Calcium-Based Wastes as Additives or Binder Substitutes in Mortars: Experimental Research with Oyster Shells or Lime Kiln Dust, Quicklime and a Modified Vinegar Solution

Lime kiln dust (LKD), a by-product of the paper industry, generates about 100 tonnes of waste per 400,000 tonnes of kraft paper produced, while global aquaculture yields more than 16 million tonnes of oysters annually, 65–90% of which is made up of shells. This study explores their valorisation in the production of more eco-friendly mortars by partially replacing hydrated lime with LKD and oyster shell powder (OSP). In addition, a vinegar solution (VS), prepared by reacting oyster shells with white vinegar (~5% acetic acid), was used as an alternative mixing liquid instead of water. The LKD and OSP were tested at different substitution levels, showing promising mechanical performance, supporting their use as sustainable alternatives in mortar production. Replacement levels of 25%, 50% and 90% achieved compressive strengths ≥ 0.4 MPa at 28 days. At 28 days, the reference lime mortar prepared with water reached 0.83 MPa, while the use of the vinegar solution increased the compressive strength to 1.86 MPa, representing an improvement of approximately 124%. Regarding binder replacement by wastes, the most efficient mechanical performance was obtained for mixtures with 50% LKD substitution, reaching 2.04 MPa at 28 days and 3.11 MPa at 60 days, increasing by 10% and 43%, respectively, while mixtures incorporating oyster shell powder showed more stable mechanical behaviour across substitution levels. Using a hot-mixing process with quicklime in the presence of the vinegar-based solution and sand may account for the higher strengths, due to the heat/steam generated during lime hydration prior to moulding and verified by microscopy. In addition, VS-containing mixes showed higher aragonite contents and detectable phosphorus-bearing compounds, which may further contribute to matrix densification and strengthening. Overall, the results indicate that the combined use of uncalcined calcium-based wastes and a vinegar-based solution can contribute to the development of calcium-based mortars with good mechanical performance, supporting circular economy strategies and the reduction in calcined-binder use in construction materials.