INTRODUCTION
1.1 Background of the study
Concrete is regarded as the primary and widely used construction ingredient around the world in which cement is the key material. However, large scale cement production contributes greenhouse gases both directly through the production of CO2 during manufacturing and also through the consumption of energy (combustion of fossil fuels). Moved by the economic and ecological concerns of cement, researchers have focused on finding a substitution of cement over the last several years. In order to address both the concerns simultaneously many attempts have been made in the past to use materials available as by product or waste. This is due to the fact that the use of by product not only eliminates the additional production cost, but also results in safety to the environment. Hence, the development and use of blended cement is growing rapidly in the construction industry mainly due to considerations of cost saving, energy saving, environmental protection and conservation of resources.
A number of investigations have been carried out with Palm oil fuel ash (POFA), an agro-waste ash, as potential replacement of cement in concrete. Sata et al. (2004) found compressive strength of 81.3, 85.9, and 79.8 MPa at the age of 28 days by using improved POFA with a reduced particle size of about 10 microns in concrete as replacement of 10%, 20% and 30% of cement respectively. They also reported highest strength at 20% replacement level. Tangchirapat [2009] observed the compressive strengths of ground POFA concrete in the range of 59.5–64.3 MPa at 28 days of water curing and with 20% replacement it was as high as 70 MPa at the end of 90 days of water curing. However, the drying shrinkage and water permeability were noted to be lower than that of control concrete with improved sulphate resistance. Past researchers also depict that both ground and un-ground POFA increase the water demand and thus decrease the workability of concrete. However, ground POFA has shown a good potential for improving the hardened properties and durability of concrete due to its satisfactory micro-filling ability and pozzolanic activity. Palm Oil Fuel Ash (POFA) is known as the by-product form from the incinerationof the palm oil fibers, shells, and empty fruit bunches in the biomass thermal power plant to generate energy. However, it was found that 5 % of the residue was then produced as the result of combustion and the wastes are then managed by disposing as landfill materials which lead to environmental hazards eventually. As stated by Aprianti et al. (2015), POFA is tagged as the environmental disruption pollutant which ends up in the atmosphere without being utilized in 20th and 21stcentury, if compared to other types of palm oil by-products.
Over the past several decades, attempt has been made to use several waste and by product material produced by the industries as potential partial replacement of cement in concrete. Investigations have been carried out through replacing part cement with industrial and other wastes such as Silica fume, ground granulated blast furnace slag, bagasse ash, rice husk ash, palm oil fuel ash, Paper mill ash, Wheat straw ash, Wallostonite, Metakaolin and many more (Karim et al., 2014). The use of these materials in concrete has significant benefits from environmental and economic stand point in comparison to traditional cement. In order to promote the utilization of POFA, many researchers established the researches regarding the POFA as the Supplementary Cementitious (SCM) in either concrete or mortar. The properties and performances of the finished products were examined and the researchers commented that the utilization of the POFA as a supplementary materials of cement is suitable. This is because the ash can increase the engineering and durability properties of either concrete or mortar. Karim et al., (2011) discovered that concrete produced using a particular level of POFA replacement achieved same or more strength as compared to OPC concrete. No significant strength reduction of concrete was observed up to about 30% replacement of POFA. Safiuddin et al., (2012) observed that the use of POFA is limited to a partial replacement, ranging from 0-30% by weight of the total cementitious material in the production of concrete. Indeed, the partial replacement has a beneficial effect on the general properties of concrete as well as cost. Sata et al., (2010) investigated that the strength development of POFA concretes with w/c ratios of 0.50, 0.55, and 0.60 tended to be in the same direction. At early ages, concretes containing POFA as a cement replacement of 10, 20, and 30% had lower strength development than control concretes while at later age 28 days, the replacement at rates of 10 and 20% yielded higher strength development. Mohammed Hussin and Awal., (2009) studied concrete replaced with POFA with a water to binder ratio of 0.45, were seen to develop strength exceeding the design strength of almost 60MPa at 28-day. Hussin et al., (2009) discovered that inclusion of 20% POFA would produce concrete having highest strength as compared to any other replacement level. Ahmad et al., (2008) studied that one of the potential recycles material from palm oil industry is palm oil fuel ash which contains siliceous compositions and reacted as pozzolans to produce a stronger and denser concrete. Pozzolanic material has little or no cementing properties. However, when it has a fine particle size, in the presence of moisture it can react with calcium hydroxide at ordinary temperatures to provide the cementing property. Ahmad et al., (2008) reported that the chemical composition of POFA contains a large amount of silica and has high potential to be used as a cement replacement. Hussin et al., (2009) have found that POFA can be used in the construction industry, specifically as a supplementary cementitious material in concrete. Hussin et al., (2009) studied the compressive strength of concrete containing POFA. The results revealed that it was possible to replace at a level of 40% POFA without affecting compressive strength. The maximum compressive strength gain occurred at a replacement level of 30% by weight of binder. Karim et al., (2011) investigated that replacing 10–50% ash by weight of cementitious material in blended cement had no significant effect on segregation, shrinkage, water absorption, density, or soundness of concrete.

1.2 Scope
Ordinary Portland Cement (OPC) concrete i.e. concrete with 100% OPC and ordinary gravel as control, and concrete with 25% – 35% POFA replacement and granite-gravel of 0% – 100% replacement having uniform water/binder (w/b) ratios of 0.5 and mixes ratio of 1:2:4 were used. Curing ages of 7 days, 28 days, 56 days and 90 days were considered.

1.4 Justification
Utilizing Palm Oil Fuel Ash (POFA) in concrete mix is a major way of turning waste to wealth. Gravel as an aggregate is cheaper than granite. Thus, obtaining an optimum combination of these materials in achieving a maximum compressive strength in concrete will go a long way in helping the construction industry.

1.5 Statement of Problem
Large scale cement production contributes to greenhouse gases both directly through the production of CO2 during manufacturing and also through the consumption of energy. Therefore, this study examines the suitability of Palm Oil Fuel Ash (POFA) replacement with cement as Supplementary Cementing Materials. One of the potential recycle materials from palm oil industry is palm oil fuel ash which contains siliceous compositions and reacted as pozzolans to produce a stronger and denser concrete

1.6 Aim
To determine the mechanical properties of Palm Oil Fuel Ash (POFA). To address the aim, the following objectives were identified:
1.7 Objectives
1. To determine concrete compressive strength of granite-gravel concrete at varying replacement of Palm Oil Fuel Ash (POFA) at different curing ages.
2. To establish an optimum replacement of Palm Oil Fuel Ash (POFA) in granite-gravel blended.