Resistance to Sulfuric Acid Corrosion of Geopolymer Concrete Based on Different Binding Materials and Alkali Concentrations

Geopolymer binder is expected to be an optimum alternative to Portland cement due to its excellent engineering properties of high strength, acid corrosion resistance, low permeability, good chemical resistance, and excellent fire resistance. To study the sulfuric acid corrosion resistance of geopolymer concrete (GPC) with different binding materials and concentrations of sodium hydroxide solution (NaOH), metakaolin, high-calcium fly ash, and low-calcium fly ash were chosen as binding materials of GPC for the geopolymerization process. A mixture of sodium silicate solution (Na₂SiO₃) and NaOH solution with different concentrations (8 M and 12 M) was selected as the alkaline activator with a ratio (Na₂SiO₃/NaOH) of 1.5. GPC specimens were immersed in the sulfuric acid solution with the pH value of 1 for 6 days and then naturally dried for 1 day until 98 days. The macroscopic properties of GPC were characterized by visual appearance, compressive strength, mass loss, and neutralization depth. The materials were characterized by SEM, XRD, and FTIR. The results indicated that at the immersion time of 28 d, the compressive strength of two types of fly ash-based GPC increased to some extent due to the presence of gypsum, but this phenomenon was not observed in metakaolin-based GPC. After 98 d of immersion, the residual strength of fly ash based GPC was still higher, which reached more than 25 MPa, while the metakaolin-based GPC failed. Furthermore, due to the rigid 3D networks of aluminosilicate in fly ash-based GPC, the mass of all GPC decreased slightly during the immersion period, and then tended to be stable in the later period. On the contrary, in metakaolin-based GPC, the incomplete geopolymerization led to the compressive strength being too low to meet the application of practical engineering. In addition, the compressive strength of GPC activated by 12 M NaOH was higher than the GPC activated by 8 M NaOH, which is owing to the formation of gel depended on the concentration of alkali OH ion, low NaOH concentration weakened chemical reaction, and reduced compressive strength. Additionally, according to the testing results of neutralization depth, the neutralization depth of high-calcium fly ash-based GPC activated by 12 M NaOH suffered acid attack for 98 d was only 6.9 mm, which is the minimum value. Therefore, the best performance was observed in GPC prepared with high-calcium fly ash and 12 M NaOH solution, which is attributed to gypsum crystals that block the pores of the specimen and improve the microstructure of GPC, inhibiting further corrosion of sulfuric acid.     

Process Development of Fly Ash-Based Geopolymer Mortars in View of the Mechanical Characteristics

This study aimed to determine the effects of design parameters, including the liquid/solid ratio (L/S), Na₂SiO₃/NaOH weight ratio, and curing temperature, on class F fly ash-based geopolymer composites. For this purpose, two disparate sources of fly ash were supplied from Çatalağzı (FA) and İsken Sugözü (FB) Thermal Power Plants in Turkey. Two different L/S ratios of 0.2 and 0.4 were used. The Na₂SiO₃/NaOH ratios in the alkaline solutions were 1, 1.5, 2, 2.5, and 3 by weight for each type of geopolymer mixture. Then, 40 different mixes were cured at two specific temperatures (70 °C and 100 °C) for 24 h and then preserved at room temperature until testing. Thereafter, the physical water absorption properties, apparent porosity, and bulk density were examined at 28 days on the hardened mortars. Additionally, compressive and flexural tests were applied to the geopolymers at 7, 28, and 90 days. It was found that the highest compressive strength was 60.1 MPa for the geopolymer manufactured with an L/S of 0.2 and Na₂SiO₃/NaOH ratio of 2. Moreover, the best thermal curing temperature for obtaining optimal strength characteristics was 100 °C for the FB.     

Effect of Solids-To-Liquids,Na2SiO3-To-NaOH and Curing Temperature on the Palm Oil Boiler Ash (Si + Ca) Geopolymerisation System

This paper investigates the effect of the solids-to-liquids (S/L) and Na₂SiO₃/NaOH ratios on the production of palm oil boiler ash (POBA) based geopolymer. Sodium silicate and sodium hydroxide (NaOH) solution were used as alkaline activator with a NaOH concentration of 14 M. The geopolymer samples were prepared with different S/L ratios (0.5, 1.0, 1.25, 1.5, and 1.75) and Na₂SiO₃/NaOH ratios (0.5, 1.0, 1.5, 2.0, 2.5, and 3.0). The main evaluation techniques in this study were compressive strength, X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electron Microscope (SEM). The results showed that the maximum compressive strength (11.9 MPa) was obtained at a S/L ratio and Na₂SiO₃/NaOH ratio of 1.5 and 2.5 at seven days of testing.     

