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Geopolymers are inorganic aluminosilicate materials synthesized at room temperature by the activation of metakaolin with an alkaline solution. The comprehension of the structure and the parameters controlling the reactivity of alkaline solutions as well as their interaction with aluminosilicate species is primordial. In this study, based on Raman and FTIR data spectroscopy, wetting angle measurement was investigated as a new technique to evaluate the depolymerization degree of the alkaline solution. Different solutions with different alkali cations (K, Na, Li) and different Si/M molar ratios were studied. The results showed that the cation influences the formation of various reactive species (rings, chains, oligomers). Moreover, the increase of the depolymerization degree of the solution was evidenced by the increase of the wetting angle. The wetting angle of geopolymer mixtures based on different solutions in presence of the same metakaolin was also investigated. Consequently, for a given metakaolin, lower wetting angle value corresponds to the formation of a single geopolymer network. However, higher wetting angles are attributable to the existence of several networks. Thus, the wetting angle can be considered as a parameter to determine the reactivity of alkaline solutions and their interaction with metakaolin. [ABSTRACT FROM AUTHOR]

Geopolymers are amorphous aluminosilicate binders showing good working properties. Despite the large and growing existing literature about these materials, the geopolymerization mechanism and the parameters that can influence still need a better and deeper understanding. The current study explores the effect of alkaline solution and metakaolin reactivities on geopolymer formation. To do this, several geopolymer samples were synthesized from two alkaline solutions and two metakaolins. The structural evolution of the formed geopolymers was investigated using 27Al static NMR and FTIR spectroscopy during the formation. The mechanical properties were measured using compression tests. The NMR data show that the geopolymerization reaction involves the conversion of the Al (V), Al (VI) of metakaolin into Al (IV) under the effect of the alkaline solution. The dissolution and formation rates strongly depend on the reactivity of the used raw materials. Furthermore, a correlation was revealed between the amount of the formed Al (IV) determined from 27Al NMR, the shift values from FTIR and compressive strengths data. The results indicate that the use of a highly reactive alkaline solution and in a lesser extent a highly reactive metakaolin may enhance the formation of the geopolymer network and, consequently, improve the mechanical strengths of the final materials. [ABSTRACT FROM AUTHOR]

The valorization of local materials in public building construction in emerging countries is a necessity from a sustainable development perspective. In Cameroon, laterite is an abundant and easily accessible local resource that can be used for the development of light and insulating materials, which until now have been made from metakaolin. Moreover, since metakaolin is not easily accessible in Cameroon, it would be interesting to compare to a model mixture based on metakaolin and hematite. The objective of this study is related to the feasibility and characterization of geopolymer foams based on laterite or on a model mixture (metakaolin-hematite) in different drastic conditions, such as fire resistance and freeze‒thaw cycles. The geopolymer foams were synthesized from a calcined laterite or metakaolin-hematite mixture, an alkaline silicate solution, a surfactant and metallic aluminum powder and then placed in an oven at 70 °C for 24 h. Structural data (X-ray diffraction) as well as determination of thermal conductivity and mechanical strength data of the foams were evaluated. The results show that the laterite and metakaolin-hematite mixture allows the elaboration of geopolymer foam, which is characterized by a low thermal conductivity (90 mW/(m. K)) and can be used as insulating materials. The freeze‒thaw treatment does not affect the thermal and mechanical properties. The treatment at 1175 °C slightly improves the mechanical strength and the thermal conductivity, leading to a good application of these materials. [ABSTRACT FROM AUTHOR]

