The Effects of Bi-Combination of GGBS and PFA on the Mechanical Properties of Concrete

Abstract

The main thrust of the current study is to examine the effects of ground granulated blast-furnace slag (GGBS), pulverized fuel ash (PFA), and bi-combination of GGBS and PFA on the mechanical properties of concrete. Seven concrete mixes were carried out in this study; including the control mix and the other six mixes had supplementary cementitious materials (GGBS, and PFA) as partial replacement of Portland cement at different replacement levels. The physical properties, oxides, and chemical composition of OPC, GGBS and PFA were experimentally investigated. The workability of the fresh concrete mixes was carried out by means of slump test and compaction index test. This study also examined the compressive strength of the different concrete mixes at different curing ages along with the splitting tensile strength. Cost analysis and the environmental impact of the different concrete mixes was also evaluated. The study results showed that the workability was significantly improved through the replacement of cement with PFA and GGBS. The utilisation of fly ash at 30% replacement level achieved the highest workability. The highest compressive strength was achieved by concrete mixes replacing 30% GGBS with cement, and a bi-combination of 10% PFA and 20% GGBS. The results also showed that the bi-combination of fly ash and GGBS at 10% and 20% replacement level was found to be favorable in terms of both cost and environmental impact.

1. Introduction

Statistics show that about 10 billion ton of concrete are produced per year in terms of global production, or in another word, about 1 ton of concrete per person per year worldwide [1]. Studies show that cement industry is a major contributor for the climate change due to carbon dioxide emissions from the cement production, in which 5 to 7% of global carbon dioxide production is produced during cement production [2]. Approximately, 50% of the CO₂ emissions come from the combustion of fossil fuel and the other 50% originates during the conversion of limestone into raw materials. Furthermore, the cement industry is a major contributor to global CO₂ emissions, with about half of these emissions coming from the calcination process, which involves heating limestone (calcium carbonate) to produce clinker, releasing CO₂ as a byproduct. The remaining emissions largely result from the combustion of fossil fuels used to heat cement kilns [3]. This clearly shows the environmental impact associated with greenhouse gas emissions from Portland cement manufacturing.

Concrete plays a significant role in achieving sustainable development. In order to reduce its environmental impact, different alternative technologies/methods have been proposed throughout concrete industry, such as using lower carbon content fuel to produce cement, such as coal or natural gas. Another alternative was utilizing a chemical absorption process that can capture carbon dioxide.

Recently, cement industry has developed strategies in order to reduce their emissions, which included the utilization of supplementary cementitious materials (SCMs). These SCMs play a significant role in energy consumption reduction required to produce clinker, which will lead to a sustainable concrete and improvement in terms of energy efficiency, mainly due to lower CO₂ footprint [4]. Some examples of these supplementary materials are ground granulated blast-furnace slag (GGBS), fly ash which is also known as pulverized fuel ash (PFA), and silica fume (SF). These materials can either be used in the production of Portland cement itself, or they can partially replace certain percentages of the Portland cement content in the concrete mix.

One of the most used approaches is adding/blending high percentages of supplementary cementitious materials, such as fly ash, silica fume, and/or ground granulated blast-furnace slag (GGBS), which all has lower carbon dioxide footprint. However, it is insufficient to consider just the CO₂ reduction. It is also necessary to consider the structural performance of the newly developed concrete and its serviceability. In other words, three interacting and interdependent parameters should be examined and considered when it comes to evaluating the sustainability of the produced concrete using supplementary cementitious materials. These are the environmental impact, the mechanical performance, and the durability.

The literature [5] stressed on that one of the most common and effective approaches to reduce Portland cement use, is the use of the partial replacement method using supplementary cementitious materials (SCMs).

The literature [6,7] reviewed the potential use of a significant number of industrial and agricultural wastes as supplementary cementitious materials in concrete production. The review included the utilization of wastes such as; fly ash, slag, silica fume, rick husk ash, palm oil fuel ash, sugar cane bagasse ash, wood waste ash, and bamboo leaf ash, in the concrete production. Moreover, the literature [8], indicated that GGBS and FA could reduce concrete CO₂ emissions by 22% and 14%, respectively, in typical concrete mixes.

