Is the Soil-Cement Brick an Ecological Brick? An Analysis of the Life Cycle Environmental and Energy Performance of Masonry Walls

Abstract

Masonry wall is a key construction subsystem, but it embodies significant environmental and energy burdens within the life cycle of buildings. Soil-cement bricks and blocks stand as an alternative low-cost masonry material, but despite the widespread claim to be environmentally friendly, more systematic investigation is lacking. This study aimed to assess the life cycle environmental and energy performance of 1.0 m² of a soil-cement brick masonry wall from cradle-to-construction in terms of carbon, energy, and water footprints, and fossil and mineral resource use, as well as compare it with conventional technologies such as ceramic and concrete block masonries in Brazil. Results showed that raw materials are a major contribution to soil cement masonry walls, followed by the joints and links with columns, in which cement stands out among other inputs. Hydraulic pressing in brick production had a negligible burden increase compared with manual pressing. The PVA mortar joint outperformed the PVA glue one, whereas resin coating performed better than cement mortar. In comparison with ceramic and concrete masonry walls, the soil cement masonry presented overall better environmental and energy performance and was the least affected by the inclusion of finishing coating layers and transport of materials in the sensitivity analysis scenarios, although improved scenarios of conventional options could be competitive, e.g., ceramic masonry with blocks produced by firing reforested wood for the carbon footprint. Scale-up analysis revealed that widespread deployment of soil cement masonry in the built environment would substantially avoid environmental and energy burdens compared with conventional technologies.

1. Introduction

The built environment is a crucial element of modern life, yet it is not necessarily sustainable. The construction sector accounts for half the available raw materials on the planet, 40% of the electricity produced, 25% of the water consumed, 45% of the waste generated and, in the case of buildings, 33% of the greenhouse gas (GHG) emitted [1,2,3]. Additionally, it occupies and transforms land, overloads landfills, and induces costs arising from environmental burdens [4], and this scenario tends to intensify considering population growth [5].

Among the subsystems of a building, masonry walls are used for internal partitioning of rooms and as part of the envelope that separates the building inside from the outside environment, whose main functions are protection, insulation (thermal, acoustic, and humidity), aesthetics, and structural in some cases [6,7]. Compared with other building subsystems, the masonry wall has shown to have significant raw material demand, waste generation, and carbon and energy footprints [8,9,10].

As an alternative material for masonry, especially in informal construction by low-income groups in emerging economies, soil-cement bricks and blocks are modular interlocking wall units composed of a pressed blend of soil, cement, and water [11,12,13]. Considering technical performance standards and market practice worldwide, suitable mechanical properties (strength, water absorption, and durability), insulation properties, and reduced cost have been extensively reported in the literature [14,15,16,17,18,19], to cite a few. Additionally, it is often referred to as an “ecological brick” based on a claim that the emissions from the curing of ceramic bricks and blocks are avoided [20,21,22]. However, the ceramic curing emissions may or may not contribute to global warming potential depending on the energy source, not to mention the cement present in the soil-cement mixture as well as joints, links with columns, and finish coating—that is a material that embodies large carbon and energy footprints—in addition to resource depletion and other environmental repercussions whose information is scarce.

Life cycle assessment (LCA) is a method widely used to investigate the environmental aspects and impacts of products and services [23]. Few studies have examined soil-cement masonry from an LCA viewpoint. Salzer et al.’s [12] LCA study reported lower carbon and energy footprints as well as resource use compared with concrete masonry in the Philippines. Similarly, Mpakati-Gama et al. [24] reported a lower carbon footprint than ceramic and concrete, but a greater energy demand than concrete in Malawi. Caldas and Toledo Filho [25] reported better performance than ceramic and concrete structural masonry in Brazil for only a few indicators within a predefined set of categories in a given characterization method.

Despite the vast techno-economic analysis of soil-cement masonry, the subjective appeal for its environmental benefits, the diverging information on its energy requirements, and scarce knowledge on additional relevant environmental repercussions justifiably linked with the product system aspects as endorsed by ISO 14044 [26], indicate the need for a more systematic investigation.

