Insights the hydration of portland cement under hydrothermal curing

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Temperature curing significantly affects the hydration of Portland cement and its subsequent properties [1]. When a water stream saturation pressure is concomitantly applied (autoclaving), the mechanism of hydration reaction and the hydrated products are quite different from those obtained under influence of single parameter of temperature [2]. When subjected to hydrothermal conditions in oil and geothermal wells, cement slurry sets under influence of combined effect of temperature and vapor pressure. CEM I 42.5 R (denoted as CEM Extra by producer) has been submitted to the effect of different temperature - vapor pressures (100°C-0.1 MPa, 165°C -0.3 MP and 220°C-1.2 MPa) in a laboratory autoclave up to 7 days. X-Ray diffraction, Simultaneous thermogravimetric and differential thermal analysis measurements (TG/DTA) were mostly used to characterize the hydration products and their transformation under different hydrothermal curing conditions. Mechanical properties were correlated with pore structure and SEM analysis. Different hydrothermal curing regimes resulted in sequential and overlapped hydration reactions with products, including portlandite, ettringite, poorly-crystalline CSH, hydrogarnet (CASH), a-C2SH, jaffeite(C6S2H3), reinhardbraunsite (C5S2H) and calcium carbonate (CaCO3). Calcium silicate hydrate, the most important substance of binder materials underwent systematic changes starting with the transformation of C-S-H gel formed at ambient temperature or under low-pressure hydrothermal conditions to a-C2SH, jaffeite and reinhardbraunsite with increasing these conditions. The gradual transition of amorphous C-S-H to a-C2SH at 160°C has engendered the refinement of pore structure and subsequent improvement of mechanical properties. However, that of a-C2SH to jaffeite and to reinhardbraunsite has caused the deterioration of pore structure with corollaries the increase of permeability and the decrease of mechanical properties. The gradual reduction of peak intensities below 200°C in TG and DTG data (Fig. 1) and the increase of peaks in temperature interval 500-600 °C in the PC cured under hydrothermal condition confirms the transformation of the amorphous C-S-H phases to crysatalline ones. Moreover, different temperature peaks from 550°C to 1000°C denoting thermal decomposition of different calcium carbonate species were depicted at DTG curves. These are ranged from low-to well-crystallized CaCO3. The micrographs of samples confirm the change from the low-crystalline microstructure to the well-crystallized artefacts.

CEM, DTG, TGA.

PALOU, M.; KUZIELOVA, E.; ŽEMLIČKA, M.; TKACZ, J.; MÁSILKO, J.

2. 9. 2018

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