Sustainable natural pozzolana concrete – A comparative study on its environmental performance against concretes with other industrial by-products

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Sustainable natural pozzolana concrete – A comparative study on its environmental performance against concretes with other industrial by-products Md. Uzzal Hossain a, Rongjin Cai a, S. Thomas Ng a,⇑, Dongxing Xuan b, Hailong Ye a a b

Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong

h i g h l i g h t s  Evaluated the techno-environmental feasibility of using natural pozzolana in concrete production,  Comparable compressive strength was found for the use of VA against conventional SCMs,  VA significantly reduced environmental impacts of concrete compared to OPC and other SCMs,  11–30% lower GHG emissions was found for VA concrete compared to other cementitious materials.

a r t i c l e

i n f o

Article history: Received 22 June 2020 Received in revised form 6 October 2020 Accepted 20 October 2020 Available online xxxx Keywords: Concrete Lifecycle assessment Natural pozzolana Supplementary cementitious material Sustainability

a b s t r a c t With increasing environmental sustainability concerns in the construction industry and a demand for producing more durable and sustainable concrete, effort is devoted to producing concrete with Ordinary Portland cement (OPC) being substituted by alternative binder materials. By considering the compressive strength, the comprehensive environmental performance of natural pozzolana in concrete was evaluated comparatively by developing six mix-designs of concrete with OPC, and commonly used industrial by-products as supplementary cementitious materials such as fly ash (FA) and ground granulated blast furnace slag (GGBS) based on the lifecycle assessment (LCA) approach. The results show that comparable compressive strengths can be achieved for the use of 20% and 30% volcanic ash (VA) against conventional SCMs. Using the volume as functional unit, the LCA results have demonstrated that higher environmental savings are associated with VA concrete compared to that of OPC, GGBS (30%) and FA (25%), but almost similar when 50% GGBS is used. For example, 15–24% and 10–19% lower global warming potential are associated with VA concretes compared to that of OPC and FA concretes, respectively, but much higher saving is found when considering the strength of the concrete, as they are about 11– 12%, 10–11% and 29–30% compared to that of GGBS, FA and OPC concretes, respectively. The results can be used as valuable guidelines for VA-based in sustainable concrete production, which would further enhance the sustainability of the concrete industry. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction Economic development has resulted in a boost in construction activities globally. While the construction industry is a major consumer of resources, it has given rise to considerable impacts to the environment. Concrete is one of the most commonly used construction materials globally. The production and use of ordinary Portland cement (OPC), a key component of concrete, has attracted

⇑ Corresponding author.

much attention as its production process is energy intensive and hence would induce significant environmental burdens [1]. Worldwide, the cement industry contributes to 8–10% of the total anthropogenic greenhouse gas (GHG) emissions [2], and this could reach 10–15% by 2020 [3] or 12–15% of the total industrial energy use [4]. The global production of cement is estimated to around 4.1 billion tons in 2019 [5], which is projected to increase by 216% by 2030 [6]. The production of cement is responsible hugely for the consumption of non-renewable resources, in particular coal, limestone and clay. The annual consumption rate of concrete is around 25 giga tons globally (i.e. over 3.5 tons per capita) [7]. Due to the

E-mail address: [email protected] (S.T. Ng). https://doi.org/10.1016/j.conbuildmat.2020.121429 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

Please cite this article as: Md. Uzzal Hossain, R. Cai, S. Thomas Ng et al., Sustainable natural pozzolana concrete – A comparative study on its environmental performance against concretes with other industrial by-products, Construction and Building Materials, https://doi.org/10.1016/j. conbuildmat.2020.121429

Md. Uzzal Hossain, R. Cai, S. Thomas Ng et al.

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assessment. For example, Robayo-Salazar et al. [29] studied the carbon emissions of alkali-activated binary concrete using VA (70%) and GGBS (30%), compared with OPC concrete in Colombia using the LCA approach. The study found that the former had 45% lower carbon footprint than the latter. Letelier et al. [30] demonstrated that adequate durability and mechanical properties of mortar can be achieved with a reduction of 20% carbon emissions for replacing 20% OPC by volcanic powder. Kupwade-Patil et al. [18] found that a reduction of 16% embodied energy from the concrete structures of a building when replacing 50% OPC by VA. However, comprehensive environmental evaluation of concrete with VA against other counterparts including FA and GGBF is yet to be conducted. Moreover, such assessment is geographic dependent, and thus localized study is necessary to obtain comparative results and facilitate the decision process. With only limited options, the cost-competitive supply of such materials is also concerning many parts of the world, especially for resource-scarce cities like Hong Kong. To promote such material in Hong Kong, this study was conducted to: (i) evaluate the suitability of using VA in concrete based on the material characteristics; (ii) design of the same grade of concrete with two different percentages of OPC replacement (e.g. 20% and 30%) based on the local specific requirements (i.e. the Civil Engineering and Development Department (CEDD) of the HKSAR Government); (iii) evaluate some initial technical performances such as workability and compressive strengths during 7 and 28-days for further promotion; and (iv) evaluate the environmental sustainability of concretes produced with OPC, conventional SCMs and natural VA as a SCM comparatively using the LCA technique. This is the first comprehensive case-specific LCA study of VA concrete, where lifecycle inventory data for VA was collected as first hand and comprehensive LCA was conducted for comparative analysis with OPC and conventional SCMs (e.g. FA and GGBS). The results can help develop valuable guidelines for the use of VA in sustainable concrete production, which would in turn promote a sustainable construction industry in specific countries or regions.

