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Abstract An aluminosilicate precursor, such as metakaolin, can be transformed into a cement-like geopolymer binder via a phosphate activation approach. This paper identifies the effect of the addition of aluminum species into the phosphate activating solution on the formation of such geopolymers, from the fresh to the hardened state. Activating solutions with Al/P molar ratios of 0, 0.1, and 0.3 were prepared by blending monoaluminum phosphate (MAP) and orthophosphoric acid (OPA). The rheological properties, fluidity, and setting times of the fresh geopolymer pastes and the compressive strength of the hardened geopolymer matrices were studied. Liquid-state 27Al and 31P nuclear magnetic resonance (NMR) measurements for the chemical environments of Al and P, and spectroscopic, thermal, and microscopic analyses revealed that the soluble aluminum in the phosphate activating solution played an important role during the geopolymerization process. Seeding of aluminum species through inclusion in the activating solution allowed a rapid sol/gel transition that improved the rheological properties and setting time of the fresh geopolymer pastes at ambient temperature. However, although the increased concentration of aluminum phosphate oligomers promoted by the soluble aluminum addition contributed to the formation of a compact matrix with high early strength, it hinders the ongoing reaction of metakaolin in the later period, which has a detrimental influence on ongoing strength development beyond 7 days of curing. [ABSTRACT FROM AUTHOR]

Abstract The direct tensile properties of ultra-high performance engineered cementitious composites (UHP-ECC) were investigated under a range of strain rates from 0.0001 to 0.05 s−1 in this study. Three kinds of polyethylene fibers with different fiber aspect ratios were used as the reinforced fiber in the UHP-ECC. The rate sensitivity of the UHP-ECC in tension was evaluated in terms of initial cracking stress, peak stress, strain capacity, energy absorption capacity and elastic modulus at different strain rates. The cracking patterns, including crack number, crack width and crack spacing, of the UHP-ECC specimens were also monitored. The test results showed a significant rate effect on the initial cracking stress and a moderate effect on the peak stress and strain capacity. The fiber aspect ratio significantly influenced the tensile properties and failure modes of the UHP-ECC. The matrix strength, single fiber tensile and pull-out tests were conducted at corresponding strain rates to further explain the test results. Finally, the micro-structure of the tensile specimens was observed using a scanning electron microscope. [ABSTRACT FROM AUTHOR]

Abstract: The enhancement of long-term durability of marine structures is a matter of interest to many researchers. The study presented in this paper examines the effectiveness of a water reducer and chloride barrier surface impregnation of the concrete cover of reinforced concrete (RC) structures, exposed to a marine environment. Specific focuses is on how surface cracks created (1) before impregnation and (2) after impregnation, affect the effectiveness of the surface treatment. The experiments are conducted in an environment which is as close as possible to the real humid subtropical marine environment. A series of reinforced concrete (RC) prisms and concrete cylinders, each treated with various commercial surface impregnation agents, were exposed to cyclic sea water shower under an outdoor environment to accelerate the dry/wet cycles for 1 year. Six types of surface impregnation agents, including four types of silane-based water repellent agents and two types of sodium silicate-based pore blockers (water–glass) were applied. Three types of RC prisms were prepared to simulate the different cracking possibilities, which may occur in surface impregnated concrete structures, during their service life. No cracks were introduced in the first prism group, while cracks were introduced before and after surface impregnation, in the second and third groups, respectively. The time-dependent water absorption of all specimens was monitored during exposure to the dry/wet cycles. Finally the specimens were split open to measure the penetration depths of the surface impregnation agents and the chloride penetration profiles. The areas with corrosion evident in the steel reinforcement in the RC prisms were also measured. Sodium silicate-based pore blockers were found to be inefficient in preventing chloride penetration of concrete under simulated marine exposures. The long-term efficiency of water repellent agents used for surface impregnation was found to be highly dependent on the type of agent and whether impregnation was carried out before or after crack formation. [Copyright &y& Elsevier]

