Sisal (see Figure 3) is a hard-leaf fiber crop that grows in the tropics. It is the most widely used natural plant hard fiber in the world today. The world’s annual production of sisal fiber can reach about 4.5 million tons (Li et al., 2000). For 1 ton of commercially used sisal fibers, 3 ton of residual fibers have been dumped. Sisal fiber has tough texture, wear resistance, salt alkali resistance and corrosion resistance. Sisal fibers under different environmental conditions show better tensile properties than other fibers (Ramakrishna and Sundararajan, 2005a), and has important application value in enhancing the performance of concrete.

Sisal fiber (see Figure 3) is low cost and easy to obtain. The incorporation of sisal fiber can significantly improve the early strength of concrete, since sisal fiber can effectively inhibit the growth of micro cracks (Bao and Meng, 2011). Adding short sisal fiber can also significantly reduce the free plastic shrinkage of cement mortar (Romildo et al., 2005). It should be noted that Sisal fiber is hydrophilic with relatively high porosity as shown in Figure 3, thus it can absorb a large quantity of water, severally weakening their binding with cement matrix. Such fiber-cement interfacial interaction can be enhanced via various strategies. For instance, Yan reported that the surface modification by silane and other coupling agents can change its hydrophilic properties, improving the interfacial properties. The surface roughness of the fiber can be enhanced by per-manganate and alkali treatment, thereby increasing the surface area in contact with the matrix. The sisal fiber can be heat treated to improve the bonding performance between the sisal fiber and the matrix and reduce the water absorption rate (Li et al., 2000).

The morphology of sisal fiber could also affect its performance in mixed materials. Long-arranged sisal fiber as a thin cement-based laminates material shows potential in semi-structural and structural applications of cement-based materials (Romildo et al., 2005). The shape of sisal fiber has an important influence on the bonding strength and interface properties of cement-based reinforced material (Flávio de et al., 2011).

Date palm, which is native to west Asia and north Africa, is one of the most planted palm trees in the world. Date palm trees could provide four types of fibers: leaf fibers in the pedicel, leaf fibers in the stem, wood waste fibers in the trunk and surface fibers around the trunk (Kriker et al., 2005). The surface fiber of date palm has the highest tensile strength in the four types of fibers. The high-performance fiber contained in date palm provides an opportunity for the development of low-cost and high-efficiency building materials. The presence of date palm fiber improves the cracking strength, toughness and performance of concrete after cracking (Kriker et al., 2005).

However, the application of these plant fibers in concrete is plagued by durability issues in alkaline environments (Kriker et al., 2008). The surface fibers of date palm trees (see Figure 3) are severely affected by alkaline solution. For example, The surface fibers of date palm trees keep 69% of the original strength after being placed in the Ca(OH)₂ solution for 6 months (Kriker et al., 2008).

The addition of date palm wood fiber can decrease the density of mortar, and improve the thermal insulation performance as well, which plays a huge potential in the research of new building energy-saving bio-composite materials. For example, Benmansour found that adding 5–15% date palm fiber makes the composite material meet the thermal and mechanical requirements of building materials (Benmansour et al., 2014).

Jute is widely cultivated in the area south of the Yangtze river in China. The fiber contained in jute has the characteristics of high tensile strength and low density. At the same time, jute fiber is also one of the lowest cost plant fibers and has good renewable properties. Among various natural plant fibers, jute fiber has a long continuous length. From the perspective of processing technology and availability, jute fiber is an ideal material among natural plant fibers to replace various synthetic fibers for composite reinforcement (Liu et al., 2013).

The incorporation of natural plant fibers has made a great contribution to the improvement of concrete mechanical properties. In particular, compared to other natural plant fibers, jute fiber has a long continuous length, good processing technology, high strength and low elongation characteristics, Jute fiber can effectively enhance the compressive performance of concrete and has good crack resistance when the dosage is 0.9 kg/m³ and a certain length (40 mm in the test) (Zhang et al., 2012). It has been reported that, when the jute fiber content of concrete is 0.5–0.6 kg/m³, the compressive and flexural strength of the concrete is enhanced most significantly (Liu et al., 2013). However, the length of the mixed fiber in concrete should not be too long (shorter than 40 mm in the test), otherwise the enhancement effect of the concrete compressive performance will be greatly reduced (Zhang et al., 2012).