increasing sustainability concerns (refereed to only environmental sustainability in this study) in the construction sector and a need to produce more durable and sustainable concrete, much effort is devoted to producing concrete with OPC being replaced by alternative binder materials [8,9]. In recent years, alternative and sustainable cement-based materials have attracted serious attention both in the academic rim and the industry [9]. Over the years, efforts have been implemented towards sustainability in the construction industry through: (i) partially replacing OPC by supplementary cementitious materials (SCMs); (ii) employing alternative materials with smaller environmental footprints; (iii) reusing waste materials / by-products; (iv) improving the energy efficiency in the production process; and (v) adopting renewable energy during production [10,11]. So far, SCMs are commonly used in the construction sector to produce more durable concrete and at the same time lower the environmental impacts. Considering the pozzolanic activities and mechanical characters, several industrial by-products such as fly ash (FA), granulated blast furnace slag (GGBS), and silica fume (SF) are commonly used as SCMs in the cement and concrete industry globally [12–17], whereas volcanic ash (VA) is a potential substitute of OPC in concrete and blended cement production [18,19], and VA is also used as natural pozzolana in many parts of the world [20]. To enhance the sustainability of the construction industry, it is imperative to use more environmentally friendly SCMs. From that perspective, VA can be an attractive alternative SCM to reduce the reliance on OPC in sustainable concrete production. VA is considered as a green and economical natural SCM for cement paste, mortars and concrete production [21]. The use of VA for partial replacement of OPC is significant due to its carbon reduction potential, regional availability and capability for producing high-performance materials [18]. VA can also improve the compressive strength and durability against acid attack, sulphate attack, chloride permeability and sorptivity [22]. Several studies have demonstrated the potential application of VA in different aspects. For example, 10–30% replacement of OPC by VA can achieve the maximum benefits in cement paste based on the mechanical and microstructural properties [23]. Grinding is an important factor for VA, as it can increase the extent of pozzolanic reaction as additional calcium silicate hydrate (CSH) related phases are formed. With finer VA (i.e. 6 lm), up to 40% of OPC replacement can be achieved with an increased compressive strength of concrete, despite 20–30% replacement is ideal for most applications [18,19]. Similarly, comparable strength was observed for 30% replacement of OPC, with control and reference FA mortars under high curing temperatures [24]. Moreover, up to 40% OPC can be substituted by 30% fine VA and 10% SF in the cement paste, as the co-existence of CSH and calcium alumino silicate hydrate gels along with other hydration products were found leading to a reduction in porosity and densification of the cement matrix [25]. Moreover, different alkali-activated binder systems were developed with conventional by-products, such as FA, GGBS and SF with activators [26,27] and VA [28]. 50% VA with 10% GGBS was blended to produce structural geopolymer, resulted in a reduction in initial setting time significantly with 85 MPa compressive strength at 28 days in room temperature [28]. Although several studies have demonstrated that the use of VA is technically feasible in concrete production [18–23], the cementitious properties of VA is source dependent, and the use of such material should meet the local characteristics and standards. In addition, it is essential to evaluate the environmental performance of materials using a life cycle assessment (LCA) technique in order to be regarded as environmentally sustainable. Notwithstanding the use of VA is considered environmentally friendly, only few studies have focused on assessing the comparative environmental

2. Materials and method 2.1. Materials and mix-designs of concrete In addition to the commonly used materials in concrete production in Hong Kong (e.g. aggregates, OPC, admixture, traditional SCMs, etc.), VA as a natural pozzolana being sourced from Indonesia was used in concrete production according to the guidelines laid down by the CEDD in Hong Kong. VA is extracted directly from the sites through open pit mining using excavator. The impurities such as soil, tree and stone are then removed. VA is handled by bulldozer while various stones are sorted by gravity from the mound of raw VA (hard rocks are slide down). The materials are stored as open stockpile (on the earth) by excavator. The VA is a rather soft material with 70% being able to pass through the 45mm sieve. To make it consistent with other cementitious materials, VA has to be grinded before being used in this experiment. In addition, drying may be necessary due to the relatively high moisture contents (up to 15%). Drying is seldom done at source, and it is usually done by air dry where enough space and sun light is available. Otherwise, mechanical drying is needed, and this would involve oven-dried of the grinded materials at 110 °C for 48 h. The materials are transported from stockpile to port using medium-sized trucks, where loaders and excavators are used for loading the VA onto the trucks. Similarly, loaders are used at the jetty to assist the vessel grabs (vessel equipment) to load the VA from the jetty to the hatch of vessel for shipping to the destination (e.g. Hong Kong) by bulk. When arrived, loaders and excavators are used to 2

