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The use of solid activators to prepare alkali-activated materials (AAM) holds great potential for on-site applications, as it eliminates the need to transport and store large quantities of concentrated alkaline solutions. This study compared the early-age properties and efflorescence between alkali-activated slag/fly ash (AASF) using solid and liquid sodium silicates as activators. The results revealed that AASF with solid sodium silicates exhibited comparable mechanical strength while possessing reduced initial setting time and flowability. All AASF mixtures were susceptible to efflorescence and carbonation, resulting in varying mineralogical compositions of carbonation products: Vaterite was detected in AASF with solid activators, while aragonite and pirssonite were identified in AASF with liquid activators containing 4 and 6 wt.% Na2O, respectively. The efflorescence of AASF exposed to bottom water was more severe than those exposed to natural conditions, as evidenced by the decalcification of reaction products, migration of alkalis, and formation of crystalline carbonates. [ABSTRACT FROM AUTHOR]

The variation of the raw materials is critical for the wide application of alkali-activated fly ash/slag (AAFS) in practice. In this study, four fly ashes sourced from different power plants were collected and analyzed. The calcium content in fly ash had insignificant impact on setting time, flowability and compressive strength of AAFS binders when the slag content was 50%. Low calcium content in fly ash promotes the formation of C-(N)A-S-H gel with high Al substitution and improves the compressive strength at later age. It seemed that the grinding process promoted the dissolution of reactive SiO2 in fly ash, resulting in reduced setting and flowability. The replacement of silica fume promotes the formation of Si-rich C-(N)A-S-H gel with reduced gel pore size. The replacement of metakaolin accelerated the initial reaction and increased the compressive strength of binders at early age due to the reduced gel pore size in C-(N)A-S-H gel. [ABSTRACT FROM AUTHOR]

Alkali-activated materials (AAMs) have gained increasing attention as potential candidates for conductive and self-sensing concretes. This study investigated the effect of fly ash microspheres and quartz sands on the piezoelectric properties of carbon fiber-reinforced alkali-activated fly ash/slag (AAFS) mortars. The rheological property, mechanical property, resistivity, and piezoelectric properties of different AAFS mortars were investigated. Our findings demonstrated that the use of fly ash microspheres (FAM) significantly reduced the yield stress and plastic viscosity of the mortars due to the "ball bearings" effect. In addition, the finer particles of FAM led to a dense gel pore structure of AAFS. The use of FAM improved carbon fiber dispersion and formed a stable conductive network. Therefore, AAFS mortars with FAM exhibited superior conductivity and piezoelectricity. The use of quartz sands exhibited adverse effects on the self-sensing properties of AAFS mortars. The incorporation of quartz sands predominantly hindered the conductive network formation of carbon fibers, resulting in a significantly higher resistivity and piezoresistivity with low strain sensitivity. • The use of FAM in carbon fiber reinforced-AAFS results in superior conductivity and piezoresistivity. • FAM improved the rheology of AAFS, facilitating the dispersion of carbon fibers. • The use of quartz sands led to a higher resistivity and poor piezoresistivity. [ABSTRACT FROM AUTHOR]

Alkali-activated materials (AAMs) are regarded as promising alternatives to Portland cement. However, the significant high shrinkage of AAMs, particularly in alkali-activated slag system, is a critical issue that hinders their wide application. This study investigated the effect of expansive admixture, i.e. CaO on the autogenous shrinkage and drying shrinkage of alkali-activated slag/ fly ash microsphere (AASFAM). The effect of CaO content on heat evolution, reaction products, microstructure development and compressive strength was systematically investigated. The results revealed that the addition of CaO can effectively reduce both the autogenous and drying shrinkage of AASFAM. Mixture with 3% and 5% CaO exhibited expansion within the first 24 hours after casting, while the drying shrinkage was reduced by 72% and 82%, respectively. The addition of CaO promoted the formation of crystalline phases including Ca(OH) 2 , hydrotalcite and calcite, leading to a lower free water content and coarser pore structure, which in turn reduced the capillary stress during the drying process. The results demonstrated the promising potential of CaO in effectively mitigating the high autogenous shrinkage and drying shrinkage of AAMs without compromising its mechanical properties. • The addition of CaO can effectively reduce both the autogenous and drying shrinkage of AASFAM. • The addition of CaO facilitates the crystalline phases in AASFAM including Ca(OH) 2 , hydrotalcite and calcite. • The formation of Ca (OH) 2 led to a larger pore size distribution. [ABSTRACT FROM AUTHOR]

