Effect of nano-silica on the chemical durability and mechanical performance of fly ash based geopolymer concrete

In this study, the effect of nano silica on the short term severe durability performance of fly ash based geopolymer concrete (GPC) specimens was investigated. Four types of GPC were produced with two types of low calcium fly ashes (FAI and FAII) with and without nano silica, and ordinary Portland cement concrete (OPC) concrete was also cast for reference. For the geopolymerization process, the alkaline activator has selected a mixture of sodium silicate solution (Na₂SiO₃) and sodium hydroxide solution (NaOH) with a ratio (Na₂SiO₃/ NaOH) of 2.5. Main objectives of the study were to investigate the effect of usability or replaceability of nano silica-based low calcium fly ash based geopolymer concretes instead of OPC concrete in structural applications and make a contribution to standardization process of the fly ash based geopolymer concrete. To achieve the goals, four types of geopolymer and OPC concretes were subjected to sulfuric acid (H₂SO₄), magnesium sulfate (MgSO₄) and seawater (NaCl) solutions with concentrations of 5%, 5%, and 3.5%, respectively. Visual appearances and weight changes of the concretes under chemical environments were utilized for durability aspects. Compressive, splitting tensile and flexural strength tests were also performed on specimens to evaluate the mechanical performance under chemical environments. Results indicated that FAGPC concretes showed superior performance than OPC concrete under chemical attacks due to low calcium content. Amongst the chemical environments, sulfuric acid (H₂SO₄) was found to be the most dangerous environment for all concrete types. In addition, nano silica (NS) addition to FAGPC specimens improved both durability and residual mechanical strength due to the lower porosity and more dense structure. The FAIIGPC specimens including nano silica showed the superior mechanical performance under chemical environment.     

Compressive strength of fly‐ash‐based geopolymer concrete at elevated temperatures

This paper presents the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures of 200, 400, 600 and 800 °C. The source material used in the geopolymer concrete in this study is low‐calcium fly ash according to ASTM C618 class F classification and is activated by sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) solutions. The effects of molarities of NaOH, coarse aggregate sizes, duration of steam curing and extra added water on the compressive strength of geopolymer concrete at elevated temperatures are also presented. The results show that the fly‐ash‐based geopolymer concretes exhibited steady loss of its original compressive strength at all elevated temperatures up to 400 °C regardless of molarities and coarse aggregate sizes. At 600 °C, all geopolymer concretes exhibited increase of compressive strength relative to 400 °C. However, it is lower than that measured at ambient temperature. Similar behaviour is also observed at 800 °C, where the compressive strength of all geopolymer concretes are lower than that at ambient temperature, with only exception of geopolymer concrete containing 10 m NaOH. The compressive strength in the latter increased at 600 and 800 °C. The geopolymer concretes containing higher molarity of NaOH solution (e.g. 13 and 16 m ) exhibit greater loss of compressive strength at 800 °C than that of 10 m NaOH. The geopolymer concrete containing smaller size coarse aggregate retains most of the original compressive strength of geopolymer concrete at elevated temperatures. The addition of extra water adversely affects the compressive strength of geopolymer concretes at all elevated temperatures. However, the extended steam curing improves the compressive strength at elevated temperatures. The Eurocode EN1994:2005 to predict the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures agrees well with the measured values up to 400 °C. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of sodium polyacrylate on mechanical properties and microstructure of metakaolin-based geopolymer with different SiO2/Al2O3 ratio

