3.1. Interpretation and Comparison of GPC and OPC Concrete
Figure 2 compares and demonstrates the distribution of CO
2 emissions in GPC and OPC.
Figure 3 displays the distributions of the ingredients and different production processes of GPC and OPC, respectively.
Compared with the CO2 emissions of GPC and OPC concrete, the minimum emission of GPC with 40 MPa is 260.14 kg CO2/m3, and the maximum is 336.54 kg CO2/m3. However, OPC concrete with 40 MPa showed a variation from 334.90 to 412.58 kg CO2 /m3, which is higher than GPC. The highest CO2 emission was recorded in C60 and C70 of GPC (333.28 kg CO2/m3 and 305.38 kg CO2/m3, respectively), which is significantly lower than the minimum in OPC concrete with the same compressive strength (405.26 kg CO2/m3). CO2 emissions from GPC at 40 MPa, 60 MPa, and 70 MPa are reduced by 20.48%, 27%, and 34.6% respectively compared to OPC. It seems that the higher the compressive strength of the geopolymer concrete, the more significant the CO2 reduction effect. In general, the production of GPC has a lower CO2 emission than that of OPC concrete. The results show that for the same compressive strength, GPC can reduce emissions by up to 166.36 kg CO2/m3 compared to OPC, which is approximately 62.73% of its carbon emissions.
Compared with the results shown in
Figure 2, it can be seen that the CO
2 emissions increased continuously with increasing the compressive strength for OPC. However, for GPC, the CO
2 emission showed a slight correlation with compressive strength [
17], which might be attributed to the fact that the compressive strength and CO
2 emission of OPC concrete is greatly affected by the amount of cement. Therefore, it can be concluded that GPC can effectively improve the environmental impact without compromising its strength. For geopolymer, the maximum impact on the environment is the alkali activator [
18]. Salas [
19] showed that the higher the compressive strength, the higher the environmental impacts of higher activator quantities. According to the distributions of CO
2 emissions in GPC and OPC shown in
Figure 2, it can be indicated that the relationship between the different phases is Csc >> Cc > Cys for GPC, while for OPC concrete, it is Csc >> Cys >> Cc. The CO
2 emission of the production of raw materials is the largest for both.
In
Figure 3a, the CO
2 emission in the raw materials production process of GPC reaches 74–80% of the total emissions, in which sodium silicate takes the most significant proportion, accounting for about 70–84%, which is followed by sodium hydroxide, making up around 10–19%. The commonality in mixes with the lowest CO
2 emission is in using less alkali activator solution. Therefore, it is suggested to employ sustainable production technology of alkali activator solution and control its usage. Bajpai [
20] proposed that silica fume, instead of sodium silicate, can further reduce the environmental impact of geopolymer concrete. On the other hand, considering that the economic value allocation procedure is used to distribute the environmental impact of fly ash, CO
2 emission of fly ash is generated, accounting for 6–10%. The environmental impact of different aluminosilicate materials varies. Using silica fume instead of fly ash in the preparation of geopolymer concrete can further reduce the environmental impact [
20]. The distribution of the CO
2 emission concerning the aggregates and water shows no striking fluctuation for all mixes. It is clear that the CO
2 emission of GPC mainly depends on transportation, heat curing, and the alkali activator solution. However, almost all emissions come from cement for OPC concrete, taking up 92–93%. It means that the CO
2 emission of OPC concrete is affected dramatically by the amount of cement.
Compared with the CO
2 emissions in concrete production shown in
Figure 3, the production of GPC resulted in high CO
2 emissions due to the high-temperature curing for 24 h, accounting for around 11–15%. It is much higher than OPC concrete, which accounts for 0.2–0.3%. It was observed that when GPC was cured under room temperature similar to OPC concrete, it will further lower the disadvantageous effect on the environment. The CO
2 emission in the transportation of GPC accounts for about 8–11%, which is not much different from OPC concrete at 7%. It mainly depends on the distance and mode of fly ash and cement transportation. The transport distance has a high impact on the global warming potential of geopolymer mixes [
20]. Therefore, it is better to purchase materials close to the plant and choose a transportation mode with low energy consumption. The influence of the materials’ source location and transport mode significantly affect both environmental impacts and production cost and thus should be a significant consideration [
9].
3.2. Interpretation of MANOVA on GPC
The above results show that the CO
2 emission of GPC does not increase proportionately and even decreases with the increase in compressive strength. For example, 1 m
3 C40 GPC emits 336.54 kg CO
2/m
3, while C60 and C70 GPC can emit less CO
2 than C40. Therefore, the growth of the environmental impact should not only be judged by the increase in compressive strength. It can be seen from some studies that various factors are affecting the strength [
21,
22,
23] and CO
2 emissions of GPC [
24]. So, it was necessary to investigate further the correlation of the various impact factors contributing to the compressive strength and CO
2 emissions.
