Radical 103 疋 stands for clothing or bales of cloth. In the Hanzi-Trainer, it is used synonymously with the correct 正, because it seems helpful for the explanations and the interpretation clothing/cloth bale would be in conflict with other radicals.
Tip: Go to the radicals that are contained in this character and learn all the characters with this radical systematically!
Vocabulary
30683664
混凝土
hùn níng tǔ
concrete
凝固
níng gù
to freeze; to solidify; to congeal; fig. with rapt attention
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!