1. Introduction
The need to develop new binders with lower or no clinker content and a low carbon footprint is a reality for today’s world if we intend to fight climate change [
1,
2]. Supplementary cementitious materials (SCMs) availability may be limited, and new SCMs must become part of these new types of cement. For example, the closure of coal-fired power plants in many countries and less coal use significantly lower fly ash availability. The production of these fly ashes is reported to be of the order of 330 Mt/year [
3], which represents a fly ash/cement ratio of 0.08 (if we accept the estimation of 4100 MT/year world cement production reported in 2023 [
4]). In this way, the fly ash/cement ratio is barely 0.08 t/t. Another waste material that is widely used in cement manufacturing is blast furnace slag (BFS). The demand for ferrous products has continued to increase in recent years, and in 2022, the demand for steel was 1790 Mt/year, with a projection of 2030 Mt/year by 2030 [
5]. BFS production in 2022 was 408 Mt, with a projection of 497 Mt by 2035 [
6]. However, the slag/cement ratio is only 0.10 t/t. The valorization of silica fume (SF) from the silicon and ferrosilicon industry is very interesting and effective due to the high pozzolanic reactivity of SF. However, its annual production is estimated at about 3 Mt because 1 t of SF is produced for every 3 t of manufactured elemental silicon [
7] and given that silicon production is 9 Mt/year [
8].
Other SCM alternatives are natural materials, such as volcanic rocks, diatomaceous earth, or clays. However, there is a set of interesting resources from the point of view of binding materials in construction: agricultural/forestry waste [
9]. One of the most widespread and highest-producing crops is
Zea mays L. (maize or corn)., world corn production is approximately estimated at 1137 Mt (data from 2019) [
10]. One part of the biomass residue is the corn cob, which remains after removing grains from spikes. Another important waste is the plant itself (stems, sheat leaves, leaves, roots). The corn cob and the rest of the plant are residual materials that can be energetically valorized by generating energy from oxidation by organic matter combustion. As the CO
2 emitted during combustion has been previously fixed by the plant, this process is practically neutral in carbon footprint terms (
Figure 1). Energy recovery can be carried out by combustion, which results in biomass ash that may have suitable properties to be used as an SCM.
Table 1 summarizes the theoretical amount of ash produced from selected crops. These crops are the world’s most popular ones: rice, wheat, sugarcane, and corn. The biomass with the highest proportion of ash is rice husk (22%), while rice straw, maize straw, and wheat straw fall within the 10–15% range. Taking into account the proportion of residue per food unit and the proportion of ash in the biomass, the theoretical quantities of ash that can be produced worldwide have been calculated. Of the seven biomasses, that which could produce the biggest amount of ash is corn straw, with 154 Mt/year. However, the corn cob only has an ash generation potential of 4 Mt/year.
Many reports [
11] have been published on the use of corn cob ash (CCA) in the formulation of cements and concrete. This ash (CCA) has shown high pozzolanic reactivity due to the presence of amorphous silica, while the sum of oxides SiO
2, Al
2O
3, and Fe
2O
3 exceeds 70%, which is one of the conditions of standard ASTM C618 [
12] to be considered pozzolanic material.
There are fewer studies on corn straw ash (CSA) [
13]. De Lima and Cordeiro [
14] published the physico-chemical properties of ash obtained at a temperature of 600–650 °C and observed the high amorphicity of ash in mineralogy terms. In addition, 62.5% of SiO
2 and a high percentage of K
2O (17.2%) were determined. They observed how the contribution to the mechanical compressive strength (Rc) of mortars with 10%, 20%, and 30% cement replacements allowed slightly higher Rc values to be obtained than in the control mortar. When CSA is washed with acid, silica content increases (94.2%), all of which is amorphous: the Rc of mortars was between 10% and 50% higher than in the control after 28 curing days. Qi et al. [
15] studied the effect of calcination temperature (500, 700, and 850 °C) on the pozzolanic reactivity of corn straw stem (stalk) ash (MSSA). In this case, the percentage of silica was between 31% and 38%, while K
2O fell within the 21–28% range. Large amounts of calcium, magnesium, and chlorides were also found. The most abundant mineral was sylvite (KCl). However, after washing ashes with water, calcite and quartz were the majority minerals, along with amorphous silica. The authors concluded that the sample with the highest reactivity was that generated at 500 °C. Raheem et al. [
16] studied several CSA samples obtained by burning stalks in an open steel container. In this case, the average SiO
2 content was high (64.26%), unlike that found in [
15]. The following contents were observed: 9.45% K
2O, 6.23% CaO and 5.28% Al
2O
3. The authors concluded that the 10% replacement of cement with CSA was the optimum one for interlocking paving stone in compressive strength terms, and the 10–25% replacement range was optimum regarding abrasion. A study was also found about corn leaf ash (CSLA) obtained at three different combustion temperatures: 500, 700 and 850 °C [
17]. The SiO
2 content fell within the 67–74% range, and the sum of SiO
2 + Al
2O
3 + Fe
2O
3 ranged from 70% to 78%. Pozzolanic reactivity was evaluated by taking electrical conductivity measurements, which revealed that the sample obtained at 500 °C was the most reactive one. The reactivity of the sample obtained at 850 °C had decreased despite having more SiO
2 because silica crystallization occurred in the form of cristobalite. The reaction of CSLA with Ca(OH)
2 solution led to the formation of gismondine and calcium silicate hydrate (C-S-H).
Previous reports have indicated studies into CSA obtained under controlled laboratory conditions. Due to the availability of this biomass in low-income environments (communities in developing countries), the aim is to study the ash obtained by the auto-combustion of the biomass to evaluate the properties of the CSA to be used in PC matrices.
