Preparation of Calcium Silicate Hydrate from Fly Ash by Alkali Fusion Method and Its Silicon Slow-Release Performance

doi:10.16552/j.cnki.issn1001-1625.2025.0861

Abstract

Abstract:

This study utilized coal fly ash （CFA） as raw material to achieve the extraction of highly reactive soluble silicon and the controllable synthesis of nano-sized calcium silicate hydrate （CSH） through an alkali fusion-hydrothermal method， providing a feasible pathway for the high-value utilization of coal-based solid waste. The specific procedure included： using Na₂CO₃ as an activator and calcining at 850 ℃ for 2 h. The calcined product was dissolved in water at a solid-liquid ratio of 1∶10. After 90 min in an 80 ℃ water bath， solid-liquid separation yielded a silicon extraction solution. This process achieves a silicon extraction rate of 45.7%. The silicon extraction solution was then reacted with CaCl₂ in an 80 ℃ water bath for 3 h to successfully synthesize CSH. Slow-release studies demonstrate that the silicon release behavior of the as-prepared CSH in citric acid solution follows the Higuchi diffusion model， with a maximum silicon slow-release concentration of 350 mg/L. It also exhibits continuous release characteristics under different pH conditions， at pH value of 6 and 8， the silicon slow-release concentration reached 150 and 100 mg/L， respectively， on the third day. This research achieves the low-cost resource conversion of CFA， and the prepared CSH holds promise for promoting the green resource utilization of solid waste and the sustainable development of agriculture.

Key words: coal fly ash, calcium silicate hydrate, alkali fusion method, slow-release, resource utilization

CLC Number:

TQ17

ZHANG Mengtian, WANG Xuekai, XU Zifang, HU Shuangyue, LI Zheng, LI Jiawei. Preparation of Calcium Silicate Hydrate from Fly Ash by Alkali Fusion Method and Its Silicon Slow-Release Performance[J]. BULLETIN OF THE CHINESE CERAMIC SOCIETY, 2026, 45(2): 517-527.

Figures/Tables 11

Table 1 Main chemical composition of CFA

Ingredient	SiO₂	Al₂O₃	Fe₂O₃	CaO	MgO
Mass fraction/%	45.999	26.218	4.962	2.750	0.462

Fig.1 Steps for preparing solution

Fig.2 Steps for testing soluble SiO2

Fig.3 Concentration-absorbance curve of soluble SiO2

Fig.4 Characterization of raw material CFA and analysis of silicon extraction rate

Fig.5 XRD patterns of calcination products at 850 ℃ and their water-leached filter residues

Fig.6 XRD patterns and SEM images of synthetic sample

Fig.7 Soluble silicon concentration in slow-release products for 3 d and short-term slow-release periods from reaction of CSH， CSH-NH4+， CSH-PO43?- under different conditions

Table 2 Kinetic fitting equations and data analysis

Kinetic model	Parameter	CSH	CSH- $N H 4 +$	CSH- $P O 4 3 -$
Higuchi equation	C	2.164	29.250	1.211
	$k H$	36.295	33.383	21.917
	$R 2$	0.990	0.950	0.971
Pseudo-second-order equation	1/（ $k 2 Q e 2$ ）	0.013	0.004	0.022
	1/ $Q e$	0.012	0.010	0.020
	$R 2$	0.966	0.999	0.986

Table 2 Kinetic fitting equations and data analysis

Kinetic model	Parameter	CSH	CSH- $N H 4 +$	CSH- $P O 4 3 -$
Higuchi equation	C	2.164	29.250	1.211
	$k H$	36.295	33.383	21.917
	$R 2$	0.990	0.950	0.971
Pseudo-second-order equation	1/（ $k 2 Q e 2$ ）	0.013	0.004	0.022
	1/ $Q e$	0.012	0.010	0.020
	$R 2$	0.966	0.999	0.986

Fig.8 Fitting curves for slow-release kinetic of CSH， CSH-NH4+， and CSH-PO43?-

Fig.9 XRD patterns and FT-IR spectra of CSH after sustained release under different conditions

