**4. Conclusion**

This study focuses on the effect of the use of POC in mortar by substituting river sand by 25%, 50%, 75% and 100% on the sound absorption properties. Two important influences on sound absorption properties namely porosity types and density, and its relation with sound absorption properties have been studied. Compressive strength is also studied to obtain adequate strength. Morphology using SEM was also used to look at the microstructure of POC concrete. The following are the results obtained from this study and the conclusion:

Although POC contains micro-pores however inclusion of 100% POC reduces interconnected porosity and total porosity. The trend vice-versa when POC inclusion is between 50 and 75%. Generally, it is found that interconnected porosity has no relation with sound absorption coefficient (SAC) which contradict with previous research. However, it was found, sound absorption coefficient at 315 Hz has good relation with percentage of POC.

Further, interconnected porosity had no good association with density. Instead percentage of POC inclusion reduced density significantly and has a good linear relationship with sound absorption at 315 Hz. This study also proved that statistically there is no association between average of sound absorption at 250 Hz, 500 Hz, 1000 Hz and 2000 Hz or noise reduction coefficient with density which opposed to the previous research findings.

Finally, POC inclusion reduces compressive strength significantly however all specimens still poses good strength of more than 5 N/mm2 fulfilling the specification standard for masonry units. It is suggested that inclusion of 50% POC produces concrete with good sound absorption at 315 Hz and 1000 Hz and may be used for alleviating the problem of noise from trains and roads. Thus, this mixture can be further suggested to be used for masonry block application for noise control materials.
