Towards Sustainable Concrete: Current Trends and Future Projections of Supplementary Cementitious Materials in South Africa

Abstract

Supplementary cementitious materials (SCMs) provide a practical solution for reducing greenhouse gas emissions associated with Portland cement production while enhancing the economy, performance, and service life of concrete and mortar. Currently, there is a significant disparity in the availability, supply, and utilisation levels of SCMs worldwide, particularly in South Africa. This paper presents an in-depth analysis of the characteristics and performance of various SCMs, including local availability, factors driving demand, production, and utilisation. The findings indicate that fly ash and limestone calcined clay are the most widely available SCM resources in South Africa, with deposits exceeding 1 billion tonnes each. Fly ash stockpiles continuously increase due to the reliance on coal-fired power plants for 85% of generated electricity and a low fly ash utilisation rate of 7%, significantly below international utilisation levels of 10–98%. Conversely, slag resources are depleting due to the steady decline of local steel production caused by energy and input costs, alongside the growing importation of steel products. Combined, the estimated production of slag and silica fume is about 1.4 million tonnes per annum, leading to their limited availability and utilisation in niche applications such as high-performance concrete and marine environments. Furthermore, 216,450 tonnes of SCM could potentially be processed annually from agricultural waste. In addition to quality, logistics, costs, and other challenges, this quantity can only replace 1.5% of clinker in South Africa, raising concerns about the viability of SCMs from agricultural waste. Based on its findings, this study recommends future research areas to enhance the performance, future availability, and sustainability of SCMs.

1. Introduction

The application of supplementary cementitious materials (SCMs), such as fly ash, slag, and calcined clay, among others, has become common in cement-based products such as Portland cement, concrete, and mortar. This can be attributed to the numerous advantages SCMs offer. First, utilising SCMs as a partial clinker replacement in cement production significantly reduces CO₂ emissions. Depending on the technologies used, up to 0.84 tonnes of CO₂ are produced while manufacturing 1 tonne of cement [1,2]. The bulk (about 90%) of this CO₂ is produced through the decomposition of limestone and the combustion of fuel during pyro-processing, while about 10% comes from electricity generation and the transportation of materials [1,3,4]. Overall, cement production accounts for approximately 8% of global CO₂ emissions [5,6,7] and contributes over 90% of the CO₂ emissions of the concrete industry [4,8]. Consequently, studies have identified SCMs as a cement replacement as a practical and far less expensive approach to mitigate CO₂ emissions than existing carbon capture, storage, and use technologies [2,5,6]. Depending on the type of SCM and the amount of clinker replacement, researchers have shown that a 13–35% reduction in greenhouse gases can be achieved by using limestone, fly ash, and slag [9,10,11]. By comparison, switching the fuels used for pyro-processing to carbon-neutral fuels or energy will not effectively reduce emissions, as modern kilns are almost at the limits of thermodynamic energy efficiency [6].

Additionally, SCMs are used to intelligently manipulate the binder phase of concrete and mortar to meet and enhance specific performance requirements, such as workability [12], strength [13,14], durability, and microstructure [15,16]. The improved performance reduces the need for frequent repairs in addition to increasing the service life of structures, thereby saving costs and conserving natural resources. This improves the overall sustainability of the construction industry.

Furthermore, most SCMs consist of industrial and agricultural waste that would typically be sent to landfills, leading to environmental and social issues, including the costs and emissions generated during transportation. In addition to the socio-environmental benefits of diverting waste from landfills, clinker is significantly more expensive than SCMs; therefore, incorporating SCMs in cement-based products results in considerable cost savings.

Despite these advantages, a few limitations should be noted with regard to the use of SCMs. Due to the low hydraulicity of most SCMs compared to Portland cement, the setting time and strength development of concrete produced with SCMs may be delayed, which is an important factor in the precast industry, where the formwork turnaround time is of the essence. This can be addressed with appropriate chemical additives such as accelerators and alkali activators. Furthermore, SCMs reduce the carbonation resistance of concrete compared to conventional Portland cement concrete. This is due to (a) the partial replacement of clinker, which reduces the overall alkalinity and carbonation buffering capacity of the binder, and (b) the consumption of Ca(OH)₂ by the pozzolanic reaction, which reduces the amount of carbonatable materials in the binder matrix. However, of the two mechanisms—chloride ingress and carbonation—that cause the corrosion of reinforcing steel, carbonation is by far the least widespread problem facing reinforced concrete globally [2]. Even in the case of carbonated reinforced concrete, the deterioration of steel may not occur, as the conditions that favour carbonation do not often favour steel corrosion. For instance, the optimal relative humidity for carbonation is 55–75%, whereas a critical relative humidity of 80% must be met before corrosion is initiated, as discussed by Richardson [17] and shown in Figure 1. Conversely, carbonation is a method of sequestrating CO₂ from the atmosphere, even though it is prolonged, typically requiring decades for the complete carbonation of concrete elements.

This study is timely and important, as cement-based products will continue to dominate the construction industry due to their durability, lack of viable alternatives, economic factors, adaptability to different environments, formability to various shapes, and the near-universal availability of constituent materials [6,18]. To buttress this, the quantities of cement produced over the past 30 years have increased 3-fold from 1.37 billion tonnes in 1994 to 4.10 billion tonnes in 2023, whereas the human population has only increased by 43% (see Figure 2). Within this period, the global annual per-capita consumption of cement has doubled from 0.24 to 0.51 tonnes per capita, and this trend will continue owing to an increase in urbanisation and infrastructural demands. Compared to competing and alternative construction materials such as steel and timber, the gap between these materials and cement production has consistently increased over time. Currently, the quantities of cement produced are greater than the combined quantities of steel and timber (see Figure 2).

