Study of different grouting materials used in vertical geothermal closed-loop heat exchangers

Borinaga-Treviño, R., Pascual-Muñoz, P., Castro-Fresno, D. & Del Coz-Díaz, J. J.

Author post-print (accepted) deposited by Coventry University’s Repository Original citation & hyperlink:

Borinaga-Treviño, R, Pascual-Muñoz, P, Castro-Fresno, D & Del Coz-Díaz, JJ 2013, 'Study of different grouting materials used in vertical geothermal closed-loop heat exchangers' Applied Thermal Engineering, vol. 50, no. 1, pp. 159-167. https://dx.doi.org/10.1016/j.applthermaleng.2012.05.029

DOI 10.1016/j.applthermaleng.2012.05.029 ISSN 1359-4311 Publisher: Elsevier NOTICE: this is the author’s version of a work that was accepted for publication in Applied Thermal Engineering. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Applied Thermal Engineering, 50:1, (2013) DOI: 10.1016/j.applthermaleng.2012.05.029 © 2012, Elsevier. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0/ Copyright © and Moral Rights are retained by the author(s) and/ or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This item cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder(s). The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. This document is the author’s post-print version, incorporating any revisions agreed during the peer-review process. Some differences between the published version and this version may remain and you are advised to consult the published version if you wish to cite from it.

Post-print version: final draft post-refereeing (released with a Creative Commons Attribution Non-Commercial No Derivatives License).

Roque Borinaga-Treviño, Pablo Pascual-Muñoz, Daniel Castro-Fresno, Juan José Del Coz-Díaz. “Study of different grouting materials used in vertical geothermal closed loop heat exchangers”. Applied Thermal Engineering 50 (2013) 159-167. ISSN: 1359-4311. https://doi.org/10.1016/j.applthermaleng.2012.05.029

http://www.sciencedirect.com/science/article/pii/S1359431112004000

Study of different grouting materials used in vertical geothermal closed loop heat exchangers

Roque Borinaga-Treviño a *, Pablo Pascual-Muñoz a 1, Daniel Castro-Fresno a 1, Juan José Del Coz-Díaz b 2

a Department of Transports, Projects and Processes Technology, University of Cantabria (UC) Avda. de Los Castros s/n, 39005 Santander, Cantabria, Spain

b Department of Construction and Engineering of Production, University of Oviedo Edif. Dep. Viesques no7, 33204 Gijón, Asturias, Spain

Abstract

As a renewable source, low-enthalpy geothermal energy is becoming more relevant in heating and cooling buildings, by using an adapted heat pump to exchange thermal energy with the ground. Vertical closed-loop geothermal systems exchange heat with the ground through a closed buried pipe system, sealed with a grouting material that ensures the stability and thermal transmission of the borehole. Different grouting materials have been tested recently, which use cement or bentonite as a base material. However, the use of recycled materials, which might contribute to the sustainability of the project, has not yet been studied. This paper analyzes the use of different natural and recycled aggregates as main constituents in cement-based mortars. Results show that all mixes fulfill the minimum consistency and strength requirements. The use of any of the aggregates proposed improves the thermal conductivity compared to the cement mortar on its own, independently of the proportion used. Limestone sand, silica sand and electric arc furnace slag enhance the thermal conductivity of the grout as its proportion of use increases. However, no satisfactory results have been obtained for Construction and Demolition Waste based mixes because of their high water requirement.

Keywords: Geothermal; Mortar; Grout; Conductivity; Recycled material.

1. INTRODUCTION

Due to the increasing fossil fuel prices and global warming effect, some leading countries have started to find new renewable energy sources. According to Omer [1], 40% of the worldwide energy is consumed in lighting, heating and cooling buildings. Most heating and cooling systems use fossil fuels or air based heat pumps, which have low efficiencies and emit CO2 and NOx, thus contributing to the global warming process. Geothermal energy has proven to be a clean, efficient source, which has, moreover been declared renewable by the European Union 2009/ 28/CE directive [2]. A geothermal heat pump system takes advantage of the year-round constant ground temperature to obtain higher efficiencies than conventional air to air or air to water heat pumps. Instead of using ambient air as a heat source or sink, geothermal pumps use the ground as a heat exchanger.

