A new study in Scientific Reports looks at a question that hits close to home for masonry crews everywhere: How do you cut material impacts without giving up the performance you need on the wall?
The research team tested a wall-coating mortar that replaces natural quartz sand with two waste streams, crushed sururu seashells (Mytella falcata) and expanded polystyrene (EPS). The materials came from an open dumpsite in the Ilha de Deus fishing community in Recife, Pernambuco, Brazil. Mortars were proportioned at a 1:3 cement-to-aggregate ratio by absolute volume with a 0.5 water-cement ratio. Three mixes were evaluated: a control mix, a 40% replacement mix (20% shells and 20% EPS by volume), and a full replacement mix (50% shells and 50% EPS by volume).
On the hardened side, density dropped from 1,752.6 kg/m3 in the control mix to 1,421.6 kg/m3 at 40% replacement, and 904.6 kg/m3 at full replacement. At 14 days, compressive strength fell from 12.6 MPa to 6.4 MPa at 40%, and 1.85 MPa at full replacement. Water absorption by immersion moved from 12.90% (control) to 10.88% (40%), then up to 21.67% (full), even though the total absorbed water mass fell from 44.4 g to 38.6 g due to the much lighter specimen mass.
For contractors, the workability results matter as much as the lab numbers. The study reports a target consistency index of 260 mm plus or minus 5 mm for coating mortar. The 40% mix hit 267 mm, but the full replacement mix dropped to 212 mm, which signals a tougher spread and finish on real jobs. The researchers also reported higher variability at 40% replacement, tying it to segregation risk when aggregates with very different densities share the same batch.
The takeaway is straightforward: the 40% blend landed closer to practical coating performance, while the full replacement mix pushed into lightweight, nonstructural territory. Before field use, the study calls for long-term durability evaluation and more microstructural testing, plus exploring admixtures and particle sizing to tighten consistency.
Read the full, original article from Nature here.