Synthesis of cement based CaO-Al{sub 2}O{sub 3}-SiO{sub 2}-H{sub 2}O (CASH) hydroceramics at 200 and 250 deg. C: Ex-situ and in-situ diffraction

Hydroceramic compositions in the CaO-Al{sub 2}O{sub 3}-SiO{sub 2}-H{sub 2}O (CASH) system have potential as geothermal well sealants as well as autoclaved construction materials. We report new data on phase compositions and reaction rates in hydrothermal syntheses at 200 deg. C and 250 deg. C using a commercial API Class G oilwell cement alone, and at 200 deg. C with additions of silica flour and of corundum (alumina). Curing times were in the range 1-240 h. We use both ex-situ laboratory X-ray diffraction and in-situ synchrotron energy-dispersive X-ray diffraction to track rates of reaction. When cement only is hydrated, jaffeite, {alpha}-C{sub 2}SH and portlandite are formed. When silica flour is added a precursory gel forms prior to the crystalline calcium silicate hydrate phases xonotlite and gyrolite. Both XRD and EDD data suggest that the addition of silica flour retards the hydration of the cement at early times (< 24 h). In alumina-containing systems the rate of consumption of clinker phases is the same as in cement only systems. Jaffeite and {alpha}-C{sub 2}SH occur as intermediates but the major end product is a siliceous katoite-type hydrogarnet. Quantitative phase analysis using Rietveld refinement of ex-situ diffraction data gives results which are mostly consistent with stoichiometric constraints in all three systems examined here.