This study aimed to assess the life cycle environmental and energy performance of 1.0 m² of soil-cement brick masonry wall and compare it with conventional technologies consisting of ceramic and concrete blocks in Brazil. Sensitivity analysis was performed to evaluate the change in hotspots and potential gains for the three technologies.

2. Methodology

This study was conducted using the attributional life cycle assessment (LCA) method based on ISO 14040 [27] and ISO 14044 [26]. The four stages of LCA were followed: goal and scope definition, life cycle inventory (LCI) analysis, life cycle impact assessment (LCIA), and interpretation.

2.1. Scope

The functional unit was 1.0 m² masonry wall, without a structural purpose, with boundaries extending from cradle-to-construction (Figure 1), considering a service life of 40 years without significant maintenance demands. The masonry wall technologies assessed were soil-cement brick, ceramic block, and concrete block (Figure 2).

Table 1. Summary of the main parameters of the assessed masonry wall technologies.

Parameter	Soil-Cement Brick	Ceramic Block	Concrete Block
Mass (kg)	2.8 [33]	1.9 [34,35,36,37]	8.7 [38]
Dimensions (length × height × width cm3)	24 × 7 × 12 [28]	14 × 19 × 9 [29]	29 × 19 × 14 [30]
Shape, number of holes	Hollow prismatic self-locking, 2 holes [28]	Hollow prismatic, 6 holes [29]	Hollow prismatic, 2 holes [30]
Composition	Sandy clay soil, cement, and water [15,39]	Clay and water [29]	Gravel, sand, cement, and water [30]
Number of bricks or blocks in 1.0 m2 of wall	60 [28,33]	37 [31]	17 [31]
Joint	PVA glue (polyvinyl acetate), or PVA mortar [40]	Cement/sand mortar	Cement/sand mortar
Function	Non-structural masonry [28]	Non-structural masonry [29]	Non-structural masonry [30]
Link with columns	Steel welded mesh	Steel welded mesh	Steel welded mesh

presents the parameters of the masonry wall technologies defined by the NBR 8491 [28] standard for the soil-cement brick, NBR 15270-1 [29] for the ceramic block, NBR 6136 [30] for the concrete block, and related literature. These parameters are consistent with typical construction practice [31] and the thermal, acoustic, resistance (not for structural function), and durability (service life) performance in accordance with NBR 15575 [32].

2.2. Life Cycle Inventory (LCI)

2.2.1. Soil-Cement Brick

The life cycle inventory of the production of a typical soil-cement brick was built based on the Brazilian standards, related literature and reports, manufacturers’ catalogues, and interviews with a local construction company (Table 2). In the soil-cement baseline scenario, the hydraulic pressing and PVA glue for the joints were considered.

A typical mix of the soil-cement brick before the curing process takes 7 to 10 parts in volume of sand-based (50% to 70%) clay (30% to 50%) soil, 1 part of Portland cement [39,41,42], and water as much as 5% to 20% of the mixture bulk mass depending on the soil [43]. Average values were used for the soil-cement baseline scenario.

Electricity is used in the sieving/crushing and mixing machines, as well as in hydraulic pressing. A manual pressing machine demands no electricity but requires more human effort, which may yield lower productivity [39,41,42], although labour was not included herein in terms of LCI environmental flows.

Table 2. Life cycle inventory of the production of 1.0 kg of soil-cement brick.

Item	Flow Type	Value	Unit	Source
Sand	Elementary	0.497	kg	[39,41,42,43]
Clay	Elementary	0.331	kg
Cement	Intermediary	0.081	kg
Water	Intermediary	0.091	kg
Electricity (sieving/crushing)	Intermediary	0.077	Wh	[39,41,42], [a]
Electricity (mixing)	Intermediary	0.462	Wh
Electricity (pressing) *	Intermediary	1.11	Wh

* Manual pressing machine does not require electric power. [a] Interview with a local company.

Table 2. Life cycle inventory of the production of 1.0 kg of soil-cement brick.

Item	Flow Type	Value	Unit	Source
Sand	Elementary	0.497	kg	[39,41,42,43]
Clay	Elementary	0.331	kg
Cement	Intermediary	0.081	kg
Water	Intermediary	0.091	kg
Electricity (sieving/crushing)	Intermediary	0.077	Wh	[39,41,42], [a]
Electricity (mixing)	Intermediary	0.462	Wh
Electricity (pressing) *	Intermediary	1.11	Wh

* Manual pressing machine does not require electric power. [a] Interview with a local company.

