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This study presents the development of a synthesis route for sodium aluminosilicate hydrate (N–A–S–H) gels with various Si/Al ratios carried out at ambient temperature. The synthesis is based on a simple "sol–gel" method, where commercial reactants are used to provide highly reactive Si and Al sources. Comprehensive characterization, including scanning electron microscope‐energy‐dispersive spectroscopy (SEM‐EDS), gas pycnometry, inductively coupled plasma (ICP), X‐ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), X‐ray diffraction (XRD), and magic angle spinning (MAS) nuclear magnetic resonance (NMR) (27Al, 23Na, 29Si, and 1H), is employed to verify the morphology, density, chemical composition, long‐range order, and local structure of the gels. Our results show that the synthesized gels are pure, amorphous, and homogeneous with Si/Al final ratio ranging from 1.22 to 2.23. Structural analysis of the gels indicates the synthesis of compounds with high degree of geopolymerization, which are representative of N–A–S–H gel formed in sodium‐based geopolymers. [ABSTRACT FROM AUTHOR]

The influence of process variables such as curing temperature and relative humidity (RH) on the shrinkage of alkali-activated fly ash (AAF) is examined in this work. The ambient conditions are varied after an initial accelerated moist curing at high-temperature. An analysis interlinking the effects of curing on AAF strength, shrinkage, reaction product content, and porosity is performed. Strength achieved and the pore structure formed for the different curing conditions depend upon the sodium alumino-silicate (N-A-S-H) gel content formed in the AAF. While water is not directly combined in the formation of N-A-S-H gel, its content is sensitive to the availability of moisture. The moisture loss due to drying during the geopolymerization reduces the N-A-S-H content formed in the AAF. Compared to the continuous moist curing at elevated temperature, there is a decrease in the N-A-S-H content on lowering the temperature or drying produced by the decrease in RH. Reducing temperature and RH following initial accelerated curing has the beneficial effect of reducing the shrinkage compared to drying at a higher temperature. Reduction in the N-A-S-H content due to decrease in temperature after the accelerated curing is more significant than the drying on lowering the RH to 50%. The autogenous shrinkage measured under sealed conditions contributes significantly to the total shrinkage in AAF. The shrinkage in the AAF is significantly lower than a comparable cement paste. While shrinkage is produced by drying, the moisture loss and shrinkage relationship is not unique. The shrinkage produced by moisture loss due to drying is primarily influenced by the pore structure formed in the AAF, which also depends on the N-A-S-H content. The influences of temperature and humidity on the strength, pore structure and shrinkage are determined by the N-A-S-H formed in the AAF. [ABSTRACT FROM AUTHOR]

This paper provides information on the global reaction path and hydration kinetics of a natural clinoptilolite-Ca zeolite blended portland cementitious system. The replacement of up to 20% as-received clinoptilolite-Ca zeolite with portland cement substantially minimized the tricalcium aluminate (C 3 A) peak heat of hydration as observed using isothermal calorimetry. Inductively coupled plasma-optical emission spectrometry results showed that the zeolite-blended samples had a higher initial chemical dissolution of calcium and aluminate; but subsequently, the consumption of both was higher compared to the neat cement control sample, likely due to the formation of new hydration reaction products, indicating an interaction between cement and clinoptilolite-Ca zeolite. Clinoptilolite-Ca zeolite transformed into clinoptilolite-Na zeolite using NaOH and NaCl solutions showed a distinct aluminate (C 3 A) peak heat flow, indicating a different reaction path compared to clinoptilolite-Ca zeolite. Micro-chemical analysis using energy dispersive spectroscopy suggests a different hydration reaction mechanism for both calcium and sodium-based zeolite, along with the possibility of reduction in the formation of C–S–H, ettringite, and monosulfate due to the formation of new hydration products. The clinoptilolite-Ca zeolite consumed significantly higher aluminate contents leading to the formation of amorphous calcium-aluminum-silicate-hydrate (C-A-S-H) gels, which consequently reduced the aluminate-related reaction (C 3 A) heat flow in the system. In contrast, the clinoptilolite-Na zeolite released sodium ions through cation exchange to produce sodium aluminosilicate hydrate (N-A-S-H) in the blended system, which did not interfere with the C 3 A peak heat flow during the hydration process. • Clinoptilolite-Ca zeolite was found to reduce the C 3 A hydration heat flow in blended portland cement systems. • Clinoptilolite-Na zeolite was found to enhance the C 3 A hydration heat flow in blended portland cement systems. • Clinoptilolite-Ca zeolite is transformed to clinoptilolite-Na zeolite using NaCl and NaOH solution through cation exchange. • Clinoptilolite-Ca zeolite produced calcium aluminum silicate hydrate (C-A-S-H) in blended cement systems. [ABSTRACT FROM AUTHOR]

