Can potassium carbonate be used to calibrate hygrometers?
Note that all salts, used in humidity calibration, dissociate to their respective hydrated cations and anions when dissolved.
From the solution crystallizes the sesquihydrate $\ce{K2CO3·​\frac 32 H2O}$ (&quot;potash hydrate&quot;). Heating this solid above 200 °C gives the anhydrous salt (Wikipedia).
It seems you have anhydrous carbonate, that hydrates itself exothermically. You can still use it, but you need to prepare rather the saturated solution with excess of carbonate, that would integrate the crystal water into itself.
Or, let the prepared saturated solution partially evaporate and crystalize.

What is important is maintained equilibrium.
$\text{Air 43% humidity }\ce{ &lt;=&gt; K2CO3} \text{ saturated solution } \ce{&lt;=&gt; K2CO3 . 3/2 H2O}$
Anhydrous $\ce{K2CO3}$, wetted by water, rather just absorbs water into its crystall lattice. If you add too little water, no solution would left, similarly what would happen to the plaster of Paris.
You need the carbonate in hydrate form, together with its saturated solution. Take some water, add there carbonate to dissolve as much of as it can, and then some. Let is settle.
If you had the hydrate, you could just moisten it. But you do not have it.
There is the third option - moistening  it enough so some liquid phase always remains. For $\pu{100 g }\ce{K2CO3}$, you need about $\ce{20 mL}$ of water just for the crystal water, so it becomes the part of the solid.
The carbonate solubility is $\pu{110.3 g/100 mL}$, so I would carefully mix some $$\pu{100 g}\  \ce{K2CO3} + \pu{50 mL}\ \ce{H2O}$$
or recalculated to  more suitable amounts.