According to Florides [3], ground heat exchangers are divided into two main groups, open and closed geothermal systems. Open geothermal systems use the existing groundwater directly to exchange heat with the ground, extracting it from a shaft or injecting it to the ground. Closed geothermal systems use a heat carrying fluid which flows through a buried pipe circuit and exchanges heat indirectly with the ground. There are two main advantages that made closed geothermal systems more interesting than open ones. On the one hand, they do not need to have an aquifer to be operational, widening their field of application. On the other hand, even when an aquifer is available their influence on the groundwater table is smaller than that of open systems, except for large geothermal borefields, where thermal affection must be further evaluated.

At the same time, closed systems are classified in two main groups, depending on the geometry of the resulting heat exchanger. Horizontal closed-loop systems, which are buried at depths up to 5 m, are affected by exterior climate conditions. Because of the low depth needed, horizontal systems are easy to install and hence, economical when a land plot is available. However, this is not applicable in many places where land is scarce. Vertical closed-loop systems, also called Vertical Ground Loop Heat Exchangers (VGLHE) are a possible option when there is little available land. A closed pipe circuit is introduced into a vertical borehole reaching depths of up to 200 m. The stability of the borehole walls is ensured by injecting a grouting material, but at the same time, this material must present good thermal properties to transmit heat from the pipes to the ground or vice versa. Philippacopoulos, Allan et al. [4,5] simulated the temperature of the grouting in a simple U shape VGLHE, evaluating the influence of the grouting, pipe placement and bonding strength on the grouting temperature distribution. They concluded that bonding quality between pipe and grout is more significant than that between grout and the existing ground. They also added that gaps diminish the heat flux in the pipe surroundings, decreasing the efficiency of the system, thus grouting with high shrinkage when drying should be avoided. Influence of the grout to formation thermal conductivity ratio in heat transfer is also analyzed by Smith and Perry [6], concluding that thermal conductivity of the grout should be equal to or greater than the formation’s not to decrease the system efficiency. Finally, Chulho Lee et al. [7] proposed a thermal conductivity range of 1.7-2.1 W/(m K) for grouts combined with most existing ground types.

According to Allan and Philippacopoulos [5], cement alone and bentonite mixes, with a thermal conductivity of 0.80-0.87 W/(m K) and 0.75-0.80 W/(m K), respectively, were predominantly used in the United States until 2000. However, these values were significantly reduced by the loss of water, due to the high porosity of the grouts. Bentonite-based grouts have been improved significantly in the last decade. Smith and Perry [6] tested the performance of bentonite-based grouts in a real installation, comparing standard bentonite grouts to the enhanced bentonite-cement sand mix. Carlson [8] compared the performance of the enhanced bentonite grout proposed by Smith with standard bentonite grouts concluding that the total borehole length required could be reduced by 10% in the former case. Chulho Lee et al. [7] improved the thermal conductivity of the grout either by using silica sand or graphite. They demonstrated greater improvement when graphite was used, obtaining up to 3.5 W/(m K) of thermal conductivity for the resulting grouting materials. Recently, Delaleux et al. [9] used Compressed Expanded Natural Graphite to enhance bentonite grouts, obtaining a thermal conductivity of 5 W/(m K), hence this grout could be used in most existing ground types. However, Delaleux et al. [9] also warned that for each 10% reduction in water content by weight there is a reduction in the thermal conductivity of the grout by 1W/(m K). In dry state, graphite enhanced bentonite grouts have values ranging from 1.5 to 2 W/(m K), due to the volume shrinkage of the mix.

The grouting materials studied in this paper are also intended to provide a preliminary analysis of the grouting materials used for thermally active foundations, which require a compressive strength similar to that of the usual structural concrete. As bentonite-based grouts do not fulfill this minimum strength requirement, cement based grouting materials are proposed instead. Geothermal mortars have not been fully investigated in the last decade. Allan and Philippacopoulos [10] presented a complete characterization of different cement-sand based mortars, studying, the influence of superplasticizer and the aggregate type on the rheological, mechanic, hydraulic and thermal properties. As a consequence, Allan and Philippacopoulos [5,10,11] presented a superplasticized cement-sand mortar to be used as grouting material, obtaining a thermal conductivity of 2.42 W/(m K) which only reduced to 2.16 W/(m K) when oven dried. However, all the proposed aggregates were silica sands or quartzite sands. Therefore, the final objective of this paper is to analyze the possibility and/or convenience of using different aggregates as the main material in geothermal grouting materials.

