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Proper curing of concrete is paramount for obtaining optimum performance from a given set of mixture proportions. For high-performance concretes, curing becomes even more important, due to the typically lower water-to-cement (w/c) ratios and the increased propensity for early age cracking due to thermal and self-desiccation stresses [1]. Besides the water which is removed due to exposure to an ambient environment, additional empty capillary porosity is created within the cement paste component of the concrete due to the chemical shrinkage that accompanies the cement hydration reactions, quantified many years ago by Powers [2], but also the subject of much recent research [3,4,5].

The creation of empty capillary pore space has two major effects on the evolving cement paste system. First, the resultant capillary pressures induce shrinkage stresses within the cement paste microstructure, causing a measurable external physical shrinkage [3,5]. The magnitude of these stresses (at relative humidities where some of the capillary pore space is still filled with water) will be influenced by the relative humidity (RH), in the following manner. The Kelvin-Laplace equation can be used to relate the relative humidity to the capillary condensation process:

where is surface tension, Vm is the molar volume of water, R is the universal gas constant (8.314 J/(mol K)), T is absolute temperature, and K is the average curvature of the surface of the condensed water (equivalent to 2/r for a spherical droplet where r is the radius of the droplet/pore). Thus, at a given RH, water will condense in all pores of curvature greater than or equal to K (pore radius less than or equal to r). The induced capillary pressure, capillary, is then given by:

According to Eq 2, the induced capillary pressure is directly proportional to ln(RH). The measured shrinkage should be a function of this capillary pressure and the moisture content (saturation) of the concrete [6,7]. Indeed, field measurements have shown that both moisture content and measured concrete shrinkage are proportional to variations in ambient RH [8]. In the case of hydration under sealed conditions, the autogenous shrinkage stresses will be influenced by the internal RH reduction due to the chemical shrinkage. As first quantified in 1940 [9], this RH reduction can be quite substantial, with RH values as low as 70% measured for low w/c (< 0.3) ratio cement paste and concrete systems [10,11]. According to Eq 2, the induced capillary pressures will be seven times greater in a system with an RH of 70% vs. one with an RH of 95%, which may be one reason why high-performance concretes are often more susceptible to early age cracking.

The second effect of the creation of empty capillary pores will be a change in the hydration kinetics for the cement paste [12,13]. Because cement hydration reactions generally proceed by a dissolution/precipitation mechanism, the empty pore space created due to drying or self-desiccation is no longer available to be filled with hydration products, so that the hydration will slow down and effectively terminate at a lower degree of hydration than that which could be achieved under saturated conditions. The remainder of this paper will focus on this latter effect, using a combination of experimental and computer modelling techniques to quantify the effects of curing conditions on the measured degree of hydration of a series of cement pastes.

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