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(in addition to in-situ casting), the average distance from the local cement manufacturing plants was calculated. The lifecycle inventory (LCI) data for aggregates production was collected from natural aggregate manufacturers and suppliers (for both fine and coarse aggregates) at Guangdong province in China as reported by Hossain et al. [41]. The data for cement production in Hong Kong was made referenced to Hossain et al. [10] and the Chinese Life Cycle Database (CLCD) [42]. The details LCI data for FA and GGBS were collected from leading construction companies in Hong Kong and respective suppliers in China (as reported in Hossain et al. [8]). In this study, the upstream impacts for SCMs were allocated on the basis of economic values, as economic allocation was mostly adopted for SCMs incorporated concrete or concrete products in different studies (e.g. [43–46]). For economic allocation of SCMs used in Hong Kong, the methodology and analysis conducted by Hossain et al. [8] was adopted in this study. The data for natural pozzolana was collected from respective suppliers in Indonesia and Hong Kong as first-handed information through a structured questionnaire survey, which includes the collection, onsite transport, processing and transport to port, and then to Hong Kong port by vessel and to the potential concrete batching plants by trucks. Due to the possibility of multiple locations, the average energy consumption for VA extraction and processing, and transport distances were considered in this study. The energy and fuel requirements for VA processing is given in Table 3. Finally, the energy consumption of concrete batching in the plants was collected from previously published literature in Hong Kong [8,47]. The LCI data of all materials for the concrete production including their sources and upstream databases / references is listed in Tables 2 and 3. The upstream data for electricity and fuel consumption for most of the processes and transportation was based on local and regional databases and references, such as CLCD and the China Light and Power (CLP) in Hong Kong. Due to the technological, temporal and spatial representation, the local and regional data / databases provide more reliable results in the LCA study [48,49]. Hong Kong as a data / database scarce region, some of the data related to materials and processes was not available in CLCD or even in case specific studies. Therefore, the European reference Life Cycle Database (ELCD) and other scientific literature were used. For example, ELCD was used for modeling the transportation by ocean ship [50], while the upstream data for the admixtures was collected from the Environmental Declaration Super-Plasticizing Admixtures [51] and Sjunnesson [52]. All materials including their processes and transportation for the studied concrete production including the production process were modeled using SimaPro 9.1.5 software. Based on the global concern and regional significance, four mid-point impact categories were selected (i.e. respiratory inorganics as PM2.5, acidification potential as SO2 eq, global warming potential as CO2 eq, and non-renewable energy consumption as MJ) and then comparatively evaluated by the IMPACT 2002 + impact method [53]. In addition, an end-point damage approach (e.g. single score) based

unload the VA from the vessel to trucks for transport to factories for further processing or to sites for concrete production. Locally produced OPC, VA and commonly used imported industrial SCMs were used to perform comparative analysis of their suitability for used as pozzolana according to the requirements of the CEDD in Hong Kong. In this study, six batches of concrete were prepared such as concrete with OPC only, with GGBS (i.e. 30% and 50% OPC replacement), with FA (i.e. 25% OPC replacement), and with VA (i.e. 20% and 30% OPC replacement) for comparative analysis (Table 1). The most acceptable proportions of GGBS and FA were considered according to different studies (e.g. [12,29,31] and also based on the CEDD guidelines [32]. In order to obtain comparable mechanical strength and workability, the total binder content and the admixture (superplasticizer) were adjusted to optimize the mixture design. By referring to the developed mix-designs, concrete samples were casted in 150  150  150 mm standardsized molds with water curing method being employed. The 7day and 28-day compressive strengths were evaluated as examples of mechanical properties according to BS Standard Methods for comparative analysis. 2.2. Environmental sustainability of concretes 2.2.1. Study aim and system boundary Environmental sustainability of concrete according to the aforementioned mix-designs was evaluated using the LCA approach. LCA is a recognized technique which consists four main steps: (i) defining goal and scope; (ii) analyzing life cycle inventory; (iii) assessing the impacts; and (iv) interpreting and analyzing the results [33,34]. The system boundary is ‘cradle-to-gate’ with two functional units, viz.: (i) environmental impacts for per 1 m3 concrete production [32,36–39]; and (ii) environmental impacts for per 1 m3 concretes divided by their corresponding 28-day compressive strengths, as this is a common functional requirement for structural applications of concrete [35,38,39]. 2.2.2. Lifecycle inventory analysis and impact assessment As a resource-scarce city, Hong Kong mostly depends on imported construction materials from different regions or countries. The raw materials (shown in Table 1) used for the studied concrete production with their imported locations are given in Table 2. The energy consumption for the production / processing of the materials with their corresponding upstream database is shown in Table 3. Locally produced OPC was considered in this study though more than half of the OPC was imported in Hong Kong in 2017. GGBS and FA sourced from China was taken into account in this study, as more than 65% of the total FA and GGBS were imported from China during the same statistical year. Almost the entire amount of aggregates used in Hong Kong were sourced from China in 2017 [40]. As there are several concrete batching plants in Hong Kong

Table 1 Mix-design (kg/m3) of different concretes used in this study. a

Materials

C1

C2

Ordinary Portland cement (OPC) Ground granulated blast furnace slag (GGBS) Fly ash (FA) Natural pozzolana (VA) Coarse aggregates Fine aggregates Water Admixture (superplasticizer)

445 0 0 0 905 745 208 1.69

333 142 0 0 935 680 221 1.81

[a, 30% OPC replacement by GGBS;

b,

50% OPC replacement by GGBS;

c,

25% OPC replacement by FA; 3

C3

b

225 225 0 0 925 720 204 1.93 d,

C4

c

C5

350 0 120 0 940 720 172 1.80

20% OPC replacement by VA;

d

356 0 0 89 1,005 730 140 4.36 e,

C6

e

311 0 0 134 1,002 730 140 5.27

30% OPC replacement by VA].

