  Kuiper belt objects are thought to be formed at least a few million years
after the formation of calcium-aluminium-rich inclusions, at a time when the
$^{26}$Al isotope -- the major source of radiogenic heat in the early Solar
System -- had significantly depleted. The internal structure of these objects
is highly dependent on any additional source that can produce extra heat in
addition to that produced by the remaining, long-lasting radioactive isotopes.
In this paper, we explore how serpentinization, the hydration of silicate
minerals, can contribute to the heat budget and to what extent it can modify
the internal structure of large Kuiper belt objects. We find that the extent of
restructuring depends very strongly on the start time of the formation process,
the size of the object, and the starting ice-to-rock ratio. Serpentinization is
able to restructure most of the interior of all objects in the whole size range
(400-1200\,km) and ice-to-rock ratio range investigated if the process starts
early, $\sim$3\,Myr after CAI formation, potentially leading to a predominantly
serpentine core much earlier than previously thought ($\leq$5\,Myr vs. several
tens of million years). While the ratio of serpentinized material gradually
decreases with the increasing formation time, the increasing ice-to-rock ratio,
and the increasing start time of planetesimal formation in the outer solar
system, in the case of the largest objects a significant part of the interior
will be serpentinized even if the formation starts relatively late,
$\sim$5\,Myr after CAI formation. Therefore it is feasible that the interior of
planetesimals may have contained a significant amount of serpentine, and in
some cases, it could have been a dominant constituent, at the time of
satellite-forming impacts.