The literature [9], examined the effects of supplementary cementitious materials (GGBS, FA, and SF) on concrete compressive strength, and in reducing CO₂ emissions from ordinary Portland Cement (OPC) concrete. They collected a comprehensive database of a significant number of laboratory concrete mixes, and they covered extensive ranges of compressive strength and SCMs replacement levels. A promising proposal was presented in their study, to serve as a guideline for the determination of: (1) the total CO₂ emission for a certain concrete mix proportion, (2) the unit binder content for a targeted compressive strength, and (3) the type and replacement level of SCMs for a targeted CO₂ reduction ratio and for a designed compressive strength.

The literature [10], investigated the optimum level of GGBS on the compressive strength of concrete. The test results proved that GGBS can improve the compressive strength of the mixed concrete, and as the level of GGBS increases the compressive strength increases. However, up to 55% of replacement of the total binder content, the compressive strength did not improve because of the unreacted GGBS, that acted as a filler material in the paste.

The literature [11], blended three types of SCMs in the concrete mix with different levels, as partially replacement for Portland cement, namely metakolin, ground granulated blast-furnace slag and fly ash. They investigated the effects of the used SCMs on the mechanical and micro-structural properties of self-compacting concrete.

The literature [12], investigated the performance of ternary blended concrete containing tremendous volume of Class-F fly ash (PFA) and ground granulated blast furnace slag (GGBFS). The test results indicated that, when blending with PFA alone to partially replace Portland cement, the compressive strength drops down as fly ash content increases from 50% to 90%. In contrast, when GGBS is incorporated alone as partial replacement of 90% of Portland cement, the obtained compressive strength was similar to that of the ordinary concrete one at 28 days of curing. On the other hand, ternary mixes containing 70% replacement of PFA to GGBS ratio of 1:2 and 50% replacement with ratio of 1:1 develop similar 28- and 90-day compressive strength compared to that of the ordinary/control specimen.

Table 1. Previous studies on using GGBS and PFA as cement replacements.

Researchers	Replacement Materials	Replacement Percentages	Water/Binder Ratio	Testing Age
[10]	GGBS	0–61%	0.4 to 1.19	7, 14, 28, 63, 119, 180, and 365
[11]	GGBS or PFA	10–30%	0.4, 0.45	7, 28, and 56
[12]	GGBS and/or PFA	23–90%	0.55	3, 7, 28, and 90
[13]	GGBS and PFA	5–30%	0.4	7 and 28
[14]	GGBS and/or PFA	25–60%	0.35	7, 28, and 60
[15]	GGBS or PFA	10–80%	0.36	3, 7, and 28
[16]	GGBS or PFA	30–70%	0.6	7, 14, and 28
[17]	GGBS and/or PFA	10–75%	0.35, 0.50, 0.65	3, 7, 28, 90, and 120
[18]	GGBS	30–50%	0.35	1, 2, 3, 5, 7, 28, and 56
[19]	GGBS and PFA	0–50%	0.44	28, 56, and 90
[20]	GGBS	0–60%	0.36 to 0.42	7, 28, 60, 90, and 365
[21]	GGBS and PFA	0–20%	0.47	3, 7, 14, and 28

summarises different recent research about using GGBS and/or PFA as a replacement for cement in concrete mixes. Replacement percentages reaches 90%, and water to binder ratio ranges from 0.35 to 1.19. testing age ranges from one day to one year. Different results were found in these studies, and sometimes with some contradictions. Hence, results of the current study will be presented along with those from research listed in Table 1 for comparison purposes.

Accordingly, this study addresses significant research gaps in terms of the application of supplementary cementitious materials (SCMs), particularly Ground Granulated Blast-Furnace Slag (GGBS) and Pulverized Fuel Ash (PFA), in concrete mixtures. Although the inclusion of SCMs in concrete has been studied extensively, a comprehensive understanding of the bi-combination effects of different SCMs on both the mechanical and environmental performance of concrete is still lacking, particularly at different replacement levels, and different w/c ratios, as shown in Table 1. The current study intends to bridge the gaps in the literature by investigating the effects of bi-combination of GGBS and PFA on key mechanical properties along with the addition of cost efficiency and environmental sustainability.