Geopolymer Concrete with Lightweight Artificial Aggregates

This article presents the physical and mechanical properties of geopolymer concrete with lightweight artificial aggregate. A research experiment where the influence of fly ash–slag mix (FA-S), as part of a pozzolanic additive, on the properties of geopolymers was carried out and the most favorable molar concentration of sodium hydroxide solution was determined. The values of three variables of the examined properties of the geopolymer lightweight concrete (GLC) were adopted: X₁—the content of the pozzolanic additives with fly ash + flay ash–slag (FA + FA-S) mix: 200, 400 and 600 kg/m³; X₂—the total amount of FA-S in the pozzolanic additives: 0, 50 and 100%; X₃—the molarity of the activator NaOH: (8, 10 and 12 M). In order to increase the adhesion of the lightweight artificial aggregate to the geopolymer matrix, the impregnation of the NaOH solution was used. Based on the obtained results for the GLC’s compressive strength after 28 days, water absorption, dry and saturated density and thermal conductivity index, it was found that the most favorable parameters were obtained with 400 kg/m³ of pozzolanic additives (with 50% FA-S and 50% FA) and 10 NaOH molarity. Changes in the activator’s concentration from 8 to 10 M improved the compressive strength by 54% (for a pozzolana content of 200 kg/m³) and by 26% (for a pozzolana content of 600 kg/m³). The increase in the content of pozzolanic additives from 200 to 400 kg/m³ resulted in a decrease in water absorption from 23% to 18%. The highest conductivity coefficient, equal to 0.463 W/m·K, was determined, where the largest amount of pozzolanic additives and the least lightweight aggregate were added. The structural tests used scanning electron microscopy analysis, and the beneficial effect of impregnating the artificial aggregate with NaOH solution was proved. It resulted in a compact interfacial transition zone (ITZ) between the lightweight aggregate and the geopolymer matrix because of the chemical composition (e.g., silica amount), the silica content and the alkali presoaking process.     

Influence of Accelerating Admixtures on the Reactivity of Synthetic Aluminosilicate Glasses

This research aims at gaining a further understanding of the impact of accelerating admixtures on the reactivity of supplementary cementitious materials (SCMs), which are widely used as a clinker replacement in blended cements. This was done on synthetic glasses with controlled composition and structure that mimic two types of real SCMs (slag and calcium-rich fly ash). The effects of DEIPA, TIPA, NaSCN and Na₂S₂O₃ on the glass dissolution, hydration kinetics and reaction products were investigated. The obtained results concluded that the pH of the NaOH solution and the composition of the synthetic glass play a key role on the effect of the admixtures. In 0.1 M NaOH (pH = 13.0), all the studied admixtures inhibited the dissolution of slag-like glasses while they enhanced the dissolution of Ca-rich fly ash-like glasses, being Na₂S₂O₃ the admixture that led to the highest increase of the dissolution rate of the Ca-rich fly ash-type glasses. In 1 M NaOH solutions (pH = 13.8), only the alkali admixtures (NaSCN and Na₂S₂O₃) enhanced the degree of reaction of both glasses. In slag-type glasses pastes mixed with 1 M NaOH, the addition of 2% Na₂S₂O₃ induced the highest increase of their reactivity as inferred by the total heat release and the amount of bound water. This is related to the formation of a high amount of S(II)-AFm, in addition to C-A-S-H, that would increase the aluminium undersaturation of the pore solution and consequently the further dissolution of the glass.     

