## 2 Agro-Wastes as Concrete Additives
### Waste from Rice Husk  
Every year, a substantial amount of rice husk is disposed of, posing a lot of problems for rice producers [58]. The burning of husks contributes to air pollution and their dumping in landfills occupies land areas [59]. Additionally, the discarded husks lead to the production of methane by microorganisms, leading to more global warming [60]. Some countries utilize rice husks as a source of energy in power plants [61]. The by-product of this burning is ash, which researchers have used as an additive compound in cement for concrete. This is due to the presence of amorphous silica and carbon in the ash [62]. These supplementary materials highly improve the compressive strength of suggested foam concrete by up to 70% [63]. Other authors have also disclosed that the addition of 5-20% husk ash to foamed concrete increases energy absorption under impact. The workability of the foamed concrete increased by 5% with a 10% sugarcane filter cake ash, but this property declined at higher levels (15% and 20%) of the ash [64]. They attributed this to the low inter-particle friction and high porosity and water absorption by the ash compared to the cement. [PERSON] et al. noted a lower thermal conductivity of the samples resulting from the inclusion of the additive [64].  
Recently, [PERSON] et al. [65] investigated the effects of micronized rice husk and ground granulated blast furnace slag, two by-products of agricultural waste, when used as additives in concrete. These materials contain high value of silica. Interaction of this type of silica with hydration products like concrete improves the formation of additional calcium-silica-hydrate gel, resulting in improved strength and durability of the concrete. The authors tested 10, 20, and 30% micronized rice husk silica in concrete and the results showed that the maximum flexural strength was 5 MPa and the split tensile strength was about 4.5 MPa more than the control sample (11% increase) (Figure 6). They ascribed this improvement to the presence of pore-filling calcium silicate hydrate, while they reported that 20% additive provided the best durability result, overall.  
In a very recent study, [PERSON] et al. [66] compared the effects of three different ashes from waste agriculture crops (olive, sugarcane leaf, and rice husk) to partially replace elements in cement and concrete. They claimed that a 50% replacement of the cement with a combination of 25% rice husk and 25% sugarcane leaf ashes could provide up to 155 MPa (for 28 days) for compressive strength, but this was not more than with the individual inclusion of the additives. In addition, the other combinations generally produced lower compressive strength values.  
In an interesting recently published paper by [PERSON] et al. [38], nano-powder derived from different wastes at very low percentages (1-3%) demonstrated a surprising improvement of up to 21% in compressive strength. This enhancement was achieved when rice husk (3%), sugarcane bagasse (3%), or cotton stalk (1%) ashes were used in the concrete as cement additives. They ascribed this improvement to the high pozzolanic impact due to the high content of amorphous silica in the additives.  
In summary, rice husk ash as a cement additive significantly enhances the compressive strength of foam concrete by up to 70% based on results reported by some researchers due to the existence of amorphous silica and carbon in the ash. Replacing 5-20% of the cement with rice husk ash has been decided as an accepted ratio. However, some other researchers do not agree to this and indicates that the maximum should be 3%.