Compressive strength and stability of sustainable self-consolidating concrete containing fly ash, silica fume, and GGBS

Abstract

This paper presents the findings of an experimental program seeking to understand the effect of mineral admixtures on fresh and hardened properties of sustainable self-consolidating concrete (SCC) mixes where up to 80% of Portland cement was replaced with fly ash, silica fume, or ground granulated blast furnace slag. Compressive strength of SCC mixes was measured after 3, 7, and 28 days of moist curing. It was concluded in this study that increasing the dosage of fly ash increases concrete flow but also decreases segregation resistance. In addition, for the water-to-cement ratio of 0.36 used in this study, it was observed that the compressive strength decreases compared to control mix after 28 days of curing when cement was partially replaced by 10%, 30%, and 40% of fly ash. However, a fly ash replacement ratio of 20% increased the compressive strength by a small margin compared to the control mix. Replacing cement with silica fume at 5%, 10%, 15%, and 20% was found to increase compressive strength of SCC mixes compared to the control mix. However, the highest 28 day compressive strength of 95.3 MPa occurred with SCC mixes in which 15% of the cement was replaced with silica fume.

Keywords

fly ash / silica fume / ground granulated blast-furnace slag / self-consolidating concrete / and sustainable concrete

Introduction

The purpose of this study is to investigate the effect of mineral admixtures on compressive strength development and segregation resistance of sustainable self-consolidating concrete (SCC) mixes. Ordinary Portland cement (OPC) mixes were partially replaced with various percentages of fly ash, silica fume, or ground granulated blast furnace slag (GGBS). OPC replacement ratios ranged from 5% to 80% by weight of total cement as described later in this paper. Hannesson et al. [1] reported that 100% replacement of cement with minerals produces very low compressive strength and indicated that some cement is necessary to produce SCC with sufficient compressive strength for practical applications. In general replacement of cement with fly ash, GGBS, and silica fume, in various combinations and percentages, reduces permeability of concrete [2] and therefore, improves durability.

Effects of GGBS, fly ash, and silica fume on concrete properties

GGBS is used in SCC mixes as cement replacement due to its inherent cementitious properties. Adding GGBS to SCC mixes produces concrete that is less permeable and more chemically stable. The chemical stability of concrete containing GGBS is attributed to the reaction of GGBS with excess soluble calcium hydroxide reducing its presence in concrete. GGBS imparts on concrete the following desirable qualities: 1) Enhanced resistance to disintegration caused by sulfate attacks and 2) Enhanced resistance to corrosion of reinforcing steel.

Fly ash is a sustainable replacement of Portland cement that is known for its slow pozzolanic reaction, leading to lower concrete strength at early ages. However, when Ultra Fine Fly Ash (UFFA) is added, high early strength can be achieved in mixes containing high volume of class F fly ash [3]. Kim and Lee [4] showed that concrete with 40% fly ash replacement by weight of of OPC exhibited increase in flow values of fresh concrete by 20% to 25% compared to a control mix. Fly ash increases concrete flow [3,4], through the decrease in water demand, which permits the use of smaller doses of superplasticizer to achieve specified concrete flow.

Naik and Singh [5] tested concrete mixes with constant w/cm ratio and 40%, 50%, 60%, and 70% class C fly ash replacements of cement. They reported that the 28-day compressive of the tested cylinders was higher than the control mix for all replacement percentages. Rashad [6] reported that High Volume Fly Ash (HVFA) concrete exhibited enhanced resistance to fire compared to control a concrete mix in which cement was not replaced with any mineral admixture.

Class F fly ash is known to control the destructive alkali-aggregate reaction when used to replace 15% to 20% by weight of Portland cement. Mohamed et al. [7] tested 50-year old concrete and found that alkali-aggregate reaction can result in loss of up to 50% of concrete strength. Fly ash in general and UFFA in particular are known to improve the chloride penetration resistance of concrete [3].

Unlike fly ash, silica fume is a pozzolan with large surface area which increases water demand, therefore, requires larger dosage of superplasticizer to maintain the desired flow of concrete [8]. In addition, when combined with fly ash, silica fume helps with improving the deficiency associated with slow pozzolanic reaction of fly ash [3].