2. EXPERIMENTAL METHODOLOGY

2.1 MATERIALS

All the mortars designed are made of water (w), cement (c), superplasticizer (sp) and different types of aggregates (si). CEM II- B (V)/ 32.5R type cement has been chosen according to EN 197-1 [12]. This cement is a mix of Portland cement and up to 35% of siliceous fly ash by weight, valid for the use in foundations. Its compressive strength is higher than 10 and 32.5 MPa at an age of 2 and 28 days, respectively.

Silica (S), limestone (L), electric arc furnace slag (EAF) and Construction and Demolition Waste (CDW) sands have been chosen as basic aggregates. Silica and limestone sands are common natural aggregates in Spain. EAF slag is a by-product of the steel making process of the Global Steel Wire Company in Santander. Its use as coarse and fine aggregate of concrete has been successfully proven by many authors. [13-15]. However, the CDW aggregate used in this research does not have any application in Spain at this time. The original CDW comes from a crushed-concrete recycled aggregate plant. The final commercial product is a 0-60 mm recycled coarse aggregate, obtained after a crushing and selection process. The first step of this process consists of cleaning the raw material, thus eliminating all the surface imperfections before crushing it. As a result, a 0-6 mm sized recycled-sand aggregate is obtained, which has until now been discarded because the current Spanish structural concrete standard (EHE08) [16] does not allow the use of CDW recycled aggregates with a size of less than 4 mm. In this research, maximum grain size is limited to 2 mm in all the aggregates to ensure the pumpability of the mix, according to Allan [11].

Table 1. Specific gravity and water absorption (EN 1097-6) and Grain-size distribution of the aggregates (EN 933-1).

Aggregate Specific gravity

Water absorption

Sieve size (mm) 4 2 1 0.5 0.25 0.125 0.063

Passing percentage L S

EAF CDW

F

2.71 0.52 100 99 61 40 28 21 16 2.65 0.16 100 100 80 67 47 26 17 3.82 1.83 100 100 39 11 5 3 2 2.57 5.07 100 100 73 48 29 19 12 2.75 N/A 100 100 100 100 97 89 75

Grain-size distribution of the aggregates by weight was determined according to EN 933-1 (Table 1) [18]. Both EAF slag and CDW present lack of fines, hence limestone filler (F) was proposed to correct their size distribution. Apparent particle density (ρs) and water absorption was also obtained according to EN 1097-6 [17], showing significant density differences between aggregates. As a consequence, volume-weighted grain-size distribution was evaluated to determine the replacement factor of EAF and CDW with limestone filler, resulting in a 25% and 10% replacement factor by weight, respectively (Table 2). While EAF-based aggregate has similar size distribution to the limestone sand, the CDW-based aggregate lies between silica and limestone values.

Table 2. Size distribution by volume of the aggregates studied.

Aggregate Sieve size (mm)

4 2 1 0.5 0.25 0.125 0.063 Passing percentage by volumen

L 100 99 61 40 28 20 16 S 100 100 79 66 47 27 18

EAF(75%)/F(25%)a 100 100 59 39 34 30 25 CDW(90%)/F(10%)a 100 100 76 53 36 25 18

a Percentage of the aggregate used by total weight of the resulting aggregate.

The superplasticizer (sp) used is a Melment F10 provided by the manufacturer, utilized to reduce the water to cement ratio, to improve the pumpability of the grout and to prevent segregation. The dosage proposed by the manufacturer is between 0.5% and 2% by unit weight of cement. Allan [19] observed a significant improvement of the rheology of the neat cement grouts with the increase of the sp dosage. Therefore, the maximum dosage recommended by the manufacturer was used for all the mixes.

