Reinventing the structural fired clayey bricks through the geopolymerisation of laterites

Fired clayey products have been successfully used as structural materials for many engineering applications as building and construction all around the world. In the tropical area, however, the most available raw clayey materials are laterites (kaolinite with associated iron minerals). The kaolinite present in the laterites based concretes is amorphous or metastable prompt to be activated with alkaline solution. In this work, the results of the investigations regarding the geopolymerisation of laterites are presented. It was found that in the presence of amorphous silica, the iron minerals of laterites reacts to form low temperature iron silicates with particularly good mechanical properties (15-35 MPa) as the results of the combination of polysialates, ferrosialates and ferrosilicates. Composites obtained can be valorized as products of substitution of structural fired clayey products

Materie prime di origine naturale nel processo di geopolimerizzazione

Geopolymers are inorganic materials prepared via a room temperature treatment (25-120?C) with alkali activation. The main applications of these materials is in the field of building materials, recycle and inertization of waste, restoration. The production and final product is optimised by a careful selection of starting materials, i.e. aluminosilicate powders. In this paper are presented aluminosilicates from natural source, as an example kaolin, clays or volcanic ash. Metakaolin is the most reactive due to its amorphous structure and aluminium coordination; its reactivity is also influenced by its mineralogical origin, morphology of grains, and calcinations method. Volcanic ash present lower reactivity and the better consolidation temperature to obtain mechanically strong materials appears to be around 400?CI materiali geopolimerici sono materiali inorganici ottenuti con consolidamento a bassa temperatura (25-120?C) mediante una reazione chimica in ambiente alcalino. Sono utilizzati per applicazioni ad alta temperatura e strutturali in edilizia, per il riciclo e /o l?inertizzazione di scorie industriali, per il restauro. L?opportuna selezione di materie prime alluminosilicatiche ? alla base dell?ottimizzazione del ciclo produttivo e del prodotto finale. Vengono presentate materie prime di origine naturale, quali il metacaolino, derivato dalla calcinazione del caolino, e le ceneri vulcaniche. Il metacaolino ? la materia prima pi? reattiva grazie alla struttura amorfa e alla coordinazione dell?alluminio: sulla reattivit? influiscono sia la morfologia e le caratteristiche del caolino di origine, sia la metodologia di calcinazione. Le ceneri vulcaniche hanno una minore reattivit? che pu? esser ovviata tramite un consolidamento a circa 400?C portando a prodotti geopolimerici con buone prestazioni meccanich

Design of inorganic polymer mortar from ferricalsialic and calsialic slags for indoor humidity control

Amorphous silica and alumina of metakaolin are used to adjust the bulk composition of black (BSS) and white (WSS) steel slag to prepare alkali-activated (AAS) mortars consolidated at room temperature. The mix-design also includes also the addition of semi-crystalline matrix of river sand to the metakaolin/steel powders. The results showed that high strength of the steel slag/metakaolin mortars can be achieved with the geopolymerization process which was particularly affected by the metallic iron present into the steel slag. The corrosion of the Fe particles was found to be responsible for porosity in the range between 0.1 and 10 μm. This class of porosity dominated (~31 vol %) the pore network of B compared to W samples (~16 vol %). However, W series remained with the higher cumulative pore volume (0.18 mL/g) compared to B series, with 0.12 mL/g. The maximum flexural strength was 6.89 and 8.51 MPa for the W and B series, respectively. The fracture surface ESEM observations of AAS showed large grains covered with the matrix assuming the good adhesion bonds between the gel-like geopolymer structure mixed with alkali activated steel slag and the residual unreacted portion. The correlation between the metallic iron/Fe oxides content, the pore network development, the strength and microstructure suggested the steel slag's significant action into the strengthening mechanism of consolidated products. These products also showed an interesting adsorption/desorption behavior that suggested their use as coating material to maintain the stability of the indoor relative humidity

