1. Introduction
The construction industry contributes significantly to any economy, and its components are vital to advancing socio-economic development goals of shelter, infrastructure, employment, and improved well-being [
1]. However, the industry has a long history of negatively affecting the environment and human health. For instance, Rheude et al. [
2] attributed high energy consumption, depletion of natural resources, high global anthropogenic CO
2 emissions, waste generation, and air pollution to construction industry activities. Notwithstanding the environmental menaces, Rheude et al. [
2] further pointed out that the construction sector is poised to build more urban space in the coming 40 years than in the last 4000 years due to increasing population and urbanisation. The architecture, engineering, and construction (AEC) projects require cement, a major constituent of mortar and concrete cum basic and widely used building material. Adekoya [
3] projected Nigeria’s local cement production capacity to hit 60 million metric tonnes per annum (MMT/a) by 2022. The report further revealed that key players in Nigeria’s cement production, including Dangote Cement PLC, has an installed capacity of 29.3 MMT/a, followed by Lafarge Africa PLC at 10.5 MMT/a and BUA Group, which stands at 9 MMT/a with expected plans to increase to 20 MMT/a at the end of 2022. The rise in cement production in Nigeria will continue to increase 0.870 GtCO
2 emissions recorded in 2015 [
4]. Nigeria’s cement production sector largely contributes to the estimated 8% of the total anthropogenic CO
2 emission from the global cement production [
5] during the clinkerisation process of converting calcium carbonate into lime at approximately 1450 °C.
Among the various strategies to reduce greenhouse gases, especially CO
2, clinker substitution, i.e., replacing carbon-intensive clinker with supplementary cementitious materials (SCMs), is one of the four major pillars for global carbon emission reduction protocols. There are economic and environmental sustainability benefits accrued from SCMs incorporation in a cement matrix. Zhang et al. [
6] posit that approximately 25–30% reduction of CO
2 emissions can be achieved via cement replacement with SCMs. In the same vein, Oyebisi et al. [
4] stress that the absence of SCMs in many concrete structures developed in the 20th and 21st centuries led to their deterioration between the first and second decades of development. SCMs are noted to react and deplete the calcium hydroxide in cement composite to form calcium silicate hydrate (C-S-H) gel responsible for more strength development of concrete [
6,
7]. Hossain et al. [
7] experimentally showed that SCMs-based concrete demonstrates excellent or equivalent physical, mechanical, durability, and microstructural properties compared to Portland cement.
In this respect, the continuous rise in urbanisation and industrialisation worldwide has demanded concrete with better strength and durability properties in super tall buildings and heavy civil engineering projects [
6]. These AEC structural projects require high-performance concrete (HPC) with better compressive, flexural, and tensile strengths, and improved durability to resist attacking ions like chloride/sulphate or chemical agents. HPC is traditionally described as concrete with improved strength, workability, and durability performance, which cannot be attained with usual materials through standard mixing, placing, and curing techniques [
8,
9,
10]. The authors agreed that HPC has more cement content than the normal strength concrete, including one or more SCMs like silica fume (SF), fly ash (FA), ground granulated blast furnace slag (GGBS), metakaolin (MK), and calcined clay (CC), rice husk ash (RHA), etc. It is achieved by lowering the water to binder ratio (W/B) between 0.2–0.38 [
11] and refining the pore structure and microstructure of cement paste with SCMs. At the same time, workability is improved with a large dose of superplasticiser [
12,
13]. The materials used are generally thoroughly picked superior quality constituents with an optimised mix design. The aggregates have to be robust, durable, and consistent with cement paste for stiffness and strength. The adoption of HPC is an effective measure to reduce the structural element’s sizes without affecting the load-carrying capacity [
6]. According to Zhang et al. [
6], HPC technology saves usable space in buildings, reducing the dead load on structures with further minimisation of foundation size and cost. Irrespective of the wide use of fly ash, silica fume, metakaolin, etc., as a cement substitute in HPCs, RHA is gaining traction, especially in rice-producing countries [
14,
15].