2.2 FRESH MORTAR CHARACTERIZATION

Mortars are the result of mixing cement, aggregates, water and additives, creating a viscous mix that hardens after a variable time. According to Gustafson [20], when mortar is in its fresh state, it displays a viscous Bingham fluid behavior that permits its injection through a pipe to the required distance or depth. The flow table consistency test (EN 1015-3) [21] is used to determine the water to cement ratio of the mix proportions to meet the fluid consistency of 260-320 mm imposed on the grout, close to the self-cast mortars. Fig. 1 shows the flow table and the mold used, as well as the resulting diameter observed in one of the tests.

To complete the characterization of the fresh mortar, bulk density (ρm-f) is also determined according to EN 1015-6 [22]. Self-compacting ability was confirmed by filling the test recipients, since no compaction energy was required for this. All the mix proportions that lead to segregation after a visual inspection of the consistency test have been directly discarded.

Fig. 1. Flow table and mold used (a). Result of one of the mixes (b).

2.3 HARDENED MORTAR CHARACTERIZATION

Three specimens were made for each of the following tests. For 24 h after making the mix, specimens were cured under ambient laboratory conditions, until molds were removed. From then, until the specified curing age is reached for each of the test specimens, they were submerged in water at 20 °C to simulate curing in water saturated ground. The container was, at the same time, in a temperature and moisture controlled chamber at 20 °C and 95%, respectively. Dry bulk density (ρm-h) of the hardened mortar is determined at an age of 28 days according to EN 1015-10 [23]. Based on the results obtained for the determination of ρm-h, water-accessible porosity (n) is calculated (Equation (1)). msat and mdry are the water-saturated and oven dried mass values, respectively. Specimen volume (Vmold) is calculated based on the mold’s volume, 256·10-6 m3.

n = (msat – mdry) / (ρw Vmold) (1)

Compressive strength of the hardened mortar is determined at an age of 7 (σc,7) and 28 (σc,28) days according to the standard EN 1015-11 [24]. No minimum strength is required for the grouts used in geothermal vertical heat exchangers, unless the grout is proposed for thermally active piles. If the grout is considered as a non-structural concrete, a minimum compressive strength of 15 MPa is recommended by the Spanish EHE08 norm [16].

Finally, thermal conductivity of the hardened mortar (λm-h) is determined at an age of 14 days with the TCi C-Therm thermal conductivity tester, as seen in Fig. 2. Transient Plane Source (TPS) method has been used to determine the thermal conductivity of mortars since no specific standard has been found for this. TPS was developed by Gustafsson [25] for the determination of thermal properties of homogeneous solid materials. It has been successfully used to determine thermal properties of highly porous building materials [26], insulation building materials [27] and hydrating cement pastes [28]. For carrying out the test, a 75mm diameter and 80 mm high cylindrical specimen was chosen. To improve the thermal contact between the sensor and the specimen, silicon oil has been used since the software of the equipment has already calibrated its influence on the measured thermal conductivity. Prior to the realization of the test, specimens were oven dried at 105 °C for 48 h and left at 20 °C, the ambient temperature of the laboratory, for at least 7 days to reach thermal equilibrium. Final thermal conductivity is calculated as the mean of the 9 tests carried out (as test is non-destructive, 3 non-consecutive test were performed per specimen, resulting in 9 test result per mix proportion).

2.4 MIX PROPORTIONS

Mix proportions have been defined by the proportion of the different materials by unit weight of cement used. Aggregate to cement ratios (s/c) of,1, 2 and 3 were set for each of the aggregate type mentioned

before. SP proportion (sp/c) was fixed to 0.02 for all the batches. Neat cement with a water to cement (w/c) ratio of 0,3 and sp/c of 0,02 is used as reference material. Finally, water to cement ratio is determined by imposing a 260-320 mm slump diameter in the flow table test (EN 1015-3) [20], to obtain similar rheological properties for all the mortars. For that purpose, a mortar laboratory mixer with a 4 l capacity was utilized. For the execution of the final valid mixture, a horizontal planetary mixer was used, with a 150 l of capacity. Both of the mixers are shown in the Fig. 3. The final mix proportions expressed by unit weight of cement are shown in Table 3.