Experimental Study of the Resistance to Influence of Aggressive Liquids on Lightweight Concrete

In light of the scientific research, the corrosion of concrete structures is one of the main problems that may reduce their durability due to the negative impact of the natural environment. The paper analyzes the influence of the type of component on the selected properties of lightweight concrete subjected to the influence of aggressive liquids. Four concrete mixes were prepared with a granular aggregate made of foamed glass (GEGA) and aggregate made of sintered fly ash (GAA) with the use of a mineral additive: silica fly ash. The prepared lightweight concrete after one year was exposed for 60 days to the following environments: strong acid—HCl, 1% and 2% concentration, weak acid—CH₃COOH, 1% and 2% concentration, and an aqueous salt solution of Na₂SO₄, 1% and 2% concentration. Then, the compressive strength was tested, and the microstructure analysis of the ready-made lightweight concrete (LWC) was performed. The degree of penetration of aggressive solutions into the cracks of the samples was assessed by means of applying 1% phenolphthalein solution. Changes in the weight of lightweight concrete samples after the test period were estimated. The obtained test results indicate that the decrease in the durability of lightweight concrete can be classified as a long-term process. Concrete with GEGA and GAA showed high resistance to aggressive environments. Moreover, the environment containing chlorides turned out to be the most aggressive, while the environment containing sulfates proved to be the least aggressive. The higher the concentration of the destructive factor was, the faster the corrosion process went. This has been proven by measuring the pH using phenolphthalein and carrying out microscopic examination. Concretes containing aggregates made of foamed glass and sintered fly ash are suitable for use both in traditional construction and in facilities exposed to an aggressive environment (e.g., in the chemical industry and at gas stations).     

Resistance of Soda Residue–Fly Ash Based Geopolymer Mortar to Acid and Sulfate Environments

The early mechanical performances of low-calcium fly ash (FFA)-based geopolymer (FFA–GEO) mortar can be enhanced by soda residue (SR). However, the resistance of SR–FFA–GEO mortar to acid or sulfate environments is unclear, owing to the various inorganic calcium salts in SR. The aim of this study was to investigate the long-term mechanical strengths of up to 360 d and evaluate the resistance of SR–FFA–GEO mortar to 5% HCl and 5% Na₂SO₄ environments through the losses in compressive strength and mass. Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS) and Fourier Transform Infrared Spectrometer (FTIR) experiments were conducted for the SR–FFA–GEO mortars, both before and after chemical attack, to clarify the attack mechanism. The results show that the resistances of the SR–FFA–GEO mortar with 20% SR (namely M10) to 5% HCl and 5% Na₂SO₄ environments are superior to those of cement mortar. The environmental HCl reacts with the calcites in SR to produce CaCl₂, CO₂ and H₂O to form more pores under HCl attack, and the environmental Na⁺ cations from Na₂SO₄ go into Si-O-Al network structure, to further enhance the strength of mortar under Na₂SO₄ attack. These results provide the experimental basis for the durability optimization of SR–FFA–GEO mortars.     

Evaluation on the Mechanical Properties of Ground Granulated Blast Slag (GGBS) and Fly Ash Stabilized Soil via Geopolymer Process

This study intended to address the problem of damaged (collapsed, cracked and decreased soil strength) road pavement structure built on clay soil due to clay soil properties such as low shear strength, high soil compressibility, low soil permeability, low soil strength, and high soil plasticity. Previous research reported that ground granulated blast slag (GGBS) and fly ash can be used for clay soil stabilizations, but the results of past research indicate that the road pavement construction standards remained unfulfilled, especially in terms of clay’s subgrade soil. Due to this reason, this study is carried out to further investigate soil stabilization using GGBS and fly ash-based geopolymer processes. This study investigates the effects of GGBS and ratios of fly ash (solid) to alkaline activator (liquid) of 1:1, 1.5:1, 2:1, 2.5:1, and 3:1, cured for 1 and 7 days. The molarity of sodium hydroxide (NaOH) and the ratio of sodium silicate (Na₂SiO₃) to sodium hydroxide (NaOH) was fixed at 10 molar and 2.0 weight ratio. The mechanical properties of the soil stabilization based geopolymer process were tested using an unconfined compression test, while the characterization of soil stabilization was investigated using the plastic limit test, liquid limit test, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The results showed that the highest strength obtained was 3.15 MPA with a GGBS to alkaline activator ratio of 1.5 and Na₂SiO₃ to NaOH ratio of 2.0 at 7 days curing time. These findings are useful in enhancing knowledge in the field of soil stabilization-based geopolymer, especially for applications in pavement construction. In addition, it can be used as a reference for academicians, civil engineers, and geotechnical engineers.     