A significant contribution of this research is the evaluation and selection of the optimal GGBS—PFA replacement levels that not only offers enhanced mechanical performance but also reduced environmental impact. The findings of this study are in particular interest of decision makers and designers as they offer a practical solution to overcome the current challenges that the cement industry is facing in terms of carbon dioxide footprint. In addition, this study provides experimental evidence on the impact of different replacements levels of GGBS and PFA on concrete’s fresh and hardened properties. Through this innovative bi-combination of SCMs, this study provides a better understanding on how alternative construction materials can be used to obtain high-performance concrete that meets both the structural and sustainability demands. Overall, the present study promotes sustainable development through the adoption of sustainable construction materials without compromising concrete performance.

2. Materials

The utilized materials for the present study consisted of concrete main ingredients; Portland cement (PC), limestone aggregate, natural sea-dredged sand and water. In addition to that, the target supplementary cementitious materials are Ground Granulated Blast Furnace Slag (GGBS) and pulverized fuel ash (PFA). The cement that was used is an ordinary Portland cement (OPC). OPC, GGBS and PFA physical properties, oxides, and chemical composition are presented in

Table 2. PC, GGBS and PFA oxide composition and some physical properties.

Oxide	OPC	GGBS	PFA
SiO2	20	35.4	47.6
TiO2	-	-	1.03
Al2O3	6	11.6	26.2
Fe2O3	3	0.35	9.4
MgO	4.2	8.04	1.42
MnO	1.1	0.45	-
CaO	63	42.0	2.4
Na2O	-	-	3.09
K2O	-	-	3.02
SO3	2.3	0.23	0.86
S3−	-	1.18	-
Loss on ignition	0.8	-	0.55
Chemical (%)
C1	0.03	-	-
Free lime	1.32	-	-
Bogue’s composition
Tricalcium aluminate (C3A)	6.48	-	-
Tricalcium silicate (C3S)	70.58	-	-
Dicalcium silicate (C2S)	6.09	-	-
Tetracalcium aluminate-ferrite (C4AF)	6.45	-	-
Properties
Insoluble Residue	0.5	0.3	-
Bulk Density (kg/m3)	1400	1200	-
Relative Density	3.15	2.9	2.45
Blaine fineness (m2/kg)	365	450	-
Color	Grey	Off white	-
Glass Content	-	90	-

.

Samples of GGBS and PFA are shown in Figure 1. As course aggregate, two groups of the crushed limestone were used. The first one is size 20/10 mm and the second one is 10/4 mm. Different physical and mechanical properties of course and fine aggregate are listed in

Table 3. Properties of the coarse and fine aggregates.

Properties	Fine Aggregate (Sand)	Coarse Aggregate (10/4 mm)	Coarse Aggregate (20/10 mm)
Water absorption (%)	0.85	1.5	1.1
Saturated density (Mg/m3)	2.82	2.68	2.65
Dry density (Mg/m3)	2.71	2.57	2.54
Shape index (%)	-	12	7
Impact value (%)	-	23	15
Specific gravity	2.6	2.7	2.7

.

The used ground granulated blast furnace slag (GGBS) was supplied by Civil and Marine Ltd., Llanwern, Newport, UK, and complies with BS EN 15167-1 [22]. GGBS is a by-product material from the blast furnaces used to make iron. On the other hand, fly ash is a by-product material that precipitated electrostatically or by mechanical means from the exhaust gases of coal-fired power stations. The utilized PFA was supplied by a local contractor and meets all the conformity criteria in BS EN 450-1 [23].

3. Research Methods

3.1. Mix Design

The present study consisted of seven design mixes; the control mix and another 6 mixes that consisted of the selected supplementary cementitious materials as partial replacement of Portland cement at different percentages. The control mix adopted the mix design of cement: sand: aggregate of 1:1.92:2.85, respectively, and water to binder ratio was about 0.49. The other mixes were developed based on the control mix design. The control mix has 100% Portland cement content, while the second mix replaces 30% of Portland cement with PFA, and the third mix replaces 30% of Portland cement with GGBS. The fourth mix incorporated 20% PFA and 10% GGBS replacement of cement, while the fifth mix replaced cement with 10% PFA and 20% GGBS. The sixth mix replaced cement with 25% PFA and 15% GGBS, while the final mix incorporated 15% PFA and 25% GGBS replacement of cement. Quantities are listed in

Table 4. Concrete mixes quantities with GGBS and PFA replacements.