The mechanical properties and microstructure of geopolymer are affected by the molar ratio of SiO₂/Al₂O₃. Meanwhile, organic polymer has the effect of improving the toughness of geopolymer, which depends on the SiO₂/Al₂O₃ ratio of geopolymer inevitably. Therefore, it is important to investigate the effect of the organic polymer on the mechanical properties and microstructure of geopolymer with varying SiO₂/Al₂O₃ ratio for using organic polymer to modify geopolymer. In this work, the SiO₂/Al₂O₃ ratios of metakaolin-based geopolymers are adjusted to 2.0, 2.5, 3.0, 3.5 and 4.0 by adding silica fume and β-Al₂O₃, with Na₂O/SiO₂, H₂O/SiO₂ being maintained at 0.2, 4.0, respectively. The geopolymers with each SiO₂/Al₂O₃ ratios are modified by addition of 0, 0.4, 0.8, 1.2 and 1.6?wt% of sodium polyacrylate (PAAS).The mechanical properties of these samples are measured and the rate of change is used to characterize the effect of PAAS on the metakalin-based geopolymers. The mechanism is also shown by 29Si NMR, XPS and FTIR. The results show that the effects of polymer on the mechanical properties of metakaolin-based geopolymer are affected by SiO₂/Al₂O₃ ratio and the effect becomes less obvious with SiO₂/Al₂O₃ ratio increasing from 2.0 to 4.0. Incorporation of PAAS can reduce the degree of polymerization of [SiO]₄ or [AlO]₄ in geopolymer and form the Si?O?C bond, which are two main reasons for polymer improving the toughness of geopolymer. But these effects decrease when the SiO₂/Al₂O₃ ratio of geopolymer increases from 2.0 to 4.0, which is corresponding to the effect on the mechanical properties. The toughening effect of organic polymer on geopolymer depends on the SiO₂/Al₂O₃ ratio of geopolymer, and only the geopolymer with lower SiO₂/Al₂O₃ ratio (no more than 2.5 in this work) can be significantly toughening modified by organic polymer. Therefore, it is necessary to consider the SiO₂/Al₂O₃ ratio of the geopolymer when geopolymer modified by organic polymer is designed.     

Synthesis of fly ash based geopolymer mortar considering different concentrations and combinations of alkaline activator solution

Geopolymerisation is a process that can transform alumina and silica rich waste materials into valuable binding materials, having excellent mechanical properties. The present experimental study shed a light on the variation in compressive strength of fly ash based geopolymer mortar by varying the molarity of sodium hydroxide as 12 M, 14 M, 16 M and accompanying by sodium silicate (Na₂SIO₃) in 2:1 (Na₂SIO₃/ NaOH) with same molarities. All the geopolymer mixes were oven cured at 80 °C for 24 h and after that kept at room temperature up to the time of testing. The compressive strength was checked subsequently at the ages of 3, 7, 14 and 28 days. The experimental results reveal that the addition of sodium silicate enhances the strength development in geopolymer mortar. The ultimate compressive strength of 40.42 MPa was obtained by incorporating sodium silicate along with 16 M concentrated sodium hydroxide. Furthermore, increasing trend of the compressive strength was found with increasing molar concentration of sodium hydroxide and curing period.     

Damage evaluation and deformation behavior of mine tailing-based Geopolymer under uniaxial cyclic compression

Geopolymerized mine tailings (MTs), as an alternative to reuse the mine wastes, can be used for construction materials (e.g., geopolymer concrete and bricks) depending on their mechanical properties. Their strength values, which can range from a couple of MPa to tens of MPa, are significant evidence for their application in the construction industry. In practice, geopolymers activated with different NaOH molarities can significantly affect the mechanical properties of MTs. The mechanical behavior of geopolymers under monotonic loading also has been widely investigated. However, the potential hazard of the exposure of geopolymer concrete/bricks to cyclic loading has received limited attention. This paper presents a study we conducted on geopolymers made by activation of MTs under cyclic loading to understand their crack and damage behaviors, including the influence of factors such as NaOH molarity and loading patterns. The influence of NaOH molarity on the elastic and plastic strains of the geopolymer specimen at different cycles was explored. A series of unconfined compression tests of cubic specimens with different NaOH molarities as well as microscopic investigations and observations via XRD, FTIR, and SEM were carried out in this study. The Young's modulus of the geopolymer was found to increase followed by a decrease with the cycles for all the selected NaOH molarities. The geopolymers activated with lowest NaOH molarity were first to start damage and activated with the highest NaOH molarity were the last to damage. The damage variable was shown to increase rapidly at the initial cycles and then gradually approached the maximum value.     

