Due to pozzolanic reactivity of the agricultural wastes discussed, various investigations on the behavior of the ashes generated from these wastes as cement replacements or aggregate replacements have been conducted. Favorable results have widely been obtained as discussed in this section.
3.1. Palm Oil Fuel Ash
The use of palm oil fuel ash (POFA) as partial cement replacement has a great influence on the microstructural development of concrete [
22,
64,
65,
66]. It has pozzolanic properties, and like fly ash and oil palm ash has potential to control heat of hydration of concrete (
Figure 6). This is because palm oil fuel ash has low pozzolanic reactivity in the early hydration stages (i.e., first 7 days [
65]). As such, palm oil fuel ash has potential to be used in massive concrete for preventing thermal cracking due to excessive heat rise.
Utilization of POFA in concrete has an effect on its fresh properties. The influence of different percentages of ultrafine oil palm ash (max size 11 μm) replacement in concrete slump was investigated by Zeyad et al. [
20], as shown in
Figure 7. It can be seen that the slump increased with the increase of POFA percentage in the mix. Similar behavior was found in other studies [
9,
19,
20].
A study of Tangchirapat et al. [
67] showed that superplasticizer was required in the mixes in order to have similar range of slump value in control mix as well as mixes with 10%, 20%, and 30% POFA. Higher amounts of superplasticizer were required (~80% higher) for concrete with 30% OPA than for the control mix. Clearly, this is not in agreement with the results found by Zeyad et al. [
20]. Tangchirapat et al. [
67] used OPA with a median size of 10.1 μm after grinding the original POFA size of 65.6 μm. The morphological analysis of original POFA showed an irregular particle shape which may lead to the lower workability as the percentages of OPA increase. Therefore, higher amounts of superplasticizer are required for similar workability. From the above discussion, it is clear that the slump of concrete with POFA is dependent on various factors and still cannot be predicted a priori. The reason of this variability in the results can be attributed to the different types, source and size of POFA used by the researchers in different countries. Therefore, a comprehensive study to develop proper guidelines is required for this waste material before it can be used in concrete production.
POFA can also be used as aggregate in concrete production, having different effects on the slump. Kanadasan and Razak [
26] investigated the use of waste POFA as coarse and fine aggregates in production of self-compacting concrete (SCC). In their study, 25% to 100% natural aggregates were replaced by POFA. As the replacement of POFA was increased 25% to 100% in the mixes, the slump value increased approximately 3% to 8%, compared with the reference mix. The higher slump value was reported by the less optimal particle packing of POFA in SCC. This correspond to the highest paste volume in the mix which provides a good coating and lubricating effects to the aggregates and thus increases the slump. This result is also proved by Mannan and Ganapathy [
68], where the slump was more than double in 100% POFA concrete compared to the control mix. The workability of concrete is also influence by the shape of POFA. Round shape of POFA may lead to improved workability of concrete compared to other shapes [
24]. However, controversial results were also reported by the researchers where slump value decreased as the percentages of POFA increased [
29]. This was due to the higher water absorption of POFA compared to natural aggregates. POFA is porous and thus absorbs water from the mix, thereby reducing the water to cement (w/c) ratio. The lower slump of POFA can be improved by adding superplasticizer in the mix, as reported in [
69,
70].
Use of palm oil fuel ash has an impact on the initial and the final setting time of concrete (
Figure 8). With increasing percentage of POFA replacement, both the initial and the final setting time increase [
19,
21]. The increase in setting times is not a concern, however, as both the initial and the final setting times are well within the requirements of American and British standards.