This work investigated the electrochemical behavior of galvanized steel bars in nano-silica modified cement mortars. The internal environmental factors of the mortar were firstly characterized. Then the passivation process of the galvanized steel bars was monitored. The results showed that the presence of nano-silica influenced the passivation process of galvanized steel bars and improved the protective properties of passive film. The morphology of passive film was changed, and the thickness was increased by the addition of nano-silica. The CHZ layer performed as a bonding agent between the galvanized steel and mortar matrix. The addition of nano-silica promoted the formation of gaps at the rebar/mortar interface, and there was no evidence for the presence of a lime-rich layer at the interface. [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the effect of using different admixtures on the one-part ambient-cured alkali activated pastes. Na 2 SiO 3 -Anhydrous powder was used as solid activator along with three superplasticizers including naphthalene, melamine and polycarboxylate with different water/precursor ratios. The flowability, setting time and compressive strength development were evaluated for all AAMs mixes. ATR-FTIR test was used to check the admixtures stability in high alkali media. All types of admixtures significantly improved the flowability of the Na 2 SiO 3 -anhydrous one-part AAMs and marginally affected the compressive strength. The results showed that polycarboxylate was effective for high water/precursor (≥0.36) while naphthalene performed better in case of low water/precursor (≤0.36) for Na 2 SiO 3 -anhydrous one-part AAMs. However, lowering the water/precursor ratio in the Na 2 SiO 3 -anhydrous one-part AAMs using superplasticizers showed no improvement or modest improvement in the compressive strength. The ATR-FTIR results revealed the relatively high stability of admixtures in Na 2 SiO 3 alkali medium of one-part AAMs (for w/b = 0.4). Highlights • The effectiveness of different admixtures in Na 2 SiO 3 -Anhydrous one-part AAMs was evaluated. • The most effective types for Na 2 SiO 3 AAMs were naphthalene when w/b ≤ 0.36 and polycarboxylate when w/b ≥ 0.36. • Minimizing the water/precursor ratio using admixtures marginally affected the compressive strength of Na 2 SiO 3 AAMs. [ABSTRACT FROM AUTHOR]

Abstract Efflorescence can be a critical issue for the application of alkali-activated fly ash–based geopolymer products, especially when the products are in a moist environment. In this study, a new method was explored to inhibit efflorescence of fly ash–based geopolymer via silane surface modifications. After the modification, the surface of the geopolymer was transformed from hydrophilic to hydrophobic, with a water contact angle of 144.1°, and the capillary absorption and diffusion of water were significantly suppressed; and as a consequence, the soluble alkali ion leaching was reduced. Analysis by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and size-exclusion chromatography demonstrated that the selected silane is attached successfully onto the surface of the fly ash–based geopolymer via chemical bonding instead of via physical absorption. A possible reaction mechanism for the silane surface modification of the fly ash–based geopolymer is proposed. [ABSTRACT FROM AUTHOR]

The addition sequence of raw materials during mixing is one factor that affects the fresh and hardened properties of ordinary Portland cement (OPC). Similar to OPC, the properties of alkali-activated materials (AAMs) may be susceptible to the mixing procedure, yet the mixing procedure's effect is still unclear. This research aims to investigate how different mixing procedures affect the fresh and hardened properties of the Na 2 SiO 3 -anhydrous alkali-activated materials along with polycarboxylate superplasticizer. Besides, a new mixing method named "hybrid-mixing" is proposed, evaluated, and compared with the conventional one-part (1 GP) and two-part (2 GP) mixing methods. It was found that the mixing procedure markedly affects the fresh and hardened properties of AAMs without changing their chemical composition. Compared with 1 GP and 2 GP binders, the hybrid-mixed binders achieved a relatively better slump-flow and compressive strength. The mechanisms governing different mixing methods are further discussed. [ABSTRACT FROM AUTHOR]