Therefore, the continuous use of cement-based materials in construction will necessitate an even higher demand for SCMs, considering the push for more environmentally friendly materials with enhanced performance. However, there is a disparity in the global availability of these SCMs, and in some cases, there is uncertainty about their future availability in some regions [2,13]. In the South African context, there is a dearth of comprehensive literature discussing the status of SCM resources, including their future availability. Previous studies have focused on the characterisation of SCMs [23], condition assessments of SCM stockpiles [24,25], assessments of SCM performance in concrete and mortar [13,14,26,27,28,29,30], and, more recently, the application of SCMs in 3D-printed concrete [31,32,33]. This paper is novel, as it critically examines the current availability, supply, and utilisation levels of different SCMs in South Africa. It highlights the advantages and limitations of each SCM, considering their characteristics and performance. This paper discusses the factors driving demand, production, and utilisation levels and makes projections for the future. This study also recommends areas of future research to enhance the performance of these SCMs and to guarantee their future availability and overall sustainability.

2. Supplementary Cementitious Materials

2.1. Fly Ash

When pulverised coal is burnt in coal-fired electric power plants, ashes are generated. These ashes comprise about 85% fly ash, or pulverised ash, and 15% coarse ash, or bottom ash [34,35]. As the name implies, coarse ash has larger particle sizes (>100 µm diameter); it drops down from the furnace and is collected at the bottom of the ash hopper of the boiler. In contrast, fly ash is collected in the electrostatic precipitator or baghouse.

The properties of fly ash vary significantly and depend on several factors, including the type, chemical, and mineralogical composition of the coal; the extent of coal pulverisation; and production technology, including collection, handling, and storage conditions [36,37]. Generally, silica, calcium, alumina, and iron oxides are present as the major compounds. These oxides are used to classify fly ash and other pozzolanic materials, with the ASTM C618 [38] designating Classes N, F, and C for these materials. Classes F and C are most applicable to fly ash, with the major difference being their CaO content, where Class F has CaO < 18% (wt%) and Class C has CaO > 18% (wt%). Due to the CaO content, Class F exhibits pozzolanic properties, whereas Class C, in addition to its pozzolanic properties, exhibits some hydraulicity. Several test results have shown that South African fly ash is generally Class F, with a high concentration of silicon, aluminium, and iron oxides [13,23,24,25,32,35,39].

Several studies have been carried out on how to utilise fly ash. These studies include but are not limited to the use of fly ash in treating industrial wastewater and remediating acid mine drainage [35,39], reinforcing metal (aluminium) composites to improve hardness and wear resistance [40], and synthesising zeolite used as an adsorbent for heavy metals and in fertiliser production [41,42]. However, these applications utilise low quantities of fly ash compared to the amount produced. Additionally, due to the energy demand of some of these applications, their economic feasibility is questionable. Further, some of these technologies are in their infancy and still require more research before they can be widely adopted.

Perhaps the most common, practical, and sustainable application of fly ash is in cement-based materials. Up to 35% of fly ash can be used as a partial replacement for clinker during cement production; such types of cement are designated as Cement II and Cement IV according to the South African National Standard [43]. The use of fly ash as a binder material in stabilising soil for road pavements [44] and in paving blocks and bricks has been reported [45,46]. For concrete applications, a 20–35% Portland cement replacement with fly ash is common. The glassy nature and spherical shape of fly ash particles create a ball-bearing effect that lubricates concrete and improves workability [12,15]. The pozzolanic reactions between the alumina and silica in fly ash, on the one hand, and the calcium hydroxide produced from cement hydration, on the other hand, form calcium aluminate silicate hydrates that contribute to long-term improvements in concrete strength [15,47], durability [12,16], and microstructural densification in concrete [15,16]. Amadi et al. [13] demonstrated this by utilising fly ash to replace 30% of the cement, significantly improving the long-term (up to 180 days) compressive strength and elastic modulus of concrete produced with fine recycled aggregate sourced from South Africa. Owing to the low reaction rates and the clinker dilution (substitution) effect, fly ash has also been demonstrated to reduce the heat of concrete hydration; therefore, it has found wide application in structures such as dams and large foundations that are susceptible to thermal and shrinkage cracks [48]. To buttress the benefits of fly ash in concrete durability, in their condition assessment of the over 60-year-old Hungry Horse Dam in Montana, USA, Montgomery et al. [49] reported alkali–aggregate reaction (AAR) in concrete cores with only Portland cement as binder but did not observe AAR in cores containing fly ash in the mix.

Despite these benefits and wide application areas, the utilisation levels of fly ash in South Africa are low, leading to issues with its handling and management. Currently, South Africa is running out of ash dams or storage space, with the construction of new storage facilities being capital-intensive, and environmental pollution in the form of dust and leaching at the dump site that may impact groundwater [50]. This ash is generated by companies that operate coal-fired electric plants and coal-to-liquid petroleum facilities. According to the last official government data, the Department of Environmental Affairs [51] reports that 33.3 million tonnes of fly ash and 5.9 million tonnes of bottom ash are generated annually, with only 7% of this “waste” recovered and used mostly for cement-based applications. This fly ash utilisation level is significantly below that of other countries, as shown in

Table 1. Comparison of fly ash generation and utilisation levels for different countries.