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Table 2 Raw materials including associated transport for concrete production in Hong Kong. Materials

Source locations

Distance (km) and transport type

Sources of data

Upstream data/ database

Cement

28 km by 30 t trucks

Survey with supplier Survey with supplier Estimated Estimated

CLCD [54]

GGBS

Local (average distance from local cement manufacturer to concrete batching plant) Dongguan in Guangdong Province (China) to Hong Kong Port To concrete batching plant (averaged) Local coal-fired power plant to concrete batching plant (averaged) Guangdong to Hong Kong Port

CLCD [55]

Admixture VA

To concrete batching plant (averaged) Guangdong to batching plant in Hong Kong (averaged) VA extraction sites to exported Port

30 km by 30 t trucks 250 km by 15 t trucks 2.5 km by 15 t truck

Exported Port to Hong Kong Port

3,702 km by ocean ship (vessel)

To concrete batching plant (averaged)

30 km by 15 t trucks

Hossain et al. [8] Estimated Estimated Survey with supplier Survey with supplier Estimated

Crushed stone (both fine and coarse aggregates) FA

50 by 30 28

km by 30 t trucks, and 128 km inland barge km by 30 t trucks km by 30 t trucks

128 km by inland barge

CLCD [54]; CLCD [55] CLCD [54] CLCD [54]

CLCD [54] CLCD [54] CLCD [54] ELCD [50] CLCD [54]

Table 3 Sources of energy for materials/processes for concrete manufacturing. Materials/processes Fine aggregates production (crushed stone) Coarse aggregates production (crushed stone) Cement production FA GGBS Admixture VA Concrete (batching) production

Energy consumption

Upstream data/ databases a

7.57 kWh/t (electricity) & 1.37 L/t (diesel) 6.07 kWh/t (electricity) & 1.37 L/t (diesel) a R* 9.3 kWh/t b 72.15 kWh/t c 2500 kWh/t b 4.86 L/t (diesel for excavation) d, 1.89 kWh/t (electricity for processing) d and 9.3 kWh/t e (electricity for grinding and drying) 2.5 kWh/t of concrete in Hong Kong f

CLCD [56,57] CLCD [56,57] Hossain et al. [10;42] CLP [58]; CLCD [56] CLP [58]; CLCD [56] CLP [58]; CLCD [56] CLP [58]; CLCD [56,57] CLP [58]; CLCD [56]

* Referred to the database/references in the right column; a Hossain et al. [41,59]; b MPA [60]; c First-hand through survey ; d Dunlap [61]; e Assuming FA equivalent energy consumption for VA processing; f Zhang et al. [47].

double than C1-C4) of admixture in C5 concrete, the slump value is lower (i.e. 28 mm) indicating the low workability of VA in concrete. However, the slump value was significantly increased for C6 concrete (i.e. 147 mm) for having additional admixture (i.e. about 17% higher than that used in C5) (Fig. 1). As mentioned earlier, VA was oven-dried at 110 °C for 48 h due to considerably higher moisture content before being used in the mixtures. This may increase the water absorption, and reduce the workability of VA concretes. Moreover, higher aggregates / binder ratio for VA concretes compared to the OPC one, may reduce the workability, and thus higher dosage of superplasticizer was needed to increase the workability.

on the selected mid-point impact categories was calculated based on the IMPACT 2002 + impact method [53], in order to compare the environmental impacts of the selected concrete designs. 3. Results and discussion 3.1. Mechanical performance of the concretes 3.1.1. Materials characterization According to ASTM C618, a good pozzolan should contain materials with the amount of SiO2 + Al2O3 + Fe2O3 higher than 70% [62]. Based on the results of materials characterization, it can be seen that this content in VA is 86.88%, which implies that VA would be a good replacement of cement for concrete production (Table 4). From the strength development, it can be found that using similar replacement amount of cement by GGBS, FA and VA, comparable concrete strength can be achieved. The 30% replacement of cement by VA would not only obtain a good early strength, but also can reach a comparable strength after 28 days (Fig. 2).

Table 4 Oxide compositions and physical properties of cement, FA, GGBS and VA. Oxides (% by mass) Na2O MgO Al2O3 SiO2 SO3 K2O CaO TiO2 Fe2O3 MnO Loss on ignition Particle size D10 (lm) D50 (lm) D90 (lm) Surface area (m2/kg)

3.1.2. Workability of fresh concretes To evaluate the workability of the concrete mixes, slump test was performed according to the CEDD guidelines. The results of workability test for fresh concrete have shown that OPC concrete was true slump as it was about 95 mm, whereas the slump value for C2 is quite high (i.e. 185 mm), and 85 mm for C3. Due to higher water absorption capacity by FA, the slump value for C4 concrete is considerably low (i.e. 18 mm) (Fig. 1). In this case, the watercement ratio can be slight increased (i.e. 0.37 to 0.40) or slightly higher admixture can be added to increase the workability of C4 concrete [63]. Even with the use of high dosages (which is about 4

OPC

FA

GGBS

VA

– 0.85 4.01 20.17 5.13 0.58 66.48 – 2.77 – 1.08

2.07 2.81 21.21 52.80 1.68 1.88 9.75 1.20 6.44 0.15 3.91

– 6.36 13.89 32.71 3.18 1.14 40.95 1.29 0.31 0.17 0.31

– – 10.81 71.31 – 7.74 4.42 0.73 4.76 0.23 3.37

6.07 19.7 43.8 542.4

3.14 15.7 82.2 879.8

2.68 14.6 53.1 920.9

5.08 30.8 104 503.7

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Fig. 1. Workability of the fresh concretes.