References 28

[1]	王晓丽，李秋义，陈帅超，等. 工业固体废弃物在新型建材领域中的应用研究与展望［J］. 硅酸盐通报， 2019， 38（11）： 3456-3464.
	WANG X L， LI Q Y， CHEN S C， et al. Application research and prospect of industrial soild waste in new building materials［J］. Bulletin of the Chinese Ceramic Society， 2019， 38（11）： 3456-3464 （in Chinese）.
[2]	JIANG J， LUO H H， WANG S F， et al. Synthesis of foamed geopolymers by substituting fly ash with tailing slurry for the highly efficient removal of heavy metal contaminants： behavioral and mechanistic studies［J］. Journal of Central South University， 2024， 31（4）： 1344-1359. DOI
[3]	XIE X R， LEI C， DAI W L， et al. Enhanced adsorption of Pb（II） ions using a magnetic mesoporous molecular sieves derived from coal fly ash and nano-Fe₃O₄ particles［J］. Journal of Industrial and Engineering Chemistry， 2025， 146： 684-696. DOI URL
[4]	CHEN X， ZHANG C X， TONG Y P， et al. Preparation of high-strength ceramsite from coal gangue， fly ash， and steel slag［J］. RSC Advances， 2025， 15（6）： 4332-4341. DOI URL
[5]	TIAN X M， GUO Z Q， ZHU D Q， et al. Recovery of valuable elements from coal fly ash： a review［J］. Environmental Research， 2025， 282： 121928. DOI URL
[6]	ZHAO X D， ZENG L， GUO J M， et al. Efficient separation and comprehensive extraction of aluminum， silicon， and iron from coal fly ash by a cascade extraction method［J］. Journal of Cleaner Production， 2023， 406： 137090. DOI URL
[7]	JU T Y， HAN S Y， MENG F Z， et al. Porous silica synthesis out of coal fly ash with no residue generation and complete silicon separation［J］. Frontiers of Environmental Science & Engineering， 2023， 17（9）： 112.
[8]	BUENO A C S O， CASTRO G L S， SILVA JUNIOR D D， et al. Response of photosynthesis and chlorophyll a fluorescence in leaf scald-infected rice under influence of rhizobacteria and silicon fertilizer［J］. Plant Pathology， 2017， 66（9）： 1487-1495. DOI URL
[9]	SARMA H H， BORAH S K， DUTTA N， et al. Impact of silicon fertilization in crop production： enhancing yield， stress and disease resistance in agriculture［J］. Journal of Advances in Biology & Biotechnology， 2024， 27（9）： 645-658.
[10]	SINGH V， MANDAL T， MISHRA S R， et al. Development of amine-functionalized fluorescent silica nanoparticles from coal fly ash as a sustainable source for nanofertilizer［J］. Scientific Reports， 2024， 14（1）： 3069. DOI
[11]	WANG G G， CHEN C Y， LI J Q， et al. A zero-emission collaborative treatment method for brown corundum fly ash and carbide slag： producing silicon fertilizer while recovering gallium and potassium［J］. Separation and Purification Technology， 2024， 332： 125739. DOI URL
[12]	TANG M C， ZHOU C C， ZHANG N N， et al. Extraction of rare earth elements from coal fly ash by alkali fusion-acid leaching： mechanism analysis［J］. International Journal of Coal Preparation and Utilization， 2022， 42（3）： 536-555. DOI URL
[13]	CHEN H， YU W Z， JIANG Z X， et al. Fe-Si alloys production and alumina extraction from coal fly ash via the vacuum thermal reduction and alkaline leaching［J］. Fuel Processing Technology， 2023， 244： 107702. DOI URL
[14]	夏文杰，金永丽，张梧祯，等. 粉煤灰制备硅铁合金并富集氧化铝的试验研究［J］. 钢铁钒钛， 2024， 45（5）： 108-115.
	XIA W J， JIN Y L， ZHANG W Z， et al. Carbon thermal reduction is used to enrich alumina from fly ash and prepare ferrosilicon alloy［J］. Iron Steel Vanadium Titanium， 2024， 45（5）： 108-115 （in Chinese）.