The development of smart concretes with self-sensing properties has gained increasing attention recently. In this study, the effect of different precursors, including blast furnace slag, fly ash, silica fume, and metakaolin, on the electrical conductivity and piezoresistive properties of alkali-activated materials (AAMs) with and without carbon fiber (CF) was investigated. The experimental results revealed that the electrical and self-sensing properties of AAMs were significantly influenced by the raw materials, both with and without carbon fiber. However, distinct trends were observed. The use of silica fume (25 wt%) was beneficial to the self-sensing property of CF-AAM mortars. Mixtures containing fly ash and pure slag exhibited higher sensitivity but with reduced repeatability of the piezoresistivity. Due to the poor dispersion of carbon fibers, as well as the porous microstructure, mixtures with metakaolin had poorer conductivity and piezoresistivity. AAM mortars had evident piezoresistivity under cyclic loading, but their stability was inferior to that of AAM with carbon fibers. The pore size distribution and gel pore quantity of the binder phase were important factors determining the conductive and piezoresistive properties of CF-AAM mortars. • AAM mortars without carbon fiber presented evident piezoresistivity but with poor stability. • The use of silica fume was beneficial to the self-sensing property of CF-AAM. • Mixtures with metakaolin had poorer conductivity and piezoresistivity. • The pore size distribution and gel pore quantity were important for the conductivity of CF-AAM. [ABSTRACT FROM AUTHOR]

The drying shrinkage of alkali-activated fly ash/slag (AAFS) is an important engineering property for the control of cracking when AAFS is utilized as a building material. In this study, results of drying shrinkage of AAFS pastes with different fly ash content and activator concentrations were presented. The weight loss of samples during the drying process was monitored. The pore structure and free water content of AAFS pastes were measured. The results showed that AAFS pastes with  30 and 50 w.t.% fly ash content exhibited a lower drying shrinkage than alkali-activated slag, while the drying shrinkage of AAFS pastes with 70 w.t.% fly ash content was the largest among all the mixtures. Besides, AAFS pastes with higher activator concentration exhibited a higher drying shrinkage. But there were no clear correlations between weight loss, pore structure and the drying shrinkage. It was assumed that other factors affect the drying shrinkage of AAFS. [ABSTRACT FROM AUTHOR]

• Chloride resistance was evaluated by natural chloride diffusion test. • W/b and slag content significantly impacted chloride transport. • Sand-aggregate ratio decided workability instead of D f. • D f was related to the threshold pore diameter and pores > 5 nm. The widespread application of alkali-activated fly ash/slag (AAFS) concrete requires the satisfaction of a series of performance criteria both from its early age properties (e.g. workability, strength) and long-term stability. In this study, long-term (till 180 days) natural chloride diffusion tests were conducted to evaluate the chloride diffusion in AAFS concretes prepared with different slag content, water/binder (w/b) ratio, alkali content, and sand-aggregate ratio. The results revealed that AAFS concretes obtained in this study were generally regarded as flow/high flow concretes (slump > 100 mm) with compressive strength > 50 MPa at 28 day. The free chloride diffusion coefficient (D f) of AAFS concretes was between 0.4 and 1.8 × 10-12 m2/s. The slag content and w/b were found as dominant parameters affecting the long-term chloride transport in AAFS concretes, while the sand-aggregate ratio presented a limited effect. The formation of more C-A-S-H gels (by increasing slag content ≥ 50%), and lower w/b (≤0.45) at Na 2 O content of ∼ 5% was preferred for AAFS concretes with good workability, strength and durability. MIP results indicated that capillary pores in AAFS reached percolation and became disconnected after 180 days. The long-term chloride diffusivity of AAFS concretes was closely related to the threshold pore diameter and volume of pores > 5 nm. The more larger pores, the higher chloride diffusion coefficient was. [ABSTRACT FROM AUTHOR]

Alkali-activated materials (AAMs) are regarded as an environmentally friendly alternative to ordinary Portland cement. AAMs reinforced with carbon fibers enable the piezoresistivity of materials that can sense its own strain, resistivity and damage. However, the high cost of carbon fiber limits the wide application in construction. In this study, a novel type of conductive aggregates is prepared by using porous ceramsites impregnated with modified agar and carbon black. Conductive alkali-activated fly ash/slag (AAFS) is prepared using carbon fibers and conductive ceramsites. Orthogonal experiment is applied to assess the effect of different parameters on the piezoresistivity and early age properties of AAFS mortars. The results reveal that with the combination of carbon fibers and conductive ceramsite, the electrical resistivity and piezoresistivity of AAFS are significantly improved. The good binding of ceramsites with AAFS matrix and carbon fibers ensured a high piezoresistivity of AAFS samples. A synergistic effect exists between the carbon fibers and ceramsites in view of the piezoresistivity. However, the workability and mechanical strength reduced with the addition of conductive ceramsites. AAFS mortar with 0.3% carbon fiber content, 25%–30% ceramsite content and 70% slag content is proposed as the optimal mixture in view of its mechanical properties and piezoresistivity. [ABSTRACT FROM AUTHOR]

