What is the term used to describe a portion of concrete containing only cement and fine aggregates?

The term that accurately describes a portion of concrete containing only cement and fine aggregates is "Cement paste." Cement paste refers specifically to the mixture of cement and water, which forms the bonding material that holds aggregates together in concrete. Fine aggregates, such as sand, are often incorporated with the cement and water to create the paste that serves as a matrix for the total concrete composition.

While "Matrix" might seem like a fitting term, it generally refers to the whole structure made up of the cement paste and aggregates working together in a composite material. Therefore, while the matrix includes the cement paste, it is not limited to just that mixture. The other terms do not correctly capture the specific components of cement and fine aggregates alone.