2. Materials and Methods
The corn straw (CS) samples were acquired from two commercial suppliers (from Pabellón de Arteaga, Aguascalientes, Mexico). The “rastrojo” term is used in Mexico to define this type of biomass. CS contains mainly leaves, leaf sheaths, and stems of the plant, but also other parts [
18], such as tassels (male inflorescences), roots, silks (stigmas), leaf blades, and corn cobs.
Figure 2 represents the most important maize plant parts.
The two commercial “rastrojo” samples were named R1 and R2. These biomass samples went from 1 mm to 3 cm and came in the form of dry chips. The auto-combustion of R1 and R2 was carried out in a cylindrical steel container.
Figure 3a shows the furnace dimensions, and
Figure 3b outlines the layout of the material inside the oven. The biomass was compacted by hand and a PVC pipe was placed so that, after removal, a chimney (loophole) was obtained for rapid and effective biomass combustion. Two thermocouples (T1 at the bottom and T2 at the cylinder’s mid-height) were used for monitoring changes in temperature.
Figure 4 shows the combustion process. After combustion and cooling (1 day) the resulting ashes, ashes were stored in airtight containers. The ashes obtained from R1 and R2 were CSA-R1 and CSA-R2, respectively.
Two other additional samples were collected (cut from fresh plants) from a crop field in Aguascalientes City (Mexico) and corresponded to the leaf and stem parts (samples MX-L and MX-S, respectively).
A furnace (model RHF 15/3 Carbolite, Hope Valley, UK) was used to determine the loss on ignition (LOI) of ashes and to prepare the selected samples from corn leaves. The thermogravimetric (TG) characterization of the biomass samples (R1 and R2) and ashes (CSA-R1 and CSA-R2) was carried out in a TGA-850 (Mettler-Toledo, Metller-Toledo S.A.E, Cornellà del Llobregat, Spain) using 70 µL alumina crucibles at a heating rate of 20 °C/min and an air gas flow of 75 mL/min. The chemical composition of cement and CSAs was determined by X-ray fluorescence with Philips MagiXPRO equipment (Philips Analytical, Almelo, The Netherlands). The mineralogical composition of raw materials was determined by XRD (Philips diffractometer PW1710, Philips Analytical, Almelo, The Netherlands) with Cu Kα radiation, 40 kV and 20 mA, from 10 to 80° (2Θ). SEM-EDX observations were performed by a JEOL JSM-6300 model (Hertfordshire, UK). Samples were covered with carbon.
To prepare mortars, CEM I 52-5R (OPC, Lafarge-Holcim, Puerto de Sagunto, Spain) and siliceous sand (Caolines Lapiedra, Liria, Spain) were used. The 10% replacement by mass of OPC with CSA-RM was carried out to have a sufficient quantity of ash to prepare the mortars for the tests at 28, 56, and 90 days of curing. This sample was prepared by mixing CSA-R1 and CSA-R2 in a 50/50 proportion (“M” means a mixture of ashes R1 and R2). CSA-RM was ground in a Gabrielli Mill-2 equipment (Gabrielli Technology, Calenzano, Italy) using 2-cm diameter alumina balls, 200 g of ash, and 20-min grinding. The particle size distribution of CSA-RM before and after grinding was determined by Mastersizer 2000 equipment (Malvern Instruments S.L., Malvern, UK) with samples suspended in deionized water for measurements (mean particle diameter d
m, and percentiles d(0,1), d(0.5), and d(0.9)). Mortars were prepared with a cement/sand ratio of 1:3 and a water/cement ratio of 0.5 according to standard EN-196-1 [
19]. For comparison purposes, limestone filler (supplied by Cementval, Puerto de Sagunto, Spain) was also used to replace 10% of CEM I 52.5R. Fresh mortars were characterized by workability (flow table spread; FTS) according to UNE-EN 413-2:2017 [
20]. The specimens in the mold were stored in a moist atmosphere and, after 24 h, were demolded and stored in a saturated lime solution. Specimens (4 × 4 × 16 cm) were tested in the flexural (3 values) and compressive (6 values) modes according to [
19].
4. Conclusions
Corn cultivation is the one that presents the greatest potential in terms of production of pozzolanic material from biomass, and it is estimated that the annual amount of ash could be more than 150 MT. The ash obtained from corn straw presents silica content close to 60% by mass and is a good candidate to play a pozzolanic role in OPC blends. The auto-combustion of this biomass produces corn straw ash (CSA), where silica is maintained in the amorphous phase because the temperature does not reach 750 °C. The total mass loss during the auto-combustion was close to 80%. The silica in CSA shows a certain reactivity level and is appropriate for blending with cement in mortars and concrete. XRD studies have shown that some soil impurities are found along with silica and these impurities can be easily removed by washing with water. CSA must be ground before being used to reduce water absorption and increase reactivity. SEM studies on ash particle morphology have shown that after combustion, the original cellular structure remains, and the resulting skeletons are composed of silica. The grinding reduced the particle size to a mean diameter close to 17.5 µm. Ashes showed adequate pozzolanic reactivity because, at 28 and 90 curing days, the compressive strengths of the mortars with 10% replacement of OPC with CSA were 92% and 98%, respectively, of the control mortar (without replacement). Ash contains a high fraction of potassium, which negatively affects the mechanical development of the mortar. Separation of leaves and stems would be an interesting approach to offer the best destination for ashes: from leaves to produce pozzolanic material (rich in amorphous SiO2) and from stems (ash rich in K2O) to produce fertilizers. This proposed auto-combustion of corn straw could be used by low-income communities to reduce Portland cement clinker use, fertilize crops, and recover waste.
Future studies should focus on the evaluation of the pozzolanic reactivity of water-washed CSA and on improving this reactivity by removing soluble potassium. On the other hand, the durability of mortars and concretes in aggressive environments should be tested.