Life Cycle Assessment (LCA) in Earth Construction: A Systematic Literature Review Considering Five Construction Techniques

Abstract

In the past decade, there has been an increase in the environmental performance assessment in earth construction through the life cycle assessment (LCA) methodology. A Systematic Literature Review verified LCA methodology trends of five earth construction techniques from 2016 to April 2022, resulting in 27 studies. The results have been analyzed through qualitative thematic analysis, considering LCA methodology. Considering embodied carbon (GWP) and embodied energy, transportation and binder content were the main factors that influenced environmental performance. Hence, earth-based constructions exhibit better results in different impact categories than conventional materials. Environmental guidelines and technical features that were presented in the LCA studies are discussed for Adobe, Cob, Rammed Earth (RE), Compressed Earth Block (CEB), and Light Straw Clay (LSC). This study presents environmental benchmarks at the unit, wall, and building scales aiming to encourage LCA methodology applied to earth construction techniques and fostering the discussion of earth construction sustainability.

1. Introduction

There is a growing concern about the consumption and production of materials for construction due to their effects on the environment and the economy [1]. Conversely, construction is among the industries that generate higher environmental impacts. Conventional building systems rely on industrially based materials with high embodied energy and significant-high rates of greenhouse gases (GHG), especially CO₂ emissions, among other potential environmental impacts [1,2].

Earth construction is one of the oldest building practices known, probably dating back to man’s sedentarization, with the development of agriculture about 10,000 years ago [3]. In most countries worldwide, earth construction can be found either old or currently under construction [4]. Although most earth construction is present in less-developed countries, this scenario is changing due to the sustainable construction concept [5].

Earth-based buildings have been reconsidered and used in contemporary construction with increasing environmental awareness. According to Dobson [4], raw earth houses are sustainable, healthy, have low carbon dioxide emissions, and have aesthetics ranging from rustic to modern. Regarding its sustainability, many authors have been studying earth construction materials as an alternative to traditional materials such as concrete, aluminum, and steel in the past decade. Some studies such as Aguilar et al. [6], Bogas et al. [7], Cuitiño-Rosales et al. [8], Millogo et al. [9], Nakamatsu et al. [10], Olacia et al. [11] are focused on enhancing the earth material to obtain a good physical and mechanical performance or focusing on constructions pathologies.

More recent earth construction studies have focused on the Life Cycle Assessment (LCA) methodology to assess and quantify environmental impacts. LCA can be defined as the compilation and evaluation of a product system’s inputs, outputs, and potential environmental impacts throughout its life cycle [12]. LCA is an accepted methodological approach for the quantitative assessment of building materials’ sustainability and environmental impact [13].

Paiva et al. [14,15] evaluated the potential of earth-based mortar reinforced with bamboo fibers to reduce GHG emissions. Using bamboo fibers reduces the mortar’s thermal conductivity, improves building thermal-energy performance, and stocks CO₂, reducing life cycle GHG emissions. Santos et al. [16] reviewed different LCA studies of mortars, including earth-based ones, and verified that the last ones have lower environmental impacts than cement and cement/lime mortars. Ventura et al. [17] assessed the existing literature about applying the LCAs to some earthen construction techniques to find critical factors. Among these factors, transport and chemical stabilizations by binders presented a major influence on environmental performance. The authors concluded that there is no universal solution for the LCA of an earthen wall.

However, we did not see any review study of LCA applied to earth construction that covers the evaluation of the techniques presented here. This work verifies LCA methodology trends in five earth-based construction techniques/products: Adobe, Cob, Rammed Earth (RE), Compressed Earth Block (CEB), and Light Straw Clay (LSC) through a systematic literature review (SLR). The SLR allows visualizing how the LCA research applied to earth construction techniques beyond state-of-the-art. A brief description of these techniques is presented below in

Table 1. Description of the earth construction techniques.

Construction Technique	Description	Representation
Adobe	Adobe is the most simple and ancient brickwork technique. It is a mix of soil and water in a plastic physical state (stabilized or not) in wooden molds shaped like bricks, without compaction, and dried in the sun [18]
Cob	Cob or stacked earth techniques require mixing earth with straw and water to build layer-by-layer load-bearing masonry walls. It is produced in a plastic state and implemented wet. It is also known as monolithic adobe and “Bauge” in France [19].
Rammed earth (RE)	RE, “pies de terre” or “Taipa” uses moist soil (stabilized or not), compacted inside a formwork (traditionally made of wood, and nowadays made of steel) [3]
Compressed Earth Block (CEB)	CEB is masonry manufactured using the compression or pressing of stabilized soil inside a mechanical or hydraulic press [20]. Interlocking compressed earth brick (ICEB) or stabilized earth brick (SCEB). ICEB does not require a mortar bed between the bricks since they are assembled dry and stacked on one another. Its size and shape depend on the construction purpose, but they all have three hollow sections in the middle of the bricks [21].
Light Straw Clay (LSC).	LSC, light clay, straw-clay, slip straw, or rammed straw composed of fiber (usually straw) coated in very wet clay. The mix can be packed into temporary or permanent frameworks. It’s used as insulation due to its workability and compatibility with other wall assemblies, but not as loadbearing [20].

.

Hereof, this work aims to encourage LCA methodology applied to earth construction techniques to instigate the discussion of earth construction sustainability by presenting embodied carbon and embodied energy values from the above-mentioned earth-based techniques considering the life cycle stages at different scales, namely unit (mass), wall scale, and building scale (m²). In addition, we hope that it brings insights for improving LCA methodological aspects applied to earth construction

2. Materials and Methods

2.1. Systematic Literature Review (SLR)

An SLR based on the PRISMA methodology [22] was executed to assess how LCA has been applied to earth construction over the past six years, from 2016 to April 2022. Articles approaching LCA to earth building assessment, comparing and discussing the environmental impacts with conventional materials were considered. This review did not consider earth-based mortars since it focuses on construction techniques once there are already specific reviews for this earth-based material, for example, the study of Santos et al. [16]. This study also does not comprise LCA studies of infrastructure works such as [23,24,25] focused on LCA applied to earth retaining walls.

Scopus and Web of Science (WOS) were the two databases selected to identify articles published in journals and conference proceedings. The exact keywords were used in both databases to ensure a consistent scope. As a first approach, the search terms “LCA,” “Earth,” and “Construction” were used in both databases. To gather more specific results, the terms “LCA” were combined individually with specific earth constructions: “adobe,” “cob construction,” “compressed earth blocks,” and “rammed earth.” For Cob, the term construction was added to improve the accuracy of the results. Ultimately, the search terms “Earth” and “Construction” and “Earth” and “Building” were also combined with “LC”.

Table 2. Search terms and results in Web of Science (WOS) and Scopus databases.

Keywords	WOS	Scopus	Total
LCA Earth Construction	34	47	81
LCA Adobe	3	3	6
LCA CEB	3	3	6
LCA Cob	3	5	8
LCA Rammed Earth	7	10	17
LCA Earthen Material	3	8	11
LCA Earth Building	42	40	82
Total	95	116	211

summarizes the search terms and results in both databases.

The search results were compiled in an excel file to organize and visualize all the chosen articles by publication year, authors, article title, and source title. After excluding duplicated articles (n = 114), the records were first selected by title, removing the ones outside the scope, such as the materials mentioned above or structures. The following step consisted of reading abstracts and the details of the LCA methodology.