In terms of the frost durability of jute fiber concrete, Chen et al. (2015) conducted a freeze-thaw cycle test on jute fiber concrete mixed with different mass ratios and found that the concrete without jute fiber has lower frost resistance than the concrete mixed with jute fiber. When the mass ratio of jute fiber is 0.9%, the mass loss rate and relative mass loss rate of concrete are the minimum.

Hemp and Flax are the earliest natural plant fibers used by humans which contain rich cellulose and show the characteristics of strong tensile force, good fineness, weak electrical conductivity, fast water absorption and dispersion, and high expansion rate. Merta (Merta and Tschegg, 2013) demonstrated that the fracture energy of hemp fiber reinforced concrete was 70% higher than that of ordinary concrete. Zhang (Zhang, 2011) used a mixture of loose flax, bentonite, and cement to successfully solve the problem of slurry leakage in the cofferdam impervious wall of Jin’anqiao hydropower Station. Adding flax fiber to ordinary cement mortar can effectively reduce the cracking caused by restrictive plastic shrinkage resulting from its good hydrophilic properties (Xu, 2013). In addition, ramie also known as “Chinese grass” by the world, is one of the important fiber crops in ancient China. The alkali-treated ramie fiber can effectively enhance the mechanical properties of concrete and prevent cracking when the content reaches a certain amount (Wang, 2013).

Besides hemp plant, rice straw from the wastes left after rice used is also a good source of natural fiber. Compared with fiber-free composite materials, the mechanical properties of rice straw fiber reinforced cement-based composite materials have been significantly improved. The addition of rice straw fibers reduces the bulk density of the composite material by 12.4–37.3% and increases the bending strength by 24.3% (Xie et al., 2015). As a very common shrub flower species in Asia, hibiscus could also provide rich natural fiber to reinforce cement-based composite materials. Researchers (Ramakrishna and Sundararajan, 2005a) found that the addition of 2.5% hibiscus fiber by weight of cement can improve the impact resistance of cement mortar. The hibiscus fiber can maintain 0%, 10–20%, and 20% of the original strength after alternating dry and wet in saturated lime, NaOH and fresh water for 60 days (Ramakrishna and Sundararajan, 2005b).

Olive waste contains olive pulp, peel and residual oil. Approximately 3 ton of pruning residues are generated each year from 1 ha of olive trees, most of which are disposed inconsiderately. Olive ash containing about 22% SiO₂ and 42% lime can be produced by burning large amounts of olive waste, and it is a new type of auxiliary cementing material (Al-Akhras and Abdulwahid, 2010).

In the production of concrete, although olive ash exerts good compaction performance and improves the compressive strength of concrete, it requires relatively high water consumption. Therefore, it is necessary to add additives to control its high water absorption (Cuenca et al., 2013). At the same time, the high-temperature performance of the concrete mixed with olive ash is better than that of ordinary concrete. In addition the excellent high-temperature resistance of olive ash makes it have the potential to be a thermal insulation material (Al-Akhras et al., 2009).

Incorporating olive ash material into cementitious mortar can improve the performance of the cementitious mortar, since it helps reduce the porosity of the bonding interface. As a partial replacement of sand, the compressive and flexural strength of mortar increase with the increase of olive ash content (see Figure 5) (Al-Akhras and Abdulwahid, 2010). However, as a partial replacement of cement, the compressive and flexural strength of mortar will be decreased with the increase of olive ash content, since the amount of cementitious materials is reduced. In addition, compared with autoclaved curing, wet-cured olive mortar exhibits better compressive and flexural strength.

Bananas with extremely large leaves are mostly grown in tropical and subtropical regions. According to statistics, in 2012, banana plants produced approximately 10 million tons of banana leaf ash (Salmabanu et al., 2019). Banana leaves must be properly burned in open air at the temperature about 900°C for 24 h to obtain high-performance banana leaf ash (Kanning et al., 2014). According to reports, the burned banana leaf ash has high pozzolanic activity after being grinded for at least 30 min (Kanning et al., 2011).

