Effect of Different Initial CaO/SiO2 Molar Ratios and Curing Times on the Preparation and Formation Mechanism of Calcium Silicate Hydrate

• Published in: Materials Vol. 16; no. 2; p. 717
• Main Authors: Wu, Jianfang, Liao, Hongqiang, Ma, Zhuohui, Song, Huiping, Cheng, Fangqin
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.01.2023; MDPI
• Subjects: 29Si MAS-NMR; C-S-H formation mechanism; C/S molar ratio; Calcium oxide; Calcium silicate hydrate; Carbon; Cement industry; Conversion ratio; Curing; Degree of polymerization; Fly ash; Hydration; Mechanical properties; NMR; Nuclear magnetic resonance; Nucleation; pozzolanic reaction; Raw materials; Scanning electron microscopy; Silica; Silica fume; Silicon dioxide; Slaked lime; Tetrahedra; Thermogravimetric analysis; Weight loss; United States > US; China
• ISSN: 1996-1944; 1996-1944
• DOI: 10.3390/ma16020717

To better understand the pozzolanic activity in fly ash used as a supplementary cementitious material in cement or concrete, calcium silicate hydrate (C-S-H) has been synthesized by adding silica fume to a supersaturated calcium hydroxide solution prepared by mixing calcium oxide and ultrapure water. Thermogravimetric analysis results have revealed the variation in the weight loss due to C-S-H in the samples and the conversion ratio of calcium oxide (the μCaO value), which represents the proportion of calcium oxide in the initial reaction mixture used to produce C-S-H, with curing time. The weight loss due to C-S-H and the μCaO value were both maximized (13.5% and 90.4%, respectively) when the initial C/S molar ratio was 1.0 and the curing time was 90 d. X-ray diffraction (XRD) analysis has indicated that C-S-H in the samples after curing for 7 d had the composition Ca1.5SiO3.5·xH2O. 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis has revealed that the degree of polymerization of C-S-H increased with an increase in curing time for samples with an initial C/S molar ratio of 1.0. The ratio of internal to terminal tetrahedra (Q2/Q1) increased from 2.29 to 4.28 with the increase in curing time from 7 d to 90 d. At curing times ≥ 28 d, a leaf-like C-S-H structure was observed by scanning electron microscopy (SEM). An ectopic nucleation–polymerization reaction process is proposed for the formation mechanism of C-S-H.