Synthesis and thermal properties of inorganic polymers (geopolymers) for structural and refractory applications from volcanic ash

The volcanic ash occurring as an abundant and readily accessible natural resource in the Central African country of Cameroon was used to synthesize aluminosilicate geopolymers using sodium hydroxide as the sole alkaline activator. Both the curing conditions and the Na₂O/SiO₂ molar ratio were found to influence the development of compressive strength of the geopolymer cement paste, which achieved a maximum strength of 55 MPa at Na₂O/SiO₂ = 0.3. The formation of a mortar by the addition of 40 wt% sand to the optimized geopolymer cement composition reduced the compressive strength to 30 MPa, still within the useful range for construction applications. The geopolymers consist largely of X-ray amorphous material with a small content of crystalline phases. Scanning electron microscopy showed a homogenously distributed mixture of lath-shaped and agglomerated morphologies, with a homogeneous distribution of Si, Al and O in the geopolymer matrix. The geopolymers are relatively stable to heat, shrinking only slowly and retaining about 60% of their as synthesized compressive strength on heating to 900 °C. The FTIR spectra of both the as synthesized and heated geopolymers show two broad absorbance bands, between 820-1250 cm⁻¹ and 450-730 cm⁻¹ assigned to the internal vibrations of Si-O-Si, and Si-O-Al respectively. The compressive strengths and the thermal stability of these materials suggest their suitability for building applications and low-grade refractories.     

Microstructure of geopolymer accounting for associated mechanical characteristics under various stress states

Geopolymer was prepared with various SiO₂/Na₂O mole ratios and mechanical tests and microstructural analyses are performed to investigate how the constituents affect its mechanical behavior in distinct stress states. Laboratory results reveal that the SiO₂/Na₂O ratio affects the polymerization by influencing the formation of silicon Q⁴(mAl) structures. The proportion of Q⁴(4Al) correlates positively with the mechanical characteristics of geopolymer, and the proportion of Q⁴(2Al) correlates negatively with the mechanical characteristics of geopolymer. The proportions of Q⁴(mAl) affect the stress–strain curve and the failure modes of geopolymer under various confining pressures. Three types of stress–strain curves with different peak strengths and plastic deformations are obtained. Incomplete polymerization generates a geopolymer with an imperfect microstructure, which determines the plastic deformation while unloading. Polymerization of a geopolymer affects its apparent cohesion and friction angle. However, the friction-induced strength declines drastically when the failure mode changes from the split mode to the shear mode.     

Fast microwave syntheses of fly ash based porous geopolymers in the presence of high alkali concentration

Microwave synthesis of porous fly ash geopolymers was achieved using a household microwave oven. Fly ash paste containing SiO₂ and Al₂O₃ component was mixed with sodium silicate (Na₂SiO₃) solutions at different concentrations of sodium hydroxide (NaOH) of 2, 5, 10, and 15 M, which were used as NaOH activators of geopolymerization. The mass ratio of Na₂SiO₃/NaOH was fixed at 2.5 with SiO₂/Al₂O₃ at 2.69. After the fly ash and alkali activators were mixed for 1 min until homogeneous, the geopolymer paste was cured for 1 min using household microwave oven at different output powers of 200, 500, 700, and 850 W. Porous geopolymers were formed immediately. Micro X-ray CT and SEM results showed that the porous structure of the geopolymers was developed at higher NaOH concentrations when using 850 W power of the microwave oven. These results derive from the immediate increase of the temperature in the geopolymer paste at higher NaOH concentrations, meaning that aluminosilicate bonds formed easily in the geopolymers within 1 min.     

Thin fly ash/ ladle furnace slag geopolymer: Effect of elevated temperature exposure on flexural properties and morphological characteristics

The flexural properties and thermal performance of 10 mm-thin geopolymers made from fly ash and ladle furnace slag were evaluated before and after exposure to elevated temperatures (300 °C, 600 °C, 900 °C, 1100 °C and 1150 °C). Class F fly ash was mixed with liquid sodium silicate (Na₂SiO₃) and 12 M sodium hydroxide (NaOH) solution using aluminosilicate/activator ratio of 1:2.5 and Na₂SiO₃/NaOH ratio of 1:4 to synthesise thin fly ash (FA) geopolymers. 40 wt% of ladle furnace slag was partially replacing fly ash to produce fly ash/slag-based (FAS) geopolymers. Thermal treatment enhanced the flexural strength of thin geopolymers. In comparison to the unexposed specimen, the flexural strength of FA geopolymers at 1150 °C and FAS geopolymers 1100 °C was increased by 161.3% to 16.2 MPa and 208.9% to 24.1 MPa, respectively. A more uniform heating was achieved in thin geopolymers which favoured the phase transformation at high temperatures and contributed to the substantial increase in flexural strength. The joint effect of elevated temperature exposure and the incorporation of ladle furnace slag further improved the flexural strength of thin geopolymers. The calcium-rich slag refined the pore structure and increased the crystallinity of thin geopolymers which aided in high strength development.     