Mix Code	Binder (kg/m3)			Coarse Aggregate (kg/m3)		Sand (kg/m3)	Water (kg/m3)
	OPC	GGBS	PFA	20/10	10/4
Control	390	0	0	701	350	780	193
70PC + 30FA	275	0	120	701	350	780	193
70PC + 30GGBS	275	120	0	701	350	780	193
70PC + 20FA + 10GGBS	275	40	80	701	350	780	193
70PC + 10FA + 20GGBS	275	80	40	701	350	780	193
60PC + 25FA + 15GGBS	235	60	100	701	350	780	193
60PC + 15FA + 25GGBS	235	100	60	701	350	780	193

.

The mix design—mix proportions was carefully selected based on a combination of practical construction application, existing literature, and the overall aim of achieving a balance between strength and workability. The ratio was designed to obtain a sufficient fine and coarse aggregate content that would result in good particle packing, workability and mechanical performance. The use of (1:1.92) sand content was intended to enhance workability which is quite often needed when SCMS are incorporated. The selection of w/c ratio was to achieve a structural concrete while maintaining a good workability, and ratio was fixed throughout all mixes to ensure consistency in the testing results. The selection of the specific replacement levels of GGBS and PFA were based on the existing literature and preliminary experiments which aimed at identifying optimal replacement levels.

3.2. Specimen Preparation

Cubes of size 100 mm × 100 mm × 100 mm, and cylinders of size 150 mm × 300 mm, were used to produce all the test concrete specimens. All test specimens were prepared in compliance with BS EN 206-1 [24], BS EN 12350-1 [25], BS EN 12390-1 [26] and BS EN 12390-2 [27].

Each design mix was carried out to develop nine cubes and two cylinders. Cubes are for compressive strength tests while cylinders are for splitting tensile tests. Specimens were prepared at the University of South Wales engineering laboratory. The samples are shown in Figure 2. It should be noted that no segregation was observed in all concrete mixes.

3.3. Testing

The workability of the fresh concrete was checked out using the slump test (BS EN 12350-2) [28] and the compaction index test (BS EN 12350-4) [29]. Tests comprised of the compressive strength test and the tensile splitting strength test. All the cube specimens undergone compressive strength test for 7, 28 and 56-day (three samples each), in accordance with BS EN 12390-3 [30] and BS EN 12390-4 [31], while all the cylinder specimens were tested for 28-day tensile splitting strength test (two samples) in accordance with BS EN 12390-6 [32].

4. Results and Discussion

4.1. Fresh Concrete Properties—Workability

Figure 3 shows the workability test results of the different concrete mixtures by means of slump test and compaction index.

The major factor that influences the workability of concrete is the water content of the mix. Another two, less important, factors are water/binder ratio and aggregate/cement ratio. The higher the water/binder content, the higher the workability. Incorporating fly ash to replace 30% of Portland cement resulted in achieving the highest slump value, achieving 180% of the control mix slump. The spherical particles shape with smooth surface of fly ash enables it to reduce friction between aggregate, sand, and cement particles, and eventually increases workability. On the other hand, the slump test results for the mix with 30% GGBS showed a slump value of 100 mm, which is higher than the control mix by 33%. This can be attributed to the fact that GGBS particles have larger fineness compared to Portland cement particles [31]. In addition, GGBS has smoother surface texture, and that postpones the chemical reaction and increases the setting time. GGBS absorbs also less water during mixing when compared with cement. Replacing 30% of the cement with fly ash achieved higher workability compared GGBS replacement. This is attributed to the lower capillary pore volume of GGBS mix that might trap some of the mixing water [12]. This means that, in terms of workability, GGBS behavior is closer to cement behavior than fly ash. The reduction in consistency may also be attributed to the accelerated reaction of the calcium species, as well as the angular shape of GGBS particles in comparison to the spherical shape of fly ash [33].