2.2.2. Ceramic Block

The inventory data used to model 1.0 kg of a typical hollow ceramic block produced in Brazil was obtained from de Souza et al. [44], whose work was based primarily on data provided by the Brazilian Nacional Ceramic Industry Association (ANICER). In the ceramic base scenario, the types of GHG emitted during the firing stage in the ceramic production were based on the ratio between native wood (53%) as soil transformation emission, and reforested wood (47%) as a biogenic emission [45].

2.2.3. Concrete Block

The Ecoinvent [46,47] inventory for 1.0 kg of a typical hollow concrete block produced in Brazil was used (

Table A1. Background inventory datasets.

Item	Intermediate Flows in Ecoinvent v3.6
Material
Portland cement	Cement, Portland {BR}| cement production, Portland | APOS, U
BFS cement	Cement, blast furnace slag 35–70% {BR}| cement production, blast furnace slag 35–70% | APOS, U
Cement mortar	Cement mortar {RoW}| production | APOS, U
Concrete block	Concrete block {BR}| concrete block production | APOS, U
Water	Tap water {BR}| market for tap water | APOS, U
Steel welded mesh	Steel, low-alloyed, hot rolled {RoW}| production | APOS, U + Wire drawing, steel {RoW}| processing | APOS, U
PVA glue	Vinyl acetate {RoW}| production | APOS, U
Resin coating	Acrylic varnish, without water, in 87.5% solution state {RoW}| acrylic varnish production, product in 87.5% solution state | APOS, U
Energy supply
Electricity	Electricity, low voltage {BR-North-eastern grid}| market for electricity, low voltage | APOS, U
Diesel	Diesel, burned in building machine {GLO}| processing | APOS, U
Wood	Wood chips, dry, measured as dry mass {RoW}| wood chips production, from industry | APOS, U
Natural gas	Heat, central or small-scale, natural gas {RoW}| market for heat, central or small-scale, natural gas | APOS, U
Consumable
Lubricating oil	Lubricating oil {RoW}| production | APOS, U
Waste for treatment
Wood ash waste	Wood ash mixture, pure {RoW}| treatment of, sanitary landfill | APOS, U
Logistics
Transport	Transport, freight, lorry 16–32 metric ton, euro5 {RoW}| market for transport, freight, lorry 16–32 metric ton, EURO5 | APOS, U *

* Conservative choice.

), which was developed using data from 25 factories in the country. In the concrete base scenario, the cement used in the default inventory consists of 100% clinker.

2.2.4. Masonry Wall Inventory

The life cycle inventory of the masonry wall of the assessed technologies in the baseline scenarios is presented in

Table 3. Life cycle inventory of 1.0 m² masonry wall of the three assessed technologies (Soil-cement, Ceramic, and Concrete) in the baseline scenarios.

Item	Value	Unit	Source
Soil-cement
Brick	169	kg	[28,33]
PVA glue or PVA mortar *	0.57	kg	[31]
Steel welded mesh	150	g	[31,48,49]
Ceramic
Block	70.3	kg	[29,31,34,35,36,37]
Cement/sand mortar	23.3	kg	[31,50]
Steel welded mesh	52.7	g	[31,49]
Concrete
Block	148	kg	[30,31,38]
Cement/sand mortar	40.0	kg	[31,50]
Steel welded mesh	57.4	g	[31,49]

* Proportion adopted: 20 parts of cement/sand mortar for 1 part of PVA glue [42,51].

, whose major data source was the National System of Costs Survey and Indexes of Construction (SINAPI) of Brazil [31].

2.3. Life Cycle Impact Assessment (LCIA)

The product system was modelled using the software Simapro v8.5 with the Ecoinvent database v3.6 library allocation at point of substitution (APOS) (Table A1) [46,47]. The impact categories for environmental performance at the midpoint level evaluated were global warming potential (GWP), fossil resource scarcity (FRS), mineral resource scarcity (MRS), and water footprint (WF) (blue water consumption); as well as cumulative energy demand (CED) for energy performance. The characterization methods used were the following: IPCC 2013 100a v1.03 [52] in GWP; Cumulative Energy Demand v1.11 [53] in CED; and ReCiPe-2016 (H) v1.1 [54] in FRS, MRS, and WF.