Country	Reference Year(s)	Production (Million Tonnes)	Utilisation Level (%)	Reference
South Africa	2017	39.2	7	[51]
Australia	2023	10.2	48	[55]
China	2019	450	70	[56,57]
India	2019–2020	205.1	83.1	[58]
Japan	2019	11.2	98.4	[59]
Poland	2021	1.3	90	[60]
Russia	2019	27.4	10.3	[59]
USA	2022	68.2	62	[61]
All production units are expressed in tonnes for uniformity.
Data for India spans from April 2019 to March 2020

. Table 1 also reveals that South African data needs to be updated, considering recent developments in which two coal-fired power stations with a combined generation capacity of 9600 MW in Kusile and Medupi were completed and became operational between 2020 and 2024 [52,53]. Within the same period, the last generating unit of the Komati power station, with a capacity of 121 MW, was decommissioned in 2022 [54]. Overall, there has been substantial net growth in coal-fired power generation since the last waste report in 2017 by the Department of Environmental Affairs [51]; therefore, the amount of fly ash generation is expected to increase accordingly.

The low levels of fly ash utilisation in South Africa can be attributed to various reasons. First, coal-fired power plants are clustered within the Mpumalanga, Gauteng, and Limpopo Provinces in the east of the country, as shown in Figure 3; therefore, transporting fly ash across the country may raise costs. Furthermore, for industrial applications, fly ash is typically used at levels below the allowable 35% cement replacement level indicated by the South African National Standard [43]. More importantly, fly ash is produced at a rate that significantly exceeds the annual production rate of cement, which is currently estimated at 15 million tonnes per annum in South Africa [62].

Owing to the low utilisation level of fly ash and the reliance on coal-fired power plants for 85% of the country’s power [64], there is currently an enormous stockpile of coal, which has been increasing annually. Using the past 44 years (1980–2024) as a reference, and assuming that a conservative estimate of an average of 23 million tonnes of fly ash is stockpiled per annum, it is estimated that over 1 billion tonnes of potentially usable fly ash are stockpiled in South Africa. This estimate is based on coal being the major source of power, with the total electricity generation capacity increasing from 23 GW in 1980 to 40 GW in 1990 [65] and 53 GW in 2024 [64].

To limit the environmental and health issues posed by the continuous stockpiling of fly ash, it is therefore imperative that the utilisation levels of fly ash be increased, including the recovery of already stockpiled materials. The latter is predicated on studies by Šulc et al. [66], who reported comparable physical and chemical properties of freshly produced fly ash and fly ash deposited in ash dams over several decades in the Czech Republic. In a related study of an ash dam in Mpumalanga Province in South Africa, where fly ash was disposed of in a brine solution, Mahlaba et al. [24] showed that the physicochemical properties of fly ash did not change significantly after 16 years of storage. Compared to freshly produced fly ash, the amorphous chemical phases of the stored fly ash decreased by only 10%, thus indicating its suitability for use as a pozzolan in cement-based materials [24].

It is worth noting that the brine originates from desalination processes in coal-power plants, and it is assumed that by disposing of it in ash dams in the form of a fly ash–brine slurry system, particulate air pollution will be eliminated, and the fly ash will act as a sink for the salts [25]. However, after 20 years of storage in the Mpumalanga ash dams, physicochemical analysis conducted by Eze et al. [25] revealed that fly ash has a low salt accumulation capacity, with most of the salts released to the environment, including leaching into the soil, which raises pollution concerns.

2.2. Slag

Slag is a by-product of heating limestone, iron ore, and coke in a blast furnace to produce pig iron for steelmaking. The molten slag is rapidly cooled with a strong water jet and ground to a fine, amorphous, glassy material called ground granulated blast-furnace slag (GGBS).

Due to quality control measures for steel production, the constituents of GGBS do not vary considerably compared to those of fly ash. GGBS comprises lime, silica, alumina, and magnesia as the main oxides. These oxides are present in quantities that give GGBS pozzolanic properties and latent hydraulic reactivity, in that it hardens slowly in water. Consequently, GGBS is predominantly used in the construction industry as a cement-based material, with applications such as a clinker replacement in blended cement, mortar, and concrete. In South Africa, the standards [43] allow for a 36–65% replacement of clinker with slag in blast-furnace cement (CEM III) and 18–50% in composite cement (CEM V). In addition to GGBS, fly ash and silica fume are the most widely used SCMs in clinker substitution in South Africa, and their substitution levels have risen from 12% in 1990 to 41% in 2009, with the goal of attaining 60% by 2030 [67,68]. The relatively large-scale use of these three SCMs has necessitated the establishment of standards governing their use in concrete and mortar, unlike other SCMs. These standards include SANS 55167 [69], SANS 50450-1 [70], and SANS 53263-1 [71] for GGBS, fly ash, and silica fume, respectively.

For concrete applications, GGBS has been reported to improve workability through its smooth and glassy surface, which creates slip planes in the paste, as well as its latent hydraulic reactivity, which makes more water available for concrete lubrication [72]. Due to the low reactivity and insufficient alkalis like Ca(OH)₂ from cement hydration for GGBS activation, there is slow early-age strength development for concrete containing up to 60% GGBS as a clinker replacement, but comparable strength properties with plain Portland cement concrete are achieved at 28 days [15]. The improvement in the microstructure and chloride binding capacity of GGBS, which leads to high resistance to the ingress of chlorides and improved concrete durability in marine environments, has been well-researched [72].

