Effects of partial replacement of quartz sand by expanded vermiculite on the properties of lightweight screed mortars

Mortars are widely used in the construction industry and account for a substantial share of Portland cement consumption. Screeds are mortar layers used to level horizontal surfaces, providing wear resistance as well as thermal and acoustic comfort, typically without a structural function. Lightweight mortars offer an alternative for reducing permanent loads while improving building performance. This study evaluated the partial replacement of quartz sand with expanded vermiculite (EV) at contents of 20, 30, 40, and 50% by volume in screed mortars. The mixtures were characterized through tests in the fresh and hardened states, including bulk density, water retention, incorporated air content, water absorption by immersion, voids index, and compressive and flexural strengths, in addition to mineralogical analysis by X-ray diffraction (XRD). The results showed that increasing EV content reduced the fresh bulk density from 1654 kg/m³ (reference mortar) to 1531 kg/m³ (50% EV), while the hardened-state density decreased from 1612 to 1445 kg/m³. Water absorption increased from 18.36% to 23.45%, and the void index rose from 29.47% to 35.71% with increasing EV incorporation. The highest mechanical performance was achieved at 20% replacement, with compressive and flexural strengths increasing from 2.51 MPa to 3.22 MPa and from 0.92 MPa to 1.06 MPa, respectively. Higher replacement levels resulted in progressive strength reductions due to the increased porosity of the mortars. XRD analysis confirmed the presence of typical cement hydration products, such as portlandite, ettringite, and calcite, while revealing a progressive reduction in quartz peak intensity and an increase in phases associated with EV. The results indicate that expanded vermiculite is technically viable as a lightweight aggregate in screed mortars, particularly at replacement levels around 20%, where density reduction can be achieved without compromising mechanical performance.