The use of alternative materials, such as fly ash from wood biomass, can reduce the consumption of cement and contribute to minimizing environmental waste. For these reasons, the aim of this study is to investigate how the use of sustainable material, namely wood biomass ash, as a replacement for cement affects the mechanical properties of micro-concrete. For this purpose, mixtures of cement and geopolymer mortars were prepared. The cement mortar mixtures were named C100P0 (100% cement) and C50P50 (50% cement and 50% ash, measured volumetrically) and were prepared with 1350 g of sand, 450 g of cement (for C100P0), or 380 ml of cement and 225 g of water. The geopolymer mortar mixtures were prepared with 1350 g of sand, 380 ml of biomass ash, and a mixture of 10-molar solutions of KOH (or NaOH) and sodium silicate solution (Na₂SiO₃) in a ratio of 1:2.5. The geopolymer mortar mixture with KOH was labeled as GP-KOH, while the one with NaOH was labeled as GP-NaOH. Chemical analysis revealed that the ash contains a higher proportion of CaO compared to cement, indicating potential pozzolanic activity. Although the pozzolanic activity of the ash becomes apparent after 28 days, the pozzolanic activity index is 27%, which is below the required value of 80%, resulting in a decrease in the strength of the mortar. Granulometric analysis showed that cement has a finer structure compared to ash. Tests of tensile and compressive strength indicated that the tensile strength decreases with increasing ash content, while the compressive strength results remained reliable. These results suggest that ash may have limited applicability in mixtures where high strength is required.