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[Display omitted] • Evaluation of activating solution influence on high temperature resistance in AAMs. • Higher availability of silicon and aluminum and better feasibility of the pastes. • Monitoring formation of Si-O-Si and Si-O-Ca networks. • Positive influence of the new crystallizations on mechanical resistance after 1000 °C. • Densification behavior of K sample contrary to the melting of that Na. Thermal performances at 1000 °C of volcanic ash-based alkali-activated materials by both sodium or potassium solutions have been tested. Once defined the feasibility for the potassium set, a multidisciplinary characterization including spectroscopic, thermal, mechanical, diffractometric before and after treatment and dilatometric analyses was applied. The formation kinetics of different networks was evaluated. A correlation between the high mechanical properties and the least amount of water was observed in non-treated samples. Treated samples at 1000 °C showed higher compressive strengths due to the crystallization of wollastonite, leucite and nepheline. Thermal stability was observed increased with the solid content in the potassium-based formulations. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ultra-dense geopolymer products were fabricated by the cold reaction sintering. • The compressive strength of the geopolymer hardened body showed maximum 425 MPa. • The relative density of the resultant geopolymer reached to maximum 98% • The key point was to use the hydrate as a raw material instead of aqueous solution. Geopolymers are attractive materials that contribute to carbon neutrality because they can significantly reduce CO 2 emissions compared to traditional Portland-based cement manufacturing. Geopolymers generally take several days to weeks to cure and have low mechanical properties. This paper proposes a new hardening method using a warm press, called "cold reaction sintering", in order to obtain a hardened body with high strength in a shorter time than conventional geopolymers. In general, to harden geopolymers use an alkaline solution such as NaOH. In this study, sodium metasilicate hydrate (Na 2 SiO 3 ·nH 2 O) was used instead of the alkaline solution. As a result of examining the composition, temperature and holding time, the maximum compressive strength value of 425 MPa was obtained when hardened at uniaxial pressure 100 MPa, 130° C for only 10 min with fly ash: Na 2 SiO 3 : Na 2 SiO 3 ·9H 2 O (68: 16: 16 wt%, respectively). In addition, dimensional change measurements during hardening, FTIR and NMR analysis were preformed to investigate and discuss the mechanism of geopolymer hardening. [ABSTRACT FROM AUTHOR]

• The use of sodium solutions permits to decrease the shrinkage. • Metakaolin has a lower impact on the shrinkage. • Wollastonite, glass fiber and sand reduce the shrinkage. • Normalized sand modifies the granular skeleton and the porosity. • The increase of liquid to solid ratio increases the shrinkage. The shrinkage of geopolymer materials is an important parameter to control for several applications. In this topic, this study aims to investigate the role of used precursors (metakaolin and alkaline solution) and different additives (sand, normalized sand, wollastonite, and glass fibers) on the shrinkage. For this, different samples were synthesized and stored at 20 °C and 100% RH. Then, shrinkage measurements were carried out at different consolidation times. The results have shown that the measured shrinkage is endogenous because it is not accompanied by mass variation and is only due to self-desiccation. It was also evidenced that the used alkaline solution controls the shrinkage. In fact, the increase of alkalinity or the use of sodium instead of potassium solutions permits to decrease the shrinkage values to less than 400 µm/m at 28 days. This result is explained by different depolymerization degrees of the solution inducing different geopolymerization rates and kinetics and affecting the pore distribution size. The metakaolin seems to have a lower impact on the shrinkage. The effect of additives was also studied. All additives reduce the shrinkage with different extents in the following order: normalized sand > sand > glass fiber > wollastonite. The normalized sand has shown the highest reduction of shrinkage (about 142 µm/m at 7 days) which can be explained by a modification of the granular skeleton and the porosity. A correlation was proven to show the increase of the shrinkage with the increase of liquid to solid ratio. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbonates persist after calcination at 600 °C. • Dolomite is partially decomposed at 750 °C. • A kaolinite gangue hinders the decomposition of dolomite at 750 °C. • Geopolymers based on high carbonates content exhibit lower shrinkage. • Crystallisation of different calcium and/or magnesium silicates was evidenced. In order to develop fire resistant geopolymer materials, it is necessary to understand the parameters controlling their thermal behavior such as the content of alkali or rare earth cations. This study highlights the effect of carbonate nature and amount on the thermal behavior of geopolymers in order to propose a model of thermal behaviour and predict shrinkage. For this, various mixtures of kaolin and calcite and/or dolomite with different percentages were calcined at 600 and 750 °C. The feasibility of consolidated materials based on these mixtures were evaluated and the thermal behavior of the obtained materials was investigated. The characterization of the mixtures revealed the persistence of carbonates after calcination at 600 °C and their partial decomposition after calcination at 750 °C. This fact was explained in the case of high dolomite content by the existence of kaolinite gangue hindering its decomposition. Consolidated materials were obtained from the different mixtures. After thermal treatment at 1000 °C, it was evidenced that the amorphous phase crystallizes to form in a major part leucite. At lower available alkaline earth species (Ca2+ and Mg2+) content, only wollastonite is formed and the shrinkage varies between 15 and 12%. However, at higher content, different calcium and magnesium silicates can be formed such calico-olivine and larnite in the case of calcite based materials and akermanite and merwinite for dolomite based materials. This fact leads to lower shrinkage value (from 6 to 8%). Thus, it is possible to control the thermal behavior and predict the shrinkage of geopolymer materials by controlling the available alkaline earth cations. [ABSTRACT FROM AUTHOR]

• Binary mixtures with pyroclastic deposits (Mt. Etna volcano) and metakaolin. • Alkaline activating solutions based on Na or K cation. • Polycondensation reaction monitoring on fresh pastes through In situ FTIR analysis. • Performance in terms of structure, compressive strengths and porosity. The huge availability of volcanic deposits in Mt. Etna volcano and the wide use of volcanic materials as aggregates in local architecture encouraged us to apply them in alkaline environment to produce geopolymeric binders to restore historical buildings of Mt. Etna area. Two clusters of samples for each volcanic precursor (volcanic ash and ghiara paleo-soil) were produced, using 10 and 20% wt. of metakaolin and two different alkaline solutions based on sodium and potassium respectively. A comparative study was made in relation to activators used, focusing on evaluation of polycondensation step (in ATR); chemical and structural characterization (FT-IR); TGA analysis, qualitative XRD analysis and mechanical compressive test after 7 and 21 curing days. ATR results confirmed the occurrence of geopolymerization for all samples; FT-IR analysis showed a higher content of carbonates in potassium-samples, decreasing at increasing of metakaolin content. A low weight lost was recorded in consolidated samples, as well as a partial dissolution of volcanic phases. Higher mechanical compressive strengths for K-samples, reaching 89 MPa as highest value after 21 days, differently to sodium-samples (51 MPa) were recorded, reflecting the porosity of the different specimens. [ABSTRACT FROM AUTHOR]