Binder chemistry – Low-calcium alkali-activated materials

Early developments in the developments of low-calcium (including calcium-free) alkali-activated binders were led by the work of Davidovits in France, as noted in Chap. 2. These materials were initially envisaged as a fire-resistant replacement for organic polymeric materials, with identification of potential applications as a possible binder for concrete production following relatively soon afterwards [1]. However, developments in the area of concrete production soon led back to more calcium-rich systems, including the hybrid Pyrament binders, leaving work based on the use of low-calcium systems predominantly aimed at high-temperature applications and other scenarios where the ceramic-like nature of clay-derived alkali-activated pastes was beneficial. Early work in this area was conducted with an almost solely commercial focus, meaning that little scientific information was made available with the exception of a conference proceedings volume [2], several scattered publications in other conferences, and an initial journal publication [3]. Academic research into the alkaline activation of metakaolin to form a binder material led to initial publications in the early 1990s [4, 5], and the first description of the formation of a strong and durable binder by alkaline activation of fly ash was published by Wastiels et al. [6-8]. With ongoing developments in fly ash activation, which offers more favourable rheology than is observed in clay-based binders, interest in low-calcium AAM concrete production was reignited, and work since that time in industry and academia has led to the development of a number of different approaches to this problem. A review of the binder chemistry of low-calcium AAM binder systems published in 2007 [9] has since received more than 350 citations in the scientific literature, indicating the high current level of interest in understanding and utilisation of these types of gels

Investigation of volcanic ash based geopolymers as potential building materials

Volcanic ash powders from Etna (Italy) and Cameroon were used as the principal source of aluminosilicate to produce geopolymers with the potential for making building products. The volcanic ash was ball milled and reacted with concentrated alkaline solutions for polymerisation and subsequent curing at 75-400 °C for 12-48 h. It was found that the gel was more viscous than a similar gel formed from metakaolin. Geopolymers made from both ashes had bulk densities of 1.7-2.0 g/cm3 and water absorption values of 20-25 %. Their compressive strength values were 25-35 MPa and the bi-axial four-point flexura! strength values ranged from 14-20 MPa. These values increased by 20 % when cured for 21 d after 90 d storage. It was also found that by curing at 200-400 °C the mechanical properties increased. Scanning electron micrographs showed that with thermal curing microcrystalline phases were present along with undissolved crystalline phases. These phases remained bound to the matrix and acted as a filler for strengthening the materials. The Ca, Mg and Fe present as impurities in the volcanic ash formed some of these crystalline phases and did not form any deleterious hydroxide or carbonate phases

Low-temperature alkaline activation of feldspathic solid solutions: Development of high strength geopolymers

Most of the natural solid solutions, as a result of the history of their formation and crystallization, present a fraction of amorphous or metastable materials that may easily be dissolved or activated in alkaline media. In this work, trachyte, granite, pegmatite and sand for comparison are used as principal solid precursors for the design of high strength geopolymers. The particularity of the solid-solution based geopolymers is the high fraction of crystalline phases incongruently dissolved that may react essentially at the surface, thus developing very resistant bonds. While working with 100 wt% of solid solution is almost unrealistic for the production of geopolymers, it was found that 15 to 30 wt% of metakaolin in replacement of the solid-solution powder conducts to low porosity (10 vol.%), high flexural strength (20-30 MPa) and compact microstructure. Preliminary resonance-based mechanical tests showed that the elastic modulus of the investigated samples ranged between 11-15 GPa, as also confirmed by instrumented micro-indentations. It was concluded that a high strength and durable matrix are a result of chemico-mechanical equilibrium of phases contained within the composites including the pore volume and pore-size distribution, which are significant for the life cycle of geopolymer composites

Investigation of the relationship between the condensed structure and the chemically bonded water content in the network of geopolymer cements

The main objective of this work was to investigate the relationship between the condensed structure and the chemically bonded water content in the metakaolin-based geopolymer network. The kaolinite clay used in this work as an aluminosilicate source was transformed to metakaolin by calcination at 700 °C. The powder of the waste glass and the silica fume were used as silica sources for the synthesis of the hardeners. The obtained hardeners were characterized by infrared spectroscopy and MAS-NMR 29Si. The metakaolin and the hardeners were used for producing geopolymers cements. The synthesized products were characterized by X-ray diffractometry, infrared spectroscopy, mercury intrusion porosimetry, scanning electron microscopy, MAS-NMR 29Si and 27Al, thermal analyses (TG and DSC) and compressive strength. The results show that the compressive strength of geopolymer cements using hardener from silica fume and the one from waste glass are 62 and 26 MPa, respectively. The microstructure (SEM observations) geopolymer cements obtained using hardener from silica fume are homogeneous, compact and dense with an average pore diameter around 10 nm. Whereas, the one obtained using hardener from waste glass are heterogeneous and contains larger pores (170 nm). MAS-NMR 29Si and 27Al results show that the specimen obtained using hardener from the silica fume contains more aluminum in four-fold coordination in its network than waste glass geopolymer, GWG. This indicates a higher degree of crosslinking of poly(sialate-siloxo) chains which could lead to a smaller pore sizes and a higher water uptake in the structure of the sample. The amount of chemically bonded water contained in the network of geopolymer cements using hardeners from waste glass and silica fume were 6.82 and 11.23%, respectively, as determined from weigth loss in the range 100-300 °C. All these results indicate that the higher content of chemically bonded water in the network of geopolymer obtained using hardener from silica fume is related to the much smaller average pore size diameter and the hydrophilic character of aluminum, which reveals obviously better mechanical and microstructural properties of the specimen. This could indicate here a higher degree of condensation using silica fume based hardeners for geopolymerisation. Please click Additional Files below to see the full abstract