Geopolymers are alkaline cements composed of aluminosilicate units. The molecular structure of some geopolymers is sodium-aluminosilicate hydrate (N-A-S-H). Impurities containing calcium and magnesium are of significant importance in various types of geopolymer materials, as these impurities affect the mechanical properties of the geopolymer. In this study, the effects of different percentages of impurities on the modulus of elasticity of two types of geopolymer structures (N-A-S-H and slag) were investigated using molecular dynamics simulations. A recently introduced and modified N-A-S-H structure was used in the simulations. The molecular structure of blast furnace slag was also used to confirm and validate the molecular simulation results. The main reason for comparing the results of molecular dynamics simulations of the N-A-S-H structure with those of slag was the availability of laboratory results for slag-based geopolymers in the technical literature. The proposed molecular structure of slag based on its constituents is (N,C)-A-S-H. This structure is the most similar to N-A-S-H. The simulation parameters were selected based on data available in the literature. For the amorphous form of N-A-S-H, several simulations were performed to determine the mean modulus of elasticity. Then, additional simulations were carried out to investigate the effect of impurities on the modulus of elasticity of geopolymer materials. The results were in good agreement with the available laboratory results. The results showed that with an increasing percentage of calcium in the N-A-S-H structure, the modulus of elasticity increased. On the other hand, with an increasing percentage of magnesium impurities in the slag and N-A-S-H structures, the modulus of elasticity decreased. The overall results showed that the molecular dynamics simulation method is a good tool to complement laboratory studies for investigating the mechanical properties of geopolymers. [ABSTRACT FROM AUTHOR]

In this study, the hydrated sodium aluminosilicate material was synthesized by one-step hydrothermal alkaline desilication using fly ash (FA) as raw material. The synthesized materials were characterized by XRD, XRF, FT-IR and SEM. The characterization results showed that the alkali-soluble desilication successfully had synthesized the sodium aluminosilicate crystalline (N-A-S-H) phase of sodalite-type (SOD), and the modified material had good ionic affinity and adsorption capacity. In order to figure out the suitability of SOD as an adsorbent for the removal of ammonium and phosphorus from wastewater, the effects of material dosing, contact time, ambient pH and initial solute concentration on the simultaneous removal of ammonium and phosphorus are investigated by intermittent adsorption tests. Under the optimal adsorption conditions, the removal rate of ammonium was 73.3%, the removal rate of phosphate was 85.8% and the unit adsorption capacity reached 9.15 mg/L and 2.14 mg/L, respectively. Adsorption kinetic studies showed that the adsorption of ammonium and phosphorus by SOD was consistent with a quasi-secondary kinetic model. The adsorption isotherm analysis showed that the equilibrium data were in good agreement with the Langmuir and Freundlich model. According to thermodynamic calculations, the adsorption of ammonium and phosphorus was found to be a heat-absorbing and spontaneous process. Therefore, the preparation of SOD by modified FA has good adsorption properties as adsorbent and has excellent potential for application in the removal of contaminants from wastewater.

