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

Synthesis of volcanic ash-based geopolymer mortars by fusion method: Effects of adding metakaolin to fused volcanic ash

International audienceThe present study aimed at improving the properties of geopolymer mortars obtained from volcanic ash as a source material. An alkali fusion process was used to promote the dissolution of Si and Al species from the volcanic ash and thus to enhance the reactivity of volcanic ash. Various amount of metakaolin (30%, 40%, 50% and 60% MK by weight) was used to consume the excess alkali needed for the fusion. The amount of amorphous phase was determined both in the volcanic ash and the fused volcanic ash and X-ray diffraction analysis was used to evaluate effect of the alkaline fusion method. Geopolymers were prepared by alkali activation of mixtures of powders of fused volcanic ash, various amount of metakaolin and river sand using a sodium silicate solution as activator. The geopolymer mortars were characterized by determination of setting time, linear shrinkage, scanning electron microscopy and compressive strength. The results of this study indicate that geopolymer mortars synthesized by the fusion method exhibit low setting time (7–15 min), low shrinkage (0–0.42%) and high compressive strength (41.5–68.8 MPa). This study showed that, by enhancing the reactivity of volcanic ash by alkali fusion and balancing the Na/Al ratio through the addition of metakaolin, all volcanic ashes can be recycled as an alternative source material for the production of geopolymers

Effect of the rate of calcination of kaolin on the properties of metakaolin-based geopolymers

Kaolin samples of the same mass were treated at 700 °C for the same duration of 30 min by varying the rate of calcination (1, 2.5, 5, 10, 15 and 20 °C/min) in order to obtain metakaolins which were used to produce geopolymers. Depending on the nature of each type of material, kaolin, metakaolins and geopolymers were characterized using thermal analysis, chemical analysis, XRD, FTIR, particle size distribution, specific surface area, bulk density, setting time and compressive strength. FTIR and XRD analyses showed that metakaolins except at 1 °C/min contained residual kaolinite whose quantity increased with the rate of calcination of kaolin and which influenced the characteristics of geopolymers. Thus as the rate of calcination of kaolin increased, the setting time increased (226 min (rate of 1 °C/min)–773 min (rate of 20 °C/min)) while the compressive strength reduced (49.4 MPa (rate of 1 °C/min)–20.8 MPa (rate of 20 °C/min)). From the obtained results the production of geopolymers having high compressive strength along with low setting time requires that the calcination of kaolin be carried out at a low rate

Reaction kinetics and microstructural characteristics of iron-rich-laterite-based phosphate binder

Due to their intriguing properties, phosphate cement, also known as chemically bonded phosphate cement, have been developed for the past decade. The need for sustainable building material with low cost and easy availability has prompted research into the use of laterites, which are abundant in Cameroon. The purpose of this research was to synthesize laterite-based phosphate cement (LPCs) with phosphoric acid as the sole activator. The parameters taken into consideration were the molar concentration of phosphoric acid, liquid to solid ratio and the type of solid precursors. Early reactivity of the LPCs was assessed using a semi-adiabatic calorimeter and the results showed an increase in the heat of reaction proportionally to the concentration of phosphoric acid, except for 10 M solutions where early reactivity was inhibited. The results of the 14-day compressive strengths of the studied LPCs were in the range of 23–98 MPa for LPE and 31–105 MPa for LPN, respectively depending on the specific formulation. For both laterite phosphate cement, molar concentrations of 6 and 8 M resulted in the optimum strength. The phase composition was determined using X-ray powder diffraction, and the amorphous and crystalline phases of the raw materials and phosphate cement were quantified using Rietveld measurement. The X-ray pattern of the laterite-phosphate cement revealed that the intensity peak of hematite decreases in the presence of phosphoric acid and led to the formation of an amorphous product, which is supported by phase analysis quantification using Rietveld refinement. The micrographs of LPCs revealed a dense matrix

Innovative porous ceramic matrices from inorganic polymer composites (IPCs): Microstructure and mechanical properties

The thermal performance of pegmatite-based geopolymer composites is investigated. Dense and compact matrix was prepared replacing metakaolin with pegmatite in the range of 70\u201385 wt% and activate with sodium hydroxide/sodium silicate solution in 1:1 vol ratio. The products of geopolymerization, cured at room temperature for 28 days, were heated at 100, 200, 400, 600, 800, 900, 1000 and 1100 \ub0C with 2 h soaking time. The high values of flexural strength (46\u201351 MPa) were observed at 1000 \ub0C as the consequences of low porosity (173 mm3/g) and water absorption (4.50\u20135.62%). The increase of the vitrification at 1100 \ub0C enhanced the liquid phase and develop porosities responsible for reduction of strength. The mechanical properties, microstructural evolution and pore size distribution were found to be influenced by the amount of fine powder of pegmatite (solid solution)