Praise be to Allah.
The majority of fuqaha' stipulated that the Jamaraat should be stoned with pebbles, which are small stones of whatever type of rock. Concrete is not rock, so it is not acceptable, according to the majority.
Ibn Qudaamah (may Allah have mercy on him) said: It is acceptable to stone the Jamaraat with anything that is called pebbles, which are small stones, whether they are black or white or red, and whether they are marble, flint, granite and so on. This is the view of Maalik and ash-Shaafai. al-Qaadi said: Marble, buraam and kadhdhaan (types of rock) are not acceptable. And we may understand from his words that marble and solid stones are not acceptable either. Abu Haneefah said: It is permissible to use dried clay or mud, or anything that comes from the earth. Something similar was stated by ath-Thawri. It was narrated from Sakeenah bint al-Husayn that she stoned the Jamarah, and a man was handing pebbles to her, and she was saying takbeer with each throw. And she dropped a pebble, so she threw her ring.
We also have a report that the Prophet (blessings and peace of Allah be upon him) stoned the Jamaraat with pebbles, and he told the people to throw pebbles the size of chickpeas, so nothing other than pebbles is to be picked up, and that includes pebbles of all types. It is not permissible to single out any particular type without evidence, or to add anything else to it, because it is a topic in which there is no room for analogy.
End quote from al-Mughni, 3/218
In al-Mawsooah al-Fiqhiyyah (15/277) it says: When stoning the jamaraat, it is stipulated that the pebbles be of stone, according to the majority of fuqaha' (Maalikis, Shaafais and Hanbalis). So it is not permissible to use gold, silver, iron, lead, wood, clay, seeds, dust, pearls, antimony or plaster in their view, because the Prophet (blessings and peace of Allah be upon him) threw pebbles and instructed us to throw pebbles the size of chickpeas, so nothing else should be picked up. The Shaafaais allowed stoning the Jamaraat with all kinds of stones.
The Hanafis were of the view that it is permissible to stone the Jamaraat with all kinds of things that come from the earth, such as stones, mud and clay, and everything with which it is permissible to do tayammum, but it is not permissible to use wood, amber, pearls or gemstones, because they are not from the earth. End quote.
Based on this, there is nothing wrong with breaking stones or large rocks and stoning the Jamaraat with them.
As for throwing concrete or pieces of brick, it is not acceptable according to the majority, although it is acceptable according to the Hanafis. However the view of the majority is more on the safe side.
Shaykh Ibn Uthaymeen (may Allah have mercy on him) was asked: What is the ruling on stoning the Jamaraat with pieces of concrete?
He replied: Some scholars think that the stones that are taken from concrete are not acceptable for stoning the Jamaraat, unless this piece of concrete contains stone. If it contains stone then there is nothing wrong with it.
End quote from Majmoo Fataawa Ibn Uthaymeen, 23/125
To sum up: If on the first day you threw pieces of concrete in which there was no stone, then your stoning was not valid according to the majority, and you have to sacrifice a sheep, the meat of which is to be distributed among the poor of the Haram; you may delegate someone to do that on your behalf.
If you cannot afford the price of a sheep, then you do not have to do anything.
And Allah knows best.
Reproduced from Islam QA