Knowledge of alkaline silicate solutions is crucial in order to optimize geopolymer properties. Geopolymers are new binders resulting from the activation of an aluminosilicate by an alkaline solution. It is well established that the solution reactivity strongly affects the geopolymerization and therefore the geopolymer working properties. As a consequence, an evaluation of the reactivity degree of alkaline silicate solutions prior synthesis is of the utmost interest. However, the determination of the solution reactivity is currently tedious, and for geopolymer commercialization, it would be necessary to find an easy way to determine it. Therefore, Raman spectroscopy, combined with chemometric techniques, is proposed as a solution to easily determine the alkaline silicate solution reactivity. To conduct this investigation, 65 silicate solutions were characterized by Raman spectroscopy, and reference values of their reactivity degree were determined. Finally, principal component analysis and partial least squares regression were performed to build a statistical model able to predict the alkaline silicate solution reactivity from Raman spectra. [ABSTRACT FROM AUTHOR]

The valorization of oyster or mussel shells toward metakaolin-based geomaterials was investigated. The aim is to better understand the feasibility and the reaction involved during the incorporation of shells (natural or calcined) into a metakaolin-based alkaline solution for potential building or contaminants sorbents applications. Mixtures are realized by (i) the substitution of metakaolin (MK) source by shells raw material (from 0 to 100% mass) or (ii) addition up to 50% mass. Substitution of MK by shells was conducted without material structure loss up to 40% and 30% addition, whether the type and treatment of shells used. In each addition or substitution, the results highlight the duality between both reactions i.e. polycondensation versus hydration. (i) using natural shells, only polycondensation reaction occurs due to the unreactivity of shells (interaction of shells with matrix by surface binding: role of aggregates or fillers); (ii) using calcined shells, coexistence of polycondensation in first followed (or stopped) by hydration reaction: presence of portlandite and C(A)SH. Then the results traduce the role of MK (Al and Si source) reactivity and calcium form (carbonate versus lime; reactivity) in MK-based alkali activated materials. The geochemical leaching approach can be used as a sensitive probe of the structural network. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate and characterize geopolymers based on laterite or metakaolin-hematite mixture model. The model has been used to reproduce laterite in areas where laterite is not available. The geopolymers were synthetized using (i) calcined laterite and (ii) a mixture of metakaolin and hematite as solid precursors and two alkaline solutions of SiO 2 /Na 2 O and SiO 2 /K 2 O equal to 1.3. The obtained results reveal that both laterite and metakaolin/hematite-based geopolymers display efficient mechanical properties, regardless of the alkaline activating solution used. Thermal conductivity and compressive strength vary almost linearly, from 0.7 to 1 W/(m.K) and from 20 to 70 MPa, respectively: Na-activated laterite (NaLc) > Na-activated metakaolin + hematite (NaMF) > K-activated metakaolin + hematite (KMF) > K- activated laterite (KLc). This order can be explained by structural modifications after thermal treatment at 1175 °C. In the case of NaLc and NaMF, the samples display weak porosity due to higher crystallinity of the powder. In contrast, KMF and KLc are more heterogeneous with higher porosity. [Display omitted] • It is possible to synthetize geopolymer based metakaolin-hematite model. • Sodium-geopolymer has higher compressive strength and lower porosity than potassium-geopolymer. • Sodium-geopolymer-laterite has higher properties than sodium-geopolymer-metakaolin-hematite. [ABSTRACT FROM AUTHOR]

The study of bitumen substitution in the field of road construction is necessary to overcome the effects of limited oil resources and economic problems. The elaboration of new materials based on geopolymer binders and aggregates was investigated in this work. Two series of formulations of geopolymer binders based on sodium or potassium were prepared with the incorporation of granite or diorite. The formulation of these binder was investigated by the study of (i) the effect of the nature of the alkaline solution by modify the pH value with base and (ii) the mass percentage of the incorporated aggregates. The time evolution study shows different aspects of the synthesized materials, which depend essentially on the composition of the activating solutions and the nature of the alkaline solutions. The formulations using these ratios (Si/Na=1.55; Si/K=1.94 and 2.96) appear the best. Moreover, it was evidenced that materials based on the potassium alkaline solution present a consolidated aspect faster than the materials based on the sodium alkaline solution. Additionally, it is possible to incorporate diorite or granite until 60 w% with some formulations. These various formulations are promising to be use not as a substitute of bitumen but in addition between bitumen and ground. [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]

• 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]