Performance of Zero Cement Concrete Synthesized from Fly Ash: A Critical Review

. Since the invention of the reinforced concrete ( 𝑅𝑅𝑅𝑅 ) technique, RC buildings have comprised the majority of extant building systems. The shift from traditional materials to green or low/zero carbon designed materials that are energy efficient, such as fly ash ( 𝐹𝐹𝐹𝐹 ), is recognized as one of the desirable approaches to reduce 𝑅𝑅𝐶𝐶 2 emissions and the climate change crisis. This review aims to summarize the performance of fly ash based Zero Cement Concrete ( 𝐹𝐹𝐹𝐹 − 𝑍𝑍𝑅𝑅𝑅𝑅 ) according to the main parameters: Fly ash types (ASTM 𝐹𝐹𝐹𝐹 Class 𝐹𝐹 and Class 𝑅𝑅 ), precursor activator, molarity (Sodium Hydroxide concentration), modulus ratio ( 𝑆𝑆𝑆𝑆𝐶𝐶 2 / 𝑁𝑁𝑁𝑁 2 𝐶𝐶 ), mixture design, mixing approach, compressive strength ( 𝑓𝑓 ’ 𝑐𝑐 ), modulus of elasticity ( 𝑀𝑀𝐶𝐶𝑀𝑀 ), splitting tensile ( f t ), curing time, and curing technique. The findings of this critical review show that the compressive strength of FA-ZCC Class 𝑅𝑅 is higher in comparison with Class F 𝐹𝐹𝐹𝐹 − 𝑍𝑍𝑅𝑅𝑅𝑅 . Ambient curing for 𝑍𝑍𝑅𝑅𝑅𝑅 made from 𝐹𝐹𝐹𝐹 Class C was more suitable compared with Class 𝐹𝐹 , which needed high-temperature curing. Increasing molarity up to 14 led to better ZCC regardless the type of 𝐹𝐹𝐹𝐹 . Modulus of elasticity and tensile strength of 𝐹𝐹𝐹𝐹 − 𝑍𝑍𝑅𝑅𝑅𝑅 was found to be similar to or lesser than those for normal cement concrete. Besides, standard approaches should be provided to enhance the mixture design technique, mixing procedure approach, mechanical properties of 𝑍𝑍𝑅𝑅𝑅𝑅 synthesized by 𝐹𝐹𝐹𝐹 .


Introduction
With a global yearly production of 20 billion tonnes, concrete is currently the most commonly utilized construction material [1,2].J. Aspdin's invention in 1824 proved that ordinary Portland cement () is a superior binder for concrete production [3].Since that,  has become one of the main global sources of energy used in combustion and chemical processes.The emissions of Carbon dioxide ( 2 ) represent about 7% to 8% from global  2 production, which is annually reach to 1.5 Giga-tons [4,5].Construction containing  has some limitations as a result of increasing environmental pollution and corrosion than blended with natural and/or by-product materials e.g.silica fumes, fly ash (), slag, etc. [6].Nowadays, concrete industry is shifting towards a green and sustainable infrastructure, and this can be done by employing natural resources or by-products that are now substituted for cement in the construction industry [4].Currently, a partial or a total replacement of Portland cement by natural Pozzolana or by-product materials is a promising choice to produce a concrete similar to  concrete.Zero Cement Concrete () is a new generation material recently used in construction industry.When compared to , the  2 emissions from  are between 50 and 80 percent lower [7].Although some intriguing and promising sustainable concrete solutions have been developed, research on cementless concrete is still in its early phases [8].
A literature survey has been conducted on the overall performance of  −  including structural behaviour, mechanical properties, mix design, reviews, stress strain relationship, economic and environmental effects (Table 1).Limited studies are available on the development and application of .Fig. 1 shows the number of published articles on  over the years, for example, SCOPUS databases showed limited publications starting from early the year 2000, to reaching their maximum in 2021 with a slow growth rate during 20 years.According to SCOPUS database [8], published documents on  between 1951 and 2021 have just increased by less than 35%.Also, a very limited number of publications on the chosen topic ranging from 2 to 11 journals.
Among the literature, the geopolymer () term and alkali-activated () materials terms appeared as synonyms or as binder without  [74], or somewhat interchangeably [68], despite the fact that the chemical reactions involved in the matrix synthesis in both  and  are fundamentally different [74].According to Ahmad et al. (2019) [66],  is an improved version of  in which the heating activation of the precursors, such as  or , is achieved by using an alkaline solution namely  and  2  3 .Provis et al. (2013) [64] differentiated between geopolymer and alkali-activated materials (); when it contains a lot amount of calcium materials, it is an alkali-activated material, but activated  with low calcium content will give geopolymer.However, Davidovits et al. (1999) [78] remarked that using the term "alkali-activated" could cause major misunderstanding and lead to incorrect assumptions regarding .To sum up, there is no confusion in terminology since the objective is unified toward cleaner and greener production of .

