**3. Results and Discussions**

#### *3.1. Vegetal Ashes and Cement*

EDS analysis led to the quantitative elementary chemical composition of CCA and SSA presented in Table 3. As it can be seen in the cement composition, Si, Al, Ca and Fe are the predominant chemical elements. Compared with cement, if we look at the composition of the vegetal ashes, it can be observed that Si lacks the B quality of SSA, Al and Fe are present only in the B quality of CCA, and Ca content is a little smaller in the SSA at A and B quality and in a much smaller rate in the CCA at A and B quality. If we look to the other two elements present in cement, namely K and S, it can be observed that the K content is much higher (around 20–23 times), whereas the S content is quite similar or a little smaller in CCA and it lacks in SSA. In contrast to the cement, in both vegetal ashes supplementary elements appear, namely small rates of C, Mg, and Cl, and in CCA, S and P.

In a comparative analysis between CCA and SSA compositions, the following observations can be drawn:


• Mg content is almost double in SSA than in CCA, whereas the Cl rates does not differ significantly between them.


**Table 3.** CCA, SSA and cement elementary chemical composition, quantitative identification.

Scanning electron microscopy (SEM) images of A- and B-quality ashes of corn cobs and sunflower stalks are presented in Figure 3, at the 50 μm scale. SSA and CCA particles are bigger and less compact than the cement ones and present an agglomerated aspect (in Figure 3, some examples are marked). The aspect of SSA at B quality is more compacted than that of SSA at A quality. In the case of CCA, the aspect of the two variants has no big differences. The agglomeration of particles can be due to the high content of K, and it is in agreement with the results observed by Shakouri et al., 2020 [28], and Kamau et al., 2016 [5]. According to Shakouri et al., 2020 [28], the K content depends on the plant species and on the fertilizers used in crops.

In Figure 4 are presented the distributions of the main identified elements through EDS analysis from Table 3, on a 0.01 mm2 area, highlighting the main components of the A- and B-quality ashes of corn cobs and sunflower stalks and their distribution on the conglomerate. Beside the general oxide mass, a few small compounds based on Si or Cl can be observed.

#### *3.2. Composite Specimens Properties*

3.2.1. Chemical Composition Analysis

In Table 4, the chemical composition of the studied concretes, according to EDS analysis, is presented from the mass and atomic percentage points of view.


**Table 4.**

Chemical

composition

 of the studied concretes, according to EDS analysis.

**Figure 3.** SEM images of CCA, SSA and cement at the magnification rate of 50 μm.

**Figure 4.** The distribution of the main identified elements in CCA and SSA, identified by EDS analysis, at the magnification rate of 80 μm.

The silicon content can be observed to be increased in almost all concretes with vegetal ashes than in the RC; only in SSA2B is it very insignificantly decreased. The carbon level is significantly decreased (up to 50%) in all mixes with vegetal ashes, except for SSA1A and SSA2B, which have very close values to RCs. The Calcium content is quite similar in CCA1A and CCA1B to that in RCs, being around 2–3% higher in CCA2A, CCA2B, and SSA1A, being almost 3% smaller in SSA2A, and it lacks in SSA1B. Aluminum levels are very close to that of RC in almost all mixes with vegetal ashes, except SSA2A and SSA1B, where they are almost four times higher. Regarding the Iron, there are no significant differences between the mixes. SSA2A contains a very small rate of Sodium, and in CCA2A, CCA2B, SSA2A, SSA1B, and SSA2B there exists small amounts of Potassium that are directly related to the strength performances of the concrete, even if its rate is very small [36].

In Figure 5, the distribution of the main identified elements in the studied concretes is presented at a magnification rate of 400 μm. In SSA1A and CCA1A, the O, Si, Ca, C, Al, and Fe elements are visible, and the greener zones containing more Si (from the aggregates composition), and the bluer ones (given by the Ca element color from the legend of each image) are correspondent to calcium-silicate-hydrate (CSH) resulting from the pozzolanic reaction within the matrix. The C content contributes to the decreasing strength of the concrete. In SSA2A and CCA2A, the distribution of O, Si, Al, Ca, Fe, C, and K elements can be seen, according to the attached legend of attributed colors to each element, the bluer zones corresponding to the CSH content. The difference between those two mixes is the Na element present in SSA2A. In SSA1B, the Si, Al, and Fe content is more pronounced than the other elements, O, C, and K. In SSA2B, Al, Si and Ca are predominant. Additionally, the K element is quite visible. In CCA1A and CCA2B, the CSH zones are visible in the left half and bottom half of the images, respectively.