As a pozzolan material, banana leaf ash can effectively improve the mechanical properties of the mortar. Kanning et al. (2014) used an artificial aging test box to evaluate the mechanical and chemical properties of concrete mixed with 10 and 20% banana leaf ash. The results show that the compressive strength and average tensile stress of the mixture with 10% banana leaf ash are nearly 25 and 10% respectively higher than those of other concrete without banana leaf ash. The compressive strength and average tensile strength of the concrete containing 20% banana leaf ash are 12 and 20% respectively higher than those of the concrete without banana leaf ash.

As a tropical grass native in Africa, elephant grass (see Figure 6) requires very little nutrients and water to grow, but can produce large amounts of ash after burning and grinding. Since, elephant grass contains a large amount of amorphous SiO₂, this ash has great potential to become a pozzolan material.

The control of combustion parameters, such as temperature, heating rate and soaking time are the main factors that determine the amount and structural state of SiO₂ in elephant grass. Cordeiro and Sales (2015) found that the pretreatment of metal oxide leaching method can increase the content of silicon oxide and reduce the burning loss of elephant grass ash. Within 28 days of curing, the use of 20% elephant grass ash cement substitutes would not change the compressive strength of 35 MPa concrete (see Figure 6). Nakanishi et al. (2014) used elephant grass materials from different origins (Napier and Cameroon) to produce elephant grass ash after being calcined at 700°C.

Bagasse ash burned after sugar production at a certain temperature is a promising pozzolanic material that can be used as an auxiliary material for Portland cement in mortar and concrete (Aprianti et al., 2015). The SiO₂ in the bagasse ash forms a new hydration product-calcium SiO₂ during the hydration reaction with cement, which improves the performance of concrete.

From the comprehensive experiment conducted by Ganesan et al. (2007), when the cement replacement rate of bagasse ash is 20%, the improvement on the compressive strength, split tensile strength, water absorption, permeability, chloride ion diffusion and resistance to chloride ion permeability of the concrete has achieved the most ideal effect. On the contrary, Rukzon and Chindaprasirt (Rukzon and Chindaprasirt, 2012) proposed that when the mixed amount of bagasse ash is 30%, the material’s resistance to chloride ion penetration can be further improved. The incorporation of bagasse ash increases the pozzolan hydration reaction and the generation of hydration products which reduces the Ca(OH)₂ in the concrete, and significantly improves the concrete’s resistance to chloride ion penetration. In addition, the particles of bagasse ash are finer than Portland cement, and the surface area of the reaction is increased while the water absorption is increased.

The difference in the results of these studies may result from the different craft and material selection for the production of bagasse ash. However, it can be proved that the incorporation of bagasse ash can significantly improve the performance of concrete.

The residue produced by burning wood waste such as sawdust, sawdust, and bark is called wood waste ash. Burning wood waste will produce an average of 6–10% wood waste ash (Siddique, 2012). According to reports, about 70% of wood waste ash is landfilled, 20% is used as a soil supplement, and the remaining 10% is used for other purposes including construction materials, metal recycling and pollution control (Siddique, 2012). As a building material, the sum of three oxides SiO₂, Al₂O₃, and Fe₂O₃ in wood waste ash exceeds the minimum limit of fly ash. In addition, the chloride contained in wood waste ash is far below the required standard, and the other obtained parameters are almost all conducive to the performance of coagulation (Ramos et al., 2013).

The physical and chemical properties of wood waste ash will be affected by factors such as tree species, origin, and burning method. In terms of concrete performance, the partial replacement of cement by wood waste ash would adversely affect concrete slump. As the content of wood waste ash increases, the water absorption capacity of concrete increases, but the strength of concrete decreases slightly. It was reported that the concrete can reach the ideal state, when 20% of cement is replaced by wood waste ash (Siddique, 2012).

Wood waste ash could be used as part of the cement substitute material in mortar. The compressive strength decreases with the increase of wood waste ash content, but it increases with the extension of the curing time (Ramos et al., 2013). The carbonization depth of wood waste ash cement mixture is greater than that of Portland cement mixture, and the carbonization effect increases with the increase of wood waste ash content. This may be due to CH reduction and consequent pH reduction (Carević et al., 2019; Carević et al., 2021).