The eligibility criteria applied in this review was the use of LCA methodology to assess the potential environmental impacts of the earth construction techniques. The selection process excluded studies focused on the LCA of insulations in earth buildings or earth-based mortars. There were also not considered publications that did not expose a transparent LCA methodology, explaining the stages as predicted in ISO 14040-2006 [12]. Other relevant scientific articles and theses were added to SLR through the snowball method. The criteria used for this inclusion were based on the geographic relevance of Portuguese material.

Figure 1 represents the flow diagram used to identify scientific publications approaching LCA in earth construction, following the PRISMA methodology [22]. There were gathered for this review a total of 27 publications. Appendix A presents the SLR references and the main categories explored in this review.

2.2. Data Analysis

Qualitative thematic data analysis was executed to compare and understand how LCA methodology has been applied to earth construction techniques.

The themes and categories are summarized in

Table 3. Thematic analysis themes and categories.

Theme	Categories
General Features	Geographic Distribution Publication year
Goal and Scope	Assess potential impact Compare the impact of earth-based assemblies Compare the impact of earth-based assemblies to conventional assemblies Compare scenarios Compare assemblies
System boundaries	Cradle-to-gate Cradle-to-site Cradle-to-grave
Life Cycle Stages (EN 15804:2012 + A1)	Product stage Construction stage Use stage End-of-life stage
Unit	Functional unit Declared unit Reference flow
Life Cycle Inventory Analysis (LCI)	Background and foreground data sources
Life Cycle Impact Assessment (LCIA)	Impact assessment methods Impact categories Allocation procedures
Embodied Carbon an dEmbodied energy	Hotspots Benchmarks
Other material features	Thermal, mechanical and durability

. The publications were organized per construction technique, the country where the study was developed, and the publication year, here defined as general features. Following the life cycle assessment framework [12], the data were categorized by the goal and scope of the studies and the adopted system boundaries, the life cycle stage (EN 15804:2012 + A1 [21]) and the adopted unit (declared, functional and reference flow). The LCI (life cycle inventory analysis) was investigated by the background and foreground data sources. There were also explored the LCIA (life cycle impact assessment), the impact categories and allocation, accomplishing the LCA methodology feature here analyzed.

The available data on embodied carbon and embodied energy from the studies considered in this review allowed us to present environmental guidelines at different scales (mass by unit, wall and building scale). Other material features considered in those studies are also briefly discussed, and a research outlook and recommendations are finally presented.

3. Results

The results are discussed here by themes, and the qualitative data are represented in Appendix A (

Table A1. Qualitative paper data by themes.

Authors	Ref	Technique	Geographic Distribution	Goal and Scope	System Boundaries	Life Cycle Stages	Unit	LCI Data Source		LCIA Methods	Allocation Procedures
								Background	Foreground
Pereira, 2017	[53]	CEB	Portugal	to assess the potential environmental impacts	cradle-to-gate	A1–A3	m2 wall	Ecoinvent 2013 v3.4	Primary	CML, CED	Not considered: CEB as single product
Florescu and Bica, 2020	[47]	Adobe	Romania	to assess the potential environmental impacts	cradle-to-gate	A1–A5	m2 wall	Ökobaudat database	Primary data	undeclared	undeclared
Mateus, R., Fernandes, J., and Teixeira, E. R., 2019	[35]	RE and CEB	Portugal	to compare the environmental impact of different earth-based assemblies	cradle-to-gate	A1–A3	m2 wall	Ecoinvent v3.3 database	Primary data company/manufacturer	CML, CED	undeclared
Fernandes et al., 2019	[40]	RE and CEB	Portugal	to compare the environmental impact of different earth-based assemblies	cradle-to-gate	A1–A5	m2 wall	Ecoinvent v3.3 database	Primary data—company and manufacturers	CML, CED	Attributional: ordinary allocation by-products
Ben-Alon et al., 2019a	[19]	Cob	USA	to assess the potential environmental impacts	cradle-to-gate	A1–A3	m2 wall	US-LCI and Ecoinvent for North America	US-LCI and Ecoinvent for North America	CML, TRACI	Attributional: ordinary allocation between by-products
Ben-Alon et al., 2021	[20]	Cob, LSC, RE	USA	to compare the environmental impact of different earth-based assemblies	cradle-to-grave	A1–B6	m2 wall	EcoInvent inventory data North America”	US-LCI.	Operational LCIA	Consequential: system expansion
Christoforou et al., 2016	[32]	Adobe	Cyprus	to compare different scenarios of raw material or transport	cradle-to-site	A1–A3	kg	GaBi database; ICE	Primary data and the literature	Undeclared	by-products
Arrigoni et al., 2017	[39]	RE	Australia	to compare different scenarios of raw material or transport	cradle-to-gate	A1–A3	m2 wall	Ecoinvent	Ecoinvent and real distances	CML, CED	Attributional: cut off system model as wasteConsequential: system expansion
Meek et al., 2021	[51]	RE	Australia	to assess the potential environmental impacts	cradle-to-gate	A1-A5	m2 wall	Ecoinvent v3.4, AusLCI database v2.8	AusLCI database v2.8; updated where more accurate or current data was available.	-	EN15804
Asman et al., 2020	[34]	Interlocked CEB (ICEB)	Malaysia	to compare the environmental impact with conventional assemblies	cradle-to-gate	A1–A5 (without A4)	m2 house	-	primary data: Bill of Quantities (BQ) in the contract document and construction data	-	undeclared
Elahi et al., 2021	[31]	CEB	Bangladesh	to compare the environmental impact with conventional assemblies	cradle-to-gate	A1–A3	m3 wall	literature	Direct data and literature; Based on the local market price for cost analysis.	-	undeclared
Alhumayani et al., 2020	[33]	Cob	Saudi Arabia	to compare the environmental impact with conventional assemblies	cradle-to-site	ND	m2 wall	Ecoinvent v3.1	Ecoinvent v3.1	ReCiPe Midpoint	undeclared
Nanz et al., 2019	[37]	RE	Germany	to compare different scenarios of raw material or transport	cradle-to-site	A1–A5	m2 wall	Ökobaudat (2017), literature and (ÖNORM B 8119-7, 2013)	Ökobaudat (2017); Austrian brick association Initiative Ziegel (2015)	-	undeclared
Meek and Elchalakani, 2019.	[50]	RE	Australia	to compare the environmental impact with conventional assemblies	cradle-to-gate	A1–A3	m2 wall	AusLCI database v2.8, Ecoinventv3.4 when AusLCI data was unavailable	Ecoinvent	-	EN15804
Shrestha 2021	[44]	Compressed stabilized earth block (CSEB)	Nepal	to assess the potential environmental impacts	cradle-to-grave	A1–C4	m2 house	Ecoinvent dataset version 3.5	Ecoinvent dataset version 3.5	CED, ReCiPe Midpoint	undeclared
Arrigoni et al., 2018	[54]	RE	Australia	to assess the potential environmental impacts	cradle to gate	A1–A3	m2 wall			ReCiPe Midpoint	waste
Ansah et al., 2020	[27]	Stabilized earth blocks façade (SEBF)	Ghana	to compare the environmental impact with conventional assemblies	cradle to gate	A1–A5	m2 house	The Inventory of Carbon and Energy (ICE) database V2.0;	Primary data (building drawings, etc.)	CED	undeclared
Zhang et al., 2020	[43]	Adobe	China	to compare the environmental impact with conventional assemblies	cradle-to-grave	A1–C4	m2 house		literature (government data and scientific publications since 1996)	CED	undeclared
Brambilla et al., 2018	[42]	CEB	Switzerland	to assess the potential environmental impacts	cradle-to-grave	A1–C4	m2 house	KBOB	KBOB	CED	undeclared
Nouri et al., 2021	[45]	RE	Iran	to compare the environmental impact with conventional assemblies	cradle-to-grave	A1–C4	1000 kg	ICE database	ICE and ISIRI7965 code		undeclared
Bonoli et al., 2018	[28]	Adobe	Italy	to assess the potential environmental impacts	cradle to grave	A1–C4	m2 wall	Ecoinvent	primary (brick factory); literature data and Ecoinvent	Impact 2002+	undeclared
Arrigoni et al., 2016	[38]	RE	Australia	to compare different scenarios of raw material or transport	cradle-to-site	A1–A3	m2 wall	Ecoinvent and the Australasian LCA;	Ecoinvent and the Australasian LCA;	CML	undeclared
Cabrera et al., 2020	[36]	CEB	Argentina	to compare different scenarios of raw material or transport	cradle-to-gate	A1–A3	kg	Ecoinvent	primary data (Mobak CEB Factory)		undeclared
Brambilla et al., 2016	[41]	CEB	Switzerland	assess the potential environmental impacts	cradle-to-grave	A1–C4	m2 house	KBOB	KBOB	CED	undeclared
Pakdel et al., 2021	[48]	Adobe	Iran	to assess the potential environmental impacts	cradle-to-site	A1–A3	m2 house	ICE	Primary data (contractor and at construction site); Building Information Modelling (BIM)		undeclared
Ben-Alon et al., 2019b	[49]	Cob, LSC, RE	USA	compare the environmental impact of different earth-based assemblies	cradle-to-site	A1–A3	m2 wall	US-LCI and Ecoinvent for North America	US-LCI and Ecoinvent for North America	CED—Traci	undeclared
Mileto et al., 2021	[46]	RE	Spain	to compare the environmental impact with conventional assemblies	cradle-to-grave	A1–A5, B5, C1–C4	m2 house	Ecoinvent	Ecoinvent	CML	EN15804