• The passive film presented bilayer structure with FeO-rich inner layer and FeOOH-rich outer layer in AAFA pore solution. • The firmly adsorbed layer with zeolite-like structure (200 nm) formed on the steel surface in AAFA pore solution. • The corrosion resistance of the passive reinforcement was significant higher in AAFA pore solution. Due to different ion types and concentration in pore solution, the passive film on the steel surface possesses different characteristics in alkali-activated fly ash (AAFA). In this study, the passive film formed in simulated AAFA pore solution was investigated by surface analysis and electrochemical measurements. The passive film possessed bilayer structure with FeOOH-rich outer layer and FeO-rich inner layer in simulated AAFA pore solution. Further, a firmly adsorbed zeolite-like layer (200 nm) formed on the steel surface. The formation of more protective passive film and adsorbed layer resulted in higher corrosion resistance of the reinforcement in simulated AAFS pore solution. [ABSTRACT FROM AUTHOR]

[Display omitted] • Products with different composition in alkali-activated flay ash were synthesized. • Chlorides were physically adsorbed on the surface of synthetic reaction products. • The chloride adsorption capacity of zeolites was lower than N-A-S-H gel. • The chloride adsorption capacity was related to the surface properties of products. Compared to ordinary Portland cement, reaction products in alkali-activated fly ash (AAF) exhibit different characteristics, dramatically influencing their chloride adsorption ability. This paper extensively characterized the chloride adsorption behavior of individual reaction products in AAF. First individual reaction products, including NASH gels, P zeolite and analcite, were synthesized and their composition, morphology and structure were extensively evaluated. The chloride adsorption ability of the above synthetic reaction products was then investigated. The synthetic reaction products mainly physically adsorbed chlorides on their surface. When Si/Al ratio of NASH gel increased, its chloride adsorption ability was enhanced, which was mainly related to the positive shift of zeta potential. The synthetic P zeolite and analcite exhibited lower zeta potential and specific surface area, thus their chloride adsorption ability was lower than synthetic NASH gels. [ABSTRACT FROM AUTHOR]

Corrosion performance of the reinforcement was dramatically affected by chemical compositions of pore solution in alkali-activated fly ash. In this study, the electrochemical behavior and surface properties of reinforcement were characterized in chloride-contaminated simulated alkali-activated fly ash pore (SAFP) solution. The results indicated that due to the high alkalinity and formation of zeolite-like adsorption layer (about 20 nm), the reinforcement was still in passive state and exhibited high corrosion resistance in chloride-contaminated SAFP solution. However, the protective effect of passivation film (consisted of mainly FeOOH outer film and FeO inner film) and corrosion resistance of the reinforcement were reduced by chlorides. Further, the chloride threshold in SAFP solution was 10 times larger than simulated cement pore solution, due to the competitive adsorption between OH−/silicates and chlorides. [ABSTRACT FROM AUTHOR]

The development of a green alternative that can partially or completely substitute the Ordinary Portland cement (OPC) as a practical construction material with low CO 2 footprint is an important subject to researchers for decades. The use of artificial pozzolanic materials, e.g. fly ash, slag, silica fume, etc. cannot meet the huge-scale demand for cement replacement. Kaolinitic clay, with its high pozzolanic reactivity after activation and abundant resources worldwide, is regarded as a promising supplementary cementitious material (SCM) for cement. Recent studies have addressed that the utilization of calcined kaolinitic clay as SCM can effectively improve the properties of cement products. This paper presents a comprehensive review of the research progress on activated kaolinitic clay as SCM in the last decade. It systematically introduces the essential properties, activation mechanism and pozzolanic reactivity assessment of kaolinitic clay. Application of calcined clay in two different cementitious systems (calcined clay blended cement and limestone calcined clay cement) is reviewed from the aspects of workability, mechanical properties, and long-term durability properties with the mechanism discussion. Finally, the environmental and economic impacts of the application of calcined clay in different cementitious systems are discussed. [ABSTRACT FROM AUTHOR]