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

Abstract

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.

Physicomechanical Behavior of High-Performance Concrete Reinforced with Recycled Steel Fibers from Twisted Cables in the Brittle State—Experimentation and Statistics

“…In contrast, HPFRC2, HPFRC11, and HPFRC12 exhibit lower intensity peaks, with values of 541, 348, and 345, respectively. Similar results have been found by other authors [99,100].…”

Study on the Modification Effect and Mechanism of a Compound Mineral Additive and Basalt Fiber on Coal Gangue Concrete

“…At the same time, this paper also gives a more accurate chemical equation of the volcanic ash reaction (secondary hydration reaction), in which Ca 1.5 SiO 3.5 •xH 2 O is a crystal form of C-S-H formed by the system. Relevant studies have found that with an increase in curing time [49], the crystallinity and polymerization degree of C-S-H formed by the reaction increase. In addition, the 'micropump effect' of coal gangue aggregate [50] absorbs some water molecules into the micropores on the surface of coal gangue aggregate during cement hydration, resulting in an increase in OHconcentration, which in turn promotes the secondary hydration reaction of FA and SF.…”

The Effect of Biological Corrosion on the Hydration Processes of Synthetic Tricalcium Aluminate (C3A)

“…A decrease in the pH of the cement matrix environment will also affect the decomposition process of hydrated calcium silicates (C-S-H phase), with the effect that the solution will tend to equalise the concentration gradient of calcium ions, which will subsequently lead to the degradation process of the cement hydration products [ 11 ]. Other clinker phases, such as tricalcium aluminate and calcium alumino-ferrite, are involved in the calcium ion release processes and will release aluminium ions into the solution as a result of changes in the solution reaction.…”