Experimental program

To examine the effect of fly ash, silica fume, and GGBS on the strength development of sustainable SCC mixes, a control mix was developed in which ASTM type I cement complying with ASTM C150 was used, without any replacements. Various sustainable mixes were then created in which cement was partially replaced with individual minerals including fly ash, silica fume, and slag. 150 mm × 150 mm× 150 mm cubes of SCC mixes were tested after 3, 7, and 28 days of moist curing. Curing method is known to affect the compressive strength of SCC mixes, but moist curing typically produces best results [9], compared to air and steam curing.

In all SCC mixes tested, a High Range Water Reducing Admixture (HRWA) based on polycarboxylic ether was used and kept constant at 1.5% of the total weight of cementitious materials. The HRWA is manufactured by BASF Corporation under the commercial name Glenium Sky 504.

Binary mixes were then created in which cement was partially replaced with various percentages of fly ash, silica fume, or GGBS. The chemical constituents of GGBS, silica fume, and fly ash are shown in Tables 1, 2, and 3, respectively.

In all SCC mixes studied in this paper, total amount of cementitious materials was approximately 480 kg/m³ and the w/c ratio was maintained at approximately 0.36. Coarse aggregates were crushed aggregate passing 14 mm sieve size with a total amount of 800 kg/m³. Fine aggregates consisted of 582.4 kg/m³ black sand and 313.6 kg/m³ dune sand. Figure 1 shows the sieve analysis results for coarse aggregates.

The filling ability of all SCC mixes was determined using the slump flow test according to ASTM C 1611M [10]. For all mixes, the slump flow test was conducted and resistance to segregation was assessed using the Visual Stability Index (VSI).

Binary mixes

Binary mixes containing cement and one mineral admixture were created to investigate the effect of pozzolan on fresh and hardened properties. 150 mm × 150 mm × 150 mm cubes of the mixes shown Table 4 were created and tested after 3, 7, and 28 days of moist curing. The w/c ratio was maintained at 0.36 for all mixes. In Table 4, mixes containing cement and fly ash were designated as FA followed by the percentage by weight of fly ash used to partially replace cement, for example FA10 refers to a mix in which 10% by weight of the cement was replaced with fly ash. Similarly, SF5 indicates a binary mix in which 5% by weight of cement was partially replaced with silica fume, GS25 indicates a binary mix in which 25% by weight of cement was partially replaced with GGBS.

The filling ability of the binary mixes was assessed after mixing for 80 min. The slump flow test included measurement of the time it takes the SCC mix to reach a diameter of 500 mm, known as T₅₀. The final diameter and the T₅₀ time are shown in Table 4, along with the VSI. All mixes shown in Table 4 were prepared with the same w/c ratio of 0.36. Clearly, all binary mixes produced final diameter larger than the control mixes indicating improved flow resulting from adding fly ash, silica fume, or GGBS. It is also observed that VSI increased to 2.0 with higher percentage of fly ash indicating segregation is a potential problem with large percentages of fly ash. Similarly, VSI generally increased to 1 or 2 for mixes in which 35% of cement was replaced with GGBS, also indicating a negative effect on segregation resistance. The effect of increased GGBS dosage on deteriorated segregation resistance is consistent with the findings of Boukendakdi et al. [11]. Similarly, increasing the dosage of silica fume increases the tendency to segregation as indicated by the VSI of “1” in SF15 compared to the lower VSI of “0” in SF5 and SF5. The result is confirmed by Mazlooum et al. [12]. However, VSI of “0” was restored in SF20. The T₅₀ time shown in Table 4 indicates mixes are highly viscous.

Experimental results and discussions

The compressive strength of binary mixes was measured after 3, 7 and 28 days of moist curing. The results are outlined and discussed in the following subsections.

Compressive strength of binary SCC mixes containing fly ash

The compressive strength of binary fly ash mixes (FA10, FA20, FA30, and FA40) shown in Table 4 was determined after 3, 7, and 28 days of moist curing. The binary fly ash mixes are the ones in which cement was partially replaced with 10%, 20%, 30%, and 40% fly ash, respectively. All replacement percentages are by weight of cement.