• Zeolite addition modifies compressive strength, high-temperature stability, and heavy-metal S/S efficiency in the FZNS specimens. • High-temperature curing can increase the pore volume fraction in the range of 25–100 nm. • New minerals (tilleyite and Friedel's salt) are formed in the FZNS40 specimens. • Zeolite addition promotes the formation of C/N-A-S-H in FZNS40-20-28d. Admixtures of pozzolanic materials or solid wastes with municipal solid waste incineration fly ash (MSWIFA) can improve heavy metal stabilization/solidification (S/S) properties in alkali-activated MSWIFA specimens. In this work, the compressive strength of MSWIFA admixes with zeolite activated by Na 2 O·1.7SiO 2 ·nH 2 O (FZNS) specimens with different curing times (3, 7, 14, and 28 d) and temperatures (20, 50, and 80 ℃) is investigated. The results reveal that a 40 wt% zeolite replacement of MSWIFA in FZNS specimens (FZNS40) not only increases the early compressive strength by 129% but also reduces strength loss during continuous high-temperature (50 or 80 ℃) curing. Moreover, zeolite addition promotes the creation of geopolymers at 28.5°–30.5°in X-ray diffraction (XRD) curves. During 50 or 80 ℃ curing, the NaCl peak disappears and a marginal Friedel's salt peak can be detected at 11.4°. The gel contents are more concentrated on C-S-H in FZNS0 and C-(A)-S-H/N-A-S-H in FZNS40, as demonstrated by differential thermogravimetry (DTG) and magic angle spinning nuclear magnetic resonance (MAS NMR) analysis. With zeolite addition, long-time and high-temperature curing can improve the heavy metal solidification ratio in FZNS specimens. After high-temperature curing, the honeycomb or cluster morphologies of the gel products, visible by scanning electron microscopy (SEM) in the FZNS specimens, reveal high heavy metal stabilization properties. It is easier to form the fibrous C-(A)-S-H/N-A-S-H gel near zeolite owing to the high content of SiO 2 and Al 2 O 3. Zeolite modifies the compressive strength, high-temperature stability, and heavy-metal S/S efficiency in FZNS specimens, which promotes the utilization of MSWIFA as an ecological resource in the future. [ABSTRACT FROM AUTHOR]

C-A-S-H (CaO-Al2O3-SiO2-H2O) and N-A-S-H (Na2O-Al2O3-SiO2-H2O) have a wide range of chemical compositions and structures and are difficult to separate from alkali-activated materials. Therefore, it is difficult to analyze their microscopic properties directly. This paper reports research on the synthesis of C-A-S-H and N-A-S-H particles with an average particle size smaller than 300 nm by applying the hydrothermal method. The composition and microstructure of the products with different CaO(Na2O)/SiO2 ratios and curing conditions were characterized using XRD, the RIR method, FTIR, SEM, TEM, and laser particle size analysis. The results showed that the C-A-S-H system products with a low CaO/SiO2 ratio were mainly amorphous C-A-S-H gels. With an increase in the CaO/SiO2 ratio, an excess of Ca(OH)2 was observed at room temperature, while in a high-temperature reaction system, katoite, C4AcH11, and other crystallized products were observed. The katoite content was related to the curing temperature and the content of Ca(OH)2 and it tended to form at a high-temperature and high-calcium environment, and an increase in the temperature renders the C-A-S-H gels more compact. The main products of the N-A-S-H system at room temperature were amorphous N-A-S-H gels and a small amount of sodalite. An increase in the curing temperature promoted the formation of the crystalline products faujasite and zeolite-P. The crystallization products consisted of only zeolite-P in the high-temperature N-A-S-H system and its content were stable above 70%. An increase in the Na2O/SiO2 ratio resulted in more non-bridging oxygen and the TO4 was more isolated in the N-A-S-H structure. The composition and microstructure of the C-A-S-H and N-A-S-H system products synthesized by the hydrothermal method were closely related to the ratio of the raw materials and the curing conditions. The results of this study increase our understanding of the hydration products of alkali-activated materials. [ABSTRACT FROM AUTHOR]

This study investigates the decomposition of alkali-activated materials (AAMs) in sulfuric acid solution. Four sodium-containing calcium aluminium-substituted silicate hydrate (C-(N)-A-S-H) and aluminosilicate hydrate (N-A-S-H) gels with designed Ca/Si and Si/Al ratios are produced with synthetic calcium aluminosilicate powders. The formation of additional calcium products in Ca-rich and Al-sufficient sample is likely helpful to retard the attack of sulfuric acid on the C-(N)-A-S-H/N-A-S-H gels. The Si O bond strength of N-A-S-H gel appears weaker than that of C-(N)-A-S-H gel. The calcium in the gels prevents the leaching of silicon from gels. The length of silicon chains in the C-(N)-A-S-H gel increases with increased Si/Al ratio while exposure to acid solution. The chain length of the N-A-S-H gel increases with reduced Si/Al ratio. C-(N)-A-S-H gels show a higher resistance to leaching than N-A-S-H gel, while the latter has a high resistance to depolymerization under acid attack. This study provides a basic route to understand the effect of AAMs composition on acid resistance. [ABSTRACT FROM AUTHOR]

