1. Introduction
Paris et al. [
1] report that approximately 4 billion tonnes of cement were produced in 2014. In 2021, this increased to 4.4 billion tonnes, and this figure is predicted to reach 5 billion tonnes by 2030 [
2,
3]. The excessive production and utilisation of cement is responsible for 5–7% of anthropogenic greenhouse gases produced annually and is currently considered unsustainable owing to its negative impacts on the environment, especially regarding climate change and its effects [
4,
5,
6]. Furthermore, the International Energy Agency (IEA) [
7] reports that there needs to be a 4% yearly decline in cement production up to 2030 to reduce these emissions and achieve net zero carbon emissions by 2050. Research has shown that the use of supplementary cementitious materials is a viable solution for reducing the amount of cement produced and used in the building industry [
1,
5,
8].
Additionally, about 2.01 billion tonnes of solid waste are generated annually from manufacturing processes, industries, and construction, and this is predicted to increase to about 3.4 billion tonnes by 2050 [
9]. Aprianti et al. [
8] estimate that by 2050, the world’s population will rise to 9 billion, which will lead to an increased demand for energy, food, housing, and clothing. This has prompted increased research into the effectiveness and availability of pozzolanic waste materials that can partially replace cement as these wastes remain in the environment, unused, and more waste is produced with continuous population growth. The use of waste materials in construction is currently growing in research globally [
10,
11,
12]. Reusing these wastes in construction is encouraged, as it will reduce the amount of raw materials used and significantly reduce landfill waste. When combined with cement, waste materials such as supplementary cementitious materials (SCMs) have also been reported to improve some of the physical properties of cement-based materials where they are used. Several studies have been conducted on the use of SCMs originating from industrial wastes [
13,
14,
15,
16,
17,
18], agricultural wastes [
19,
20,
21,
22,
23,
24,
25], and other general or natural wastes like plastics and glass [
26,
27,
28,
29,
30,
31]. However, research is growing regarding the reuse of agricultural wastes for construction purposes. This is because agricultural wastes typically release carbon dioxide during calcination, which is offset by the carbon dioxide absorbed by the plants throughout their lifecycle [
32], thus making them a more sustainable alternative to other SCMs.
Rice husk ash, derived from rice husks, has been selected for this study due to the large quantities being produced in tropical countries and their ability to combine with hydrated cement to form compounds possessing cementing properties. Additionally, as many of these countries are fast developing, there will be an increase in population, economic development, and urbanisation, which will result in increased construction needs and higher energy demands in the years to come [
12,
33]. Rice is considered a staple food in most tropical countries and is grown multiple times a year, with worldwide production increasing from 650 million tonnes in 2010 to about 787 million tonnes in 2021 [
1,
34]. The resulting waste from rice production, rice husks, from which rice husk ash (RHA) is derived, make up 20–23% of harvested rice, are typically dumped in landfills during rice production, and have no economic value [
35,
36,
37].
In addition to rising global cement consumption and waste production, there is a corresponding increase in global energy consumption. Due to the continuous depletion of finite natural resources as a result of growing population and urbanisation, in addition to excessive waste production, global energy consumption, which stands at 176,431 Twh, is predicted to increase to 197,000 Twh by 2030, with carbon emissions also rising to 40.4 billion tonnes by 2030 [
38,
39]. Furthermore, pursuing new energy sources to cater to increasing global energy consumption will further contribute to environmental degradation and the reduction of resources [
40]. With operational building energy consumption contributing to 55% of global energy consumption [
41], prioritising building performance and developing energy-efficient buildings, starting with the optimisation of the building envelope, remains crucial. The indoor thermal environment is actively controlled through the use of heating, ventilation, and air conditioning (HVAC) systems to regulate the temperature and overall climate. Although these systems ensure the thermal comfort of occupants, they can significantly impact building energy consumption and costs for households. According to Xie et al. [
42], there is a growing increase in the use of energy to improve occupant well-being and indoor thermal comfort, which creates a need to improve the energy efficiency of buildings.