Investigations on the Performance of Shotcrete Using Artificial Lightweight Shale Ceramsite as Coarse Aggregate

In this study, a type of artificial lightweight shale ceramsite (ALSC) was used as the coarse lightweight aggregate for shotcrete (LAS), of which the mechanical properties, chloride penetration ion resistance, and rebound behavior were investigated. Based on the experimental results on compressive, tensile, and bond strength, LAS meet the strength requirements, and the replacement rate of fly ash (FA) and silica fume (SF) are suggested to be kept at ~15% and 10%, respectively, to result in the best mechanical properties of LAS. Adding FA and SF to the mixture significantly improved the chloride ion penetration resistance (CPR) of LAS because of morphology effects and secondary hydration of FA and SF that lead to a denser microstructure of the mixture. The electric flux and chloride ion migration coefficient (D_RCM) of LAS decreased by 56% and 67%, respectively, with FA increasing from 0 to 10%. The electric flux and D_RCM further decreased by 71% (153C) and 66% (3.24 m²/s), respectively, with FA increasing from 10 to 20%. As 5–10% SF was further added, the electric flux and D_RCM of LAS decreased to extremely low levels; for instance, with FA = 10% and SF = 10%, D_RCM = 1.61 m²/s, and the electric flux was too small and could be ignored. The contact stresses between aggregate and shotcrete mixtures were measured to investigate the rebound trend of ALSC in shotcrete. According to the analyses of the theoretical model of the rebound behavior of aggregate in shotcrete proposed by Armelin and Banthia, because of the reduced contact stresses between ALSC and mortar and the smaller density of LAS compared with normal-weight aggregate, the rebound rate of ALSC was about half of that of normal-weight aggregate in the shooting process of the shotcrete.     

Hydration Mechanisms of Alkali-Activated Cementitious Materials with Ternary Solid Waste Composition

Considering the recent eco-friendly and efficient utilization of three kinds of solid waste, including calcium silicate slag (CSS), fly ash (FA), and blast-furnace slag (BFS), alkali-activated cementitious composite materials using these three waste products were prepared with varying content of sodium silicate solution. The hydration mechanisms of the cementitious materials were analyzed by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The results show that the composite is a binary cementitious system composed of C(N)-A-S-H and C-S-H. Si and Al minerals in FA and BFS are depolymerized to form the Q⁰ structure of SiO₄ and AlO₄. Meanwhile, β-dicalcium silicate in CSS hydrates to form C-S-H and Ca(OH)₂. Part of Ca(OH)₂ reacts with the Q⁰ structure of AlO₄ and SiO₄ to produce lawsonite and wairakite with a low polymerization degree of the Si-O and Al-O bonds. With the participation of Na⁺, part of Ca(OH)₂ reacts with the Q⁰ structure of AlO₄ and the Q³ structure of SiO₄, which comes from the sodium silicate solution. When the sodium silicate content is 9.2%, the macro properties of the composites effectively reach saturation. The compressive strength for composites with 9.2% sodium silicate was 23.7 and 35.9 MPa after curing for 7 and 28 days, respectively.     

Investigation of the Geopolymerization Potential of a Waste Silica-Rich Diabase Mud

Diabase mud (DM) is a silica-rich residue yielding from aggregate crushing and washing operations in quarries. This work focuses on identifying the geopolymerization potential of a diabase mud through characterization of its mineralogical composition, investigation of its reactivity, and assessment of the early compressive strengths of alkali activated mixtures formulated based on the mud’s dissolution results. The findings suggest that considerably low amounts of Al and Si metals were dissolved following the dissolution tests conducted on DM, however, the incorporation of small quantities of CEM I, gypsum, and metakaolin (MK) moderately at a Na₂SiO₃:NaOH ratio of 50:50 and with a molarity of NaOH of 4 M enhanced the geopolymerization compared to low L/S ratio mixtures cured at different conditions. When M was increasing, the high L/S ratio mixtures exhibited fluctuations in strengths, especially beyond a 10 M NaOH molarity. Maximum strengths of mixtures at equivalent molarity of 10 were achieved when the Na₂SiO₃:NaOH ratio reached 30:70, regardless of the ambient conditions and the presence of CEM I. The curing conditions, the ratio of Na₂SO₃:NaOH, and the presence of CEM I in the DM-based mixtures did not appear to significantly affect the mixture when NaOH concentration was between 2 M and 4 M; at higher molarities, however, these enhanced the strengths of the geopolymerized DM.     