In the South African context, different variants of slag enjoy widespread use in concrete applications. For instance, ground granulated Corex slag (GGCS), otherwise known as Corex slag, was introduced in 1999. It has been reported to produce good concrete strength and improve durability properties [14,73] and is suitable for 3D concrete printing [33]. The improved strength and durability were achieved through the Corex manufacturing process, which slightly increased the concentration of calcium and aluminium oxides at the expense of silica, thereby increasing hydraulic reactivity. Similar technologies were subsequently employed to produce other variants of slags, such as ground granulated FeMn arc furnace slag (GGAS), which confers specific performance qualities in concrete [14,30].

Currently, there is no data on the amount of GGBS produced in South Africa per annum. An approximate approach involves estimating GGBS data using the quantity of steel produced. According to the South African Iron and Steel Institute [74], 4.7 million tons of steel were produced in 2024. Using an average figure of 0.3 tons of slag produced for every ton of steel [75], it is therefore estimated that about 1.41 million tons (1.26 million tonnes) of GGBS were produced in South Africa in 2024.

In recent years, local production quantities of steel have been consistently declining, mainly due to energy and input costs, thereby resulting in the import of finished steel to meet local demand [76]. This is particularly highlighted in the closure of the Saldanha steel plant in the Western Cape Province in 2020. There is also an imminent closure of two major steel plants in Newcastle and Vereeniging if the government does not intervene with a stimulus package and policy changes [77,78]. Before closure, Saldanha Steel was producing about 310,000 tons of Corex slag per annum [79]. These developments raise major concerns about the future availability of slag in the country.

2.3. Silica Fume

Silica fume is a by-product of the manufacturing of silicon and ferrosilicon alloys. The process involves the reduction of high-purity quartz to silicon at about 2000 °C in an electric arc furnace, thereby releasing silicon dioxide vapour. At the top of the furnace, the vapour oxidises and condenses in the low-temperature zone to form ultrafine spherical particles called silica fume, micro silica, or condensed silica fume [80,81].

Silica fume consists of over 85% amorphous silica, with an average particle size of less than 0.3 µm, approximately two orders of magnitude smaller than cement particles. Due to the superfine particle size, silica fume has a high specific surface area, which can lead to particle agglomeration and raise the water demand of fresh concrete, necessitating superplasticisers [81]. Also, the ultrafine and highly amorphous nature of silica fume makes it a very reactive pozzolan, which contributes to the early and high strength and improvement in the aggregate–binder bond of mortar and concrete [15,80], pore filling and microstructural densification of concrete [15], reduced permeability, and improved concrete durability [82]. The typical replacement level of cement with silica fume in concrete is 5–20%. In Cem II/A–D blended cements, silica typically replaces 6–10% of the clinker [43].

Globally, the quantity of silica fume production has been fairly constant in recent years, with an estimated 1 to 1.2 million tonnes produced in 2023. China, Norway, the USA, Russia, and Japan accounted for over 75% of this total [83]. The 2020 to 2024 data reveal that South Africa contributes less than 10% of global production, translating to approximately 50,000 to 100,000 tonnes of silica fume annually [83], with a significant portion of this quantity being exported.

Compared to the production of fly ash in South Africa, the quantities of silica fume are significantly lower. Furthermore, silica fume is approximately six to ten times more expensive than slag and fly ash, respectively. Due to these factors, along with the challenges of handling, transporting, and mixing the ultrafine particles, the use of silica fume is often restricted to niche applications in high-performance concrete.

2.4. Natural Pozzolans

Natural pozzolans are a wide range of amorphous aluminosilicate materials, including but not limited to volcanic ash, natural zeolite, tuffs, pumice, scoria, and perlite. Most of these pozzolans are of volcanic origin and thus can be found as natural deposits in geologic areas of volcanic activity. They are designated as Class N pozzolans according to ASTM C618 [38]. The South African standard [43] specifies two types of natural pozzolans: natural calcined pozzolana, which has been activated by thermal treatment, and uncalcined natural pozzolans, which are simply referred to as natural pozzolans.

2.4.1. Natural Pozzolans (Uncalcined)

The application of natural pozzolans can be traced to the ancient Romans and Greeks, who used volcanic tuffs and their variants, such as Santorinian earth, Milos’s earth, and Skydrian earth, as hydraulic binders to construct infrastructure and monuments that are still in existence today [84]. Natural pozzolans such as zeolites and diatomite may increase the water demand of fresh concrete due to the combined effect of their porous internal structure and higher fineness compared to cement, which increases the specific surface area and water absorption characteristics [84,85]. The purpose of grinding these pozzolans finer than cement is to increase their otherwise low reactivity. Locally in South Africa, Sinngu et al. [86] demonstrated that using 20% natural zeolite as a clinker replacement in blended cement met the ASTM C618 [38] Class N pozzolan requirement. The results showed that mortars containing 20% zeolite reduced workability but improved the long-term (180-day) compressive strength compared to Portland cement mixes and mixes containing 30% fly ash. Furthermore, 20% zeolite mixtures proved effective in reducing drying shrinkage and mitigating alkali–silica reactions and sulphate attacks [86]. In related studies, Sharbaf et al. [87] demonstrated that the compressive strength, elastic modulus, and chloride and abrasion resistance performance of self-compacting concrete containing 15 and 22.5% vitrified rhyolite (a type of volcanic tuff) as an SCM were comparable to, and in some instances better than, the control concrete, especially over long curing periods. Other studies on natural pozzolans include a corrosion assessment of steel rebars embedded in mortars containing scoria as an SCM [88], while Seraj et al. [89] demonstrated that reducing the particle size of pumice as an SCM increased the rates of cement hydration, pozzolanic reaction, and compressive strength of mortars while also increasing mixture viscosity.

