1. Introduction
This paper outlines the findings of a study to evaluate the compressive strength of self-consolidating concrete (SCC) in which up to 90% of ordinary Portland cement (OPC) is replaced with industry by-products including GGBS, fly ash, and silica fume. These pozzolanic materials convert calcium hydroxide (Ca(OH)
2), a hydration by-product of OPC, to calcium silicate hydrate (C-S-H) gel [
1]. In this study, the amount of GGBS used to replace cement is the largest compared to fly ash and silica. This is intended to support the recycling of the vast amount of GGBS produced by the steel industry. GGBS comes out of blast-furnaces as molten slag at temperatures as high as 1400 °C to 1500 °C and is then rapidly quenched into a glass state [
2]. Rapid quenching with large quantities of cool water, applied through high-pressure water jets, produces granulated slag with a high glass content [
2]. In addition, GGBS was shown to enhance the durability and strength development in SCC. The increase in the strength of concrete containing GGBS as a partial replacement of OPC is due to the increased hydration products caused by pozzolanic reactivity, and due to the finer pore structure [
3].
GGBS reduces the expansion of concrete associated with alkali-silica reaction (ASR) and prevents the loss of mechanical properties [
4]. Hogan and Meusel [
2] tested SCC containing GGBS with replacement ratios up to 65% and demonstrated that concrete expansion due to ASR decreases as the replacement ratio increases. A loss in compressive strength due to ASR may be as high as 50% after 50 years of ASR reaction [
5].
Sengul and Tasdemir [
6] concluded that the partial replacement of OPC with fly ash or slag is more effective in decreasing the chloride penetration permeability than reducing the w/b ratio. Compared to mortar samples without GGBS, 20 mm cubic samples in which OPC was partially replaced with GGBS (5%, 15%, 25%, and 35%) performed better in terms of a loss in compressive strength when placed in sodium chloride (NaCl) and sodium sulfate (Na
2SO
4) solution [
3]. After 180 days of exposure to NaCl/Na
2SO
4 solutions, decrease in compressive strength was the least in mortar samples where 25% or 35% of OPC was partially replaced with GGBS. One way partial replacement of OPC with GGBS enhances durability of concrete is through the reaction of tricalciumaluminate (C
3A) with free chloride ions Cl
− found in pore solution, in the presence of Ca(OH)
2, to produce stable chloroaluminate compounds such as Friedel’s salt (C
3A·CaCl
2·10H
2O) and Kuzel’s salt as shown in Equations (1) and (2) [
3]. This process, known as chemical chloride binding, is exhibited in cement pastes containing supplementary cementitious materials (SCMs) with high content of C
3A. Physical chloride binding occurs when ions are absorbed directly by C-S-H gel. The high amount of C
3A is a characteristic of GGBS while Ca(OH)
2 is a cement hydration product.
Chloride binding, which contributes to enhancing the durability of reinforced concrete, is largely due to the formation of Friedel’s salt, and this binding capacity generally increases with the content of C
3A in the SCM, such as GGBS [
7].
Partial replacement of OPC with a combination of glass powder (GP) and GGBS may enhance the durability of concrete by decreasing water absorption as measured by the sorptivity test. It was demonstrated that a partial replacement of 50% OPC with a combination of 15% glass powder (GP) and 35% GGPS decreased water absorption of concrete [
8]. Under an elevated temperature as high as 700 °C, a loss in compressive strength and mass in concrete made with 100% OPC was comparable to concrete made with up to 50% GGBS [
9]. However, under elevated temperatures, an increase in carbonation depth was observed in concrete with GGBS replacement up to 50% compared to control concrete with 100% OPC.