Globally, several solutions have been proposed to address this issue, such as policy change and implementation, the adoption of renewable energy systems, and the use of phase-change materials [
43]. However, in most developing countries, where the majority of their population is in the low-income group, applying these strategies to address energy consumption remains a challenge [
29,
42]. More people are concerned about the upfront costs of buildings, and little consideration is given to building performance, which leads to many householders experiencing thermal discomfort within their homes [
44,
45]. Ochedi and Taki [
44] explain that the major factors influencing energy usage in buildings are the building envelope and materials, occupant behaviour, climate, building design, artificial lighting and ventilation systems, and appliances. Furthermore, according to Oyekan and Kamiyo [
46] and Danso [
47], the materials chosen to construct buildings have an impact on the amount of energy they consume and their cost. Research is currently advancing to include more solutions to address building energy performance, starting at the building envelope level [
42]. Building envelope typically refers to building walls, windows, roofs, etc., based on their function and location. The building envelope is responsible for heat loss or gain from or into a building. The thermal transmittance of building elements can be used to compare the heat loss or gain through the building fabric from different building elements, such as the roof, windows, or walls [
48,
49]. Various researchers [
48,
49], report that the ratio of heat loss from the building envelope is 35–40% for external walls, 13–25% for ceilings, floors, and roofs, and 25–47% for windows and doors. Building elements with high thermal transmittance coefficients typically result in high heat gains or losses and high energy consumption for building occupants. In tropical climates, this heat gain usually results in the use of mechanical cooling systems to improve occupants’ thermal comfort, thereby increasing building energy consumption [
45,
50]. According to Harish and Kumar [
51], optimising the building envelope design can reduce building energy consumption by up to 20–50%. Controlling the heat gain or loss from the building envelope is therefore crucial to improving the energy efficiency of buildings.
Alqahtani et al. [
52] explain that sustainable materials are being sought after to minimise the embodied and operational energy costs of buildings and reduce their associated carbon footprint. Masonry units serve as primary construction materials for external walls in many countries, and the energy efficiency of a building can be significantly improved by adopting masonry units that have better mechanical and thermal properties [
53]. Previous research has reported that the use of rice husk ash (RHA) for the sustainable production of cement-based masonry units increases the strength with longer curing periods, reduces the density due to RHA having a lower specific gravity than cement, and reduces the thermal conductivity of the final product where it is incorporated [
54,
55,
56,
57,
58,
59], although there is an increase in water demand. However, studies have reported that the water absorption of these masonry blocks is below the maximum of 15% stipulated by ASTM C90-09 for medium-weight concrete masonry units [
25]. Ferraro and Nanni [
58] investigated the effects of using RHA blended with cement to produce mortar. They observed that using up to 15% off-white RHA to partially replace cement resulted in a 19% decrease in the thermal conductivity of mortar samples. The study also noted a 15% increase in compressive strength, a 9% increase in tensile strength, and a 1% reduction in water absorption after 28 days of curing. Likewise, Carig et al. [
54] produced hollow concrete masonry units in their study using 5–15% rice husk ash to partially replace cement. They observed similar or lower values of water absorption using 5–15% RHA replacement when compared to the control sample. They also achieved up to a 43% increase in compressive strength, although this started to decrease after 10% RHA was introduced. Similar to Ferraro and Nanni [
58], they reported up to a 13% reduction in the thermal conductivity of the RHA masonry block samples. Likewise, Onyenokporo et al. [
55], who employed an experimental study to investigate the effect of rice husk ash on the thermal properties of cement-based masonry blocks, also observed a reduction of up to 17% in the thermal conductivity of the samples using 15% RHA replacement by weight of cement. In their study, Selvaranjan et al. [
57] replaced river sand with rice husk ash at varying replacement values of 10–50% by weight of sand. They found that compressive strength decreased with increasing replacement values, but samples with up to 30% RHA still met the minimum required value for mortar after 28 days. Additionally, thermal conductivity decreased up to 67% with controlled-burnt RHA and up to 73% with open-burnt RHA. Selvaranjan et al. [
57] explain that there is an increased number of pores in mortars containing RHA, which trap air and improve the overall thermal insulation of samples. Moreover, this reduction in thermal conductivity has a positive impact on the building envelope in terms of building energy consumption to address heat gains or losses.