C-A-S-H (CaO-Al2O3-SiO2-H2O) and N-A-S-H (Na2O-Al2O3-SiO2-H2O) have a wide range of chemical compositions and structures and are difficult to separate from alkali-activated materials. Therefore, it is difficult to analyze their microscopic properties directly. This paper reports research on the synthesis of C-A-S-H and N-A-S-H particles with an average particle size smaller than 300 nm by applying the hydrothermal method. The composition and microstructure of the products with different CaO(Na2O)/SiO2 ratios and curing conditions were characterized using XRD, the RIR method, FTIR, SEM, TEM, and laser particle size analysis. The results showed that the C-A-S-H system products with a low CaO/SiO2 ratio were mainly amorphous C-A-S-H gels. With an increase in the CaO/SiO2 ratio, an excess of Ca(OH)2 was observed at room temperature, while in a high-temperature reaction system, katoite, C4AcH11, and other crystallized products were observed. The katoite content was related to the curing temperature and the content of Ca(OH)2 and it tended to form at a high-temperature and high-calcium environment, and an increase in the temperature renders the C-A-S-H gels more compact. The main products of the N-A-S-H system at room temperature were amorphous N-A-S-H gels and a small amount of sodalite. An increase in the curing temperature promoted the formation of the crystalline products faujasite and zeolite-P. The crystallization products consisted of only zeolite-P in the high-temperature N-A-S-H system and its content were stable above 70%. An increase in the Na2O/SiO2 ratio resulted in more non-bridging oxygen and the TO4 was more isolated in the N-A-S-H structure. The composition and microstructure of the C-A-S-H and N-A-S-H system products synthesized by the hydrothermal method were closely related to the ratio of the raw materials and the curing conditions. The results of this study increase our understanding of the hydration products of alkali-activated materials.

Alkali-activated materials (AAMs) are usually recognized having better acid resistance compared to ordinary Portland cement (OPC), however, the detailed mechanism has not been well studied due to the complexity of raw materials and hardened matrix. In this study, two typical binding gels in AAMs, sodium aluminum silicate hydrate (N-A-S-H) and calcium (sodium) aluminum silicate hydrate (C-(N)-A-S-H), were chemically prepared by laboratory synthesized aluminosilicate powders. The microstructure and composition evolution of the two gels exposed to sulfuric acid were investigated. The results showed that the C-(N)-A-S-H gel had higher degree of structural order than the N-A-S-H gel, while the N-A-S-H gel showed higher polymerization. The AlIV in N-A-S-H gel transferred completely to AlV during sulfuric acid attack, while trace amount of AlIV was still detected in the exposed C-(N)-A-S-H gels. Both gels had increased silica polymerization degree after exposure to sulfuric acid solution. Due to rapider dealumination of N-A-S-H gel than C-(N)-A-S-H gel, the Si/Al ratio increased much higher in the former. The molecular framework changes of N-A-S-H gel caused by dealumination was found to have less effect on the integrity though than that of C-(N)-A-S-H gel, in which coarse gypsum crystalline grains formed and led to destructive stress in hardened matrix. This paper provides an insight at microstructure level of the two typical gels, which is essential for the manufacturing and application of alkali-activated materials. [ABSTRACT FROM AUTHOR]

Abstract: Sodium aluminosilicate hydrate (N-A-S-H) gel, the main reaction product of the alkali-activated aluminosilicates, differs of the aluminium-modified calcium silicate hydrate (C-A-S-H) gel of PC pastes. Increasing the level of SCM to reduce PC content of binders are being considered to address reduction in CO2 emissions, activation of the additional SCM content by alkali activation represents a possible environmentally sustainable solution. Therefore, mixtures of C-A-S-H and N-A-S-H gels might be anticipated and the present study assesses the compatibility relationships between them. Compositional diagrams are provided to indicate phase compositional ranges and the phase assemblages obtained under equilibrium conditions. In calcium-rich formulations (pH in excess of 12), C-A-S-H and C2ASH8 form as stable phases. However, in the lime poor part of the diagram an amorphous gel (N,C)-A-S-H precipitates but its stability is dependent on system pH and available Ca. (N,C)-A-S-H gels are de-stabilised by Ca to give C-A-S-H gels in suitable systems. [Copyright &y& Elsevier]

