Impact of Curing Temperature on the Life Cycle Assessment of Sugarcane Bagasse Ash as a Partial Replacement of Cement in Mortars

Abstract

Sugarcane bagasse ash (SCBA), a biomass waste resulting from sugarcane bagasse burning for electricity production, has shown to be a viable alternative option as a partial cement replacement due to its chemical composition and physical properties. Besides, previous research indicates that higher curing temperature may improve the mechanical properties of mixes containing SCBA as cement replacement. However, the environmental assessment of those mixes is lacking in the literature. This study aims to understand how curing temperature impacts the Life Cycle Assessment (LCA) of SCBA as a partial replacement of cement in mortars. An LCA was performed from the extraction of the raw materials to the material production part of the life cycle, including transport. This study shows that the reduction of environmental impact when using SCBA highly depends on the curing temperature. When mortars were cured at 45 °C, the use of SCBA reduced the environmental impact of mortars two times with respect to the reduction at 21 °C (31% reduction when cured at 45 °C vs. 14% at 21 °C, with a 20% replacement). This difference is mainly related to the fact that the higher the curing temperature, the better SCBA mortars perform in terms of strength, thus, net savings of cement required to achieve a given performance are higher. Results indicate that the sustainability of SCBA utilization as a partial replacement of cement will be better when mortar is poured in hot regions or during days with higher ambient temperatures. Likewise, the advantages of using SCBA in terms of sustainability will decrease if the external temperature is low.

1. Introduction

1.1. Cement Industry and the Need of Waste Valorization

The demand for construction materials has surged due to the enormous development in the construction industry and the increase in the world’s population. This has been translated into a worldwide increase of 175% in cement production over the last 20 years [1]. Regrettably, cement manufacturing consumes a great deal of energy and negatively impacts the environment by consuming raw materials and releasing a massive amount of CO₂ [2]. Furthermore, besides the consumption of natural resources for building materials production, continuous waste generation has become one of our society’s most prominent environmental issues. Hence, much endeavor is being made to design viable solutions to solve these problems, including developing more sustainable cementitious composites with the inclusion of industrial waste, by-products, or alternative aggregates [3,4]. The partial or total replacement of ordinary Portland cement with supplementary cementitious materials (SCMs) and natural aggregate by recycled aggregates are potential pivotal approaches to reduce the environmental concerns and carbon footprint deriving from the construction industry [4].

Nowadays, there is a large availability of artificial pozzolanic materials that are by-products and, most often, waste from industrial or agricultural applications and processes [5,6]. These include coal fly ash, ground granulated blast furnace slag, silica fume, rice husk ash, and sugar cane bagasse ash (or others biomass combustion ash). In particular, the use of coal fly ash as a supplementary cementitious material for concretes and mortars production has been extensively studied, implemented in standards, and is widely accepted by the concrete industry [7]. However, the current trend of closing coal-based power plants in developed countries will reduce the availability of this valuable substitute for cement [8]. As a result, recent attention has been shifted to the utilization of alternative ashes from agricultural waste in cementitious composites, which may reduce the environmental impact of constituent materials such as cement [9]. Besides, the valorization of these agricultural wastes avoids negative impacts which may arise from their disposal.

1.2. Sugarcane Bagasse Ash as an Alternative SCM

In this context, sugarcane bagasse ash (SCBA) has shown to be a viable alternative option as a partial cement replacement due to its chemical composition and physical properties. SCBA is a biomass waste resulting from sugarcane bagasse burning (calcination) for electricity production [10,11]. Sugarcane bagasse is a by-product of the sugarcane industry in sugar and ethanol production [11]. The sugarcane industry plays an important role and holds a significant share in the economy of many countries [12]. More than 50% of the countries are involved with sugarcane crops [10,12], totaling an annual global production of about 2 billion metric tons. Brazil is the world’s undisputed leader, with 757 million tons of sugarcane production in 2020, followed by India (370 million tons) and China (108 million tons) [13,14,15]. As its production increases [16], the amount of ash (waste) generated will also increase, contributing to the ongoing issue of biomass waste management [17,18]. These ashes are often disposed of in landfills without any environmental control or, in some cases, used as fertilizer [19,20,21]. However, several investigations [21,22,23,24] have already determined the feasibility of using ashes resulting from agro-industrial by-products (such as SCBA) as supplementary cementitious material (SCM). Thus, there is an opportunity to reduce both the environmental impact of cementitious materials and the generation of waste. Numerous recent research efforts have been undertaken to incorporate SCBA to produce sustainable mortar and concrete [10,19,20,22,25,26,27,28].

Literature has shown that the mechanical performance of cementitious composites containing SCBA may range from 80% to 160% compared to the same without SCBA, depending on the ash mineral and morphological characteristics [10]. Nevertheless, more systematic investigations will be essential to understand further the pozzolanic behavior of SCBA. Moreover, a crucial task will be finding the most ecological post-processing for the SCBA, with the lowest energy demand and CO₂ emissions, that yields the higher reactivity. It will help instill more confidence in the concrete and construction industry stakeholders for adopting SCBA as an alternative pathway for sustainable concrete production [10].