In this study, seawater sea-sand Engineered Geopolymer Composites (SS-EGC) were developed and investigated for the first time. The developed EGC achieved high compressive strength (over 140 MPa) and high tensile ductility (around 8%) simultaneously. Emphasis was placed on understanding the influence of seawater and sea-sand (compared to freshwater and washed sea-sand) on the matrix properties and tensile performance of EGC, with two fly ash-to-slag ratios (8:2 and 2:8) considered in the matrices. Results showed that the use of seawater hindered the reaction of EGC matrix and led to a slight reduction of compressive strength (compared to the freshwater counterpart). It was found that the content of hydrotalcite phases in SS-EGC matrix was higher than that of freshwater EGC. In addition, using seawater was found to increase the average modulus of matrix obtained from nanoindentation, leading to a higher fiber/matrix bond strength. The tensile strain capacity of SS-EGC was slightly lower than that of freshwater EGC. The developed SS-EGC showed superior crack resistance and better sustainability than the cement-based counterpart from the literature (with similar compressive strength). The findings of this study provided useful knowledge for the design and development of high-strength high-ductility SS-EGC towards sustainable and resilient marine infrastructures. • Seawater Sea-sand EGC (SS-EGC) were developed and investigated for the first time. • Using seawater and sea-sand slightly decreased the compressive strength of EGC. • The content of hydrotalcite phases in SS-EGC matrix was higher than that of freshwater EGC. • Using seawater and sea-sand in EGC increased the tensile strength but lowered the tensile strain capacity. • The developed SS-EGC showed better sustainability and lower cost than the cement-based counterpart in the literature. [ABSTRACT FROM AUTHOR]

Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC) incorporating artificial geopolymer aggregates (GPA) were developed and over-saturated cracking (i.e., average tensile crack spacing smaller than the theoretical limit) was observed in this novel material. The developed UHS-ECC exhibited an ultra-high compressive strength (over 150 MPa) and an ultra-high tensile ductility (over 8%) simultaneously. The influences of GPA size on the matrix properties, tensile performance, micromechanics, and cracking behavior of UHS-ECC were systematically investigated. Over-saturated cracking and double-stage crack evolution (i.e., a bilinear relation between average crack width and tensile strain) were observed in UHS-ECC with GPA size smaller than 0.60 mm, while saturated cracking and single-stage crack evolution (i.e., a linear relation between average crack width and tensile strain) were observed in the other groups. Finally, the mechanism of over-saturated cracking and double-stage crack evolution was illustrated. The findings of this study extend the fundamental knowledge of ECC technology, which is meaningful for designing and developing UHS-ECC materials towards ultra-high tensile ductility. [Display omitted] • GPA-based UHS-ECC were developed with both ultra-high strength (over 150 MPa) and ultra-high ductility (over 8%). • Over-saturated cracking was observed in UHS-ECC with GPA size smaller than 0.60 mm. • UHS-ECC with over-saturated cracking exhibited a double-stage crack evolution. • The mechanism of the over-saturated cracking of the GPA-based UHS-ECC is revealed. • UHS-ECC with GPA size of 0.30–0.60 mm exhibited the highest strain-hardening potential. [ABSTRACT FROM AUTHOR]

This paper provides a detailed underlying mechanism of gel composition of hybrid alkali-activated fly ash/slag (AAFS) pastes prepared by utilizing the air pollution control residue (APCr, a highly alkaline waste containing 38.3% of Na 2 O content by mass) as a partial replacement (8%, 16% and 24% replacement by mass) of energy-intensive commercial sodium silicate (CSS) activator. AAFS pastes containing APCr as a partial replacement of CSS showed the strength, micromechanical and microstructure properties equivalent to the reference AAFS paste without APCr (R-0). SEM-EDS, FTIR spectral subtraction, and nanoindentation results showed that reaction products in AAFS pastes were composed of geopolymer gel (N-A-S-H), cross-linked gel (C–N-A-S-H) and alkali-activated gel (C-A-S-H) contributing to 60–66% of paste volume. Reaction products were more dominated by calcium-rich gels (C-A-S-H or C-(N)-A-S-H) as compared to N-A-S-H gel in all AAFS pastes. The heat of hydration and thermogravimetric results confirmed the slow rate of reaction of APCr-AAFS pastes at the early age but a higher rate of reaction at the silater stage which contributed to achieve a similar strength as control paste (R-0). [ABSTRACT FROM AUTHOR]

Integrating phase change materials (PCMs) into building materials has been widely used to improve the energy efficiency of buildings, in which microencapsulation and shape stabilization of PCMs are considered as two most effective solutions. In this study, artificial geopolymer aggregate (GPA) was employed as a novel PCM carrier for energy storage purposes. Detailed investigations were conducted into the physical, mechanical, and thermal properties of GPA-PCM, which can be engineered through different raw material selections (e.g., slag content, water/binder ratio, and incineration bottom ash (IBA) content). It was demonstrated that increasing the IBA content is an efficient means to increase the porosity of GPA, an index of the capacity to accommodate PCM. Up to 16 wt% PCM could be absorbed into the GPA through vacuum suction, resulting in a significant melting enthalpy of 24.74 J/g. Besides, GPA-PCM could achieve an excellent mechanical strength greater than 53.2 MPa and thermal conductivity of 0.510–0.589 W/mK. The time-temperature history curves of GPA revealed that up to 10.5 °C of thermal regulation was achieved due to PCM impregnation. The developed GPA-PCM composites facilitate an innovative and low-carbon solution for utilizing PCMs in construction for temperature-regulating and energy-saving purposes. [ABSTRACT FROM AUTHOR]

