Mechanical and Physical Properties of Fluorogypsum-Cement-Fly Ash Blends for Artificial Reef and Coastal Protection Structures

Abstract

The State of Louisiana faces severe soil erosion along its coastlines. Coastal protection structures most commonly are made of limestone and concrete. However, in Louisiana, these materials are not widely available, which makes it significantly expensive to protect the coastlines and marine life. This study proposes to use locally available materials that have lower cost and lower density than limestone and concrete to reduce the economic burden of protecting and enhancing coastlines. This study presents an extensive investigation of strength and durability properties of a low-cost concrete-like composite material made of pH-adjusted fluorogypsum (FG), class C fly ash (FA), and type II Portland cement (PC). A series of compressive strength tests and volumetric expansion measurements were conducted on specimens after 28 days of curing. The experimental results were used to develop response surface models (RSMs). A composition with 62% pH-adjusted FG, 35% FA, and 3% PC, (62-35-3), was selected based on strength and volumetric expansion properties to conduct additional experimental studies. A statistical characterization of the compressive strength and relative volumetric expansion of the FG-based blend after 28 days of curing was performed through goodness-of-fit tests, which indicated that these properties could be described by using lognormal and normal distributions, respectively. The long-term compressive strength of the FG-based blend was investigated under prolonged immersion in seawater, demonstrating that the material strength does not degrade below its 28-day value after one year of immersion and exposure to field environmental actions. The compressive strength under field conditions was compared to the material kept under laboratory conditions. A visual examination of the immersed specimens showed that aquatic organisms covered most of the FG-based samples’ surface, which indicated that these organisms are attracted to the proposed material. A small artificial reef structure made of a FG-based blend was built and placed underwater at a site in Grand Isle, LA and investigated for stability and settlement for nine months. Monitoring of the settlement of the structure over the immersion period indicated that the structure was stable. A preliminary cost analysis was also performed to compare the cost of artificial reefs constructed with the FG-based blend, crushed limestone, or crushed recycled concrete. Additionally, based on experimental results obtained from the first part of the study, two compositions of controlled pH-adjusted FG, FA, and PC, i.e., (62-35-3) and (60-34-6), were studied for mechanical and physical properties. The use of controlled pH-adjusted FG in FG-based blends significantly improved