In today’s fast growing urbanization, environmental sustainability is a significant factor that cannot be ignored by architects, engineers, researchers and, above all, by the construction industry; one of the means to achieve the balance in sustainable development is through the utilization of locally available waste or recyclable materials. The alarming rate of concrete production that consumes a vast amount of natural resources around the world signifies the need for sustainability through the use of alternate materials. It is estimated that the production of concrete consumes about 27 billion tonnes of raw materials or four tonnes of concrete per person per year [1
]. The quarrying and manufacturing process of massive quantities of aggregates, in addition to about 2.8 billion tonnes of cement products manufactured every year [2
], cause around 5–7% of the planet’s total CO2
]. Consequently, the problem is likely to get worse, as it is foreseen that by 2025, about 3.5 billion tonnes of CO2
will be emitted from the manufacturing of cement [4
]. Further, it is predicted that by 2050, concrete production will reach four-times the level as that of 1990 [5
]. It is to be borne in mind that the main ingredients in concrete, namely binders and aggregates constitutes about 10–15% and 60–80% of the total volume, respectively [6
]. The quarrying activities around the world to produce coarse aggregates have drastically changed the ecological balance, and hence, it is indispensable to search for sustainable alternatives to replace both the binder and the aggregates that are being used in concrete to reduce the adverse effects due to excessive use of virgin materials. On the contrary, many waste materials are dumped in open fields and underutilized; one such waste known as construction and demolition waste that could be a potential recyclable material is valuable recycled concrete aggregate (RA).
RA is available in many developed and developing countries due to the demolition of aged buildings and structures; further, in many war-torn countries, many structures have been the target of bombing, and structures have become redundant. This results in huge quantities of RA being heaped as piles of rocks, and thus, RA has a role to play in sustainable development; RA has become increasingly important in the field of construction as an alternative to primary (natural) aggregates. Nevertheless, many studies concluded that utilization of RA in concrete will affect the hardened properties negatively [7
]. The results obtained by Akça et al. (2015) [10
] showed that utilization of concrete made from 100% RA is limited, due to the reduction in the hardened strength ranging from 15–25%. Moreover, after reviewing the effect of RA on concrete, Safiuddin et al. (2013) [11
] found that the reduction in the strength of concrete made from RA was attributed to the existing porous adhered mortar on its surface, which has higher water absorption.
Researchers showed that in spite of the reduction in the mechanical and durability performance of RA concrete, the concrete could attain considerably enhancement by utilization of traditional supplementary cementitious materials (SCMs) with high pozzolanic activity, such as fly ash (FA), silica fume (SF) and ground granulated blast slag (GGBS) [12
In recent decades, the usage of traditional SCMs or pozzolans has become more intense in the concrete industry due to their better long-term properties. Hence, concerns over the plentiful availability of the traditional SCMs led to contemplation about other sustainable sources as pozzolanic materials [14
]. Sustainable sources of SCMs including rice husk ash (RHA), palm oil fuel ash (POFA) and palm oil clinker (POC) have been utilized by researchers in the production of normal, high-strength and lightweight concretes [15
]; however, the utilization of these sustainable SCMs has been limited to some properties, and very few literature works related to SCMs are available. Hence, more research works have to be carried out to investigate the utilization of SCMs in the development of RA concrete.
The rice industry generates millions of tonnes of rice husk during milling of paddy rice, which comes from the fields. RHA is a by-product generated from burning of the rice husk at a temperature range of about 800–900 °C in the biomass plants that use rice husk as fuel for power generation. It was estimated that about 156 million tonnes of rice husk are generated globally, of which 2.14 million tonnes in Malaysia and 1.81 million tonnes in the USA annually; it is estimated that 18–22% of rice husk weight will be converted into RHA after burning in boilers [18
]. Thus, rice husk has the potential to produce 26–34 million tonnes of RHA containing over 90% (up to 95%) amorphous silica that could be used as an alternative SCM [21
]. The commercial viability of RHA is not prevalent, and hence, dumping of RHA in the vicinity of the agricultural lands is a considerable threat to the environment. Hence, research works on the utilization of RHA as sustainable SCM in different types of concrete have been examined [22
]. It was reported that, due to its highly pozzolanic nature, RHA can be used up to 20% as SCM without affecting the strength and durability properties of concrete [16
]. Another research work on the development of high-strength concrete showed that a compressive strength up to 80 MPa could be achieved by incorporating 10–30% of RHA [24
One of the latest additions that could be considered as a potential SCM is POFA, a by-product obtained from palm oil mills. It is produced by burning of oil palm shell, fibers and empty fruit bunches at temperatures between 800 and 1000 °C for electricity generation during the palm oil extraction process [25
]. Annually, the amount of palm oil residues produced globally is about 184 million tonnes, with 53 million tonnes in Malaysia, the world’s second largest producer and exporter of palm oil; and it is estimated that the expansion of palm oil plants would increase by 5% every year [26
]. The resulting ash after combustion, i.e., POFA, is 5% by weight of the original solid materials. According to these statistics, the annual production of POFA is about 10 million tonnes around the world. Studies concluded that the waste materials obtained by the palm oil industry can be reused in lightweight concrete production, including blast-resistant concrete [28
] and geopolymer concrete [29
]. In addition, the investigation on the potential use of POFA as SCM concluded that POFA is a good pozzolanic material since it has a high amount of silica content (50–70%) [30
]. An early study by Safiuddin et al. (2016) [31
] indicated that concrete containing 20% POFA has a 28-day compressive strength satisfying the strength requirement for high-strength concrete. Further, Johari et al. (2012) [32
] concluded that utilization of POFA tends to reduce the early mechanical properties, while the strength at a later age was comparable to the control specimens due to the pozzolanic mechanism of POFA.
POC is another by-product from the palm oil industry. The difference between POFA and POC is that POFA is collected in the form of ash, while POC is collected as large chunks. Attempts have been made by Kanadasan and Abdul Razak (2015) [33
] to utilize the POC powder (POCP) as a cement replacement material in self-compacting mortar; they found that the replacement of 50% of POCP with a similar particle size as that of cement could produce compressive strength of about 70% of control specimens.
There are very few and limited literature works available on the utilization of RHA and POFA with RA concrete [34
]; however, there is no research work on the use of POCP as SCM in the development of RA concrete. Thus, this study investigates the effect of sustainable SCMs, namely RHA, POFA and POCP on concrete made from 100% RA. Using a total of 11 concrete mixes, the effect of these SCMs on the fresh and hardened properties, including workability, compressive strength, ultrasonic pulse velocity (UPV), splitting tensile strength, flexural strength and modulus of elasticity, of concrete made from RA was determined; the replacement levels of RHA, POFA and POCP were maintained at 10%, 20% and 30% for conventional cement. In all mixes, the crushed granite aggregate was replaced wholly with RA and compared with one control mix, developed using conventional cement and normal aggregate.