Search

The growing demand and use of geopolymer binders have been of high interest in recent times due to their sustainability and economic benefits. However, locally available materials must be used in the production of geopolymer binders in order to maximize their benefits. In this study, a laterite soil which is a locally available material in many parts of the world was used as the aluminosilicate precursor. In order to produce the geopolymer binders, the laterite soil was activated thermally through calcination and the resulting calcined laterite was activated with an alkali activator composed of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The influence of calcination temperature and the concentration of NaOH on the properties of the developed geopolymers was investigated. Calcination temperature used ranged between 550 °C to 750 °C and the NaOH concentrations used were 8 M and 10 M. The results from this study indicate that some locally available laterite soils can be utilized in the production of geopolymer binders if they undergo calcination. Increasing the calcination temperature from 550 °C to 750 °C resulted in the transformation of phases and an increase in the reactivity of the laterites, resulting in material with improved properties. The use of laterite calcined at 750 °C and activated with 8 M NaOH solution resulted to an increase in the 28 days compressive strength by 35.3 MPa compared to when laterite calcined at 550 °C was used. Increasing the concentration of the NaOH solution was also found to yield higher material performance. Microstructural investigations were also carried out to confirm the macrostructural properties.

This paper investigates the resistance of alkali-activated binder from volcanic ash (VA) + Municipal Solid Waste Incinerator Fly Ash (MSWI-FA) system exposed into sulphuric acid solution and artificial seawater at 56 days. The effect of these aggressive media on the physical, structural, mechanical and microstructural properties of alkali-activated mortars was discussed. The degradation was studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The decrease of compressive strength after 56 days was 9.10%, 22.08%, 32.5% in artificial seawater, 2% H2SO4 and 4% H2SO4, respectively. The loss of strength in both media is primarily due to the fact that the water molecules present in these media penetrated into the pore cavities between binders and aggregates, making them weak. In an acidic medium, it is also due to the decalcification and depolymerization reactions whereas in seawater medium, it is attributed to the movement of content ions and probable formation of Friedel’s salt. The results disclose that VA + MSWI-FA based alkali activated materials resist better in seawater than in sulphuric acid solution. Synergistic use of volcanic ash and MSWI-FA for construction materials through alkaline activation looks like the upcoming trend to valorize these wastes.

This work assessed the potential of synthetic acid aluminum phosphate solutions for the enhancement of the characteristics of metakaolin phosphate geopolymer binders obtained at room temperature. The main parameters dealt with are the concentration of the initial phosphoric acid solution (40 wt%, 50 wt%, and 60 wt%) and the molar ratio Al/P (1/3 and 1.4/3) of the synthesized acid aluminum phosphate solutions. The prepared solutions have different contents and types of mono aluminum phosphate compounds (MAP) and their reactivity is pH-dependent. This is because of the continuous neutralization of the protons due to the dissolution of aluminum hydroxide, which raises the pH and decreases the conductivity. Thus the acid aluminum phosphate solutions with molar ratio Al/P of 1/3 are the most reactive. They have significantly enhanced the compressive strength of the resulting phosphate geopolymer binders. But, when compared to phosphate geopolymer obtained with pure phosphoric acid of the same concentration, the highest rate of compressive strength improvement is recorded for acid aluminum phosphate solutions having an initial concentration of phosphoric acid of 40 wt%. Thus, the modification of the composition of the phosphoric acid with the addition of the appropriate amount of aluminum is beneficial for enhancing the characteristics of phosphate geopolymer binder at any age.

The aim of the present project deals with the use of the available agricultural by-product groundnut shell ash (GSA) as replacement (0, 10, 20 and 30 wt.%) to metakaolin for the synthesis of lightweight geopolymer. Two alkaline solutions (8 and 10M) have been used as activator solution. The resulting product was characterized using XRD, FT-IR, SEM-EDS analyses as well as physico-mechanical properties. The results revealed that the addition of the GSA literally decreased the compressive strength from 21 to 14 MPa (8M) and 30 to 15 MPa (10M). Bulk density was ranged between 1.21 and 1.10 g.cm− 3 and 1.51–1.20 g.cm− 3. Increasing of the groundnut shell ash content up to 30 wt.% leads to the high porosity (38.13 and 40% vol). SEM images revealed that the microstructure of reference geopolymer (MKGA0) appeared denser than those containing additive (MKGA20) justifying the deterioration in the mechanical strength. The end-products appeared as promising candidate for non-structural application in construction.

