Internal curing can be used to counteract the effects of self-desiccation in low water-cement ratio mixtures. Internal curing uses prewetted lightweight aggregate (LWA) as a reservoir that can release water as it is needed to keep the hydrating cement paste structure saturated. While many developments have been made in advancing the field of internal curing, this work examines the composition of the early-age pore solution. The effect of the pore solution composition on the relative humidity is also discussed. The paper performs desorption measurements of the lightweight aggregate to better understand how fluid is released from the lightweight aggregate. An approach is suggested to use solutions rather than pure distilled water to improve the relevance of the measurement of desorption at high relative humidities.