Category selection criteria were based on the environmental mechanisms significantly affected by the product system both in the foreground (direct burdens) and background (indirect burdens) over its life cycle. The carbon footprint encompasses the GHG emissions that contribute to global warming; for instance, emitted during power generation, transport, block-making, and production of raw materials such as cement. The energy footprint proxies for renewable and non-renewable energy demand for environmental issues, e.g., in the production of fuels and energy carriers, and even evokes the energy embodied in the supply chain of non-energy materials (as a primary function) like minerals. Fossil and mineral resource scarcity pertains to resource use efficiency and thus depletion of reserves; for example, of petroleum, lime, and clay utilized in energy and material supply chains. The water footprint concerns water use efficiency and, therefore, its availability, e.g., considering evaporation, pollution, and trade in processes, and incorporation into products.

2.4. Sensitivity Analysis

Hotspot analysis was performed to investigate the impact of changes in plausible key input parameters and assumptions. For soil-cement bricks, the use of electricity for manufacture (manual or hydraulic press), as well as the joint options (PVA glue or PVA mortar) in the wall construction were examined. In the ceramic block manufacture, the type of GHG emissions as a function of the origin of the wood used (reforested or native), as well as the option for natural gas as a heat source (454 kcal∙kg⁻¹ including efficiency losses) [55] were evaluated. For concrete blocks, two types of cement were studied: 95% to 100% clinker blended with 6% to 10% calcite (namely CP-V in Brazil); and 25% to 65% clinker blended with 35% to 70% blast furnace slag (BFS) and 0% to 10% calcite (namely CP-III) [30,56].

The inclusion of finish coating layers was also examined. A layer of acrylic resin coating was considered on each side of the soil-cement wall with replacement every 5 years [25], and cement/sand mortar coating on each side of the wall for the three technologies, being 1.5 cm thick for soil-cement and concrete, and 3.0 cm thick for ceramic due to its smaller width [32,48].

Additionally, the effect of transportation was evaluated for the resin-coated soil-cement and mortar-coated ceramic and concrete masonry walls in the baseline scenarios considering a 10 km and 50 km distance from the material supplier and construction site, modelled as a round-trip, i.e., the truck loaded on the way to the construction site and empty on the return.

3. Results

The life cycle environmental and energy performance of soil-cement brick, ceramic block, and concrete block masonry walls in the baseline scenarios is presented, followed by the sensitivity analysis on hotspots, finish coating layer, and transport, then benchmarking, scale-up, multifunctionality, and research prospects are discussed.

3.1. Environmental and Energy Performance

The soil-cement masonry wall outperformed the ceramic and concrete walls in GWP, CED, and FRS (

Table 4. Environmental and energy performance of 1.0 m² masonry wall of the assessed technologies in the baseline scenarios.

Masonry Wall Technology	GWP	CED	MRS	FRS	WF
	(kg CO2 eq)	(MJ)	(kg Cu eq)	(kg Oil eq)	(L)
Soil-cement	13.2	109	0.63	2.00	104
Ceramic	33.0	178	1.02	2.25	66.8
Concrete	19.9	169	0.11	2.99	110

GWP: global warming potential, CED: cumulative energy demand, FRS: fossil resource scarcity, MRS: mineral resource scarcity, WF: water footprint.

; Figure 3;

Table A2. Environmental and energy performance contribution per group of the masonry wall technologies in the baseline scenarios.