[15]	HOU D S， WU D， WANG X P， et al. Sustainable use of red mud in ultra-high performance concrete （UHPC）： design and performance evaluation［J］. Cement and Concrete Composites， 2021， 115： 103862. DOI URL
[16]	陈娜，严云，胡志华. 用粉煤灰制备SiO₂-Al₂O₃气凝胶的研究［J］. 武汉理工大学学报， 2011， 33（2）： 37-41.
	CHEN N， YAN Y， HU Z H. Research of SiO₂-Al₂O₃ aerogel with fly ash［J］. Journal of Wuhan University of Technology， 2011， 33（2）： 37-41 （in Chinese）.
[17]	水利部. 二氧化硅（可溶性）的测定（硅钼黄分光光度法）：SL 91.1—1994［S］. 1995.
	Ministry of Water Resources. Determination of silica （dissolved）（molybdosilicate method）：SL91.1—1994［S］. 1995 （in Chinese）.
[18]	TUNSTALL L E， SCHERER G W， PRUD’HOMME R K. A new hypothesis for air loss in cement systems containing fly ash［J］. Cement and Concrete Research， 2021， 142： 106352. DOI URL
[19]	GUO C B， ZOU J J， WEI C D， et al. Comparative study on extracting alumina from circulating fluidized-bed and pulverized-coal fly ashes through salt activation［J］. Energy & Fuels， 2013， 27（12）： 7868-7875. DOI URL
[20]	LI J， ZHANG J S， WU X， et al. A nanocomposite paper comprising calcium silicate hydrate nanosheets and cellulose nanofibers for high-performance water purification［J］. RSC Advances， 2020， 10（51）： 30304-30313. DOI PMID
[21]	FUJIWARA M， SHIOKAWA K， KUBOTA T. Direct encapsulation of proteins into calcium silicate microparticles by water/oil/water interfacial reaction method and their responsive release behaviors［J］. Materials Science and Engineering： C， 2012， 32（8）： 2484-2490.
[22]	石勤，刘珂，后王新，等. 沸石基缓释肥料的研究进展［J］. 硅酸盐通报， 2022， 41（6）： 2181-2190.
	SHI Q， LIU K， HOU W X， et al. Research progress of zeolite-based slow release fertilizers［J］. Bulletin of the Chinese Ceramic Society， 2022， 41（6）： 2181-2190 （in Chinese）.
[23]	李二庭，靳蕊含，刘向军，等. 准噶尔盆地彩南地区天然气干燥系数异常高原因分析［J］. 石油实验地质， 2023， 45（2）： 347-355.
	LI E T， JIN R H， LIU X J， et al. Reason for abnormally high drying coefficient of natural gas in Cainan area， Junggar Basin［J］. Petroleum Geology & Experiment， 2023， 45（2）： 347-355 （in Chinese）.
[24]	宋华玲，郭雪飞，谭红琳，等. 岛状硅酸盐宝石矿物的近红外光谱特征研究［J］. 硅酸盐通报， 2019， 38（11）： 3592-3596.
	SONG H L， GUO X F， TAN H L， et al. Near infrared spectrum analysis of nesosilicate gemstone minerals［J］. Bulletin of the Chinese Ceramic Society， 2019， 38（11）： 3592-3596 （in Chinese）.
[25]	梁起睿，叶金蕊，颜丙越，等. 芳纶纤维与树脂基体改性对复合材料剪切性能的影响［J］. 高分子材料科学与工程， 2024， 40（2）： 91-100.
	LIANG Q R， YE J R， YAN B Y， et al. Effect of modification of aramid fiber and resin matrix on shear properties of composites［J］. Polymer Materials Science & Engineering， 2024， 40（2）： 91-100 （in Chinese）.
[26]	张欢欢，方双明，付娟，等. 电解锰渣中锰的柠檬酸浸出工艺及机理研究［J］. 硅酸盐通报， 2023， 42（6）： 2071-2080.
	ZHANG H H， FANG S M， FU J， et al. Leaching process and mechanism of manganese from electrolytic manganese slag by citric acid［J］. Bulletin of the Chinese Ceramic Society， 2023， 42（6）： 2071-2080 （in Chinese）.
[27]	RAJARAMAKRISHNA R， BOTTA R， NUNTAWONG N， et al. Structural studies of transition metal ions doped in biomass ash as silica source for glass production in Thailand［J］. Journal of Physics： Conference Series， 2018， 1120： 012104.
[28]	邹家伟，刘志超，王发洲. 基于γ-C₂S的蜂窝陶瓷常温制备与性能研究［J］. 材料导报， 2025， 39（4）： 53-59.
	ZOU J W， LIU Z C， WANG F Z. Room temperature preparation of γ-C₂S-based honeycomb ceramics and their properties［J］. Materials Reports， 2025， 39（4）： 53-59 （in Chinese）.