ingenieur 2020 Vol 83 | Page 62

Table 4 . Physical properties of concrete mixes

Figure 4 . Effect of coal waste on compressive strength of concrete

Fresh properties of concrete Table 4 shows the fresh properties of concrete obtained from different mix proportions . From the results obtained , coal waste concrete mix has a 90mm slump whereas control concrete mix has a 140mm slump . The result of slump for the fully bottom ash replacement concrete mix was much lower than the slump result for the control concrete mix . This was due to the high water absorption properties of bottom ash , so the workability of the fully bottom ash replacement concrete mix is decreased . Similar findings were reported by previous researchers using coal bottom ash as a fine aggregate replacement [ 19 , 20 ]. The fresh density for coal concrete and control concrete are 1950kg / m ³ and 2385kg / m ³ , respectively . The density of coal concrete is lower than the control concrete which consists of granite and sand as aggregates . The reason for this observation is the volume of fine aggregates is less than that of the coarse aggregates and that of the density of sand but granite is higher than that of the bottom ash aggregates . Consequently , air bubbles were driven out when the cement paste filled the pores after being absorbed on the surface of the bottom ash [ 21 ].

Compressive Strength The compressive strengths of the 100mm x 100mm x 100mm cubes at seven , 14 and 28 days after being placed in water are shown in Figure 4 . It shows that coal concrete and control concrete display similar patterns where the compressive strength increases with the increase in the curing period . At a period of 28 days the compressive strength for coal concrete was 37.28MPa while that of the control concrete was 36.28MPa . The coal concrete shows comparable strength with the control concrete at 28 days of curing . This is probably due to the high amount of filler in bottom ash aggregates that filled the spaces between the pores , thus increasing the bonding strength between the aggregates and the cement . This is due to the fineness of coal fly ash as a reactive

60 VOL 83 JULY-SEPTEMBER 2020