In addition, wood waste ash can effectively inhibit the harmful expansion caused by the alkali-silicon reaction whilst the increasing content of wood waste ash can reduce the expansion rate. However, due to delayed hydration of free and dead-burned CaO and MgO, the ash content greater than or equal to 20% will lead to severe swelling (Ukrainczyk et al., 2016). Compared with ordinary concrete, the concrete containing wood waste ash has higher corrosion resistance when exposed to monoacid solution. When the concrete is under dibasic acid solution corrosion, the situation is just the opposite. The mineral admixture mixed with 20% wood waste ash and 80% fly ash can significantly improve the chloride ion diffusion resistance of concrete at the level of 25% cement substitution (Cheah and Ramli, 2011).

The annual production of bamboos all over the world is about 20 million ton, mainly in Asia and Latin America. Bamboo (see Figure 7) with the advantages of good mechanical properties, high flexibility, fast growth, light weight, and low purchase cost, is a promising building material (van der Lugt et al., 2006).

In the 1980s, Ramaswamy et al. (1983) verified the possibility of bamboo fiber blending into concrete. Compared with ordinary concrete, the impact strength of bamboo fiber concrete is effectively improved and the shrinkage characteristics are significantly reduced. Xie et al. (2015) produced a cement paste mixed with bamboo fiber for performance tests. With the increase of time, the accumulation of Ca(OH)₂ on the surface of the bamboo fiber is increased, which significantly reduces the fracture performance of the composite material. The bamboo fiber concrete shows a sufficiently high stability and the optimal fiber content is 16%.

Bamboo leaves can be calcined at 600°C to make bamboo leaf ash, which contains 78.7% of SiO₂. Tested by the volcanic ash activation method, bamboo leaf ash is a highly active volcanic ash (Frías et al., 2012). Bamboo leaves activated at 600°C and controlled holding time of 1.2 h can prepare a pozzolan suitable for the future production of type II/A cement (Frías et al., 2012). With the increase of bamboo leaf ash content, the demand for water in the mixture increased. As the curing age increases, the increase of bamboo leaf ash content would increase the compressive strength, porosity and water absorption, but decrease the bulk density of the cementitious mortar (Umoh and Odesola, 1979).

Bamboo also has the potential to replace steel bars to reinforce concrete structures. Through a series of performance tests, the bamboo slat concrete (see Figure 7) shows good performance. Treating bamboo as a reinforcement material, the bearing capacity of the beam could also be significantly improved (Agarwal et al., 2014).

Straw containing the properties of volcanic ash is a multi-purpose renewable biological resource. In addition, most straws are rich in crude fibers that could be used to improve the performance of concrete.

Rice husk ash by burning rice husk which not only is low cost, but also has good pozzolanic effect and micro-aggregate effect, can be used to improve the strength and durability of cement matrix and concrete (Zheng et al., 2018).

With the increase of rice husk ash content, the initial setting and final setting time of the cement mixture are prolonged. When rice husk ash is ground for 45 min and the content is 10%, the setting time of the mixture is close to that of cement (Wang et al., 2017).

For concrete, the addition of rice husk ash can significantly improve the internal structure of concrete and make it more compact, thereby enhancing the mechanical properties, frost resistance, chloride ion penetration resistance and acid erosion resistance of rice husk ash concrete (Zhang, 2015). In terms of mechanical properties, rice husk ash fine particles can fill the internal pores of concrete, whilst the rice husk ash contains reactive amorphous SiO₂, which will react with Ca(OH)₂ generated after cement hydration to increase the strength of concrete. This activity is mainly reflected in the later stage of cement hydration (Wang et al., 2017). When the mixing amount of rice husk ash reaches 20%, the reinforcement effect of rice husk ash on concrete is the strongest (Zhang, 2015).