In this study, the hydrated sodium aluminosilicate material was synthesized by one-step hydrothermal alkaline desilication using fly ash (FA) as raw material. The synthesized materials were characterized by XRD, XRF, FT-IR and SEM. The characterization results showed that the alkali-soluble desilication successfully had synthesized the sodium aluminosilicate crystalline (N-A-S-H) phase of sodalite-type (SOD), and the modified material had good ionic affinity and adsorption capacity. In order to figure out the suitability of SOD as an adsorbent for the removal of ammonium and phosphorus from wastewater, the effects of material dosing, contact time, ambient pH and initial solute concentration on the simultaneous removal of ammonium and phosphorus are investigated by intermittent adsorption tests. Under the optimal adsorption conditions, the removal rate of ammonium was 73.3%, the removal rate of phosphate was 85.8% and the unit adsorption capacity reached 9.15 mg/L and 2.14 mg/L, respectively. Adsorption kinetic studies showed that the adsorption of ammonium and phosphorus by SOD was consistent with a quasi-secondary kinetic model. The adsorption isotherm analysis showed that the equilibrium data were in good agreement with the Langmuir and Freundlich model. According to thermodynamic calculations, the adsorption of ammonium and phosphorus was found to be a heat-absorbing and spontaneous process. Therefore, the preparation of SOD by modified FA has good adsorption properties as adsorbent and has excellent potential for application in the removal of contaminants from wastewater. [ABSTRACT FROM AUTHOR]

• High strength binary (BW + CW) and ternary (BW + CW + SCM) geopolymers were developed at ambient curing. • Optimized strengths were reached at balanced molar ratios of SiO 2 /Al 2 O 3 = 8.4 and Na 2 O/SiO 2 = 0.18. • The fresh properties of geopolymers are highly affected by SiO 2 /Al 2 O 3, Na 2 O/SiO 2 and L/S of the geopolymer system. • The effects of high temperature and the addition of supplementary cementitious materials were considered. • The detailed microstructural study proved the efficiency of the employed mix design based on pre-targeted parameters. This study investigates the use of binary system of concrete waste (CW) and red clay brick waste (BW) to develop geopolymer binders with appropriate fresh properties and high mechanical strengths at ambient curing. Binary waste geopolymers were adjusted based on the contents of BW and CW precursors between 20% and 80% and predetermined liquid/solid (L/S) and oxide ratios of SiO 2 /Al 2 O 3 and Na 2 O/SiO 2. The effect of oxide ratios on the fresh and mechanical properties were considered by assessing the flow spread diameter, initial and final setting time and compressive strengths of BW + CW binders. Further optimization of the optimal binary BW + CW mixture was also explored by partial replacement of BW + CW precursors with 15%, 30% and 45% wt.% of ground granulated blast furnace slag (GGBS), metakaolin (MK), class C fly ash (FA-C), class F fly ash (FA-F) at room temperature curing and by applying 24 h initial high temperatures of 50 °C, 75 °C, and 100 °C. Extensive microstructural study with X-ray diffraction and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and Fourier transformed infrared spectroscopy was performed to characterize the optimized compositions at various conditions. The results indicate that 40% CW + 60% BW, at SiO 2 /Al 2 O 3 of 8.4 and Na 2 O/SiO 2 of 0.18, was the optimal binary mix, which reached high mechanical properties at ambient curing. The addition of SCMs with elevated alumina or calcium contents permitted the geopolymer mixture to attain significant strength enhancements with very high mechanical strengths of more than 100 MPa at 28 days at room temperature. An advanced degree of geopolymerization and enhanced formation of intermixed C-A-S-H/C-S-H and N-A-S-H products was confirmed in relation to the well-adjusted SiO 2 /Al 2 O 3 and Na 2 O/SiO 2 ratios and optimized strength geopolymers. [ABSTRACT FROM AUTHOR]

• Recent advances in MD simulations of N-A-S-H type gel are reviewed. • The currently available modeling methods are described in detail. • Thermodynamics, structural characterization, and dynamics properties of N-A-S-H type gels are summarized. • Future opportunities and challenges are also discussed. Geopolymer binders have attracted considerable attention in experimental research, in which their formation mechanism, molecular structure types, and the evolution of their amorphous products have been identified to a wide extent. However, significant challenges and barriers were found in the numerical modeling of such kind of materials. This paper reviews the recently released molecular dynamics (MD) simulation studies associated with the sodium-aluminate-silicate-hydrate (N-A-S-H) type gels that are the main products in geopolymer binders. The data related to the modeling methods, thermodynamics, structural properties, and dynamics behavior of N-A-S-H type gels are collected and evaluated. Despite the limited available literature, the utilization of MD simulation enables exploring the micro-scale properties and structural evolution of geopolymer materials. Future perspectives show that understanding N-A-S-H type gels through MD modeling lays a theoretical foundation for the practical engineering applications of geopolymer materials. [ABSTRACT FROM AUTHOR]

