α‐Dicalcium Silicate Hydrate: Preparation, Decomposed Phase, and Its Hydration

Abstract

ol-C,SH can be synthesized by hydrothermal treatment of lime and silicic acid for 2 h at 200°C. When heated to 390-490"C, a-C,SH dissociates through a two-step process to form an intermediate phase plus some y-C,S. This appears to be a new dicalcium silicate different from known dicalcium silicate-, a'", p, and y phase-and is stable until around 900°C. At 920-96OoC, all the phases are transformed to the atL phase. The intermediate phase has high crystallinity and is stable at room temperature. 29Si MAS NMR measurements indicate the possibility that it contains both protonated and unprotonated monosilicate anions. The intermediate phase that has passed through the a' phase at higher temperature yields p-C2S on cooling. The intermediate phase is highly active, and completed its hydration in 1 day ( w / s = 1.0, 25°C). Among the crystalline calcium silicate hydrates with Ca/Si = 2.0, it is hillebrandite that yields p-C,S at the lowest temperature.

Research review of cement clinker chemistry

“…The most reactive polymorph is indeed an XRD-amorphous phase [52] with the same composition as the crystalline polymorphs. The other very reactive polymorph is x-C 2 S [53]. These polymorphs have the identical chemical composition but a much higher reactivity than polymorphs known from common cement manufacture.…”

“…Synthesis of highly reactive belite polymorphs. The production of highly reactive C 2 S polymorphs was pioneered by Ishida [53] and Garbev et al [63]. Supported by the presence of foreign ions it is based on a two-step process that involves the synthesis of α-Ca 2 SiO 4 ⋅ H 2 O in an autoclave at 150-200°C as a first step and its conversion into highly reactive belite polymorphs by tempering at 400-800°C in a second step, see Fig.…”

Low Temperature Synthesis of Belite Cement Based on Silica Fume and Lime

“…On the other hand, XRD analysis illustrates that the crystallographic properties and density of dissociation products of α -C 2 SH depend on the calcination temperature [20]. α -C 2 SH dissociates at 390–490°C forming an intermediate phase plus γ -C 2 S. The intermediate phase then transforms to α L ′ phase at 920–960°C and yields β -C 2 S on cooling while hillebrandite yields β -C 2 S at the lowest temperature [21]. Hillebrandite synthesized by mechanochemical treatment of amorphous precipitated silica/lime decomposed into β -C 2 S below 1000°C [18].…”

Self-repairing properties of OPC clinker/natural zeolite blend in water and alkali carbonate environments at 270°C

“…Table 5 lists the weight losses in different temperature ranges for specimens cured in water and carbonate solution and Figure 4 shows the DTG curves for the control samples (cured for 24 h at 270°C) and the samples cured for 10 d. The weight losses up to 400°C can be associated with (sodium, calcium) silica aluminate and CSHs; zeolite decomposition may also contribute to the weight loss in that temperature range (analcime, mordenite and thomsonite (Foldvari, 2011)). Between about 400 and 550°C, calcium-rich CSHs contribute to the weight loss (Ishida et al, 1993), along with calcium hydroxide; the recrystallisation of aragonite to calcite occurs at around 450°C. The weight losses at temperatures above 550°C are, for the most part, due to the decomposition of carbonates such as calcite ($600°C), magnetite ($620-650°C), calcium magnesium carbonates (600-650°C, huntite), magnesium iron calcium carbonate ($700-750°C) and dolomite (750-800°C) (Foldvari, 2011).…”