The group calls itself Comal Residents for Rural Heritage.
"Once somebody gets a permit and gets started, you can't stop them," Kermit Fox told the crowd. "It just goes on and on. So we need to stop them before they get the permit."
More than 800 people have signed a petition to the Texas Commission for Environmental Quality, opposing granting a permit for the plant.
Aggregate Industries, a subsidiary of the Swiss firm Holcim, has applied for a TCEQ air quality permit to operate the plant adjacent to the existing Cemex Plant.
But residents said the quarry could threaten the nearby Comal Springs, and harm their neighborhoods with blasting, dust and traffic.
Annalisa Peace, executive director of the Greater Edwards Aquifer Alliance, told the crowd the site is in the Edwards Aquifer recharge zone and is likely habitat for endangered golden-cheeked warblers.
Frederic Warner, a spokesman for Aggregate Industries, also spoke to the crowd. He was challenged with questions as he spoke.
"Can your company guarantee that it would do no damage to the aquifer, that it would do no damage to air quality?" asked Daniel Boone, a candidate for Texas House District 73.
"Our goal is there will be the minimum amount of impact," Warner began, before being drowned out by derisive laughter.
Peace urged the crowd to pressure elected officials, the U.S. Fish and Wildlife Service and the TCEQ to support their cause.