High strength metahalloysite based geopolymer

Abstract

The unique properties of aluminosilicates had made them valuable in the wide range of industrial applications. One branch of application represents building materials. A significant factor of the level of the quality of engineering properties of building materials anyhow is the water/binder ratio. It is well known that its decreasing value effectively increases strength and quality of other engineering properties of the material. Due to the accompanying effect impairing workability of the mixture for processing the pressure compaction is needed.

The subject of the paper are the results of the study of the properties of metahalloysite based geopolymer prepared under the use of the combination of very low water/metahalloysite ratio (0.08), pressure compaction of the fresh mixture by applying an uniaxial compressive stress of 300 MPa and alkali activation. The effect of preparation conditions was systematically studied by thermal analysis (DTA, GTA), mercury intrusion porosimetry, scanning microscopy and compressive strength estimation of the geopolymer. The used metahalloysite was a product of calcination of the source material at 650 °C 4 h. It was an amorphous material showing an increased thermodynamic unstability and herewith an increased reactivity in comparison with the unheated solid.

The metahalloysite based geopolymer pressure compacted paste reached after 24 h of the hardening compressive strength of 76.2 MPa whereas the reference paste only 0.03 MPa. It represents 2540 times increase in behalf of the pressure compacted paste. High compressive strength was evidently the consequence of the found high homogeneous and dense pore structure of the pressure compacted paste.

The regression analysis shown for the geopolymer binder systems a polynomial empirical equations as an appropriate configuration. For the alkali activated slag and Portland cement systems the exponential configuration was suitable. The cause of the difference may be apparent phase composition. It was pronouncedly amorphous structure of the geopolymer binder systems opposite to the combined amorphous and crystalline structures distinctive for alkali-activated slag and Portland cement binder systems. Then the resulted different destruction mechanism under the loading, at the strength test of the material.

For the elucidation of the causality more detailed research is needed.

Introduction

Geopolymers are a class of aluminosilicates formed by reaction of alkali solution with dehydroxylated clay or analogous solid waste materials under high alkaline conditions. The hardening mechanism involves the chemical reaction of geopolymeric precursors, such as alumino-silicate oxides, with alkali polysilicates yielding polymeric Si–O–Al bonds. The result of the hardening mechanism is a three dimensional zeolitic framework unlike traditional hydraulic binders [1], [2], [3], [4], [5], [6], [7].

This circumstance is a cause of significant differences in the quality and variety of the engineering properties of the composites based on geopolymer and current cements. Geopolymers can gain strength more rapidly then Ordinary Portland cement. The dependence of the engineering properties of concretes and other cement composites on the value of water/cement ratio (w/c) is very well known. Also the fact that their strengths, durability and the quality of other engineering properties are increased when the w/c values are decreased [1], [2], [3]. A significant factor limiting the use of this positive effects is the decrease of the workability of the fresh composite mixture and the decrease of the quality of the engineering properties of the hardened composite. A limited solution enables the use of the plasticizers or the superplasticizers. An illustration of the influence and significance of the w/c ratio used shows Fig. 1.

A very attractive solution represents the combination of super low w/c ratios, under w/c 0.1 resulting in a submicroscopic pore structure and an adequate high quality of the engineering properties of the composite [4], [5], [6], [7], [8], [9], [10]. It appears that the low porosity cement composites have a great potential of reconsideration and modification of composition and structure [11]. All mentioned dependences and effects are common also to geopolymer systems distinguishing from the related cement materials only by the specific properties of geopolymer as a binder. The significance of the water: cement ratio for the geopolymer systems is reported by Barbosa et al. [12]. According to the authors the effects of the changes of water:cement ratio on the properties of the geopolymers are the same like at cement materials. Similar is the effect of ambient temperature increase showing the setting time decrease as a consequence of increase of rapidity of polymerization and the hardening more rapidly than OPC cement. But only the temperature increase to 90 °C seems to be optimal [13], [14], [15], [16], [17]. The aim of the presented work was to study on the possibilities of the use of effects of the combination of very low w/c ratio and pressure compaction in the geopolymer cement systems. As a model the alkali activated metahalloysite system was used. Metahalloysite used was a product of its heating. Halloysite is an end member of kaolin group of clay minerals and it is composed of kaolin like kaolinite. The properties of the heated halloysite may vary due to the influence of mineral composition which is complicated by presence of minor elements and impurities in the clay lattice and by nonclay minerals [18], [19].

