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This study explored a novel cementitious material developed from subway tunnel muck (STM) intended for ecological concrete (EC) preparation. The effects of three alkaline activators (NaOH, KOH, and CaO) on the properties of the cementitious materials were systematically examined. The results indicated that NaOH exhibited the most effective activation performance, followed by KOH, with CaO being the least effective. The NaOH-activated materials exhibited the highest compressive strength (reaching up to 12.15 MPa), the densest microstructure (characterized by the lowest porosity and smallest average pore size), the most substantial gel formation (evidenced by the highest mass loss in thermogravimetric analysis), and the optimal gel structure (indicated by the pronounced peak sharpening in Fourier transform infrared spectroscopy) after a 28-day curing period. Moreover, the crystallization of potassium salts under KOH activation detrimentally impacted the microstructure of KOH-activated materials. To balance the need for structural strength and nutrient provision, NaOH + KOH-activated materials were selected for the preparation of EC. Notably, the application of NaOH + KOH-activated materials resulted in a significant increase in K+ concentration in the soil layer, compared to common soil. Furthermore, NaOH + KOH-activated materials exhibited a slow-release effect, thereby offering sustained nutrient support conducive to plant development. [ABSTRACT FROM AUTHOR]

Geopolymers are green and sustainable cementitious materials that are a possible alternative to cement due to lower energy consumption and emissions. Volcanic ash can be used as raw material to prepare geopolymers by alkali activation owing to its high silica and alumina content. However, research on volcanic-ash-based geopolymers is still limited and no consensus on the optimal alkali activator and reaction mechanism has been obtained. This study investigates the effect of alkali activators with different cations on the mechanical properties and microstructure of volcanic-ash-based geopolymers. The flexural and compressive strengths were evaluated. X-ray diffractometry, Fourier transform infrared spectrometry, scanning electron microscopy coupled with energy-dispersive spectroscopy and silicon-29 (29Si) magic-angle spinning nuclear magnetic resonance were used to characterise the microstructural properties. The results showed that the alkali activation effect of sodium hydroxide (NaOH) on volcanic ash was superior to those of lithium hydroxide (LiOH) and potassium hydroxide (KOH), particularly at a concentration of 8 mol/l, with 28-day flexural and compressive strengths of 3.0 and 28.3 MPa, respectively. Microstructural results indicated that the sodium hydroxide-activated geopolymers formed dense, continuous, highly polymerised silica–aluminate gels. This paper provides a proper ratio scheme for activating the volcanic ash from similar deposits and promoting practical applications conducive to sustainable engineering development in the future. [ABSTRACT FROM AUTHOR]

This study explored a novel cementitious material developed from subway tunnel muck (STM) intended for ecological concrete (EC) preparation. The effects of three alkaline activators (NaOH, KOH, and CaO) on the properties of the cementitious materials were systematically examined. The results indicated that NaOH exhibited the most effective activation performance, followed by KOH, with CaO being the least effective. The NaOH-activated materials exhibited the highest compressive strength (reaching up to 12.15 MPa), the densest microstructure (characterized by the lowest porosity and smallest average pore size), the most substantial gel formation (evidenced by the highest mass loss in thermogravimetric analysis), and the optimal gel structure (indicated by the pronounced peak sharpening in Fourier transform infrared spectroscopy) after a 28-day curing period. Moreover, the crystallization of potassium salts under KOH activation detrimentally impacted the microstructure of KOH-activated materials. To balance the need for structural strength and nutrient provision, NaOH + KOH-activated materials were selected for the preparation of EC. Notably, the application of NaOH + KOH-activated materials resulted in a significant increase in K+ concentration in the soil layer, compared to common soil. Furthermore, NaOH + KOH-activated materials exhibited a slow-release effect, thereby offering sustained nutrient support conducive to plant development.