). Furthermore, the numeric data regarding the embodied carbon and embodied energy retrieved from the here analyzed studies are available in the Supplementary Materials (Table S1).

3.1. General Features

Among the 27 scientific papers approaching LCA in earth construction, 37% of the earth construction assemblies were performed in European countries. However, this number was increased by snowball reference inclusions. Combined with Asia (26%) and Oceania (19%), these studies comprise 81.5% of the publications. The American continent has four publications, and Africa has one.

It points to a trend in the use of the LCA methodology to assess earth construction that does not reflect the geographic distribution of the here considered constructions techniques. However, when it is not applied solely to building restoration, considering also new constructions, their geographic distribution reflects some trends.

Despite earth as a building material being shared across all continents, its vernacular features influence which technique is more common in certain regions. For instance, in the Iberian Peninsula, the development of raw earth construction went back to its origins with the Phoenicians, Carthaginians, and Romans and was enhanced by the Muslims [3]. Nevertheless, it is possible to notice a geographic distribution of construction. Specifically in Portugal, from the lower Tejo to the Algarve, rammed earth construction predominates; the adobe technique prevails in the coastal area between Setúbal and Aveiro, usually associated with hydrographic basins of Vouga and Mondego rivers; in the north region, stone-based houses (granitic and schists) are more common [18]. Therefore, construction is susceptible to a plurality of conditions—climatic, geological, economic, and cultural—since many generations have been involved in adapting techniques to regional characteristics and technologies [2].

RE and CEB are more consolidated techniques regarding modern earth constructions, reflecting its widespread reproduction through the continents. According to Vyncke et al. [26], RE technology has evolved using sophisticated materials and machines. It allows the RE technique to be designed to achieve mechanical features such as compressive strength and fulfill aesthetic requirements. Conversely, CEB allows less water use, and its manufacture is flexible, allowing being produced in large industrial presses or manual molds. Nevertheless, the presses also can be equipped with different mold shapes [26].

In Figure 2a, it is possible to visualize that in Europe and Asia, LCA in earth construction is applied to a higher diversity of earth constructive systems, including adobe, RE, and CEB. In Oceania, namely in Australia, all the LCA studies are focused on the RE technique. Meanwhile, the research group of Ben-Alon et al. in the USA is on LSC, Cob, and RE techniques. The only African LCA study included in this revision is on CEBs [27].

RE and CEB are the most used in contemporaneity, reflected in the number of publications in Europe, Asia and Oceania. Adobe is one of the most ancient techniques, and some works on restoration and heritage such as Bonoli et al. [28] also present LCA as methodology. Regarding Cob, the LCA studies are more specific in the American continent.

Cob technique is considered global [29], been very expressive in countries such as France, the United Kingdom, and Germany. Despite being used for native Americans [5], some authors attribute Cob widespread to America and New Zealand through colonization [5]. Since it is considered the simplest earth-building technique [29], Cob has gained space mainly due to sustainable demands. However, there are a few works focused on the environmental impacts of these techniques from a life cycle perspective [19].

LSC derives from wattle-and-daub, a construction found throughout Europe and Asia. The light version was introduced after World War II in Europe. In Germany, LSC (“leichtlehmbau”) doubled the wall thickness and introduced more straw, resulting in a significant thermal performance increase. It was taken to America only in the 1990s by Laporte, a natural builder, and later adapted to the North American demand [30]. Beyond being an excellent insulator that can be applied to retrofit works, its workability and compatibility can be used with other earth-based building techniques around windows, doors, and other openings [20,30].

Figure 2b exhibits the annual LCA publications per construction technique, pointing to a higher number of publications on this theme with more diversification of approached techniques, mainly after 2019. Looking specifically at each construction technique, RE gathered the highest number of publications in 2019 approaching the LCA methodology. Adobe and CEB´s publications are higher in 2020, and Cob and LSC publications occur only after 2018.

3.2. LCA Methodology Framework

3.2.1. Goal and Scope

Among the selected articles, almost a third of them propose a broader goal that relies on the assessment of potential environmental impacts of the construction techniques. The other two-thirds compare the environmental impact of assemblies or scenarios (Figure 3).

The most frequent comparing studies (9) are those that compare earth-based assemblies with constructions that use conventional materials, such as [31,32,33,34]. Other authors compare different earth-based assemblies. For example, Mateus et al. [35], compare CEB and RE in a Portuguese scenario. Ben-Alon et al. [20] compare LSC, RE, and CEB with conventional assemblies.

Regarding the scenarios, Cabrera et al. [36], Nanz et al. [37], and Arrigoni et al. [38,39] compare different scenarios of raw material sources or vary the composition of the mixtures, to evaluate how it affects the impact categories analyzed.

3.2.2. System Boundaries

Figure 4a synthesizes the distribution of construction techniques studies according to the adopted system boundaries. All techniques present at least one study that adopted cradle-to-grave as a system boundary. CEB did not present any publication with a cradle-to-site boundary, and LSC was not analyzed in a cradle-to-gate system boundary. Nevertheless, it is essential to highlight that some studies declare themselves in a system boundary but can present further analysis in other phases outside. Fernandes et al.’s [40] research on “cradle-to-gate” yet presents some scenarios for other life cycle stages.

