This thesis reports on a materials science appraisal for recycled construction materials in roadways, that supports engineering decision-making. Inconsistent performance criteria for roadway materials and the variable nature of material source have prompted the need for this research. The aim of the study is to investigate the application of a materials science appraisal to recycled construction materials for use in roadways. The investigation is undertaken through a literature review of roadways, conceptual development of the materials science appraisal methodology, and demonstration of the application of the materials science appraisal to recycled construction materials; and how this supports engineering decision-making. The literature review revealed that there are numerous and proven uses of recycled and secondary materials in roadway applications but there was a lack of necessary integration of materials into categories by material-type, which can better describe behaviour in an engineering situation. Three novel fundamental material types (ceramic, metallic, and polymeric) have been described. The conceptual development of the innovative and novel materials science appraisal, based on material-type, has defined how materials science through a systematic step-by-step procedure can be used to achieve engineering sustainability in roadways and provide support in engineering decision-making. The application of the novel concept of the materials science appraisal to recycled construction materials is shown through the essence of laboratory testing. The results from the materials science appraisal, together with sensitivity analysis, give an informed engineering decision on product choice. The appraisal is novel in that it is proposing a new theory on materials science and developed a paradigm shift in the evaluation of recycled construction materials. The appraisal overcomes the absence of materials science thinking in the field of roadway engineering. The materials science appraisal is of benefit to various stakeholders (client, consultant, supplier and contractor) as it provides a method for addressing material uncertainties. A system now exists for introducing into designs and any contract the principles of the materials science appraisal that will be of great benefit to industry.