strength was 53.8 MPa and 70.7 MPa for C5 compared to 47.7 MPa and 59.23 MPa for C1, respectively at 7 and 28 days. However, it was about 8% and 9% at 7 and 28 days for replacing 30% of OPC by VA (e.g., C6 concrete). The results indicate that even using 30% of VA in concrete, comparatively higher strength can be achieved. The results also indicate that the early strength development (e.g. 7 days) is relatively faster when using VA in concrete and achieved 76–80% of the total 28-day strengths (for both C5 and C6). Compared to C2, about 24% higher strength was observed for C5 at 7 days, which is about 21% for C6. Similarly, the strength was 70.7 MPa for C5 at 28 days, which was 11% and 9% higher than those of C2 and C3, respectively. For C6, the strength was almost similar to C3, but slightly (about 3%) higher than C2. At 28 days, the strength was 3% higher for C5, but 5% lower when using a higher amount of VA (e.g. 30%) compared to 25% of FA in concrete production. The results are also consistent with other studies [18,19,23,24]. The overall results indicate that by replacing 20–30% of OPC with VA, acceptable or even considerably higher strength (9–16% compared to OPC concrete) can be attained which would meet the standards specified by CEDD, and is indeed comparable and even higher to its counterparts including the GGBS (3–11%) and FA concretes. The results also have demonstrated that substituting 20% of VA can get a higher strength, and it could reduce to 8% when 10% more VA is used in the concrete, but it is still higher than the concrete produced with OPC and other industrial by-products (Fig. 2). As the size of the aggregates was the same in the studied concrete mixtures, the water-to-binder ratio may significantly influence the compressive strength development in addition to the use of different binders [63–66]. It can be seen that C4-C6 concretes have considerably lower water content compared to C1-C3 concretes, resulting in an increase of mechanical strength, but significantly lower the workability (Fig. 1). Thus, higher superplasticizer was used to account for the workability loss, particularly for C5 and C6 concretes. Compared to OPC and SCMs concretes, natural pozzolana concretes (C5 and C6) have considerably higher

3.1.3. Compressive strength of concretes The compressive strength of different mixed designs of the studied designed concrete at 7 days and 28 days are shown in Fig. 2 and Table S1. Compared with the control concrete (C1), about 14% lower compressive strength was observed at 7 days for replacing 30% and 50% OPC by GGBS (i.e. C2 and C3 concretes), respectively. However, it was about 6% higher for C2 and 8% higher for C3 at 28 days indicating that early strength is comparatively slower for the use of GGBS, but it could gain higher strength later (e.g. 19% higher for OPC concrete but more than 35% for C2 while 36% for C3 at 28 days compared to the 7-day strength). The results also indicate that the compressive strength is significantly higher than that of FA concrete. For example, about 12–14% higher strength was observed for C4 compared to that of C1 at 7 and 28 days. This may be due to the higher cementitious materials (OPC, GGBS and FA) used in C2-C4 concretes compared to C1 (Table 1). In addition, low water-to-binder ratio may increase the strength of FA concrete compared to C1-C4 concretes. Compared with the control concrete (C1), about 11% and 16% higher strengths were observed for replacing 20% of OPC by VA (i.e. C5 concrete) at 7 and 28 days, respectively. The average

Fig. 2. Compressive strength of different mix-designs of concrete. 5

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observed for C4 compared to C1. Although only 25% of FA were used, the contribution to the total GHG emissions was high (i.e. about 14% due to its higher upstream burden according to economic allocation). However, natural pozzolana can reduce GHG emissions significantly at both 20% and 30% (C5 and C6), as the emission was 432 kg CO2 eq and 390 kg CO2 eq for C5 and C6, respectively compared to C1. The results indicate that the savings were 15% for C5 and 26% for C6 (Fig. S1). VA induces significantly lower impacts compared to OPC, GGBS and FA due to less processing and the absence of any upstream burden (contributing only to 1–2% of the total emissions, which is associated with transportation, grinding and drying). Therefore, C5 and C6 concretes are associated with about 15–26%, 2–9% and 9–18% lower GHG emissions compared to C1, C2, and C4 concretes, but 4–13% higher than that of C3 despite around half of the VA was used in C5 and C6 (Table 5). In the category of non-renewable energy, about 3,516 MJ energy was needed for the production of C1, which is about 4% and 13% higher than that of C2 and C3 concretes, respectively (Table 5 and Fig. S1). However, higher energy consumption was associated with C4 (i.e. about 2%) compared to that of even C1. This is because of the upstream impacts of FA due to the burning of coal for electricity generation, and its subsequent shared to FA according to economic allocation (Fig. 3). The results also show that about 9% and 15% lower non-renewable energy consumption was associated with C5 and C6 concrete production compared to C1, due to the significantly lower energy consumption for VA processing. Overall, about 7–13% and 11–17% less energy was consumed when using natural pozzolana (C5 and C6) compared to the use of GGBS (C2 and C3) and FA (C4) in Hong Kong (Fig. S1). The LCA results show that about 6% lower acidification impact was associated with C2 concrete compared to that of C1 (Fig. S1). About 8.25 kg SO2 eq emissions was associated with per m3 of C1 concrete production, which is about 15% lower than that of C3 concrete (i.e. 6.98 kg SO2 eq). Due to the use of 30% VA in C6 concrete, the savings of acidification potential was also higher (i.e. about 14%), as 7.12 kg SO2 eq was associated with C6, compared to that of C1 concrete, and 8% greater saving when using 20% natural pozzolana. The use of FA contributed to the highest acidification impact due to their upstream allocated impacts, as it induced 16% higher than that of OPC concrete (C1) (Table 5). It can be seen that OPC has contributed the highest percentages of SO2 eq emissions for all types of designs. Compared to VA, GGBS and FA contributed much higher amount of SO2 eq emissions due to the allocation of impact from their upstream products and processes. Although admixture contributed higher in C5 and C6 concretes due to the use of higher dosages of admixture compared to that of C3-C4 (Fig. 3), acidification impact was similar to that of GGBS concrete (C2 and C3), despite about 24–30% lower than that of FA concrete (C4) (Table 6).

