1 Introduction

Concrete has an estimated global production of 10 billion m3 per year, and its widespread use causes several environmental problems, including harmful gas emission to the environment during the extraction, transportation, and application of the material [1]. According to the International Energy Agency [2], manufacturing Portland cement requires the calcination of limestone at high temperatures, making this process responsible for 7% of global CO2 emissions. The Global Cement and Concrete Association [3] states that the average CO2 emissions per ton of cement worldwide were 635 kg and 564 kg in Brazil. The production of one ton of cement consumes around 1600 kg of natural resources [4] and, as well as being responsible for the emission of polluting gases, producing cement contributes to acid rain [5, 6].

In Brazil, Portland cement production in 2021 was around 65.8 million tons [7]. Data from the Ministry of Mines and Energy [8] indicate that per capita cement consumption in Brazil in 2020 was 286 kg/habitant. This number would be linked to its relatively low cost and the diversity of the products that make it up. However, the increase in input prices, such as thermal and electrical energy, freight, packaging, and refractories, has made its production more expensive [7]. This fact is linked to the COVID-19 pandemic, which has impacted the global economy and reduced Brazil’s Gross Domestic Product (GDP) by 4.1% [9], causing a misalignment between supply and demand and the devaluation of the real.

In the world’s context, several authors have researched using concrete, mortar, or paste, replacing cement with supplementary cementitious materials, such as granulated blast furnace slag, rice husk ash, and active silica, among others [10,11,12]. Abd-Elrahman et al. [13] investigated the use of 5% peanut husk ash (PHA) as a partial substitute for ordinary Portland cement (OPC) in ultra-high-strength concrete (UHSC). The optimal treatment temperature for PHA was found to be 600 °C. The results showed that this substitution achieved high mechanical characteristics, demonstrating the potential of PHA as a sustainable alternative in UHSC applications, repurposing agricultural waste, and reducing reliance on traditional cement. In turn, Hakeem et al. [14] combined industrial and agricultural waste to create affordable and sustainable ultra-high-performance concrete (UHPC). These authors found that replacing 20% of OPC with wheat straw ash (WSA) yielded the best mechanical characteristics. Additionally, increasing the WSA replacement content while keeping a glass particles (GP) substitution content fixed significantly reduced drying shrinkage values. These results demonstrated the potential of using GP and WSA to produce sustainable UHPC with significant environmental benefits. Similarly, Mohamed et al. [15] focused on evaluating the suitability of replacing OPC with 5%, 10%, and 15% of activated alum sludge (AAS) waste as a pozzolanic material. They investigated the inclusion of copper ferrite (CuFe2O4) spinel in various OPC–AAS-hardened composites, which enhanced their physicomechanical properties at normal curing ages and improved their stability against firing. The composite containing 90% PC, 10% AAS waste, and 2% CuFe2O4 demonstrated significant economic and environmental benefits. Ibrahim et al. [16] investigated the impact of polymer impregnation on the fire resistance and mechanical properties of cement-based composites, especially those with high-volume fly ash (HVFA) at water/cement (w/c) ratios of 0.35, 0.40, and 0.45, cured up to 90 days. They found that reducing the w/c ratio produced denser, stronger cement pastes. Polymer-impregnated samples showed decreasing compressive strength with higher w/c ratios, while OPC and OPC-FA pastes with a 0.35 w/c ratio had higher compressive strength when treated up to 600 °C after 14 and 28 days. In this sense, there has been a great effort by the scientific community to reduce the environmental impact associated with using OPC in construction, with several approaches being developed recently [17,18,19,20].

In this scenario of innovative sustainable materials, one addition that has gained attention from the technical and scientific community is oil shale ash (OSA), which has pozzolanic characteristics demonstrated by silica, hematite, and alumina in its composition. Furthermore, OSA contributes to concrete’s strength since the soluble silicon dioxide (SiO2) and aluminum oxide (Al2O3) present in its composition react with the calcium hydroxide (Ca(OH)2) precipitated during cement hydration and produce calcium silicate hydrate (C–S–H) [21]. This chemical compound influences the physical and mechanical properties of cementitious materials. As such, OSA can be used in various construction applications, such as concrete aggregate, partial cement replacement in concrete and mortar, and asphalt mixtures [22, 23].