Studies on the compressive and tensile strengths of concrete when POFA was used show a reduction in strength with increase in the POFA content in the mix, as illustrated in
Figure 9 [
26,
30,
71]. For 20–40% cement replacement by POFA, the compressive strength of concrete at 28 days reduced to 31–66% of the control mix concrete. This trend of strength reduction was also observed in concrete with 100% of POFA. As the ratio of POFA in the mixes increased, the compressive strength of concrete gradually decreased [
70]. For low replacement levels (10%), it was shown that strength at later ages (1 year) can be equal to the reference mixture [
21]. In general, lower strength of POFA concrete can be attributed to the lower density of POFA as well as its porous nature. Although the concrete strength reduces with increase in POFA, the strength is still suitable for development of lightweight structural concrete [
70]. Furthermore, continuous hydration of ultrafine POFA concrete cured in water resulted in higher strength compared to air curing [
70].
At lower water-to-binder (w/b) ratio (e.g., 0.3), concrete made with 30% cement replacement by POFA showed a similar range of compressive strength as concrete without POFA [
71]. In this case, 100% POFA was used as coarse aggregate. In a similar study [
22], the authors claimed that it is possible to make structural lightweight concrete using 100% POFA, satisfying the required concrete properties.
Long-term and durability properties of concrete are also affected by addition of palm oil fuel ash as partial cement replacement. It has been shown that POFA has a positive effect on durability: it decreases the chloride diffusivity of the mix [
47,
72,
73], lowers water absorption [
20,
67,
74] and water permeability [
75,
76,
77], and increases sulfate resistance [
67,
78]. Furthermore, it has been identified that partial cement replacement by POFA can be used to suppress alkali silica reaction [
79].
Although some properties of concrete (most notably strength and setting time) are adversely affected by POFA, research has shown that it can be used in many applications. As such, it can be a great resource in developing countries. Future research needs to promote its use in structural applications.
3.2. Rice Husk Ash
Considering that rice husk ash has a very high specific surface area, it has an important effect on the hydration of blended concrete [
80]; it has a high pozzolanic reactivity [
81,
82,
83]. As with other pozzolanic materials, calcium hydroxide is consumed by the pozzolanic reaction, leading to reduced porosity [
80] and even an improved interfacial transition zone (ITZ) compared to reference concrete [
84]. This causes the failure to occur through the aggregate which is intended as this phase has higher strength than the transition zone and improves the mechanical properties of the concrete including compressive, tensile and flexural strengths [
85]. The addition of rice husk ash was found to increase the degree of cement hydration at later ages, which can be attributed to its internal curing ability [
80].
Use of RHA as supplementary cementitious material affects the fresh properties of concrete. While some studies showed that the addition of rice husk ash was found to increase slump compared to reference (i.e., Portland cement) concrete [
86], others found a moderate decrease in slump [
87]. The effect of RHA on the setting time (initial and final) is still under debate: while some authors found that the setting times are increased in proportion to the RHA addition percentage [
88], others found a moderate decrease in setting time proportional to the RHA addition percentage [
89]. RHA addition has other positive contributions to the properties of fresh concrete: according to Le and Ludwig (2016) [
51], bleeding and segregation of self-compacting high-performance concrete can be reduced by incorporation of RHA. The incorporation of RHA improves the physical structure of binders like cement and increases the plastic viscosity [
85]. This effect is more noticeable at higher percentages of RHA content in the mix. The macromesoporous structure of RHA (see
Figure 4) could induce great intermolecular attraction forces and can be used as viscosity modifying agent in concrete [
24]. As a pozzolanic material, RHA reacts with calcium hydroxide during the hydration process of cement and improves the aggregate–paste connectivity [
85]. This improves connection of aggregate–paste forces failure to occur in the aggregates when load is applied. Hence, the mechanical properties of concrete including compressive, flexural and tensile strength improve when RHA added.
The addition of RHA to concrete causes an increase in compressive strength, as shown in
Figure 10. However, no noticeable difference in strength was observed with RHA content between 10 to 20%. It was also noted that 20% of RHA content in concrete increased its strength by ~20% higher than that of OPC [
90]. The compressive strength development up to 365 days showed about 13% higher strength in 10% RHA concrete than in control mix of OPC [
91]. This was corroborated by Hesami et al. [
85], who found 14% higher strength than OPC was reported for 10% RHA replacement. Similar result was also reported by Habeeb & Mahmud [
92], where maximum strength was found at 10% RHA replacement and no difference in strength was reported with 20% RHA when compared to the 10% RHA mix.