There is an increase in publications over the years considering cradle-to-grave as a system boundary ([41] in 2016, [28,42] in 2018, [43] in 2020 and [20,44,45,46] in 2021), nevertheless, cradle-to-gate is still the most common system boundary. In total, 13 studies used cradle-to-gate as a system boundary, followed by 8 with cradle-to-grave and 6 with cradle-to-site (Figure 4b).

3.2.3. Life Cycle Stages

According to NP EN 15804:2012 [21], the product category rules for LCA include building production and construction services. The adopted life cycle stages in the here-analyzed studies can be visualized in Figure 5. Most studies focused on the product stage (A1–A3) since it is required for standard compliance. CEB, RE, and Adobe techniques presented some studies that approached from A1 to A5 stage (construction stages). Ben-Alon et al. [20] included usage stages regarding A1 to B6 (including usage stage) to Cob, LSC, and RE.

Some of the studies comprise specific stages, such as Asman et al. [34], which have adopted the A1–A5 (product and construction stages) for CEBs but do not consider transport during the construction stage (A4). Other studies considered the end-of-life stages (C stages), such as Bonoli et al. [28] and Zhang et al. [43] adobes´, Shrestha [44], Brambilla et al. [41,42] CEB´s, and RE’s study of Nouri et al. [45] and Mileto et al. [46]. Hence, two studies did not declare the stage of the production system [31,32]. The discrepancy in the considered life cycle stages may lead to different results, which can be an obstacle to comparing studies, even for the same constructive technique.

3.2.4. Unit

The adopted units in the here-considered studies are compiled in Figure 6a. Although monolithic and brickworks and structure techniques are compared, the majority of LCA studies are at wall scale, square meters specifically. However, some brickworks studies yet consider constructive units (mass), such as Christoforou et al. [32] for adobes, Nouri et al. [45] for a ton of produced material for RE, and Cabrera et al. [36] for each CEB of 4 kg.

There is an increase throughout the years of publications that consider the building scale in LCA studies (Figure 6b), which is coincident with the increase in cradle-to-grave adopted system boundaries previously discussed (Section 3.2.2). It might indicate some concern in providing more detailed LCA studies involving not only the raw material´s choices and studies at wall scale but also the building project and its use, getting to the end-of-life of the construction.

Regarding the use of declared and functional unities (Figure 7), the here-analyzed studies indicate the function regarding the adopted scale. Namely the production unit itself, the wall function, or the floor area. However, some studies use thermal properties as the building function.

Nanz et al. [37], Fernandes et al. [40], Florescu and Bica [47], and Pakdel et al. [48] consider the U-value to compare the environmental indicators of earthen assemblies with usual building materials such as fired clay bricks or concrete. Thermal resistance and transmittance (U-value) correlate the thermal resistance or conductivity of the material with the thickness of the evaluated unit (block or m² of a wall).

Nanz et al. [37] consider the U-Value and the thermal and hygroscopic properties of RE to evaluate how it can reduce energy demand for building operations. Pakdel et al. [48] compare U-values from the building components of the green guest house (adobes included) compared to a conventional building. In this study, the earth-based house did not have any HVAC system, and the thermal comfort was achieved by passive techniques and by the thermal features of the building materials. These options will directly influence energy consumption and carbon emissions evaluated throughout the LCA methodology during the usage stage.

Fernandes et al. [40] used a functional unit 1 m² of the wall for RE, ceramic hollow brick and lightweight concrete, considering mortar and render, with a similar U-value. In the same sense, Florescu and Bica [47], used similar U-values to compare two wall stratifications: conventional ceramic blocks with vertical holes insulated with mineral wool with unburned bricks produced to contemporary standards insulated with wood fibers.

The category named load-bearing wall with insulation value (Figure 7) gathers works that that has an insulation value meeting or exceeding the requirements of the International Energy Conservation Code in the climatic zone that they were conducted. In it are included Ben Alon et al.’s works [19,20,49] in the USA and Meek and Elchalakani’s [50] in Australia.

The studies gathered in the wall thermal properties category in Figure 7 use other thermal features than U-Value as the functional unit. However, it is essential to highlight that some of the publications only show the adopted unit (mass or wall system) but do not clearly indicate the specific function of the product. For example, Bonoli et al. [28] declare the adopted functional unit as one square meter of wall. Yet, there is considered the use of the walls, including the thermal energy needed to maintain during the winter an internal temperature of 20 °C and a temperature of 26 °C in the summer, which characterizes a functional unit.

Brambilla et al. [42] use thermal inertia as a strategy to evaluate the effects on LCA by reducing energy consumption during the building and operational phases. The study compares in different scenarios the reduction of heating promoted by better thermal inertia in comparison with the embodied energy of the chosen building materials, including CEB.

Arrigoni et al. [44] studied hygroscopicity in rammed earth via Moisture Buffer Value (MBV) since humidity is an essential aspect of indoor thermal comfort. The RE mixes chosen for this test performed better than the usual material, and unstabilized earth results were better than stabilized earthen material applied to the RE technique.

3.2.5. Life Cycle Inventory Analysis (LCI)

The data sources were observed for inventory analysis, dividing them into background and foreground sources. The background data sources were mainly generic data from the Ecoinvent database, except for Adobes. In 6 of 15 studies, the Ecoinvent was used together with one more specific1database (Australasian, US-LCI, and AusLCI, for example) regarding its geographic location or with more specific literature data (Figure 8a).

Adobe is the construction technique that presents government data sources and more literature in preference to database sources. Since RE is the most studied building technique, it offers more diversity in background sources. For example, RE is the only technique that uses data from AusLCI [50,51] and Australasian databases [38]. Cob background data are only Ecoinvent´s or from US-LCI. KBOB databases were only used in CEB studies by Brambilla et al. [41,42].

Considering that earth construction is not a conventional technology, there are data available in some databases such as Ecoinvent (the most used). In the last versions (Ecoinvent database v.3.7.1, Zurich and Ecoinvent database v.3.8, Zurich), it is possible to find clay plaster and cob. This finding shows that the evaluation of non-conventional materials has aroused interest.

As foreground data sources (Figure 8b), not the majority of the publications could rely on primary data. The ones that did declare the source from constructors, contractors and at the construction site, manufacturers, technical documentation, bill quantities in contract, building drawings, material specifications, sources of materials, transportation modes, method statements, electricity, and fuel consumption for plants and equipment.

Ecoinvent, as for background data, is the most used. However, the literature and local data from other databases are also used to seek accuracy.

3.2.6. Life Cycle Impact Assessment (LCIA) Methods

Regarding LCIA methods (Figure 9), cumulative energy demand (CED) is a single impact method common in all scientific publications that quantify primary energy usage. CML baseline is the second most used for CEB and RE (CML-IA database). Impact2002+ method was used only by Bonoli et al. [28] adobe study.

As expected, there is a geographic trend in the LCIA method where TRACI is used by Ben Alon et al. [19,20] in American publications since this method was developed considering North America’s context. CML is most used in Europe, and the ReCiPe midpoint has an equal distribution in Europe, Oceania, and Asia. The last three LCIA methods can be explained because they are methods with global characterization. Specifically, CML can be used since it is recommended by the old version (2012) of EN 15804, a standard devoted to applying LCA to the construction sector.

3.2.7. Impact Categories

Figure 10 indicates the use of the midpoint impact categories parameters per construction technique. The most common to all construction techniques midpoint impact category approached is Global Warming, characterized by the category parameter Global Warming Potential (GWP) or Climate Change. As expected, the six most used impact category parameters are those required in EN 15804:2012 [21]. The only selected study that used endpoint impact categories is Bonoli et al. [28] adobes´, using the LCIA method Impact2002+.