Besides, to effectively evaluate the environmental impact of cementitious composites by using SCBA and define their best application, an environmental assessment that accounts for the effects of their substitution in cement is crucial. Many methodologies exist today to evaluate the environmental impact of processes and products. The most widely acknowledged and standardized is the so-called Life Cycle Assessment (LCA). According to the U.S. Environmental Protection Agency (EPA), an LCA is a tool for evaluating the environmental burdens associated with a product, material, process, or activity. The ISO standard 14040, the standard that describes the principles and framework for LCA, defines it as “the compilation and evaluation of the inputs, outputs and potential environmental impacts of a product system throughout its life cycle” [29]. In other words, LCAs identify and quantify energy and material used and pollutants released into the environment to model the complex processes included in the life cycle of a product. It can cover a specific fragment, or the entire product’s life-cycle, from materials acquisition, manufacturing, use, and final disposition (disposal or reuse). However, it must be reminded that an LCA is a relative tool intended for comparison and not absolute evaluation, thereby serving to help decision-makers to compare all major environmental impacts when choosing between alternative courses of action [30].

Remarkably, the use of LCA to evaluate the environmental impact of SCBA in cementitious composites has been found as lacking in the literature. Very few studies have been found evaluating the environmental impact of SCBA as a binder replacement in cementitious composites. Two studies [31,32] only evaluated the potential reduction of greenhouse gasses (mainly CO₂) emissions as a single factor in utilizing SCBAs as partial cement replacement. Whereas a third study [33] performed an LCA of SCBA as partial cement replacement in concrete, using as a functional unit a school building constructed as a 3-D digital model employing building information modeling (BIM) and considering 17 impact categories. All of the previous studies concluded that the utilization of SCBA can indeed contribute to the net reduction of greenhouse gasses (GHG) emissions and almost all other impact categories.

Nevertheless, to make a realistic estimation of the environmental footprint of cementitious composites, the functional unit used in the assessment should include a reference performance. For instance, some authors suggest that the appropriate functional unit to assess the effect of admixtures and recycled materials in cementitious composites should have the same quantity of material (e.g., 1 m³) with a given fixed value of compressive strength [34,35]. In this way, it is possible to compare the sustainability of using recycled materials, accounting for their effects on one of the main target properties of a cementitious composite, its strength. Thus, accounting for the effect of SCBA substitution on the strength and the changes in the cement content to achieve the same performance as the reference material (without SCBA) will be key to assessing the real benefits of using this biomass waste as a partial replacement of cement.

Besides, external factors may also affect SCBA replacement’s impact on the mechanical properties of cementitious composites. A previous study has pointed out that the effect of another type of biomass ashes (from the paper industry) on the concrete strength highly depends on the curing temperature [36]. Curing conditions strongly impact the hydration of cementitious composites, significantly influencing their strength, porosity, and durability [37], consequently, the environmental impact during their life-cycle. The effect of curing temperature has been extensively investigated in plain Portland cement mortar and concretes. In particular, research has shown that curing temperature is a key factor influencing the evolution of the compressive strength of cementitious composites [36,38,39,40,41,42], indicating that the higher the curing temperature, the faster the development of strength at early ages. However, after 28 days, the strength may be lower in samples cured at higher temperatures (crossover effect) [42,43]. The same may be true in cementitious composite containing SCMs. Indeed, research suggested that different curing temperatures and processes may increase the compressive strength and reduce the permeability of concrete and mortars where cement is replaced by SCMs [36,44,45].

Velay-Lizancos et al. [36] indicated that curing temperature influences the effect that biomass ashes from the paper industry have on concrete compressive strength, with higher curing temperature showing better results in terms of biomass ashes’ impact on compressive strength. More importantly, their results suggest that replacing cement with biomass ash might mitigate the crossover effect due to higher curing temperature on the compressive strength evolution of eco-concrete. However, the effect of curing temperature has been little investigated in mixtures containing SCBA as a partial cement replacement. Murugesan et al. [46] analyzed the effect of eight different curing methods on concrete specimens, including accelerated hot water curing at 100 °C for one day, obtaining the lowest compressive strength among the studied methods. Rajasekar et al. [47] studied the effect of three different curing regimes on the compressive strength of ultra-high-strength concrete (UHSC) with SCBA replacements: (i) normal water curing at 27 °C ± 2 °C, (ii) steam curing for 24 h at 90 °C, and (iii) heat curing for 24 h at 160 °C. They concluded that heat curing seems more efficient in improving concrete performance than normal and steam curing. Thus, based on previous research, higher curing temperature may improve the mechanical properties of mixes containing SCBA as cement replacement.

If curing temperature changes the effect of SCBA on the composites’ compressive strength, it can be an important factor that affects the actual change in sustainability produced by the use of biomass ash in cementitious composite production and must be included in the environmental assessments. Therefore, this research aims to understand and quantify the impact of curing temperature on the environmental performance of mortars with different replacements of cement by SCBA.