In order to enhance the utilization of alkaline carbide slag (CS) and steel slag (SS) in solid waste-based cementitious materials (SWBCM). In this study, slag (GGBS) was utilized as the primary material, phosphogypsum (PG) as the sulfat activator, and carbide slag (CS) and steel slag (SS) as the alkali activators to prepare carbide slag-steel slag-slag-phosphogypsum composite cementitious material (CS-SS-GGBS-PG). The impact of partial SS (mass fraction) substitution by CS on the flowability, mechanical properties, softening coefficient, pore solution pH value, hydration heat, and microstructure of CS-SS-GGBS-PG was studied. The hydration products, microstructure, and pore structure of CSSS-GGBS-PG were studied via XRD, FTIR, TG-DTG, SEM, and MIP. The results show that CS replacement with SS decreases the flowability of CS-SS-GGBSPG compared to the specimen without CS doping (A0). The addition of CS provides an abundance of Ca2+ and OH- to facilitate the hydration reaction in the system, prompting CS-SS-GGBS-PG to form more C-(A)-S-H gels and ettringite (AFt) in the early stages. This significantly enhances the 3 and 7 days strength of CS-SS-GGBS-PG. Compared to A0, the sample group with 5% CS substitution for SS showed a respective increase of 400% and 1,150% in flexural strength at 3 and 7 days, and an increase of 800% and 1,633% in compressive strength. However, if CS is substituted for SS in excess, the system's late hydration process will be inhibited, increasing the volume of harmful pores in the specimen's microstructure and lowering its degree of compactness. This leads to a decrease of 28 days of strength and a deterioration of the water resistance of CS-SS-GGBS-PG. It is advised that the blending ratio of SS to CS be 3:1 in order to maintain equilibrium between the early strength enhancement of CS-SS-GGBS-PG and the stability of strength and water resistance in the latter stage. This study has a positive impact on improving the utilization of CS and SS in SWBCM, reducing the dependence of SWBCM on traditional high-angstrom chemical alkali activators, and promoting the development of SWBCM. [ABSTRACT FROM AUTHOR]

In this study, the changes in the phase and microstructure of alkali-activated slag (AAS) in ultralow-temperature environments (−170°C) was experimentally studied by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric (TG), scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS), Si29 magic-angle spinning nuclear magnetic resonance (MAS NMR), and pore structure. The results show that as the modulus of water glass increased, the mass loss of AAS after the ultralow-temperature freeze–thaw cycles (ULT-FTC) decreased, the freeze–thaw resistance increased. The ULT-FTC caused the internal structure of the AAS samples using different activators to slip and rearrange. Partial calcium-(alumina)-silicate-hydrate gel [C─ (A)─ S─ H] gel was decalcified. The gel structure formed using 2.0 M water glass as the activator was the most stable. The dense structure with a lower Ca/Si ratio enables the AAS to maintain a relatively stable microstructure after undergoing ULT-FTC. [ABSTRACT FROM AUTHOR]

Coal gangue (CG) is a residual product from coal mining and washing processes. The reutilization of CG to produce geopolymers is a low-carbon disposal strategy for this material. In this study, the calcined CG (CG700°C) was used as aluminosilicate precursors, and the effects of alkali activators (i.e., Na2SiO3/NaOH, NaOH concentration, and liquid–solid) on the mechanical characteristics and microstructure of CG700°C-based geopolymers were investigated. The findings indicated that the specimens with a liquid–solid ratio of 0.50 (G2.0-10-0.50) exhibited a compact microstructure and attained a compressive strength of 24.75 MPa. Moreover, increasing the Na2SiO3/NaOH mass ratio has shortened the setting times and facilitated geopolymer gel formation, resulting in a denser microstructure and improved compressive strength. The higher NaOH concentrations of alkali activators facilitated the dissolution of CG700°C particles, and the geopolymerization process was more dependent on the condensation of SiO4 and AlO4 ions, which promoted the formation of geopolymer networks. Conversely, an increase in the liquid–solid ratio from 0.50 to 0.65 had a negative impact on compressive strength enhancement, impeding the polycondensation rate. Examination through scanning electron microscopy and mercury intrusion porosimetry revealed that employing a lower Na2SiO3/NaOH mass ratio (G1.2-10-0.55), smaller NaOH concentrations (G2.0-8-0.55), and a higher liquid–solid ratio (G2.0-10-0.65) led to the presence of larger pores, resulting in decreased 28 days compressive strength values (15.87 MPa, 13.25 MPa, and 14.92 MPa, respectively), and a less compact structure. The results suggest that the performance of CG700°C-based geopolymers is significantly influenced by alkali activators. [ABSTRACT FROM AUTHOR]