Ternary blended concrete mix compositions that consisted of different levels of fly ash and GGBS exhibited higher workability than the control mix. Mixes with higher fly ash content achieved a higher slump than the ones with higher GGBS content. The slump decreased with an increase in the GGBS content. For instance, the mix composition with 20% fly ash and 10% GGBS achieved a 150 mm slump whereas the mix composition with 10% fly ash and 20% GGBS achieved a lower slump of 135 mm. Apparently, the presence of GGBS in the fly ash concrete tends to lower the slump value compared to that of only fly ash concrete.

The compaction index test results are seen to be inversely related to slump test results. They measure the compact ability and the stability of fresh concrete, respectively. Any increase in the slump value for any mix composition is reflected by a reduction in the compaction index value. The lower the compaction index value, the better and higher the workability. For instance, the highest slump value was obtained for the mix with 30% fly ash and it obtained the lowest compaction index value.

Considering (BS EN 206-1: 2000) [24] classification for slump: S1 (10 to 40 mm), S2 (50 to 90 mm), S3 (100 to 150 mm), S4 (160 to 210 mm), and S5 (larger than 220 mm), the control mix is considered class S2. All ternary mixes along with 30% GGBS replacement mix are classified as S3. Only 30% fly ash replacement mix is classified as S4.

4.2. Hardened Concrete Properties—Compressive Strength and Splitting Tensile Strength

Concrete cubes were tested for compressive strength at 7, 28, and 56-days, and the results are shown in Figure 4. As expected, the compressive strength increased as the age of curing increases. When Portland cement hydrates, it produces high quantities of portlandite crystal (Ca(OH)₂) and amorphous calcium silicate hydrate gel (C₂S₂H₃/or so called C-S-H). Fly ash comprises mainly of silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃). These components of fly ash react pozzolanically with the produced lime (Ca(OH)₂) during Portland cement hydration [30]. Fly ash is a finely divided amorphous pozzolanic material, that consists of alumino-silicate and variable amounts of calcium. When fly ash is introduced to Portland cement in the existence of water, it will react with the released calcium hydroxide from the hydration process of Portland cement. This will lead to delayed strength evolution, which is clear in the results, where all 7 days strength for mixes containing fly ash is lower than the control mix. This agrees with the known behavior of fly ash, which delays strength evolution of concrete mixes.

On the other hand, the mix containing 70% Portland cement and 30% GGBS contains more silica and less lime than OPC only concrete, and hence more C-S-H and less lime are produced, and the microstructure paste of the hydrated cement is denser. This yields a higher strength at later ages of 28 and 56 days (42.8 and 47.5 MPa, respectively) compared to the control mix (39.1 and 44.4 MPa, respectively). However, since GGBS hydrates very slowly during its initial hydration, it shows no significant difference at 7 days’ strength compared with control mix. According to the literature [34], the high strength development of concrete contains levels of GGBS is due to the blocking of pores of GGBS-Portland cement system. ‘Pore-blocking’ results in developed long-term hardening of the cement-GGBS paste, and it happens when GGBS reacts with Portland cement and water. This starts when Portland cement hydrates, the hydration products mainly are Ca(OH)₂ and C-S-H gel, whereas GGBS when hydrate, produces more complex hydration products because it consists of high alumina and silica content. Followed by these hydrations, the system exhibits precipitation of calcium silicate hydrate (C-S-H) and calcium aluminate hydrates.

All the ternary mixes developed a lower compressive strength at early and late ages (7 to 56 days of curing) compared to that of the control mix, except for one designated mix, which is the mix containing 10% fly ash and 20% GGBS. It obtained a higher compressive strength by 5% and 4.5% at 28 and 56 days respectively, but slightly lower value at 7 days of curing. This high gain in strength is attributed to the higher content of SiO₂ and Al₂O₃ compared to that of the control mix. This will enable the rapid growth of the hydration products of calcium silicate hydrate (C-S-H).

Figure 5 shows the tensile strength of the different concrete mixes. It is noticed that control mix concrete has a tensile strength of 3.34 MPa, compared to 3.11 MPa for the 30% fly ash mix (7% lower), while 30% GGBS mix has 3.44 MPa (3% higher) tensile strength. The differences are generally negligible. This may be ascribed to the slow hydration and hence affecting the strength development of fly ash. This finding is in consistent with the findings obtained from [16], who pointed out that incorporating 30% of fly ash to partly replace Portland cement, attained a lower tensile splitting strength in comparison with control concrete.