2. Materials and Methods

2.1. Materials

Mortars were produced with Portland cement Type I (Buzzi Unicem, Greencastle, IN US with a specific gravity of 3.02. The fine aggregate used was natural siliceous sand compliant with ASTM C33/C33M [48] and had a fineness modulus of 2.99, an absorption of 1.97%, and a relative density (specific gravity in saturated surface dry-SSD-condition) of 2.45. The sugar cane bagasse ash (SCBA) was supplied by a sugar plant located in Valle del Cauca (Colombia), and is a waste generated after the combustion of sugarcane bagasse for energy production. The received ash was ground with a RETSCH ZM-1 Lab Benchtop Ultra Centrifuge Mill Grinder (passing an 80 μm mesh) for 2 min to reduce the particle size and improve homogenization and reactivity. The original and final particle size was obtained with a PSA 1090 (Anton Paar, Graz, Austria) and is reported in

Table 1. SCBA particle size.

Type	D10 (μm)	D50 (μm)	D90 (μm)	Mean Size (μm)
As received	8.0	31.9	71.6	37.6
Ground	3.7	18.7	47.5	23.6

.

The chemical configuration of the ash was obtained by a Lab X500 XRF analyzer (Hitachi, Tokyo, Japan) and is reported in

Table 2. Chemical composition of SCBA.

Compound	Percentage (%)	Standard Deviation—σ
SiO2	54.97	0.026
Al2O3	13.85	0.019
CaO	9.98	0.013
Fe2O3	8.568	0.0054
MgO	2.074	0.0123
TiO2	1.595	0.0064
K2O	1.385	0.0017
Mn2O3	1.245	0.0067
SO3	0.926	0.0015
Na2O	0.509	0.0052
P2O5	0.213	0.006
SrO	0.158	0.0005
CrrO3	0.089	0.0006
ZnO	0.087	0.0003
LOI	16.5	-

. The SCBA used in this study satisfied the minimum requirement of 70% in the sum of SiO₂ + Al₂O₃ + Fe₂O₃ pozzolanic oxides conform to the ASTM C618 [49]. However, the loss of ignition (LOI) was about 7% higher than the minimum 10% requirement [49].

2.2. Mix Design, Sample Manufacturing, and Curing Conditions

The mortar formulation used as reference was prepared with a 1:3:0.5 (cement:sand:water) weight ratio. Two percentages of cement replacement with SCBA were selected for this study based on previous literature [10]. The amount of cement replaced was 10% and 20% (by mass of cement). Water to binder (cement + SCBA) and binder to aggregate ratios were constant for all mixes at 0.50 and 3, respectively. Mix proportions of the composites are presented in

Table 3. Mix proportions (per m³ of mortar).

Component	Reference	10% SCBA	20% SCBA
Cement (kg)	486.39	437.75	389.12
SCBA (kg)	0.00	48.64	97.28
Sand (SSD) (kg)	1459.18	1459.18	1459.18
w/b	0.50	0.50	0.50

.

Six cubes with dimensions of 50.8 × 50.8 × 50.8 mm³ were cast per each mortar mixture. All the samples were cured in their molds at room temperature (21.5 °C and 60% RH) covered with a plastic sheet during the first day. After demolding, three samples per mixture were cured at 21 °C, while another three were cured at 45 °C, until the testing day.

2.3. Methods

2.3.1. Compressive Strength Test

Compressive strength tests were carried out according to ASTM standards C349 [50]. An MTS (Eden Prairie, MN, USA) machine with a load capacity of 300 kN, under a displacement control of 0.05 mm/s, was used. The mortar compressive strength was examined at 28 days.

2.3.2. Life Cycle Assessment (LCA) of Mortars

The framework of the LCA performed in this study is presented in Figure 1. The LCA methodology of this study follows the ISO standards 14040 and 14044 [29,51]. According to these standards, four steps are identified:

• Goal and scope

This research aims to understand and quantify how curing temperature affects the environmental performance of eco-mortars with different percentages of sugarcane bagasse ash (SCBA). Three mixes with 0%, 10%, and 20% replacement of SCBA by mass of cement and two different curing temperatures (21 °C and 45 °C) were studied. A “cradle-to-gate” analysis was considered as the system boundary, which allows for the quantification of the embodied environmental impacts of the material from the extraction of raw materials (cradle) to the mortar production (gate) stages. The boundary of the LCA is illustrated in Figure 2. The functional unit (FU) selected for this study was 1 m³ of mortar with the same compressive strength as the reference mixture (plain Portland cement mortar without SCBA cured at the same curing temperature). To compare the environmental impact of the investigated mortars, an experimental campaign was performed beforehand to obtain a curve exhibiting the development of compressive strength as a function of cement content for the reference mortar. This curve estimates the variations in the cement content needed for achieving the same compressive strength as the reference mortar when SCBA is used to replace part of the cement.

• Life cycle inventory (LCI)

In this stage, energy and raw materials data (input) and emissions and wastes data (output) must be identified and allocated for each material production. In order to facilitate the LCI phase, numerous databases have been developed. These databases provide essential inventory data such as raw materials, electricity generation, transport, and waste generation data that are essential in every LCA and are based on average data representing average production and supply conditions for products and services. Since no global database exists, different sources were used in this study depending on their availability and reliability. Cement and natural fine aggregate production and total transportation impact data were obtained from the European Platform on Life Cycle Assessment (ELCD database) [52]. Mortar production input data was assumed to be equivalent to concrete production based on their essentially identical composition (aggregate, cement, and water) and because of the more abundant and reliable data on concrete production. The main difference between the two products is the aggregate size (with mortar using only the fraction passing through a 4.75 mm sieve). These data were obtained from a previous study [53]. Waste generation was estimated considering the density of the studied mortars, where the average density of all hardened samples was 2220 kg/m³ (with a standard deviation lower than 2.1%). Finally, transport distances were collected from the Life Cycle Inventory of Portland Cement Concrete by the Portland Cement Association (PCA) [54].