Indicator	Masonry Technology	Brick or Block Raw Materials	Brick or Block Manufacturing	Joint and Link with Column	Total
GWP (kg CO2 eq)	SC	11.4	0.1	1.7	13.2
	CE	0.9	26.3	5.8	33.0
	CO	9.1	0.9	9.9	19.9
CED (MJ)	SC	64.9	2.5	41.9	109.4
	CE	13.0	120.2	45.0	178.1
	CO	71.9	21.1	76.4	169.4
MRS (kg Cu eq)	SC	0.612	0.000	0.019	0.632
	CE	0.956	0.015	0.049	1.021
	CO	0.027	0.001	0.082	0.110
FRS (kg oil eq)	SC	1.126	0.030	0.841	1.997
	CE	0.278	1.152	0.815	2.246
	CO	1.344	0.262	1.384	2.989
WF (L)	SC	62.9	4.5	36.9	104.4
	CE	15.8	28.7	22.2	66.8
	CO	52.2	20.7	36.9	109.8

GWP: global warming potential, CED: cumulative energy demand, FRS: fossil resource scarcity, MRS: mineral resource scarcity, WF: water footprint, SC: soil-cement, CE: ceramic, CO: concrete.

in the Appendix A). Soil-cement presented lower MRS than ceramic and lower WF than concrete. Raw materials were the key contributors to soil-cement and concrete, and the manufacturing process was prominent for ceramic. Joints and links with columns were relevant for the three technologies in most categories.

In the soil-cement masonry wall, Portland cement was the major single-flow contribution to most categories (86% GWP, 59% CED, 56% FRS, 45% WF) and soil to MRS (92%). The PVA glue used in the joints also stood out (10% GWP, 34% CED, 39% FRS, 30% WF). Water used in the soil-cement mix amounts to 15% of the WF. Electricity used in the brick manufacture and steel mesh used in the link with column together represented up to 5% in all categories, except WF with 10%.

The ceramic masonry wall was notably dependent on mineral resources (94% MRS). The joint mortar presented a significant contribution in most categories (17% GWP, 25% CED, 35% FRS, 30% WF). Biomass from wood yielded a high share of renewable energy sources (47% CED) but presented a substantial WF (36%) and GWP (82%), where the latter can be attributed to the direct emission from block manufacturing originated from native wood burning. Steel mesh for masonry and column link represented up to 3% in all categories.

The concrete masonry wall was the technology that used the most cement and can alone be associated with the largest burdens in all categories (76% GWP, 48% CED, 83% MRS, 79% FRS, 35% WF). Electricity presented a substantial contribution to WF (37%) and CED (17%). Sand (10% GWP, 20% CED, 25% FRS, 20% WF) and gravel (7% CED, 8% MRS, 6% WF) were also significant in some categories. Steel mesh used in the link with the column represented up to 5% in MRS and less than 1% in all other categories.

3.2. Sensitivity Analysis

3.2.1. Hotspots

The environmental and energy performance indicators of the sensitivity analysis on hotspot parameters and assumptions are presented in Figure 4 and

Table A3. Hotspot sensitivity analysis scenarios compared with the baseline scenarios of the masonry wall technologies.

Scenario		GWP	CED			MRS	FRS	WF
			Total	Fossil	Biomass
		(kg CO2 eq)	(MJ)			(kg Cu eq)	(kg Oil eq)	(L)
SA	SC: manual press, PVA glue	13.1	107.7	90.5	8.2	0.631	1.977	101.3
B	SC: hydraulic press, PVA glue	13.2	109.4	91.4	8.5	0.632	1.997	104.4
SA	SC: hydraulic press, PVA mortar	12.0	74.2	58.8	8.2	0.628	1.284	75.1
SA	CE: reforested wood	10.2	178.1	90.2	67.0	1.021	2.246	66.8
B	CE: reforested and native wood	33.0	178.1	90.2	67.0	1.021	2.246	66.8
SA	CE: native wood	53.3	178.1	90.2	67.0	1.021	2.246	66.8
SA	CE: natural gas	22.3	315.4	308.2	7.2	1.024	6.656	50.0
B	CO: clinker cement	19.9	169.4	137.1	18.4	0.110	2.989	109.8
SA	CO: clinker and BFS cement	16.7	156.5	137.1	18.4	0.213	2.757	107.6

GWP: global warming potential, CED: cumulative energy demand, FRS: fossil resource scarcity, MRS: mineral resource scarcity, WF: water footprint, B: Baseline scenario of the technology, SA: sensitivity analysis scenario, SC: soil-cement, CE: ceramic, CO: concrete.

in the Appendix A.