Components of 𝒁𝒁𝒁𝒁𝒁𝒁
is a revolutionary and environmentally friendly building material that is utilized as an alternative to  concrete (Fig. 2).Basically, the main constitutes of  are namely, as shown in Fig. 3, precursor/binder material (rich aluminosilicate binder (e.g., FA), alkaline activators (e.g., a combination of  with  2  3 or  with  2  3 ), Aggregates (coarse and fine), extra water (potable or distilled or deionized) and admixture (e.g., superplasticizer) if needed.It was reported that the best commonly suitable aluminosilicate materials for producing the  are  and GGBFS, which both have been shown to offer affirmative effects [50].The main binder materials are natural or by-products, which are rich in alumina-silicate minerals [23].The raw materials used to make geopolymer are determined by criteria such as availability, pricing, application type and end-user demand.Alkaline liquids are made from soluble alkali metals, commonly Sodium or Potassium.Sometimes, superplasticizers (  ) may be used as a constitute of  [51].According to the testing results, the  dosage was effective for binder content levels ranging from 0.8 to 1.5 percent [26].

Precursors
Precursors are a wide range of aluminosilicate materials (, , calcined clays, metakaolin, silica fume, volcano ash, boiler ash, other Pozzolana, etc.), with varying availability, responsiveness, cost and price around the world [65]. and  are found to be the most common aluminosilicate appropriate precursor resources for the production of , which have been demonstrated to give good results [50].For each precursor material, there have been benefits and drawbacks.Metakaolin has a white colour.The high dissolvability of metakaolin in the reactant solution results in a controlled Si/Al ratio in the  [79].Metakaolin is costly to generate a large quantity since it needs a high temperature (500 C-700 C) for several hours for its calcination.In comparison, using waste materials such as  is economically cost effective [80].

Fly Ash (FA)
is classified into two classes ( and ), which can be measured by  −  fluorescence () results [81], based on  C618 and  M295 [82] (Fig. 4).When compared to  Class ,  Class  has a higher calcium content (more than 20%) [3].This quantity of  is lower than  content (Fig. 5) [83].In general,  is regarded as dangerous to individuals and the environment since it contains acidic and poisonous materials, making it a pollutant [59].Aquatic life may suffer if  is improperly disposed of in the ocean, rivers, or ponds.Annually,  production is about 900 : 500  for China, 140  for India, 115 for  and the  and 14.5  for Australia.Soon,  production is expected to reach 2000  [1].
Depending on the amount of unburned carbon in the ash, the colour of  can range from tan (low  content) to gray (mid  content) to black (high  content) (Fig. 6) [84].The physical properties of  consist of tiny spherical particles, hollow or solid and primarily glassy in composition.Angular particles make up the carbonaceous material in the .Most bituminous coal's  particle size distribution is often comparable to that of silt.(≤ sieve No. 200).Bituminous coal  is slightly finer than sub-bituminous  in spite of that subbituminous coal  is silt-sized as well. has a specific gravity of (2.1-3.0) and a specific surface area ranging from 170 to 1000  2 / [83].conducted an experimental work to investigate the best modulus of Sodium Silicate, which is called molar ratio (modulus ratio= 2 / 2 ).They concluded that the molar ratio higher than 1.5 to 2 resulted in strength reduction and prevent the reaction process and hence, the optimum molar ratio is 1.5.Moreover, the ratio of alkaline solution to  (/) plays a huge role in the properties of  −  and hence lower / led to high strength and good permeability values [59].