This work aimed to evaluate the role of the addition of blast furnace slag for the formation of reaction products and the strength development of volcanic ash-based phosphate geopolymer. Volcanic ash was replaced by 4 and 6 wt% of ground granulated blast furnace slag to accelerate the reaction kinetics. Then, the influence of boric acid for controlling the setting and kinetics reactions was also evaluated. The results demonstrated that the competition between the dissolution of boric acid and volcanic ash-slag particles is the main process controlling the setting and kinetics reaction. The addition of slag has significantly accelerated the initial and final setting times, whereas the addition of boric acid was beneficial for delaying the setting times. Consequently, it also enhanced the flowability of the paste. The compressive strength increased significantly with the addition of slag, and the optimum replaced rate was 4 wt% which resulted in 28 d strength of 27 MPa. Beyond that percentage, the strength was reduced because of the flash setting of the binder which does not allow a subsequent dissolution of the particles and their precipitation. The binders formed with the addition of slag and/or boric acid are beneficial for the improvement of the water stability of the volcanic ash-based phosphate geopolymer.

The physical, mechanical and microstructural properties of metakaolin/volcanic ash-based geopolymer composites were investigated, as well as the influence of the incorporation of a reactive silica from rice husk ash (RHA) in their performance. The geopolymer composites were designed by replacing metakaolin with volcanic ash, VA (10–30 wt%) and RHA (0–20 wt%). Physical and mechanical testing, FTIR, XRD, optical microscopic and SEM were used to characterize the geopolymer composites. The results showed that the mechanical strengths and physical properties were mostly affected by the VA content as well as RHA content where the optimal mechanical strengths (19.3 and 60.73 MPa for flexural and compressive strength, respectively) were obtained with the composite sample containing 20 wt% of VA and 10 wt% of RHA. This was due to the additive that enhances the compactness and microstructure of geopolymer matrices obtained and also the increase of SiO2/Al2O3 ratio with the addition of RHA. Synergistic use of metakaolin, volcanic ash, and rice husk ash for construction materials through alkaline activation looks like the upcoming trend to valorize these materials.

This research reports on the influence of particle size and distribution on the physical, mechanical and microstructural features of solid solutions (feldspathic materials) based inorganic polymer composites (IPCs). Both granite and pegmatite were ground to different degree of finess making four different granulometry with particles of 63, 80, 125 and 200 μm. The respective mixes receive 15 wt% of metakaolin and were activated with a well designed alkaline solution. Matrices obtained showed high compressive and flexural strengths in the range 101.2–131.3 MPa, and 29–35.5 MPa, respectively. It was observed that the optimum mechanical performance of these matrices can be achieved through a mix-design of different grades of granulometry. This was suggested by mechanism combining reactivity and particles packing. In fact, although it can be expected that the finess of the combination of the particles size under 63 μm might present the better reactivity, it is showing that the combination of fine, medium and coarse particles is efficient in achieving denser and tougher microstructure. Lower cumulative pore volume (17 mL g−1) of the composites based on pegmatite, value not far from that of natural stones, resulted in a higher impact resistance of 3.03 J. It was concluded that designing the feldspathic rock-based composites with high strengths appear as sustainable, low energy consumption and environmentally-friendly materials for the structural construction.