The SCC mix with 20% fly ash replacement produced the highest compressive strength compared to the SCC mixes with 10%, 30%, and 40% fly ash replacement. The 28-day compressive strength of the SCC mix with 20% fly ash replaced was 67.96 MPa, compared to 61.33, 56.5, and 55.75 MPa for 10%, 30%, and 40% replacements, respectively. Figure 2 shows that the control mix produced higher compressive strength after 3 and 7 days of moist curing compared to other mixes. After 28 days of moist curing, all binary mixes gained strength approaching the control mixes.

However the mix with 20% fly ash replacement produced higher strength at 28-days compared to all mixes including the control mix.

The 28-day compressive strength of SCC mixes with 30% and 40% fly ash replacements was 56.5 and 55.75 MPa, respectively. Replacing cement with up to 40% fly ash produces a competitive compressive strength high enough for many practical structural design applications.

Compressive strength of binary SCC mixes containing silica fume

The compressive strength of SCC mixes SF5, SF10, SF15, and SF20 was measured after 3, 7, and 28 days of curing. The corresponding strengths after 28-days of curing are 72.8, 81.11, 95.3, and 75.83 MPa, respectively which are all higher than the control mix. Therefore, as shown in Fig. 3, the optimum dosage of silica fume for this particular SCC mix was 15% (SF15) producing strength of 95.3 MPa. This is 44.2% higher than the control mix where only Portland cement was used. The optimum silica fume percentage of 15% was also reported by Elahi et al. [13]. SF15 produced higher strength at 3, 7, and 28 days, compared to all mixes including the control mix. After 3 and 7 days of curing, the control mix produced higher strength than all mixes containing silica fume (SF5, SF10, and SF20), except the optimum SF15 mix.

Compressive strength of binary mixes containing GGBS

In this section, the compressive strength of SCC mixes containing GGBS is reported. These are the mixes in which cement was partially replaced by GGBS at seven dosages, namely, 10%, 25%,, 35%, 45%, 50%, 60%, 70%, and 80%. All replacement percentages are calculated by weight. The 28-day compressive strength for each mix was 66.75, 77.53, 81, 74, 75.66, 62, and 50.45 MPa, respectively. As shown in Fig. 4, 35% is the optimum GGBS replacement ratio producing a high 28-day compressive strength of 81 MPa. The optimum GGBS value depends on the mix designed. For instance, the mix studied by Oner and Akyuz [14] showed the optimum replacement ratio of cement with slag is 55%.

The SCC mix, GS80, in which 80% of the cement was replaced with slag, produced the lowest compressive strength of 50.45 MPa in this family of binary mixes. In addition, tendency to segregation is observed on mixes where GGBS replaced 60% or more of the cement.

Summary and concluding remarks

Sustainable SCC mixes that produce medium to high compressive strength were developed and studied. In these mixes up to 80% by weight of the cement was replaced with silica fume, fly ash, and blast furnace slag. In this study, water-to-cement ratio was maintained at 0.36, therefore, all findings pertain to this particular water-to-cement ratio.

Outlined below are the major findings of the study:

1) Partial replacement of cement with fly ash and GGBS increases concrete flow, but may lead to segregation at higher replacement ratios.

2) Replacing cement with the optimum value 20% fly ash produced the highest compressive strength after 28-days of curing compared to the other three fly ash replacement ratios of 10%, 30%, and 40%. When cement is partially replaced with 20% fly ash, the compressive strength after 28-days of curing (67.8 MPa) was almost similar to the control mix with 0% fly ash replacement (66.08 MPa).

3) Replacing cement with the optimum of 15% silica fume produced a high 28-day compressive strength of 95.3 MPa, which is 44% higher than the compressive strength of the control mix. Replacing cement with 5%, 10%, and 20% silica fume produced lower 28-day compressive strength than the 15% replacement ratio. However, all silica replacement ratios produced SCC mixes with higher 28-day compressive strength compared to the control mix.

4) Replacement of cement with the optimum value of 35% GGBS produced that highest 28-day compressive strength, compared to GGBS ratios ranging from 10% to 80%.

5) Binary SCC mix in which 80% of the cement was replaced with slag reached a 28-day compressive strength as high as 50.45 MPa, which is suitable for many practical structural design applications.

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