Geopolymer is relatively newer binder developed as an alternative to conventional cementitious binders. It lowers CO2 emissions, consumes less energy, and effectively utilises industrial wastes. The alkali activator solution, used in traditional two-part geopolymer to initiate the polymerisation reactions, is highly viscous and corrosive, which limits the effective utilisation of geopolymer concrete in construction. Consequently, a single-component "just add water" geopolymer system was developed to address the issues of cast in situ applications of two-part geopolymer concrete. This paper discusses the diverse examinations of the various source materials and alkali activators employed over the years in synthesising single-component geopolymer (SCG) along with their detailed chemical analysis. In addition to the different constituent types, the effect of various curing conditions, e.g., ambient or elevated temperatures, admixtures were also presented on the properties (physical, mechanical, and durability) of SCG-based paste/mortar/concrete. Such detailed analysis of the chemical composition and engineering properties of various SCG mixes would help the researcher and material designer to choose a suitable mix proportion based on the targeted performance. Further, based on an in-depth analysis of the various bindings phases (e.g., CSH and CASH) formed in SCG, the schematic representation of the reaction mechanism was highlighted. This paper also highlighted the life-cycle cost analysis (LCCA) and current challenges in the production of SCG. Therefore, the present review article would serve as a source of information for the scientific community to understand the current research trend in the development of SCG binder, its design and consequent prediction of various engineering properties made with different constituent materials and would serve as a source of information for further research and development which is one step towards the goals of net zero emissions by 2050 through a sustainable approach. [ABSTRACT FROM AUTHOR]

Coal gangue (CG) is a residual product from coal mining and washing processes. The reutilization of CG to produce geopolymers is a low-carbon disposal strategy for this material. In this study, the calcined CG (CG700°C) was used as aluminosilicate precursors, and the effects of alkali activators (i.e., Na2SiO3/NaOH, NaOH concentration, and liquid–solid) on the mechanical characteristics and microstructure of CG700°C-based geopolymers were investigated. The findings indicated that the specimens with a liquid–solid ratio of 0.50 (G2.0-10-0.50) exhibited a compact microstructure and attained a compressive strength of 24.75 MPa. Moreover, increasing the Na2SiO3/NaOH mass ratio has shortened the setting times and facilitated geopolymer gel formation, resulting in a denser microstructure and improved compressive strength. The higher NaOH concentrations of alkali activators facilitated the dissolution of CG700°C particles, and the geopolymerization process was more dependent on the condensation of SiO4 and AlO4 ions, which promoted the formation of geopolymer networks. Conversely, an increase in the liquid–solid ratio from 0.50 to 0.65 had a negative impact on compressive strength enhancement, impeding the polycondensation rate. Examination through scanning electron microscopy and mercury intrusion porosimetry revealed that employing a lower Na2SiO3/NaOH mass ratio (G1.2-10-0.55), smaller NaOH concentrations (G2.0-8-0.55), and a higher liquid–solid ratio (G2.0-10-0.65) led to the presence of larger pores, resulting in decreased 28 days compressive strength values (15.87 MPa, 13.25 MPa, and 14.92 MPa, respectively), and a less compact structure. The results suggest that the performance of CG700°C-based geopolymers is significantly influenced by alkali activators.

In this research, the effect of a polycarboxylate based superplasticizer on the strength of Geopolymer Concrete (GPC) was investigated. A fixed amount of superplasticizer (1.5 % of Fly Ash weight) was utilized along with alkali activators Sodium Hydroxide (NaOH) and Sodium Silicate (Na2SiO3). The Ultrasonic Pulse Velocity Test determined the strength development of concrete. The quality of the GPC improved when a higher concentration of alkali activator was applied based on the Concrete Quality Designation. In detecting the color development of GPC, samples were put through MATLAB and specimens became lighter as time passes due to dehydration (a process where water escapes from the sample). Stress strain diagrams were generated which generally indicate that GPC specimens are ductile. The researchers were able to assess the workability of the mix designs using a rating from 1 to 5, with 1 being the least workable and 5 being the most workable.