The successful use of oil shale (OS) as a building material depends, in part, on using large quantities of residual ash produced in its combustion process. With a chemical composition linked to the rocks, they have good pozzolanic properties, i.e., OSA and other OS residues can produce eco-friendly composites with lower cement contents [24, 25]. With this in mind, a thermochemical conversion process to transform OS residues into valuable composite materials has been proposed while addressing environmental issues related to OS residues [26].

According to Yihdego et al. [27], OS is a fine-grained sedimentary rock that can produce liquid hydrocarbons like oil when subjected to destructive distillation at around 600 °C. However, its extraction process is more costly, as the substances in OS are solid and, therefore, cannot be pumped directly from the ground. This rock is found in several countries, such as Estonia, Russia, Jordan, Brazil, Australia, New Zealand, and China, totaling an approximate worldwide reserve of 4 trillion barrels, more than 2 trillion barrels of oil. Furthermore, by 2035, OS would contribute 6% of total energy demand, favoring the major importers [28].

Given the growing concern about the environmental impacts associated with the use of cement by the construction industry, the field lacks a comprehensive understanding of the possibility of using OSA as a cement substitute in cement-based materials because of its potential benefits. While cross-sectional studies have provided some gaps and existing opportunities, there is still a need to examine what has been studied so far and consider the earliest discoveries on the subject, not rather than just the most recent ones. Therefore, as a novelty, this review proposes using the ProKnow-C (Knowledge Development Process—Constructivist) for the first time to study cement-based materials with added OSA. Specifically, the aim is to answer the following questions: (i) What is the ideal percentage for replacing cement with OSA? (ii) What is the optimum temperature for burning OS to obtain the ash? (iii) What is the influence of OSA’s specific surface area when it is incorporated into cement-based materials? (iv) How does the OSA’s chemical composition influence the performance of Portland cement-based composites? (v) What is the relationship between the OSA content and the physical properties of the cement mixture in the fresh state? (iv) How does incorporating OSA into cementitious materials influence their mechanical and durability properties? Thus, a systematic literature review was adopted to find answers to the questions posed and identify areas needing further investigation.

2 Systematic literature review

The ProKnow-C method [29] was used to carry out the Systematic Literature Review (SLR). This method is a structured approach for selecting relevant studies, synthesizing evidence, and identifying knowledge gaps, which has been widely used in the literature [30,31,32]. Figure 1 shows the main stages of the SLR on cementitious materials with added OSA.

Fig. 1
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SLR steps using ProKnow-C

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Initially, the keywords “oil shale”, “ash”, “concrete”, “cement-based material”, “Portland”, and “mortar” were searched in pairs using the filters available in the selected databases—Science Direct, Web of Science, and Scopus. A period of 42 years (from 1980 to December 2023) was adopted as the time frame to broaden the search’s scope and to cover the need for more recent studies on the topic. The search for papers (full articles published in journals) in the databases indicated initially returned 1979 articles which were submitted to the following selection stages prescribed by ProKnow-C (Fig. 1). As a result, the 14 manuscripts chronologically presented in Table 1 were selected to make up the bibliographic portfolio.

Table 1 Bibliography selected using ProKnow-C
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Once the portfolio had been selected, the questions previously presented were used as research lenses to study (i) the cement-based material type, detailing the OSA content used in the mixes, the temperature at which it was obtained, its specific surface area and the chemical composition, (ii) the physical properties in the fresh state and (iii) the compressive strength and durability properties of the materials containing OSA in their composition.

3 Results and discussion

3.1 Cement-based material type

Table 2 depicts the results of the bibliographic portfolio, highlighting the types of cementitious materials studied in the selected bibliography, the type of modification (partial replacement of cement or addition to the mix), and other additions. It also specifies some OSA characteristics, which will be discussed later.

Table 2 Details of the cementitious materials used in the selected bibliography
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The most used cement-based material in the selected bibliography was concrete (60% of the studies), followed by mortar (40%) and cement paste (33%). It is understood that the scientific community’s greater interest in investigating concrete is related to practical application since concrete is one of the most widely used building materials in civil construction.