Five different alkali activators were prepared by mixing sodium hydroxide with molar concentrations of 8, 10, 12, 14 and 16M and sodium silicate, Na2SiO3 in volume ratio 1:1. The obtained alkaline activators were used for producing metakaolin/feldspathic rock-based geopolymer composites. These geopolymer composites were obtained by mixing different solid precursors of natural solid solutions (trachyte, pegmatite and granite) in the range of 70 to 85wt% with metakaolin and adding each alkaline solution at a liquid/solid mass ratio of 0.37. The pastes were cured at room temperature for 28days before testing. The influence of NaOH concentration on structural modifications was investigated using Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD). Water absorption for open porosity, Environment Scanning Electron Microscope (ESEM) and compression tests were adopted to observe the morphology and mechanical properties. The results indicate that the NaOH concentration of 12M was optimum for solid solution of trachyte with 98–106MPa due to content of higher amorphous phases compared to solid solutions of pegmatite (94–105MPa) or granite (101–108MPa) where the better results were achieved at 10M.

Natural pozzolans are tremendous source of reactive silica and alumina required for geopolymer synthesis as an alternative binder for ordinary Portland cement. They are available at a comparatively low cost and generate a low ecological footprint through their simple extraction. Following review paper summarizes the mechanical and durability characteristics as well as the microstructural properties of natural pozzolan based geopolymers and their potential as binding material. The microstructural characteristics are based on the advanced analytical techniques used so far to understand geopolymerization, the phase composition, mechanical and durability properties evolution. From this review is obvious that significant analytical techniques have been successfully carried out on fresh and hardened natural pozzolan based geopolymer obtained in alkaline medium, which helped to understand underlying chemical reaction and external factors affecting final properties. These analyses have shown that natural pozzolan based geopolymer has potential to be used as sustainable building materials. But, for its large scale utilisation there is need to upscale the manufacturing process at a pilot-plant in order to address the macroscopic challenges that may arise at that level.

The work aimed to classify a raw brown colour fly ash (RFA) into coarse grey fly ash (GFA) and fine brown fly ash (BFA) using a laboratory air classifier and to study the properties of grey fly ash...

Coal Bottom Ash (CBA) is one of the widely-produced residues of coal incineration in thermal power plants. The annual extraction of such a huge amount of waste needs a massive transfer field, which constitutes a threat to the environment. However, the utilization of such residue as a fine aggregate in concrete can be an environmentally-friendly opportunity. Hence, the main purpose of this paper is to evaluate the microstructure and mechanical properties of the usage of BA as a fine aggregate replacement in mortar mixtures. It is observed that BA has a reactive fraction with pozzolanic characteristics. This provides stronger mortar when a medium volume of the material (up to 40%) is substituted. In addition, the CBA based mixture can be classified as lightweight concrete. The compressive strength value of the specimen using 40% CBA at 56 days was 55 MPa, which reached the same strength limit of control mortar mixture.

Two different kaolins (Kao1, Kao2) containing about 11 and 28% of gibbsite were applied for producing metakaolin-phosphate-based geopolymer cements. These kaolins were transformed to metakaolins by calcination them at 700 °C. Gibbsite contained in these kaolins were transformed to γ-Al 2 O 3 during the calcination. The hardener used in this work was a phosphoric acid solution with molar concentration 10 M. It could be observed that the compressive strength of metakaolin-phosphate-based geopolymer cements (54.41/36.01 MPa) decreases in the course GMK1/GMK2. The both geopolymer cements indicate the formation of berlinite which is well dispersed in GMK1 and therefore reinforced the structure of the specimen. The micrograph images of phosphate-based geopolymer cements GMK2 is heterogeneous while the one of the GMK1 is homogeneous and more compact microstructure. The higher content of gibbsite in Kao2 affects negatively the mechanical and microstructural properties of metakaolin-phosphate-based geopolymer cements.

