Effect of sea sand and recycled aggregate replacement on fly ash/slag-based geopolymer concrete

The recycled coarse aggregate crushed and screened from the old grade C25–C30 concrete building and natural granite aggregate were also used as a reference, as shown in Figure 1. The density, fineness modulus, shell content, and chloride ion content of SS were determined according to JGJ/52-2006. The materials used are the V1500-I type FA, V500 (S95) type slag, and P.O. 42.5 cement, for which the detailed physical and chemical compositions are given in Tables 1 and 2. The geopolymer alkaline activator was formed by a mixture of NaOH and Na₂SiO₄ solutions with a mass ratio of 1:2.5. This ratio of 1:1.5 has also been widely used by previous researchers as an optimum value [24,25,26]. Relevant performance indices of the materials are shown in Tables 3 and 4.

Fig. 1

The mechanical and microstructural properties of GPC depend on different mixing parameters, such as the alkali activator concentration and the ratio of FA to slag. To determine the optimum mix proportion of the slurry and to optimize the strength and workability of the GPC, an orthogonal array design is implemented. Based on the recent and available literature [24,25,26], three main factors that alter the strength and workability of geopolymer were identified as the RA replacement ratio, NaOH concentration, and binder to activator ratio. Geopolymer mixes were prepared using 10%, 20%, and 30% slag ratios in FA/slag-based GPC. The alkaline activator used was a mixture of sodium silicate (Na₂SiO₄) and sodium hydroxide (NaOH) at three concentrations (8 mol/L, 12 mol/L, and 16 mol/L). The ratio selected was based on the findings in the literature [24,25,26], which gave the best results of GPC. Moreover, water-binder ratios of 0.3, 0.4, and 0.5 at 0%, 25%, 50%, 75%, and 100% artificial aggregate replacement with two different types of sand (SS and RS) were used to prepare the GPC. A similar replacement ratio has also been adopted in the following literature [12, 20]. Based on the optimum mix proportions, geopolymer paste (GPP) and geopolymer mortar (GPM) were prepared to study the effect of seawater and SS on GPP and GPM without coarse aggregates.

FA, slag, and aggregate were first dry mixed in a 100-L mixer for at least 2 min to ensure that all aggregates were coated with the powder mix. Then, the premixed alkali solution was blended into the dry mixture and mixed for approximately 3–4 min. Then, the composite paste/mortar/concrete was poured into 100 mm × 100 mm × 100 mm cube molds and 100 mm (diameter) × 200 mm (height) cylindrical molds, and finally, it was vibrated for 1–2 min using a vibrating table. Thereafter, the samples were demolded after 24 h and cured in a standard curing room until testing.

After obtaining the optimal mix designs, GPP, GPM, and GPC were cast in a similar manner. GPP and CPM were cast into 70.7 mm × 70.7 mm × 70.7 mm cube molds, while GPC was cast into 100 mm × 100 mm × 100 mm cube molds, 100 mm (diameter) × 200 mm (height) cylindrical molds and 100 mm × 100 mm × 400 mm prism molds. The OPCC was made in a manner that was similar to that of the GPC samples.

The slump test, cube compressive strength test at 3 days and 28 days, cube splitting tensile strength test, and cylinder compressive strength test at 28 days were carried out on nine groups of specimens. To compare the influence of individual factors on the test results, the test results were processed and regrouped; these have been presented in Table 8. The mean value of the 3 days compressive strength of groups 1, 4, and 7 were used to represent the 3 days compressive strength when the NaOH solution concentration was 8 mol/L (A1 in Table 8). Similarly, the mean values of groups 2, 5, and 8, and those of groups 3, 6, and 9 represent the 3 days compressive strength when the NaOH solution concentration was 12 mol/L (A2 in Table 8) and 16 mol/L (A3 in Table 8). The results of the slag content (B in Table 8) and water-binder ratio (C in Table 8) were calculated according to the above method and are recorded in Table 8.

The influences of various factors on the mechanical properties of GPC are shown in Figures 2–6. It can be seen that the NaOH solution concentration and slag content have little influence on the slump and Poisson’s ratio values of GPC, whereas the influence of the water-binder ratio is significant. The NaOH solution concentration, slag content, and water-binder ratio considerably impact the compressive strength and elastic modulus, but the influence law and mechanism are not exactly the same. Moreover, as the NaOH solution concentration and slag content increased and the water-binder ratio decreased, the splitting tensile strength increased.

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

The results of the orthogonal analysis presented above and the results of Xie et al. [26] and Nuaklong et al. [28] can be used to propose optimum mix proportions, that is, a 12 mol/L NaOH solution concentration, 30% slag content, and 0.3 water-binder ratio.

