Assessment of ground dune sand (GDS) in the production of air lime mortars for historic buildings restoration

Traditional historic buildings are primarily constructed with lime mortar. Due to climate-induced wear, these materials, used for joints and coating, require repair with compatible mortar, preferably formulated with locally sourced, low-carbon resources. Given that 75 % of Algeria's territory consists of desert areas abundant in dune sand, this resource presents a widely available alternative for restoration work. This paper explores the design of a new restoration mortar formulation based on air lime, dune sand and ground dune sand (GDS). A grinding process was employed enhance the reactivity of dune sand, aiming to evaluate its potential as pozzolanic material, with the ball-to-sand masse ratio (b/s) varying from 7 till 14. Thirteen blends were prepared, incorporating GDS as a partial lime replacement at rates of 20 %, 40 % and 60 % alongside four different b/s ratios. Both micro-and macro-level analysis were conducted on these blends, including characterization of the GDS and the mortars. The results in the fresh state show that GDS acts as a plasticizer in air lime mortars due to the fine particle effect, which enhances workability and reduces internal friction, leading to better compaction. FTIR, XRD and TGA analyses did not conclusively demonstrate a pozzolanic reaction between GDS and air lime. This may be attributed to the presence of limestone aggregates, carbonation, and the partial activation of GDS, which could have masked the reaction. However, when 60 % of the lime is replaced by GDS, a reduction in carbonation was observed, making the pozzolanic reaction and C-S-H formation more apparent. Still, the structure of C-S-H formed in this case is less organized than typically seen with other active pozzolanic materials. This structural alteration accounts for the reduced mechanical strength of the mortar at this substitution rate. Nevertheless, the overall filling effect of fine GDS particles, combined with carbonation, improves the mechanical properties and durability of lime mortars by reducing porosity and increasing density. This effect is particularly pronounced with a b/s ratio of 10 and a GDS content between 20 % and 40 %. These results confirm the role of GDS as both a filler and as a partially reactive material, ensuring mortar compatibility with heritage restoration requirements.