These findings are the same as other LCA construction sector review studies, such as Bahramian et al. [52], that already verified that usually, the impact category most analyzed is related to greenhouse gas emissions (GHG) and energy use. This can be explained since climate change and global warming are the leading global challenge (environmental, economic and social), and a significant amount of GHG emissions came from fossil fuels.

3.2.8. Allocation Procedures

The allocation procedures (Figure 11) are here divided into consequential and attributional approaches. The attributional’s cut-off is divided into allocatable (by-products and between by-products), waste, and EN15804. Seventeen of the publications did not declare the use of allocation procedures. Pereira [53] avoided allocation by considering CEB as a single product.

Among the declared ones, most of them adopt an attributional methodology, except 3. The economic allocation adopted by Ben-Alon et al. [19] Cob´s study was between wheat and straw since straw is considered a co-product of wheat production. For Fernandes et al. [40], the financial allocation was chosen to allow the comparison among different construction systems (CEB, RE, and conventional construction works) since a percentage of revenue generated by each activity was considered.

Arrigoni et al. [38,54] RE studies avoid allocation, considering by-products as waste under an attributional approach. Arrigoni et al. [39] also analyses RE´s stabilizers in a consequential approach to explore the consequences on the environment caused by a change in the choice of the mixture’s components.

Moreover, in a consequential allocation approach, Ben-Alon et al. [20] applied a market-based economic allocation for straw used in LSC and Cob, performing the allocation between wheat and straw to capture a scenario where straw is not a simple by-product of cereal production but a valuable building material.

These differences, especially in modeling and evaluating the impacts of co-products and wastes, can lead to very different results. Since raw earth is naturally a material with lower environmental impacts, using materials with a higher impact can prejudice the material’s environmental performance. Therefore, it raises the need for some “rules,” such as developing product category rules (PCRs) for earth construction techniques.

3.3. Embodied Carbon and Embodied Energy

The embodied carbon (kg CO₂ eq/unit) is represented in Figure 12. It takes into consideration the adopted unities and the life cycle stage. As discussed by Ventura et al. [17], the embodied carbon results change drastically with different scales. Building scale is much more complex and introduces new elements, such as coverings, openings, etc., that can significantly influence the results. On the other hand, it is more difficult to have LCA data from buildings, especially when comparing different techniques.

Nevertheless, when the studies consider mass (kg) unities for adobes, CEB or material for RE, the embodied carbon values are under 1 kg CO₂ eq. At the wall scale, the here presented studies comprise all five techniques. The values range from 10 to 288 kg CO₂ eq/m². At the building scale, the values range from 1000 to 15,000 kg CO₂ eq/m², not including LSC and Cob techniques at this scale.

In a general overview, it is possible to visualize the effects of stabilizers and transport in this parameter. Considering the mass unit (kg) and the life cycle stages A1–A3, it is possible to visualize in Figure 12 that the embodied carbon values increase with transport distance and with the % of stabilizers. Cabrera et al. [36] compare the effects of adding different % of Portland cement and lime as stabilizers in CEBs, resulting in a higher kg CO₂ eq/kg as the % of stabilizers increases. The decrease of kg CO₂ eq due to the use of local soil is also noticed by Christoforou et al. [32] values for adobes (A1–A3).

Regarding the life cycle stage of cradle-to-site, Meek et al. [51] compare cement as a stabilizer with lime-activated RE and alkali-activated RE in Australia, concluding that avoiding cement as a stabilizer reduces the kg CO₂ equivalent.

For the m² wall unit for RE, the values for A1–A5 go from the Portuguese scenario presented by [35,40] around 20 kg CO₂ eq/m², and Meek et al. [51] values around 80 CO₂ eq/m² for stabilized RE with lime and cement.

The values for adobes from [47,48] are around 100 kg CO₂ eq/m². As expected, Florescu and Bica [47] values refer to A1–A5 as life cycle stages at the wall scale, presenting smaller than the cradle-to-site house scale values presented by Pakdel et al. [48]. Yet, during the usage stage, Pakdel et al. [48] study in the Iranian scenario presented 288 kg CO₂ eq/m² without HVAC, revealing a decrease of over 60% compared to the conventional building with insulation. Florescu and Bica [47] also compare with a traditional hollow brick wall achieving a 40% reduction in embodied carbon, not only because of the use of earth-based unburnt bricks but also by replacing the glass fiber insulation with wood.

For CEBs at wall scale, Mateus et al. [35] and Fernandes et al. [40] also provide CEBs´ data per mass for A1–A3 stages in a Portuguese scenario around 0.39 kg CO₂ eq/unit and around 20 kg CO₂ eq/m². Pereira [53] presented 20 kg CO₂ eq/m², and from that 13.4 kg CO₂ eq/m² is related to raw materials extraction and production. Considering only the raw materials, around 80% of the GWP in the CEB production is attributed to the cement used in its stabilization. Since Elahi et al. [31] present the values in m³, it is not possible to compare them directly with the other authors.

At the building scale, CEBs embodied carbon values from Ansah et al. [27] for A1–A3 stages attributed to the material production phase over 50% to the cement added to the stabilized blocks. Asman et al. [55] compared CEBs with conventional construction in a study case in Malaysia. Since the CEB system eliminates the formwork, beam, and column during construction, reducing the usage of cement and mortar leads to a reduction of 35% in embodied carbon at the building scale. Shrestha [44] compares different construction systems in Nepal, getting to lower values of GHG emissions when stone and more when earth base materials are used.

The embodied energy values refer to CED values. Despite there is still no consensus if CED is an indicator of LCI or belongs to the LCIA, the cumulative demand analysis allows the comparison of results of a detailed LCA study to others where only primary energy demand is reported [56]. Some authors even consider it a starting point to the life cycle thinking [57]. Nevertheless, EN 15,804 [21] also covers the topic of energy consumption with nine indicators.

The cumulative demand analysis does not replace the life cycle assessment and should be used along with other indicators to provide the full picture of the analyzed product [57]. Figure 13 comprises the embodied energy per unit and construction technique. As to embodied carbon, the embodied energy values have different scales according to the adopted unit. At the product mass (kg), the embodied energy values range from 0.029 to 0.7 MJ/kg for adobes, CEB, and RE; at wall scale, from 45 to 395 MJ/m² for all techniques except adobes. Considering the building scale, adobe and CEB techniques presented embodied energy values from 770 to 1300 MJ/m².

As for embodied carbon, the embodied energy values increase with the transport of soil [32] and the use of stabilizers [39], and also with the increase of constructive elements, such as insulation [37].

Regarding the studies that considered both indicators, the materials that present lower embodied carbon values are those with lower embodied energy as well. Since the hotspots are related to the transport phase, the fuels are mentioned by the authors as the main contributor to both impact categories.

Figure 14 highlights the processes pointed out as primary sources of GWP and energy are raw material transport for embodied carbon and energy and electricity sources for operational energy.

Specifically for each technique, the modules that contribute the most to Adobe´s embodied carbon and energy during the embodied phase are raw material transport (e.g., soil, clay, aggregate transport) [32,47] or adobe blocks transport to the construction site [32].

For RE, the hotspots for embodied carbon and energy are raw material transport [35,39,40,46] and the amount and type of stabilizer [36,39,40,50,53,58]. Using local soil, choosing near raw material sources, and adding a lower percentage of stabilizers contribute to lower values for both indicators [39].