Soil-cement masonry alternative scenarios yielded benefits in all indicators. The scenario considering joints of PVA mortar stood out in CED, FRS, and WF compared with the baseline scenario for soil-cement wall as well as the ceramic and concrete walls. Opting for manual or hydraulic pressing had a negligible effect in all categories assessed.

Ceramic block manufacturing using biomass from planted forests–i.e., reforestation or sustainable forestry wood whose carbon content is considered biogenic–as the energy source, resulted in the ceramic masonry scenario with the smallest GWP compared with all others. The low cost of firewood is the key factor for its predominant use in Brazil [55], but also reflects the socio-political-economic landscape concerning the widespread exploitation of native wood and its multiple environmental repercussions; for instance, the native biomass-based scenario was the most impactful in GWP compared with all others. Natural gas as an energy source in block manufacturing increased FRS and non-renewable CED indicators substantially compared with the baseline scenario, although it yielded lower GWP and WF.

The concrete block techno-economic performance is closely associated with the type of cement used, which also affects its life cycle environmental and energy performance. The BFS cement incorporates an industrial by-product as supplementary cementitious material that yields equivalent technical performance for the purposes evaluated in this study and better environmental and energy performance in most categories (GWP, CED, FRS, and WF), except MRS due to the allocated burdens from ironmaking.

3.2.2. Finish Coating Layer

The environmental and energy performance indicators of the sensitivity analysis scenarios comprising finish coating layers are shown in Figure 5 and

Table A4. Coating layer sensitivity analysis scenarios compared with the baseline scenarios of the masonry wall technologies.

Scenario		GWP	CED	MRS	FRS	WF
		(kg CO2 eq)	(MJ)	(kg Cu eq)	(kg Oil eq)	(L)
B	SC: baseline	13.2	109.4	0.632	1.997	104.4
SA	SC: acrylic resin	15.6	150.5	0.832	2.796	140.4
SA	SC: mortar	28.9	230.7	0.755	4.196	156.2
B	CE: baseline	33.0	178.1	1.021	2.246	66.8
SA	CE: mortar	64.5	420.7	1.267	6.644	170.5
B	CO: baseline	19.9	169.4	0.110	2.989	109.8
SA	CO: mortar	35.7	290.7	0.233	5.188	161.7

GWP: global warming potential, CED: cumulative energy demand, FRS: fossil resource scarcity, MRS: mineral resource scarcity, WF: water footprint, B: Baseline scenario of the technology, SA: sensitivity analysis scenario, SC: soil-cement, CE: ceramic, CO: concrete.

in the Appendix A. The inclusion of coating layers presented a striking increment to the ceramic masonry while the resin-coated soil-cement masonry was the least affected. The inclusion of coating layers in the analysis did not change the soil-cement ranking compared to ceramic or concrete but did differentiate it against the ceramic wall even further.

Resin coating in soil-cement masonry presented 16% of the overall contribution to GWP and between 24% and 29% to the other indicators. Mortar coating was as impactful as all other contributions to the raw-surface masonry in GWP, CED, and FRS, while it was less impactful in MRS (16%) and WF (33%). Ceramic masonry coating was at least half of the overall burdens in GWP, CED, and WF, and two-thirds in FRS. The mortar coating represented from 32% to 53% of the overall contributions to the concrete masonry. The MRS category was the least affected by the inclusion of finish coating in the analysis.

3.2.3. Transport

The environmental and energy performance indicators of the transportation sensitivity analysis scenarios are presented in Figure 6 and

Table A5. Transport sensitivity analysis scenarios of the masonry wall technologies for different distances.

Indicator	SC Masonry	Transport		CE Masonry	Transport		CO Masonry	Transport
		10 km	50 km		10 km	50 km		10 km	50 km
GWP (kg CO2 eq)	15.6	0.6	3.0	64.5	0.8	3.9	35.7	0.9	4.4
CED (MJ)	150.5	9.6	47.9	420.7	12.3	61.3	290.7	13.9	69.3
MRS (kg Cu eq)	0.8	0.0	0.0	1.3	0.0	0.0	0.2	0.0	0.0
FRS (kg oil eq)	2.8	0.2	1.0	6.6	0.3	1.3	5.2	0.3	1.5
WF (L)	140.4	1.1	5.3	170.5	1.4	6.8	161.7	1.5	7.7

