# Eco-Efficient Cement-Based Materials Using Biomass Bottom Ash: A Review
## 1 Introduction  
More and more countries are now generating electricity from the direct combustion of biomass waste [1; 2]. Biomass combustion is considered an important source of energy today, producing 14% of the world's energy supply [3]. [PERSON] et al. [4] estimate the high availability of biomass at 220 billion tons per year. Other world studies estimate that by 2100, between 25% and 46% of energy from biomass combustion will be used [5].  
Different technologies are used for biomass combustion. [PERSON] et al. [6] showed the most appropriate technology for biomass combustion, the so-called fluidized bed, mainly due to its versatility in the type of fuel it consumes. The technology used influences the characteristics of the biomass ash produced, with different properties depending on the type of biomass used for energy generation.  
Two main types of ash are produced during energy generation process by biomass combustion: bottom ash and fly ash.  
Bottom ash includes the coarse fraction of ash produced in the grill, in the lower bed, and in the primary combustion chamber. Normally it is mixed with mineral impurities contained in the biomass fuel and slag that has not fully combusted, whereas fly ash is the ash fraction with a smaller particle size, carried by the flue gas, which is collected and precipitated in filters [7; 8].  
Biomass combustion is considered a clean energy without CO\({}_{2}\) emissions, therefore more plant facilities have been built in recent years [9]. This increase in biomass combustion power plants result in the amount of ash generated, without having a recycling alternative.  
There are different works that have studied the use of biomass fly ash (BFA) as incorporation in the manufacture of new construction materials. The use of BFA in the manufacture of mortars has been studied [10; 11; 12; 13; 14]; the use of BFA in cements, conventional and self-compacting concrete has also been studied with positive results [15; 16; 17].  
However, there are few studies in which biomass bottom ash (BBA) is incorporated into construction materials; its composition is very heterogeneous [7] so its application is more restricted. [PERSON] et al. [11] reported that the use of BBA in cement, mortar, or concrete is feasible. [PERSON] et al. [18] studied the mechanical behavior of mortars manufactured with BBA, both reporting interesting results when analyzing the influence of certain parameters on mechanical behavior and defining trends and relationships among the different properties. Other authors have studied the physical, chemical, and mechanical properties of conventional concrete made with BBA [19]. [PERSON] et al. [20] showed the possibility of reusing BBA mixed with natural aggregates and recycled aggregates for cement-treated granular materials in road bases and sub-bases, and the possibility of stabilizing soil by reducing the use of lime [21].  
The positive results obtained in the different works reveal the possibility of using BBA as an eco-efficient material, eliminating the negative impact of this waste on landfill.  
In this work, a review of previous studies in which biomass bottom ash has been applied in cement-based materials, mortars, and concrete was analyzed. The different application possibilities were studied, from the characterization properties of different typologies of BBA, to the revision of the possibilities of applying them to these cement-based materials, including mechanical properties, durability, etc.