In another study [
93], natural sand (average particle size 95 μm; specific gravity 2.59) was replaced with RHA (average particle size 28 μm; specific gravity 2.13) in a range of 25 to 100% to produce autoclaved aerated concrete (AAC). Inclusion of RHA negatively affected the compressive strength of AAC. Approximately 22–58% lower compressive strength was observed in AAC for 25–100% RHA content. The lower strength of AAC could be ascribed to the higher water requirement of RHA, which negatively affected the compressive strength of the AAC [
93].
Durability of concrete was found to improve with incorporation of RHA as partial cement replacement. This can be attributed to the refinement of the pore structure caused by the pozzolanic activity. Concrete with RHA has shown to be more resistant to chloride ingress [
72,
87,
94], to have lower water permeability [
95] and water absorption [
96], improved sulfate resistance [
97,
98], and lower susceptibility to alkali silica reaction [
98,
99].
Other concrete properties such as thermal conductivity and autogenous shrinkage were also investigated when RHA was incorporated in the concrete mix [
36,
100]. Although the thermal conductivity of the RHA insulators was found lower, it was still higher than a commercial thermal insulator made from diatomaceous silica, used as reference [
36]. Similarly, RHA was found to mitigate the autogenous shrinkage of ultrahigh-performance concrete (UHPC). The mesoporous structure of RHA absorbs the free water from the mix and reduces the effective w/b and thus improving bleeding, segregation, workability, and compressive strength [
100]. Furthermore, very fine (nano-)particles of RHA improved heat evolution during the early-age cement hydration. Researcher also produced nanoparticles from the agricultural waste, such as OPA, RHA, etc., and used them to improve the heat evolution during the early-age cement hydration. It was reported that nano-OPA and nano-RHA accelerate the hydration process at faster rate than the OPC [
101]. These nanoparticles help rapid formation of calcium silicate hydrate (C–S–H) in the binding paste thus improve mechanical and durability properties of concrete [
102,
103].
3.3. Sugarcane Bagasse Ash
Like the other waste materials discussed, sugarcane bagasse ash influences the hydration and microstructural development of blended concrete. Sugarcane bagasse ash has pozzolanic properties [
57,
104,
105]. Use of sugarcane bagasse ash as partial cement replacement causes a reduction in hydration heat compared to the reference concrete [
55]. The reduction of temperature rise is proportional to the percentage of sugarcane bagasse ash replacement (
Figure 11). Due to such properties, sugarcane bagasse ash can be used for temperature control in mass concrete.
Workability of concrete reduces with the increase in the amount of SCBA content in the concrete mix [
107]. The loss of workability is attributed to the fineness of SCBA (which is lower than cement), which absorb more water from the mix, leaving concrete drier, and consequently, less workable [
108]. Other studies, however, reported an increased concrete workability with increase in the SCBA content [
109]. For concrete with 25% SCBA, about 128% higher slump was reported than the control mix. The authors concluded that the addition of SCBA in concrete reduces the water demand. Another study also reported that the replacement of cement with SCBA increased the workability of concrete; therefore, no extra superplasticizer would be needed [
110]. The initial and final setting time of concrete also reduced ~15–20% when 15% SCBA was replaced in concrete [
108]. This lower setting time can be useful for specific applications, e.g., in concrete repair applications [
111,
112].