The concrete mixes containing bi-combinations of FA and GGBS, particularly the 70PC + 20FA + 10GGBS and 70PC + 10FA + 20GGBS, showed higher tensile strength compared to the mixes with only PFA or GGBS. This indicates that the combination of SCMs can complement each other’s properties resulting in a better strength development. The concrete mixes with 60% PC content (60PC + 25FA + 15GGBS and 60PC + 15FA + 25GGBS) showed contrasting behavior. Whereas one mix (60PC + 25FA + 15GGBS) showed a significant drop in strength, the other mix (60PC + 15FA + 25GGBS) performed almost as well as the best-performing mix. This suggested that the balance between the replacement level of FA and GGBS is critical when the PC content is reduced, with higher GGBS replacement levels being more effective for maintaining tensile strength. This is in line with the literature [16].

The reduction in tensile strength for the 60PC + 25FA + 15GGBS mix could be ascribed to, the early hydration process being slowed down by the high FA replacement level along with the slow reaction of PFA. This indicates that using PFA above 20% replacement level could result in adverse effect on tensile splitting strength while the addition of slightly higher GGBS replacement level could maintain similar performance to the control mix even at lower PC content [35].

Data collected from several research about the effect of PFA to binder ratio on compressive strength of concrete, along with current study results are illustrated in Figure 6. It is seen that at low water/binder ratio (0.35), PFA replacement ratio between about 10% and 30% has what can be considered a negligible effect on compressive strength. At water/binder ratio of 0.4 and larger, PFA is seen to have an adverse effect on compressive strength, where it reduces the strength significantly.

It can also be noticed that the best combination of different factors that could provide the best conditions for the development of high strength concrete are with low W/B ratios (0.2 to 0.4), moderate aggregate to binder ratios (8–10) and low to moderate PFA content (0–20%). High content of PFA above 30% tends to reduce concrete strength, especially at higher W/B ratios. High W/B ratios above 0.6 consistently lead to lower strength regardless of PFA content or aggregate ratio. At higher aggregate to binder ratio (3–4.5), strength of concrete seems to be optimal, whereas excessively high aggregate to binder ratio could weaken the bonding behavior. Overall, low w/b ratios, moderate PFA replacement levels, and balanced aggregate to binder ratios are well favored to achieved high strength.

Figure 7 shows the effect of GGBS/binder ratio on compressive strength. Similar to PFA, the highest strength was found at water/binder ratio of 0.35. However, unlike PFA, GGBS replacement ratio between 25% and 50% shows generally slight enhancement to compressive strength at this low water/binder ratio. At higher water/binder ratio of 0.45, enhancing replacement ratio of GGBS reduces to be between 10% and 30%. Above that water/binder ratio, GGBS does not show significant effect on compressive strength (neither increasing nor decreasing), which is already low. This indicates that one can get low strength concrete with high aggregate/binder ratio and high GGBS replacement percentage without compromising compressive strength, with much lower environmental impact due to low cement content.

Figure 8 presents the combined effect of GGBS and PFA on compressive strength of concrete. It is seen that the degradation in compressive strength due to PFA addition defeats the GGBS ability to increase strength. For example, yellow bubbles with PFA/binder ratio around 20% has lower strength than orange bubbles which has around 10% PFA/binder ratio, even at low water/binder ratio of 0.35.

Furthermore, results showed that the synergistic effect of PFA and GGBS is most effective at low w/b ratio (0.2–0.4) and moderate replacement levels (e.g., 20–30% GGBS and up to 15% PFA). On the other hand, there is an adverse effect when both PFA and GGBS are used in high amounts.

4.3. Cost Analysis and Selection Criteria

Cost analysis was carried out to assess the economic viability of concrete mixes produced with GGBS and fly ash, by comparing them with the control mix. Tentative prices for each concrete ingredient including GGBS and fly ash were obtained from searching through trading companies.

Table 5. Price breakdown of concrete mixes.

