Strength, mineralogical, microstructural and CO2 emission assessment of waste mortars comprising excavated soil, scallop shells and blast furnace slag

The construction sector is actively seeking alternatives to Portland cements, with a view to decarbonising cementitious construction materials, such as mortars and concretes. Using pozzolanic industrial mineral wastes (e.g. fly ash and slags) as supplementary cementitious materials (SCMs) is an effective strategy. However, further investigation is required to identify more SCMs that have longevity in supply, with a view to benefiting the circular economy. Excavated soil and scallop shell wastes are becoming increasingly abundant due to urbanisation and aquaculture sectoral expansion. Managing these wastes through their valorisation as construction materials is more sustainable and therefore highly desirable over disposing them to landfill. This study investigates the use of excavated soil and scallop shell wastes as raw materials for manufacturing low-carbon cementitious mortars. Mechanochemical and thermal techniques were investigated as alternatives to alkali reagents for activating strength-gaining properties of the soil waste. Mixtures comprising mechanochemical+thermally activated soil waste with 10% blast furnace slag produced the highest strengths (2.43 MPa) after 28-days and generated ∼50% less CO2 emissions compared with CEM-I-treated mixtures. Furthermore, the compressive strengths performance recorded for mechanochemical+thermally activated soil waste with 10% scallop shells (1.07 MPa) were only 8% less compared with strengths for equivalent mixtures containing 10% type 1 Portland cement. Based on results from regression analyses, Si/Al was the most effective elemental ratio for predicting 7- and 28-day compressive strength of mortars based on filter cake that had been mechanochemical and/or thermally treated through linear functions. Exponential functions were more appropriate for predicting strength based on other elemental ratios, which considered one or a combination of Ca, Na, K and Fe.