Transformation of the geopolymer gels to crystalline bonds in cold-setting refractory concretes: Pore evolution, mechanical strength and microstructure

Two K2O-MgO-Al2O3-SiO2 based geopolymer gels with bulk chemical composition corresponding to cordierite (Co) and 1:1 mullite-cordierite (MuCo) were successfully transformed to crystalline bonds in high temperature service of cold-setting made refractory concretes. Kyanite aggregates changed the flexural strength of the gels from11 to 28 MPa due to the development of good adhesive bonds. Under thermal cycles, up to 1250 °C, the cumulative pore volume remained at 0.09 mL/g, as fromthe absence of important densification/shrinkage. However, the behavior of the cumulative pore volume curves changed from that of a matrix with a wide range of distribution of pore sizes to that, of matrix, consisting of relatively coarse grains. The latter exhibits a rise at 10 μm as void spaces created around the contact points among the coarse kyanite grains and that at 0.054 μm as poreswithin the crystalline phases (cordierite, kalsilite, leucite,mullite, enstatite) formed. Themicrostructural observations confirmed the transformation of gel pores (size around 0.01 μm) to interparticle and intergranular pores due to the crystallization. The flexural strength of refractory concretes increased from28MPa to 40MPa in agreement with the increase in the elastic modulus from 9 to 30 GPa. The crystallization was enhanced by the MgO content (being important in Co compared to MuCo) and the kyanite concentration as particles of kyanite effectively acted as phase separation and nucleation site

Geopolymer Development by Powders of Metakaolin and Wastes in Thailand

Geopolymer has been developed as an alternative material to Portland cement. Geopolymer is based on the polymerization of alkaline activation and oxide of silicon and aluminium. These oxides can be found in many pozzolanic materials such as metakaolin and the wastes from industries and agricultures in Thailand, e.g., fly ash, bagasse ash and rice husk ash. Pozzolanic materials were selected as source materials for making geopolymers into 4 different types. Sodium hydroxide concentration of 10 Molar (10MNaOH) and sodium silicate (Na2SiO3) solutions were used as alkaline activators by the mass ratio of Na2SiO3/NaOH at 1.5. The mixtures were cast in 25×25×25 mm. cubes. After casting, the geopolymers were cured at 80๐C for 24 hrs. in an oven and then at room temperature for 7 days. The pozzolanic materials effects, the Si/Al molar ratio and the Na/Al molar ratio were studied and characterized. An X-ray fluorescence (XRF) was chosen to determine the percentages of silica and alumina in order to verify the proper ratio of the fly ash, Rice husk ash, Bagasse ash and Metakaolin.The study also included the impact on mechanical and physical properties such as compressive strength, water absorption, density and porosity

Cold-setting refractory composites from cordierite and mullite-cordierite design with geopolymer paste as binder: Thermal behavior and phase evolution

Cordierite and mullite-cordierite based geopolymer binders were prepared as refractory bonds to embed coarse kyanite aggregates. Alkali activation was used to consolidate the refractory at room temperature and generate a reactive interface between the matrix and the aggregates. The flexural strength at room temperature of consolidated materials was 11 MPa without kyanite addition, whilst it reached 28 MPa when kyanite was added. The latter showed thermal expansion near to zero up to 500 \ub0C, followed by an expansion of 3c0.5% up to 1000 \ub0C, then a decrease between 1000 and 1050 \ub0C, concluding with a constant value of expansion <0.5% up to 1250 \ub0C. The increase in expansion was found to be linked to the transformation of the geopolymer into ceramic bond with the formation of crystalline phases, while the decrease was apparently related to the liquid phase sintering. The stability of mullite, cordierite and leucite formed up to 1250 \ub0C was responsible for the constant expansion observed up to this temperature. The mechanical properties, the phase evolution and the thermal behavior of the K2O-MgO-Al2O3-SiO2 bonds, together with the refractory behavior of kyanite aggregates, were found suitable for the design of sustainable cold-setting refractory composites