In terms of durability, the concrete mixed with rice husk ash can effectively resist damage during freeze-thaw cycles, and the relative dynamic elastic modulus can be basically guaranteed to be greater than 60% at 200 freeze-thaw cycles (Zhang, 2015). Rice husk ash fills the gap among cement particles and fly ash particles, increasing the density between the powders, and improving the resistance to chloride ion penetration of concrete (Wang et al., 2017). When the concrete mixed with 20% rice husk ash, the anti-chloride ion penetration performance of the concrete is exerted to the best (Zhang, 2015). Mixing rice husk ash can also effectively enhance the acid resistance of concrete, whose compressive strength loss rate, mass loss rate and thickness loss rate are all lower than the ordinary concrete specimens. Similarly, when the rice husk ash content is 20%, the resistance to acid erosion of concrete is the best (Zhang, 2015).

In addition, rice husk could also be used to replace natural fine and coarse aggregates in the mixture of structural lightweight concrete and heat-insulating concrete to achieve the purpose of insulation (Shafigh et al., 2014).

Corncob has the characteristics of uniform organization, suitable hardness, good toughness, strong water absorption and good wear resistance. Corncob ash by burning corncob under certain conditions exerts good pozzolanic properties (see Figure 7) whose content of SiO₂ is more than 65% (Shafigh et al., 2014).

As the content of corncob ash increases, the SiO₂ content in the mixture increases at the same time, therefore corncob ash concrete needs more water to make the mixture workable (Adesanya and Raheem, 2009a). In addition, as the content of corncob ash increases, the stability and setting time of cement increase, since the increase of corncob ash reduces the surface area of Portland cement, hinders the hydration process, and adversely affects the combination of Portland cement (Adesanya and Raheem, 2009b).

With the increase of corncob ash content, more SiO₂ can react with the lime produced during cement hydration, thereby producing more cementitious material, which helps to improve the compressive strength of the concrete (Adesanya and Raheem, 2009a). The best blending amount of corncob ash in concrete is 8%, and more than 10% may not be conducive to the development of concrete compressive strength, and the best curing time is 120 days (Augustine and Tiza, 2016). The specimens mixed with 6% corncob ash wear more slowly than other specimens, so is the sulfate resistance (Binici et al., 2008). This may be due to the formation of a good interface between the ash and the agglomerated matrix.

At the same time, corncob particles also have great potential as aggregates for lightweight concrete. Pinto et al. (2012) found that corncob concrete (see Figure 7) prepared with a ratio of 6:1:1 (corncob particles: Portland cement: water) shows acceptable mechanical properties. Compared corncob concrete with the concrete made of expanded clay, the density and thermal characteristics of corncob concrete are consistent with those of expanded clay concrete, except for the compressive strength. This may be related to the corncob particle size, component ratio and curing time. At present, corncob concrete is only suitable for non-structural applications, and further research is needed on how to improve the strength of corncob concrete.

Coconut shell is one of the most promising agricultural wastes and can be used as coarse aggregate in concrete production. Compared with palm shell (PKSC), the compressive strength of coconut shell concrete (CSC) is higher (see Figure 9). From an economic point of view, coconut shell is also more suitable as a substitute for concrete aggregate than palm shell (Olanipekim et al., 2006).

Theoretically speaking, coconut shells can replace natural coarse aggregates to improve the compressive strength of concrete. However, due to the shape, rough texture and excessive density of coconut shell, the strength of coconut shell concrete is difficult to exert. Therefore, the appropriate amount of coconut shell can be used to prepare workable concrete with good strength (Tomas and Ganiron, 2013).

Compared with ordinary concrete, coconut shell concrete has higher ductility and almost the same crack width under the initial cracking torque with corresponding reinforcement ratio (Gunasekaran et al., 2013a). The flexural performance of coconut shell concrete is comparable to other lightweight concretes. Under normal conditions of use, the deflection and cracking characteristics of coconut shell concrete are equivalent to those of ordinary concrete (Gunasekaran et al., 2013b). At the same time, under the scanning electron microscope, the pore structure in the coconut shell (see Figure 10) appeared as a reservoir (Gunasekaran et al., 2014). Due to aging, the cracks tend to shrink, indicating that the cohesiveness between coconut shell and cement paste is good (Gunasekaran et al., 2012). Coconut shell aggregate concrete under intermittent curing has the highest strength, followed by full water curing, and air-dried curing.

