Concrete is used because its fixed composition makes its properties more predictable.
Glass walls are used because they can help support the skeleton of the building.

In today's session, I'd like to start out with a brief overview of developments since the birth of the skyscraper. Builders and engineers have continuously looked for ways to improve building methods and materials, in order to make structures, these new structures, stronger, taller, and lighter. Skyscrapers are, and must be, built to last, so they must be made of materials that are strong, durable, resistant to the sun, wind, rain, frost, and snow, and affordable. Concrete is one of the most common materials, beyond the steel supports, because it's enormously versatile. Its composition can be changed depending on the, on the needs of the building. It can be reinforced making it stiffer, you know, well, what I mean is stronger, by setting steel mesg or bars into the concrete. And additives can make it set to a harden faster or slower depending on the needs of the design. And this will be a key concern that you guys will ultimately spend a lot of time in your careers thinking about, you know, really grappling with this idea.