	Constituent
Mix	OPC	PFA	GGBS	Aggregate	Water	Concrete Cost (£/m3)	Concrete Cost/Control
Estimated Price (£/t)	60	30	55	10	1
Control (£/t)	23.4	-	-	18.3	0.34	42.04	100%
70PC + 30FA (£/t)	16.5	3.6	-	18.3	0.34	38.74	92%
70PC + 30GGBS (£/t)	16.5	-	6.6	18.3	0.34	41.74	99%
70PC + 20FA + 10GGBS (£/t)	16.5	2.4	2.2	18.3	0.34	39.74	95%
70PC + 10FA + 20GGBS (£/t)	16.5	1.2	4.4	18.3	0.34	40.74	97%
60PC + 25FA + 15GGBS (£/t)	14.1	3	3.3	18.3	0.34	39.04	93%
60PC + 15FA + 25GGBS (£/t)	14.1	1.8	5.5	18.3	0.34	40.04	95%

presents the average prices of one cubic meter of concrete for all the mix compositions. It can be said that these prices may be considered representative for bulk purchase in the UK market.

It is clear that the cost for the production of 1 cubic meter of an ordinary concrete mix (the control mix) is the most expensive mix among all the mix compositions, since it consists of 100% Portland cement, which is the most expensive ingredient. The most less expensive produced concrete mix was the mix where 30% fly ash replaced Portland cement (38.74 £/m³), with 92% of the control mix cost. It can be seen that cost saving for these mixes ranges from 1% to 8%. It should be noted that prices given in Table 5 was based on 2018 prices in the UK.

Different criteria usually control the selection of the most appropriate concrete mix. The most important three key criteria are: (1) concrete mechanical properties (workability and compressive strength); (2) cost of production; (4) Environmental impact (embodied CO₂ footprint).

To measure the environmental impact of the different concrete mixes the embodied CO₂ of the different concrete mixes was calculated by using the Equation (1) obtained from Italian Environmental Product Declaration Database.

Embodied CO₂, ECO₂ = Σmi × ECi

(1)

where, ECi is the ECO₂ coefficient and mi is the mass of ingredients. The estimated embodied CO₂ values of different concrete mixes are shown in

Table 6. Embodied CO₂ of the different concrete mixes kgCO₂/kg. Data has been collected from [36].

Concrete Mix	Ingredient	Eci (kgCO2/kg)	Mass Required (kg/m3)	ECO2 Emitted (kgCO2/kg)
Control	OPC	0.788	390	307.3
	Sand	0.005	780	3.9
	Aggregate	0.01	1100	11
Total 322.22
70PC + 30FA	OPC	0.788	275	216.7
	Sand	0.005	780	3.9
	Aggregate	0.01	1100	11
	PFA	0.004	120	0.48
Total 232.08
70PC + 30GGBS	OPC	0.788	275	216.7
	Sand	0.005	780	3.9
	Aggregate	0.01	1100	11
	GGBS	0.067	120	8.04
Total 239.6
70PC + 20FA + 10GGBS	OPC	0.788	275	216.7
	Sand	0.005	780	3.9
	Aggregate	0.01	1100	11
	GGBS	0.067	40	2.68
	PFA	0.004	80	0.32
Total 234.6
70PC + 10FA + 20GGBS	OPC	0.788	275	216.7
	Sand	0.005	780	3.9
	Aggregate	0.01	1100	11
	GGBS	0.067	80	5.36
	PFA	0.004	40	0.16
Total 237.12
60PC + 25FA + 15GGBS	OPC	0.788	235	185.2
	Sand	0.005	780	3.9
	Aggregate	0.01	1100	11
	GGBS	0.067	60	4.02
	PFA	0.004	100	0.4
Total 204.52
60PC + 15FA + 25GGBS	OPC	0.788	235	185.2
	Sand	0.005	780	3.9
	Aggregate	0.01	1100	11
	GGBS	0.067	100	6.7
	PFA	0.004	60	0.24
Total 207.04

.

Table 7. Selection matrix for the various concrete mixes.

