Lime has been used in masonry mortars for millennia, and many old buildings remain stand- ing today with purely lime technology, despite its low compressive strength compared with ce- ment. Modern approaches tend towards stronger and more durable materials to minimise repair cycles and labour costs, but at the detriment of traditional buildings. Cement -based mortars used to repair historical masonry has accelerated the decay of old buildings, which had previously been in reasonable condition for decades. This has seen the conservation industry return to con- sider lime technologies as a more compati ble solution for solid masonry buildings. Life cycle analysis is used to assess environmental impacts of materials across various cate- gories throughout the different stages of their interaction with humans and the wider environ- ment. Despite an increasing number of life cycle analyses investigating mortars, few studies compare the impacts of cement -based and lime -based mortars , and those that do rarely consider the implications for heritage repair and maintenance. Life cycle analysis is used to estimate global warming potential associated with a repair in- tervention taking historical sandstone ashlar masonry as a case study , and considering iterative replacement cycles under two different repair methods . Global warming potential per unit area (m²) over 100 years for cement -based mortar is found to be between 43 – 348 % greater than scenarios using lime -based mortars. Sensitivity analysis finds the choice of functional unit to greatly impact outputs, such that when comp aring mortars, that with highest emission intensity can change. Careful selection is needed of the most relevant functional unit for a specific case. ing materials as the usage and demolition stages can vary depending on the product and its ap- plication (Churcher, 2013) . According to the English Housing Survey (Ministry of Housing, 2020) over 73 % of dwell- ings in England were more than 39 years old and 20.1 % were built before 1919, while 29.2 % of dwellings had solid walls. Traditional buildings of solid wall construction allow moisture transport through liquid capil- lary flow and through vapour diffusion , however they avoid damp and mould through heating, ventilation and through the use of materials that allow rapid drying such as traditional lime mor- tar (Hughes, 2020, Little et al., 2015) . More recently it has been found that modern cement m ortars are accelerating substrate decay when used on solid masonry and are often considered incompatible with traditional masonry buildings (British Standards Institution, 2013, Nežerka et al., 2015) . Compatible repair materials should prolong the lifetime of the substrate and therefor e be sacrificial in nature (Hughes and Valek, 2003) . An increased decay rate due to incompatible materials is evident on many mason- ry structures (Mitchell, 2007, van Hees et al., 2004) . This consequently increase s the amount of material needed for subsequent repairs , compounding environmental impacts , however , this im- pact has not yet been quantified. 1.1 Goal and scope This study aims to compare the impact of commonly used cement mortar to a best practice re- pair mortar for the conservation of historical sandstone structures. This study consider s only the impact category global warming potential (GWP100) which is a characterization model for ra- diative forcing over 100 years, measured in mass of carbon dioxide equivalent emissions (kgCO 2eq), as defined by ISO/TR 14047 (BSI, 2012) . The lifecycle stages consider ed are: extraction of raw materials, processing, manufacturing and use stages, including carbon sequestration and maintenance cycles, however transport and demolition stages are ignored as these are assumed equal in all cases. Emissions associated with binders are combined with those for other mortar components at relevant mix ratios. Two dif- ferent masonry repair approaches are applied along with deterioration rates to find the impact of a traditional lime conservation approach compared to a cement mortar considered incompatible. 