Gehlenite hydrate

C2A8H8, known by its mineral name of stratlingite and also as gehlenite hydrate , is well established as a natural mineral, hydration product of i certain types of composite cements and laboratory product. Its crystal data... 

Metakaolinite is the high quality mineral addition to cement or concrete because of the excellent pozzolanic properties. In reaction with caldinn hydroxide C-S-H, C AHjj and gehlenite hydrate ate produced [183], These reactions are rapid and as soon as after 3 days these phases are detectable by XRD [184], The gehlenite hydrate is imstable in the presence of calcium hydroxide, while CjAHg is the stable phase, however, the rate of CH reaction with C2ASH3 is low and the non-equilib-rimn state is existing [185]. 

Kurdowski, W., Duszak, S., Sorrentino, R Corrosion of Gehlenite Hydrate in strong choride solution. In Mangabhai, R.J., Glasser, RP (eds.) Calcium Aluminate Cements, p. 371. lOM Communications, London (2001)... 

Locher [13] foimd that the gehlenite hydrate is not stable in the calcium hydroxide containing mixture and is transformed into hydrogamet CjASH. CjASH is not the stable phase in the gypsum and calcium hydroxide solutions and in this condition is transformed into ettringite. However, the gehlenite hydrate is stable in the presence of gypsum and NajSO [13]. 

In the presence of calciitm sulfate and calcium hydroxide, ettringite rather than calciitm aluminates is formed in the hydration of both and CjjA7Cap2. Small amorrrrts of monosulfate may also be formed. In pastes made of C2S and C,2A2 hydrated for 16 years gehlenite hydrate (strathngite, C2ASHjg) was also found among the products of hydration (Hannawayya, 1992). 

Dicalcium silicate in calcium aluminate cements does not yield a C-S-H phase and free calcium hydroxide, as occurs in Portland cement. Instead, stratlingite (also called gehlenite hydrate, C2AHg) is formed as the product of hydration. 

In mixes with Portland cement as the main constituent, the setting is due to a rapid formation of ettringite and the hydration of calcium aluminate cement. The hydration of the calcium silicates has little influence on the setting process, but contributes to the subsequent strength development (Gu et al., 1994). In addition to the phases formed in the hydration of pure Portland and calcium aluminate cements, stratlingite (gehlenite hydrate, C2SAHg) may also be formed in the hydration process. 

MacDowell, J.F. (1986) Hydrogamet—gehlenite hydrate cement from Ca0-Al203-Si02 glasses, in Proceedings 8th ICCC, Rio de Janeiro, Vol. 4, pp. 423-428. 

MacDowell, J.F., Sorrentino, F., and Capelle, M. (1991) Hydrogamet-gehlenite hydrate equilibrium in glass cements, in Proceedings 13th International Conference on Cement Microscopy, Tampa, pp. 131-146. 

In the later stages the hydration progresses similarly to that of pure Portland cement in OPC-rich mixes and also to that of pure calcium aluminate cement in CAC-rich mixes. In mixes of intermediate composition stratlingite (gehlenite hydrate, C2SAHg) may also be formed. 

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