In this study, the extraction of RHA was evaluated as a partial replacement for Portland cement in cementitious mixes. Then, the use of cement with RHA was assessed in concrete production.

The department of Tolima-Ibagué (Colombia) was selected as the study area (Figure 1) to evaluate the environmental impacts of RHA extraction and the subsequent partial replacement of Portland cement in cementitious mixes and in concrete.

1 m³ of concrete was chosen as the functional unit. For cement and RHA, 1 kg of material was chosen.

Primary data were obtained from interviews with companies and organizations in Colombia (rice mill, concrete, cement and aggregate plants in the department of Tolima). These data were supplemented with the Ecoinvent V3 database and adapted to the Colombian context. The location data and transport distances, which could not be obtained during the visits that were carried out, were supplemented using Google Earth Pro.

To define which impact categories to evaluate, other LCA studies that address the use of RHA in concrete were considered (Table 6). SimaPro 9.3.3 software was used to organize the inventory data and perform the impact assessment. SimaPro is a popular life cycle analysis tool that can be used to quantitatively measure the environmental impact of a product or service.

The result of the inventory data obtained with SimaPro 9.3.3 is a long list of emissions and resource consumption. The software includes several methods for interpreting this list. The method that is best suited for the impact categories chosen in this study is Recipe2016 Midpoint (H). This method has been used by several well-known authors on this topic, such as: (64, 66, 67, 69, 73, 76, 77). This approach was chosen because it provides unambiguous values that can be used to compare concrete with different alternatives, as used in the study by (77). Midpoint effects are considered more precise because they correspond to a higher level of empirical evidence. As in (77), the midpoint effect results were considered to evaluate the different forms of effects produced by each constituent of the concrete and its process.

Electricity and heat are generated through the process of obtaining ash through the gasification of RH. The processes that have impacts in all categories when RHA is obtained are electricity (high voltage-cogeneration) and synthetic gas, see Figure 4.

The greatest impact from the use of electricity required during the electricity and heat cogeneration process occurs in the ozone layer depletion category at 43%, followed by 35% in the mineral resource scarcity category and 6 % in the water consumption category. Syngas production has impacts on water consumption (4%), mineral resource scarcity (3%) and eutrophication (3%). These impacts are due to the use of electricity and water during the process. There is a minimum impact of 0.2% on land use due to waste disposal in landfills. These results coincide with the study by (67) who found impacts from biomass gasification in the mineral resource scarcity category and impacts on land use.

The process of obtaining rice husk ash in the global warming category was improved at environmental level. This process saves energy due to the fact that heat and electricity are generated by cogeneration (renewable energy). Therefore, the energy savings also prevent the generation and release of carbon dioxide into the environment, which minimises the impact of global warming. The impacts in this category (global warming) were insignificant with respect to the savings or impacts avoided in it since the production of electricity from fossil fuels was avoided.

According to (66), the production of 1 MJ from RH has lower impacts on global warming, acidification and eutrophication categories than the production of 1 MJ from coal. This corroborates the advantages of energy production from RH (in a cogeneration process from husk) that has ash as a by-product. According to (66), if RH is used as a source of thermal energy instead of coal, the environmental impact decreases by 97% in the global warming category for each MJ generated. The results also suggest that gasification has up to 12 times lower impacts per kWh than combustion (67). In general, providing energy from residual biomass in small farming communities would significantly reduce environmental impacts and improve waste management practices (67). Likewise, the category of water consumption had a low environmental impact since the process of cogeneration of heat and electricity requires lower water consumption than that required to produce electricity from fossil energy sources (Figure 4).

Figure 6 shows that the cement alternative that has the greatest environmental impact is PC100-RHA0. The greatest impacts were on global warming (100%), mineral resource scarcity (100%), fossil resource scarcity (100%), fine particulate matter formation (100%), human carcinogenic toxicity (100%), eutrophication (97%) and acidification (85%) categories. As the clinker percentage decreases and the RHA percentage increases, the environmental impacts decrease in all impact categories.

The global warming category decreases as the RHA content increases. The reason for these results is that when ash is obtained, impacts are avoided due to the electrical energy and heat that are generated during the process and due to the impacts avoided by the non-disposal of RH. Although electricity consumption is necessary during the cogeneration process, the amount is much less than the electrical energy generated during the process. These positive results of lower CO₂ emissions for RHA compared to cement coincide with the study carried out by Hu (57), in which 157 kg CO2 eq/ton are required for the RHA compared to that required for cement, which is 801.6 kg CO2 eq/ton. Likewise, in the study by Hu (57), the energy required to obtain RHA was -353.5 MJ/ton, in terms of savings.

The process of obtaining Portland cement contributes to impacts in the global warming category due to the high energy consumption in the process of obtaining clinker. However, obtaining rice husk ash does not generate impacts in this category due to the energy savings from the cogeneration of electricity and heat during its production process.

The cement alternative with the highest RHA replacement (PC75-RHA25) was found to lead to savings in all impact categories compared to PC100-RHA0. The greatest impact was on global warming. However, these results were much lower than those obtained for PC100-RHA0. The total greenhouse gases emission and energy consumption in the cement industry can be reduced by using waste materials to replace virgin materials (clinker/coal). This agrees with what was found by (78).