Artificial geopolymer aggregate is an emerging technology in the field of solid waste recycling, aiming to ease the exploitation of natural aggregates as well as reduce the environmental burden of industrial/urban/agricultural waste and by-product accumulations. In this study, geopolymer aggregates (GPA) were strategically utilized to enhance long-term tensile performance and sustainability of high-strength Engineered Cementitious Composites (ECC). Accelerated aging test was conducted to evaluate the long-term performance of GPA-ECC, with the conventional fine silica sand ECC (FSS-ECC) as the control group. It was found that after accelerated aging (i.e., to simulate long-term curing condition), the compressive and tensile strengths of both GPA-ECC and FSS-ECC increased. In addition, owing to the flaw effect of GPA, the long-term tensile ductility of GPA-ECC was maintained, while that of FSS-ECC decreased significantly. Compared with FSS-ECC, GPA-ECC showed better multiple cracking behavior, higher strain energy density, and finer crack width under both short- and long-term conditions. Finally, a cost analysis of ECC matrix was conducted to exhibit the cost-efficiency and sustainability of GPA-ECC. This study provides a sustainable approach for enhancing the long-term tensile performance of high-strength ECC based on artificial aggregates. • Geopolymer Aggregate (GPA) Engineered Cementitious Composites (ECC) maintained its tensile ductility after accelerated aging. • The tensile strain energy density of GPA-ECC was improved after accelerated aging, while that of FSS-ECC had little change. • GPA-ECC showed a better crack resistance than FSS-ECC in both short- and long-term conditions. • The use of GPA could reduce the cost of ECC matrix and improve the sustainability. [ABSTRACT FROM AUTHOR]

It is well known that an increase in the compressive strength of cementitious composites is usually accompanied by a loss of tensile ductility. Designing and developing ultra-high-strength cementitious composites (e.g., ≥200 MPa) with high tensile strain capacity (e.g., ≥3%) and excellent crack resistance (e.g., crack width ≤100 μm) remain challenging. In this study, a series of ultra-high-strength Engineered Cementitious Composites (UHS-ECC) with a compressive strength over 210 MPa, a tensile strain capacity of 3–6% (i.e., 300–600 times that of ordinary concrete), and a fine crack width of 67–81 μm (at the ultimate tensile strain) were achieved. Hybrid design of fiber reinforcement and matrix for UHS-ECC was adopted by combining the ECC and ultra-high-performance concrete (UHPC) design concepts, and the effect of fiber hybridization and aspect ratio on the mechanical behavior of UHS-ECC was comprehensively investigated. The overall performance of UHS-ECC was assessed and compared with the existing high-strength ECC and strain-hardening UHPC, and it was found that the currently designed UHS-ECC recorded the best overall performance among the existing materials. Finally, the multiple cracking behavior of UHS-ECC was analyzed and modeled based on a probabilistic approach to evaluate its critical tensile strain for durability control in practical applications. The results of this study have pushed the performance envelope of both ECC and UHPC materials and provided a basis for developing cementitious composites with simultaneously ultra-high compressive strength, ultra-high tensile ductility, and excellent crack resistance. [Display omitted] • Hybrid design of fiber reinforcement and matrix for UHS-ECC was introduced by combining the UHPC and ECC design concepts. • UHS-ECC with a compressive strength over 210 MPa and a tensile ductility over 6% was developed. • UHS-ECC with 2% 18-mm PE fiber and 1% 13-mm steel fiber recorded the best overall performance. • Cracking behavior was modeled by a probabilistic approach to estimate the critical tensile strain of UHS-ECC. [ABSTRACT FROM AUTHOR]