An Evaluation of the Physical and Chemical Stability of Dry Bottom Ash as a Concrete Light Weight Aggregate

Compared to the bottom ash obtained by a water-cooling system (wBA), dry process bottom ash (dBA) makes hardly any unburnt carbon because of its stay time at the bottom of the boiler and contains less chloride because there is no contact with seawater. Accordingly, to identify the chemical stability of dBA as a lightweight aggregate for construction purposes, the chemical properties of dBA were evaluated through the following process of the reviewing engineering properties of a lightweight aggregate (LWA). Typically, river gravel and crushed gravel have been used as coarse aggregates due to their physical and chemical stability. The coal ash and LWA, however, have a variety of chemical compositions, and they have specific chemical properties including SO₃, unburnt coal and heavy metal content. As the minimum requirement to use the coal ash and lightweight aggregate with various chemical properties for concrete aggregate, the loss on ignition, the SO₃ content and the amount of chloride should be examined, and it is also necessary to examine heavy metal leaching even though it is not included in the standard specifications in Korea. Based on the results, it is believed that there are no significant physical and chemical problems using dBA as a lightweight aggregate for concrete.     

Use of Gypsum as a Preventive Measure for Strength Deterioration during Curing in Class F Fly Ash Geopolymer System

This study discusses strength deterioration during the curing process of fly ash geopolymer and the use of CaSO₄·2H₂O (gypsum) as a deterioration remedy when the ash was synthesized using a 10M NaOH and Na-silicate solution. The strength decline was mainly due to the widespread formation of nanometer-sized cracks that were related to excessive Na and Si concentrations at an early age. Use of 2 wt% CaSO₄·2H₂O resulted in the best measured strength by temporarily reducing Na and Si concentrations; Na was absorbed by SO₄²⁻, up to 11% in the matrix within one day, and formed Na₂SO₄ (thenardite), which gradually dissolved over time, slowly releasing Na ions. However, more than 4% gypsum suppressed overall strength development because too many Na ions were locked into Na₂SO₄ and could not participate in geopolymerization. The addition of gypsum impeded glass dissolution and even halted the process when more than 4% gypsum was used.     

Influence of Silica Modulus and Curing Temperature on the Strength of Alkali-Activated Volcanic Ash and Limestone Powder Mortar

This present study evaluates the effect of silica modulus (M_s) and curing temperature on strengths and the microstructures of binary blended alkali-activated volcanic ash and limestone powder mortar. Mortar samples were prepared using mass ratio of combined Na₂SiO_3(aq)/10 M NaOH_(aq) of 0.5 to 1.5 at an interval of 0.25, corresponding to M_s of 0.52, 0.72, 0.89, 1.05 and 1.18, respectively, and sole 10 M NaOH_(aq). Samples were then subjected to ambient room temperature, and the oven-cured temperature was maintained from 45 to 90 °C at an interval of 15 °C for 24 h. The maximum achievable 28-day strength was 27 MPa at M_s value of 0.89 cured at 75 °C. Samples synthesised with the sole 10 M NaOH_(aq) activator resulted in a binder with a low 28-day compressive strength (15 MPa) compared to combined usage of Na₂SiO_3(aq)/10 M NaOH_(aq) activators. Results further revealed that curing at low temperatures (25 °C to 45 °C) does not favour strength development, whereas higher curing temperature positively enhanced strength development. More than 70% of the 28-day compressive strength could be achieved within 12 h of curing with the usage of combined Na₂SiO_3(aq)/10 M NaOH_(aq). XRD, FTIR and SEM + EDX characterisations revealed that activation with combined Na₂SiO_3(aq)/10 M NaOH_(aq) leads to the formation of anorthite (CaAl₂Si₂O₈), gehlenite (CaO.Al₂O₃.SiO₂) and albite (NaAlSi₃O₈) that improve the amorphosity, homogeneity and microstructural density of the binder compared to that of samples synthesised with sole 10 M NaOH_(aq).     