Table 4. Life inventory data used for this study.

	Cement (kg)	Sand (kg)	SCBA (kg)	Mortar (m3)	Transport (t·km)
INPUTS Fossil fuels (kg)
Diesel	3.56 × 10−2	3.29 × 10−4	0.00	2.00 × 10−2	2.06 × 10−2
Gas	8.53 × 10−3	1.24 × 10−4	0.00	5.30 × 10−2	1.13 × 10−3
Soft coal	2.67 × 10−2	3.13 × 10−4	0.00	5.25 × 100	7.31 × 10−5
Hard coal	4.83 × 10−2	2.18 × 10−4	0.00	4.01 × 10−2	9.01 × 10−5
OUTPUTS Emissions in the air (kg)
CO2	8.85 × 10−1	2.34 × 10−3	0.00	4.59 × 100	6.40 × 10−2
CO	2.14 × 10−3	4.19 × 10−6	0.00	8.81 × 10−4	1.10 × 10−4
CH4	5.80 × 10−4	3.72 × 10−6	0.00	2.19 × 10−3	6.25 × 10−5
C2H4	3.95 × 10−10	9.24 × 10−12	0.00	7.36 × 10−8	3.70 × 10−10
CFC-11	5.22 × 10−9	1.75 × 10−10	0.00	2.09 × 10−15	6.08 × 10−11
CFC-114	5.35 × 10−9	1.79 × 10−10	0.00	3.67 × 10−9	6.23 × 10−11
SOx	1.05 × 10−3	9.49 × 10−6	0.00	5.34 × 10−2	3.41 × 10−5
NOx	1.79 × 10−3	1.52 × 10−5	0.00	8.01 × 10−2	5.39 × 10−4
N2O	2.22 × 10−6	3.81 × 10−8	0.00	2.20 × 10−5	7.32 × 10−7
NH3	3.91 × 10−2	7.24 × 10−9	0.00	3.27 × 10−7	4.00 × 10−7
NMVOC	2.26 × 10−1	1.37 × 10−6	0.00	9.20 × 10−5	3.20 × 10−5
HCl	1.99 × 10−2	1.80 × 10−7	0.00	4.00 × 10−4	8.20 × 10−8
N (water)	1.16 × 10−4	4.23 × 10−9	0.00	4.81 × 10−6	2.35 × 10−7
PO4−3 (groundwater)	5.14 × 10−7	1.23 × 10−8	0.00	3.85 × 10−3	5.88 × 10−7

presents the LCI data considered in this study.

As shown in Table 4, no allocation (waste status) was applied to the SCBA. This scenario is based on the following assumptions: (i) SCBA results from a by-product (sugarcane bagasse) of a multifunctional process (sugarcane processing) and thus is a waste; (ii) even if considered as a by-product, the resulting impact is likely to be very small, therefore, the environmental impact of its production will not be relevant; (iii) it is believed that the CO₂ emissions from biomass burning do not contribute to the greenhouse effect, since the carbon released from crops already existed in the atmosphere and was absorbed during the plant growth, thus, the net CO₂ associated with the whole process can be considered neutral [31,55]. Besides, the authors deliberately avoided estimating of the cost for the industrial implementation of SCBA because it was out of the scope of the present paper, which was to study the effect of curing temperature on the embodied environmental impacts of the material. An economic analysis should consider market variables such as availability, local cost (which in the very first moment would be zero–waste value), and demand (like other residual pozzolans, the cost of the material will increase with the increasing demand), as well as the cost of further processing, but also potential revenues from carbon credits.

• Life cycle impact assessment (LCIA)

The LCIA phase is crucial to evaluate the importance and relevance of the environmental impact of a product based on the LCI results. In this study, the TRACI methodology (mid-point approach) was employed to assess the total environmental impact of the investigated mortars. The life cycle impacts were assessed by analyzing nine environmental impact categories. The categories (from TRACI) used to analyze and compare the environmental impact of the studied mixes, and their units are displayed in

Table 5. Designated impact categories from TRACI (last two added).

Category	Units
(i) Global warming potential (GWP)	kg CO2 eq
(ii) Ozone depletion potential (ODP)	kg CFC-11 eq
(iii) Eutrophication potential (EP)	kg N eq
(iv) Acidification potential (AP)	kg SO2 eq
(v) Smog formation (S)	kg O3 eq
(vi) Respiratory effects (RE)	kg PM2.5 eq
(vii) Energy consumption (EC)	MJ surplus
(viii) Waste generation (WG)	kg
(ix) Abiotic depletion potential (ADP)	kg Sb eq

. Among these, ecotoxicity was excluded from the analysis since no evidence of this impact has been found in the main components of mortars [56,57]. Moreover, the characterization factors, which estimate the relative contribution of each substance (inputs and outputs in the LCI) to each impact category in its corresponding unit, were considered. The characterization factors were obtained from the study of Ryberg et al. [58].