Figure 7 shows the results obtained for each group. At 7 days, the compressive strengths of seawater paste (SWP) and sea sand mortar (SSM) were 15.8% and 14.7% lower than those of distilled water paste (DWP) and river sand mortar (RSM), respectively, indicating that seawater and SS reduced the early strength of the geopolymer slurry. At 28 days, the compressive strength of SWP was 14.1% lower than that of DWP, but the compressive strength of SSM was 16.3% higher than that of RSM. This may be because the chloride ions in seawater and SS reduced the pH value of the slurry in the early stage, which hinders the hydration reaction of the geopolymer, and then reduced the early compressive strength of the specimens. However, this hindering effect would fail in the later stages, and the strength would be improved.

Fig. 7

Figure 8A–8E shows the failure modes corresponding to sea sand geopolymer concrete (SS–GPC) with RA replacement ratios of 0%, 25%, 50%, 75%, and 100% at 28 days, respectively, and Figure 8F corresponds to SS-OPCC with RA replacement ratios of 100%, where each photo is the most representative specimen selected from the three specimens in each group. As can be seen from Figure 8, all of the specimens were destroyed because of a long crack running through the top and bottom of the specimens. Compared with SS-OPCC, there were more fine cracks near the main crack of the SS–GPC. The strength of NA is higher than that of RA; therefore, when the crack develops to aggregate, it bypasses the NA but directly penetrates the RA (as shown in Figure 9). As a result, the failure surface of the specimen becomes flatter as the RA replacement ratio increases.

Fig. 8

Fig. 9

Figure 10A–10C, respectively, shows the failure modes corresponding to river sand geopolymer concrete (RS–GPC) with RA replacement ratios of 0%, 50%, and 100% at 28 days, and Figure 10D corresponds to the RS-OPCC with an RA replacement ratio of 100%. The failure mode of RS–GPC is similar to that of SS–GPC with an increase in the replacement ratio. Compared with GPC (Figure 10C), the cracks of the OPCC specimens (Figure 10D) are wider.

Fig. 10

It can be clearly seen from Table 9 that the slump of the GPC is higher than that of the OPCC, indicating that it is a high-fluidity concrete and has better working performance. In addition, the slump of SS–GPC is slightly higher than that of RS–GPC, because SS has a more rounded appearance with fewer edges and corners, as well as a smaller fineness modulus, resulting in greater slurry flowability. Simultaneously, the slump in all groups increased with an increase in the RA replacement ratio. Because RA reached the saturated surface dry (SSD) state before pouring, a small portion of water in the aggregate cracks was released during mixing, which is equivalent to increasing the water-binder ratio [29, 30]. However, many edges and corners of the RA are polished during the manufacturing process, enabling the slurry to flow more easily over the surface of RA, resulting in an increased slump.

In Figure 11, it can be seen that the cube compressive strength (f_cu) of SS–GPC (SG-0, SG-25, SG-50, SG-75, and SG-100) remains relatively stable with an increase in the RA replacement ratio, while the influence of the RA replacement ratio on the compressive strength of SS–GPC at 28 days is more significant than that at 7 days (13.1% and 5.3% difference between the maximum and the minimum, respectively). Simultaneously, the compressive strength of SS–GPC was lower than that of RS–GPC at 7 days. In contrast, the compressive strength of RS–GPC (RG-0, RG-50, and RG-100) exhibited a considerable regular downward trend with the increase in the RA replacement ratio, which is consistent with the experimental phenomena of Xie et al. [31] and Shaikh et al. [32]. In addition, the decline was greater at 28 days than that at 7 days. The strength of the SS–GPC slurry was lower than aggregate at 7 days, owing to an insufficient hydration reaction. Then, the specimens had started to become damaged from the slurry (Figure 12). As the reaction proceeded, the strength of the SS–GPC slurry exceeded that of RA at 28 days. When the RA replacement ratio increased, the weak part gradually transferred from slurry to RA (Figure 13). In contrast, the strength of RS–GPC exceeds that of RA, so the specimens begin to be damaged from the interface transportation zone (ITZ) or RA (Figure 14). During the preparation of RA, a large number of microcracks are generated, and some old mortar with low strength was attached, resulting in poor quality of RA compared with NA. At the same time, the ITZ between the old mortar and old aggregate in RA was conducive to the more rapid development of cracks, thus reducing the bearing capacity of the specimens [33, 34].