The orthogonal design method was used in the experimental design stage because it is necessary to simultaneously study the effect of multiple impact factors and save costs/time in testing. Taking the sodium hydroxide concentration (C
NaOH), sodium silicate to sodium hydroxide ratio (SS/SH), and alkali activator solution to fly ash ratio (S/F) as three variable factors, orthogonal experiments for the three factors and four levels were designed. The test results are shown in
Table 4.
The idea of MANOVA is to examine the contribution of different sources of variation to the overall variation based on experimental data to assess each parameter’s importance. Therefore, the F test and significant value were performed. Generally, the parameter change has an essential effect on the experiments when the F value is large or the sig value is close to zero.
- (1)
Sig < 0.05, R2(a) was 0.999 and R2(b) was 0.962, which show strong positive correlation. The experimental error was deficient (0.1% and 3.8%), confirming that this model has an excellent fitting effect.
- (2)
According to the sig values, it is concluded that the S/F ratio has a significant influence on CO2 emission (Sig = 0.000), which is followed by SS/SH (2 × 10−6) and CNaOH (1.6 × 10−4). As for compressive strength, CNaOH (Sig = 0.000) has a remarkable impact on it, which is followed by the SS/SH (Sig = 0.008) and S/F ratio (Sig= 0.050).
- (3)
The percentage contribution in
Figure 4 confirms the same conclusion.
To further investigate the differences and significance of different levels, Bonferroni and Tukey–Kramer analyses were performed using SPSS.
According to
Table 6 and
Table 7, increasing C
NaOH from 8 to 14 mol/L showed a gradual increase in compressive strength and CO
2 emission. However, the increase in C
NaOH within the range 10–12 mol/L showed no evident difference in the CO
2 emission. When other C
NaOH changed, it showed a stronger and obvious influence. As for compressive strength, when C
NaOH varied from 8 mol/L or 10 mol/L to other concentrations, it changed significantly. Meanwhile, the effect of 12 mol/L and 14 mol/L showed a slight difference, which means that the increase in compressive strength slows down after C
NaOH reaches 12 mol/L. The compressive strength increases with the increase in concentration of sodium hydroxide. However, beyond a specific range, too much OH
- can affect the dissolution of fly ash and thus negatively affect the mechanical properties [
25,
26].
From
Table 6 and
Table 7, the effect of SS/SH on CO
2 emissions showed a big difference with the change of varied ratios within 2 to 4. Meanwhile, CO
2 emissions increased gradually by increasing the ratio from 2 to 4. On the contrary, as the ratio of SS/SH declined from 4 to 2, the compressive strength gradually increased. Overall, different ratios exerted different influences on the compressive strength. They are mainly embodied in the ratios going from 4 to 2.5 and 4 to 2. However, the effects of the adjoining ratios on compressive strength are not different. Obviously, the smaller the SS/SH ratio or the higher the C
NaOH is, the higher the PH and alkalinity of the solutions. Previous studies showed that relatively high alkalinity and pH could provide an alkaline environment conducive to the polymeric reaction and improve the activation effect of fly ash to obtain a compact structure and promote strength growth [
27,
28,
29].
As shown in
Table 6, the sig value of CO
2 emission is exceptionally close to zero, which means that the CO
2 emission changes significantly as the S/F changes within the range from 0.4 to 0.52. Meanwhile, CO
2 emission increases as it increases from 0.4 to 0.52. However, the change of S/F showed an insignificant effect on the compressive strength. In addition, its increase will prompt the compressive strength, while when exceeding a certain value (0.48), it will decrease. It might be attributed to the increase in Si/Al ratio due to alkali activator solution usage, leading to a higher compressive strength with a more compact structure [
30]. Meanwhile, many studies proved that a too high S/F ratio significantly reduces the compressive strength and the excessive amount of the alkaline activator that leads to inhibition of the geopolymerization process [
31,
32].
According to MANOVA analysis, the optimum mix based on CO
2 emission and compressive strength is shown in
Table 8.
3.3. Interpretation of GRA on GPC
According to Equations (3)–(8), the gray relation coefficient and gray relational degrees of the three parameters to CO
2 emission and compressive strength are calculated and shown in
Table 9. The gray relational degree of compressive strength showed r
1 (1) > r
1 (2) > r
1 (3) > 0.5, indicating that sodium hydroxide concentration exerts a significant impact on the compressive strength, which is followed by the SS/SH ratio and S/F ratio. Concerning CO
2 emission results, it showed 0.5 < r
2 (1) < r
2 (2) < r
2 (3), confirming that the S/F ratio is the most notable impact factor among the three studied parameters. Thus, the sequences of the gray relational degree of compressive strength and CO
2 emission results are just the opposite. Therefore, it can be suggested that CO
2 emissions will not increase continuously by increasing the compressive strength as they do for OPC concrete.