Molarity
Sodium Hydroxide is marketed as pellets (granules) or flakes with a purity range of 96% to 98.6%; the price of the product is based on the material's purity [17].Sodium silicate () also known as water glass is available in the market in gel form.The strength of  mortar is significantly influenced by the proportion of  2 and  2  in  gel.Typically, a ratio between 1 and 1.5 produces results that are adequate [17].It has been advised to make the alkaline activator solution ( and ) one day (24 hours) in advance of use to ensure adequate solution mixing [38], Since during the mixing  solution, a great heat is produced and the polymerization occurs by reacting with one another, acting as a binder in the geopolymer mortar.It is recommended that after combining the  and  solution, the solution should be utilized within 36 hours because after that it becomes semi-solid [17].
It was found that the strength characteristics produced better and higher outcomes when increased the molarity dosage of the  solution utilized in the  mix design [88].Higher molarity causes higher ' and less workable materials.According to the literature, high strength in  can be attained when the molarity of the  solution is between 10M and 16M and the ratio of / is between 0.5 and 2.5 [51].It was concluded that the workability was decreased by raising the / ratio from 0.67 to 3.0 and the  molarity from 10M to 20M.The / ratio of 0.67 to 1.0 produced the significant ' [84].Geopolymer with relatively high strengths of 60-70 MPa was obtained when 10M and 15M ; / of 1.0 [89].It was also found that The forerunner of  class  with the maximum ' was  with a molarity of 15M and / of 1.0 and 2.0.Additionally, for mixes with an / ratio of 1.0, the setting time and workability shrank as the molarity increased [84].Nevertheless, the ' of the GPC is unaffected by the mass ratio of alkaline solution to  (/) [88].

Mixture Proportion Design
For decades, unlike , the standard guideline for designing mixture proportions of the conventional  concrete is available such as  111.1 [90].Because of the narrow investigation on  mix design, there appears to be no precise technique that takes into account all of the important parameters.
Li et al. [35] reviewed many papers concerning various mix design procedures of zero cement concrete/mortar based on trials and errors attempts.They concluded that there are three major methods: (1) Target strength method, which involves fixing the content of either water or binder (2) Performance-based method and (3) Statistical factorial model method, which involves Taguchi methods and multivariate regression model.Among these methods, the target strength procedure is the most popular and proper method.They also recommended that the optimal design procedure should be selected based on the situation, demand and required specification of the  production [35].

Mixing procedure
There is currently no standard mix design procedure for  [29].From the literature review, many researchers have used various procedures for a mix design methodology to manufacture the  by trial and error to determine the optimal mix process as shown in Table 2.The parameters were varied around mixing time and which material was mixed first.Gomaa et al. (2018) [84] utilized 4 and 8 mixing procedures to produce 215 mortar mixtures (using one type of  class ) and 80 different mixtures of  (employing four types of  class  with different  percentages) respectively.The mixing procedures are illustrated in Table 3.They concluded that the best mixing steps were procedure number 4 and 8 for  mortar and , respectively, taking into account that mixing dry materials before adding the  was significant to produce good performance mixture including setting time, workability and compressive strength.Besides.increase the mixer speed (from 136 rpm to 281 rpm) and mixing the activator solution prior adding them to the mixture gradually for 5 minutes instead of 1 minute during mixing enhance also the compressive strength, setting time and workability.1  [84] used different regimes of curing: The first regimen involved a high-heat curing process lasting 24 hours in an electric oven set to 70 °C.The second regime included 7 days of ambient temperature (23 ± 2) °.For both curing types, two methods were used to store the specimens after demoulding them: (1) Samples were kept in the lab at room temperature until the testing age as per  C39/2016.(2) To stop moisture loss, specimens were placed in plastic bags as per  C39-2016.Wallah et al. (2006) [80] used two curing approaches for low calcium  based , heat curing either, dry curing ( 24 hr for 60° oven-curing) or curing in steam chamber and ambient curing of the laboratory conditions without any heat-curing.Although the curing regime has been affected by mixture design and chemical and/or physical properties of  − , it is well concluded that heat (oven) curing and ambient curing (or moisture curing) were appropriated for  synthesized by  type F and type C, respectively [84].It should be noted that samples with dry curing produced an ' greater than samples cured in steam hall.
For concrete made with , moisture curing with tab water or lime-saturated water hydrated lime (() 2 ) creates a saturated solution when less than 0.2 percent of the material has melted.This only amounts to (0.9 pounds/55 gallons) or (2 grams/litre) [105].To stop leaching of calcium carbonate ( 3 ) from the concrete, lime is added to the water.The specifications can be found in the edition of  C511-2013 [106].Many researchers have used several types of curing regimes for  −  including an oven or heat curing (with/without steam), curing at ambient laboratory/room temperature and moist curing (using tab water or lime saturated water.For appropriate  −  curing, they recommended the following: • Each specimen rested for two hours at room temperature of (23 ± 2) ºC after the concrete had been cast in the plastic cylinders.