Fig. 11

Fig. 12

Fig. 13

Fig. 14

Similar to the conclusions made by many other scholars, the compressive strength of SG-0 is lower than that of RG-0, and this is mainly because the fineness modulus of SS is smaller; that is, the specific surface area is larger, reducing the amount of slurry per unit surface area, thus reducing the concrete strength [35,36,37]. However, the 28-day compressive strength of SS–GPC was higher than that of RS–GPC when the RA replacement ratio was high (>50%). According to the mortar test, the strength of SSM was lower at 7 days but higher at 28 days compared with that of RSM (Figure 7), which indicated that SS improved the hydration reaction in the later stage, thus improving the bonding performance between the GPP and aggregate. Simultaneously, it was found that the bonding performance between the SS–GPC paste and RA is stronger than that of the RS–GPC paste when comparing the ITZ in Figures 13E and 14C. The presence of SS improves the performance of the ITZ between the geopolymer and RA, and eventually leads to a higher compressive strength of SS–GPC than that of RS–GPC with a high RA replacement ratio.

The compressive strengths of SG-0, SG-25, SG-50, SG-75, and SG-100 increased by 37.29%, 38.99%, 41.10%, 41.62%, and 28.52% from 7 days to 28 days, respectively. With an increase in the RA content, microcracks in the aggregate will see the infusion of more water, causing a more alkaline excitation of the hydration reaction of the solvent, which can make the reaction more intense and complete, thus, promoting the development of its late strength. However, the strength loss caused by a defect in the RA is greater at a 100% replacement ratio. The compressive strengths of RG-0, RG-50, and RG-100 increased by 19.38%, 15.92%, and 10.15%, respectively, from 7 days to 28 days. This is mainly because the aggregate is the weak part of the whole specimen, and its strength significantly impacts the overall strength of the specimen, leading to a decrease in the rate of increase in strength with an increase in the RA replacement ratio.

As the bonding force between GPC and aggregate is much higher than that between OPCC and aggregate, making ITZ denser, the compressive strength of GPC was higher than OPCC. At the same time, the compressive strengths of SPC-100 and RG-0 were higher than those of RPC-100 and SG-0, indicating that SS increased the compressive strength of RA-OPCC and decreased that of NA-GPC to some extent.

The influence of the RA replacement ratio and sand type on the elastic modulus (E_c) of the concrete cylinder specimen is shown in Figure 15. It can be seen that the variation rates of the two groups were similar to that of the compressive strength. Observing the curves of the SS–GPC, the effect of the RA replacement ratio on the elastic modulus at 7 days was very small (12% difference between maximum and minimum), while the effect at 28 days increased first and then decreased. By observing the RS–GPC curve, the elastic modulus curves show a significant downward trend with an increase in the RA replacement ratio.

Fig. 15

In the SS–GPC, the improvement rates of the elastic modulus from 7 days to 28 days were 61.11%, 60.16%, 47.86%, 50.40%, and 30.22%, respectively, which decreased with an increase in the RA replacement ratio. In the RS–GPC, the improvement rates were 52.56%, 97.73%, and 68.18%, showing a trend of first increasing and then decreasing. However, the elastic modulus of OPCC (SPC-100 and RPC-100) is much higher than that of GPC (SG-100 and RG-100), but the improvement rates were only 5.8% and 4.8%, respectively, which indicates that the elastic modulus of SPC-100 and RPC-100 has matured in the early stage.

Figures 16 and 17 show the complete stress–strain curves of SS–GPC and RS–GPC at 7 days and 28 days, respectively. Figure 18 shows the complete stress–strain curves of GPC and OPCC at 7 days and 28 days under the condition of 100% RA replacement ratio. In all images, the vertical axis represents the stress, the positive X-axis represents the longitudinal strain, and the negative X-axis represents the transverse strain.

Fig. 16

Fig. 17

Fig. 18

In Figures 16 and 17, the change rules of the cylinder compressive strength of the GPC are similar to that of the cube compressive strength. The reason for this rule is similar to that of the cube compressive strength. The rising segments of the longitudinal strain and transverse strain curves of SS–GPC almost coincide completely, indicating that the RA replacement ratio has little effect on the strength and stiffness. It can also be seen that each curve has a considerable decreasing section, where the specimens show ductile failure characteristics at 7 days but brittle failure characteristics at 28 days. In contrast, with an increase in the RA replacement ratio, the slope of the longitudinal strain rising section decreased, but the ductility was enhanced in RS–GPC. The specimens with a higher strength exhibited greater stiffness. At the same time, the transverse strain showed a similar trend.

In Figure 18, the curves of OPCC are steeper than those of GPC. OPCC has a higher stiffness but lower strength compared to GPC, indicating that there is no relationship between stiffness and strength. The use of SS improves the stiffness of the GPC or OPCC. Over time, the stiffness of the RC group can develop to be similar to that of the SS group (see Figure 18B).