Due to gross misconduct, I had to let one of my construction executives go. I need to address his co-workers but need some advise on what to say and how to present this to them.

You must be extremely careful about what you tell your other employees about your reasons for firing someone. If you say too much, you risk a defamation lawsuit from the terminated employee. You also risk giving that employee ammunition for a wrongful termination lawsuit.
Because of these risks, prudent construction employers should resist the temptation to explain their actions to their other employees. Simply tell them that you must respect the fired employee's privacy and not discuss the termination.
The drawback to this strategy is that your remaining employees might become nervous that they will be the next to go. This could have a poor effect on morale. Meet with each of your remaining employees privately and reassure them. Talk about their performance and about why you want to keep them in the business.