Sorbents for SO2 removal from flue gas were prepared on the basis of fly ash and calcium hydroxide at different slurring times; they were tested in the desulfurization reaction at low temperature and characterized before and after the reaction, by determining the particle size, specific surface area, and pore size distribution in order to study the influence of the slurry time in the microestructure of the sorbent and to establish the main changes in the desulfurization reaction at low temperature. Small macro- and mesopore volumes were the structural properties of the sorbent related to the conversion values, with a maximum in conversion and porosity for the sorbent at a slurring time of 15 h despite the continuous increase in the specific surface area (microporosity). These results confirm that the pore size distribution and volume are the main variables which can be related to the maximum utilization of the sorbent at low temperatures. The increase in the specific surface area of the sulfated sorbents at high relative humidity (80%), due to the microporosity increase, may be explained by taking into account that the partial condensation of water can happen in the pore structure and a gas-liquid-solid reaction, with a fast nucleation step, would produce the observed microporosity increase.