Mix	Performance		Initial Cost Saving	Environmental CO2 Impact
	Workability (Slump Test)	28 Days Compressive Strength
Control	100.0%	100.0%	-	100%
70%OPC + 30%PFA	280.0%	82.3%	8%	70.0%
70%OPC + 30%GGBS	133.3%	109.3%	1%	72.8%
70%OPC + 20%PFA + 10%GGBS	200.0%	95.8%	5%	70.9%
70%OPC + 10%PFA + 20%GGBS	180.0%	104.4%	3%	71.9%
60%OPC + 25%PFA + 15%GGBS	200.0%	85.4%	7%	61.4%
60%OPC + 15%PFA + 25%GGBS	153.3%	100.5%	5%	62.3%

shows a selection matrix for the various mixes based on the three aforementioned criteria. Consistency results are taken from the slump test results, with reference to the control mix. Compressive strength and initial cost saving are also computed with reference to the control mix. In addition, the environmental impact of fly ash and GGBS is also considered. Fly ash and GGBS are becoming more and more widely used in concrete because they are able reduce negative environmental impact of Portland cement in terms of carbon emission and energy consumption. For instance, the indicative embodied CO₂ for OPC is 788 kg-CO₂/tonne, while is it 67 kg-CO₂/tonne for GGBS, and 4 kg-CO₂/tonne for PFA (Italian Environmental Product Declaration Database). It is clear that fly ash has negligible environmental impact compared to cement and GGBS. Cement has the highest environmental impact among the three materials, and hence the mixes with 60% cement has the lowest environmental impact, for example, the mix 60%PC + 15%FA + 25%GGBS has 37.7% less CO₂ effect when compared with the control mix. According to the selection matrix, the mix 60%PC + 15%FA + 25%GGBS would be a good candidate that does not compromise the strength, and at the same time has good cost saving and significant reduction in environmental impact. In addition, it has reasonable improvement in workability.

5. Conclusions

This study has presented the results of an experimental investigation on the properties of fresh and hardened concretes comprising various levels of fly ash, GGBS, and their bi-combination as Portland cement replacement. The environmental impact in terms of CO₂ production and the economic viability of different concrete mixes containing PFA and GGBS and their bi-combination were also studied. The findings of this study are quite significant in terms of promoting of sustainable construction practices through the reduction of reliance on Portland cement and thus minimizing carbon dioxide footprint.

The following list summaries the key conclusions that were obtained from this investigation:

• The workability increased highly when using fly ash and GGBS, represented by the slump test. Fly ash has a higher capability in increasing workability compared with the same amount of GGBS.
• Fly ash is not recommended to be used with water/binder ratio larger than 0.35, since it has tendency to decrease the compressive strength of concrete. On the other hand, GGBS showed a tendency to increase concrete compressive strength, and can be used with water/binder ratio up to 0.55 without compromising concrete compressive strength. When both fly ash and GGBS are used in one mix, the compressive strength will depend on the proportions of each substance. Higher GGBS/fly ash ratios provide higher values in compressive strength. This applies to compressive strength of concrete at all studied ages.
• For low strength concrete, high aggregate/binder ratio and good GGBS replacement percentage can be used to obtain concrete with much lower environmental impact.
• The results of the tensile splitting test indicated that fly ash and GGBS have indefinite effect on tensile strength of concrete.
• Cost analysis and CO₂ impact of the mixes showed that a mix of 60%OPC + 15%FA + 25%GGBS has around 5% saving in initial cost, with about 38% reduction in CO₂ footprint, while maintaining the same level of compressive strength. The adoption of such concrete mix presents a very promising alternative that will help to significantly control the usage of Portland cement, reduce the dumping problems of such by-products materials, and reduce the associated environmental impact problems, hence contributing tremendously towards the achievement of the sustainable development.

Overall, this study offers an insight on an optimal use of bi-combination of GGBS and PFA, which can significantly minimize the impact of concrete on the environment while maintaining its structural integrity. It contributes to the ongoing efforts and work on developing sustainable concrete technology and offers a valuable information to the decision makers and designers in materials selection via balancing performance, cost and environmental benefits.

Future research is recommended to further investigate other concrete properties, such as drying shrinkage and creep, as well as investigating the long-term related-durability properties such as sulfate attack and chloride penetration.