Purity of Forehead and Hand's Dorsal in Tayammum
It is not remote that it is not necessary for body parts involved in tayammum to be pure. However, it is based on caution that forehead and the back of the hands, whenever possible, should be pure.

The Things on Which to Make Tayammum
Doing tayammum on anything that is considered as the earth — like gypsum, limestone, etc. — is valid and it is not remote that doing it over baked gypsum and limestone, and bricks is also correct.

Doubt Regarding Sufficient Time for Wuḍū’/Ghusl
A person, who doubts whether or not there is enough time to make wuḍū’/ghusl, should make tayammum and say their prayer.

To Make Tayammum instead of Ghusl Again
A person, who is the excused from performing ghusl, performs tayammum as a substitute for ghusl of janābah and a wuḍū’ invalidator occurs later, then as long as the excuse remains, it is obligatory caution for him to perform tayammum instead of ghusl for every act that requires being in a state of purity and then to do wuḍū’, as well. If he is excused from wuḍū’, he is to perform another tayammum instead of wuḍū’.

Not Being Able to Make Wuḍū’ nor Tayammum
A person who is neither able to perform wuḍū’ for prayer nor tayammum, he should offer his prayer, as per obligatory caution, within its specific time without them and make it up in qaḍā’ with wuḍū’ or tayammum later on.