In the South African context, SANS 50197 [43] allows for the incorporation of 6–35% natural calcined and uncalcined pozzolans as a clinker replacement in Portland-composite and pozzolanic blended cement, denoted as CEM II and CEM IV, respectively. However, there is limited data on the types, quantities, and sources of natural pozzolans used in cement manufacturing in South Africa. Nevertheless, the literature reports deposits of natural pozzolans in different parts of South Africa, including volcanic tuff in the Karoo Basin [90], pumice in the Eastern Cape province [91], and natural zeolite in the Western Cape and KwaZulu-Natal provinces [86].

A major issue limiting the large-scale industrial application of natural pozzolans in South Africa, and by extension most parts of the world, is the cost of quarrying, processing, and transportation, which is significantly higher than the cost of waste-derived SCMs such as fly ash and slag [85]. Also, not all natural material sources that meet the ASTM C618 [38] requirement are pozzolanic, as has been demonstrated in the literature, where quartz and some natural minerals were shown to meet ASTM C618 [38] specifications but did not pass pozzolanic tests [92]. This highlights the variability that exists with natural pozzolans, wherein the sum of silicon, iron, and aluminium oxides may meet the 70% ASTM C618 requirement, but these oxides are not present in suitable proportions to possess pozzolanic properties. Consequently, there is a need to test potential sources of natural pozzolans to assess their suitability in cement, mortar, and concrete applications.

2.4.2. Limestone Calcined Clay

Calcination at 700–850 °C can activate kaolinite clay through dehydration, dihydroxylation, and the breakdown of the crystalline structure, producing metakaolin. Metakaolin is an amorphous alumino-silicate material, typically used as an SCM because of its pozzolanic reactions that generate calcium alumino-silicate hydrates (C-A-S-H) and other aluminate hydrates, which enhance strength and durability in mortar and concrete. However, the market price of metakaolin is approximately three times that of Portland cement due to its alternative applications in the paper, ceramics, and refractory industries, as well as the high production costs and stringent requirements regarding colour and purity [93]. Consequently, to improve the economic viability of calcined clay in cement-based materials, the use of otherwise “dirty clays” with a kaolin content of at least 40% has been proven to be suitable as an SCM [27,93]. Furthermore, to optimise environmental benefits, the clinker content or clinker factor must be reduced as much as possible by increasing the metakaolin content beyond the typical 5–20% threshold without compromising concrete performance. To achieve this, Antoni et al. [94] investigated a blend of metakaolin and limestone in a 2:1 ratio and reported the optimal performance of mortar with 30% and 15% of metakaolin and limestone contents, respectively. The study showed that mortars containing this blend exhibited better mechanical properties at 7 and 28 days than the reference 100% Portland cement mortar [94]. According to Sharma et al. [95], this marked the beginning of a new type of ternary binder system, often referred to as limestone calcined clay cement (LC³).

The LC³ binder is based on the synergistic chemical reactions among the constituent materials in LC³, where the calcined clay participates in pozzolanic reactions with calcium hydroxide, while the alumina in metakaolin reacts with limestone to form carboaluminate hydrates, in addition to the filler effect of limestone [85,95]. Following this, researchers have optimised the LC³ composition to achieve a clinker replacement of up to 50% and above and have demonstrated comparable, and sometimes even better, mechanical properties and durability of mortars and concrete [27,95,96,97]. Another advantage of LC³ is that the calcination temperature is much lower than the 1450 °C required for clinker production, and there are no significant CO₂ emissions associated with the calcination process, leading to substantial cost and environmental benefits [98]. Additionally, since clay is a raw material in cement production, kaolinite clays may be readily available around cement plants, and the clay calcination process can occur in existing cement manufacturing kilns, thereby resulting in savings in new capital investment and transportation [93].

On the downside, calcined clays can exhibit various colour shades, including brown, grey, yellow, white, and red, depending on the iron content [27,95], which can influence the colour of the final LC³ blend. Colour, as a quality control parameter, can pose concerns for cement producers and consumers. Recent advancements in calcination technologies can address this, as demonstrated by Martirena et al. [99], who controlled the calcination environment, particularly the oxidation occurring during the cooling phase, thereby producing black LC³ instead of red. Both colour variants of LC³ possess similar properties, as observed in their reactivities and the strength of the resulting mortars [99]. Additionally, the high surface area of LC³ increases the water demand of fresh concrete, but this can be managed by using appropriate superplasticisers.

In South Africa, there are significant deposits of limestone resources in all nine provinces of the country, with the largest deposit occurring in a 150-km-long belt along the Northern Cape boundary [100]. Further, over 20 deposits of kaolinite clay have been reported in different parts of the country, including vast areas of the Western Cape Province, Grahamstown in the Eastern Cape Province, and the Pretoria/Bronkhorstspruit area of Gauteng [27,101]. Combined, these deposits are estimated to have about 1.1 billion tons (1 billion tonnes) of clay reserves [27,97]. Other identified deposits include Namaqualand in the Northern Cape and other provinces such as Limpopo, Mpumalanga, North West, and Kwa-Zulu-Natal [101].

