You first need to see which puts is used (cement based/kalk based etc) call the ones who built the house or check if there is any house booklet you have received when you biught thw house. you dont want the new mixture be harder than the orginial. thimat would make the surrounding crack. use the mixture (ie serpo 261) to fix the crack then use the same paint to paint it over when it dried (tex silikatfärg). no big deal. i have attached the one that cane with my house booklet.