Although current research shows the effect of rice husk ash additions on the thermal properties of cement-based masonry units, there is a dearth of literature properly quantifying the effects of these RHA masonry blocks on building energy performance when used as a building material for external walls. As the external walls constitute a major part of the building envelope, it has major implications for the overall energy performance of buildings and the resulting carbon emissions. So far, only Hitawala and Jain [
60] have conducted energy performance analysis of a prototype building using rice husk ash for the building envelope. They combined rice husk ash insulation (88.28 wt% rice husk ash (RHA), 9.29 wt% of bentonite, and 2.41 wt% of exfoliated graphite) and rice straw ash blocks (60% paddy straw, 28% fuel ash, and 12% binder) for comparative analysis with burnt clay brick masonry wall assembly. They recorded a 22% reduction in energy performance index and a 48% reduction in embodied carbon emissions using the external wall and roof incorporating rice husk ash.
Due to the dearth of literature quantifying the effects of these RHA masonry blocks on building energy performance, this paper, therefore, contributes to the existing body of knowledge within the field. This study critically investigates the effect of rice husk ash masonry blocks on building energy performance when used as a walling material. Through the use of EnergyPlus interface in DesignBuilder v7 to carry out a simulation study, a prototype building from the context was selected to quantify this impact. The computer simulation allowed for a comparative analysis of the prototype building(s) using the RHA masonry blocks and conventional cement masonry blocks to evaluate the effects of rice husk ash on overall building performance in terms of heat gains through the walls, energy consumption for cooling, occupants’ thermal comfort, and carbon emissions.
4. Discussion
Based on previous studies using RHA for partial cement replacement, this study focused on quantifying the energy performance of the RHA masonry blocks, as the topic is still under-researched. In addition, previous studies had focused on the strength and other physical properties of the blended blocks, with not much thought given to the thermal properties that affect the building’s energy performance after the blocks have been used for the construction of building walls.
It is noteworthy to mention that although the effect of the RHA blended blocks may not seem very significant in terms of operative temperature, cooling load, and carbon dioxide production, the effect was more significant for reducing the heat gains through the external walls, as these were the only parameters of the existing building adjusted in DesignBuilder. Based on the observation of the prototype buildings, it is evident that the houses were not built with passive design strategies such as solar shading and the use of greenery to reduce direct solar radiation into the buildings. This is also reported by Adaji et al. [
45], who agree that mechanical cooling is used in most homes in Nigeria and other Sub-Saharan African countries to improve their thermal comfort. This is an unsustainable approach to achieving long-term thermal comfort in these houses because it is both costly and energy-intensive. Moreover, for a country like Nigeria, which is situated directly at the equator, the sun is directly overhead, especially in the daytime, resulting in high levels of solar heat gain through the building envelope. The incorporation of passive design strategies into the building can significantly improve its energy performance [
50]. When these passive strategies are combined with the change to external walls, as demonstrated in this study, it will result in a more significant improvement to the overall building’s energy performance.
Nigeria is a country with sporadic electricity supply, which is greatly augmented by off-grid power generators to run cooling mechanical equipment in a bid to improve indoor building temperature. With buildings and households recording huge amounts of energy consumption, especially with the use of electrical devices and cooling equipment, the importance of improving building energy performance cannot be overstated. The results of this study will go a long way towards reducing building energy consumption as well as carbon emissions from these activities. Coupled with the use of waste materials to reduce building costs, the improvement to the thermal performance of the building fabric will contribute to reducing both embodied energy and operational energy costs, making buildings more affordable to build and operate. Also, waste reuse and recycling contribute to the growth of a circular economy [
53]. The potential of using this waste material in construction is therefore evident and would go a long way towards reducing global energy consumption. Although this study only focused on the external walls, the use of RHA can also be extended to include both internal walls and other cement-based components such as mortars, plasters, and concrete floors. The sheer impact of this will drastically result in a reduction in building energy consumption and carbon emissions.