•
The recommended curing time is 6 -6 hrs, but it has also been said that curing times of more than 48 hours are insignificant.

•
Longer curing times result in concrete with advanced ultimate strength and enhanced durability.

•
Under low temperatures (< ambient of 21 to 23 °),  −  needs higher than 24hrs to set because the rate of reaction is slower.

•
Although it is practically hard to perform in situ, an oven curing regime at high temperatures of between (600-900 °) improves the polymerization progression and leads to well gel development, which improves ' and durability characteristics.

•
Longer curing times result in concrete with higher ultimate strength and durability.

•
To stop the water from evaporating after demoulding, for oven and ambient curing, the  −  samples should be wrapped in plastic bags because it stops moisture from evaporating while curing.

•
To develop  −  strength, it is advantageous to apply curing at room temperature for a longer period of time.
•  −  and conventional concrete are approximately similar or lesser in mechanical properties such as modulus of elasticity, compressive and tensile strengths, •  synthesized by    has higher strength than  made from  .
• Precursors for ambient-cured and thermal-cured (high temperature), have been used in class  and class , respectively.• High calcium content of  class , gives an extra hydration reaction that affects strength development.• High temperature curing (30-90) ° for  −  Class F is more convenient than Class F, which preferred an ambient curing temperature.• Compressive strength, modulus of elasticity and splitting tensile strength obtained from  −  can be similar to that of normal cement concrete.• Long curing period led to better strength of  −  regardless of the type of FA.
• Modulus ratio (molar ratio=  2 / 2 ) equal to 1.5 resulted in optimum strength for  −  • Higher ratio of / up to 2.5 led to higher strength.• Higher Molarity up to 14 headed to better strength.Despite the fact that many experimental data are presented by various authors to understand the  − , the lack of codes of practice is impeding its widespread adoption.Thus, there is a necessity to provide a standard method to support the design procedure and principles of mixture design, stress-strain relationship, mechanical properties, etc. of  − .

Fig. 1 .
Fig. 1.Selected journals that published articles on  as per SCOPUS (modified and redrawn from Wasim et al. [8])

Table 1 .
Literature survey on  −

Table 2 .
Mixing mechanism steps available in the literature for  and  mortar

Table 3 .
[84]a et al. (2018)ocedures using various  class  for  and  mortars adopted byGomaa et al. (2018)[84] Gomaa et al. (2018)nclude curing type and period.Curing period is one of many factors (including properties of raw constituents, activator solution, molarity, temperature and  stage) that affect the concrete properties (fresh and hardened states), durability, acid resistance and behaviour of [103].The experimental outcomes showed that as the curing age increases, the compressive strength of  increases (e.g., 7 to 28 d and 28 to This action is similar to conventional concrete, especially beyond 90 d of curing, the development of -rich gel phases can also lead to significant improvement in later age strength[104].Gomaa et al. (2018)