Besides, two other categories (not measured in TRACI) were considered to account for the potential reduction of raw materials due to the replacement of cement by SCBA: (i) waste generation (WG) and (ii) abiotic depletion potential (ADP). Waste generation quantifies the net kilograms of material produced that can become waste at the product’s end of life. Abiotic depletion potential is defined as the consumption of non-renewable mineral (raw materials) resources. Therefore, if ADP and WG are not considered in the environmental assessment of mixes with SCBA, the real environmental benefits of using this waste in mortars will not be fully reflected.

• Interpretation of the results

The last phase of the LCA is the interpretation of the results obtained from the impact assessment. This phase is not strictly defined as the previous ones, allowing practitioners to apply findings from the LCA to various situations, knowing the uncertainty and the assumptions used to generate the results. In this study, the analysis was conducted following two different approaches:

• Single categories analysis: Single impact categories were analyzed, and the results were discussed to appreciate the potential implications of replacing cement with SCBA as a function of curing temperature.
• Normalized unified index: Environmental impacts of single categories were normalized using the normalization factors (NFs) recommended by Ryberg et al. [58] to relate the environmental impact results of each category to a common reference. The goal of this normalization is to put each environmental impact in relation to the impact of society’s production and consumption activities. As a result, normalized values will better reflect the product system’s contribution to each category’s environmental impact compared to those of the reference system. The reference system used was the environmental impacts of each category per year in the US [58]. The normalized values were calculated by dividing the environmental impact of each category by its corresponding normalization factor. Note that this section did not consider WG and ADP since there are no NFs available for these two categories.

3. Results and Discussion

3.1. Compressive Strength and Binder Content Variation Based on Designated FU

Figure 3a displays the compressive strength results of the studied mortars after 28 days of curing at 21 °C and 45 °C. Results showed that the replacement of cement with SCBA at standard curing temperature (21 °C) had a slightly negative effect on the compressive strength. The higher the replacement level, the higher the reduction of compressive strength. Nevertheless, the recorded loss was lower than 8% as shown in Figure 3b (6.3% and 7.3% for 10 and 20% replacement, respectively) compared to the reference mortar cured at the same temperature. However, when the curing temperature was higher (45 °C), the replacement of SCBA positively influenced the compressive strength when the results were compared to the reference mortar cured at 45 °C. Mortars with 10 and 20% SCBA replacement exhibited an increase in compressive strength of 24.5% and 22.2% (Figure 3b) compared to the plain mortar cured at the same temperature, respectively. Besides, the use of SCBA mitigated the high-temperature crossover effect observed in the reference mortar. It means that replacing Portland cement with SCBA may be beneficial when the external curing temperature is high or when a lower heat of hydration is required.

Thus, results show that curing temperature modifies the effect of SCBA on the mortar’s compressive strength. Therefore, a secondary experimental campaign was performed to assess the influence of partial replacement of cement by SCBA on the compressive strength of the mortar with different water to cement ratios. The objective of this secondary experimental campaign was to estimate the variation of cement content required to obtain a compressive strength equal to the compressive strength of the reference mortar (no SCBA) for each SCBA replacement level. Thus, the savings or additions of cement content needed for each case can be estimated. The results of this additional experimental campaign contributed as input data to the LCA.

Two extra reference mixtures (besides the one listed in Table 3) were made, varying the cement content (w/c ratio). Figure 4 displays the compressive strength of the reference plain mortar (no SCBA) as a function of the cement content for two different curing temperatures. This graph was used to estimate the cement content required in the modified mixes to obtain the same compressive strength as the reference mortar (at the same curing temperature). First, the compressive strength obtained from the modified mixtures was introduced in the graph to estimate the corresponding cement content of a mixture with the same strength if SCBA was not used. Next, the difference between this estimate and the cement content needed to achieve the reference strength was calculated. Then, this variation was added to the initial cement content used for the specific mortar. The result is the estimated cement content required to achieve the same compressive strength as the reference mortar (without SCBA). The binder content of each modified mixture is the estimated cement content plus the SCBA content (applicable). The results are presented in

Table 6. Estimation of cement content to obtain the FU.

Mixture	Curing Temperature (°C)	Compressive Strength, fc (MPa)	Initial Cement Content (kg/m3)	Estimated Binder [Cement, SCBA] Content to Achieve Reference fc (kg/m3)
Reference	21 °C	45.20	486.4	486.4 [486.4, 0]
BA10		42.37	437.8	497.1 [448.5, 48.64]
BA20		41.88	389.1	499.0 [401.7, 97.28]
Reference	45 °C	33.44	486.4	486.4 [486.4, 0]
BA10		41.62	437.8	414.1 [365.5, 48.64]
BA20		40.88	389.1	420.6 [323.4. 97.28]

, which displays the initial and the required binder content to achieve the same compressive strength as the reference mortar at 28 days when SCBA is used, as a function of the curing temperature. The aggregate content was kept constant, and therefore, the changes in the binder content to achieve the desired strength only affected the water-to-binder ratio.

