Insights into the hydration of Portland cement under hydrothermal curing

Abstract

The combined effect of temperature and vapor pressure on hydration reactions of three different types of Portland cements was studied using a laboratory autoclave. Oil well Portland cement Class G high sulfate resistant (HSR), Dyckerhoff Portland cement and Portland cement CEM I 42.5 R Extra were cured under hydrothermal conditions (165 °C–0.5 MPa and 220 °C–2.0 MPa) up to 7 days. In parallel, hydration reactions at laboratory conditions (25 °C–0.1 MPa) of these samples were also studied. Simultaneous thermogravimetric and differential thermal analysis measurements (TG/DTA) were mostly used to characterize the course of hydration under different curing conditions. X-ray diffraction, scanning electron microscopy and mercury intrusion porosimetry were used to identify the hydration products and to characterize the coupled effect of temperature and vapor pressure on microstructure and pore structure development. Also, mechanical properties were correlated with pore structure and scanning electron microscopy analysis. Different hydrothermal curing regimes resulted in sequential and overlapped hydration reactions with products including portlandite, ettringite, poorly crystalline C–S–H, hydrogarnet (C–A–S–H), α-C2SH, jaffeite (C6S2H3), scawtite (C7S6$${\bar{\text{C}}}$$H2) and reinhardbraunsite (C5S2H). Calcium silicate hydrate underwent systematic changes starting with the transformation of C–S–H gel formed during the non-equilibrium phases or under low-pressure hydrothermal conditions to α-C2SH, jaffeite and reinhardbraunsite with increasing hydrothermal temperatures. The gradual transition of amorphous C–S–H phases to α-C2SH, C6S2H3, C7S6$${\bar{\text{C}}}$$H2 and C5S2H has caused the deterioration of pore structure with corollaries of the increase in permeability and the decrease in mechanical properties. Moreover, different temperature peaks from 600 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.