Sugar cane bagasse ash from SOSUCAM company in Cameroon was used to synthesize sodium waterglass as a new alternative hardener. The new hardener was used to prepare metakaolin-based geopolymer cements. The compressive strength of the resulting geopolymer cement cured at room temperature for 28 days was 32.9 MPa. Samples soaked for 28 days in water in parallel experiments revealed a strength of 31.4 MPa. This shows that exposure of water does not lead to any weakening. The value of water absorption was 7.1% in the water-soaked cements, indicating the presence of fewer pores and voids than in the dry cements. However, in SEM micrographs, the microstructure of geopolymer cement appears rather homogeneous and compact without any change by water soaking. It can thus be concluded that sodium waterglass from sugar cane bagasse ash can be used as an alternative hardener or reactive ingredient for producing geopolymer cement with a high degree of cross-linking geopolymer framework. The use of this low-value silica-rich waste for producing sodium waterglass results in environmental benefits including a significant reduction of CO 2 emission and energy consumption compared to the production of commercial sodium waterglass.

The progress achieved with the use of volcanic ash for geopolymer synthesis has been critically reviewed in this paper. This consists of an overview of mineralogy and chemistry of volcanic ash. The role of chemical composition and mineral contents of volcanic ash on their reactivity during geopolymerization reaction and, consequently, mechanical properties have been accessed. An attempt has been made to establish a relationship between synthesis factors and final properties. A critical assessment of some synthesis conditions has been addressed and some practical recommendations given along with suggestions of future works that have to be done. All this has shown that there are still many works such as durability tests (carbonation, freeze-thaw, resistance, etc.), life cycle analysis, etc. that need to be done in order to satisfy both suitability and sustainability criteria for a large-scale or industrial application.

Iron rich brown fly ash has been segregated into (a) coarse, and (b) fine size fractions using air classification. The suitability of fine iron-rich fly ash (CFA) has been evaluated for the synthesis of geopolymer. The behavior of CFA based geopolymer in relation to reactivity, NaOH concentration and sodium silicate to sodium hydroxide (S/H) ratio has been studied. The concentrations of NaOH (6, 8, 10 and 12 M) and S/H ratios (0.33, 1.0 and 3.0) were the variable parameters. Calorimetric studies revealed that the total heat release increased with NaOH concentration, but decreased with the increase in S/H ratio. The geopolymer synthesis was done by thermal curing at 60 °C for 24 h. It was found that the compressive strength increased with an increase in the NaOH concentration. The increase in S/H ratio from 0.33 to 1.00 resulted in a significant increase in the compressive strength, but a further increase from 1.00 to 3.00 resulted in a slight decrease in compressive strength. High strength geopolymer mortar up to 70 MPa at 28 days was obtained, when the mixture was formulated with 12 M NaOH and S/H ratio of 1.00. From SEM-EDS studies, an increase in the degree of reaction and formation of geopolymer gel was observed with increase in NaOH concentration. Also it revealed that the geopolymer gel obtained was ferro-sialate based polymers with chemical formula close to [Ca, Na]-[-Fe-O-] x -[-Si-O-Al-O-] 1−x -[-Si-O-] y , wherein “x” is less than or equal to 0.5, and “y” is a value between 0 and 25. Formation of the compact microstructure has resulted in improved strength properties of geopolymer.

The mechanical properties and durability of volcanic ash based geopolymer mortars synthesized at 27 °C and 80 °C were evaluated. The water absorption and apparent porosity increase up to 28 days, then become almost constant with time. The bulk density of specimens cured at 27 °C decreases continuously with time, whereas it doesn’t change considerably with time for specimens cured at 80 °C. Geopolymer specimens cured at 27 °C achieved the compressive strength of 20.5 MPa after 28 days and it no longer increases significantly with time. While a maximum strength of 37.9 MPa is reached at 90 days for specimens cured at 80 °C. Specimens performed well in wetting and drying conditions. Geopolymer specimens obtained at 27 °C developed a better resistance to 5% sulfuric acid than the ones cured at 80 °C. The microstructure of geopolymer specimens after acid exposure revealed the formation of gypsum as a secondary phase due to the reaction between sulfuric acid and calcium from geopolymer gel. The presence of Na-rich gel in specimen cured at 27 °C enhances its acid resistance. The pore structure and permeability are the key factors affecting the durability of volcanic ash based geopolymer mortars.