compressive strengths with lower water-to-binder ratio, and higher superplasticizer and aggregate contents. In addition to the effects of equal water-to-binder ratio and aggregates contents, and the consistent use of superplasticizer need to be evaluated for further promotion of natural pozzolana in concretes for designing similar grades of concretes. Because the addition of superplasticizer may increase the strength of concrete in addition to its workability [67]. 3.2. Environmental impacts of concrete with different alternatives 3.2.1. Comparative analysis based on volume The LCA results on the selected environmental impact categories for per m3 functional unit is given in Table 5. The results show that respiratory inorganics are almost similar for both C1 and C2 concretes, despite it is slightly lower for C3 (i.e. about 5%) but significantly higher for C4 (i.e. about 18%), compared to C1 (Fig. S1). About 0.308 kg PM2.5 eq particle emissions as respiratory inorganics was associated with per unit of C5 concrete production, which is about 7% lower than that of C1 concrete production. Due to the use of higher amount of VA (30%) in C6, the saving of respiratory inorganics was even higher (i.e. about 12%), as 0.292 kg PM2.5 eq was associated with C6, compared to 0.333 kg PM2.5 eq for C1. It can be seen that OPC has contributed the highest percentages of PM2.5 emissions for all types of design. Despite different aggregate contents had been used, almost similar emissions were found for all designs. Compared to VA, GGBS and FA have contributed much higher amount of PM2.5 due to the allocation of impact from their upstream product and process (i.e. steel and electricity, respectively). However, admixture contributed higher in C5 and C6 concretes due to the use of the high dosages of admixture compared to C3-C4 (Fig. 3). Overall, C5 and C6 concretes are associated with about 7–12% lower respiratory impact compared to OPC concrete (C1), 7–13% lower compared to GGBS concrete (C2 and C3), and about 27–32% lower than that of FA concrete (C4) (Table 5). The results show that CO2 eq emissions as global warming potential for C1 concrete production is considerably high in Hong Kong, due to the higher carbon emission factor of OPC (about 1 kg CO2 eq/kg OPC) [10,48]. This is because coal is the principal fuel for burning clinker and most of the materials and fuels are imported to Hong Kong. Therefore, more than 500 kg CO2 eq GHG was emitted for the production of the studied concrete produced with OPC only (Table 5). As mentioned earlier, OPC has contributed to about 89% of the total GHG emissions, whereas 10% by aggregates and less than 1% by others such as admixture and production process (Fig. 3). The use of GGBS would significantly reduce the GHG emissions for the same grade of concrete production compared to OPC. For example, GHG emissions for C2 and C3 were 446 kg CO2 eq and 366 kg CO2 eq, respectively, which was about 13% and 28% lower than that of C1 (Fig. S1). Although not same as OPC, GGBS has also significantly contributed to the total emissions due to its higher upstream burden. Comparatively, the use of FA does not significantly reduce the GHG emissions of designed concrete, as only 6% GHG emission reduction was

3.2.2. Analysis based on the strength of the concrete As the volume and strength of concrete differ when different materials are used, especially for SCMs even for producing the same grade of concrete, it is important to consider the mechanical performance when analyzing and comparing their corresponding

Table 5 Environmental impacts for per m3 of concrete production. Concrete types

Respiratory inorganics (kg PM2.5 eq)

Global warming (kg CO2 eq)

Non-renewable energy (MJ eq)

Acidification potential (kg SO2 eq)

C1 C2 C3 C4 C5 C6

0.333 0.336 0.316 0.391 0.308 0.292

511 446 366 479 432 390

3,516 3,371 3,062 3,573 3,200 3,003

8.247 7.745 6.978 9.569 7.554 7.116

6

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Fig. 3. Contribution analysis for different mix-designs of concrete.

