Abstract
Understanding the role of curing conditions on the microstructure and phase chemistry of alkali-activated materials (AAMs) is essential for the evaluation of the long-term performance as well as the optimization of the processing methods for achieving more durable AAMs-based concretes. However, this information cannot be obtained with the common material characterization techniques as they often deliver limited information on the chemical domains and proportions of reaction products. This paper presents the use of PhAse Recognition and Characterization (PARC) software to overcome this obstacle for the first time. A single precursor (ground granulated blast-furnace slag (GBFS)) and a binary precursor (50% GBFS–50% fly ash) alkali-activated paste are investigated. The pastes are prepared and then cured in sealed and unsealed conditions for up to one year. The development of the microstructure and phase chemistry are investigated with PARC, and the obtained results are compared with independent bulk analytical techniques X-ray Powder Fluorescence and X-ray Powder Diffraction. PARC allowed the determination of the type of reaction products and GBFS and FA’s spatial distribution and degree of reaction at different curing ages and conditions. The results showed that the pastes react at different rates with the dominant reaction products of Mg-rich gel around GBFS particles, i.e., Ca-Mg-Na-Al-Si, and with Ca-Na-Al-Si gel, in the bulk paste. The microstructure evolution was significantly affected in the unsealed curing conditions due to the Na+ loss. The effect of the curing conditions was more pronounced in the binary system.
Highlights
Alkali-activated materials (AAMs) are multi-phase materials composed of crystalline and amorphous phases, which together with alkalis and water react over time [1]
The representative backscattering electron (BSE) image of raw granulated blast-furnace slag (GBFS) was divided into nine fields, as shown in Subsequently, a data set from each Spectral Imaging (SI) image field was selected and imported into 11 of 36 described in PhAse Recognition and Characterization (PARC) for the preliminary phase characterization according to the procedure Section 2.2.1
The amorphous phase content of the sealed cured pastes is similar at 1, 7, and 28 days, but it has increased after 1 year
Summary
Alkali-activated materials (AAMs) are multi-phase materials composed of crystalline and amorphous phases, which together with alkalis and water react over time [1] This is followed by a net decrease in volume due to chemical shrinkage, i.e., the volume of the reaction products is less than the initial volume of the unreacted particles and the liquid. Despite numerous efforts to characterize AAMs in the literature, rigorous definitions on the reaction products (N-A-S-H, C-A-S-H, C-(N-)A-SH) and possible secondary products, such as zeolites and layered double hydroxides, in these materials do not exist yet [6,10,11] This is due to the complexity of the chemistry (element distribution gradients within phases) and microstructural features of AAMs (phase distribution)
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