Fig. 2. Thermal conductivity test. Fig. 3. Laboratory mixer for mortar (a) and horizontal planetary mixer (b).

Table 3. Mix design proportions calculated for the fixed flow table consistency.

Mix s1 s2 s/c s1/c s2/c sp/c w/c 1 0 0 0 0 0 0.02 0.30 2 L 0 1 1 0 0.02 0.30 3 L 0 2 2 0 0.02 0.37 4 L 0 3 3 0 0.02 0.43 5 EAF F 1 0.75 0.25 0.02 0.30 6 EAF F 2 1.50 0.50 0.02 0.40 7 EAF F 3 2.25 0.75 0.02 0.49 8 CDW F 1 0.90 0.10 0.02 0.40 9 CDW F 2 1.80 0.20 0.02 0.62 10 CDW F 3 2.70 0.30 0.02 0.93 11 S 0 1 1 0 0.02 0.34 12 S 0 2 2 0 0.02 0.50 13 S 0 3 3 0 0.02 0.54

3. RESULTS AND DISCUSSION

3.1 FRESH STATE PROPERTIES

Fig. 4 shows fresh mortar bulk densities for different aggregates and proportions, respectively. All the aggregates, except the CDW one, display higher densities as their proportion of use increases, denoting the influence of the apparent particle density of the aggregates. The decrease observed in the CDW mix is because the high water absorption of the aggregate, 5.07%, significantly increases the water to cement proportion required. The influence of the aggregate in the fresh bulk density is directly dependent on its type and proportion of use. L aggregate presents similar values for the s/c = 1 and 2 mix proportions, attributed to the w/c ratios of the mixes (s/c = 1 mix has a w/c ratio of 0.34 and while s/c = 2 mix is 0.5). As a consequence, the higher density of the aggregate for the s/c = 2 mix is compensated by the higher density of the cement paste of the s/c = 1 mix. For all the cases evaluated, fresh mortars are found to be denser than the bentonite-water mix present in most of the boreholes 1050 kg/m3 [29]. Therefore, it is not expected to have any difficulties in pumping the bentonite-water mix used during the drilling process out of the borehole.

Machine

Specimen

Sensor

Silicon gel

Fig. 4. Fresh mortar densities for different aggregates and proportions.

3.2 HARDENED STATE PROPERTIES

Compressive strength at 7 and 28 days is shown in Fig. 5. Compressive strength decreases as aggregate to cement ratio increases for all the aggregates evaluated. All the observed values are above the minimum of 15 MPa required by the EHE08 norm [16]. Therefore, all the mixes evaluated could be used taking into consideration this criterion. However, the resistance value of CDW mixes decreases sharply compared to the rest of the aggregates, indicating bad bonding strength in the cement paste because of its high water absorption. Results also show that the improvement in compressive strength at 28 days over the results observed for 7 days of curing is insignificant. Hence, it is sufficient to determine compressive strength at 7 days of curing for design purposes, shortening the testing time. L, S and EAF-based grouting materials display sufficient compressive strength to be used in thermally activated foundations, but further investigation is necessary to determine their validity.

Fig. 5. Compressive strengths at 7 (a) and 28 (b) days of curing age.

Once mortar is hardened, part of the water used in the mix necessary to meet the required consistency is lost, affecting the dry bulk density and the resulting mortar porosity. A higher water to cement ratio implies an excess of water that increases porosity and hence decreases dry bulk density. This effect is also

present in this contribution, as is shown in Fig. 6. Unlike the fresh mortar bulk density, dry bulk density has higher values for s/c = 2 than for s/c = 3, indicating a greater loss of water in the latter. For the same reason, a minimum porosity value is obtained at s/c = 2 for all the aggregates analyzed. In the CDW mortars, a special increase of the accessible porosity is observed, due to the high water to cement ratio required by the mix.

Fig. 6. Aggregate proportion dependent hardened dry bulk density (a) and porosity (b).