Mechanical,microstructural and mineralogical evaluation of alkali-activated waste glass and stone wool

Mineral waste wool represents a significant part of construction and demolition waste (CDW) not yet being successfully re-utilized. In the present study, waste stone wool (SW) and glass wool (GW) in the form received, without removing the binder, were evaluated for their potential use in alkali activation technology. It was confirmed that both can be used in the preparation of alkali-activated materials (AAMs), whether cured at room temperature or at an elevated temperature in order to speed up the reaction. The results show that it is possible to obtain a compressive strength of over 50 MPa using SW or GW as a precursor. A strength of 53 MPa was obtained in AAM based on GW after curing for 3 days at 40 °C, while a similar compressive strength (58 MPa) was achieved after curing the GW mixture for 56 days at room temperature. In general, the mechanical properties of samples based on GW are better than those based on SW. The evolution of mechanical properties and recognition of influential parameters were determined by various microstructural analyses, including XRD, SEM, MIP, and FTIR. The type of activator (solely NaOH or a combination of NaOH and sodium silicate), and the SiO₂/Na₂O and liquid to solid (L/S) ratios were found to be the significant parameters. A lower SiO₂/Na₂O ratio and low L/S ratio significantly improve the mechanical strength of AAMs made from both types of mineral wool.     

Fire resistance of geopolymer concrete produced from Elazığ ferrochrome slag

This paper presents the effect of elevated temperatures up to 700 °C on compressive strength and water absorption of two alkali‐activated aluminosilicate composites (one of them is river sand aggregate geopolymer concrete; the other one is crushed sand aggregate geopolymer concrete) and ordinary Portland cement based concretes. To obtain binding geopolymer material, Elaz?? ferrochrome slag was ground as fine as cement, and then it was alkali activated with chemical (NaOH and Na₂SiO₃). Geopolymer concrete samples were produced by mixing this binding geopolymer material with aggregates. At each target temperature, concrete samples were exposed to fire for the duration of 1 h. Fire resistance and water absorption of geopolymer and ordinary Portland cement concrete samples were determined experimentally. Experimental results indicated that compressive strength of geopolymer concrete samples increased at 100 °C and 300 °C temperatures when compared with unexposed samples. In geopolymer concrete samples, the highest compressive strength was obtained from river aggregates ones at 300 °C with 37.06 MPa. Water absorption of geopolymer concrete samples increased at 700 °C temperature when compared with unexposed samples. However, a slight decrease in water absorption of concrete samples was observed up to 300 °C when compared with unexposed samples. SEM and X‐ray diffraction tests were also carried out to investigate microstructure and mineralogical changes during thermal exposure. Copyright © 2016 John Wiley & Sons, Ltd.     

Kinetics of geopolymerization: Role of Al2O3 and SiO2

The early-stage reaction kinetics of metakaolin/sodium silicate/sodium hydroxide geopolymer system have been investigated. The setting and early strength development characteristics, and associated mineral and microstructural phase development of mixtures containing varying SiO₂/Al₂O₃ ratios, cured at 40 °C for up to 72 h, were carefully studied. It was observed that setting time of the geopolymer systems was mainly controlled by the alumina content. Essentially, the setting time increased with increasing SiO₂/Al₂O₃ ratio of the initial mixture. Up to a certain limit, the SiO₂/Al₂O₃ ratio was also found to be responsible for observed high-strength gains at later stages. An increase in the Al₂O₃ content, i.e. for low SiO₂/Al₂O₃ ratio, led to products of low strength, accompanied by microstructures with increased amounts of Na-Al-Si-containing “massive” phases (grains). EDAX analyses showed that the SiO₂/Al₂O₃ ratios of geopolymer gel phases were quite similar to those of the starting mixtures, but with an overall lower Na content. Most importantly, this study clearly demonstrates that the properties of resulting geopolymer systems can be drastically affected by minor changes in the available Si and Al concentrations during synthesis.     