CEB technique emissions are also related to transported soil and the use of stabilizers. According to Shrestha [44], around 60% of energy consumption in CEBs is from fossil fuels, a source of GHG emissions. In this study, in terms of embodied energy comparatively with other constructions such as burnt-brick masonry (BBM), those values are lower (4% less). However, in GWP emissions, CEB´s emissions are around 30% less than BBM´s.

Another study from Elahi et al. [31] compares CEBs with fired clay bricks (FCB) and attributes higher GWP values to the amounts of stabilizers, namely cement. However, the CEB mix analyzed under the LCA methodology contains only 5% cement and 20% of supplementary cementitious materials (SCMs), such as fly ashes, resulting in almost five times smaller GWP emissions and 3.8 times less energy consumption for construction with CEB compared to FCB values.

Cob´s hotspots are related to the raw material process and transport during the embodied phase [19,20]. Alhumayani et al. [33] discuss the environmental assessment of large-scale 3D printing of Cob compared to concrete. The energy demand and GWP values presented here are different due to 3D printing (3DP), which requires more energy during the building phase. In this study, conventional Cob has shown a much higher overall performance over 3DP Cob, mainly to the high electricity demand required for this construction methodology to operate the robotic arm, affecting both global warming and fine particulate matter formation. However, when compared to concrete 3DP, the environmental performance of 3DP Cob is 80% better than 3DP concrete and 91% better in the GWP impact category.

LSC studies are restricted to the same research group (Ben-Alon et al.). The LSC-specific hotpots are attributed to straw, both for GWP and energy demand. Straw’s environmental impact is related to its production, which requires chemicals. Emissions of methane (CH₄), sulfur dioxide (SO₂), and nitrogen oxides (NO_x) are associated with the use of pesticides and fertilizers during the straw production stage [49].

Raw material transportation in specific soil corresponds to significant GHG emissions and energy consumption during the embodied phase due to the mass amount used in the construction and its distance from the building place. In the case of adobes, for example, Pakdel et al. [48], the adobe bricks were built 12 km far from the construction site, and the embodied energy and carbon pointed out that transportation energy is higher than manufacturing embodied energy for raw materials (gravel, mud and glazing).

Mateus et al. [35] compare CEB and RE production, concluding that CEB’s potential environmental impacts could be minimized if produced in the same place of earth extraction, which is the case in the RE. Fernandes et al. [40] RE LCA study reveals that environmental impacts categories and embodied energy especially have a small contribution (around 10%) from raw material extraction (A1) and transport (A4) since the material is produced on the construction site. In this case, the construction stage (A5, mainly due to fuel for equipment and stabilizer (hydraulic lime)) is above 60% in all impact categories analyzed in the study.

Even when the soil is not from quarries but is from other construction sites, such as excavated soil from a tunnel construction studied by Nanz et al. [37], it increases GWP and embodied energy. However, in this case, using soil as a recycled material instead of being destined for a landfill is considered a form of credit, reducing the embodied energy and carbon during the transport (A2 to A4 stages).

Yet regarding embodied phase, Shrestha’s [44] results point to higher energy and GHG emissions due to the use of cement and fossil fuels in burnt bricks and concrete-based building elements compared to CEBs and interlocked bricks. The emissions of earth-based solutions can be lower when using locally available materials, attributing to material selection in the structural system the significant impact GWP midpoint category and energy demand.

Considering the stabilizers used in mixes, cement is the second more cited source of GHG emissions. In Arrigoni et al. [39], the choice of stabilizer affected overall environmental impact more significant than the choice of the inert fraction. The mixes that incorporated cement had the highest environmental impact. When the cement was eliminated as a stabilizer, the environmental impact could be reduced by 85% compared to the base case. A reduction in the environmental impacts by between 50 and 100% per category can be achieved using alternative types of stabilizers, in comparison with the base case consisting of crushed limestone stabilized with 10% Portland cement by mass of dry substrate [38].

Pereira [53] uses local soil for CEBs. However, the stabilizers contribute to GWP for 95% of kg CO₂ eq (75% from cement and 20% from hydraulic lime) considering the production of one CEB. Regarding embodied energy, over 93% are from non-renewable sources, which is attributed to the fuel used in transport and raw material extraction.

Meek et al. [51] discuss not only the cement effect as a stabilizer in RE environmental impacts but also the use of stabilizers by combining the substrate of the RE mixes (composed of aluminosilicate materials, namely concrete aggregate and crushed bricks) with chemical activators (alkali-activated with 12 M NaOH solution and lime-activated with hydraulic-lime). There are also added SCMs such as fly ashes, ground granulated blast furnace slag, kaolin, and silica fume. The lime and alkali-activated GWP values are similar, indicating good flexibility of these materials to adapt to local supply networks. The sensitivity analyses conducted by authors also point to the use of local materials, namely earth components, due to their amount in the mixes. The use of local material is more relevant for GWP than the distance of stabilizers since it is a small component in the total mass of the mixes. However, since one-third of emissions are directly related to NaOH production, it is recommended to use only the minimum amount needed to achieve the unconfined compressive strength of 12 MPa (reference value achieved by control cement stabilized mix).

Among electricity sources during the operational stage, Pakdel et al. [48] consider a 77.2% and 85.7% reduction in natural gas and electricity from an adobe house heated with a fireplace compared to a conventional building with HVAC (heating, ventilation, and air conditioning) equipment to ensure similar thermic comfort in both constructions.

For Cob, LSC, and RE, Ben-Alon et al. [20] did a thermal-energy performance simulation that accounts for heating and cooling demand during building operation for each wall technology, for a 50-year lifespan, in six different climate scenarios. The results showed that the operational energy demand in the LSC chamber was lower for all climates, followed by the insulated RE. Except for the hot desert and desert climate, the authors highlight the significance of the heating energy demand. For that reason, the electricity for heating and cooling energy provided during the operational phase is also the primary energy demand.

3.4. Technical Features Considered in LCA Studies

The LCA approach should not be conducted without comparing other materials’ features to characterize the construction technique [51]. As already presented in Section 3.2.4, the technical features are part of the LCA analysis; however, this does not exclude the possibility of presenting along with LCA studies more technical features that can contribute to the choice of the materials in earth construction.

This review has identified the use of technical features is approached from two different perspectives. The first one uses technical performance as a functional unit in order to establish a comparison with other materials, as expected to perform an LCA assessment. The second approach considers these features along with the environmental parameters along with the design, by comparing the technical features in different amounts (% wt.) of stabilizers, for example, to choosing the best raw materials combinations and later applying the LCA methodology.

Figure 15 indicates, per construction technique, the technical features that were presented in the LCA studies. Here are included features that characterize the product function and other characteristics that were also further investigated. They are grouped as mechanical, durability assessment tests, hygroscopic, thermal, and microstructural. The mechanical group tests include unconfined compressive strength, flexural strength, and tensile strength. The durability tests comprise the accelerated erosion test (AET), wire brush test and modified wire brush test, the cyclic dry wet tests, and also the submersion and efflorescence assessment. The thermal properties group includes thermal transmittance (U-Value), thermal resistance (R-Value), and specific heat capacity. The microstructure comprises the porosity assessment only since other microstructural analyses such as SEM-EDS can be considered as a complement to mineralogical characterization, as presented by Aubert et al. [59].

The Supplementary Material (Table S1) presents the details and properties of the earthen techniques presented by the here studied authors, including the technical data.