In the studies that used concrete, the modifications made were the partial replacement of cement or the addition of OSA to the mix. Due to the chemical composition of ash, its use in Portland cement-based composites favors the cementitious and pozzolanic character of the sample. Given this, OSA can be used as an addition or a substitute for cement, with the content varying according to the particle size, composition, and specifications in each paper.

However, some authors suggest that the influence of OSA is better evaluated in compacted mortars since the initial porosity is more controlled and, therefore, the cementing characteristic of the ash reflects differences in the samples’ strength [33]. As with concrete, OSA is used in mortars both as an addition and as a substitute for cement, with the percentage varying according to the specific parameters of the study.

In addition, previous research has reported that cement paste is a more complex material to characterize and study, as it involves a highly reactive cementitious matrix and a limited amount of water. This complexity can make studies more challenging and less common than concrete [33].

3.1.1 OSA content

Figure 2 presents the frequency of each OSA content in the selected articles portfolio.

Fig. 2
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Frequency of each OSA content in the selected bibliography

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The most frequent contents in the selected literature were 10%, 20%, and 30% (eight articles). The intermediate content of 15% was found in two papers, while 40% was tested in three papers. The 50% to 90% range contents were tested in two papers each. The contents of 5%, 18%, 25%, 96%, and 100% were used only once each.

The studies showed that OSA can replace cement in concretes and mortars by up to 30% of the cement mass, defining the 10% replacement content as ideal [21, 38, 39, 41]. Furthermore, OSA as a cement substitute in pastes and mortars has been limited to 20%—all grades are in % by weight of cement from now on [36, 37, 44, 46]. Other studies, on the other hand, have used OSA as an addition in pastes and concretes, varying its concentration between 15%wc and 30%wc [34, 40, 42]. In this context, it was shown that increasing the ash content reduced the concrete’s compressive strength [39, 41, 42].

It should be emphasized that determining the OSA content to be incorporated into cementitious materials is of fundamental importance to guarantee adequate performance and, not least, the efficiency, sustainability, and compatibility of these materials. However, the studies identified the need to present explicit conclusions, generating the need for further studies on this topic. The analysis found that 10% and 30% OSA contents are commonly considered suitable for replacing or adding OSA to concrete and mortar. In addition, increasing the OSA content can lead to a reduction in the concrete’s compressive strength. The choice of OSA content must balance the cementitious and pozzolanic benefit of the ash and maintain the desired mechanical properties of the final material, which justifies greater efforts to determine it.

3.1.2 OS burning temperature

The OS burning temperature is critical for obtaining an OSA with a uniform composition because shale from different regions has different geological characteristics, i.e., equal burning temperature ranges can produce ashes with different chemical compositions [40]. Even so, OS combustion represents an opportunity to standardize the physical and chemical characteristics of the OSA. In this sense, Fig. 3 shows the frequency by burning temperature range in which OSA was obtained in the selected articles.

Fig. 3
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Frequency by OS burning temperature range in the selected bibliography

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It can be seen that OS burning temperatures between 500 and 1100 ºC have been tested in the literature, with the most common being between 600 and 700 ºC (50% of the papers). Temperatures between 700 and 800 ºC and between 900 and 1100 ºC were used in 21% and 14% of the articles, respectively. The other cases were infrequent in the portfolio.

In an experimental study, OSA was burnt in the laboratory at temperatures between 600 and 1100 ºC for 15 min, showing that increasing the temperature reduced the concentration of CaCO3, favoring the formation of free lime. In addition, lime compounds were obtained from 600 ºC, and calcium sulfate (CaSO4) and calcium silicate (Ca2SiO4) were formed between 700 and 800 ºC. This investigation showed that OSA’s grindability—the ratio between the surface obtained and the energy consumed to obtain it—between 600 and 800 ºC is much higher than the same characteristic of the OSA obtained above this temperature. As for compressive strength, better results were achieved using ashes obtained between 700 and 900 ºC [38].