In this research, the effect of a polycarboxylate based superplasticizer on the strength of Geopolymer Concrete (GPC) was investigated. A fixed amount of superplasticizer (1.5% of Fly Ash weight) was utilized along with alkali activators Sodium Hydroxide (NaOH) and Sodium Silicate (Na2SiO3). The Ultrasonic Pulse Velocity Test determined the strength development of concrete. The quality of the GPC improved when a higher concentration of alkali activator was applied based on the Concrete Quality Designation. In detecting the color development of GPC, samples were put through MATLAB and specimens became lighter as time passes due to dehydration (a process where water escapes from the sample). Stress strain diagrams were generated which generally indicate that GPC specimens are ductile. The researchers were able to assess the workability of the mix designs using a rating from 1 to 5, with 1 being the least workable and 5 being the most workable. [ABSTRACT FROM AUTHOR]

Phosphorus slag (PS) is an industrial waste discharged during the refining of yellow phosphorus by a high-temperature electric furnace. The slag is generally deposited on the surface in situ, and this uses up considerable areas of land and causes environmental pollution problems. The glass phase content in electric furnace PS is over 90%, indicating that the PS has potential cementitious property. As a cement admixture, PS has a retarding effect on cement, thus, its application to cement has become less adopted. Based on the hydration characteristics of PS and the application conditions of mine filling, the feasibility of alkali-activated cement–PS used as cementitious material (CPCM) to cement sulfur tailings was studied. Quicklime, NaOH, and Na2SiO3 were used as the activators to investigate the CPCM curing performance in a laboratory, and the compatibility of CPCM with sulfur tailings was evaluated. The results show that when the PS-to-cement mole ratio is 1∶1, quicklime is 3% of PS, the final setting time of CPCM is 300 min, and the CPCM strength for 28 days is 40.6 MPa, indicating that the material can replace P.O 42.5 cement in the subsequent filling process. When the Na2SiO3 is 4% of PS, the final setting time of CPCM is 39.3% less than that of cement, and its strength for 7 days is 31.1% higher than that of cement. Compared with cement, CPCM shortens the setting time of backfill by 8 hours, and its strength does not deteriorate after curing for 28 days. Therefore, CPCM is suitable for cementing sulfur tailings. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) analyses show that the CPCM hydration products are mainly Ca(OH)2 and C−S−H. Moreover, the PS hydration resulted in a decrease in the Ca(OH)2 content, and a compact C−S−H gel with a low Ca/Si mole ratio is formed. This not only improves the later strength of CPCM but also prevents the strength deterioration of the backfill.

Inorganic polymers are considered to have a much smaller CO2 footprint than traditional Portland cements. The possible environmental impact reduction was calculated to be up to 77% of a CEMI ordinary Portland cement (OPC)-based mortar, while the CO2 emissions were reduced by up to 83%. The highest contribution to the total CO2 footprint of inorganic polymers is related to the manufacturing of alkali activators. Within this context, the current work discusses the synthesis of inorganic polymers made from Fe-rich precursor and alternative silicate solutions. The obtained results indicate that there is no significant difference between an inorganic polymer prepared from alternative and commercially available solutions, which confirms that the waste glass can be use as alternative raw material in the production of sodium silicate.

The scientific community has devoted considerable attention to alkali-activated slag binder systems, due to the low-carbon footprint and eco-friendly attributes. However, the regulation of setting time poses significant challenges to the widespread application in practical engineering environments. This study proposes an innovative approach to encapsulate alkali activators in capsules or tablets, followed by the incorporation into the alkali slag concrete matrix. This strategy aims to regulate and control the early dissolution and hydration rate of slag, thereby altering the setting time of the alkali-activated material. Specifically, this method can prolong the setting time by approximately 40%. Importantly, this intervention does not significantly damage the mechanical properties of materials over the 28 days due to the continuous outward diffusion of the active ingredients of the alkali activator. This encapsulation technique harbors the capacity for quantitative modulation of the setting time in alkali-activated slag concrete. The findings of this research offer valuable theoretical insights, paving the way for the application of alkali-activated hydration systems in diverse engineering domains. • Proposed an alkali activators encapsulation technology to regulate setting time. • This method did not observably compromise mechanical properties at 28d. • This method effectively prolonged the hydration induction period. [ABSTRACT FROM AUTHOR]