**Figure 5.** The distribution of the main identified elements in the studied concretes, at the magnification rate of 400 μm: (**a**) CCA concretes, (**b**) SSA concretes.

#### 3.2.2. Compressive Strength

The compressive strength determined at 28 days is presented in Figure 6. As it can be seen in the graph, CCA1A, CCA1B, and SSA1A registered similar values for compressive strength, being around 14.7 ± 0.3% smaller than RC. Therefore, in the case of CCA, the qualities of the two types of ash led to almost the same result, and between CCA and SSA at A quality there are no big differences. CCA1A and CCA2A registered the best results between the ash mixes due to the Si content. SSA1A good result can be attributed to the smaller content of C in SSA than in CCA. Instead, B quality of SSA determined the biggest decrease in compressive strength than RC, with 42% in the case of concrete compositions with 2.5% ash. From the mixes with 5% vegetal ash, the best result was registered by SSA2A, followed by CCA2B and CCA2A, with 25.30%, 27.80%, and 30.00% smaller values than RC.

In conclusion, regarding the compressive strength measured at 28 days, for the 2.5% replacement rate of cement, the best result was given by the A quality of CCA, and for the 5% replacement rate, by the A quality of SSA.

Regarding the compressive strength evolution from 28 days to 3 months, according to Figure 6, it can be observed that the biggest increase was achieved by SSA2B, namely 14.74%, with around 12.5% bigger than that of RCs. SSA1A also had a good evolution, with a 11.40% increase, but with around 1% under the RCs. SSA2A, SSA1B, and CCA2A also registered good results, scoring 9.40%, 8.30%, and 8.10% more than the initial compressive strength. CCA at B quality instead determined a decrease in the time of this parameter, with around 3.5%.

**Figure 6.** Compressive strength values at 28 days and 3 months age (N/mm2).

If the value obtained at 28 days and evolution in time are considered together, the best result was obtained by SSA1A, which had a decrease in compressive strength of 14.70% than RC and a strength gain in time very close to that of RC's, being only 0.9% smaller.

#### 3.2.3. Flexural Tensile Strength

The vegetal ash used in the concrete composition led to a decrease in the flexural tensile strength (Figure 7). CCA1A and SSA2B registered the best results regarding this parameter, being only 0.95% and 1.30% smaller than RC. This can be attributed to the Si presence in CCA, and the higher content of Ca plus a much smaller C content in SSA, respectively. In CCA group, the ash at B quality led to smaller values than that at A quality, at around 12%. The SSA group registered more inferior results than the CCA one, especially in the case of using A-quality ash. SSA1A and SSA2A achieved a smaller flexural tensile strength with 29.5% and 33%, respectively, than RC. SSA at B quality obtained better results for this parameter than that of A quality, especially in the case of the 5% replacement rate.

**Figure 7.** Flexural tensile strength values [N/mm2].

In conclusion, the A-quality ash from corn cobs led to the best values among the studied mixes with vegetal ash, regarding the flexural tensile strength.

### 3.2.4. Splitting Tensile Strength

The corn cob and sunflower stalk ash had significant negative effects on the splitting tensile strength of the RC (Figure 8). The best result among the mixes with vegetal ash was obtained by SSA1A, being around 31% smaller than the RC's results, followed by CCA2A, with a decrease of 39.4%. The best result for SSA1A can be attributed to a combination of elements: the higher Ca content in SSA at A quality for all ashes, its smallest C content, and Si presence. CCA1A, CCA1B, and SSA2A registered smaller values, with 43%, 44%, and 46.6%, respectively, than the reference. CCA2B obtained the biggest decrease, at around 54%.