It is, however, important that clay sources be tested to ascertain their suitability for use as SCMs, since it has been reported that only clays with a minimum of 40% kaolinite content can be utilised to obtain good reactivity [27,93]. Studies [27,102] have shown that clays from Bronkhorstspruit in Gauteng and Hopefield in the Western Cape are suitable as SCMs, but samples collected from Grahamstown in the Eastern Cape do not meet the requirements for use as SCMs due to their kaolinite content of 30%. Further studies are needed to characterise other clay deposits in the country, including Grahamstown. This is because the kaolinite clay deposit in Grahamstown is a field with at least six occurrences [101]; thus, the material properties may vary across the field. Furthermore, recent studies have shown that low-kaolinite clays (<40%) can be utilised for low-strength and mild-durability exposure conditions [103] and/or by reducing the limestone and calcined clay content in the LC³ blend to 30% [104,105]. These approaches could increase the utilisation potential of clay from Grahamstown and other local sources that do not meet the 40% kaolinite content threshold.

2.5. Agricultural Waste

Agricultural activities generate a substantial amount of waste, commonly referred to as biomass waste. Biomass waste is managed through various processes, including burning to generate electricity, production of biofuels such as bioethanol and biodiesel, utilisation as insulation materials, or even open-pit fires. Studies by Chipfupa and Tagwi [106] reported that open-field burning is a predominant practice in South Africa, with only a small portion of biomass waste utilised for soil fertilisation and livestock feed. The burning process creates ash, and the yield and chemical composition vary depending on the crop. These ashes can contain amorphous silica (20–90% by weight) and other cementitious oxides, making them suitable for use as SCMs [107]. Consequently, the most researched ashes are produced from rice husks and straws, sugarcane bagasse and leaves, cassava peels, palm kernels, wheat straw ash, coffee husk ash, and corn cobs.

Farrant et al. [108] investigated the performance of sugarcane bagasse ash sourced from South Africa in binary and ternary concrete mixes and reported that, in terms of strength and durability, a ternary blend comprising 60% Portland cement, 10% silica fume, and 30% sugarcane bagasse ash yielded good performance. Using the central composite design method, Iro et al. [109] demonstrated that replacing 30% of Portland cement with cassava ash produced the optimal compressive and flexural strength of concrete. Van Tuan et al. [110] showed that rice husk ash and silica fume have similar chemical compositions, with high silica contents of 88% and 97%, respectively, largely comprising amorphous silica and a very high specific surface area. However, unlike ultrafine silica fume particles, the high specific surface area of rice husk ash is due to its internal porosity. Compared to mortars containing 100% cement and those containing 20% silica fume as a cement replacement, studies have shown that mortars containing 20% rice husk ash require a higher superplasticiser dosage to attain the desired workability. The porosity of rice husk ash resulted in internal curing, leading to a higher compressive strength than the reference cement and silica fume mortars at 7, 28, and 91 days [110]. Other studies have reported mixed results on the performance of agricultural waste as SCMs. For instance, compared to the reference 100% cement mix and the 80–20% cement–fly ash mix, Shakouri et al. [111] reported early-age (2–3 h) accelerated hydration and heat release in the 80–20% cement–corn cob ash binder paste. In the concrete test, the results showed that the compressive strength and bulk electrical resistivity of the 80–20% cement–corn cob ash mix were significantly less than those of the reference mixes, and the differences in these test parameters became more pronounced as the test ages progressed from 7 to 112 days. The authors attributed these results to the high alkali (potassium oxide) content of corn cob ash, which may have impacted hydration kinetics [111].

In the South African context, potential sources of crops for SCMs include sugarcane, corn, and wheat, with production quantities of 17.9, 16.4, and 2.2 million tons, respectively, as of 2023 [112]. Combined, these crops account for about 80% of South Africa’s total crop production by volume, with sugarcane and corn being the first and second most widely produced crops in the country, respectively [112]. Based on this, it is estimated that a total of 238,600 tons (216,450 tonnes) of ash can be processed from agricultural waste, comprising 118,400 tons of sugarcane bagasse ash, 71,600 tons of corn cob ash, and 48,600 tons of wheat ash. These calculations are based on literature studies that report that 1 ton of sugarcane produces 6 kg of bagasse ash [113], whereas 1 ton of corn produces 180 kg of cobs [114], with the ash content of corn cobs being 2.2% [115] and the ash content of wheat being 2.21% [116]. The data make a major but unfeasible assumption that all the sugarcane bagasse, corn cobs, and wheat produced in South Africa are converted to ash for SCMs at the expense of other applications of agricultural waste.

Compared to the estimated 15 million tonnes of annual cement consumption in South Africa [62], the total potential ash production of 238,600 tons (216,450 tonnes) can only replace about 1.5% of clinker. This value, which cannot significantly contribute to mitigating the CO₂ emissions associated with cement production, is within the 0.4–5.9% limit of the potential cement replacement capacity of agricultural waste in most African countries, as reported by Schmidt et al. [117]. Other challenges of agricultural waste as SCMs include the seasonal occurrence of crops that produce ash, variability in material properties and supply, lack of infrastructure for material processing, and the complex and costly logistics required to collect and homogenise the ashes from different producers across the country [117,118,119]. Additionally, the absence of these SCMs in the market and in commercial quantities, which is largely due to their non-availability in a ready-to-use form, is a major challenge to their widespread adoption [120].

