My experience with heating concrete is that it bursts into pieces - it breaks into its components before it becomes hot enough to melt, and that the "clinker" that is left when it cools down will unmistakenly not be concrete. Pumice is what comes to mind.
The police museum exhibit was built to spread the lie that not only did the concrete melt, but after it cooled, it turned back into concrete. So in addition to the fairy tale that fires fueled by office materials and human beings became so intense they melted the concrete in their path (without a blast furnace, figure THAT one out), the lava cooled back into concrete without melting the guns that were encased therein!
And now you come trotting out with the tired old thermite argument - surely the NY police museum exhibit that indoctrinated school children and adults alike into believing ridiculous imopossibilities wasn't wrong after all...no...the only thing that can melt both concrete and steel is thermite, so this is proof that not only were the police reports of molten concrete true, so were the FDNY reports of rivers of molten steel. The "hot spot" images, and the fires raging for months after the collapse are all true. The authorities are absolved. Huzza!
Snort.