*3.1. Compressive Strength*

Figure 6 represents the compressive strength improvement at 7, 14 and 28 days of curing cylindrical specimens made with date palm fronds biochar and compared with a biochar-free control mix. The compressive strength of the control mix was noted to be 28.2, 36.6, and 43.5 MPa at an interval of 7, 14, and 28 days, respectively. The incorporation of biochar showed a linear rise in compressive strength. For instance, adding biochar with a dosage of 0.75%, 1.00%, and 1.50% increased compressive strength by 11%, 12%, and 14%, respectively, while at 0.25% and 0.50%, biochar addition represents similar strength to control mix. A similar result on compressive strength due to biochar addition was also indicated at the 14-day and 28-day ages of concrete. It is observed that

the addition of 0.50%, 0.75%, 1.00% and 1.50% of biochar indicated the strength improvement of 17%, 23%, 24% and 28%, respectively, at 14-day age and 16%, 28%, 26% and 29%, respectively, at 28-day age. However, a 0.25% biochar addition represents no significant change in strength compared to the control mix. The increase in compressive strength due to biochar's addition was mainly associated with the biochar's high surface area, porosity, and water retention capability [3,18]. Dry biochar particles absorbed some of the mixing water during concrete mixing, resulting in a reduced free water–cement ratio in the concrete matrix. The presence of capillary water causes the cementitious matrix to have a high capillary porosity, which has a negative impact on strength development [31]. During the initial hardening of concrete, the water absorption through porous biochar resulted in the increasing density of the cement matrix by lowering the available water in the pores. The water absorbed in the pores of the biochar was eventually provided internally to assist cement hydration via internal curing, which contributed to the cementitious matrix's strength development [4]. Additionally, the smaller particle size of biochar exhibited a filler effect, which helps to minimise voids and gaps between cement particles and aggregates [3]. The findings discovered that the optimum biochar dosage was 0.75% and 1.50%, representing compressive strength improvement at 7, 14, and 28-day age compared to control specimens.

**Figure 6.** Compressive strength of concrete cylinder specimens.
