**1. Introduction**

The world's cement production has been estimated to around 4.1 billion metric tons [1]; it emitted around 2.3 gigatons of CO2 in 2019, this value being around 7% of total CO2 emissions [2], and it is expected to reach 4.83 billion metric tons in 2030 [3]. A solution for decreasing the CO2 emissions determined by the cement industry is its replacement by supplementary cementitious materials (SCMs). These SCMs can contribute to reduce the environmental impact and costs related to the cement and concrete industry, since they need less process heating and emit smaller CO2 levels [4], enhance sustainability, and improve some of the concrete's properties [5]. The SCMs category includes, among many others, the vegetal ashes obtained from the burning of different plants. A few examples of such vegetal ashes that were studied as SCMs in concrete composition are: rice husk ash [6–8], sugarcane bagasse ash [9,10], corn cobs [5,11,12], wheat [13,14], and many others. Previous studies showed that lignocellulosic ash containing significant rates of silica and alumina can develop pozzolanic activity in the presence of calcium hydroxide, leading to improved properties of concrete [15,16]. Maize and sunflower, important crops that have an annual

**Citation:** S, erb ˘anoiu, A.A.; Gr ˘adinaru, C.M.; Muntean, R.; Cimpoes,u, N.; S, erb ˘anoiu, B.V. Corn Cob Ash versus Sunflower Stalk Ash, Two Sustainable Raw Materials in an Analysis of Their Effects on the Concrete Properties. *Materials* **2022**, *15*, 868. https://doi.org/10.3390/ ma15030868

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 20 December 2021 Accepted: 18 January 2022 Published: 24 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

global cultivated area of around 178 × <sup>10</sup><sup>6</sup> ha [17], and 26.03 × <sup>10</sup><sup>6</sup> ha [18], respectively, represent significant and widely spread sources of lignocellulosic materials. Maize crops are cultivated mainly in China, USA, South Africa, and Eastern Europe, whereas sunflower crops are grown in southern South America, Southern Europe, South Africa, and South and European Russia [19].

Corn cob ash is a SCM studied quite intensively lately, as it has proven pozzolanic properties in various studies that have been conducted on the use of corn cobs as a cement partial replacement in concrete, but the results have been mixed. Adesanya, 1996 [20], and Binici et al., 2008 [21], showed that corn cob ash (CCA) determined less water absorption and improved resistance to sulfate attack. The studies led by Adesanya, 1996 [20], Adesanya and Raheem, 2009 [12], Kamau et al., 2017 [22], and Memon and Khan, 2018 [23], concluded that CCA determined a significant decrease in concrete compressive strength. Adesanya and Raheem, 2009 [12], found that CCA decreases the concrete slump, whereas Kamau et al., 2016 [5], found that it increases it. In 2012, Olafusi and Olutoge [24] studied the strength characteristics of corn cob ash concrete and concluded that the concrete did not reach its design strength after 28 days, and this depends on the pozzolanic activity of the corn cob ash. From the results of the various tests carried out by Mujedu et al., 2014 [25], it can be concluded that the combination of corn cob ash (CCA) and sawdust ash (SDA) is a suitable material for pozzolana, because it has a combination of more than 70% (SiO2 + Al2O3 + Fe2O3) to meet the requirements of this material. The compressive strength of concrete increases with curing time and decreases as the percentage combination of CCA and SDA increases up to 10% of ordinary Portland cement in concrete to obtain the greatest strength gain. Although the strength of CCA-SDA concrete is lower than that used as reference, it can still be used for general concrete projects where strength is not important, such as floors, mortars, and mass concrete [25]. According to Ahangba and Tiza, 2016 [26], when 10% CCA is used instead of cement, the cement setting time increases from 258 min to 277 min). Substitution beyond this range will reduce the strength of concrete and cannot be controlled. This kind of substitution can also be used in building walls and beam units to reduce the use of cement and its high cost. Analyzing the research of Owolabi et al., 2015 [27], it can be pointed out that the workability of fresh corn cob ash concrete decreases with the increases in corn cob ash content, its compressive strength of concrete decreases with increasing CCA replacement rate, but increases with increasing age of cure, and to obtain the best compressive strength of the concrete, it is recommended to replace 5% of the cement with corn cob ash. The studies developed on CCA concrete focused mainly on the mechanical properties of fresh and hardened material, and on the resistance to sulphate attack. Fewer studies were conducted on the hydration process and chloride resistance of this type of concrete. Shakouri et al., 2020 [28], studied the effects of CCA as a cement replacement between 3% and 20% in concrete, and they found that it negatively affects the cement hydration and decreases the compressive strength and the chloride ion permeability of the concrete. The durability of CCA concrete to chloride corrosion represents an important problem that can reduce infrastructure service life worldwide [28].

Sunflower stalk ash (SSA) is a SCM much less studied than CCA, to our knowledge. Akso ˘gan et al. (2016) [14] studied the compressive strength, abrasive resistance, and linear absorption coefficient of concrete made with barite, colemanite, and SSA as a cement replacement and they found that the optimum replacement rate was 2.5% of SSA. A higher rate of 5% of SSA increased the concrete resistance to 5% sodium sulfate solution during an 180 days test. They also found that SSA improved the concrete behavior during the freeze– thaw process [14]. Darweesh (2020) [29] studied the physical, chemical, and mechanical properties of cement pastes with SSA, and found that SSA increases the compressive strength, the CSH amount, and decreases the free lime content, and that 24 wt% was the optimum rate of cement replacement with SSA, with higher rates as 30 wt% having negative effects on the cement paste properties.

The aim of this study was to study the effects of corn cob ash and sunflower stalk ash obtained by two methods on the mechanical and some durability properties of the normal concrete. The study novelty consists of the comparative analysis of these two types of vegetal ashes, since to our knowledge very few studies were carried out on concrete with sunflower stalks, and we did not find any with which to compare directly their performances. For obtaining the CCA and SSA, the same methods were applied in order to have a clear view of the differences given by the two plants. The research implied the development of eight mixes with 2.5 vol.% and 5 vol.% cement replacement with CCA at A and B quality, and with SSA at A and B quality. The objective was to obtain a type of concrete with comparable performance to ordinary concrete, but adapted to the new requirements of cost, energy efficiency, and sustainability. The tests performed included compressive strength at 28 days and 3 months, flexural tensile strength, splitting tensile strength, and the resistance to the action of repeated freeze–thaw cycles and to the action of hydrochloric acid.
