# Evaluation of Rice Straw Ash as a Pozzolanic Addition in Cementitious Mixtures
## 1 Introduction  
One of the most important challenges in the construction industry is to reduce environmental impacts, produced principally by widespread ordinary Portland cement (PC) use, given its associated large carbon footprint. For the production of 1 kg of PC, approximately 1 kg of CO\({}_{2}\) is emitted [1]. The latest research suggests that around 8% of global anthropogenic CO\({}_{2}\) is due to PC fabrication [2]. The use of Supplementary Materials from industrial waste can contribute to reducing the carbon footprint. The most widely used Supplementary Materials are fly ash (FA), blast furnace slag (BFS), silica fume (SF), among others [3, 4, 5, 6, 7]. Given the pozzolanic/hydraulic reactions of these materials in PC mortars and concretes, mechanical behavior and durability properties are enhanced. Other Supplementary Materials used as substitutions or additions in PC-based mixes are ashes from agricultural waste. In the last few decades, different research groups around the world have studied using such wastes, but their application is still scarce. Agricultural waste mainly comprises straw (leaves and stems) and fruit shells, while other types include bagasse, cobs, and pod seeds and shells. With proper selection and treatment, this waste can be used as a PC substitute in the manufacture of suitable building materials. Controlling biomass combustion is key. Biomass is produced in large quantities worldwide. However, due to their ill-defined reuse and poor management in open-aircombustion or kiln combustion processing treatments, the obtained biomass ashes offer no optimal usage. Nowadays, much interest is being shown in biomass as a fossil fuel replacement because of its high calorie value [8]. Forecasts of the use of bioenergy indicate that it is likely to increase in the future, but the policy markets of each country complicate predicting its future use [9]. Biomass reuse can be considered a CO\({}_{2}\)-neutral or quasi-neutral process, as the carbon released to the atmosphere during combustion is previously fixed by the plant during the photosynthetic process. The ash produced in selected cases has pozzolanic properties and can, therefore, be valuable in the construction industry. Rice straw (RS) is waste from rice production, the annual production of which is 67 million tons worldwide [10]. This waste is difficult to manage, especially in natural environments such as wetlands where it usually develops. In the Albufera Lagoon of Valencia (Spain), around 75,000-90,000 tons of RS are produced every year [11]. Such waste is generated in a very short time and at a given time of the year, and is traditionally subjected to burning in the same cultivation field. Farmers believe that this practice helps to destroy fungal spores, such as _Pyricalaria oryzae_, as well as some bacteria and weed seeds. It also facilitates the re-application of certain nutrients to the soil. However, this practice not only poses serious health problems in the areas surrounding cultivation areas, but also considerable environmental damage. Straw burning is an important gas emitter, with emissions in the form of carbon dioxide (CO\({}_{2}\)), methane (CH\({}_{4}\)), nitrogen oxides (NO\({}_{\text{x}}\)), sulfur oxides (SO\({}_{\text{x}}\)), hydrocarbons, dioxins and particles of different natures. Their possible controlled burning and the valorization of the corresponding ash can be possible solutions to these problems. Many ashes from agricultural waste have proven their feasibility in the concrete industry as Supplementary Materials in the cement matrix and/or for manufacturing new building materials [12; 13; 14; 15; 16; 17]. RS is an agricultural waste formed by leaves, stems and roots composed of fibrous cellulose containing high silica content [18]. Some research works have focused on using RS ash (RSA) as a supplementary component in cements. [PERSON] et al. studied ashes from different RS parts and their chemical composition: rice leaf ash (RLA), rice leaf sheath ash (RLSA) and rice stem ash (RSA).