The present paper investigated the reactivity of volcanic ash in alkaline medium and reported the effect of synthesis conditions on microstructural and mechanical properties of volcanic ash-based geopolymers. The reactivity of volcanic ash was carried out by leaching it under different NaOH concentrations (8, 10, and 12 M) and temperatures (27, 60, and 80 °C), and chemical composition of the filtrate measured by ICP-OES. The influence of silica modulus (1.4, 1.5, and 1.66) and curing temperatures (27, 60, and 80 °C) on microstructural characteristics of the resulting geopolymers was assessed by Calorimetry test, XRD, FTIR, 27Al and 29Si MAS-NMR, TGA, and FESEM–EDX. The dissolution behavior of Si is influenced by NaOH concentration, while Al is more sensitive to temperature. The low amount of dissolved species is correlated with the low amount of heat released. XRD, FTIR, 27Al and 29Si MAS-NMR, and TGA have shown the evidence of changes occurred during geopolymerization of volcanic ash and the extent of reaction with varying silica modulus and curing temperature. FESEM and EDX have shown that geopolymers obtained are poly(ferro-sialate-siloxo), poly(ferro-sialate-disiloxo), and poly(ferro-sialate-multisiloxo) binder types with Ca2+, Mg2+, and Na+ as charge-balancing cations. These structures are irrespective of silica modulus and curing temperature. The curing temperature is the main factor affecting the early compressive strength.

This study aims at determining the setting and hardening behavior of volcanic ash-phosphate cement (VAPC) at room temperature using various concentrations of phosphoric acid. The properties of the fresh and hardened pastes of the VAPCs were assessed. The results reveal that iron is the main element taking part in the hardening reaction followed by aluminum, calcium, and magnesium, while silicon remained inert in phosphoric acid. The initial setting time is delayed when the concentration of phosphoric acid is increased (low pH). The low solubility of calcium and magnesium at high pH fostered the rapid formation of the binder, which hinders the further dissolution of the remaining particles. Whilst the high solubility of iron and aluminum at low pH slows down the rate of reaction but is beneficial for the extensive dissolution of reactive elements leading to the formation of a binder with high compressive strength. The microstructural analyses revealed that the binder is X-ray amorphous with a homogeneous distribution of all elements except silicon which is present in very few amounts.

This work aimed to investigate the possibility of using binder obtained from calcined laterite and phosphoric acid as a novel stabilizing for the manufacturing of compressed earth bricks (CEB). Various proportions of binder from up to 20 wt% were used to stabilize the soils. Fourier Transformed Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) thermal analysis (TGA-DSC), Scanning Electron Microscopy (SEM), water absorption and compressive strength tests were used to characterize the obtained products. The results showed that CEBs developed are highly resistant (similar to concrete) with dry compressive strength ranging from 17 MPa to 38 MPa and wet compressive strength from 6.3 MPa to 16.9 MPa. All the CEBs are also water-resistant and the water absorption was between 9.36-14.18 %. The increase of the binder content improves the performance of the compressed earth bricks. The new binder developed, which is based on the ferro-silico-aluminophosphate network, is effective for the stabilization of the lateritic soil for producing compressed earth bricks.

Porous composites with the principal class of porosity in the range of those presented in the literature as ideal for the moisture control capacity of building environment are described. In the course of the design of the matrices, micrometric pores are introduced to give to the pore systems a bi- or multimodal characters with the aim of improving the phases percolation during the course of desorption and make the moisture accumulation–desorption behavior of the porous composites essentially function of weather and environment. The porous composites present size of pores in the range $$0.001{-}1\, \upmu \hbox {m}$$ for the gel pores and peak centered at $$10\,\upmu \hbox {m}$$ for the micrometric pores which insure the matrices efficiency in moisture control capacity and durability. The results of cycles of moisture absorption–desorption in the course of various seasons of the year permit to identify the activities of gel pores meanly efficient in the extreme environment: absorption when the temperature is under $$11\,^{\circ }\hbox {C}$$ ; relative humidity is $$>\,60{\%}$$ and desorption when the temperature is above $$18\,^{\circ }\hbox {C}$$ . At ambient conditions, the pores more active are micrometric pores, while gel pores enter in activity only in the extreme environment conditions. The proposed porous geopolymer composites appeared promising candidates for the management of the moisture while improving the thermal insulation of residential building particularly in the regions with important fluctuation of weather. The use of geopolymerization process for the production of those porous composites, the choice of recycling industrial and municipal inorganic wastes appears ideal solution, environmentally friendly, eco-efficient and sustainable for the design of newly materials for the moisture control capacity in building environment.