**Figure 8.** Splitting tensile strength values [N/mm2].

#### 3.2.5. Resistance to Repeated Freeze–Thaw Cycles

After 50 freeze–thaw cycles, RC registered a 34.80% decrease in compressive strength (Figure 9). A 2.5% replacement rate of cement with SSA of A quality led to a significant improvement of this parameter, this composition obtaining a 11.90% decrease only compared with control samples. The SSA1A performance can be attributed to a combination of chemical elements of SSA of A quality than the other ashes, namely the highest Ca rate, the smallest C content, and Si presence. The 5% B-quality ash of CCA and SSA also had positive effects, these mixes registering a 13.3 ± 0.2% decrease. CCA1B also obtained better freeze–thaw resistance than RC, but in a smaller rate than CCA2B and SSA2B, respectively; therefore, the conclusion can be drawn that a higher replacement rate of cement with B-quality ash for both plant cases led to improved freeze–thaw resistance. The good results obtained by CCA1B and CCA2B can be due to the Al and Fe content in the CCA at B quality. Regarding the A-quality ash, a higher replacement rate in the case of CCA led to an improvement of about 9%, but in the case of SSA led to a significant decrease from 11.9% up to 32%. Only CCA1A registered a smaller resistance, at around 2% (Figure 9).

#### 3.2.6. Resistance to HCl Chemical Attack

All compositions with vegetal ash registered an improved resistance to chemical attack of 18% HCl solution, with more than 29.50% (Figure 10). The best results were obtained by SSA1B, CCA1A, SSA2B, and SSA1A, registering higher values with 45.5 ± 1.5% than RC. The good results obtained by CCA1A can be attributed to the highest Si content of CCA at A quality, whereas those of SSA1B and SSA2B to the combination of high content of Ca and small C content of the SSA at B quality. CCA2A and CCA2B registered a 40 ± 1% improvement than RC, whereas the results for CCA1B and SSA2B were 33% and 29.50%, respectively.

**Figure 10.** Mass loss due in case of chemical attack of 18% HCl solution [%].

In CCA group, A quality ash led to better results than the B one, and an increased replacement rate of this ash type led also to improved resistance. In SSA group, B quality ash registered better values for resistance to the action of 18% HCl solution than the A one, but the improvement decreased as the replacement rate increased (Figure 10).

In conclusion, 2.5% of A quality of CCA and B quality of SSA led to the best values of resistance to 18% HCl solution (Figure 10).

### **4. Conclusions**

This study aimed to analyze the effects of two different qualities of ashes obtained by corn cobs and sunflower stalks free burning, on some properties of a micro concrete, by applying 2.5 vol.% and 5 vol.% replacement of the cement. The analyzed properties were compressive strength, flexural tensile strength, splitting tensile strength, freeze–thaw resistance, and resistance to chemical attack of hydrochloric acid solution:


Most concrete and reinforced concrete applications are based on the idea of improving the strength properties. However, the role of using corn cob and sunflower stalk ash as partial substitutes for cement is not to improve the mechanical strength, but to improve other properties of concrete, such as resistance to repeated freeze–thaw cycles or resistance to various chemical agents, properties that make them more economical in terms of production costs and more environmentally friendly by reducing the amount of energy incorporated in their production. If engineers stop thinking about strength, it will not be difficult to find many other areas of application for these new materials, as well as improving them at the same time.

Regarding applications for concrete with the studied vegetal ashes, the chloride content of CCA and SSA limits their use to unreinforced concrete applications or to applications with non-corrosive reinforcement or non-structural ones.

As future directions to be researched, the effects of bigger rates of CCA and SSA as cement replacement or as additive material on the freeze–thaw resistance can be studied; in general, in this study, bigger improvements were observed as the vegetal ash content increased.

**Author Contributions:** Conceptualization, C.M.G., A.A.S, ., R.M., N.C. and B.V.S, .; formal analysis, C.M.G.; resources, A.A.S, ., R.M. and B.V.S, .; investigation, C.M.G., N.C. and A.A.S, .; methodology, C.M.G. and N.C.; writing original draft, C.M.G., N.C.; writing review, C.M.G. and R.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Acknowledgments:** Special thanks to the Sika Romania representatives for their technical support and for providing the necessary additives in order to accomplish this research.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

