The development of coral concretes and their upgrading technologies: a critical review

The utilization of locally available raw materials is extremely necessary for the offshore island construction in ocean. Coral can be used as aggregate but is usually light and porous, having rough surface, weak adherence to the attachments and high concentration of sea salts. These characteristics affect the workability, mechanical properties, volume stability and durability of resulting concretes. This review paper discussed the instability of Portland cement hydration products under the ocean environment, the low strength and stiffness of coral aggregates, their large connected porosity and the weak interface microstructure between the cement matrix and coral aggregates. To overcome these problems, such as low grade of strengths, inadequate corrosion resistance and high brittleness, this paper proposed some perspective techniques: modification of coral aggregate, development of new cementitious materials, fiber reinforcement and mix proportion design

A gentle acid-wash and pre-coating treatment of coral aggregate to manufacture high-strength geopolymer concrete

The coral reef fractures could be potentially used as aggregates for marine infrastructure construction, providing an alternative option to the high-cost gravel and sand aggregates. Coral aggregates are rough, porous and of low strength, which have a weak layer of minerals and microorganisms on the surface and a proportion of quartz inside pores. In the present study, using low concentration of acetic acid to pretreat coral aggregate was proposed to enhance the interface binding. When Portland cement was used as binder, the flexural and compressive strengths of cleaned coral aggregate concrete increased by 22.5% and 7.3%, respectively. Furthermore, the coral aggregate geopolymer concrete (CAGC) was prepared, given geopolymer acts as a strong binding and low porosity matrix, which is a promising material for the marine environment. To manufacture high-strength CAGC, a combined treatment method by gentle acid-wash cleaning and pre-coating of coral aggregate was proposed. The results showed that the mass loss rate of coral aggregate increased with the increase of acetic acid concentration and soaking time; the release of CO2 gas was helpful to remove the embedded sediment particles from the coral aggregates. Excessive acetic acid concentration and prolonged soaking time have adverse effects on the mechanical properties of CAGC, which was attributed to the corrosion of the internal skeleton of coral aggregate and the increase of connected pores. By using the gentle acid-wash and pre-coating method, a high-strength CAGC with compressive strength up to 60 MPa at 28-day was successfully manufactured, meeting the mechanical requirements of the most offshore construction requirements