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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]

• 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]

[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]