Since global warming and green buildings have been the concern of the scientific society, efforts were made toward achieving more green and sustainable buildings. One of the efforts was to utilize waste materials such as Coal Bottom Ash (CBA) as cement replacement in the concrete mixture to reduce the harmful effects of cement manufacturing procedure and dumping of coal burning products from utilities industry. This paper studies the effect of replacing cement in concrete mixture by CBA. The porosity of four different concrete mixtures including control mixture was studied using different percentages of CBA, which are 0%, 10%, 20%, and 30% as cement replacement. The results have shown improvement in porosity at 10% and 20% replacement. However, 30% replacement was chosen as the optimum percentage since it showed a 6.825% percentage of porosity which is slightly similar to control mixture, therefore, it utilize larger amount of CBA while providing normal concrete quality, which contribute to the sustainable waste management target of Malaysia.