Research shows that fly ash partially replacing cement, coconut shell replacing coarse aggregate, and cullet glass sand replacing fine aggregate could be used to make economical and environmentally friendly concrete which could be used for lightweight structures (Phatak et al., 2014). Even though coconut shells show good mechanical properties in concrete, it is still necessary to conduct sufficient research on coconut shell concrete before practical application, especially in terms of durability (Reddy et al., 2014). Besides, the fiber extracted from coconut shell can be used in mixing cementitious mortar. The initial strength retention rate of coconut shell fiber is higher than other fibers after exposure to different media for a certain period of time.

Palm shell (see Figure 7) is discarded during the extraction of palm fruit (Olanipekim et al., 2006). Annually 4 million tons of oil palm shell produced by Malaysia which is expected to rise further by the year 2020. Since 1984, a lot of research has been carried out on palm shell as a lightweight aggregate. As aggregate, palm shell has very good wear resistance, which is about 80% lower than traditional coarse aggregates. In addition, the impact value and crush value of palm shell are low, indicating its high shock absorption capacity (Shafigh et al., 2014). Under scanning electron microscopy, it can be seen that there are a large number of micro-pores with a size of 16–24 μm on the outer surface of palm shell (see Figure 11).

For palm shell concrete, the different curing conditions will also affect the compressive strength of palm shell concrete. According to reports, the ideal curing condition is full water curing (Shafigh et al., 2011b). Under full water curing conditions, it can produce palm shell high-strength lightweight concrete with a 28 days compressive strength about 43–48 MPa and a dry density about 1870–1990 kg/m³. Palm shell high-strength lightweight concrete is very sensitive to insufficient curing, so at least 7 days of wet curing is required (Shafigh et al., 2011b). When reducing the water-cement ratio, increasing the palm hull content or reducing the cement content, the sensitivity of palm hull concrete under insufficient curing conditions can also be reduced (Shafigh et al., 2012a). Under all curing conditions, the compressive strength of palm shell concrete increases with age, but it is still lower than that of ordinary concrete. In general, the compressive strength of palm shell concrete is 49–55% lower than that of ordinary concrete (Mannan and Ganapathy, 2004) resulting from the smooth surface of the shell which will cause poor bonding between the palm shell and the cement matrix (Shafigh et al., 2012b). On the positive side, compared with ordinary concrete, palm shell concrete also has higher ductility and polymer interlocking properties (Alengaram et al., 2013). The ductility index of palm shell concrete is 2.8 times higher than that of ordinary concrete (Ahmmad et al., 2014). Therefore, palm shell lightweight concrete is more suitable for the application of structural seismic performance than ordinary lightweight concrete. To improve the bonding force between palm shell and mortar, Shafigh et al. (2011a) also proposed to crush palm shell to 0.15–8 mm particles, and used a screen to remove smaller-than-2.36 mm particles to prepare high-strength concrete.

The porous structure of palm shell provides palm shell concrete good thermal insulation properties (Shafigh et al., 2012a). Under heat treatment, the surface quality of palm shell concrete can be changed, and the expansion and contraction of the material can be reduced, thereby improving the dimensional stability and bio-resistance, permeability of palm shell concrete, and reducing the equilibrium moisture content (Yew et al., 2014). With the increase of heat treatment temperature and heat treatment time, the workability of palm shell concrete could also been improved (Yew et al., 2014).

Malaysian annual production of rubber seed is projected to be 1.2 million metric tons there by indicating that freely availability of this material can be used in concrete industry. Since rubber seed shell (see Figure 7) is lighter, Muthusamy et al. (2014) studied the possibility of using rubber seed shells as an alternative material as coarse aggregates in concrete. As the number of rubber seed shells increases, the performance of concrete gradually decreases. Besides the low density of the concrete, this performance degradation is also related to the inability of the cement slurry to cover the aggregate uniformly. About 10% of the rubber seed shells are suitable as coarse aggregates for concrete preparation. In addition, Luhar et al. (2019) used fibers extracted from waste rubber tires to replace the sand in the mixture, which opened a new development path for the development of rubber-based concrete.