3.2. Life Cycle Assessment (LCA)

3.2.1. Environmental Impact of Single Categories

Figure 5 displays the environmental impacts of each LCA category considered for the studied mortars as a function of curing temperature differentiating the contribution of each component. In the graphs, for each material (including mortar production), the transport impact data was included in the results. The amount of component materials (cement, natural aggregate, and SCBA) used in the mix design is according to tests performed for determining the mix proportions of the modified mortars that would have the same compressive strength as their reference (without SCBA).

Table 7. Environmental impacts for 1 m³ of mortars cured at 21 °C (* modified mortar that would have the same compressive strength as their reference).

Mortar	Cement	Natural Aggregate	SCBA	Production	Transportation of Components	Total
GWP—Global warming potential (kg CO2 eq)
Reference	437.84	3.57	0.00	4.65	21.04	467.10
BA10 *	403.73	3.57	0.00	4.65	21.24	433.19
BA20 *	361.62	3.57	0.00	4.65	21.28	391.12
ODP—Ozone depletion potential (kg CFC-11 eq)
Reference	5.14 × 10−6	5.17 × 10−7	0.00	3.67 × 10−9	3.94 × 10−8	5.70 × 10−6
BA10	4.74 × 10−6	5.17 × 10−7	0.00	3.67 × 10−9	3.98 × 10−8	5.30 × 10−6
BA20	4.24 × 10−6	5.17 × 10−7	0.00	3.67 × 10−9	3.98 × 10−8	4.81 × 10−6
EP—Eutrophication potential (kg N eq)
Reference	4.13 × 10−2	1.03 × 10−3	0.00	1.27 × 10−2	8.16 × 10−3	6.32 × 10−2
BA10	3.81 × 10−2	1.03 × 10−3	0.00	1.27 × 10−2	8.24 × 10−3	6.01 × 10−2
BA20	3.41 × 10−2	1.03 × 10−3	0.00	1.27 × 10−2	8.25 × 10−3	5.61 × 10−2
AP—Acidification potential (kg SO2 eq)
Reference	1.16	2.96 × 10−2	0.00	1.10 × 10−1	1.32 × 10−1	1.43
BA10	1.07	2.96 × 10−2	0.00	1.10 × 10−1	1.33 × 10−1	1.35
BA20	0.96	2.96 × 10−2	0.00	1.10 × 10−1	1.34 × 10−1	1.23
S—Smog formation (kg O3 eq)
Reference	22.02	0.56	0.00	1.99	4.31	28.88
BA10	20.31	0.56	0.00	1.99	4.36	27.21
BA20	18.19	0.56	0.00	1.99	4.36	25.09
RE—Respiratory effects (kg PM2.5 eq)
Reference	3.91 × 10−2	1.01 × 10−3	0.00	3.84 × 10−3	1.93 × 10−3	4.58 × 10−2
BA10	3.60 × 10−2	1.01 × 10−3	0.00	3.84 × 10−3	1.95 × 10−3	4.28 × 10−2
BA20	3.23 × 10−2	1.01 × 10−3	0.00	3.84 × 10−3	1.95 × 10−3	3.91 × 10−2
EC—Energy consumption (MJ surplus)
Reference	136.48	4.09	0.00	0.96	42.44	183.98
BA10	125.84	4.09	0.00	0.96	42.86	173.76
BA20	112.72	4.09	0.00	0.96	42.93	160.71
ADP—Abiotic depletion potential (kg Sb eq)
Reference	9.00 × 10−6	1.69 × 10−9	0.00	0.00	0.00	9.00 × 10−6
BA10	8.30 × 10−6	1.69 × 10−9	0.00	0.00	0.00	8.30 × 10−6
BA20	7.43 × 10−6	1.69 × 10−9	0.00	0.00	0.00	7.43 × 10−6

reports the contribution of each material and process to the total environmental impact of the mortars cured at 21 °C. In the table, the transportation contribution to the environmental impact was separated from the impact of the materials.

Results show that cement is the most significant contributor to all impact categories. The main reason is the great deal of CO₂ emissions due to the calcination of limestone and fossil fuel and energy use required for clinker production. Thus, as expected, the reduction of the cement content produced by the utilization of SCBA reduced the environmental impact of mortars, not only on the global warming potential (which is expressed in kg CO₂ eq) but also on all other categories, as can be seen in Table 7. The impacts from cement production are, for each category, at least one order of magnitude higher than the other two largest contributors (natural aggregate and mortar production). The contribution of aggregate and mortar production does not change, regardless of the amount of cement replaced by SCBA, because the amount does not change in the mortar formulation considered for the LCA in this study.

Besides cement production, transportation of component materials is the major contributor to the environmental burden of cementitious materials. Transport impacts directly depend on the transportation type and distance. While transportation impact data was obtained from the European Platform on Life Cycle Assessment (ELCD database) [52], transport distances were obtained from the Life Cycle Inventory of Portland Cement Concrete by the PCA [54] and multiplied by two (round-trip). In Figure 5, transportation was added up to the impact of each material. Therefore, the impacts coming from SCBA are none other than the impact of their transportation to the mixing location. That leaves the transport phase as a possible source of varying the environmental impact of these eco-friendly mortars. In this analysis, the transportation distance for SCBA was assumed to be the same as the transportation distance for Portland cement, fly ash, and other SCMs indicated in the Life Cycle Inventory of Portland Cement Concrete [54].