2.6. Recycled Concrete Fines

Recycled concrete fines (RCFs) are the powder (<0.15 mm) materials produced when recycled aggregates (RAs) are processed from the concrete and mortar components of construction and demolition waste (CDW). Generally, the cement paste adhering to RAs becomes concentrated in the RCF fraction, making RCFs suitable for use as an SCM. This cement paste comprises phases such as Ca(OH)₂, C–S–H, AFm, AFt, and gypsum, and depending on age and exposure conditions, may contain unhydrated cement and CaCO₃. To demonstrate the use of RCFs as an SCM, researchers [121,122,123] employed a wet carbonation process involving the introduction of about 10% CO₂ at ambient temperature (20 °C) in the presence of NaOH or Na₂SO₄. This process produced calcite and amorphous alumino–silica gel, the latter being a pozzolanic material. Zajac et al. [121] investigated the performance of composite cement pastes incorporating carbonated cement paste as SCMs versus limestone cement, both used to replace 40% of clinker. The results showed that, at all test ages (1–180 days), the carbonated cement paste mixes exhibited higher compressive strength. Meanwhile, the calcium carbonate reacted with the available alumina to form monocarbonates and stabilised ettringite [121]. In other studies, Li et al. [124] used untreated RCFs as an SCM. The study involved three fractions of RCFs, with maximum particle sizes of 0.045 mm, 0.075 mm, and 0.15 mm, used to replace 10, 20, and 30% of cement in mortar. Mortars containing 10% RCFs with a maximum particle size of 0.045 mm achieved the best compressive strength, comparable to the control mix with 100% Portland cement. The results also revealed that incorporating RCFs increased the porosity of mortar, and this effect became more pronounced with larger RCF particle sizes.

Although RCFs represent a promising source of SCM, the lack of recent data on CDW in South Africa [125] makes it difficult to determine the quantity and viability of RCFs as an SCM. According to the last published data from 2017 [51], about 4.5 million tonnes of CDW are generated annually, comprising concrete, bricks, metals, glass, asphalt, timber, paper, ceramics, plastics, and gypsum. Based on this data—although some researchers [125,126] argue it is largely underestimated—concrete and mortar materials are assumed to account for 30% of CDW. Following the findings of Rangel et al. [127], RCFs make up 3% of CDW, which suggests that about 40,500 tonnes of SCM could be processed annually from CDW. However, this quantity is not substantial, as it constitutes less than 0.3% of South Africa’s annual cement requirement of 15 million tonnes. Furthermore, beneficiation processes such as crushing CDW, detaching the attached cement paste, thermal activation, and sieving may increase costs. The variability in RCF properties—due to differences in binder type in the CDW concrete—poses challenges for its application as SCM. The practical implications of this variability include inconsistent chemical composition, fineness, and reactivity, which could hinder large-scale industrial use.

3. General Discussion and Future Projections of SCMs

The estimated production quantities and stockpile of SCMs, as discussed in Section 2, are summarised and presented in

Table 2. Estimated availability of SCMs in South Africa.

SCM	Annual Production (Million Tonnes)	Stockpile/Deposit (Million Tonnes)	Comment
Fly ash	>39.2	>1000	Continuous increase in the stockpile
Slag	1.3	NA	Continuous decrease in annual production, raising concerns about future availability
Silica fume	0.05–0.1	-	Production is forecast to remain largely unchanged in the foreseeable future
Calcined clay	-	>1000	Great potential but largely unutilised
Agricultural waste	0.22	-	Largely unutilised, with limited viability
Recycled concrete fines	0.04	-	Limited viability

. Based on Table 2, it can be inferred that the available quantities of slag, silica fume, agricultural waste, and RCFs are insufficient to meet the current and future SCM needs of South Africa. This inference is based on their combined production quantities, which amount to only a possible 10% cement replacement, given a cement consumption of 15 million tonnes per annum in South Africa. For context, Table 2 presents a best-case scenario for agricultural waste, which is practically unfeasible due to several barriers. These include alternative applications of agricultural waste such as energy generation, biofuel production, soil fertilisation, livestock feed, and insulation materials, among others. Other challenges include variabilities in quality and supply, seasonal availability, logistics, and the absence of SCMs in a ready-to-use form [117,118,119]. Regarding slag, the forecast indicates that due to energy and input costs, as well as competition from steel imports, local production of slag is expected to decline steadily, raising concerns about future availability. In the case of silica fume, future production quantities are not expected to change significantly, aligning with global trends. Overall, due to limited availability, cost, and performance, the use of silica fume and slag will remain restricted to niche applications such as high-performance concrete and marine environments. On the other hand, compared to the other SCMs discussed in this study, RCFs are the least viable, considering their quantity, variability, and the cost of beneficiation.

Table 2 also shows that fly ash and limestone calcined clay represent the most promising sources of SCMs for the future. Each of these SCMs is estimated to have a deposit of 1 billion tonnes. Consequently, researchers, the industry, and policymakers should focus on how best to utilise and optimise the performance of these materials in cement-based applications.

4. Recommendations and Areas of Future Research

4.1. Exportation of Fly Ash

It is recommended that some of the excess/unused SCMs generated in South Africa should be exported, especially to other African countries. This is predicated on the following arguments. First, although there is scope for increased local SCM utilisation in cement-based applications, particularly blended cement, the production rate of fly ash, as well as the existing stockpile, far exceeds the annual production rate of cement. Additionally, exporting fly ash has the dual benefit of generating revenue and reducing the volume of fly ash that would otherwise be deposited in scarce landfills or ash dams, with their attendant environmental and socio-economic consequences. Furthermore, the reported unavailability of SCMs such as fly ash and slag in most African countries causes reliance on imports from outside the continent [117,118]. Therefore, importation from South Africa will meet local demand at a reduced cost and with lower emissions due to shorter travel distances.

