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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]

Cl− adsorption and intercalated ions release processes during LDHs are highly intricate phenomena. In this study, ion exchange performance of LDHs with specific cation interlayers and intercalated anions (CaAl–NO 3 -LDHs, CaAl–NO 2 -LDHs, CaFeAl–NO 3 -LDHs and CaFeAl–NO 2 -LDHs) were extensively investigated. The results indicate that in the presence of low chloride concentration, the intercalated anions are prematurely released due to partial dissolution of LDHs in neutral deionized water and ion exchange with OH− in alkaline simulated concrete pore solution. Therefore, Cl− adsorption of LDHs and release process of intercalated anions in LDHs are not completely synchronized. Compared to partial LDHs dissolution, the accelerating effect of OH− exchange on intercalated anion release in simulated concrete pore solution is more pronounced. Further, higher stability of LDHs in testing solutions is mainly corresponding to higher binding energy between intercalation ions and cation interlayers, thus saturated Cl− adsorption content of NO 3 -LDHs with high binding energy is higher than NO 2 -LDHs with low binding energy, and saturated Cl− adsorption content of CaAl-LDHs with high binding energy is higher than CaFeAl-LDHs with low binding energy in testing solutions. • Introducing Fe into cationic interlayers reduces the stability of the formed LDHs. • LDHs are partially dissolved in deionized water to maintain high alkalinity. • Intercalated anions in LDHs exchange with OH− to form OH-LDHs in SCP solution. • Asynchronous Cl− adsorption and intercalated anions release exist in solutions. • Higher E B prohibits LDHs dissolution and OH− exchange, improving Cl− adsorption. [ABSTRACT FROM AUTHOR]

The mechanical properties, phase evolution, microstructure development and (re)hydration properties of limestone calcined clay cement (LC3) paste and mortar were investigated to understand their thermal stability under moderate temperatures (100–400 °C). The residual compressive strengths of LC3 and Portland cement (PC) mortars kept constant or slightly increased from ambient to 300 °C, and sharply dropped at 400 °C. At 100 °C, monocarboaluminate (Mc) and ettringite (AFt) converted into hemicarboaluminate (Hc) and monosulfate (Ms), and caused partial dehydroxylation of C-(A)-S-H. As the temperature increased, at 200–300 °C, due to the decomposition of large fraction of hydration products, macroscopic cracks and microstructure damage occurred. At 400 °C, the mean chain length (MCL) of C-(A)-S-H gel decreased, accompanied by the shift of Si and Al-coordination number and the formation of active sites. Under water saturation conditions, the microstructure showed self-repairing performance. [ABSTRACT FROM AUTHOR]

Carbonation treatment can effectively improve the performance of recycled concrete aggregate and fines due to the reactions of CO 2 with CH and C–S–H gel of cement paste. To better understand the mechanisms involved in the performance improvement, the surface properties of carbonated recycled cement paste powder (CRP) and its effect on the rheology, hydration and strength development of cement paste was studied. The results showed that during the carbonation, the surface of CRP was covered by a layer of amorphous silica gel. The generated CaCO 3 was wrapt by the silica gel and seldom exposed. The silica layer led to the poor flowability of CRP-cement paste due to that the silica gel on the surface of CRP has a strong affinity for H 2 O. During the very early hydration, the silica gel dissolved and then CaCO 3 was exposed. CaCO 3 is capable of chemically absorbing Ca2+, which facilitated the nucleation of C–S–H nuclei and stabilized the C–S–H phase. As a result, the C–S–H grew densely, uniformly and perpendicularly on the surface of CRP. In addition, the chemically absorbing Ca2+ enabled the chemical bond to be formed between CaCO 3 and C–S–H. Due to increased C–S–H resulted from reactions of silica gel with CH at the interface and the stronger bond formed between CaCO 3 and C–S–H, the interface between CRP and hydration products was much stronger than that between recycled cement paste powder (RP) and hydration products. [ABSTRACT FROM AUTHOR]