Abstract
Abstract The silicate AFm, strätlingite, has been shown to be stable in high aluminosilicate cement systems but its stability with respect to the anion content of hydrated Portland cement paste is unknown. The stability of strätlingite in the presence of sulfate and carbonate phases relevant to cement systems are reported. Results show that strätlingite persists at the sulfate activity conditioned by gypsum, ettringite and at carbonate activity conditioned by the presence of calcite, carbonate AFm, or carbonate AFt. Structural incorporation of anions such as carbonate or sulfate in strätlingite was not observed in the temperature range 20–85 °C.
Highlights
It is a common practice to add gypsum to Portland cement [21]
The X-ray Powder Diffraction (XRD) pattern of the resulting solids (Fig. 3a) shows that stratlingite persisted in the presence of gypsum
The Scanning Electron Microscope (SEM) image (Fig. 4a) of the stratlingite–gypsum– water mixture cured at 55 °C shows the clear monoclinic gypsum grains dispersed in stratlingite powder, Fig. 3 XRD patterns of the resulting solids from a Stratlingite– gypsum slurry cured for 30 days in supersaturated gypsum solution; b Stratlingite cured in saturated gypsum solution for 70 days: stratlingite persisted in the presence of gypsum suggesting coexistence of both phases, in agreement with the XRD pattern
Summary
It is a common practice to add gypsum to Portland cement [21]. most novel cements such as the calcium aluminosulfate cements contain substantial amounts of sulfate. Cement could be blended with limestone, mainly CaCO3, and atmospheric CO2 is readily uptaken by wet cements pastes cured in the open In this way, sulfate and carbonate phases form normally in the hydration process of cement paste. High saturation in sulfate –rich service environments and elevated temperature may lead to the formation of gypsum and calcite. The impact of these sulphur and carbon bearing phases on the stability of the crystalline phases occurring in the CaO–Al2O3– SiO2–H2O system is important as reactive supplementary materials are often added with the result that the composition of the reactive fraction of concrete varies over wide limits of composition
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