that of OPC concrete, as the emissions were 6.12 kg CO2 eq and 6.03 kg CO2 eq for C5 and C6, respectively. The results indicate that VA can induce significantly lower impacts compared to OPC and other industrial SCMs even when considering the strength of concrete. Compared to C2, 11–12% lower GHG emissions were associated with C5 and C6 concretes, and 10–11% lower than that of FA concrete (C4), but slightly higher (4–5%) than the C3 concrete (Table 6). It should be noted that 50% OPC was replaced in C3 concrete whereas it was only 20% and 30% for C5 and C6 concrete, respectively. In the category of non-renewable energy, about 59.37 MJ energy was needed for the production of C1, which is about 10%, 20% and 12% higher than that of C2, C3 and C4 concrete, respectively (Table 6 and Fig. S2). For C5 and C6 concretes, about 24% and 22% lower non-renewable energy consumption were associated when compared with that of C1. This is due to the significantly lower energy consumption for VA processing (Fig. 3), while there is no upstream allocated impact of VA. When considering the strength of concrete, about 2–14% lower energy was consumed for the use of natural pozzolana (for C5 and C6 concretes) compared to the use of GGBS (C2 and C3) in Hong Kong, whereas it was about 10–12% lower compared to the use of FA in concrete (C4) and 22–24% lower than that of OPC concrete (Fig. S2). When considering the strength of concrete, the LCA results show that almost similar acidification impact was associated with

environmental impacts. Based on the second functional unit, the LCA results of the designed concretes (environmental impacts for per 1 m3 concretes divided by their corresponding 28-day compressive strengths) are shown in Table 6. The results show that about 5.61 g PM2.5 eq emissions as respiratory impacts is associated with per strength of C1 concrete production, which is about 5% and 12% higher than concretes containing GGBS (e.g. C2 and C3, respectively). Due to higher upstream impacts and lower strength, concrete containing FA induced higher respiratory impacts (i.e. about 2%) compared to that of C1 concrete. However, more than 20% lower respiratory impacts were observed for both C5 and C6 concretes containing natural pozzolana than that of C1. Compared to VA concrete, about 8–17% and 22–24% higher respiratory impacts were observed in GGBS and FA concrete, respectively (Fig. S2). Due to the use of higher OPC in C1 concrete, CO2 eq emissions was considerably higher than all other concretes (Table 6). Even when considering the strength of concrete, C2 and C3 induced about 18% and 34% lower GHG emissions than that of C1 (Fig. S2), as about 7.07 kg CO2 eq and 5.68 kg CO2 eq GHG emissions were associated with per strength (MPa) of C2 and C3 concretes, respectively, whereas it was about 8.62 kg CO2 eq for C1 concrete (Table 6). Similarly, the use of FA could reduce GHG emissions by about 19% compared to that of C1. Natural pozzolana could reduce GHG emissions for about 29% and 30% compared to

Table 6 Environmental impacts of concrete production based on strength. Concrete types

Respiratory inorganics (g PM2.5 eq)

Global warming potential (kg CO2 eq)

Non-renewable energy (MJ eq)

Acidification potential (g SO2 eq)

C1 C2 C3 C4 C5 C6

5.614 5.326 4.916 5.706 4.353 4.501

8.62 7.07 5.68 7.00 6.12 6.03

59.37 53.50 47.56 52.16 45.25 46.34

139.24 122.93 108.40 139.69 106.84 109.82

7

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Fig. 4. Damage assessment associated with concrete for different mix-designs.

lower dosages of admixture, perhaps by adding other auxiliary materials, such as GGBS or limestone. To achieve the equivalent compressive strength (as one of the functional units in this study), different binder mass and volume were used in this study, particularly for GGBS and FA concretes (C2-C4). It should be noted that C2-C4 concretes were designed for comparative purpose only. The binder mass was consistent for VA concretes (C5-C6) when compared to control (C1), which is the main aim of this study (to promote VA as a sustainable material). In addition, higher aggregate contents and lower water-to-binder ratio were used for VA concretes (C5 and C6). Thus, further investigation is needed for designing the optimum use of those materials with consistent water-to-binder ratio for VA concrete with the control and other SCMs. It is also essential to ensure the effective drying and grinding of VA to obtain the maximum benefits. Moreover, integral designs associated with VA and industrial by-products to substitute different percentages of OPC in different grades of concrete can be tested for feasibility analysis, and the environmental performance can be evaluated for further promotion.

C4, but significantly lower in C3 and C4 concretes compared to that of C1 (Fig. S2). About 139 g SO2 eq emissions was associated with per strength of C1 concrete production, whereas it is about 123 g SO2 eq and 108 g SO2 eq for C2 and C3. Due to the use of 20% and 30% VA in C5 and C6 concretes, the savings of acidification potential was 21–23% respectively compared to that of OPC concrete. Compared to VA, the use of GGBS induced 1–10% higher acidification impact, but the saving was more than 20% compared to that of FA concrete (C4) when considering their strengths (Table 6). 3.2.3. Damage assessment of concretes For comparing the LCA results of different mixtures of concrete effectively, a damage score was obtained based the associated characterization, damage and weighting factors provided by the IMPACT 2002 + impact method (given in the Supplementary Information, Table S2–S4). The total environmental burden represented by a dimensionless eco-point measured in units of milli-points (mPt), which indicates the potential number of people affected by the environmental impacts in a period of one year [53]. Based on the FUs, the LCA results are shown in Fig. 4. It can be seen that the net environmental burden for OPC concrete production is about 108 mPt/m3 which is 7% higher than that of C2 concrete, but much higher than (18%) when considering 50% OPC replacement by GGBS (C3). Again, the net environmental burden of FA concrete is almost equal or even higher (i.e. about 3%) than that of C1. However, the saving is about 12% for substituting only 20% VA (C5), and much higher (about 18%) when replacing 30% VA (C6) than that of OPC concrete (Fig. 4). However, much higher impact savings of environmental load for all concretes were observed when considering the strength of concrete. For example, about 13%, 25% and 11% lower for C2, C3 and C4 concretes, respectively. Due to higher strength of VA concrete, the saving is even much higher, as it is 26% and 25% for C5 and C6, respectively than C1 concrete. It is almost similar to C3 concrete, but 14–17% higher than C2, and 15–17% higher than C4 concrete, respectively (Fig. 4). Despite the benefits of the use of VA in terms of controlling the alkali-silica reaction and reducing shrinkage [22], it may have several negative impacts including an increased bleed, additional set time, permeability, etc. [68], and thus, more tests are necessary to promote VA as a supplementary material. Moreover, studies reporting on VA applications (e.g. cement paste, mortar and concrete) are in different regions with different sources [69], and thus those results cannot be adopted in other regions, as VA is heavily dependent on its origin, mode of formation and chemical composition [23,70,71]. This study considered limited technical parameters including the material characterization, workability of fresh concrete and the mechanical strengths to analyze the preliminary feasibility with only one type of concrete production, and thus more tests should be carried out comprehensively. Because of the low workability of VA concrete, higher amount of admixture was used. Further research is necessary to increase the workability with

