What should a technician use to cap fresh concrete cylinders?

The appropriate method for capping fresh concrete cylinders is to use high-strength gypsum paste. Capping serves to provide a flat, smooth surface for testing the compressive strength of the concrete. High-strength gypsum paste is specifically designed for this purpose because it adheres well to the concrete, creates a uniform surface, and has a known compressive strength that does not interfere with the strength testing of the concrete sample itself.

While neat cement paste and sulfur mortar are alternatives in some contexts, they do not provide the same level of consistency and reliability as high-strength gypsum. Neat cement paste can sometimes result in variability due to its potential for shrinkage and differences in quality, which may affect test results. Sulfur mortar, while also commonly used, has its own set of requirements and considerations, such as needing proper curing and temperature control, which may not be ideal for all situations.

Using any available adhesive is not a good practice, as many adhesives may introduce variables that can skew test results. They may not provide the necessary strength, could react chemically with the concrete, or result in inconsistent surfaces that do not accurately represent the cylinder's strength when tested.