Another temperature range (500 ºC and 1030 ºC) was also used to burn the ashes in the laboratory, revealing that the compressive strength, fineness, and pozzolanic activity of the sample made with OSA obtained at 700 ºC were higher when compared to the others. It occurs because C–S–H developed better in the samples that used OSA obtained at this temperature. The active silica in this material reacts quickly with calcium hydroxide and forms hydrated calcium silicate, favoring the sample’s strength [40].

It is worth mentioning that the OSA used in the other papers in the portfolio were obtained already burnt, i.e., without analyzing the OS burning temperature or duration. For this reason, there is no need to discuss these studies in this section.

Based on the studies cited above, there are indications that the optimum OS burning temperature for obtaining OSA is around 600 to 800 ºC. This temperature range was the most commonly used in the literature and favored the compressive strength, grindability, and pozzolanic activity of the cementitious material in the analysis. This topic can be used for future research.

3.1.3 OSA specific surface area

Analyzing the OSA specific surface area is necessary, as this characteristic influences the material’s reactivity and water absorption. Considering the portfolio, ten articles showed this characteristic, as shown in Fig. 4.

Fig. 4
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Frequency by OSA specific surface area range in the selected bibliography

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The incidence of using OSA with a specific surface area between 5000 and 8000 cm2/g was around 60%, followed by an area between 10,000 and 15,000 cm2/g (30%) and, lastly, values between 3000 and 3500 cm2/g, corresponding to around 10% of the portfolio.

It has been shown that the pozzolanic activity of ash is favored by increasing its fineness [35] and that adding OSA with a specific surface area between 6000 and 8000 cm2/g can contribute to the concrete’s strength since it increases its reactivity due to the area available for precipitation of cement hydration products [21]. It has also been shown that the OSA fineness influences mortars’ strength, with ash particles with an average size of 10 µm favoring mortar’s compressive and flexural strength when added to this material. In addition, it has been shown that the fluidity of the mix is impaired, and the saturation point increases by adding OSA [44].

In this context, it is worth mentioning that the grinding stage of cement production takes place more quickly and with less energy consumption when OSA particles smaller than the clinker are added to the process [45]. It has been reported that the concrete’s compressive strength decreases with increasing OSA content since it increases the water demand and increases the w/c ratio of the mixes, corroborating other results in the literature [39, 41, 42].

It has also been reported in selected literature that the release of heat from concretes containing OSA is slower and more gradual than that of concretes with no addition, leading to a delay in the initial strength gain of the sample [38, 41, 42].

From the above, it can be concluded that OSA is favored in cementitious compounds due to its high specific surface area compared to the cement fineness of 3500 cm2/g [38]. In other words, increasing the OSA fineness favors its pozzolanic activity. Analysis of the bibliographic portfolio suggests that adding OSA with a specific surface area between 6000 and 8000 cm2/g is ideal for improving concrete strength.

3.1.4 OSA chemical composition

According to Table 2, the OSA chemical composition varies according to the temperature at which the ash is obtained and, above all, the mineralogical origin of the rock from which it is extracted. Therefore, assessing these characteristics before applying the ash is essential.

Lime (CaO), silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3) are the main constituents of Portland cement, and these substances are also present in OSA, giving the material its cementitious characteristics [47]. The Brazilian standard NBR 12653 defines pozzolanic materials as those that react with calcium hydroxide and form compounds with binding properties when they have a high fineness and are in contact with water [48]. Given this, studies have attributed a pozzolanic character to OSA [21, 37, 40]. However, only in two studies in the portfolio did the OSA composition meet the chemical requirements for pozzolanic materials in this standard, exposing the mineralogical differences in each region and highlighting the importance of analyzing the material chemically [21, 37].

From this perspective, it is essential to evaluate the ability of OSA to act as a binder in cementitious compounds. In general, OSA is expected to improve the durability of these materials through several mechanisms, reducing porosity to block the sulfate ions’ penetration, promoting a pozzolanic reaction that reduces permeability by filling empty spaces, absorbing alkaline ions to prevent alkali-aggregate reactions, and increasing electrical resistivity, which indicates lower porosity. Furthermore, by reducing permeability, OSA minimizes the formation of efflorescence by limiting the migration of water and salts [48].

