Alkali-activated Fly Ash-slag Pastes Research Articles

A comprehensive intelligent framework for interpreting and quantifying the microstructure of waste-derived alkali-activated materials: A case study of fly ash/slag pastes

The alkali-activated fly ash/slag (AAFS) pastes exhibit complex compositions and heterogeneous spatial distributions of phases, necessitating a comprehensive tool capable of quantifying phase assemblage, characterizing ion diffusion, and unveiling microstructure evolution. However, existing techniques have limitations in providing interpretability and visual analyses. This study introduces a versatile analysis methodology for energy dispersive X-ray spectroscopy (EDS) images, encompassing data preprocessing, data interpretation, and four downstream applications: phase inference, phase quantification, ion diffusion characterization, and quantification of phase spatial distributions. Through the analysis of five representative elements, this study successfully identifies and quantifies three components in precursors (slag, quartz, mullite) and four reaction products (slag-products, quartz-products, mullite-products, mixture products) in the AAFS pastes. Moreover, by characterizing ion diffusion and quantifying spatial distributions, insights into the interactions among various precursors and reaction products are elucidated. The proposed EDS image analysis methodology is expected to yield valuable understandings of the reaction mechanisms and phase evolution involved in utilizing solid waste as cementitious materials.

Multiscale study of microstructural evolution in alkali-activated fly ash-slag paste at elevated temperatures

To promote the high-temperature application of alkali-activated fly ash-slag (AAFS), it is vital to thoroughly understand the performance of AAFS at elevated temperatures. This paper presents a systematic study on the multiscale microstructural evolution in AAFS paste in terms of nanostructure, chemical composition and morphology changes characterised using 29Si nuclear magnetic resonance, X-ray diffraction and Fourier transform infrared spectroscopy, as well as the pore structure evolution by means of mercury intrusion porosimetry, backscattered electron microscopy and X-ray microcomputed tomography. Results indicate that the decomposition of C-A-S-H and N–C-A-S-H gels occurs while gel pores are filled at elevated temperatures up to 800 °C, along with the crack development, whereas micro-cracks are healed by melting and viscous sintering. This study provides an in-depth insight into the damage mechanisms of AAFS paste at elevated temperatures from a multiscale viewpoint, accounting for Level I: solid gel particles, Level II: gel matrix, and Level III: AAFS paste.

A multiscale study on gel composition of hybrid alkali-activated materials partially utilizing air pollution control residue as an activator

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 Na2O 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).

In-situ X-ray tomographic imaging of microstructure evolution of fly ash and slag particles in alkali-activated fly ash-slag paste

This paper presents an in-situ investigation of the microstructure evolution of fly ash and slag particles in alkali-activated fly ash-slag paste using X-ray microcomputed tomography. Results indicate that the dissolution of fly ash and slag particles is not uniform especially for the particles with large size due to the heterogeneous distribution of chemical composition and initial defects. The dissolution of the particles with small size is faster than that of large particles owing to the relatively high specific area. The formation of reaction products (i.e., inner products) is mostly accumulated within the boundary of original particles, which become a barrier to prevent the further dissolution of unreacted particles. The fly ash-slag interaction in terms of microstructure evolution is not obvious at early ages (1–3 d) but becomes apparent at later ages (7–28 d), which can be attributed to the continuous transport of dissolved ions between fly ash and slag.

Behavior of alkali activated fly ash slag paste at room temperature

Abstract This study involves to explores the mechanical strength effect of Alkali Activated Fly Ash Slag paste (AAFS) at normal room temperature and its microstructure properties. Weight mixing percentages of Fly Ash (FA) and Ground Granulated Blast-furnace Slag (GGBS) are varying as 30/70, 50/50, and 70/30. Each proportion was activated by a combination of sodium hydroxide (NaOH) of 3 M, 6 M, 9 M, and 12 M and sodium silicate solution (Na2SiO3) keeping the ratio as 1:1. Each sample gets cured at normal room temperature. The result shows the strength variation with the molar concentration of NaOH solid mix proportion, the weight percentage of Na2O/SiO2 of alkali activated (AA) paste. Maximum strength is observed in AAF30S by the combined solution of N12W i.e. 88.65 MPa; in AAF50S by N9W i.e. 71.62 MPa; in AAF70S by N6W i.e. 41.89 MPa. Maximum percentage of GGBS leads to increase of calcium percentages with a high pH value. This cause to high strength achieved in AAF30S than AAF50S and AAF70S.

Internal curing of alkali-activated fly ash-slag pastes using superabsorbent polymer

To mitigate autogenous shrinkage that may cause early-age cracking of alkali-activated fly ash-slag (AAFS) concrete, internal curing using superabsorbent polymers (SAP) is employed in this study. AAFS pastes with different SAP dosages (0–0.5%) and slag replacement ratios to fly ash (15–30%) are investigated. Experimental results indicate that with the addition of SAP workability of fresh paste is improved while compressive strength is comparatively reduced. As SAP dosage increases from 0.2% to 0.5%, chemical shrinkage and autogenous shrinkage of AAFS pastes are reduced by around 18% to 45% and 76% to 85%, respectively. Internal curing of SAP is found to lower the heat peak and shift the peak to the right. This indicates the slower hydration rate corresponding to the lower chemo-mechanical deformation (chemical shrinkage), which contributes to the mitigation of autogenous shrinkage. Therefore, internal curing by means of SAP is an efficient method for mitigating autogenous shrinkage in AAFS pastes.