The main objective of this work was to investigate the potential of using granite waste (GW) as raw material for geopolymer synthesis. The disposal of GW might cause many serious environmental issues. Due to the low reactivity of this aluminosilicate material in its natural state, GW was activated through alkali fusion with different amounts of sodium hydroxide. Fused materials obtained were then used to synthesize geopolymers at room temperature using sodium silicate as activator. The products were characterized using compressive strength, water resistance, XRD, IR spectroscopy and SEM/EDS analyzes. The results showed that the reactivity of GW is improved by alkali fusion method. The amount of reactive phase significantly increases with the amount of Na 2 O during the pre-activation process. Compressive strength results of geopolymer mortars are varying between 6.25 and 40.5 MPa depending on the amount of Na 2 O used during the alkali fusion process.

This paper looks at the possibility of using low reactive volcanic ash for making geopolymer cement. The research is directed towards (a) alteration of the reactivity of volcanic ash by mechanical activation, and (b) use of mechanically activated volcanic ash for the synthesis of a geopolymer. The effect of mechanical activation was quite visible on particle size distribution and the degree of crystallinity. The disappearance of some anorthite peaks and appearance of quartz peaks in volcanic ashes milled for 120 min demonstrate the change in mineralogy. The appearance of an intense carbonate band with milling time could be related to sorption of atmospheric CO2 on the grains surface during mechanical activation. The manifestation of mechanical activation of volcanic ash was prominent on (a) the reaction kinetics, (b) microstructural development, and (c) physico-mechanical properties of the geopolymer product. The rate constant and extent of geopolymerization increased with milling time but decreased with curing temperature. This decrease is in non-conformity with other alumina-silicate materials used for geopolymerization such as metakaolin and fly ash. FEG-SEM and EDAX results revealed that the geopolymer gel obtained is mixture of poly(ferro-sialate-siloxo) and poly(ferro-sialate-disiloxo) binder type with a formula close to [Ca,Na,K,Mg]–[–Fe–O–]x–[Si–O–Al–O–]1−x–[–Si–O–]y. The physico-mechanical properties changed significantly. Setting time reduced by >95% in samples milled for 60 min or more. The compressive strength which was negligible for 0–30 min milled volcanic ash reached 29–54 MPa after 60–120 min of milling time. Heat curing influenced the early age (7 and 28 days) compressive strength but the 90 day compressive strength of both ambient and heat cured samples were comparable.

Volcanic scoria (Z G , Z K ) and kaolin (MY 38 ) were applied for producing geopolymer mortars using two methods such as fusion and conventional methods. Kaolin was transformed (700 °C, 1°/min, 4 h) into metakaolin and used to consume the excess alkali needed for the fusion and also to increase the amount of amorphous phase in volcanic scoria. In order to investigate the effect of structural water contained in volcanic scoria on their reactivity, only volcanic scoria were used to produce geopolymer pastes using conventional method. X-ray pattern of volcanic scoria Z G shows the presence of muscovite and TG curves indicate that Z G and Z K content approximately 6 wt% and 0 wt% of water respectively. Geopolymer pastes from Z K (Z K G) obtained only with volcanic scoria were handled easily after 7 days at ambient temperature. Whereas geopolymer pastes from Z G (Z G G) at the same curing condition were handled after 24 h. The compressive strength of GZ K (geopolymer mortars from Z K using conventional method), GfZ K (geopolymer mortars from Z K using fusion method), G Z G (geopolymer mortars from Z G using conventional method), GfZ G (geopolymer mortars from Z G using fusion method), Z G G and Z K G are 25, 15, 39, 17, 33 and 10 MPa respectively. It can be concluded that the conventional method is a good process to synthesize geopolymers using volcanic scoria. The structural water from muscovite (clayey mineral) influences positively the properties of volcanic scoria based-geopolymers.