The thermal properties are the features most commonly used to define the functional unit, as in [19,20,37,40,47,49], fulfilling the local or international requirements for the climatic zone allowing the comparison with other materials. Brambilla [42] also uses the thermal properties of CEBs in different insulations and ventilation scenarios in a double office room, applying simplification of the LCA methodology. The authors used CED and GWP to calculate the indicator LCER (life cycle efficiency ratio) for each scenario. However, the LCER indicator did not allow comparing the GWP and CED data with the other works.

Regarding the mechanical features, UCS is the material feature most investigated among the LCA studies. The compressive strength is used for testing the effects of aggregates or stabilizer additions in earth-based mixes. However, only CEB and RE studies perform these tests.

Arrigoni et al. [54] also present the mechanical characterization of the RE mixes concerning RCA (recycled concrete aggregate) content, which results in a nonlinear decrease of the UCS values with the increase of RCA as aggregate. The authors also used porosimetry and microscopy to understand and explain the hygroscopic and mechanical behavior of the mixes.

Arrigoni et al. [39] compares relative environmental impact in terms of GWP with UCS (mean relative impact score) and concluded that UCS is not a good indicator of durability. Hence, all the stabilized mixes acquired the minimum of both mechanical and durability tests, but those with the highest UCS values have worse environmental performance. They concluded that the environmental performance is influenced by cement and mostly by transport. However, eliminating the stabilizers neither nullified the environmental impact since transportation was still necessary. Meek et al. [51] discuss the UCS and other mechanical characteristics (bond strength, flexural, and shear strength) to verify the behavior of rammed earth mixes. UCS indicates an influence of stabilizers type. However, other tests, such as bond strength, were more affected by substrate changes.

When the UCS results along with embodied carbon values are taken into consideration, Meek et al. [51] highlight the complexity around choosing the materials and how important is to take both under consideration. By comparing two RE mixes by changing only the substrate (crushed brick BC and crushed limestone CL) and both alkali-activated to stabilize, the authors concluded that a less amount of stabilizer could be used in BC as the substrate to achieve the UCS. That choice implies reducing the material emissions and becoming environmentally beneficial to use BC as the substrate instead of CL which had a minor environmental impact in the first approach.

Elahi et al. [31] evaluate UCS with different proportions of stabilizers (fly ashes and cement) in CEBs, considering flexural strength and tensile strength to get to an optimum ratio of stabilizers. The authors stated that the amount of cement in CEB mixes could be reduced with the addition of FA, resulting in less embodied carbon.

Considering the CEBs studies that presented embodied carbon or embodied energy values along with mechanical features, namely Asman et al. [55], Cabrera et al. [36], and Elahi et al. [31], the data comparison is unfeasible since the adopted unities in LCA assessment are different (m² house, m³ wall and kg, respectively). Regarding the RE, it is possible to visualize at the m² wall scale for an m² external load-bearing wall the increase of UCS and EE with the % of stabilizers, in specific cement (Figure 16). Without stabilizers, the EE values range from 45 to MJ/m² and UCS from 1.3 to 2.0 MPa. With different proportions of stabilizers (cement and fly ashes), EE ranges from 60 to 170 MJ/m², and UCS goes from 2.9 to 6.7 MPa. With 10% cement as a stabilizer, EE values range from 210 to 230 MJ/m² and UCS from 8.7 to 13.8 MPa. Therefore, we can see least and more energy-mechanical efficient mixtures in terms of each gained 1 MPa, for example, values ranging from 16.77 MJ/m².MPa (Mix #0: CL + 10% C) to 53 MJ/m².MPa (Mix5#ELS). Smaller values of “MJ/m².MPa” indicate more energy-mechanical efficient products [14].

The comparison for embodied carbon is not possible, since the available data are from the same research group, namely Meek and Elchalakani [50] and Meek et al. [51].

Durability tests were performed in CEBs by Elahi et al. [31] and in RE [38,39] by Arrigoni et al. research group. Elahi et al. [31] performed detailed durability laboratory tests, including cyclic dry wet tests (mass loss (%), dry CS (MPa), wet CS (MPa), and strength retention (%)) and also submersion and efflorescence tests along with LCA. The authors compare the performance of CSEB (with different% of fly ashes (FA) and cement as stabilizers) with fired clay bricks (FCBs). The durability tests allow choosing the best CSEB mix composition, namely a 5% cement and 20% FA (optimum cement-FA combination), to assess the environmental impact of the mix in comparison with FCBs through LCA.

Arrigoni et al. [39] and Arrigoni et al. [38] assessed durability in RE from the perspective of comparing the environmental impact of the chosen stabilizers. There was performed accelerated erosion due to sprayed water and mass loss due to wire brushing tests, along with UCS results.

In this sense, using material features in LCA studies beyond as functional units allows a fair comparison between construction techniques, being commonly translated like environmental-performance indicators as in Paiva et al. [14].

Materials with lower embodied environmental impacts in the production phase may not be the least impactful when the total life cycle or material application is considered in a building element or whole building scale. It can be attributed to many factors, such as maintenance, replacements, or energy performance. Therefore, the use of additional data related to materials properties and indicators can help in decision-making and should be encouraged in LCA studies.

4. Research Outlooks and Recommendations

Based on the founded hotspots, some research outlooks and recommendations are provided in

Table 4. Recommendations for environmental performance improvement based on hotspots.

Strategies	Description	Construction Techniques
		Adobe	CEB	Cob	LSC	RE
Use local earth and aggregates	Using local materials results in smaller carbon emissions; the higher the proportion (usually earth and mineral aggregates) in the mixture, the more relevant the transportation’s environmental impact due to fossil fuel burning.	x	x	x	x	x
Reuse of excavated earth	Other sources should be considered if the soil needs great amounts of aggregates or clays to enable the technique. Regarding the distance from the construction site, using excavated soils can reduce the environmental impact.	x	x	x	x	x
Decrease the amount of binder	Cement as a stabilizer has a higher impact regarding the indicator; hence it should be used in small amounts (around 5% wg).	x	x	x	x	x
Use of SCMs replacing conventional binders	SCMs such as fly ash, blast furnace slag, and other by-products should be used to provide the required features for the earthen material.	x	x	x	x	x
Use of alkaline activators (in small concentrations) to replace conventional binders	Alkaline activators as chemical stabilizers should be used in the less concentration possible to give the required features for the earthen material.	-	-	-	-	x
Use bio-based materials as reinforcement.	Bio-based reinforcements are renewable sources and can absorb and store CO2. Additionally, they can improve some materials’ properties, such as thermal resistance.	x	-	-	-	x

for the evaluated construction techniques.

Some strategies that can be used for all earth construction techniques are using local materials, reusing excavated earth, decreasing the amount of binder, and using SCMs instead of conventional binders. Transportation and binder content was the most cited causes for the increase in environmental impacts of earth construction techniques in this literature revision. Hence, they can be directly linked.

It is known that the rise of distances of earth and/or aggregates will increase the environmental impacts due to the burning of diesel or other fuel for transportation. This will be critical principally when transportation is not efficient, which is the case in many developing countries [60,61], where earth construction is used as a current practice. In some cases, especially in terms of earth stabilization, it is preferable to take materials from longer distances if they have a better chemical composition that will reduce the number of chemical stabilizers (usually cement or lime).

LCA can do this type of evaluation, and the results tend to vary according to the different environmental impacts. For some impacts, e.g., climate change and fossil fuels depletion can be preferable to have more extended materials traveling distances if this earth requires fewer binders [62]. One option for reducing transportation distances is the reuse of excavated earth from buildings or infrastructure sites, as presented by Nanz et al. [37]. However, the quality of the material and the need for stabilization must be considered.