Importantly, future studies should assess the economic and environmental benefits of exporting fly ash from South Africa to other African countries. These studies should include a comparative analysis of sourcing fly ash from South Africa vs. the current practice of importing fly ash from outside the continent.

4.2. High-Volume Fly Ash

Following the enormous deposition of fly ash in ash dams across the country, it is recommended that future research focus on utilising high-volume fly ash (HVFA) in mortar and concrete. HVFA refers to 50% or more fly ash used as a replacement for cement. The use of HVFA results in longer setting times and slow early-age (1–7 days) strength development. The main concerns in this case lie within the precast concrete industry, where early strength development is essential for removing formwork, managing project timelines, and controlling costs. Future research should aim to address this limitation using various techniques highlighted in the literature [128,129], including but not limited to high-temperature curing; the use of ultrafine materials such as fly ash, silica fume, and limestone; the application of chemical activators like calcium hydroxide, sodium silicate, or sodium sulphate; and the incorporation of fibres.

4.3. Inclusion of LC³ in the Cement Standards

Considering the significant deposits and widespread availability of limestone and kaolinite clays in South Africa, their potential for large-scale (50% or more) clinker replacement, and their performance in concrete and mortar, it becomes imperative that LC³, as a type of cement, should be included in the South African National Standards (SANS). It is acknowledged that SANS 50197 [43] allows for the incorporation of 6–35% natural pozzolans as clinker replacements in CEM II and an even higher level of clinker replacement of up to 55% in CEM IV. However, according to SANS 50197 [43], the up to 55% clinker replacement in CEM IV can be achieved by blending natural pozzolans with fly ash and silica fume. This implies that SANS 50197 [43] needs to be updated to reflect recent research findings showing that LC³ can be used to replace 50% of clinker or more, without blending with fly ash and silica fume. The development of LC³-specific standards will guide and promote higher levels of clinker replacement, enabling their widespread use. To achieve this, further research is needed to characterise clay resources in South Africa, including their short- and long-term performance in cement-based materials.

4.4. Government Incentives and Policies

The government should implement policies and fiscal incentives that encourage the widespread adoption of SCMs in construction. One approach could involve funding pilot LC³ projects to showcase the performance and economic benefits of LC³. This will raise awareness and build confidence among industry stakeholders, including engineers, architects, contractors, and artisans.

Additionally, the Carbon Tax Act No. 15 of 2019 [130] should be enforced within the cement and construction sectors. The Carbon Tax Act, introduced by the South African government, offers credits and tax offsets to organisations operating below their greenhouse gas emission targets. Enforcing this law will incentivise cement producers to adopt large-scale clinker replacement via LC³ and high-volume fly ash as practical methods to reduce greenhouse gas emissions.

5. Conclusions

Based on the discussions presented in this paper, the following conclusions can be drawn:

1. Cement-based materials are the most widely used construction materials and will continue to dominate the construction industry in the future. Therefore, the use of SCMs is important as a practical approach to curbing environmental emissions associated with clinker generation.

2. Fly ash and limestone calcined clay are the most abundant SCMs in South Africa. With an estimated deposit of 1 billion tonnes each, these SCMs represent the most promising supplies for the future.

3. In the case of fly ash, the utilisation level stands at 7%, which is significantly below the levels in most other countries, leading to continuous increases in stockpiles deposited in ash dams. Conversely, clay deposits, like most natural pozzolans, may require investment for exploration, quarrying, processing, and research to assess their suitability for cement-based applications. However, limestone calcined clay can be used at higher clinker replacement levels than fly ash.

4. Exporting fly ash to other African countries is a practical solution to address concerns regarding the growing fly ash stockpiles. The benefits of exportation include generating revenue for the South African economy, meeting the SCM needs of the African continent, and addressing the social and environmental pollution issues caused by fly ash storage.

5. Slag resources are depleting due to the steady decline of local steel production resulting from energy and input costs, along with the increasing importation of steel products. This has led to the closure of a major steel plant in recent years, and there is a further possibility that two plants will close soon if the government does not intervene with a stimulus package and policy changes. These developments raise major concerns about the future availability of slag in the country.

6. Due to the limited availability of silica fume and slag, with a combined estimated production of 1.4 million tonnes per annum, their utilisation is restricted to niche applications in high-performance concrete and marine environments.

7. Up to 216,450 tonnes of SCM could be processed annually from agricultural waste. However, this source of SCM may not be viable for several reasons, including competing/alternative applications of agricultural waste, limited production quantity, non-homogeneous quality, seasonal availability and supply, and the absence of this SCM in a ready-to-use form.

8. Compared to the other SCMs discussed in this study, recycled concrete fines has limited viability due to its quantity, variability, and the cost of beneficiation.

9. This study recommends the development and inclusion of LC³-specific cement standards to guide and promote widespread use. Also, to raise awareness and build stakeholders’ confidence, the government should fund pilot projects to demonstrate the performance and economic benefits of LC³. Furthermore, future research should address limitations such as the longer setting time and slow early-age strength development of HVFA for use in mortar and concrete. This will increase the utilisation levels of fly ash and address the social, economic, and environmental issues caused by fly ash disposal. Additionally, enforcing the Carbon Tax Act will incentivise cement producers to adopt LC³ and HVFA as practical methods to reduce greenhouse gas emissions.