Purpose – The purpose of this paper is to examine the use of a waste marine sea shell product incorporated into a concrete mix as an aggregate replacement. Utilising shells reduces the storage of shell waste, also reducing the need for quarried aggregate and has potential benefits of adding a different material to a design mix concrete mix design for improved performance. Design/methodology/approach – The test methods used to evaluate the concrete were, British Standard tests for compressive strength (BS EN 12390-3:2002) and porosity (BS EN 12390-8:2009). A paired comparison test was carried out examining two different partial replacement shell aggregate mixes against a plain concrete control sample. Findings – The results showed a reduction in compressive strength when 50 per cent of sea shells were used as an aggregate replacement, for both sand and gravel, compared to the control sample. Crushed and graded sea shells used in concrete displayed a lower porosity/permeability than plain concrete. Originality/value – Whilst there is existing work relating to the compressive strength of concrete using sea shells, the porosity of concrete using sea shells has not been widely addressed and the paper investigates this aspect of sustainable concrete research.