Regarding binder content, the conclusion is non-trivial, since it is related to the earth-based technique and to the type of product and building characteristics, namely if it is desirable to have more resistant materials to have less global material consumption in the building element. Therefore, we can have lower environmental impacts considering just the material level. In some cases, the increase of binder led to lower global environmental impact, especially when mechanical performance and durability are the main design requirements [17]. On the other hand, depending on the technology and design (e.g., high-rise buildings), the increase of binder content will be just a waste of material and an increase in environmental impacts, without any performance gain. Van Damme and Houben [63] verified that the addition of binder content of stabilized compressed earth blocks is much more efficient than RE and adobe. In some cases, and applications, the more intelligent strategy uses other types of material or technology instead of using earth.

In situations that require a high amount of binder, the use of SCMs and/or alkaline activators can be potential solutions for different techniques, as seen for adobe [32,64,65], CEB [66] and RE [39,67,68]. For SCMs is desirable to use waste materials that will not need to have a very difficult/intensive treatment, that tend to expend energy and generate additional environmental impacts. There are several options for waste-based SCMs with minor environmental impacts, namely: fly ashes, rice husk ashes, sugar cane bagasse ashes, and municipal waste ashes [69,70], among others.

Considering the use of alkaline activators, the literature about LCA has already demonstrated that they can be problematic if used in large quantities. One option is using waste-derived activators, such as rice rusk ash [71].

Finally, one strategy especially important for embodied carbon reduction is using bio-based material as reinforcements since these materials can sequester and store CO₂, which can be used for adobe, Cob, RE, and LSC. Additionally, they bring extra benefits in thermal performance and operational energy reduction during building operation for air-conditioning [14,15] and help combat crack propagation, directly related to the durability and service life of buildings’ components.

5. Research Limitations

Regarding SLR, there are some limitations identified in this review, comprising the number of studies and the gathered data.

As expected, there are a few studies applied to this topic which led to the temporal limitation being over 5 years. Yet, during the search process and in the search tools (WOS and Scopus), the results could not be refined to only peer-review papers, been included conference papers and proceeding of conferences.

The keywords applied in this review provided results in a broader sense than expected since earth construction can comprise constructive techniques and earth retaining walls, tunnels, and other earth-based materials such as plasters and mortars. This led to excluding many studies (n = 60) from the initial results (n = 97).

Beyond the environmental impact quantification, the LCA applied to infrastructure studies might be relevant for government and other stakeholders since some of these studies also consider the economic feasibility of the earthen walls, for example [23,25]. However, it was chosen not to consider them along with the earth earth-based techniques explored here. As aforementioned, discussing the results in terms of constructive type and building scale exhibits limitations, and adding retaining walls and tunnels would demand more categories to consider that could not allow a discussion since they result in different benchmarks.

The LCA methodology allows a range of adopted unities, and the specificity of each reference flow makes difficult the direct data comparison. Yet, by combining five different techniques, this study indicated that different technical features can be used in LCA methodology applied to earth construction, leading to difficult direct comparisons.

As presented in Section 3.3, it is interesting to visualize the values for embodied carbon and energy for the five earth techniques and the stabilizers’ effects; however, the comparison between them is not trivial, and some aspects should be taken into consideration. Since some techniques are monolithic and other brickworks, and require different thicknesses and materials such as mortars and other elements even at the same scale (unit, wall, or building). Even when it is possible to compare the results based on the reference flow, the number of publications included in this review is too small or belongs to the same research group, which can lead to bias.

Furthermore, some embodied carbon and embodied energy values are not presented in kg CO₂/unit or MJ/unit, respectively but as % or normalized to the base case. This also reduces the numeric data for comparisons.

6. Conclusions and Final Remarks

The Systematic Literature Review (SRL) revealed that life cycle assessment (LCA) methodology had been increasingly used in the scope of environmental impact characterizations promoted by civil construction and a crescent interest in earth-based techniques.

The following earth-based techniques were evaluated: Adobe, Cob, Rammed Earth (RE), Compressed Earth Block (CEB) and Light Straw Clay (LSC). The most investigated technique was RE, and the least was LSC, mainly due to the geographic distribution of the technique and the frequency of use in the market.

Considering the LCA methodology, it was found that the adopted scope goes from a broad concept of assessing the environmental impact of the technique by comparing the earthen techniques or materials with conventional assemblies, such as fired clay bricks.

Most of the here analyzed studies are focused on the product stage (EN 15804:2012) once it is required for standard compliance. Nevertheless, there is a slight increase in publications approaching cradle-to-grave as a system boundary.

Besides adopting similar or the same functional unities, comparing the construction techniques on an equivalent basis is impossible due to the reference flow, as discussed by Ventura et al. [17]. Different amounts of earth, stabilizers, plaster, and other components are necessary to deliver the same performance described by the functional unit. Hence, from the unit (mass), passing through the wall scale and getting to the building scale, the unfeasibility of this comparison between the techniques increases.

The SLR highlighted that the functional unities are commonly related to thermal properties, such as U-value, to compare the environmental indicators of earthen assemblies with usual building materials. Hence, other materials are also investigated along with LCA studies, namely mechanical characteristics, and some durability tests. The LCA complemented with these studies, can provide a more complete evaluation of the earth-based technique. Regarding the technical features beyond the functional unit characterization, special attention needs to be paid to the durability tests performed with mechanical tests to quantify the amount of different stabilizers in the mixes. These studies contribute to the reduction in the use of cement and consequent improvement of earth-based construction recyclability.

The here-gathered results for embodied carbon and embodied energy allow to visualize some initial benchmarks at mass, wall, and house scale. For embodied carbon at the unit (mass) scale, the values are under 1 kg CO₂ eq/kg of material. On the wall scale, it increases by 10² kg CO₂ eq/m², and at the building scale, embodied carbon increases by 10³ kg CO₂ eq/m². Regarding the embodied energy, the values begin around 10⁻³ MJ/kg (unit scale), increasing 10⁵ MJ until reaching values at 10² MJ/m² (wall scale). At the building scale, embodied energy values reach 10³ MJ, performing a range of 10⁷ MJ/unit.

When the comparison is made between brickworks, namely adobes and CEBs and between monolithic earth structures (RE and Cob), the aforementioned considerations about unities need to be taken into consideration. However, among the brickworks, adobes present smaller embodied carbon and energy values than CEBs, probably due to their manufacturing being less industrialized. Similarly, considering the monolithic structures, Cobs exhibit smaller embodied carbon and energy than RE.

For all evaluated techniques, the authors from the here analyzed studies describe raw material obtention or production as primary embodied carbon and energy. The activities responsible for these values (hotspots) are related to the fuels expended during the transportation and processing and the stabilization by chemical binders, especially Portland cement.

Hence, this work confirmed that earth construction techniques present better environmental performance than conventional construction materials for embodied carbon and embodied energy.

Taking into consideration the relevance of local features and the differences in LCA methodology assumptions, the main contribution of this study is to provide insights for exploring the LCA topic in earth construction. As suggestions for future works, the here-considered out of scope topics such as insulation and infrastructure could be studied through SLR. Future review works can explore technical features (e.g., thermal, mechanical, fire resistance, water resistance, etc.,) along with LCA to guide the choice of raw materials and stabilizers for specific earth-based techniques.