In other impact categories evaluated for PC75-RHA25, the following savings were found: SOD (-100%), A (-100%), E (-100%), human non-carcinogenic toxicity (HNCT) (-100%), LU (-100%), WC (-100%), FPMF (-95%), MRS (-71%), HCT (-53%) and FRS (-37%). This is because when the Portland cement process was compared with the process for obtaining RHA, the pozzolanic material had lower impacts in these categories (Figure 5).

The results show that Portland cement had higher environmental impacts mainly due to the use of raw materials and fossil fuels. The use of ash as an alternative material helps to reduce the environmental impacts. This is in line with the results of (78). Mendes (79) also quantified the number of environmental aspects and impacts of mortar production with and without RHA and compared these impacts. They found fewer impacts in the mortar with ash compared to the mortar without RHA, in the processes of generation of rice husk ash, beneficiation of rice husk ash and RHA transportation. Once again, this shows the advantages of using RHA in binders. Impacts related to air emissions (CO2, NOx, PM, CO and SO2) decreased with the increase in supplementary cementitious materials according to (76). The use of pozzolanic cement as a binder for concrete preparation reduces the raw material consumption and reduces the environmental impacts of cement production (77).

Regarding water consumption, cementitious mixes with a higher percentage of Portland cement contribute to impacts in this category. By increasing the percentage of rice husk ash in the mixes, savings are found in this category. This is due to the fact that in the process of obtaining rice husk ash, savings result from the cogeneration of electricity and heat (and therefore these processes and the associated water consumption are avoided). This differs from obtaining Portland cement, which requires high energy consumption, and therefore water, in the clinker production process.

Likewise, the impacts on the mineral resource scarcity category were lower with the increase in RHA content, which resulted in 100%, 94%, 89%, 83%, 77% and 71% in PC100-RHA0, PC95-RHA5, PC90-RHA10, PC85-RHA15, PC80-RHA20 and PC75-RHA25, respectively. This is because, when the process of obtaining cement with obtaining rice husk ash was compared, it was found that obtaining Portland cement generates higher impacts in this category than obtaining rice husk ash. This is due to the consumption of raw materials extracted from nature to manufacture cement (limestone, clay, gypsum and iron ore), unlike the process of obtaining rice husk ash that uses a by-product of the rice industry.

Obtaining rice husk ash uses a by-product of the rice industry. Therefore, when it is compared to cement production, it presents savings in the scarcity of natural resources category. In addition, (74) found that there are environmental benefits as the impacts due to the disposal of RHA are reduced, since a residue becomes a valuable product and therefore a natural resource.

As can be seen in Figure 6, concrete with Portland cement (C-PC100-RHA0) had the greatest environmental impacts in all the categories evaluated. The greatest impacts were in the global warming (100%), fine particulate matter formation (100%), acidification (100%), eutrophication (100%), human carcinogenic toxicity (100%), mineral resource scarcity (100%), fossil resource scarcity (100%) and water consumption (100%) categories. The process that contributed the most to these impacts in all the categories evaluated was the production of Portland cement. However, concrete with a higher percentage of pozzolanic cement had lower impacts in the global warming and fossil resource scarcity category than concrete without RHA. This coincides with Chen Lo (80) in their study in which the use of ash in concrete also reduced the carbon footprint in relation to conventional concrete between 10% and 20% and in which the process that contributed the most to the impacts on concrete was Portland cement. Manjunatha (77) also found that these impacts decreased considerably in comparison to conventional concrete. Better results were obtained with a higher RHA replacement percentage (25%). However, this replacement percentage in the cement should not be increased, since previous studies show (Table 2) that if this ash replacement percentage is increased in the cement, the properties of the cement or concrete can be compromised by the addition of pozzolan.

Likewise, concrete with Portland cement (C-PC100-RHA0) had the lowest impact in the land use category (56%). This is because in this category the impact of cement production is minimal (11%) compared to the process of sand extraction (82%). The concrete with the lowest environmental impacts was the concrete with the highest RHA replacement percentage (C-PC75-RHA25). This concrete had the greatest environmental impacts in the mineral resource scarcity (73%), water consumption (57%) and global warming (29%) categories, also due to the production of Portland cement and the process for obtaining gravel. Environmental savings were also present in the other categories. The greatest savings were in the human non-carcinogenic toxicity (-100%), land use (-100%) and stratospheric ozone depletion (-100%) categories. This was due to the savings caused by the use of RH cement in the concrete, in agreement with what was found by other authors (77).

In general, all the types of concrete that were evaluated had impacts in the global warming and water consumption category. These impacts were greater when cementitious mixes with higher Portland cement content were used. When cementitious mixtures with a higher rice husk ash content were used, the impacts in this category were lower. This is because the process of obtaining rice husk ash is associated with the cogeneration of electricity and heat from rice husks. Therefore, the generation of electricity by means of fossil fuels is avoided, which implies lower CO₂ emissions and less water consumption.