The use of SCBA produced important changes in the environmental impact of several impact categories, especially on the ADP and the GWP. The GWP quantifies the greenhouse gases released into the atmosphere (carbon dioxide (CO₂), methane (CH₄), and nitrous oxides (N₂O)) as a function of the amount of CO₂ that would have the same impact over a 100-year period [59]. When the curing temperature was 21 °C, BA20* presented a decrease of over 16% in GWP compared to the reference mortar while keeping the same estimated strength. Moreover, the ODP impact was reduced by 7% and 16% for BA10* and BA20*, respectively, for the same curing temperature. The ODP refers to a decrease in the ozone density through the thinning of the stratospheric ozone layer due to anthropogenic pollutants (e.g., halocarbons). That implies an increased exposure of human skin to UV light, which may lead to a higher risk of melanoma [60]. Besides, for the same curing temperature, the total energy demand was reduced by 6% and 13% in BA10* and BA20*, respectively, compared to the energy demand for the reference mortar.

The S, AP, RE, and EP impact categories were also reduced by 13%, 14%, 15%, and 11%, respectively, when SCBA was used (BA20*) and when the curing temperature was 21 °C. It is important because of the consequences that those impact categories have on the environment. The S impact category ponders the air pollution caused by the reaction between sunlight and emissions from fossil fuel combustion during the production of raw materials. As a result, the formation of other chemicals is promoted (e.g., ozone and peroxide), which leads to an increase in ground-level ozone concentration [61]. The AP impact category is also associated with the combustion of fossil fuels (e.g., nitrous and sulfide oxides), which may leach in the presence of oxygen and water and endanger the surrounding ecosystems [62]. The EP impact category relates to the natural or artificial discharge of nutritional elements to a body of water. These compounds decrease the oxygen available in aquatic systems, consequently reducing the water quality. The water contaminated by these organisms can be a threat to public health and biodiversity, thus, it must be monitored [63].

Finally, regarding abiotic depletion and waste generation, the replacement of cement by SCBA also reduced the impact of these two categories. The ADP impact category reflects the exhaustion of non-renewable resources and consequent environmental impacts, thus, a reduction of cement used for mortar production will have a positive effect. ADP and WG were reduced by 18% and 4% for BA20*, respectively, compared with the corresponding values for the reference mortar, both with 21 °C curing temperature.

Table 8. Environmental impacts for 1 m³ of mortars cured at 45 °C (* modified mortar that would have the same compressive strength as their reference).

Mortar	Cement	Natural Aggregate	SCBA	Production	Transportation	Total
GWP—Global warming potential (kg CO2 eq)
Reference	437.84	3.57	0.00	4.65	21.04	467.10
BA10 *	328.98	3.57	0.00	4.65	19.65	356.85
BA20 *	291.08	3.57	0.00	4.65	19.77	319.08
ODP—Ozone depletion potential (kg CFC-11 eq)
Reference	5.14 × 10−6	5.17 × 10−7	0.00	3.67 × 10−9	3.94 × 10−8	5.70 × 10−6
BA10	3.86 × 10−6	5.17 × 10−7	0.00	3.67 × 10−9	3.68 × 10−8	4.42 × 10−6
BA20	3.42 × 10−6	5.17 × 10−7	0.00	3.67 × 10−9	3.70 × 10−8	3.97 × 10−6
EP—Eutrophication potential (kg N eq)
Reference	4.13 × 10−2	1.03 × 10−3	0.00	1.27 × 10−2	8.16 × 10−3	6.32 × 10−2
BA10	3.11 × 10−2	1.03 × 10−3	0.00	1.27 × 10−2	7.62 × 10−3	5.24 × 10−2
BA20	2.75 × 10−2	1.03 × 10−3	0.00	1.27 × 10−2	7.67 × 10−3	4.89 × 10−2
AP—Acidification potential (kg SO2 eq)
Reference	1.16	2.96 × 10−2	0.00	1.10 × 10−1	1.32 × 10−1	1.43
BA10	0.87	2.96 × 10−2	0.00	1.10 × 10−1	1.23 × 10−1	1.14
BA20	0.77	2.96 × 10−2	0.00	1.10 × 10−1	1.24 × 10−1	1.04
S—Smog formation (kg O3 eq)
Reference	22.02	0.56	0.00	1.99	4.31	28.88
BA10	16.55	0.56	0.00	1.99	4.03	23.12
BA20	14.64	0.56	0.00	1.99	4.05	21.24
RE—Respiratory effects (kg PM2.5 eq)
Reference	3.91 × 10−2	1.01 × 10−3	0.00	3.84 × 10−3	1.93 × 10−3	4.58 × 10−2
BA10	2.93 × 10−2	1.01 × 10−3	0.00	3.84 × 10−3	1.81 × 10−3	3.60 × 10−2
BA20	2.60 × 10−2	1.01 × 10−3	0.00	3.84 × 10−3	1.82 × 10−3	3.26 × 10−2
EC—Energy consumption (MJ surplus)
Reference	136.48	4.09	0.00	0.96	42.44	183.98
BA10	102.55	4.09	0.00	0.96	39.64	147.25
BA20	90.73	4.09	0.00	0.96	39.90	135.69
ADP—Abiotic depletion potential (kg Sb eq)
Reference	9.00 × 10−6	1.69 × 10−9	0.00	0.00	0.00	9.00 × 10−6
BA10	6.76 × 10−6	1.69 × 10−9	0.00	0.00	0.00	6.76 × 10−6
BA20	5.98 × 10−6	1.69 × 10−9	0.00	0.00	0.00	5.98 × 10−6

presents the environmental impacts of the production of 1 m³ of the studied modified mortars cured at 45 °C, with the same target compressive strength as the reference. The environmental results of the mortars cured at 45 °C followed the same trend of mortars cured at 21 °C, with the environmental impact decreasing as the replacement of cement increased. However, at 45 °C, the use of SCBA reduced the environmental impact of mortars to a greater extent than at 21 °C, as is clearly shown in Figure 6.

