Optimization, microstructure, and durability of sawdust ash concrete modified with Conplast SP430 superplasticizer

The use of sawdust ash (SDA) as a supplementary cementitious material offers an opportunity to valorize biomass waste and reduce dependence on Portland cement, but its incorporation can affect workability, strength development and durability. This study investigates the combined effects of SDA replacement and Conplast SP430 dosage on the fresh, hardened and durability performance of concrete. SDA replacement levels of 0–40% and Conplast dosages of 0–7.6 L/m 3 were investigated, followed by microstructural and elemental characterization of the resulting concrete using BSE-SEM and EDS. The optimum formulation, identified within the investigated mixture range, was 10% SDA with 5.7 L/m 3 Conplast SP430, which achieved a 28-day compressive strength of 24.7 MPa, representing a 51.5% increase relative to the control. Continued strength development was observed at later curing ages, with the optimum mixture reaching 25.8 MPa at 90 days. SEM observations showed comparatively favourable matrix morphology at 10% SDA, while higher SDA replacement levels were associated with increasing morphological discontinuity and interfacial defects. EDS analysis showed a progressive increase in the Ca/Si ratio from 0.345 in the control to 0.586 at 40% SDA, with the 10% SDA mixture exhibiting an intermediate value of 0.422. The increase in Ca/Si was negatively associated with compressive strength within the investigated mixtures. Durability assessment of the optimum mixture after 56 days showed 76.7% strength retention in Na₂SO₄, whereas MgSO₄ and the acidic environments produced substantially greater deterioration. Overall, the results indicate that 10% SDA with 5.7 L/m 3 Conplast SP430 provided the most favourable balance of mechanical, microstructural and durability performance within the experimental range investigated. The findings provide experimentally based guidance for SDA–Conplast concrete while recognizing that the observed microchemical relationships are specific to the materials and exposure conditions examined.