The replacement ratio of cement with GGBS affects the extent of improvement in mechanical properties and durability. Mohamed and Najm [
10] tested several SCC mixes with w/b of 0.36 where OPC was partially replaced with GGBS in the range of 0% to 80%. The authors noted that the highest 28-day compressive strength was obtained when 35% of the cement was replaced with GGBS and surpassed the control mix prepared with 100% OPC. Replacing cement with more than 35% GGBS in binary OPC-GGBS SCC mixes results in a reduction of compressive strength compared to the compressive strength corresponding to the optimum replacement ratio, and possibly lower than the control mix. This is consistent with the findings of Sengul and Tasdemir [
6] that suggest replacing 50% of OPC with GGBS produced 28-day strength lower than the control mix when w/b was 0.6, higher than the w/b of 0.36 used in the study by Mohamed and Najm [
10]. Mansour et al. [
11] demonstrated that the compressive strength of SCC cubes after 7 days and after 28 days of water curing increased with an increase in the replacement percentage of OPC with GGBS from 5% to 25%. Dadsetan and Bai [
12] tested 100 mm cubes SCC samples (w/b = 0.45) in which OPC was partially replaced with 10%, 20%, and 30% GGBS, and demonstrated that the compressive strength increased with replacement ratio and surpassed the control mix. In a concrete strength/durability study where 50% of OPC was partially replaced with various combinations of glass power (GP) (45% to 5% in decrements of 5%) and GGBS (5% to 45% in increments of 5%), the optimum GGBS replacement for maximum compressive strength was found to be 35% (along with 15% GP) after 3, 7, and 28 days of curing [
8]. It is important to note that the 35% optimum GGBS replacement ratio occurred in studies using 150 mm concrete cubes. However, in 20 mm cubic mortar samples, it was shown that the 35% GGBS replacement ratio provides the highest compressive strength after 180 days of curing [
3]. The trend of increasing the compressive strength with an increase in GGBS replacement ratio up to an optimum replacement ratio includes concrete made consistent with studies that use non-recycled aggregates or mixing water. Majhi et al. [
13] prepared and tested concrete mixes that use recycled aggregates and reported a decrease in compressive strength with an increase in GGBS replacement ratios from 10% to 60%. Dinakar et al. [
14] developed a mix design method for SCC in which GGBS is used to replace OPC with percentages ranging from 20% to 80%. The authors also noted that 35% is the optimum GGBS replacement of OPC that produces the highest 28-day compressive strength.
The effective curing of concrete structures is critical for the hydration of binders to proceed and for the design compressive strength to be achieved [
15]. Water curing is the most commonly used method in construction practice, where water is applied all over the freshly cast structural elements, that are often covered with burlap, continuously for three to seven days. The purpose of water curing is to retain water in the structural elements in order to entrap the moisture needed for hydration of the binder [
16]. American Concrete Institute (ACI) 301 [
17] recommends the minimum curing period to be the time required to achieve 70% of the specified compressive strength. Traditional curing methods serve their intended purpose but often require significant amounts of water to cure all structural elements [
16]. Membrane-forming curing compounds are known to enhance durability but their effect on compressive strength development may vary depending on the type of curing compound. Curing compounds provide a membrane to prevent moisture loss which allows hydration reaction to proceed and strength development to continue. ASTM C309 [
18] refers to two types of curing compounds, clear compounds which may or may not contain dyes, and white pigmented compounds.
Xue et al. [
16] indicated that concrete samples cured using acrylic-based, paraffin-based, silicate-based, or composite-based, all demonstrated lower drying shrinkage and permeability. The membrane-forming curing compounds resulted also in an enhanced compressive strength, flexural strength, and crack resistance. The investigators observed that acrylic-based curing compounds outperformed the other compounds included in their study, in terms of improved durability and mechanical properties. However, Saarthak et al. [
19] concluded that compressive strength is not sensitive to the type of curing compound used. Ibrahim et al. [
20] concluded that when curing compounds are used, the durability and strength of the concrete are the same or better than those of concrete cured using the typical water-based method. The investigators argued that the curing method has a pronounced effect on durability but a limited effect on the concrete’s compressive strength. Yash et al. [
21] argued that membrane-forming curing compounds produce concrete with strength and durability properties that are 80% to 90% better than concrete cured by exposure to air in room temperature, or concrete under wet cotton mat burlap. To the contrary, Al-Gahtani [
22] observed that the 28-day compressive strengths of concrete cured under wet burlap were higher than concrete cured using acrylic-based or water-based curing compounds.
The objective of this study is to evaluate compressive strength development in concrete in which 90% of OPC is replaced with supplementary cementitious composites including high volume GGBS and smaller amounts of silica fume and fly ash. In addition, it is desired to evaluate the effectiveness of various curing methods in developing the compressive strength of the sustainable mixes evaluated in this study. Curing method affects the rate of hydration which is important because concrete containing a high volume of GGBS exhibits slower strength development at an early age [
23]. Three curing methods are examined in this study, water-curing, air-curing, and curing using membrane-forming chemical compounds.