Considering the results for each analyzed impact category, Figure 6 displays the reduction of the environmental impacts of modified mortars in comparison to the reference (no SCBA) for both 21 °C and 45 °C. Besides, the LCA accounted for the deviation of compressive strength obtained by the tested samples (Figure 3a). The environmental impact for each impact category was calculated assuming the average compressive strength (solid lines) as well as the minimum and maximum strength results to determine the range of uncertainty (shadowed regions). The results of each impact category for mortars containing SCBA were normalized to the corresponding results for the reference mortar (no SCBA) assuming its average compressive strength in the calculation. The result is a dimensionless value equal to 1 for the reference–the lower the value, the lower the environmental impact.

Figure 6 shows that replacing cement with SCBA can help reduce the environmental impact of cementitious composites and that the higher the replacement, the lower the environmental footprint of mortars in the studied range of SCBA replacement. While WG impact did not change, all other categories showed important differences produced by curing temperature. Whereas at 21 °C, BA20* presented a reduction of single impact categories ranging between 11.23% and 17.40% (in comparison to reference mortar), at 45 °C, the reduction of the environmental impacts due to the use of SCBA doubled, ranging from 22.61% to 33.51%. The LCA results can be explained by looking at the compressive strength results and the functional unit chosen for the LCA. The reference mortar cured at 45 °C presented a compressive strength 26% lower than the same mixture cured at 21 °C. This tendency of high curing temperatures to induce lower strength after 28 days is well documented in the literature as a consequence of the poorer quality of the hydration products [64], and it is known as the crossover effect. However, the data of the present study showed that SCBA was effective in mitigating the crossover effect.

3.2.2. Environmental Impact Assessed with Normalized Unified Index

Normalization factors (NFs) may be used as an optional aid for the interpretation of the results of the LCA. LCIA results for each impact category were first normalized by the NFs. A unified index was calculated by adding the normalized impacts of each category. Next, the normalized unified index was calculated as the unified index of each mixture divided by the unified index of their reference mortar. While previous results considered ADP and WG, as mentioned in the methods section, these two impact categories were not considered in the analysis to obtain the normalized unified index due to the lack of NFs.

The normalized unified index represents with a single score the relative environmental impact of each mixture compared to the reference one. Figure 7 displays the results of this approach. For a curing temperature of 21 °C, BA10* and BA20* reduced their environmental impact by 6% and 14%, respectively, compared to the reference mortar cured at the same temperature. Nevertheless, the best results in terms of environmental performance were obtained when the curing temperature was higher (45 °C). At 45 °C curing temperature, the reduction in the environmental impact was more pronounced on BA10* and BA20*, with values of 23% and 31%, respectively, compared to their reference mortar cured at 45 °C.

4. Conclusions

The impact of curing temperature on the sustainability of sugarcane bagasse ash as a partial replacement of cement in mortars was assessed in this study. Nine impact categories were analyzed for each mixture and curing temperature. Based on the results of this study it was concluded that:

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Among the impact categories, waste generation is the only impact category that was not affected by curing temperature. For all of the others analyzed categories, when mortars were cured at 45 °C, the use of SCBA reduced the environmental impact of mortars two times with respect to the reduction at 21 °C.

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At 45 °C, a replacement of 97 kg of cement with SCBA (per m³ of mortar) produced a reduction of the environmental impact (presented with the normalized unified index) of 31%, while the reduction produced by the same amount of SCBA with a curing temperature of 21 °C was 14%.

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The reduction of environmental impact when using SCBA as a partial replacement for cement highly depends on the curing temperature. The results clearly indicate that the sustainability of SCBA utilization as a partial replacement of cement will be better when mortar is poured in hot regions or during days with higher ambient temperatures.

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The advantages of using SCBA in terms of sustainability will decrease if the external temperature is low. Therefore, external curing temperature is an important factor that should be considered when the sustainability of cementitious composites containing SCBA is assessed.

5. Future Directions and Limitations

This study focused on the effect of curing temperature on the environmental impact of using SCBA as a partial replacement of cement in the production of mortars (cradle-to-gate). Future directions are (i) extending the analysis considering a cradle-to-grave analysis, thus, including durability as a parameter, and (ii) assessing the effect of temperature on the sustainability of other alternative SCMs and wastes. Besides, this study was focused on mortar. Therefore, the analysis of other cementitious composites such as concrete or plaster, as well as a comparison between them to assess the best usage of this waste, will be of great interest. Finally, the principle of this research should be applied to other types of waste.