4. Conclusions In this study, the technical feasibility and environmental sustainability of the use of natural pozzolana in concrete were evaluated. For comparative analysis, six batches of concrete were prepared such as concrete with OPC only (C1), with 30% OPC substitution by GGBS (C2), with 50% substitution by GGBS (C3), with 25% OPC replacement by FA (C4), with 20% OPC substitution by VA (C5), and with 30% OPC substitution by VA (C6), and compressive strengths at 7 days and 28 days were evaluated according to the BS Standard Methods. Based on the first-hand and casespecific data, a comprehensively LCA of the considered six types of concrete (C1-C6) for evaluated environmental impacts were comparatively conducted based on per m3 of concrete and their corresponding strengths. Based on the findings, the following conclusions can be drawn:  By using 20–30% VA in concrete, an acceptable or even considerably higher strength can be attained (e.g. 9–16%, 3–11% and 3% compared to C1, C2-C3 and C4, respectively). Despite a reduction of 8% strength when using an addition of 10% of VA (e.g. 30%) compared to C5, the compressive strength was still higher when compared to concrete produced with OPC and other industrial by-products.  With the system boundary and considerations, VA could induce significantly lower impacts compared to OPC, GGBS and FA due to less processing and lack of any upstream burdens. For example, C5 and C6 concretes were responsible for about 15–24%, 9– 18%, and 2–11% lower GHG emissions compared to C1, C4, and C2 concretes, respectively, but it is almost similar to that of C3 concrete where 50% OPC is replaced by GGBS. 8

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 However, the savings are much higher when per strength functional unit is considered. For instance, C5 and C6 concretes could reduce about 29–30%, 11–12%, and 10–11% of the total GHG emissions compared to C1, C2, and C4 concretes, respectively.  The net environmental burdens of C5 and C6 concretes with VA were considerably lower than that of OPC concrete and the concretes made with conventional industrial by-products.

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Based on the preliminary technical and comprehensive LCA results, VA can be a potential substitute of OPC in concrete production which should enhance the sustainability performance of the concrete industry. However, more tests are necessary to warrant the mechanical performance and durability of the VA-based concrete. CRediT authorship contribution statement Md. Uzzal Hossain: Conceptualization, Data curation, Investigation, Methodology, Software, Visualization, Writing - original draft, Writing - review & editing. Rongjin Cai: Conceptualization, Formal analysis, Investigation, Visualization. S. Thomas Ng: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Software, Supervision, Writing - review & editing. Dongxing Xuan: Conceptualization, Validation, Visualization, Writing - review & editing. Hailong Ye: Conceptualization, Methodology, Project administration, Resources, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors would like to thank the Peakward Enterprises (Holdings) Ltd. Hong Kong for providing the natural pozzolana for the experimental analysis, and the necessary information for conducting the LCA of natural pozzolana. The financial support of The University of Hong Kong through the 40th Research Assistant Professor/Postdoctoral Fellowship Scheme is also gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2020.121429. References [1] W. Shen, L. Cao, Q. Li, W. Zhang, G. Wang, C. Li, Quantifying CO2 emissions from China’s cement industry, Renew. Sustain. Energy Rev. 50 (2015) 1004–1012. [2] K.L. Scrivener, R.J. Kirkpatrick, Innovation in use and research on cementitious material, Cem. Conc. Res. 38 (2008) 128–136. [3] C. Ouellet-Plamondon, G. Habert, Life cycle assessment (LCA) of alkali activated cements and concretes, in: Handbook of Alkali-Activated Cements, Mortars Concretes, (2015) 663–686. [4] N.A. Madlool, R. Saidur, M.S. Hossain, N. Rahim, A critical review on energy use and savings in the cement industries, Renew. Sustain. Energy Rev. 15 (2011) 2042–2060. [5] Cement Statistics and Information, The U.S. Geological Survey, Mineral Commodity Summaries, 2020. https://pubs.usgs.gov/periodicals/mcs2020/ mcs2020-cement.pdf [6] B.L. Damineli, F.M. Kemeid, P.S. Aguiar, V.M. John, Measuring the eco-efficiency of cement use, Cem. Conc. Com. 32 (2010) 555–562. [7] A.P. Gursel, E. Masanet, A. Horvath, A. Stadel, Life-cycle inventory analysis of concrete production: a critical review, Cem. Conc. Com. 51 (2014) 38–48. 9

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