3.2 Physical properties in the fresh state

Analyzing the fresh-state physical properties of cementitious materials containing OSA in their composition is essential since the material’s applicability is directly related to the amount of water used in the mixture. However, of the bibliographic portfolio selected, only two studies reported this analysis.

It was observed that the demand for water and superplasticizing additives has increased to maintain the fresh properties of the concrete within acceptable workability ranges. In addition, it was shown that concretes with added ash had satisfactory values in terms of fluidity, viscosity, and resistance to segregation. However, when the replacement of the cement mass by OSA exceeded 10%, there was a negative impact on the material’s compressive strength [41].

On the other hand, it has been shown that incorporating OSA into the mix increases the cementitious material’s saturation point and reduces its fluidity, making it necessary to use suitable additives to improve the concrete’s workability [44].

The analysis of the results shows that the mix’s physical properties in the fresh state are directly related to the OSA content used. Higher contents increase the water demand required to achieve adequate workability. Therefore, the OSA content added to cementitious materials should be controlled, and water-reducing additives can be considered so as not to impact the w/c and, consequently, influence the sample’s compressive strength. This topic could be used in future research.

3.3 Compressive strength and durability

Evaluating cementitious materials’ compressive strength and durability is extremely important, as these characteristics are related to structural performance. Considering the portfolio, 13 articles analyzed the mechanical performance of OSA mixtures, corresponding to around 93%. However, only 43% of the authors discussed the durability of these materials.

Smadi and Haddad [38] concluded that using OSA to replace 30% of the mass of sand or cement in concrete and mortar without changing the sample’s w/c reduces the material’s compressive strength. The authors state that using substitution levels of 10% favors the cementitious material’s compressive strength.

Chan and Ji [37] compared the influence of OSA and active silica on the concrete’s compressive strength. They concluded that both additions increase the samples’ mechanical performance, with silica proving to be more effective, mainly because adding OSA increases the water demand of the mixture and consequently acts negatively on the compressive strength. In addition, the authors revealed that water absorption and chloride diffusion in concrete decrease with increasing OSA content. Using 20% OSA in the mix reduces permeability and contributes to the material’s durability. These results can be explained by the pozzolanic reaction between SiO2, Al2O3, and Ca(OH)2 precipitated during cement hydration, which produces C–S–H and reduces the concrete’s porosity.

Ish-Shalom et al. [33] evaluated the compressive strength of compact samples containing only OSA in their composition, with 8% water about their mass. The authors concluded that the samples strengthened as the curing and hydration period increased, revealing the material’s cementitious characteristics, which aligns with other studies [35, 39]. The different reaction speeds of the calcium silicates in the hydrated cement paste can explain this gain in strength.

According to Bentur et al. [34], the high demand for water due to the OSA fineness means that the composite’s compressive strength with added OSA is lower compared to mixtures without added OSA as the w/c increases, corroborating the literature [35, 41, 44].

Yeĝinobali et al. [39] revealed that replacing cement with OSA in mortars and concrete helps to reduce alkali-silica expansions, mainly due to the high fineness of the ash and the low alkali content compared to other pozzolans. On the other hand, according to Vatin et al. [42], increasing the amount of OSA in concrete increases the material’s thermal expansion due to the high concentration of sulfur trioxide (SO3). When reacting with the tricalcium aluminate present in cement, this compound generates a highly expansive calcium hydrosulfoaluminate. However, the shrinkage of concretes containing OSA was lower.

The cement-based materials’ porosity was also discussed. Bentur et al. [34] concluded that the porosity curve of hydrated cement paste and ash paste is similar, with the latter having a lower compressive strength due to the greater demand for water, which is related to its fineness. According to Feng et al. [21], replacing cement with OSA in concrete reduces the volume of pores larger than 100 nm. It increases the concentration of pores smaller than 50 nm in the hydrated cement paste, thus contributing to the concrete’s compressive strength. Baum and Soroka [43] showed that although OSA paste has higher total porosity than cement paste, their compressive strength is similar. This fact suggests that the gel produced when the ash is hydrated is denser than hydrated cement’s gel. The authors base this conjecture on the fact that, at early ages, the increase in porosity of ash pastes results in a decrease in hydration products, leading to the formation of denser solids than those produced by cement paste.