SC: soil-cement, CE: ceramic, CO: concrete, GWP: global warming potential, CED: cumulative energy demand, FRS: fossil resource scarcity, MRS: mineral resource scarcity, WF: water footprint.

in the Appendix A. A 10 km distance would add up to 4% to the masonry burdens and a 50 km distance would increase this to up to 22%. The FRS was the most affected indicator, followed by CED, then GWP, being the use and burning of fossil fuel as the major reason. The MRS category had minimal influence, followed by WF. The soil-cement masonry was the least affected by the inclusion of logistics in the analysis, followed by ceramic, then concrete, which can be explained by their total material demand ranking.

4. Discussion

4.1. Benchmarking

Results of this study were put into perspective with comparable findings of the state-of-the-art literature; yet it is noteworthy that potential major LCA modelling differences should be acknowledged, thus this is an indicative comparison with no intended definitive assertions. In this study, soil-cement masonry bricks produced using a hydraulic press, PVA mortar wall joints, and resin coating, with 10 km-distant suppliers resulted in 16 kg CO₂ eq∙m⁻² and 160 MJ∙m⁻². As for the carbon footprint, the consulted literature has reported results either in the same order of magnitude: 7 kg CO₂ eq∙m⁻² [12] and 12 kg CO₂ eq∙m⁻² [24]; or one order of magnitude higher: (80–100) kg CO₂ eq∙m⁻² [25]. Similarly, in terms of the energy footprint, the consulted literature reported 138 MJ∙m⁻² [24] and (700–800) MJ∙m⁻² [25].

4.2. Scale-up

The soil-cement masonry wall presented an overall better environmental and energy performance with few exceptions compared with both the ceramic and concrete walls in the baseline scenarios (Figure 3) as well as when including finishing coating layers as observed in the sensitivity analysis (Figure 5). Concerning large-scale production, sensitivity analysis showed that the hydraulic pressing in soil-cement brick manufacturing adds insignificant burdens to the indicators assessed (Figure 4) and a single hydraulic machine can yield up to 3000 bricks in a work day [41,42], in contrast with up to 1500 bricks per work day from a manual pressing machine [39].

To understand the large-scale implications, soil-cement masonry with bricks produced using a hydraulic press, PVA mortar wall joints, and resin coating was compared with conventional mortar-coated ceramic and concrete masonries of the baseline scenarios assuming 10 km-distant suppliers considering a 1000 people community and the masonry area per number of occupants of low-standard (27.7 m² per capita), medium-standard (46.8 m² per capita), and high-standard (66.0 m² per capita) single-family homes [31]. The avoided burdens of soil-cement coated masonry compared with the ceramic and concrete walls are shown in

Table 5. Avoided environmental and energy burdens of the soil-cement masonry wall compared with conventional baseline ceramic and concrete masonry walls for a 1000 people community of low-, medium-, and high-standard (st.) houses.

Indicator	Soil-Cement vs. Ceramic			Soil-Cement vs. Concrete
	Low-St.	Medium-St.	High-St.	Low-St.	Medium-St.	High-St.
GWP (106 kg CO2 eq)	1.4	2.3	3.2	0.56	0.95	1.3
CED (106 MJ)	7.6	13	18	4.0	6.8	9.5
MRS (106 kg Cu eq)	0.012	0.020	0.029	−0.017	−0.028	−0.039
FRS (106 kg oil eq)	0.11	0.18	0.26	0.069	0.12	0.16
WF (106 L)	0.84	1.4	2.0	0.60	1.0	1.4

GWP: global warming potential, CED: cumulative energy demand, FRS: fossil resource scarcity, MRS: mineral resource scarcity, WF: water footprint.

.

The deployment of soil-cement coated masonry wall on a large scale would substantially avoid environmental and energy burdens compared to conventional technologies. Mineral resource exploitation is an exception in the comparison with concrete masonry; however, ceramic masonry is the technology most dependent on such a resource, to which soil-cement masonry presented a significant advantage comparatively, (12 to 29) t Cu eq avoided. Furthermore, using soil-cement masonry in the built environment of a 1000 people community considering from low- to high-standard residential buildings could avoid the emission of (0.6 to 3.2) kt CO₂ eq, the demand of (4 to 18) TJ of energy, and the use of (0.1 to 0.3) kt oil eq and (0.6 to 2) thousand m³ of water.