1.2 Literature review Santos et al. (2021) reviewed LCA s on mortars and although they found 124 studies, the ma- jority focused on cement -based mortars , while many did not provide detailed boundary condi- tions . Compar ison between studies with undeclared or different boundary conditions is difficult , highlighting the importan ce of LCAs that compare multiple mortar mixes to the same boundary conditions. Several LCA studies have compare d modern cement -based and traditional lime- based mortars, a summary of these is found in Table 1. In their cradle to gate LCA study Diaz -Basteris et al. (2022) compared mortars of cement and lime over 100 years, and found that the binder production had the highest impact on GWP. Pineda et al. (2017) studied both physical and environmental performance of grouts using dif- ferent binders: cement , hydrated air lime and natural hydraulic limes (NHL). Usage and demoli- tion were excluded from the LCA, however , as the intended use was in historical structures an incompatibility factor was developed based on chemical and mechanical characteristics. Silva et al. (2015) also studied compatibility of mortars comparing similar binders and included physical properties such as pore size distribution, water absorption and drying index . However, neither of these studies related compatibility to environmental impacts. Table 1. Summary of lifecycle assessments compar ing global warming potential (GWP) of cement -based and lime -based mortars. Values in parenthesis denote GWP after sequestration due to lime carbonation . Reference Life cycle type Life cycle inventory data source Material s (fuel source in pa- renthesis) Binder GWP Mix ra- tio (mass based) Mortar GWP kgCO 2eq/ton kgCO 2eq/t on (Pineda et al., 2017) Cradle – gate Ecoinvent Hydrated air lime 766.4 Cement 833.4 Hydrated air lime, pozzo- lan, limestone 1:1:0.48 350 Cement, silica fume 1:0.33 500 (Diaz -Basteris et al., 2022) Cradle – grave Collected by survey, interview & produc- tion site visits Hydrated lime (coke) 1518 (983) Hydrated lime (gas) 1193 (658) Cement (coke) 1538 Cement (gas) 1193 Hydrated lime, sand (coke) 1:2.33 587 Cement, sand (coke) 1:2.33 737 (Moropoulou et al., 2005) Cradle – grave n.a. Hydrated air lime, sand 1:2.33 0.17 Hydrated air lime , sand + 5% metakaolin (pozzolan) 1:2.8:0.2 0.16 Cement, sand 1:2.33 0.23 2 METHODOLOGY When using LCA studies to inform material choices each study must be reviewed carefully to ensure that the comparison is relevant to the specific application. Mortar mixes are herein de- signed to represent the most common cement -based approach compared to a repair mix recom- mended by conservation literature. The cement mortar is a 1:5 volume ratio sand and cement, using local soft building sand and widely available Multicem CEMII. The lime mortar is intend- ed to replicate a traditional mix using CL90 quick lime pieces ( 5-0mm), well graded washed sharp sand sieved to a maximum of 2mm, and wood ash from a commercial biomass boiler sieved to 2mm as a pozzolanic additive (Copsey, 2020, English Heritage, 2011) . The mortar mix proportions are detailed in Table 2 (not the exothermic reaction of water and lime produced steam) . Table 2. Mortar mix proportions Mortar mix Components Volume ratio Mass ratio Binder mass per mortar volume Water/binder ratio Flow rate kg/m3 kg/kg mm Cement Cement, building sand 1:5 1:5.8 234 1.5 143.5 Lime -ash Quicklime, sharp sand, wood ash 1:3:0.1 9 1:2.9:0.1 338 2.1 132.7 Sand s were dried to constant mass at 70°C ensuring water content was controlled. Dry ingre- dients (sands and binder) were first mixed before water was added meaning a strong exothermic reaction took place for the lime mortar and steam was released. Mortars were mixed by hand for 5 minutes following BS EN 1015 -2 (BSI, 2007a) , flow rate was determined by procedures in BS EN 1015 -3 (BSI, 2007b) . 3 INVENTORY ANALYSIS Life cycle inventory (LCI) figures tend to account for cradle to gate emissions including materi- al extraction, transport to plant and manufacturing. Figures for GWP of lime and cement binders vary considerably according to (Anderson and Moncaster, 2022) and as seen in Table 1, high- lighting the importance to use data for the actual materials used . Inventory cradle -gate figures are taken for the specific products used . GWP for quicklime is taken from the manufacturer’s report (Lhoist, 2022) , and although there is no explanation of calculation methods this figure is very similar to the value commissioned by the European Lime Agency (EuLA ) presented in the GABI database (EESAC, 2019) , which is fully compliant with the ILCD Data Network and standards ISO 14040 and 14044 . The GWP for cement (BRE Global Ltd., 2019) is taken from an environmental product declaration (EPD) verified by BRE Global to be compliant with BS EN 15804 (based on EN ISO 14044) and is a similar value to many other EPD’s for similar products . For sand the GWP is taken from a cradle to gate LCA conducted by Grbeš (2015) of silica sand following the ReCiPe method , assum ing here all sand followed the basic wet pro- cessing . The GWP of water is taken from Ecoinvent database. The wood ash pozzolan is a waste product so is considered to have zero GWP . Several functional units are used in this study: initially the GWP per mass of mortar is calculat- ed (kgCO 2eq/kg) without considering maintenance or replacement cycles , then GWP is calcu- lated per unit area of masonry (kgCO 2eq/m2) for a particular case considering replacement cy- cles as explained in section 5. 