The technology of Geopolymer Concrete (GPC) is gaining importance in the present era because of the reduced carbon dioxide emission and low embodied energy, when compared with the conventional cement concrete. GPC is obtained by the activation of silica and alumina rich source materials in the presence of alkali activator. This paper evaluates the strength and durability characteristics of fly ash based GPC with ferrochrome slag aggregates. The fine and coarse aggregates in the GPC were replaced with ferrochrome slag in various proportions, and it was noted that the coarse aggregate replacement performed better than the replacement of fine aggregates. The optimum replacement percentage that gives the maximum strength in terms of coarse aggregate replacement was arrived at. The result concluded that the incorporation of ferrochrome slag as coarse aggregate at optimum level enhanced the strength characteristics in comparison with the conventional GPC with normal aggregates. The durability characteristics of the various replacement mix was carried out, and the results of the optimum replacement mix was compared with the conventional geopolymer mix. Apart from these, the feasibility of utilising ferrochrome slag as aggregates in concrete were aiso ascertained by leaching tests. [ABSTRACT FROM AUTHOR]

Purpose: Alkali-activated materials, also known as geopolymers, are considered promising assets in the sustainable materials industry. Given the excellent properties in terms of thermal stability and low thermal conductivity, geopolymer-based matrices can effectively substitute cementitious binders in the preparation of passive fire protection (PFP) systems. The present study aims at evaluating the environmental footprint of a newly proposed geopolymer-based fireproofing material. The results are compared to a reference commercial lightweight cement-based coating with equal PFP performance. Methods: The boundaries of the system assessed were based on a cradle-to-grave life cycle. A preliminary scale-up of the laboratory protocol allowed the evaluation of the industrial production of the geopolymer-based PFP mix. An ancillary life cycle analysis was performed, comparing the environmental footprint of a geopolymer-based concrete block to the relevant literature studies for the same system in order to validate the approach of the present study. The functional unit of the main study was defined, taking into account the material performance in terms of resistance to heat exposure, allowing a functional comparison with lightweight cement-based PFP. The impact assessment phase used the CML-IA methodology as a characterization method. Results and discussion: The ancillary LCA confirmed the alignment of the assumptions of the current study with previous analyses. The analyzed geopolymer-based fireproofing material exhibited a life cycle impact which is 27% lower than the lightweight concrete reference in terms of the global warming indicator, mainly thanks to the avoided CO2 emissions from the clinker process in cement manufacturing. Therefore, the greenhouse gas reduction described in previous studies on geopolymer application as a strong environmental advantage of the geopolymer technologies is also confirmed in this case. However, the other considered impact categories, such as resource depletion, acidification, eutrophication, and human toxicity, resulted in indicator values higher than the reference, as a consequence of the energy-intensive production process for the alkali activators (in particular, sodium silicate). Conclusions: Though the reduction of greenhouse gas emissions is confirmed, the overall sustainability of geopolymers for PFP applications is hindered by the relevant environmental footprint of the sodium silicate production process. However, a substantial reduction of the impacts could be achieved by selecting the production process of sodium silicate which takes advantage of renewable energy supplies (e.g., hydrothermal route) or by reducing the amount of sodium silicate in the geopolymer recipe in favor of waste-based alkali activators. [ABSTRACT FROM AUTHOR]

Foamed geopolymer (inorganic polymers materials) were successfully produced from alkali activated glass waste powder, after thermally treated at temperatures between 500 and 700°C for 1 hour. These geopolymers were synthesized by mixing the mixed color glass waste powder with (potassium hydroxide and sodium silicate) solutions. Thermal treatment of these new materials at temperatures ranging from 500°C to 700°C recorded an important volume increase 18-41%, during to a foaming process specific for sodium or potassium silicate (aluminate) hydrates. For these compositions and due to foaming process, the increase of volume is noticed at 600°C and partial melting occurs at 700°C. The formation of glass foams leading to large changes in volume associated with different sizes of open porosity. This method for creating a foaming geopolymers represents a novel reuse of the waste glass in engineering applications as thermal and sound insulations coupled with low cost and environmental benefits. [ABSTRACT FROM AUTHOR]