Alkali-activated cements prepared from aluminosilicate powders, such as blast furnace slag and fly ash, are rapidly attracting attention as alternatives to cement because they can significantly reduce CO2 emissions compared to conventional cement concrete. In this study, we investigated the relationship between the physical and chemical changes by accelerated carbonation conditions of alkali-activated cements. Alkali-activated cements were prepared from binders composed of blast furnace slag and fly ash as well as alkali activators sodium silicate and sodium hydroxide. Physical changes were analyzed from compressive strength, pH, and neutralization depth, and chemical changes were analyzed from XRD, TG-DTG, and 29Si MAS NMR. The C–(N)–A–S–H structure is noted to change via carbonation, and the compressive strength is observed to decrease. However, in the case of Na-rich specimens, the compressive strength does not decrease by accelerated carbonation. This work is expected to contribute to the field of alkali-activated cements in the future. [ABSTRACT FROM AUTHOR]

The objective of this study was to optimize the composition of concrete waste (CW)-based geopolymer binders at ambient environment. The goal was to evaluate the reliability of a new algorithmic mix design method that relies on pre-targeting ratios of SiO 2 /Al 2 O 3 , Na 2 O/SiO 2 and liquid/solid of CW precursor powder and sodium silicate and sodium hydroxide alkaline reagents. The effects of elevated temperature curing and the addition of supplementary cementitious materials (SCMs) were also considered. The optimized CW-mixes at ambient curing were subjected to an initial 24 h curing at 50, 75, and 100 °C, and Class C fly ash (FA-C), Class F fly ash (FA-F), metakaolin (MK) and ground granulated blast furnace slag (GGBS) replaced CW at wt.% of 15%, 30% and 45%. The relationship between SiO 2 /Al 2 O 3 and Na 2 O/SiO 2 molar ratios and the fresh properties, including flowability, initial and final setting times, and mechanical strengths were assessed. Furthermore, an extensive microstructural study using X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDS) spectroscopy and Fourier transformed infrared (FTIR) spectroscopy was performed for various mix compositions and curing conditions. The results proved that fresh and mechanical properties are highly dependent on the targeted chemical ratios. Enhanced mechanical strengths of CW-compositions were achieved at adjusted ratios of SiO 2 /Al 2 O 3 = 12.9 and Na 2 O/SiO 2 = 0.23, which were significantly increased by incorporating SCMs with high calcium content. Microstructural assessments indicate that the degree of geopolymerization of CW-based geopolymers and related C(N)-A-S-H formations can be optimized at ambient environment. • New algorithmic mix design based on targeted chemical factors is proposed. • Normal and high strengths CW geopolymers were developed while using ambient curing. • The effects of high temperature and the addition of pozzolanic materials were considered. • Balanced ratios of SiO 2 /Al 2 O 3 = 12.9 and Na 2 O/SiO 2 = 0.23 are important to optimize the compositions. • The detailed microstructural study confirmed the reliability of the algorithmic mix design. [ABSTRACT FROM AUTHOR]