Another category in which all the concretes that were evaluated had impacts is the mineral resources extraction category. In this category, as the CP content in the concretes increases, the impacts increase. This is due to the consumption of raw material extracted from nature to obtain Portland cement. By increasing the percentage of rice husk ash, the impacts decrease because a by-product of the rice industry is used to obtain this concrete, instead of the raw material extracted from nature in the case of Portland cement. The use of rice husks as a replacement for cement in the production of concrete is a research area with great environmental benefits in Colombia, as has been shown by other studies (20, 27, 39-42). RHA is an agricultural residue generated during the milling of rice. Therefore, it can help reduce the environmental impact and promote sustainable practices, under a life cycle approach. In the department of Tolima, current annual production of rice is 406,737 tons, of which it is estimated that 20% is RHA (3, 6). Therefore, it could be said that the department of Tolima has replacement potential in the manufacture of 81,347 tons of RHA concrete, as a pozzolanic material that favours the formation of cementitious compounds.

Sensitivity analyzes were performed to assess the influence of greater distances traveled by the RHA from the rice mill to the cement plant. This sensitivity analysis for transportation distances has been evaluated by other authors such as (76). The transport distance that was considered in Sections 3.2 and 3.3 is 14 km (Table 1). In this section, only the processes are compared when this distance is varied, to determine how viable the replacement of rice husk ash is by increasing its transportation distance. The cement production with the highest percentage of ash replacement (25%) at a distance of 14 km was compared with this same production process, with the distance varied to 60 km and 100 km (Figure 7). Transport turned out to be a variable that was negligible compared to the positive environmental impacts of the use of RHA in cement, as shown in Figure 7. When RHA transported over a distance of 100 km from the mill to the cement plant is used in the cement to make C-PC75-RHA25 concrete, it still has environmental advantages, as shown in the Figure 8. In this case, the environmental impacts were compared in the categories of concrete C-PC75-RHA25 in which the ash was transported 14 km, or 60 km to 100 km. These results indicate that the variation in the transport distance of rice husk ash to the cement plant (up to 100 km) does not negatively affect the environmental impacts of cement and concrete production with this pozzolanic material (Figure 7 and Figure 8).

The economic criteria were evaluated considering the production costs for the products that were manufactured. The method used to calculate the economic costs was based on an economic study of construction materials conducted by (81, 82) used this method to study a new concrete with recycled gypsum cement and recycled aggregates. Other authors such as (83) have used this method to analyze different concrete alternatives with recycled aggregates from construction and demolition waste (CDW). The objective of this study was to determine and then compare the approximate costs of the alternatives for cement and concrete production.

The reference costs for aggregates, cement and concrete were calculated by adding the cost of producing each material. The production costs for the materials were determined based on the costs reported by the companies and plants surveyed in Colombia. The data were collected in different years from different companies or plants with similar production capacities. These data were supplemented with secondary information on material production costs. The transportation distance was considered a constant since it was the same for all alternatives. For this reason, it was not included in the total price of the concrete. The production and sales costs of the materials that make up the concrete are shown in the Table 7, which indicates the source of the information.

Different combinations of PC with RHA in different proportions were made and the price of each alternative was determined using the cost of the material PC and RHA given in Table 7. We managed to substitute 25% of PC with RHA because the resistant property of cement was maintained. A larger substitution would mean losses in its resistance, as presented in Table 4. Only Chao (84) presented an economic study of alternatives.

The six proposed cement alternatives are listed in Table 8.

The methodology for calculating the economic criteria was based on data for the production of 1 kg of Portland cement (PC) for the alternatives of mixing with rice husk ash (RHA). From the results obtained, it appears that the alternative with 100% PC (A1) is cheaper than the other alternatives. In the second alternative (PC95-RHA5), the final price of cement/kg is reduced by 2.3%. In the last alternative (PC75-RHA25), the final cost was reduced by 11.5% (Table 8). On the other hand, the production costs of the six proposed concrete alternatives are listed in Table 9.

In this case, the economic criteria are based on the required data for the final price of 1 m³ of concrete for the cement alternatives proposed above (A1-A6) together with the price of pozzolanic cement ($/350 kg). For this reason, there are six alternatives (A7 to A12). The first alternative A7 with 100% PC (A1) was cheaper than the other alternatives.

In concrete there are economic advantages between 2.1% and 10.3% of the final price when PC and RHA are used compared to A7 (100% PC), estimated at $112.3 m³. That is, a difference between $2.3 and $11.6. Alternative A12 is the most economical because less cement is used in the mix with RHA. When Chao (84) replaced 30% RHA, the concrete had mechanical properties closer to those of conventional concrete, and the total cost of concrete was reduced by 7.16%. Partial replacement of cement with rice husk ash can thus lead to significant cost savings and reduce the negative environmental impact of cement production. In addition to the economic advantages mentioned above, a great possibility opens up in Colombia and in the department of Tolima. Utilising RHA in concrete production may provide an opportunity to reduce reliance on traditional cementitious materials that are energy intensive and have a high carbon footprint (16). This is in line with Colombia’s commitment to sustainable development and climate change mitigation. Furthermore, the use of RHA can contribute to the development of a circular economy by turning agricultural waste into a valuable resource.