Pastes and mortars of alkali-activated palm oil fuel ash (POFA) are synthesized with three types of alkaline solutions and are characterized for molecular functional groups by Fourier transform infrared (FTIR) spectroscopy, microstructure observation, and chemical composition by scanning electron microscopy with energy dispersive X-ray (SEM-EDX) and mechanical strength using a universal mechanical tester. The alkali activators chosen for this investigation are 12 M sodium hydroxide (NaOH), 3 M sodium silicate (Na2SiO3), and a combination of both activators (Na2SiO3 and NaOH) by mass ratio of 3:1. POFA with Na2SiO3 solution yields the highest compressive strength in comparison to the other activators. The main gel binder dominantly composed of silica with varying calcium/silica (Ca=Si) ratios is analyzed by analytical techniques through FTIR and SEM-EDX. [ABSTRACT FROM AUTHOR]

Geopolymers have several applications and concrete is one of the materials that can be produced with geopolymer as binder. Since industrial byproducts/wastes such as fly ash, iron slag, micronized biomass silica, silica fume, red mud, etc. can be used as a binder instead of Portland cement, geopolymer concrete (GPC) has generated lot of interest among the scientific and engineering community. This has also resulted in reduced carbon footprint of concrete and an effective method of disposing industrial waste. In this study GPC with a blend of class-f fly ash and ground granulated blast furnace slag as binder has been developed, and its flexural and fracture characteristics have been studied. The GPC developed has a 28-day compressive strength in the range 40-50 MPa. Incorporation of steel fibres resulted in increased flexural strength, enhanced fracture properties and ductility. The residual strength of steel fibre reinforced GPC was also determined in the study. [ABSTRACT FROM AUTHOR]

• High-strength geopolymer can be produced with clinker aggregates in 8 h. • The increase in SS/SH positively contributed to strength development. • Geopolymer concrete at 900 °C has a compressive strength of approximately 20 MPa. • The produced geopolimer can be used in facilities requiring radiation shielding. Geopolymers are a new generation of building material that has recently been popular in reducing carbon emissions. As a result of its amorphous alumina-silicate-based reaction product, it has many superior properties. This study investigated the mechanical and physical properties and high-temperature resistance of ground-granulated blast furnace slag and metakaolin-based geopolymer concrete fabricated with clinker aggregates produced in a short curing time (8 h). In alkali activators, the sodium silicate to sodium hydroxide (SS/SH) mass ratios were 2, 2.25, 2.5 and 2.75. Two curing methods were used, namely ambient (25 °C) and oven curing (80 °C). The observed unit weights vary between 2275–2392 kg/m3 for the samples of the geopolymeric composites. Porosity and water absorption vary between 4.92 and 8.84% and 3.58–7.26%, respectively. The heat curing application decreased the mixtures' water absorption, unit weight, and porosity values. The compressive strength of the samples subject to ambient curing varied between 39.2 and 59.8 MPa. Withal, the samples cured at 80 °C for 8 h were between 66.9 and 112.4 MPa. The increased SS/SH ratio also increased the observed strength values. Capillary water absorption of all samples is below 0.6 kg/m2. Heat-cured geopolymers decreased capillary water absorption to 0.1 kg/m2. The weight loss after high temperature decreased as the SS/SH ratio increased. The N-A-S-H gel in the zeolitic structure was observed in SEM analysis. The FT-IR analysis visualized the carbonation in the heat-cured mixtures. As a result, over 100 MPa compressive strength values were observed with the clinker aggregates at 80 °C after 8 h of curing time. Withal, a high-temperature refractory composite was developed with the clinker aggregates. Moreover, the nuclear radiation shielding abilities of geopolymer concretes were investigated experimentally. The gamma shielding parameters of the samples were evaluated by transmission measurements in the range of 0.081–0.661 keV. Also, neutron dose measurements were performed, and the samples produced as neutron attenuator capacity was evaluated. The radiation shielding capabilities were improved slightly with increasing sodium silicate solution content. [ABSTRACT FROM AUTHOR]

