Properties of Concrete Incorporating Recycled Post-Consumer Environmental Wastes

Crushed dolomite aggregate with nominal maximum aggregate size of 20 mm and well-graded local sand were used as coarse and fine aggregates (FAs), respectively. The aggregate gradation is performed according to ASTM C136 and illustrated in Fig. 1. The relative specific gravity and compacted density at saturated surface dry condition of the coarse aggregate (CA) were 2.64 and 1.7, respectively, whereas FA had a relative specific gravity of 2.73, and compacted density of 1.55, and a fineness modulus of 3.07. Type I Portland cement having a specific gravity of 3.15, initial and final setting times are 3.14, and 143 min, respectively and conforming to ASTM C150 requirements. Different binders incorporating ASTM Type I cement and SCMs including class C fly ash, and silica fume (SF) were also used in all mixtures other than the controls. All SCMs, including fly ash and silica fume, conform to ASTM C618 and ASTM C1240 standards, respectively. The fly ash and SF specific gravity values of 2.6, and 2.2, respectively. Table 2 shows all proportions of all concrete mixtures. The water-to-cement ratio of 0.50 was kept constant for all mixtures. The coarse and FA weights were also kept constant, and taken as 1070 and 710 kg/m³ respectively.

The corncob was produced by grinding dried corncobs to about 6.00 mm diameter to enhance adequate combustion and to reduce the carbon content which affects the pozzolanic properties. The ground corncobs were burnt in open air using a local furnace. A measured quantity of corncob was collected, sprinkled with a kerosene fuel and set on fire. The temperature of the corncob was measured during burning as it turned from cob to burn and to powder to with a measured temperature of 600 °C in about 10 h. Olive’s seeds were also prepared by washing at first then drying by placing the seeds in the oven for 24 h at temperature of 40 °C. The second stage was grinding the dried seeds on three stages till it reaches the ash sizes. The compacted bulk and loose densities were measured to be 735 and 705 kg/m³, respectively. The specific gravity was measured and taken as 1.25.

Testing was carried out in two different phases. The first phase consisted of evaluating the properties of all fresh concrete mixtures to ensure that the concrete is workable, they were assessed using the standard slump test (ASTM C143). In the second phase, the compressive and split tensile strengths of all concrete mixtures were measured at various ages (7 and 28 days) to investigate the effect of using SCMs on the hardened concrete properties. The compressive strength was conducted on (150 × 150 × 150) mm cubes (Egyptian code standards) and cylindrical specimens of 200 mm diameter and 400 mm height were used to perform the split tension test.

The slump test was performed for all mixtures and the results are summarized in Table 3. It is clear that mixture (M11), which has 30 % of cement, replaced by olive’s seed ash experienced the largest slump value. Figure 2a shows the effect of different SCMs on the slump values for all mixtures. The least slump value is found to be for mixture (M16), which has 30 % of cement, was replaced by CCA.

It is also noticed that replacing the cement by olive’s seed ash and CCA has a significant effect on concrete workability. It is shown that the olive’s seed ash increased the slump by more than 200 % compared to the control mixture (M1). The effect of replacing cement by fly ash is much less than other cementitious materials. Figure 2b shows the normalized slump of all mixtures compared to the control case, and it shows that as the SF content increases, the slum decreases. The results showed that the concrete slump increased by an average of 38 % as the CCA content increased from 5 to 30 %, indicating that concrete becomes more workable, meaning that more water is required to make the mixtures more workable. Figure 3 shows the slump test performed for some mixtures.

The preparation of concrete cubes, curing and the compressive test of one of the samples are shown in Fig. 4. Figure 5 summarizes the effect of the inclusion of silica fume, olive’s seed ash, CCA, and fly ash in all mixtures. The addition of SF with 5, 10, 15, 20 and 30 % in mixtures M2 through M6 has significant effect on the 7 and 28 days compressive strength. The 7 days compressive strength of the mixture that has 10 % SF (M3) was 97.9 % of the control mixture strength (M1), while the 28 days strength was 91.3 % of the control mixture strength respectively. The effect of olive’s seed ash on the compressive strength is shown in Fig. 5b. The olive’s seed ash replaced the cement content by 5, 10, 15, 20 and 30 %, respectively. The compressive strength of the mixture that has 5 % olive’s seeds (M7) decreased by 39.7 and 27.9 % at 7 and 28 days compared to the control mixtures, respectively. It is concluded that the chemical composition of olive’s seed ash must be investigated to address its effect on the hydration process. The CCA effect on the compressive strength is investigated by replacing the cement content by 5, 10, 15, 20, and 30 %. The 30 % replacing decreased significantly the compressive strength compared to the control mixture (M1). The CCA will be considered an emerging waste-by-product material in the near future, but it is recommended to limit of replacement to be 15 % as a result of the present study. The replacement of cement by 5 and 15 % of CCA (M12 and M14) decreased the 28 days compressive strength by 35.9 and 68.5 % compared to the control mixture as shown in Fig. 5c. Figure 5d shows the effect of fly ash replacement on the compressive strength at 7 and 28 days. The replacement ratios were 5, 10, 15, 20 and 30 % from the cement weight. The compressive strength increased by 13 % due to a 5 % of cement replacement, however increasing the fly ash content to 30 % affect adversely the compressive strength as shown in Fig. 5d.

The effect of percentage replacement and mixture proportions on the compressive strength of concrete at the curing ages of 7 and 28 days are presented in Fig. 6a, b respectively. The results at 7 days for 5, 10, 15, 20, and 30 % mixture proportions are presented in Fig. 6a. The compressive strength is found to be decreasing as the percentage of cement replacement is increasing and that for the results at 7 and 28 days as well. The recorded observations indicate that blended cement concrete especially with olive’s seed ash and CCA gain strength slowly at early curing age. The present study results are in agreement with the findings from previous studies, which stated that pozzolanic reaction at room temperature is slow, thus a longer curing period is needed to observe its positive effects.

Split tensile test with typical failure mode is shown in Fig. 7. The results are arranged to study the effect of replacing cement partially by silica fume, olive’s seed ash, CCA, and fly ash in ordinary Portland concrete. The results clearly indicate that the split tensile strength dropped due to the addition of all SCMs. Two different trends were dictated in the split tensile strength reduction. In the case of mixtures with silica fume, the tensile strength decreased by increasing the percentage cement replacement from 5 to 10 %. The present study results indicate that the optimum parentage of SF to be used is 5 %. All other supplementary materials affect adversely the tensile strength as shown in Fig. 8.