Would sea shells and beach sand make a good combination for concrete mixes?
It is possible to make concrete with beach sand and shells as aggregate, provided the materials are clean and are properly graded, though it may not prove economical. Because shells are usually flat and sharpedged they make concrete with poorer workability than more commonly used gravel or crushed stone. This lack of workability increases the amount of mixing water needed for placeability. If the quality of the concrete mixture is to be maintained much more cement will have to be used. This will increase the cost of the mixture. Beach sand is often of poor gradation; this fact will increase the water requirement further and call for still more cement, again increasing the cost. There is also the likelihood that beach aggregates will have high salt content, often higher than in seawater. Significant amounts of salts in concrete may lower the strength and decrease the quality of the concrete. If concrete is reinforced and if the salt content is sufficiently high, the reinforcement may corrode. For these reasons it is likely that local ready mixed concrete suppliers can deliver freshly mixed concrete to the jobsite at lower cost than a mix of comparable quality can be made from beach materials. A local testing laboratory could evaluate the beach materials in trial mixtures and determine the proportions needed for the strength required.