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Volume 112
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Resource recovery from fly ash via alkali fusion: Synthesis and silicon sustained-release behavior of calcium silicate hydrate
Zifang Xu a b, Mengtian Zhang a, Xuekai Wang a c *, Chenzhen Cao a, Jiawei Li a, Zheng Li a, Shuangyue Hu a
a School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, 232001, China
b National Key Laboratory of Safe Mining and Environmental Protection in Deep Coal Resources, Huainan, 232000, China
c Anhui Industrial Generic Technology Research Center for New Materials from Coal-based Solid Wastes, Huainan, 232001, China
10.1016/j.partic.2026.03.002
Volume 112, May 2026, Pages 183-196
Received 5 December 2025, Revised 2 February 2026, Accepted 1 March 2026, Available online 17 March 2026, Version of Record 24 March 2026.
E-mail: wangxuekai@aust.edu.cn
Highlights

• Developed an acid-free Na2CO3 alkali fusion process for CFA, producing functional C-S-H.

• Synthesized C-S-H slow-release silicon fertilizer, regulated by an in-situ forming CaCO3 barrier layer.

• Elucidated the silicon phase transformation mechanism in CFA during alkali fusion, highlighting side-reaction inhibition.


Abstract

The large-scale accumulation of coal fly ash (CFA) poses serious environmental risks. Its high-value utilization remains a key challenge for sustainable development. This study employed an acid-free process to extract silicate ions from CFA and convert them into high-value calcium silicate hydrate (C-S-H) with silicon-releasing functionality. An optimal silicon extraction yield of 45.7 ± 0.6% was achieved at a calcination temperature of 850 °C with a CFA:Na2CO3 mass ratio of 1:1. The maximum citric acid-soluble silicon release of C-S-H reached 88.9 ± 3.9 mg/g. The release kinetics analysis indicated that the silicon release was controlled by diffusion through an in situ formed calcium carbonate barrier layer. This acid-free approach introduces an environmentally friendly alternative for utilizing CFA. The obtained C-S-H material acts as an efficient slow-release fertilizer, whose nutrient release can be tuned by the surrounding pH. Additionally, the well-crystallized β-nepheline byproduct also holds promising industrial prospects. All of these research findings together provide a theoretical basis and a practical approach for converting CFA into high-value products, as well as for developing pH-responsive controlled-release materials to serve sustainable agriculture.

Graphical abstract
Keywords
Coal fly ash; C-S-H; Alkali fusion; Sustained-release fertilizer; Silicate release kinetics; Nepheline
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