**1. Introduction**

Self-consolidating concrete (SCC) was invented in 1980 as a promising solution to cast concrete for structures with dense reinforced formwork sections [1,2]. Technically, SCC can be placed and consolidated in congested reinforced sections under its weight and flow around reinforcement by improving filling capacity. The cohesiveness of the concrete obtained from optimized mixture design and proper handling of concrete during pouring facilitates casting concrete without segregation and bleeding [1,3–7]. In addition, the characteristics of SCC provide more technical solutions by eliminating vibrating equipment, reducing noise pollution, and lowering labor costs and construction time. Applying SCC in the construction industry showed more positive aspects, such as reduced labor associated with lower human risk in construction sites. From the microstructural point of view, the

**Citation:** Salih, M.A.; Ahmed, S.K.; Alsafi, S.; Abullah, M.M.A.B.; Jaya, R.P.; Abd Rahim, S.Z.; Aziz, I.H.; Thanaya, I.N.A. Strength and Durability of Sustainable Self-Consolidating Concrete with High Levels of Supplementary Cementitious Materials. *Materials* **2022**, *15*, 7991. https://doi.org/ 10.3390/ma15227991

Academic Editor: Frank Collins

Received: 7 October 2022 Accepted: 9 November 2022 Published: 11 November 2022

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proper mixture design of the SCC improves the interfacial transition zone (ITZ) between aggregate, reinforcement, and bulk cement paste, which enhances durability by decreasing permeability [2,7–9]. Given all the advantages of SCC in practice, emphasis has been placed on optimizing its constituent composition by incorporating supplementary cementitious materials (SCMs). Moreover, emphasis has been placed on investigating the effect of the water-to-binder ratio in the mixture design with the use of chemical admixtures such as superplasticizers and viscosity-modifying admixtures. SCMs and mixture design with the type of applications collectively affect SCC's fresh and hardened properties [10–12]. However, with the increasing global trend towards sustainable development in construction, more research is indispensable to reduce the high content of Portland cement used in SCC production. SCC cost was considered one of the major drawbacks in the production process due to the high content of the OPC utilized in the mixture [13]. It has been estimated that cement production was 4100 million tons [14]. The production process of Portland cement releases at least 930 kg/ton of carbon dioxide into the atmosphere [15], which is considered one of the main challenges that many countries have targeted by adopting long-term measures to minimize CO2 emissions [16]. Furthermore, consuming natural resources for the constituent components of concrete exerts a considerable impact annually that jeopardizes sustainability. Due to the increasing population worldwide and rapid urbanization, there is global demand for Portland cement, which augments a massive demand in the construction industry for infrastructural development [17]. Therefore, inspecting more sustainable and environmentally friendly construction materials is crucial in developing advanced concretetech binders. Developing green concrete by incorporating SCMs such as fly ash, GGBS, and microsilica is a promising solution for producing environmentally friendly concrete by reducing mixtures' OPC quantity and lowering CO2 emissions [18–22]. The SCMs have long-lasting effects on the environment because of their nature as non-biodegradable waste materials. Incorporating pozzolanic materials can improve concrete durability and increase the life span of the structures by reducing the required maintenance and repair in addition to cement reduction [23,24]. The most commonly used SCMs for replacing OPC in SCC are ground granulated blast furnace slag (GGBS), fly ash (FA), silica fume (SF), and Microsilica (MS) [25–27]. Fly ash, GGBS, and microsilica are by-products generated from different manufacturing processes and are not produced intentionally. The SCMs have been used as essential constituents to enhance concrete performance and durability when exposed to different aggressive environments [28,29]. Previously, fly ash, GGBS, and microsilica have been applied to replace OPC partially in SCC to enhance fresh and hardened properties and reduce its carbon footprint owing to the high content of binder used in its mixture design [30–33]. This technique was intended to lower CO2 emissions associated with OPC production and improve concrete durability [34–36]. Moreover, it is intended as a method to enhance the environment by applying a green combined binder with sustainability in addition to the durability factor [37]. However, increasing the cement replacement level while maintaining the engineering properties and the durability of SCC is still challenging [38]. Previously, it has been found that 10% silica fume and 10% GGBS gave the best results for the durability and mechanical properties in SCC; however, the recommendations were that 6% silica fume and 8% GGBS should be incorporated as a partial replacement for the OPC separately for better performance [39]. A silica fume to OPC ratio was used in three different percentages (4.85%, 10.5%, and 14%) to produce SCC; however, a better mechanical performance was exhibited in comparison to the normal vibrated concrete [40]. Zhao et al. [41] incorporated 20–40% FA as a partial replacement for OPC to investigate its performance in SCC and concluded that a decrease in mechanical properties was registered at 7 and 90 days in both mechanical properties. In another study, Liu [42] investigated the substitution of FA as a partial replacement for cement to study its effect on SCC. The research showed a decrease in compressive strength as FA increased from 20% to 80%. The results showed that 40% replacement with FA revealed insignificant compressive strength loss. Siddique's results [43] also showed that up to 35% replacement with FA in SCC resulted in compressive strength reduction as well as split tensile strength.

