3.1. Compressive Strength
The compressive strength of concrete is the most critical index of its design and performance.
Figure 1 shows the compressive strength of concrete with CG and metakaolin with curing ages of 3, 7 and 28 days and
w/
c ratios of 0.55 and 0.45. It can be noticed that there is a steady drop in the compressive strength of the sample with the increase in CG addition regardless of curing ages and
w/
c ratios. With a
w/
c ratio of 0.55, the 3 day compressive strengths of the samples (without metakaolin) containing 25%, 50%, 75% and 100% CG were 17.95 MPa, 17.38 MPa, 17.00 MPa and 16.31 MPa, respectively, presenting a 2.70%, 5.79%, 7.86% and 11.60% decline in compressive strength relative to the control sample (CG0MK0). The 7 day compressive strengths of the sample without metakaolin ranges from 23.20 MPa to 26.99 MPa. There is a 14.70% decrease in the 7 day compressive strength of concrete with 100% CG compared to the control sample. The 28 day compressive strengths also show a similar variation trend. The 28 day compressive strength of the control sample is 35.45 MPa, which declines to 33.77 MPa, 32.43 MPa, 30.72 MPa and 28.04 MPa for the samples containing 25%, 50%, 75% and 100% CG (CG25MK0, CG50MK0, CG75MK0 and CG100MK0), respectively indicating a 4.81%, 8.59%, 13.42% and 20.96% drop in compressive strength for the corresponding addition of CG. At the 0.45
w/
c ratio, a similar compressive strength degradation is found with the increase in CG, as shown in
Figure 1b. The inclusion of 25%, 50%, 75% and 100% CG in concrete results in a 3.81%, 7.92%, 10.63% and 14.78% decrease in the 3 day compressive strength of concrete. The 7 day compressive strength of samples with 100% CG is reduced by 30.08% compared to that of the control sample (32.27 MPa). Similarly, it can be observed that the incorporation of 25%, 50%, 75% and 100% CG in samples reduces the 28 day compressive strength to 47.50 MPa, 41.83 MPa, 39.78 MPa and 38.17 MPa, respectively, which is 5.30%, 16.61%, 20.73% and 23.90 lower than that of the control sample, respectively.
It can be concluded that the incorporation of CG has an obvious negative effect on the compressive strength of concrete. To some extent, porous structures of CG absorb moisture from the mortar aggregate interface, reducing the
w/
c ratio and thus leading to finer and denser interfacial transition zone structures. However, it seems that this effect contributes little to concrete strength, whereas the relatively lower stiffness and elastic modulus of CG than those of gravel (coarse aggregate) may be the main reason for the decline in concrete strength. The stiffness and modulus of CG are derived from microscopic mechanical properties, which has been confirmed by Li [
4]. Meanwhile, the highly porous microstructure of CG directly leads to stress concentration, which plays an important role in the strength of concrete. In general, there is an inverse relationship between the compressive strength of concrete and the content of CG. The results are in line with [
43,
44]. In addition, the degree of decline in the strength of concrete containing CG is also related to the
w/
c ratios and curing ages, as shown in
Figure 1. Higher water cement ratios correspond to a milder strength deterioration. This may be due to the large amount of water absorbed by CG at the high
w/
c ratio. On the other hand, the degradation of strength increases with the curing age, which might be related to the low solidness and crushing value of CG.
Another result is that metakaolin improves the compressive strength of concrete, as shown in
Figure 1. It can be found from
Figure 1a that the addition of 10% metakaolin in sample (without CG) exhibits 16.73%, 13.54% and 17.36% enhancement in compressive strength as compared to control samples at 3 days, 7 days and 28 days, with a 0.55
w/
c ratio. In addition, the inclusion of metakaolin in the sample with a 0.45
w/
c ratio (CG0MK10) indicates 13.68%, 20.14% and 13.38% improvements in the 3-day, 7-day and 28-day compressive strengths, respectively. In addition, it is noticed that the incorporation of metakaolin can mitigate the strength degradation of concrete with CG regardless of curing ages and the
w/
c ratio. At the 10% percentage replacement level and the 0.55
w/
c ratio, the 7-day compressive strengths of the samples are 21.53 MPa, 20.01 MPa, 13.06 MPa, 17.95 MPa and 17.14 MPa for 0%, 25%, 50%, 75% and 100% coarse aggregate replacement levels (CG0MK10, CG25MK10, CG50MK10, CG75MK10 and CG100MK10), respectively, which are 16.71%, 11.47%, 5.60% and 5.09% higher than that of the corresponding samples (with the same replacement level of coarse aggregate) without metakaolin. The 7-day compressive strengths of the samples containing 0%, 25%, 50%, 75% and 100% CG with 10% metakaolin increase by 13.7%, 7.42%, 9.49%, 6.50% and 7.06%, respectively. At the 28 day curing age, the improvement effect of metakaolin on the compressive strength of the samples is between 15% and 21%. The enhancement can be also found in concrete containing 20% metakaolin. However, the enhancement is weakened due to the dilution effect. Comparing
Figure 1a,b, it can be easily deduced that metakaolin is also beneficial as the
w/
c ratio decreases from 0.55 to 0.45.