4 IMPACT ASSESSMENT The GWP for each mortar is presented in Table 3. As a higher proportion of binder was used in the lime -ash mortar this made up a higher proportion of the impact . Table 3 Global warming potential and carbonation percentages for mortars used. Mortar GWP per mass binder % Process emissions % Carbon- ation Mass binder per volume mortar GWP per mass mortar GWP per volume mortar kgCO 2eq/kg % % kg/m3 kgCO 2eq/kg kgCO 2eq/m3 Lime -ash 1.204 65.7% 84.7 338 0.112 184 Cement 0.826 n.a. n.a. 234 0.105 198 Carbonation of the lime -ash mortar is slightly reduced due to the calcium hydroxide con- sumed by the mass of ash pozzolan ( 1.294 g Ca (OH)2 consumed by 1 g ash , value taken from Pavlíková et al. (2019) who used the Chapelle test on wood -based biomass ash). As quicklime (CaO) was used which expanded with the addition of water, the percentage of the available free lime consumed by the ash was calculated based on experimental results. It is assumed that 90% of the remaining available lime carbonates before end of life (as reported by EuLA (Ecofys, 2014) and in agreement with Figueiredo (2018) ), giving 85% carbonation of the Ca(OH) 2 avail- able in 1 kg CaO . 5 COMPATIBILITY IMPACT The historical precedent viewed in heritage structures around the world suggest s that traditional lime pointing can remain in good condition for many decades, however , this depends on appli- cation efficacy , curing conditions, exposure and weather conditions (Papayianni and Hughes, 2018) . Pointing mortars applied correct ly are expected to last at least 30 years, and normally 50 - 100 years according to Maurenbrecher et al. (2001) , whereas Boynton and Gutschick (1964) suggest 15 years for cement tuck pointing and 50 -500 years for lime mortars. A conservative es- timate therefore is to assume repointing is necessary every 35 years, meaning raking out of old mortar and repointing would be carried out three times over 100 years. Despite the widely stated incompatibility of cement mortars on historical masonry structures, only one study has been found to focus on quantifying the effect of cement repointing on the de- terioration rates of such structures. To estimate deterioration rates of medieval sandstone churches André et al. (2014) took masonry sections that retained their original finish as (non - decayed) reference points and compared these to more degraded areas using three dimensional LiDAR laser scanning. For white sandstone t hey found that areas retaining traditional lime mor- tars decayed at a rate of 3.5-4.8 mm/century while areas repointed with cement mortar in the 20th century degraded between six to seven times faster (21.2, and 35.5 mm/century), and in some areas up to ten times (André et al., 2014) . Masonry repair work can follow different strategies , and here two opposing strategies are proposed , named as method A and method B . Method A would follow the remov al of deterio- rated stone and mortar, repoint ing joints flush with the substrate and use of minimal “plastic ” (mortar) repair to ensure rain water drains off. Method B involves restoring the stone -work back to the original face using “ plastic ” mortar repair, thus large areas of decayed stone are rendered and joints highlighted by inscribing a line into the mortar. An example of a historical sandstone masonry wall can be seen in Figure 1 with representations of methods A and B depicted. These were used to estimate percentage area and volume of mortar needed per function al unit (1 m2) of stone masonry under each repointing method . Figure 1. Masonry decay and repair methods on representative masonry at the grade II* listed Elizabethan Lymm Hall (Author, 2021) In the example case study (Fig. 1) : method A require s repointing joints and plastic repair ac- counting for 8 % of the area, whereas method B requires that all degraded areas are covered with mortar , at 26% of the area . Joints are on average 3mm wide and assumed repointed to 20mm deep, plastic repairs are assumed on average 5mm deep and one deeper area (highlighted in yel- low in Figure 1) is assumed 30mm deep. For repair method A this deeper area is filled with stone tiles taking up half the volume (leaving the other half filled with mortar), whereas for method B the whole volume is filled with mortar. For each repair method the total volume of mortar needed is calculated for the current condi- tion. Decay rates from (André et al., 2014) were used taking the average traditional mortar de- cay rate 4.15 mm/century for lime-ash mortar , and the average decay rate after cement repoint- ing 28.35 mm/century for cement mortar . For each this was applied for a 35-year intervention and then a 70-year intervention and aggregated along with the initial repair to determine the ma- terial required for three interventions over one century. This enabled calculation of GWP for each method . 