Aggregate Absorption (Desportion)

The second key issue in providing internal curing via saturated lightweight aggregates is how much of the water contained in the aggregates is readily available to migrate to the hydrating cement paste during the curing process. Assuming that all of the water in the LWA will be available has been shown to be an incorrect assumption. ^{10, 19} A more reasonable approach is to saturate the aggregates in a fashion similar to that which will be employed in the actual proportioning of the concrete, and then measure how much water is "released" at a reduced relative humidity. As low w/c cement paste hydrates under sealed conditions, the internal RH can drop to the range of 85 % to 90 %.¹⁰ As water surrounding a saturated LWA is "attracted to" a hydrating cement particle, LWA-absorbed water replaces the pool of mixing water, maintaining saturated conditions in the hydrating cement paste.¹⁰ For the water in the LWA to effectively participate in the hydration process, it must surely be released from the LWA before these lower RH levels are reached internally.

Starting from the saturated state, the LWAs can be conveniently exposed to known equilibrium relative humidities that are maintained via saturated salt solutions. The water "desorbed" from the LWA in these exposures provides the values that should be used in equation (1) to characterize the absorption of the aggregates (_LWA) at complete saturation (S=1). Exposures to saturated salt solutions of potassium sulfate and potassium nitrate will provide equilibrium relative humidities of about 97 % and 92 % at 25 ºC, respectively. ¹⁰ Thus, use of the former salt solution could be viewed as a conservative approach to internal curing, while the latter should provide a more liberal estimate. Figure 2 provides representative desorption isotherms for an expanded shale lightweight aggregate, measured at a temperature of 20 ºC at NIST. From Figure 2, it can be observed that a minimum of a one week to two week exposure to the saturated salt solution may be necessary to obtain an equilibrium mass for the "saturated" LWA, based on exposing 2 g to 3 g samples of saturated LWA to the salt solutions. This particular LWA loses over 95 % of its absorbed water at relative humidities as high as 93 %, a beneficial characteristic for its application in internal curing. It should be noted that not all candidate LWA materials have this desirable property.¹⁹ An alternative to measuring desorption isotherms for the LWA may be to measure the cumulative absorption over different time intervals, as the rate of absorption may relate to the ease of desorption.²

Figure 2- Desorption isotherm vs. RH and exposure time for Hydrocure® LWA. * RH values of 97 %, 93 %, and 85 % were obtained using saturated salt solutions of potassium sulfate, potassium nitrate, and potassium chloride, respectively. RH of 0 % was obtained with 40 ºC oven drying, followed by placement in a desiccator with a desiccant.