Although the focus of the research was on the thermal properties of the RHA masonry blocks and their effect on building energy performance, the study also determined the physical properties of the RHA masonry blocks, such as density, compressive strength, and water absorption, as these are very important parameters to consider for building components, and this data is useful for their adoption as well as for use in further studies. Full details can be found in Onyenokporo, Taki, and Zapata Montalvo [
25] and Onyenokporo et al. [
55]. It is noteworthy to mention that one major limitation to the commercial adoption of rice husk ash as a partial replacement for cement is its slow early strength gain and increased water absorption when compared to conventional concrete masonry blocks. However, solutions to these have been provided in previous studies. Ettah et al. [
56] recommend that adequate attention be given during the curing process and a chemical activator be used to improve the strength. Similarly, Trejo and Prasittisopin [
78] explain that the water absorption of RHA block samples can be increased by reducing the particle size of the rice husk ash, either through mechanical grinding or the chemical alkali extraction method. Future work will therefore consider curing for longer periods than 28 days and also using a chemical activator or superplasticizer to improve the strength of RHA block samples. Further reduction in the RHA particle size to less than 45 µm, as used in this study, should also be considered. As recommended by Trejo and Prasittisopin [
78], reducing the cellular, honeycomb-shaped structure may cause a decrease in water absorption properties, meaning that fresh concrete mixtures containing smaller RHA particles will have improved workability and lower water requirements compared to those containing larger RHA particles.
5. Conclusions
This study focused on the potential of using rice husk ash (RHA) masonry blocks for external building walls in tropical climates, using Abuja, Nigeria, as the study context. For the bungalow, using the RHA10% and RHA15% blocks reduced the heat gains through the external walls by 5.1% and 9.9%, respectively. This represents a reduction of about 3.4% and 6.5%, respectively, in terms of cooling load and a 2.1% and 4.1% reduction in carbon emissions. However, in terms of indoor operative temperature, the reduction was not very significant, with approximately a 1% reduction in the operative temperature using RHA15%. This was similarly observed for the duplex/storey building. Using the RHA10% and RHA15% blocks resulted in a reduction of heat gains through the external walls by 5.5% and 11.3%, respectively. This also translates to a reduction of about 2.0% and 4.2%, respectively, in terms of cooling load and a reduction of 1.4% and 2.8% in carbon emissions.
The findings from this study demonstrate the potential of using rice husk ash masonry blocks for external building walls in tropical climates, which can help improve building energy performance. The prospects of improving the building envelope through the use of RHA masonry blocks will contribute towards reducing the operational costs spent on cooling in most households, reducing carbon emissions from the process, and improving the thermal comfort of building occupants. An increase in population leads to an increase in energy demand as well as an increase in demand for and use of natural resources. The significance of the research outcomes cannot be overstated, as they provide evidence to justify the utilisation of these supplementary cementitious materials, like rice husk ash, for sustainable building construction. This research will prove useful in encouraging the adoption of this waste material, reducing landfilled waste, and encouraging a circular economy. It will also add to the existing knowledge on design strategies to minimise building energy consumption. The outcomes of this research will prove useful to householders, researchers, architects, and policymakers in their decision-making processes. In addition, this study will be beneficial in bridging the knowledge gap as well as introducing new methods that can be adopted for similar studies.
The need for continued research in this field cannot be overemphasised, as it has the potential to foster the development of more energy-efficient construction materials. Additionally, the impact of this research will be further strengthened if an actual RHA masonry building is built. Producing a building prototype of an RHA building will help to strengthen the results obtained from the building simulation study and provide a real-life example of a rice husk ash masonry building. Furthermore, a post-occupancy survey can be conducted to gauge the influence on building energy performance and compare this to the simulation results. For future experiments as well as building simulation studies, it will be useful to consider using RHA in concrete, mortar, and plaster, as these are also major cement-based components of the external envelope. This will provide a bigger picture to demonstrate the effect of rice husk ash additions to the building envelope.