The addition of RHA as an SCM in the cement matrix looks attractive to managing the large quantity of waste, especially in Nigeria, with rice production growth of approximately 70% in the last decade [
16]. Ugbede et al. [
17] reported that rice production in Nigeria increased from 5.5 million tonnes in 2015 to 5.8 million tonnes in 2017, with a 5% annual increase following federal government intervention policy on agriculture and economic development. For every one-tonne rice paddy, approximately 40 kg of rice husk presents environmental problems from widespread land occupation and disposal burning [
18]. Rice husk is a by-product of rice production, and its high silicon content makes it difficult to degrade naturally [
15]. It is mostly utilised as a fuel in electric generation, firing of clay bricks, and rice processing plants. About 20% of rice husk converts to RHA through open field burning or a controlled incinerator system, for which temperature and duration are either monitored or not [
19,
20]. Adnan et al. [
20] considered rice husk to be typically preeminent in ash compared to other biomass fuels. A highly reactive RHA particle size and surface area depends on the controlling temperature and burning environment the rice husk is subjected to. The attractive property of RHA in blended cement comes from 87–97% of non-crystalline silica content, which is fibrous and lightweight, with an external high surface area [
20]. The addition of RHA brings about denser microstructure and homogeneity in concrete, preventing water and influencing chloride/sulphate ions and chemical agent migration [
18]. Consequently, Hu et al. [
21] reported a compressive increase of 11.3% at 28 days of hydration with a 15% replacement ratio with a high reactive RHA.
From the above, the use of highly reactive RHA with well-graded size distribution and low W/B in a blended cement matrix can achieve a close microstructure and improved durability in HPC. HPC is optimally designed with various fine materials, leading to a very dense microstructure and low permeability. The low transport of moisture and attacking ions impermeability generally improves the durability performances of HPC. Typical durability evaluation of RHA-based cement blend has been determined through chloride and sulphate ion penetration, acid attack, carbonation, shrinkage, sorptivity, porosity, and water absorption resistance tests. Rumman et al. [
14] evaluated the chloride permeability resistance of high strength concrete at varied cement contents of 500, 425 and 350 kg/m
3 by RHA contents of 0, 55 and 70 kg/m
3, respectively. The W/B was fixed at 0.4 and 0.5 for each mix to propose a new mix design method that incorporates durability as a design factor and RHA as an SCM. Their results revealed that up to 20% RHA content, chloride permeability was reduced to approximately 63%. Hossain et al. [
7] investigated the water absorption and sorptivity of a high strength mortar containing RHA as an SCM. A mix ratio of 1:3, with W/B of 0.4 and 0.5 at 5–20% cement replacement of five steps, was adopted. The hardened mortar samples were subjected to two different curing schemes (standard and high-temperature curing) for 1, 7, and 28 days. The water absorption and sorptivity test results revealed that RHA blended mortar with a high-temperature curing method showed an overall decline in water absorption and sorptivity.
Sahoo et al. [
18] assessed the durability properties (water permeability, sorptivity, porosity, chloride permeability, carbonation resistance, and weight loss due to acid attack) of a normal strength concrete blended with RHA. At 15% RHA content cured for 28 and 60 days, sulphur acid attack reduced by 7% whereas the water permeability reduced by 15% at 90 days. The carbonation resistance result showed approximately 0.13 cm/day
0.5 in the magnitude of co-efficient of carbonation at 90 days. Chloride ion penetration recorded a reduction of approximately 15% at 10% RHA content, whereas 15% RHA content increased to 26.1%. Similarly, De Silva et al. [
19] examined the effect of waste RHA residual ash from rice husk fueled brick kilns on the durability properties of a plastering mortar. With 550 kg/m
3 cement content, RHA was varied from 5–10% in five steps. Their finding showed a reduction of 54% and 70% in sulphate exposure for 5% and 10% RHA content. Weight loss in an alkaline environment reduced from 3.13% to 2.4% in 10–15% RHA content. Swaminathan et al. [
22] investigated the influence of a ternary mix of Portland cement, metakaolin, and RHA on the durability properties of an M60 grade HPC. Their results revealed that the durability dimensions of water absorption, sorptivity, porosity, and permeability penetration improved due to RHA’s presence in the mixture. Finally, Wang et al. [
15], in their review of RHA performance on cement-based materials under various W/B, pointed out within 10–25% RHA content, chloride penetration resistance is optimised, carbonation resistance best improved at 20%, and shrinkage resistance optimisation stands at approximately 10–20% cement replacement. Wang et al. [
15] placed sulphate resistance optimisation at 40 and 50% RHA content in the cementitious material. Generally, the authors noted that tested concrete and mortar samples’ durability and mechanical properties improved within 10% and 20% of RHA content.
RHA has attracted a lot of research on the strength properties of normal strength concrete and mortar. However, few studies are available on durability dimensions of HPC, especially in developing countries like Nigeria. Hence this study investigates the durability properties of water absorption, sorptivity, and chemical attack of a fibrous RHA blended in HPC mixtures. The aim is to understand the feasibility of incorporating a controlled incinerated RHA into an HPC matrix in fulfilling its long-term durability properties. The information will be relevant for further RHA use in sustainable construction materials and the industry’s growth.