Three different kaolins (K1, K2, K3) from sources in Cameroon were applied for producing geopolymer cements. The kaolins differ significantly in their gibbsite and quartz contents. Thermal transformation (700 °C, 4 h) into their metakaolins MK1, MK2, MK3 shows the total loss of crystalline kaolinite and reveal the typically rather broad bump in the X-ray pattern. Gibbsite becomes dehydrated into γ and χ-Al 2 O 3 . Geopolymer cements (GP1, GP2, GP3) were obtained using freshly prepared sodium silicate solutions (NWG) with a ratio NWG/MK = 0.87. It could be observed that the initial (60/80/90 min) and final (90/140/160 min) setting time increases and their 28 day compressive strength (49/39/30 MPa) decreases in the course GP1/GP2/GP3. It is discussed that the higher content of quartz in K1 (up to 22 wt.%), compared to K2 (10 wt.%) and K3 (8 wt.%) promotes higher strength values and decreased setting times. Gibbsite was not present in K1, but up to 11 wt.% in K2 and 28 wt.% in K3, transformed in its dehydrated forms remains unreacted during geopolymerization. Therefore, the higher content of gibbsite in the kaolinite could be related to a lower strength. The reacted volumes and compositions of the geopolymer become almost the same in all three cases. A content of 30–50% of unreacted metakaolin was proved in all cases.

This paper studies the alkali activation of iron-rich aluminosilicates (laterites). Three activating alkaline solutions were prepared from sodium hydroxide solution (8, 10 and 12 M) with sodium silicate (Na2SiO3) in order to obtain the sodium silicate solutions with moduli of SiO2/Na2O equal to 0.75, 0.92 and 1.04; H2O/Na2O = 9.78, 10.45 and 12.04. The effects of above-defined solutions on the setting time, physical and microstructural properties of geopolymer binders from calcined laterite (600 °C), containing metakaolinite, as the sole binder at room temperature are reported and discussed. A laterite from Eloumden and one from Odza were used. The synthesized products were labelled GPEL(i=1.04, 0.92 and 0.75) and GPOD(i=1.04, 0.92 and 0.75) series. The dry compressive strength measured after 7 and 28 days were 4–10 and 10–18 MPa, respectively. It was typically found that the geopolymer paste from sodium hydroxide with molar concentration 12 M and the molar ratio SiO2/Na2O of the silicate solution equal to 0.75 produced the highest compressive strength (~ 18 MPa). These samples also have a denser matrix. The dry bulk densities of both series increased with the decrease of silica moduli and were in the range 2.31–2.43 and 2.32–2.52 g/cm3 and the water absorptions were in the range of 8.21–11.40% and 7.23–13.03% for geopolymers GPEL and GPOD series, respectively. The setting time decreased with increasing molarity of NaOH solutions. The physicochemical properties and the mineralogy of both iron-rich aluminosilicates were influenced by the silicate modulus of activating solutions and the best compositions were achieved with characteristic SiO2/Na2O = 0.75 and H2O/Na2O = 9.78.

