:1 layer type. A study of the effect of the calcination temperature of kaolin on the properties of inorganic aluminosilicate polymers showed that geopolymers with optimal characteristics are obtained in a temperature range between 973 and 1073 K. Furthermore, it was found that the kaolin calcination heating high rate (K min−1) results in higher losses in the metakaolin, which are related to an increase in the residual kaolin. In the synthesis of geopolymers, the final properties are influenced by both the amorphous metakaolin phase and the residual kaolin content. Higher kaolin contents adversely affect the mechanical strength of the product. Therefore, to produce a geopolymer with high mechanical strength, kaolin calcination between 973 and 1073 K should be carried out at a low heating rate . On the other hand, as demonstrated by differential scanning calorimetry (DSC), the calcination time has little effect on the reactivity of the material obtained during isothermal processing at 1023 K. The heat released during the polycondensation reaction for the raw material calcined for 2 h at 923 K was about 92% of the heat emitted as a result of the process for the material calcined at 1023 K for 4–6 h. The reactivity of the material after calcination for 2 h at 823 K was lower because the heat released was about 85% of the maximum registered for materials calcined at 1023 K .
The second step is the alkaline activation of metakaolin. The process of alkaline activation involves three types of reactions . The first type leads to the dissolution of metakaolin into silicate monomers and aluminate monomers. This phenomenon has a strong exothermic character. The second type includes the polymerisation processes of these monomers into aluminosilicate oligomers, which immediately polymerise into small geopolymeric fragments or ‘proto-zeolitic nuclei’. They are, however, thermodynamically metastable and incompletely cross-linked. Therefore, the third stage involves their combination into larger molecules, i.e. into aluminosilicate inorganic polymer gels and crystallised phases, which allows obtaining a material with stable properties . One major factor affecting the composition of alkali-activated material binding phases is the calcium content; N–A–S–H, C–(N)–A–S–H and C–A–S–H gels are formed in low-, intermediate- and high-calcium systems, respectively. However, in high-calcium systems, the main binding phase is crystalline C–S–H . The reactivity of metakaolin also depends on the Al2O3 and SiO2 content; the higher the Al2O3 and SiO2 content, the more reactive the metakaolin . Moreover, altering the Si/Al molar ratio allows the synthesis of materials with different structures as the aluminium atoms cross-link chains of SiO4 and MAlO4 tetrahedra (where M is a monovalent cation, typically Na+ or K+). The polymer formation rate is also influenced by alkali concentration. An analysis of DSC curves showed that the polymerisation reaction (starting after the dissolution process) increased with the NaOH solution concentration . Furthermore, various parameters, such as the material’s specific surface area, initial solids content, purity and reaction temperature, result in the creation of different structures and networks [6, 7, 8, 9, 10, 11, 12].
Finally, the materials obtained need to be cured at ambient (approximately 298 K) or elevated (313–353 K) temperatures depending on precursors, mix design, relative humidity, etc. Generally, heat curing accelerates the early strength development of materials ; however, curing at advanced elevated temperatures for a prolonged period causes specimen deterioration due to the destabilisation of the silicate–Si–O–Al–O–bond.
Due to the depletion of natural metakaolin resources and aspects related to reasonably managing post-process waste (with respect to environmental protection), research is being carried out on using other aluminosilicate raw materials such as fly ash, blast furnace slag, volcanic ash [13, 14, 15, 16, 17] and post-mining waste [18, 19, 20]. The different sources of materials used to produce AAMs and geopolymers generate a very common production problem; their diverse and unique chemical composition results in unpredictable product physical and mechanical properties. There are also no simple and inexpensive test methods for the unambiguous quality determination of such raw materials in terms of their suitability for geopolymerisation processes or for producing AAMs. Nevertheless, this article presents a new method using negative temperature coefficient (NTC) thermistors that allows measurement of the thermal effects occurring during the polymerisation process of materials from different sources and estimation of the mechanical properties of these materials.
Materials and methods
Three types of metakaolins (M1, M2 and M3), from two different commercial companies (Astra, Poland, and the Sedlecký Kaolin A.S., Czech Republic), were used for the test. For the M1 and M2 materials, the procedure for their production was similar and based on calcination in rotary kilns, while the calcination of M3 metakaolin took place in flash-type furnaces.