Effect of Cementitious Materials on the Engineering Properties of Lightweight Aggregate Mortars Containing Recycled Water

With the trend toward taller and larger structures, the demand for high-strength and lightweight cement concrete has increased in the construction industry. Equipment for transporting ready-mixed concrete is frequently used to bring concrete to construction sites, and washing this equipment generates a large amount of recycled water, which is an industrial by-product. In this study, we recycled this water as the pre-wetting water for lightweight aggregate and as mixing water, and we substituted blast furnace slag powder (BS) and fly ash (FA) as cementitious materials (Cm). In addition, we evaluated the fluidity, compressive strength, tensile strength, drying shrinkage, and accelerated carbonation depth of lightweight ternary cementitious mortars (TCMs) containing artificial lightweight aggregate and recycled water. The 28-day compressive strengths of the lightweight TCM specimens with BS and FA were ~47.2–51.7 MPa, except for the specimen with 20% each of BS and FA (40.2 MPa), which was higher than that of the control specimen with 100% OPC (45.9 MPa). Meanwhile, the 28-day tensile strengths of the lightweight TCM specimens containing BS and FA were ~2.81–3.20 MPa, which are ~13.7–29.5% higher than those of the control specimen. In this study, the TCM specimen with 5% each of BS and FA performed the best in terms of the combination of compressive strength, tensile strength, and carbonation resistance.     

Combined Freeze-Thaw and Chloride Attack Resistance of Concrete Made with Recycled Brick-Concrete Aggregate

The objective of this study was to investigate the physico-chemical properties of concrete made with recycled brick-concrete aggregate, which was the mixture from waste concrete and waste clay brick in a 7:3 ratio. Specifically, this paper investigated the mechanical properties, freeze-thaw resistance, and distribution of water-soluble chloride ions of concrete containing RBCA and fly ash (FA) against combined freeze-thaw and sodium chloride attack. Concrete containing RBCA replacement of natural coarse aggregate and fly ash replacement of Portland cement was subjected to 45 freeze-thaw cycles containing sodium chloride solution. It was discovered that the mechanical properties and freeze-thaw resistance to sodium chloride attack gradually decreased with increasing RBCA content. At the same time, a replacement level of 15% FA by weight resulted in significant improvements in compressive strength and resistance to combined freeze-thaw and chloride attack. Furthermore, using a replacement of 30% FA by weight markedly improved the resistance to chloride ion penetration of concrete due to the lowest water-soluble chloride content.     

Recycling of Sustainable Co-Firing Fly Ashes as an Alkali Activator for GGBS in Blended Cements

This study investigates the feasibility of co-firing fly ashes from different boilers, circulating fluidized beds (CFB) or stokers as a sustainable material in alkali activators for ground granulated blast-furnace slag (GGBS). The mixture ratio of GGBS and co-firing fly ashes is 1:1 by weight. The results indicate that only CF fly ash of CFB boilers can effectively stimulate the potential characteristics of GGBS and provide strength as an alkali activator. CF fly ash consists of CaO₃ (48.5%), SiO₂ (21.1%), Al₂O₃ (13.8%), SO₃ (10.06%), Fe₂O₃ (2.25%) and others (4.29%). SA fly ash consists of Al₂O₃ (19.7%), SiO₂ (36.3%), Fe2O3 (28.4%) and others (15.6%). SB fly ash consists of Al₂O₃ (15%), SiO₂ (25.4%), Zn (20.6%), SO₃ (10.9%), Fe₂O₃ (8.78%) and others (19.32%). The mixtures of SA fly ash and SB fly ash with GGBS, respectively, were damaged in the compressive strength test during seven days of curing. However, the built up strength of the CF fly ash and GGBS mixture can only be maintained for 7–14 days, and the compressive strength achieves 70% of that of a controlled group (cement in hardening cement paste). The strength of blended CF fly ash and GGBS started to decrease after 28 days, and the phenomenon of ettrigite was investigated due to the high levels of sulfur content. The CaO content in sustainable co-firing fly ashes must be higher than a certain percentage in reacting GGBS to ensure the strength of blended cements.     