Replacing cement with FA was investigated by Uysal and Sumer [44]; they concluded that FA up to 25% may result in more developed compressive strength compared to the 100% OPC SCC. Previous studies investigated the influence of silica fume and fly ash on the performance of the SCC. Cement was replaced with 10% SF and 30% FA, and the results showed an improvement in the compressive strength [45]. The impact of GGBS and SF on SCC compressive strength was determined after replacing the cement with 30%, 50%, 65%, and 80% GGBS. SF was incorporated with 50% GGBS in three percentages, 5%, 10%, and 15%. The study showed that SF has a recognized impact on compressive strength when used with 50% GGBS [46]. In another study, SF was incorporated as a partial replacement for OPC up to 25%. The results showed an enhancement in tensile strength, whereas a decrease in compressive strength was registered; however, researchers concluded that no more than 5% SF may be used as an enhancement factor in SCC [47]. Micro- and nanosilica were investigated as replacements for OPC in high-performance SCC, and the results showed the dominancy of nanosilica in its effect on the strength properties due to its high reactivity. It is concluded that particle size distribution with a wider range may create low porosity and low water demand and enhance packing density [48]. Higher resistance of sorptivity characteristics of SCC was registered when combined with FA and SF; however, partial replacement of OPC with only 20% FA showed a reduction in sorptivity [49].

In general, fewer and limited studies have been performed regarding the durability of the SCC with the maximum amounts of binary blended replacement of OPC by SF and GGBS; moreover, fewer studies have been conducted to investigate the durability of SCC using SF and FA. In the present experimental investigation, this study investigates up to 70% cement replacement with binary mixtures of microsilica, fly ash, and GGBS yet aims to maintain the engineering properties of SCC for infrastructure applications. Essentially, the novelty in this work is the sustainable mixture design associated with high-strength and durable SCC with high content of SCMs as a partial replacement for cement. This research will achieve two significant goals: the first one is the sustainability of SCC as a high-strength building material, and the second one is the advanced durability which will provide protection against an aggressive environment. Two groups of SCCs were designed to study fresh properties such as flowability and viscosity, in addition to compressive strength as the hardened property. In order to evaluate SCC durability and service life [50], a water absorption test, water permeability test, and chloride ion penetration test were applied. The first group of mixtures contained a binary system with up to 38.74% low calcium fly ash having 0.12 CaO/SiO2 in addition to the microsilica. The second group contained GGBS at up to 71.16% having 1.33 CaO/SiO2 in addition to the microsilica. Microsilica was incorporated in a constant quantity of 30 kg/m<sup>3</sup> in all mixtures, which is equal to 6.4%. The main objective of the current work is to confirm the possibility of producing high-strength and durable SCC by incorporating a high percentage of SCMs as a partial replacement for OPC.

#### **2. Materials and Methods**

This study used ordinary Portland cement with 42.5 N grade in compliance with BS EN 196 [51] and standard BS EN 197–1:2000 CEM I [52]. The chemical and physical properties of OPC are shown in Tables 1 and 2. The supplementary cementitious materials were ground granulated blast furnace slag GGBS complies with the BS EN 15167–1:2006 [53]S, Indian low calcium fly ash (FA), and microsilica (MS). Microsilica (MS) was used in a constant quantity (30 kg/m3) in all mixtures. Tables 3 and 4 show the chemical analysis for all SCMs and the residue on 45 micron sieve, respectively. Polycarboxylate highrange superplasticizer (HRSP) type F and G [54,55] compatible with the ASTM C494 and BSEN [55,56] was used to produce SCC. It is a high-performance concrete superplasticizer based on modified polycarboxylate ether, and it has a unique carboxylic ether polymer with long lateral chains. The superplasticizer, an effective cement dispersant and high-range water reducer, was used to fix constant water content and control flow in all mixtures. In addition to that, it can produce high-flowing concrete without segregation, high early

strength, and high workability with lower water content and lower permeability. It is a high-range superplasticizer that can be used for ready-mix concrete, self-consolidating concrete, precast concrete, and underwater concreting. Moreover, it is used for concrete containing microsilica, GGBS, and fly ash with extremely low w/c [57]. Polycarboxylate high-range superplasticizer (HRSP) was used for the admixture in this research.

#### **Table 1.** OPC physical properties.


**Table 2.** OPC chemical analysis.


**Table 3.** Chemical composition for cementitious materials.


**Table 4.** Residue on 45 micron sieve for cementitious materials.


In order to overcome the problem of natural fine sand shortage, a mixture of fine washed sand and dune sand was used as part of the concrete ingredients in all mixtures. According to the sieve analysis, the dune sand particle size is 50% passing sieve size with 0.150 mm and 1% passing sieve size with 0.075 mm. Coarse aggregate was used in two sizes, 20 mm and 10 mm.