• Uncalcined clay was utilized to produce low-cost binders. • The fresh, mechanical, permeability and microstructural properties were evaluated. • Slag was used to improve the performance of clay-based binders. • Sustainable activators were utilized to produce the binders. The need to reduce the negative environmental impact and cost of construction binders have resulted in an imminent need to utilize locally available materials as alternatives. An experimental study was carried out to explore the use of locally sourced raw clay as a binder for construction applications. This approach was implemented to develop a sustainable alternative to Portland cement which is known to be detrimental to the environment and consume a large amount of raw materials. Thus, in this study, clay binders were produced by using various alkali activators and incorporating slag to form a blended alkali activation system. The mechanical and physical characteristics of the developed clay binders were measured at 28 days and the effects of different activators on the physical and mechanical properties of clay binders were assessed. The results of the extensive experimental program indicate that the alkali activation of clay soil and the clay-slag system are feasible options to produce an effective, hardened, and low-cost clay binder. The 28-day compressive strength as high as 30 MPa was achieved with the use of sodium carbonate to activate a clay-slag blend. The sodium silicate and calcium hydroxide solutions showed a positive impact on the strength and durability characteristics of a binder made only with clay soil. However, a clay-slag blended binder activated with sodium silicate, sodium carbonate, and calcium hydroxide solutions showed enhanced strength and durability properties. [ABSTRACT FROM AUTHOR]

Abstract: The influence of using different types of alkali activators namely: sodium sulphate, sodium silicate, sodium carbonate and sodium hydroxide on the hydration characteristics and physic-mechanical properties of special blended cement systems containing oil shale ash were investigated. Compressive strength values of the blended cement pastes containing 10% oil shale ash I and 30% oil shale ash II in absence of activators about 83–80% of Portland cement paste respectively. The addition of various alkali-activators to the same two mixes improves the mechanical strength of both cement mixes. The various cement alkali-activators used in this study followed this order (major to minor) with respect to the mechanical strengths Na2SO4 >Na2SiO3 >Na2CO3 >NaOH. The results showed that SAII would have better pozzolanic properties than SAI referring to chemical composition and the degree of crystalinity and these findings were confirmed by DTA, XRD and SEM. [Copyright &y& Elsevier]

Abstract: This research investigated the immobilization potential of alkali-activated lime–rice husk ash (RHA) for synthetic Cr(OH)3, Fe(OH)3, Zn(OH)2 and zinc cyanide plating sludge. The binder consists of hydrated lime and RHA at a weight ratio of 45:55. Waterglass (Na2SiO3) with SiO2/Na2O≈3 and anhydrous sodium carbonate (Na2CO3) were used as alkali activator between 0 and 8 wt.% of the binder. Results showed that Zn(OH)2 addition causes a considerable strength development in control and sodium silicate-activated samples but only after 14 days. Similar observations were found for the sample loaded with 10 wt.% plating sludge but this only occurred after 28 days. A possible explanation for these phenomena is that the initial formation of calcium zincate, which has a set retarding effect, inhibits early strength development. At later ages, calcium zincate dissolves and Zn is taken up in the formation of C–S–Z–H solid solutions leading to strength development. These phenomena were not observed from the sodium carbonate-activated lime–RHA matrices. In these, it is believed that zinc/calcium carbonates readily form inhibiting calcium zincate and C–S–Z–H formation. Despite this, carbonate-containing mixes with up to 30 wt.% plating sludge gave a 14-day strength and Cr concentration in TCLP leachate that meet the regulatory limit for landfilling. [Copyright &y& Elsevier]

• By-product alkali activators are comparable to commercial alkali activators. • Slag activated by RHA-sodium silicate is cost efficient and high performance. • Cost of silica fume-sodium silicate is lower than conventional sodium silicate. • Sonication is hi efficient to produce sodium silicate from silica fume. • Lime kiln dust is combination of calcium oxide and calcium carbonate activators. The use of Ground Granulated Blast Furnace Slag (GGBFS) as binder for concrete had been reviewed extensively in previous reviews. However, the influence of various types of alkali activators on the behavior of GGBFS based concrete had yet to be consolidated in a comprehensive review. Hence, this paper presented a comprehensive review of the effect of various commercial and by-product types of alkali activators on the fresh properties, mechanical properties, and durability of Ground Granulated Blast Furnace Slag-based concrete. Commercial alkali activators prescribed are those hydroxide-based, silicate-based activators, carbonate-based and oxide-based alkali activators. By-product activators can be obtained directly or derived from the industrial by-product. The by-product derived sodium silicate yielded almost similar mechanical properties to GGBS as compared to commercial sodium silicate. Lime kiln dust had similar properties as oxide and carbonate-based alkali activators in alkali activation of alkali-activated materials. [ABSTRACT FROM AUTHOR]