In case of mechanical properties, a study with SCBA replacement in concrete showed equal or marginally better strength than the control mix of concrete, even at early age of three days [
55]. At 28 days, with 20% SCBA, a maximum 11% higher compressive strength than control mix was found. The results also indicated that up to 25% of SCBA replacement could be used in concrete production without sacrificing the strength. The optimum level of SCBA content (i.e., no strength difference from the control mix) was 20% in their study [
113]. In another study, 5% and 10% of total cement content was replaced by SCBA, and then the compressive strength was compared with control mix without any SCBA [
110]. It was observed that a maximum 12% higher strength was found in concrete with 5% SCBA content. With 10% SCBA content, the strength was 4% higher than the control mix. Furthermore, tensile and flexural strengths were also improved when SCBA was added to the mix. With 10% SCBA, a maximum 50% and 12% higher tensile and flexural strengths of concrete respectively than the control mix were observed. However, there was reduction in strength when SCBA content increased to 25%. At this level, similar strengths were noticed in both SCBA concrete and in control mix.
The addition of sugarcane bagasse ash has a positive effect on concrete durability. The durability performance of concrete with different SCBA content against chloride, gas and water penetration was also investigated by the researchers [
55,
114]. Resistance to chloride ingress increases with the increase in SCBA content (see
Figure 12). Regarding transport of water, contradictory findings have been reported: while some authors found a marginal increase in sorptivity with the increase in SCBA content [
55], others reported a decrease in water permeability [
106]. Some authors suggested that the sorptivity increase is due to porous nature of SCBA and the impurities in it [
114].
From the above discussion, it can be said that the SCBA influences the quality of concrete. It has a good chemical composition and physical properties such as fineness, setting time and compressive strength. However, there is uncertainty about the optimum content of SCBA in concrete as the current results show variability in concrete strength with SCBA replacement ranging from 5% to 25%. Nevertheless, the use of SCBA in addition to the concrete is a very feasible option in improving the mechanical properties of the concrete, besides providing a suitable destination to agroindustrial by-product [
107].
3.4. Bamboo Leaf Ash
The use of bamboo leaf ash in concrete is less studied compared to other agricultural wastes discussed. Singh et al. [
61] investigated the hydration of bamboo leaf ash blended Portland cement. They found that replacement of cement with 10% BLA delayed the initial and final setting of concrete by approximately 29% and 37% compared to the control mix. At 20% BLA replacement, these setting times reduced and were equal to those of the control mix. However, in another study, it was found that the setting of concrete was delayed as the percentages of BLA content increased as shown in
Figure 13. The maximum reached 49% and 30% higher initial and final setting times of concrete were reported with 25% BLA [
115]. The pozzolanic activity of BLA increased with the increase in time and temperature.
In terms of concrete workability, the higher the percentages of BLA replacement, the lower the slump was observed [
116]. At a given w/b, the lower percentages of BLA (10–20%) showed improvement in workability by reducing bleeding and segregation. However, when BLA replacement was 30–40%, the concrete was unworkable. The reason was the cellular bamboo leaf ash particles and the higher fineness of BLA compared to Portland cement. The reduced workability was also observed in the study by Singh et al. [
61]. Therefore, in order to improve the workability and consistency of the mix, a proper dosage of additional water or superplasticizer are required.
When the mechanical properties of BLA concrete are concerned, the majority of studies reported that the strength of concrete reduces with increase in the BLA content [
59,
116,
117]. As shown in
Figure 14, the compressive strength of concrete gradually reduces as the amount of BLA replacement increases. However, for high-strength mortars, the strength of mortar with BLA replacement was similar to that of the control mix at 28 days. The mixes with BLA had high water demand [
118]. At 10% and 20% BLA replacement in concrete, the physical and mechanical property requirements for concrete were in accordance with EN 197-1 standard [
118]. In another study, the optimum replacement of OPC with BLA for the selected grade was found to be 15%. At 28 days, an ~10% reduction in the strength was observed with BLA, which was found acceptable by the authors [
118].
Concrete durability properties such as acid resistance and chloride resistance are considerably improved at 10% replacement of cement with BLA [
59]. The capillary suction test of 15% BLA concrete showed that the sorption coefficient was less than that of the control concrete. It was concluded that the presence of 15% BLA in the mix improves the durability of concrete by filling the voids in the cement [
117]. Therefore, from durability point of view 10–15% BLA replacement with cement in concrete can be considered beneficial. Clearly, more studies are needed on BLA concrete.