The influence of the compositional parameters Na 2 O/Al 2 O 3 = 0.60–1.08, SiO 2 /Al 2 O 3 = 2.42–3.17 and of 20–60% limestone on the compressive strength, gel composition and structure of alkali-activated metakaolin-limestone cements was studied. The reference formulation of SiO 2 /Al 2 O 3 = 3.17, Na 2 O/Al 2 O 3 = 1.08 and 20%limestone, formed sodium aluminosilicate hydrate (N-A-S-H) intermixed with a Ca-containing N-A-S-H (N-(C)-A-S-H), both with 3D-network structure. A reduced SiO 2 /Al 2 O 3 = 2.42 resulted in the formation of Zeolites and calcium-aluminosilicate-hydrate (C-A-S-H) as secondary phases that reduced the strength. A lower Na 2 O/Al 2 O 3 = 0.60 reduced the reactivity of metakaolin and the Al uptake in the gels, forming silica gel and reduced the strength of mortars. Increasing the limestone load to 60% increased the Ca2+ uptake in N-(C)-A-S-H via ion exchange Na+↔Ca2+ as charge balancers, without modifying the 3D-network structure nor the strength of pastes. Limestone can be considered as a precursor; its addition reduced the alkali consumption, changed the chemical composition and the assemblage of the reaction products, while preserving their 3D-network structure. • Compositional parameters were studied on Limestone-Metakaolin alkaline cements. • The main reaction products were amorphous 3D structured N-A-S-H and N-(C)-A-S-H. • Using 60% limestone increased Ca2+ uptake in N-(C)-A-S-H via ion exchange Na+↔Ca2+. • The 3D-network of N-(C)-A-S-H was preserved with the higher Ca2+ contents. • Reduced SiO 2 /Al 2 O 3 formed zeolites and C-A-S-H and reduced the strength. [ABSTRACT FROM AUTHOR]

• Molecular dynamics simulation of the newly introduced model of N-A-S-H geopolymer. • Using of N-A-S-H structure model validated by experimental results of previous literatures for modulus of elasticity. • Extracting much more realistic outcomes relative to the results of simulations performed in the past. Geopolymer as a new type of alkaline cement with low CaO is produced by the synthesis of aluminosilicate materials with an alkaline solution. The identical structure of geopolymers is still unclear, although a number of structures have been considered. N-A-S-H compound as one of these introduced structures consists of different molecule components. The latest N-A-S-H model for geopolymers includes structural aluminosilicate molecules with various motifs of charge balancing of Q 4 (4 A l) S i seems to be appropriate for molecular dynamic simulations. Since the optimum ratio of Si/Al = 2 is reported by different researchers, in this paper the N-A-S-H model is modified by reconsidering this ratio for molecular dynamics simulations to obtain its modulus of elasticity. Simulation parameters were chosen based on the findings of the past literature review. In order to consider the randomness of the proposed amorphous structure of N-A-S-H several simulations were conducted to find the mean value of the modulus of elasticity. The modulus of elasticity was calculated by an existing stress-strain script in the Material Studio® software. The simulation results showed good agreements with the existing experimental ones. This finding proves that the proposed modified N-A-S-H model is capable of being considered for further research and investigations. [ABSTRACT FROM AUTHOR]

C-A-S-H (CaO-Al₂O₃-SiO₂-H₂O) and N-A-S-H (Na₂O-Al₂O₃-SiO₂-H₂O) have a wide range of chemical compositions and structures and are difficult to separate from alkali-activated materials. Therefore, it is difficult to analyze their microscopic properties directly. This paper reports research on the synthesis of C-A-S-H and N-A-S-H particles with an average particle size smaller than 300 nm by applying the hydrothermal method. The composition and microstructure of the products with different CaO(Na₂O)/SiO₂ ratios and curing conditions were characterized using XRD, the RIR method, FTIR, SEM, TEM, and laser particle size analysis. The results showed that the C-A-S-H system products with a low CaO/SiO₂ ratio were mainly amorphous C-A-S-H gels. With an increase in the CaO/SiO₂ ratio, an excess of Ca(OH)₂ was observed at room temperature, while in a high-temperature reaction system, katoite, C₄AcH 11 , and other crystallized products were observed. The katoite content was related to the curing temperature and the content of Ca(OH)₂ and it tended to form at a high-temperature and high-calcium environment, and an increase in the temperature renders the C-A-S-H gels more compact. The main products of the N-A-S-H system at room temperature were amorphous N-A-S-H gels and a small amount of sodalite. An increase in the curing temperature promoted the formation of the crystalline products faujasite and zeolite-P. The crystallization products consisted of only zeolite-P in the high-temperature N-A-S-H system and its content were stable above 70%. An increase in the Na₂O/SiO₂ ratio resulted in more non-bridging oxygen and the TO₄ was more isolated in the N-A-S-H structure. The composition and microstructure of the C-A-S-H and N-A-S-H system products synthesized by the hydrothermal method were closely related to the ratio of the raw materials and the curing conditions. The results of this study increase our understanding of the hydration products of alkali-activated materials., Competing Interests: The authors declare no conflict of interest.