A major scientific challenge for carbon neutral, environmental friendly future energy production is the development of renewable energy production to technological readiness. One example are solar thermal power plants. Since their energy generation is intermittent, they demand for a feasible storage solution for which thermochemical reaction systems are considered. The present work subjects the thermochemical reaction system CaO / Ca(OH)2 and its structural-mechanical correlations impacting the powder bulk performance upon thermochemical cycling. On exemplified Ca(OH)2 crystals is shown, that during the first de- and rehydration process, the entire crystal morphology is disintegrated. The underlying mechanism is evaluated by theoretical considerations on the layered structure of Ca(OH)2 and validated by scanning electron microscopy (SEM) on the probed material before and after dehydration as well as after rehydration. The obtained findings are transferred to the technically relevant powdery storage material, where they are capable to explain the phenomenon of agglomeration, which is proven by measurement of secondary particle size distribution over a number of ten thermochemical reaction cycles. From SEM imaging performed on the samples it is found, that agglomerates consist of cohering smaller particles. The inferred insights can help to deduce necessary amendments of reactor design or material modification also for other thermochemical reaction systems.