Table 3 details the physicochemical properties evaluated and the main results achieved by the studies selected for the portfolio. In general, the compressive strength of cementitious materials containing OSA in their composition is favored as long as its use does not alter the mixture’s w/c. It was also observed that some durability properties, such as thermal conductivity, permeability, and chloride diffusivity, benefited from using ash. Table 3 also shows the lack of studies on other mechanical properties of cementitious materials containing OSA, such as static and dynamic modulus of elasticity, tensile strength, and flexural strength, as well as essential durability properties such as ultrasonic pulse velocity, electrical resistivity, carbonation, among others.

Table 3 Properties studied and main findings of the selected bibliography
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3.4 Recommendations for future research

The increasing worry over the environmental effects of utilizing Portland cement in construction has led to several investigations into incorporating alternative materials and additions to cementitious composites. However, there was little interest from the scientific community in investigating the behavior of cement mixtures incorporating OSA, demonstrated by the bibliographic portfolio used in this research—only 4 of the 14 selected articles were published in the last ten years. Therefore, discussions regarding the selected literature highlighted several gaps in knowledge about using as a substitute for cement, mainly in concrete and mortars, and the following topics could be suggested:

  • The OSA performance in different dosages and cement replacement proportions;

  • The influence of OSA production temperature on its pozzolanic activity and the properties of cementitious composites;

  • The effect of OSA particle size and fineness (or specific surface area) on the properties of cementitious materials;

  • The impact of using OSA with different chemical compositions on Portland cement-based materials;

  • The effect of OSA addition on mechanical properties, such as compressive strength, tensile strength, flexural strength, and static and dynamic modulus of elasticity of cementitious composites;

  • The influence of incorporating OSA into cement-based materials on physical properties, such as porosity and water absorption, and durability, such as ultrasonic pulse speed, electrical resistivity, carbonation, and resistance to aggressive agents, such as chlorides;

  • The economic and environmental viability of using OSA on a large scale in the construction industry.

4 Conclusions

This research addressed the growing concern about the environmental impacts of the construction sector’s use of cement and alternative solutions for developing innovative sustainable materials. With this perspective, a SRL was carried out on the influence of incorporating OSA into cementitious materials, and the following conclusions were obtained:

  1. i.

    Contents between 10 and 30% are suitable for replacing or adding OSA in concrete and mortars. This margin tends to balance the cementitious and pozzolanic benefits of the ash with the maintenance of the desired mechanical properties.

  2. ii.

    The optimum OS burning temperature for obtaining OSA is 600 ºC and 800 ºC. This temperature range is the most frequently used in the literature and promotes improvements in the cementitious material’s compressive strength, grindability, and pozzolanic activity.

  3. iii.

    Using OSA in cementitious compounds is advantageous due to its high specific surface area, which is greater than that of ordinary cement. This fact means that the finer the ash, the better its pozzolanic activity. Based on the analysis of the studies presented, it is concluded that adding OSA with a specific area between 6000 and 8000 cm2/g is ideal for improving the concrete’s strength.

  4. iv.

    As a binder in cementitious compounds exposed to humidity, OSA tends to improve several properties, such as resistance to sulfates, reduced permeability, mitigation of the alkali-aggregate reaction, increased electrical resistivity, and reduced efflorescence.

  5. v.

    The OSA content directly relates to the mixture's physical properties in the fresh state. As higher levels require more water to obtain adequate workability, controlling the OSA content added to cementitious materials and considering using water-reducing additives to avoid impacts on the w/c and, consequently, on the sample’s compressive strength is essential.

  6. vi.

    Using OSA in cementitious materials generally improves compressive strength as long as it does not affect the mixture’s w/c. Furthermore, durability properties such as thermal conductivity, permeability, and chloride diffusivity are also beneficial. However, more studies must be conducted on other mechanical properties, such as modulus of elasticity, tensile and flexural strength, and additional durability properties, such as ultrasonic pulse speed, electrical resistivity, and carbonation.

These conclusions indicate that the development of cementitious materials incorporating OSA requires a complete understanding of the effects of such modification. Therefore, all topics covered in this study can be used in future research. It is worth mentioning that these conclusions are limited to the selected bibliographic portfolio.