The Properties of Lightweight Aggregates Pre-Coated with Cement Pastes and Their Suitability for Concrete

One of the biggest technological problems connected with the production of lightweight concretes made of porous aggregates is their much higher water absorption, which may cause on the one hand workability loss, and on the other hand excess water content in concrete. The aim of this research was to assess the effect of impregnation of lightweight aggregates (LWAs) with cement paste on their properties and to verify its effectiveness in concretes. Three types of lightweight aggregates differing in porosity and pore structure (sintered fly ash Lytag, expanded clay: Leca and Liapor) were selected for the tests. The following parameters were taken into consideration in the research program: LWA type and size, LWA initial moisture content, strength, and rheology of cement pastes. The tests of 22 different aggregates, plain and coated with cement paste, included density, crushing strength, and development of water absorption in time. The research program proved that porous aggregates, due to their impregnation with cement pastes, may be effectively sealed and strengthened. All tested LWAs showed a considerable decrease in water absorption by up to 71%. However, only Lytag aggregate showed a visibly enhanced crushing strength. Verification of effectiveness of aggregate sealing on the enhancement of concrete properties showed both a considerable reduction in water absorption of composites (by up to 52%) and a very high increase in their strength (by up to 107%).     

Effect of Nano-SiO2/PVA on Corrosion Behavior of Steel Rebar Embedded in High-Volume Fly Ash Mortar under Accelerated Chloride Attack

In this paper, the influence of Nano-silica (NS) and Polyvinyl alcohol (PVA) fibers on the corrosion behavior of steel rebar embedded in high-volume fly ash cement mortars under accelerated chloride attack was studied by using an impressed voltage technique. The PVA fibers used were 1.0 vol.%, and two mass fractions of cement (50 and 60 wt.%) were replaced by fly ash. Four NS mass fractions (0, 0.5, 1.0, and 1.5 wt.%) were utilized in this paper. In addition, the mono and hybrid effects of NS and PVA on the mechanical properties and water absorption of mortar were also studied. The results showed that the incorporation of PVA and nano-SiO₂ can improve the flexural and compressive strengths of high-volume fly ash mortar. Generally, the flexural and compressive strengths increased with the increase of nano-SiO₂ content. Moreover, the incorporation NS can also reduce the capillary water–absorption rate of cement mortar. The impressed voltage corrosion test indicated that the composite incorporation of nano-SiO₂ and PVA can significantly delay the deterioration process of steel bars in mortar, effectively reducing the steel rebar’s corrosion level and increasing the exposure time of the surface crack. With hybrid-incorporation 1.0 vol.% PVA and 1.0 wt.% nano-SiO₂, the steel rebar had the lowest corrosion degree, which exhibited a mass loss of 49% and increased the broken time by 71% as compared to the control mortar.     

Mechanical Properties and Microstructural Characterization of Metakaolin Geopolymers Based on Orthogonal Tests

Metakaolin was used as a raw material for the preparation of geopolymers, where two types of alkali activators (Na₂SiO₃ + NaOH and Na₂SiO₃ + NaOH) were used to prepare metakaolin geopolymers at room temperature. The mechanical properties and microstructures of the metakaolin geopolymers were analyzed. A three-factor, four-level orthogonal test was designed to investigate the mechanical properties of the metakaolin geopolymer with different ratios. The compressive and flexural strength of different specimens were tested for 7 and 28 days. Both the Na-based and K-based geopolymers exhibited excellent mechanical properties, but the K-based geopolymer had better mechanical properties. The optimal compressive strength and flexural strength of the K-based geopolymer were 73.93 MPa and 9.37 MPa, respectively. The 28-day optimal compressive strength of the Na-based polymer was 65.79 MPa, and the flexural strength was 8.71 MPa. SEM, XRD, and FTIR analyses showed that the mechanical properties of the geopolymers could be greatly improved by using a higher alkaline solution concentration, proper Na₂SiO₃/MOH mass ratio, and proper mass ratio of alkali exciter to metakaolin. Amorphous silicoaluminate was more favorable for the dissolution of silicon–alumina raw materials, promoted the formation of an amorphous silicoaluminate gel, and caused the internal structure of the geopolymer to be more compact.