Progressive heating of clays leads to the removal of free, adsorbed and structural water, which is followed by the formation of new minerals. In open atmospheric conditions include reversible dehydration of loosely bound water when dry-heated between 100 °C and 300 °C, irreversible removal of interlayer water and loss of swell potential at temperature above 400 °C, dehydroxylation or release of hydroxide groups from the crystal structure between 500 °C and 1000 °C, silicate recrystallization and the formation of new minerals above 800 °C [20], [21], [22].

Some physico-chemical properties of clays such as swelling, plasticity, cohesion, compressibility, strength, cation exchange capacity, particle size, adsorptive properties, pore structure, surface acidity, and catalytic activity as well mineralogy are greatly affected by thermal treatment. The investigation of these effects and their influence on the properties of the clays has a great importance [23], [24], [25].

The thermal transformation of clays is dependent on the heating parameters such as temperature, heating rate and time, as well as, cooling parameters significantly influence the dehydroxilation process. The major quantitative criterion for evaluating the performance by thermal analysis is a degree of dehydroxilation. In dehydroxilation region the activity of heated material increases along with the temperature but decreases abruptly in the “spinel” region. The activities can be measured by the compressive strength of prepared geopolymer.

The behavior of geopolymers at high temperatures exposure is significantly dependent on the chemical composition of the binder itself, especially on the type of alkali ion, and on the type of high-temperature phases built up [26]. The type of charge-compensating cation, Si/Al ratio and crystal structure themselves are the significant factors also [27], [28].

The metakaolin geopolymer suffered strength loss exposed to temperatures up to 800 °C. It is interesting that fly ash geopolymer gained strength after the same high temperature exposure. Results show a strength drop of 34% in metakaolin geopolymer while fly ash-synthesized counterparts had strength increase of 6% after elevated temperature exposures. According to the authors the cause is the different pore structure character of the materials. The metakaolin geopolymer pores are predominantly composed of mesopores whereas fly ash geopolymer pores contain higher proportion of micropores than metakaolin geopolymer [29].

While numerous studies on the calcination of kaolinite have been conducted much less attention has been paid to the thermal transformation of halloysite, nevertheless the heating has undoubtedly a significant influence on the properties of halloysite [30], [31]. The situation is evidently a consequence of the fact that the reaction mechanism under heating of halloysite has been postulated to be analogous to that of kaolinite because halloysite is structurally and chemically similar to kaolinite. But halloysite possesses a tabular morphology in contrast to the plate-like nature of kaolinite. The textural and morphological characteristics of heated halloysite, accordingly, should be different from those of kaolinite [18], [30], [31]. It is evident that heating causes extensive changes in a broad spectrum of the properties also of halloysite, offering potential uses from the production of versatile ceramic and novel clay mineral nanocomposites including construction materials [12], [32], [33].

The significant fact is that the burning of clays produces material, that harden when mixed with lime and water. It is due to the fact that clay gains a pozzolanic activity when burned at temperature between 600 °C and 900 °C. This artificial pozzolana is mostly composed of silica and alumina [34].

Thermally treated or calcined clays display a higher reactivity during geopolymerization than non-calcined material. Calcification activates material by changing their crystalline structure into amorphous structure to store extra energy and increase its activity and increasing compressive strength [35]. The calcining temperature of the clay affects the pozzolanic reactive state. Calcination also affects the amount of Al and Si released from source material. Higher the temperature used during calcination process, shorter the time is needed to obtain the material that gives the maximum compressive strength [36], [37].