Dicalcium phosphate dihydrate (DCPD) and hydroxyapatite (HAP) were prepared from oyster shell powder and phosphoric acid solution as raw materials using sol-gel process in order to study the influence of calcium phosphate compounds on the mechanical and microstructural properties of metakaolin-based geopolymer cements. The synthesized DCPD and HAP with molar ratio Ca/P equal to 1.00 and 1.65, respectively, were used as additives for replacement of metakaolin (0, 2, 4, 6, 8 and 10 wt%) and the resulting powders were used for producing geopolymer cements. The compressive strengths of geopolymer cements containing DCPD were in the range 40–48 MPa with increasing DCPD content, but when 10 wt% was used, the strength fell to 26 MPa. Similar results were obtained for HAP addition with a decrease of the compressive strength at 8 wt%. The ESEM images of geopolymers containing 4–10 wt% of HAP showed some non-reacted or partially reacted particles that do not well connect to the matrix. Samples of geopolymer with 4–8 wt% of DCPD present a lower number of unreacted particles and the matrix appeared denser than that of the control geopolymer cements. The use of calcium phosphate compounds such as brushite and hydroxyapatite led to the overall improvement in compressive strength and contribute to the densification of the structure of geopolymer cements. On the other hand, the addition of 4% of HAP and 10% of DCPD resulted in a large amount of hydroxyapatite and brushite, respectively in the systems. The matrix appear less dense indicating that a fraction of calcium phosphate addition at the aforementioned level was excessive.

Amorphous aluminium hydroxide and aluminium oxy-hydroxide were used as replacements in the synthesis of kaolin based-geopolymers. Physical and mechanical properties of geopolymers cured at 60 °C and exposed at elevated temperatures (300–1200 °C) were investigated. It was found that when cured at 60 °C, specimens with replacements exhibited low compressive strength as compared to those without replacement. Conversely, when heated as from 1100 °C, both compressive strength and volume stability of specimens with replacement were improved. Thus, specimens without replacement were morphologically damaged at 1150 °C whereas those with 10% by mass of amorphous aluminium hydroxide or with 30% by mass of aluminium oxy-hydroxide showed good volume stability at 1150 and 1200 °C respectively. Additionally, geopolymers with 30% by mass of aluminium oxy-hydroxide that were heated at 1100 °C showed compressive strength of 29.1 MPa whereas those with 10% by mass of amorphous aluminium hydroxide gave 60.2 MPa at 1150 °C. This was attributed to the improvement of compactness of specimens along with the formation of stable crystalline phases. Yet, heating geopolymers at 1200 °C led to partial dissolution of nepheline and carnegieite which generates closed pores hence decrease of compressive strength. Partial replacement of amorphous aluminium hydroxide or aluminium oxy-hydroxide in the synthesis of kaolin-based geopolymers allows both improvement of compressive strength and thermal stability of heated kaolin-based geopolymers.

The aim of this work is to valorize volcanic scoria by using them as starting material for geopolymers production. Nevertheless, volcanic scoria possesses low reactivity. Various amounts of metakaolin (5%, 10%, 15%, 20% and 25%) were added into two volcanic scoria (Z D and Z G ) in order to improve their reactivity. Two alkaline solutions were used to activate the aluminosilicate materials. The starting materials were characterized by particle size distribution, specific surface area, chemical and mineralogical composition. The geopolymers were characterized by the setting time, XRD, FTIR, SEM and compressive strength. The results indicated that volcanic scoria have low specific surface area (2.3 m 2 /g for Z D , 15.7 m 2 /g for Z G ), high average particle size ( d 50 = 13.08 μm and 10.68 μm for Z D and for Z G respectively) and low glass phase contents. Metakaolin have a smaller average particle size ( d 50 = 9.95 μm) and high specific surface (20.5 m 2 /g). The compressive strength of geopolymers increased in the ranges of 23–68 MPa and 39–64 MPa for geopolymers from Z D –MK and Z G –MK respectively. This study shows that despite the low reactivity of volcanic scoria it can still be used to synthesize geopolymers with good physical and mechanical properties.

This work reports the mechanical, durability and insulating properties of novel geopolymer composites made of iron-rich laterite and sugarcane bagasse fibers with sodium silicate as a hardener. The results showed that the addition of fibers was beneficial for improving the fracture behavior of iron-rich laterite based geopolymer from brittle to ductile. The 28 days compressive strength ranges from 50 MPa to 14 MPa with the content of fibers. The modulus of elasticity was improved by 50% with only 3 wt% of fibers. The geopolymer composites perform well after 20 wet-dry cycles with the improvement of its ductility. The 28 days thermal conductivity decreases from 0.77 W/mK to 0.55 W/mK with the fibers content.