Use of glass waste as an activator in the preparation of alkali-activated slag. Mechanical strength and paste characterisation

The alkaline activation of aluminosilicates yields alkaline cements, eco-efficient alternatives to ordinary Portland cements. Alkaline cements and concretes exhibit highest strength and longest durability when activated with a solution of alkaline silicate hydrates (waterglass). To obtain these alkaline silicates, however, an aqueous solution of the proper proportion of carbonate and silica salts must be heated to temperatures of around 1300 °C. The present paper explores the feasibility of using urban and industrial glass waste as a potential alkaline activator for blast furnace slag (AAS).AAS pastes were prepared with three activators: waterglass, a NaOH/Na₂CO₃ mix and the solutions resulting from dissolving glass waste in NaOH/Na₂CO₃. Mechanical, mineralogical (XRD, FTIR) and microstructural (porosimetry, NMR and SEM/EDX) trials were conducted to characterise the pastes obtained.The findings proved the feasibility of using glass waste to alkali activate slag. Treating glass waste with NaOH/Na₂CO₃ (pH = 13.6) favours the partial dissolution of the Si in the glass into its most reactive monomeric form.The solutions resulting from the treatment of glass waste act as alkaline activators, partially dissolving vitreous blast furnace slag. The composition and microstructure of the reaction products identified in the two types of paste were similar. Strength and microstructural development in the pastes activated with glass waste were also comparable to the parameters observed in AAS pastes prepared with conventional activators.     

Alkali activation of a novel calcium‐silicate hydraulic binder with CaO/SiO2 = 1.1

A quasi‐amorphous low‐calcium‐silicate hydraulic binder, with an overall CaO/SiO₂ (C/S) molar ratio of 1.1, was produced. This cementitious material was then hydrated with aqueous solutions containing 3 wt% alkalis (either NaOH, Na₂CO₃ or Na₂SiO₃). The evolution of the hydration processes of the samples were monitored by compressive strength testing, XRD, FTIR, ²⁹Si and ²⁷Al MAS NMR, isothermal calorimetry and TGA. It was found that the nearly exclusive hydration product formed was a C‐S‐H phase with a semi‐crystalline structure. More importantly, the paste prepared with the Na₂SiO₃ solution developed compressive strength values similar to those of ordinary portland cements (OPC) with faster early age kinetics. In addition, the isothermal calorimetry results indicated that these new hydraulic binders present much lower heat of hydration values compared with a traditional OPC. The results presented here open the possibility of producing cement with a compressive strength comparable to that of OPC but with lower CO₂ emissions during the production process and with lower hydration heat related problems during the production of concrete structures.     

Assessment of mechanical and microstructural properties of geopolymers produced from metakaolin,silica fume,and red mud

Geopolymers have been studied as viable alternative to traditional Portland cement-based products, given the use of industrial by-products as raw materials. This work evaluated the mechanical and microstructural properties of geopolymeric mortars produced with sodium hydroxide solution, metakaolin, silica fume, and red mud. The mixtures were produced by means of dosages with different molar ratios and curing conditions. The raw materials were characterized by granulometry, chemical, mineralogical, and thermal analysis. The characterization of mortars was performed by scanning electron microscopy (SEM) and axial compressive strength tests. The precalcination at 850°C of the red mud was sufficient to make it more reactive and suitable for use in geopolymers. Noteworthy, the best mechanical strengths of metakaolin mortars for curing at 50°C, and with the lowest SiO₂/Al₂O₃ ratios. In the mortars with incorporated red mud, there was a decrease of strength at thermal curing conditions and with the increase of residue content, whose microstructure indicates the formation of more pores in the geopolymer matrix. The thermal curing promoted the formation of sodalite crystals, and the significant presence of Na particles on the surface suggests that part of the added NaOH did not react with the precursors.     

One‐Part Geopolymers Based on Thermally Treated Red Mud/NaOH Blends

In this study, one‐part “just add water” geopolymer binders are synthesized through the alkali‐thermal activation of the red mud which is relatively rich in both alumina and calcium. Calcination of the red mud with sodium hydroxide pellets at 800°C leads to decomposition of the original silicate and aluminosilicate phases present in the red mud, which promotes the formation of new compounds with hydraulic character, including a partially ordered peralkaline aluminosilicate phase and the calcium‐rich phases C₃A and α‐C₂S. The hydration of the “one‐part geopolymer” leads to the formation of zeolites and a disordered binder gel as the main reaction products, and the consequent development of compressive strengths of up to 10 MPa after 7 d of curing. These results demonstrate that red mud is an effective precursor to produce one‐part geopolymer binders, via thermal and alkali‐activation processes.     

