Structural lightweight mortar incorporating heat-treated Algerian diatomite and expanded clay sand: mechanical performance and acid resistance

The technical feasibility of partially replacing ordinary Portland cement (OPC) with heat-treated diatomaceous earth (HtDe) in structural lightweight mortar (SLWM) was investigated. HtDe, sourced from diatomite deposits in Mascara province of Algeria and calcined at 850°C, was incorporated as a supplementary cementitious material at substitution levels of 0%, 8%, 10%, 12%, 14% and 16% by mass of OPC. In all formulations, expanded clay sand was used as the sole fine aggregate, resulting in six SLWM mix designs in which the only variable was the HtDe replacement level. The fresh-state properties, hardened unit weight, ultrasonic pulse velocity, compressive strength (Rc) and flexural strength (Rf) at 7, 28 and 90 days, water absorption and acid resistance (mass loss under hydrochloric acid (pH 3) and sulfuric acid (pH 3.5) over 90 days) were systematically evaluated. The results showed that an increase in HtDe content progressively reduced workability and unit weight, while improving mechanical performance up to the 14% substitution level. The SLWM with 14% HtDe achieved the highest Rc and Rf values and exhibited the lowest mass loss under acid exposure of all the tested formulations, effectively meeting the structural lightweight criteria defined in ACI 213R-14. Furthermore, water absorption increased with an increase in HtDe content, a trend attributed to the inherent open porosity of the diatomite skeleton, which did not preclude the concurrent mechanical improvements driven by the pozzolanic densification of the cementitious matrix.