It can be found from
Figure 1 that the addition of metakaolin has a positive effect on the compressive strength of concrete. There are both physical and chemical improvement mechanisms for metakaolin. The most important physical mechanism is the filler effect due to the fine particle size of metakaolin. According to the density theory, partial harmful pores in the structure of the concrete can be transformed into gel pores by adding appropriate fine metakaolin particles. Hua proved that gel pores were harmless in exploring the relationship between pores and compressive strength [
45]. On the other hand, it is well-known that the pozzolanic reaction is representative of chemical improvement mechanisms. The reactive SiO
2 in metakaolin can react with Ca(OH)
2 to produce additional hydrated calcium silicate (C-S-H) [
46,
47]. The C-S-H gels can not only have a filler effect, by forming a more stable pore structure, but also have superior mechanical properties and chemical stability compared to Ca(OH)
2 [
48]. However, there is a slight decrease in the strengthening effect on the compressive strength of concrete containing 20% metakaolin replacement level, which is associated with the dilution effect. Furthermore, the strengthening effect is affected by the
w/
c ratio. The improvement effect of metakaolin is more obvious in the samples with a high
w/
c ratio, which may be due to the greater defects in the samples with the 0.55
w/
c ratio.
3.3. Ultrasonic Pulse Velocity
Figure 3 shows the variation of ultrasonic pulse velocity of concrete samples with different metakaolin and CG replacement levels. It can be clearly seen that metakaolin improves while the CG decreases the ultrasonic pulse velocity of sample. The ultrasonic pulse velocity displays a little fall for the sample with less than 50% CG, which sees a steep decline when the CG addition beyond 50%, however. At
w/
c ratio of 0.55, The ultrasonic pulse velocities are 3.07 km/s, 2.85 km/s, 2.73 km/s, 2.68 km/s and 2.44 km/s respectively for the control samples and the samples containing 25%, 50%, 75% and 100% CG (CG0MK0, CG25MK0, CG50MK0, CG75MK0, CG100MK0). The ultrasonic pulse velocity of control sample with a 0.45
w/
c ratio is 4.25km/s, and the corresponding declines in ultrasonic pulse velocity of concrete with 25%, 50%, 70% and 100% CG are found to be 5.73%, 11.01%, 18.19% and 26.38%, respectively. With a metakaolin replacement level of 10%, the samples containing 100% CG displayed a decrease of 17.77% and 16.90% compared with the samples without CG at
w/
c ratios of 0.55 and 0.45. Analogously, there are drops of 17.31% and 22.56% in the corporation of 20% metakaolin and 100% CG in samples with mixed up 0.55 and 0.45
w/
c ratios. Obviously, metakaolin results in an improvement in ultrasonic pulse velocity, which is consistent with Dabbaghi [
30]. Whilst, the addition of CG in sample leads to the attenuation of ultrasonic pulse velocity, which is attributed to the porous structure of CG. As reported in ref [
50], ultrasonic pulse velocity is an index reflecting the integrity of the internal structure of concrete materials. This means that the attenuation of ultrasonic wave velocity represents the more discrete microstructure of concrete due to the incorporation of CG.
3.5. Open Porosity
The variation in the open porosity of the samples are tabulated in
Table 3. It can be observed that there is a steady increase in the open porosity as the percentage of CG increases in the sample. At a 0.55
w/
c ratio, a fall in the open porosity values by 6.58%, 9.68%, 13.64% and 23.32% is noticed for the replacement level of coarse aggregate at 25%, 50%, 75% and 100% CG, respectively. a similar trend is found in samples with a 0.45
w/
c ratio. It can be observed that the open porosities of the samples incorporating 25%, 50%, 75% and 100% CG are 8.75%, 9.49%, 9.89% and 10.63%, respectively, which are 11.60%, 21.04%, 26.18% and 35.58% higher than that of the control sample. Open porosity is negatively correlated with density, cracks and uniformity. The presence of the porosity of CG increases the pore system in concrete. Unsurprisingly, the percentage of open porosity is significantly reduced due to the addition of metakaolin. However, the improvement in the open porosity increased first and then decreased as the percentage of metakaolin increased. The inclusion of 10% metakaolin achieves a superior performance in terms of open porosity. A drop in the open porosity values by 10.20% and 5.23% is realized for the control samples with 0.55 and 0.45
w/
c ratios, respectively. It is seen from the addition of 20% metakaolin in
Table 3 that the open porosity is 7.04% and 7.43%, respectively, which represent a 9.18% and 5.23% reduction in the control specimen mixed with 0.55 and 0.45
w/
c ratios, respectively. This decline in open porosity is due to the filler effect by finer particles and pozzolanic reaction of metakaolin.