Evaluating the Mechanical Properties of M20 Concrete Hollow Blocks with Coconut Husk Ash (CHA) and Crushed Coconut Shell (CCS) as Cement and Fine Aggregates’ Replacement

Concrete is considered the second most widely used material in building construction. Its high demand quickly depletes natural resources, causing ecological imbalance. This study explores the potential of utilizing CHA and CCS as replacements for cement and fine aggregates in load bearing CHB. It characterizes the physical properties of CCS, chemical properties of CHA, and identifies the effective proportions of CHA-CCS for mechanical and thermal performance following ASTM standards. Five (5) different mix designs were developed using an M20 mix ratio, with CHA and CCS replacement levels of 5% and 10%, respectively. A total of 150 CHB samples were manufactured and tested for compressive, water absorption, and thermal conductivity after 7, 14, and 28 days of curing. The results demonstrate that CHA is a good pozzolanic material with 73.97% SiO2 + Al2O3 + Fe2O3 by mass. Mix C5-A0 shows the highest compressive strength by up to 8.94% and Mix C0-A5, with 5% CCS, substantially decreases strength by 15.53%. The results support the inverse relationship between water absorption and compressive strength, as well as the correlation between density and thermal conductivity. The study confirms that a 10% replacement of CHA and CCS results in a feasible combination of strength and thermal performance. However, exceeding this replacement level is not recommended as it decreases the strength of the block.