Metakaolin characterisation methods
In order to determine the morphology of materials originating from different sources (M1–M3), tests were carried out on a JEOL JSM 820 Scanning Electron Microscope (SEM) with an energy-dispersive spectrometer (EDS) IXRF Systems Model 500 Digital Processing. The metakaolin particles were dried to constant mass, attached to coal tape and covered with a thin layer of gold using the JEOL JEE-4X vacuum evaporator. The mineralogical phases present in the sample were determined using X-ray diffraction analyses (Rigaku X-ray diffractometer) applying the following settings: CuKα radiation, a reflective graphite monochromator, 45 kV lamp voltage, 200 A lamp current, 0.05 2θ step and 1 s counting time per step. For phase identification, measured values of the lattice parameters were analysed against the ICDD (International Centre for Diffraction Data 2016) catalogue in the Xrayan program. The laser diffraction particle size analysis of metakaolin was performed in accordance with ISO 13320 using an Analysette 22 (Fritsch). The specific surface areas were determined with the BET (Brunauer–Emmett–Teller) method of multilayered nitrogen (99.999% purity) adsorption using an Autosorb iQ. Material density was measured using a helium Pycnomatic ATC.
To produce geopolymer materials, metakaolin and a sand mixture (1:1) were used. A NaOH solution (10 M, Sigma-Aldrich) with an aqueous sodium silicate solution (R-145, molar ratio 2.5, density 1.45 g cm−3) was applied for the activation process. The solid-to-liquid ratio was 2. The metakaolin, sand and alkaline solution were mixed for 15 min to form a homogeneous paste in a low-speed mixing machine. The prepared mixtures (M1–M3) were moved into a cylindrical form of the same volume, in accordance with EN 13791.
Geopolymerisation process characterisation methods
A Vicat needle apparatus was used to determine the initial setting time of mortar geopolymer pastes during polymerisation in the furnace at 348 K, according to PN-EN 196-3:2016-12. The beginning of the setting process was determined by the penetration behaviour of a steel needle into a geopolymer paste sample at a 300 s frequency.
Geopolymer characterisation methods
Spectroscopy analyses of geopolymers after 28 days of curing were carried out using an FT-Raman Nicolet NXR 9650 (Spectro-Lab) equipped with a Nd:YAG3 + laser with a 1064 nm wavelength. For each material, a minimum of 3 spots were measured (50 µm).
Compressive strength tests were carried out according to EN 13791 on cylindrical forms (Ø = 50 mm; h = 100 mm). The test was performed after 28 days of curing in 50% humidity and about 294 K on a Matest (Italy) testing machine, model 3000 kN.
Results and discussion
Taking into consideration all the above results, one can assume that materials M1 and, to a lesser degree, M2 will be most useful in geopolymerisation processes. M2, due to the presence of residual kaolinite, was probably calcined at a low temperature or the calcination time was too short. At low temperatures (below 973 K), calcination produces residual kaolinite clays that make the product less reactive, whereas at higher temperatures (above 1123 K), re-crystallisation occurs, leading to the formation of non-pozzolanic materials such as spinel, silica and mullite . In contrast, M3 was probably calcined at temperatures that were much higher than required, as evidenced by the presence of a mullite phase. As was proven in a detailed thermodynamics study of the kaolinite–mullite transformation, which was made at 1223–1773 K, the most stable transformation in this temperature range was the one that yields mullite rather than γ-alumina .
Particle size distribution and density of three types of metakaolins M1, M2 and M3
2.544 ± 0.002
2.609 ± 0.002
2.696 ± 0.001
The oxides composition of three types of metakaolins M1, M2 and M3
Thermal effects recorded with the NTC thermistor system for three types of metakaolins M1, M2 and M3, and the subtraction between maximal thermal effects recorded during metakaolin geopolymerisation and in counterpart geopolymers
The time of max. thermal effects/s
End-set time of thermal effects/s
Subtraction between max. thermal effects during geopolymerisation and in geopolymer/ΔT/K
Compressive strength of three types of geopolymers G1, G2 and G3, after 28 days of curing
48.52 ± 3.06
56.34 ± 2.50
4.24 ± 0.62
Based on the results, the applicability of using an NTC thermistor probe system as a low-cost method for analysing phenomena occurring during geopolymerisation processes was confirmed. The presented method, together with other analyses, allowed unambiguous determination of the quality of raw metakaolin materials in terms of their suitability for geopolymerisation processes and producing AAMs. Even though all materials were referred to as metakaolin, the results of this study showed that calcined materials can significantly differentiate the geopolymerisation process and final physical and mechanical properties of AAMs. The NTC thermistor system allowed prediction of the mechanical properties of produced materials, e.g. their compressive strengths.
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