Effect of applied hydrostatic stress on the hydration of Portland cement and C3S

Synopsis The hydration behavior of Portland cement and C 3 S with and without stress applied were studied at water to solid (w/s) ratios of 0.35 and 0.50. A greater degree of hydration and denser microstructure were obtained with hydration under stress at an early stage of the hydration process, normally within 48 hours. This observation was confirmed by the determination and comparison of CH content, TGA results, surface area and pore-size distribution of samples hydrated with and without applied stress. Environmental SEM examination provided the evidence that micro-cracking occurred only in samples hydrated under applied stress. It is suggested that this facilitated the migration of water into the protective layers (around cement grains) that form in the early stages of hydration accelerating the reaction with the unreacted cores of the particles. The hydration and hydration kinetics of Portland cement and C 3 S have been extensively studied [1-4]. Numerous techniques have been used to determine the fraction of anhydrous materials including spectroscopic and thermoanalytical methods [5]. The hydration reactions reported are however generally monitored in a stress free state, i.e. no external and continuously applied stress is maintained during the process. In practice, hydration often takes place in concrete that is in a stressed state. It is, therefore, important to have an accurate estimation of the influence of stress on reaction kinetics and the development of time-dependent engineering properties. For example, the prediction of early age creep of concrete requires an estimate of the time dependence of the degree of hydration [6]. It has recently been suggested, for cement paste and C 3 S paste specimens, that the extent of reaction of the cement and C 3 S as a function of time may be significantly different if hydration occurs under a uniaxially applied stress [7]. The objective of this work was to confirm (in a systematic manner) that applied stress affects the hydration kinetics (especially at early ages) of Portland cement and C 3 S. The work in reference [7] was extended to a hydrostatic stress condition. Experiments were designed to permit hydration of paste specimens (kept continuously saturated) subjected to a hydrostatic stress. The extent of hydration was monitored using thermal analytical methods. These were supported by pore-size distribution and surface area determinations. ESEM examination was conducted on wet specimens to ensure that the observations were compatible with the objectives of the study. The fineness of the …