4.3. Multifunctionality

Masonry walls may or may not be designed for structural support. Concrete presents significant compressive strength by nature, being more often employed in load-bearing masonry, yet certain ceramic and soil-cement bricks and blocks are also used for structural purposes [11,13,57]. From an LCA standpoint, masonry technologies are multi-functional in essence, having common overlapping functions such as protection, insulation, and aesthetics [6,7]. However, when designed for structural support as an extra function in this context, life cycle performance comparison issues arise from functional unit comparability and two immediate potential solutions are system expansion and allocation. System expansion would enlarge the boundary to comprise not only masonry but also essentially load-bearing elements as columns, beams, and maybe slabs, thus would not only divert the comparability focus on the masonry by aggregating multiple elements and thus results, but also increase the complexity of the analysis and require additional effort.

Allocation of burdens between functions by physical means is intrinsically complex and subjective in this case as there is no linear causality relation—LCA baseline assumption—between the masonry system mass or volume and load capacity, which is actually more closely related to material chemical composition and properties. One may argue that increasing the mass of a given brick or block would increase the masonry load capacity; however, this would also increase the load of the whole building and thus its structural requirements, resulting in larger life cycle burdens. This feedback loop becomes important in the case of, for example, a multi-floor building. Instead, it would be simpler to use a more suitable material capable of resisting a specified load with lower material demand. For instance, hypothetically assigning a 50% allocation factor to an extra structural function for the concrete masonry examined herein, a non-structural soil-cement masonry would be outperformed in all categories assessed. On the other hand, considering structural soil-cement bricks and normalizing the functional unit for compressive strength, it would demand nearly 70% more raw material per m²∙MPa—of which cement is a major portion—than concrete blocks [25,30], compared with 10% lower material demand without structural function as shown in this study.

4.4. Research Prospects

There has been extensive effort to recycle by-products from other industries by incorporation into soil-cement bricks and blocks materials such as coconut fibre waste [58], polymeric waste [59], granite cutting powder [60], clay and cement waste [61], mining waste and tailing [62,63], rice husk [64], foundry sand [65], and water treatment sludge [64,66], to cite a few. Even though brick and block raw materials are key contributors to the masonry life cycle environmental and energy performance, other aspects should also be factored in. For instance, in this study, joints and links with columns stand for the second largest contribution in all categories, as similarly observed in related literature [12,24]. Additionally, the finish coating layer may be a significant contributor, especially when using cement mortars, as similarly reported in related literature [25]. Alternatively, other coating options such as gypsum plaster, plasterboard, and ceramic tile finishing layers [48] may be systematically evaluated compared to the resin option. In this way, further research with a focus on reducing the environmental and energy aspects and impacts of soil-cement masonry should consider improving the masonry system as a whole in addition to its “building block” raw materials.

5. Conclusions

Soil-cement masonry walls overall outperformed conventional masonry technologies such as ceramic and concrete in terms of carbon, energy, and water footprints, and fossil and mineral resource use, although improved scenarios of conventional options could be competitive for specific indicators, e.g., ceramic masonry with fired reforested-wood in block manufacturing for carbon footprint. Large-scale employment of soil-cement masonry showed the potential to greatly reduce environmental and energy burdens in comparison with conventional options.

The inclusion of finish coating layers and material transport did not change the environmental and energy performance ranking of the technologies comparatively, with the soil-cement masonry being the least affected. Raw materials were a major contribution to soil-cement masonry, followed by the joints and links with columns, where cement was a hotspot. Benefit-to-detriment of hydraulic pressing in brick production notably outweighs manual pressing. Using PVA mortar joint in lieu of PVA glue, as well as resin coating instead of cement mortar showed significant gains.

Future research should consider a systematic and holistic evaluation of the masonry system, incorporate the notion of non-overlapping multifunctionality to ensure analysis consistency, and encompass other wall technologies such as lightweight steel and wood stud frame board walls, and finish coating layer options.