Abstract: This paper aims to investigate the durability properties of recycled aggregate concrete treated with silane-based water repellent agents, since the high water absorption of recycled aggregate concrete is a major factor jeopardizing its durability. Different dosages of silane-based water repellent agents were either coated on the surface of the concrete (hereafter “surface silane treatment”) or integrally added into the concrete mixture (hereafter “integral silane treatment”). The mechanical and durability properties of the treated concrete were evaluated. It was found that integral silane treatment can improve the durability of recycled aggregate concrete, but may lead to reductions in compressive strength; surface silane treatment is more effective in improving the resistance of recycled aggregate concrete to capillary water absorption, carbonation and chloride penetration than integral silane treatment. [Copyright &y& Elsevier]

In this study, Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates were successfully developed with high strength and high ductility. A multi-scale investigation was conducted to gain an in-depth understanding of the microstructure and ductility enhancement mechanism of geopolymer aggregate ECC (GPA-ECC). The use of geopolymer fine aggregates enabled the high-strength ECC to achieve higher tensile ductility and finer crack width compared to existing ones with similar compressive strength in the literature. It was found that the GPA reacted with the cementitious matrix, and the width of the GPA/matrix interfacial transition zone (ITZ) was larger than that of the silica sand/matrix ITZ. Moreover, the GPA achieved a strong bond with the cementitious matrix and could behave as "additional flaws" in high-strength matrix, resulting in saturated multiple cracking and excellent tensile ductility of ECC. This study provides a new avenue for developing high-performance fiber-reinforced cementitious composites based on artificial geopolymer aggregates. • Geopolymer fine aggregates were successfully applied to develop high-strength high-ductility ECC with fine crack width. • Geopolymer fine aggregates reacted with cementitious paste, resulting in a strong interfacial bond. • Geopolymer fine aggregates acted as "additional flaws" in high-strength ECC matrix, leading to saturated multiple cracking. • Compared with existing ambient-cured high-strength ECC, geopolymer aggregate ECC exhibited superior tensile ductility. [ABSTRACT FROM AUTHOR]

Geopolymer binders have good potential in the repair of existing concrete structures due to their superior durability and mechanical performances. Understanding the interfacial transition zone (ITZ) between geopolymer mortars and ordinary Portland cement concrete is a crucial step to implement such an environmentally-friendly technology. This paper focuses on the failure modes of the bond interface and the microscopic and chemical interactions between various geopolymer mortars and concrete substrates at ITZs. The occurrence of newly-formed gels and second hydration mechanisms at ITZs are analyzed and discussed. [ABSTRACT FROM AUTHOR]

Engineered Cementitious Composite (ECC) is an advanced fiber-reinforced concrete exhibiting multiple-cracking and strain-hardening under tension. This study aims to explore the feasibility of producing high-strength seawater sea-sand Engineered Cementitious Composites (SS-ECC) for marine and coastal applications facing the shortage of freshwater and river/manufactured sand. The effects of key composition parameters including the sea-sand size (1.18/2.36/4.75 mm), the polyethylene fiber length (6/12/18 mm), and the fiber volume dosage (1.0/1.5/2.0%) on the mechanical performance of SS-ECC were comprehensively investigated. SS-ECC with tensile strength over 8 MPa, ultimate tensile strain about 5%, and compressive strength over 130 MPa were achieved. Using seawater and sea-sand had almost no negative effects on the 28-day mechanical properties of high-strength ECC. For SS-ECC, increasing fiber length and dosage enhanced the tensile strain capacity, and sea-sand size had limited effects on the tensile performance; these phenomena were interpreted by the micromechanical analysis. A probabilistic-based method was proposed to analyze the reliability of the tensile strain capacity of SS-ECC, and it showed good agreement with the experimental results. The findings provide new insights into the design and applications of ECC in marine and coastal infrastructures for improving safety, durability, sustainability, and reliability. • SS-ECC with compressive strength >130 MPa for marine and coastal applications was developed. • High-strength SS-ECC achieved tensile strength >8 MPa and tensile strain capacity about 5% at 28 days. • Using seawater had almost no negative effects on the 28-day mechanical properties of high-strength ECC. • The effects of sea-sand size, fiber length and fiber dosage on mechanical properties were investigated. • A probabilistic-based model was proposed to analyze the reliability of tensile strain capacity of ECC. [ABSTRACT FROM AUTHOR]