Synthesis of geopolymers from volcanic ash via the alkaline fusion method: Effect of Al2O3/Na2O molar ratio of soda–volcanic ash

The alkaline fusion method was used to enhance the reactivity of volcanic ash for geopolymer synthesis. To that end, different mixtures of fused soda–volcanic ash (fused volcanic ash) were used to assess reactivity for geopolymer synthesis. The amount of amorphous phase was determined both in the volcanic ash and the fused volcanic ash and X-ray diffraction analysis was used to evaluate effect of the alkaline fusion method. Different geopolymer mortars were prepared by alkaline activation of mixtures of powders of fused volcanic ash and metakaolin and river sand using sodium silicate as activator. Metakaolin was considered as consumer of excess of alkali contained in the fused volcanic ash. The geopolymer mortars were characterized by determination of setting time, linear shrinkage, compressive strength and scanning electron microscopy. The amount of amorphous phase and excess of fused soda content of the fused volcanic ash depended on molar ratio of Al₂O₃/Na₂O and played a key role for geopolymer synthesis. The most convenient Al₂O₃/Na₂O molar ratio of fused volcanic ash to produce effective geopolymer mortars ranged between 0.13 and 0.18. This study showed that volcanic ash can be used successfully as an alternative raw material for production of geopolymers via alkaline activation of fused volcanic ash.     

Study on modification of the high-strength slag cement material

The influence of the slag powder's fineness, the amounts of activator, type and contents of modification addition on the dry-shrinkage and strength of the high-strength slag cement material was investigated. The experimental data showed that adding 9% Na₂SiO₃ activator and 10% Portland cement (PC) made the ratios of drying-shrinkage of high-strength slag cement material similar to the ratios of Portland cement and the compressive strengths as higher. The main hydration products are calcium alumina-silicate gels and a little CH; the gel ratio of CaO/SiO₂ is close to 1 and includes a little Na₂O and MgO for high-strength slag cement material, as shown by means of scanning electron microscope (SEM) and energy-dispersive X-ray analyzer (EDXA).     

Solidification of electroplating sludge using alkali-activated pulverized fuel ash as cementitious binder

This work investigated the potential for utilization of alkali-activated PFA as solidification binder to treat electroplating sludge. The sludge was solidified using 30 wt.% of lime and 70 wt.% of PFA. Two alkali activators, Na₂SiO₃ and Na₂CO₃, were added at 0, 4, 6, and 8 wt.%. Results showed that early strength development of lime-PFA cements with Na₂SiO₃ and Na₂CO₃ was considerably higher than those without. Addition of electroplating sludge resulted in reduced strength. The strength reduction was greater when 4% Na₂SiO₃ activator was used than when 8% Na₂CO₃ activator was used. A higher pH of Na₂SiO₃ solution (pH=13.5) compared to that of Na₂CO₃ solution (pH=11.9) resulted in resolubilization of metal hydroxides from the electroplating sludge, which competed with calcium ion for soluble silicate. In addition, Pb, Cd, and Cu were not found in the toxicity characteristic leaching procedure (TCLP) leachates. Cr, Zn, and Fe were detected and in some cases Cr exceeded U.S. EPA allowable limits.     

Microstructural and strength improvements through the use of Na2CO3 in a cementless Ca(OH)2-activated Class F fly ash system

This study explores the beneficial effects of Na₂CO₃ as an additive for microstructural and strength improvements in a Ca(OH)₂-activated fly ash system. NaOH-activated fly ash samples were also tested to compare the effect of Na₂CO₃. Compressive strength testing, XRD, SEM/BSE/EDS, ²⁹Si/²⁷Al MAS-NMR, MIP and TGA were performed. The testing results indicate that the use of Na₂CO₃ for Ca(OH)₂-activation led to a noticeable improvement in strength and microstructure, primarily due to (1) more dissolution of raw fly ash at an early age, (2) more formation of C–S–H [or C–S–H(I)], (3) porosity reduction, and (4) pore-size refinement. We also found that (1) an early high alkalinity from the NaOH formation was not a major cause of strength, (2) geopolymer was not formed despite the early NaOH formation, and (3) no visible pore-filling action of CaCO₃ was observed. However, Na₂CO₃ did not produce any improvement in strength for NaOH-activated fly ash.