6 RESULTS & DISCUSSION It is plausible that a laborer unaware of compatibility issues would choose method B as a lack of research would imply little desire to retain the aged historical patina, whereas method A would be a more conservative approach, following guidance form those such as the Society for the Protection of Ancient Buildings (Slocombe, English Heritage, 2011) . Therefore, Table 4 pre- sents impacts for GWP100 for the lime -ash mortar under method A, along with that for cement mortar under method B. A sensitivity analysis evaluated the impact of cement mortar under re- pair method A. A further sensitivity check used the LCI data of Diaz -Basteris et al. (2022) for lime and cement binders , with results reported in Table 4 . Table 4 Volume of mortar used and GWP100 for mortars under different repair methods [1] (Diaz -Basteris et al., 2022) The results emphasize the importance of both the selection of LCI data and the choice of repair method , altering the GWP by 1.43 and 2.18 times respectively in the most significant cases. Comparing results to those of Diaz -Basteris et al. (2022) who found that a cement -based mortar produced 26% more emissions than a lime -based mortar per ton over 100 years , here the GWP of the cement mix was 43 – 348 % higher than the lime mix per m2 of masonry . There are sev- eral differences between the studies , most notably the present study considered maintenance cy- cles and the impact of cement on masonry decay . If mortars are compared using repair method A for both, the GWP of the cement mortar was 1.43-2.06 times that of the lime -ash mortar, de- pending on the LCI source data chosen. In all cases the cement mortar has a higher GWP than the lime -ash mortar, however, the choice of repair method can have a greater influence on GWP than the binder type. The approach illustrated in the present study has developed that of Pineda et al. (2017) by in- corporating the impact of incompatibility on GWP . The present work has considered one case study for repair and although it was intended to be representative, the calculation of impacts depend on the initial condition. Furthermore, the de- cay factors are extracted from the study of a different building and ideally would be determined on a case -by-case basis. 7 CONCLUSION S This study set out to compare GWP100 of a commonly used cement mortar to a best practice lime mortar for the repair of one meter squared of historical sandstone masonry. The approach incorporated the increased deterioration caused by incompatible cement mortars compared to a standard deterioration rate applied to the traditional lime mortar. The results imply that cement mortars have a higher GWP100 by a factor of 1. 43 to 4.48, depending on the LCI source data and repair method used. In the case study presented the u se of lime instead of cement mortar saves up to 8.93 kgCO 2eq per meter squared of stone masonry over 100 years (using product LCI data) . Sensitivity analysis investigated the impact of the repair strategy used as well as the source LCI data, finding that both have a significant effect on the results, thus emph asizing the need for a case -by-case approach if accurate GWP is sought when compar ing mortars. In all scenarios the cement mortar remained the most emission intensive , highlight ing the im- portance of selecti ng appropriate and compatible materials not only for the longevity of histori- cal masonry, but also to minimize GHG emissions associated with the volume of material re- quired for repairs. Mortar Repair method Volume of mortar used in 100 years per m2 masonry Total GWP100 for 1m2 stone masonry % increase over low- est GWP Total GWP100 for 1m2 stone ma- sonry using LCI data from [1] % increase over lowest GWP m3/m2 kgCO 2eq/m2 % kgCO 2eq/m2 % Lime -ash A 0.0034 4.3 lowest 4.2 lowest Cement B 0.0188 13.2 209 18.9 348 Cement A 0.0103 6.1 43 8.7 106 8 ACKNOWLEDGEMENTS The Low Carbon Eco -Innovatory Research Programme funded through the European Regional Development Fund at the University of Liverpool as well as Ecospheric Ltd. are acknowledged for financial support. Any opinions, findings and conclusions expressed in the manuscript are those of the authors and do not necessarily reflect the views of the funders. This is a preprint of the following chapter: Dickens, L.A. & Di Sarno, L., Global Warming Potential Comparison of Lime and Cement -Based Masonry Repair Mortars , published in To- wards a Carbon Neutral Future , edited by Papadikis, K., Zhang, C., Tang, S., Liu, E., Di Sarno, L., 2024, Springer, reproduced with permission of Springer Nature Singapore Pte Ltd. . The final authenticated version is available online at: https://doi.org/10.1007/978 -981-99-7965 -3_59 .