Thermal conductivity is the main parameter that defined the thermal behavior of the grouting material. Depending on the ground conditions at installation, grout can be water saturated or in dry state. Since water is a better thermal conductor than air, dry state thermal conductivity should be considered as a more conservative value. First of all, influence of aggregate type and proportion of use on the resulting thermal conductivity of the grout are evaluated, as shown in Fig. 7.

Fig. 7. Thermal conductivities for different aggregate types and proportions.

Mean value of the thermal conductivity for s/c = 1, 2 and 3 is calculated for each aggregate type, showing a variation with respect to the s/c = 1 material of 12.8%, 6.6%, 20.1% and 14.5% is observed because of

its proportion of use. However, an increment of up to 202%, 234%, 150% and 142% is achieved due to the use of L, S, EAF and CDW aggregates, respectively, when compared to the pure cement reference material, 0.9 W/(m K). Therefore, the addition of any aggregate to the grout significantly improves thermal conductivity. Moreover, the type of aggregate used causes a greater influence on the thermal conductivity than its proportion, for the proposed mix proportions.

Mortar can be considered as a composite material which consists of aggregate solid particles, grout and air-filled or water filled voids. Although this relationship is unknown, it can be supposed that when specimens are oven dried, final thermal conductivity of the grout (λm-h) would depend on the thermal conductivity of the air (λa), the c-sp-w hardened paste (λp) and thermal conductivity of aggregate particles (λs) as well as on the volumetric proportion of each of them (Va, Vp and Vs). These volumetric parameters are explained in the Appendix A. The thermal conductivity of air of 0.026 W/(m K) at 300 K is obtained from the existing bibliography [30], significantly lower than would be expected for either the solid or the c-sp-w paste. Effective thermal conductivity of the c-sp-w paste is unknown. However, according to Allan [10], thermal conductivity of the pure cement grout decreases as the water to cement ratio increases.

Finally, thermal conductivity of the aggregate particles must be estimated. For that purpose, different models are used to estimate the effective thermal conductivity of the composite materials [31,32]. Since not all the variables are measurable in mortars, their validity cannot be proven. However, the proposed parameters could be used to seek tendencies. There is not any standardized method to evaluate this property. The best option would be to create a solid specimen from the uncrushed raw material. However, this option was not possible for either the EAF or especially for the CDW, because of their production process. In this contribution, water-saturated aggregate-alone effective thermal conductivity (λsa,eff) is measured to determine the final thermal conductivity of the particles, according to the ASTM D-5334 standard [33].

According to Zhang [31], series (2), quadratic parallel (3) and geometry-weighted models (4) are proposed to estimate the thermal conductivity of the solid particles.

𝜆𝜆𝑠𝑠𝑠𝑠,𝑒𝑒𝑒𝑒𝑒𝑒 = (1 − 𝜙𝜙𝑤𝑤)𝜆𝜆𝑠𝑠,𝑠𝑠𝑒𝑒𝑠𝑠𝑠𝑠𝑒𝑒𝑠𝑠 + 𝜙𝜙𝑤𝑤𝜆𝜆𝑤𝑤 (1)

𝜆𝜆𝑠𝑠𝑠𝑠,𝑒𝑒𝑒𝑒𝑒𝑒 = �(1 − 𝜙𝜙𝑤𝑤)(𝜆𝜆𝑠𝑠,𝑞𝑞𝑞𝑞𝑠𝑠𝑞𝑞𝑠𝑠𝑠𝑠𝑞𝑞𝑠𝑠𝑞𝑞1/2 ) + 𝜙𝜙𝑤𝑤𝜆𝜆𝑤𝑤

1/2�2 (2)

𝜆𝜆𝑠𝑠𝑠𝑠,𝑒𝑒𝑒𝑒𝑒𝑒 = 𝜆𝜆𝑠𝑠,𝑔𝑔𝑒𝑒𝑔𝑔𝑔𝑔𝑒𝑒𝑞𝑞𝑠𝑠𝑔𝑔(1−𝜙𝜙𝑤𝑤) ∙ 𝜆𝜆𝑤𝑤

𝜙𝜙𝑤𝑤 (3)

Where 𝜆𝜆Rs and 𝜆𝜆Rw are the thermal conductivities of aggregate particles and water, respectively. The thermal conductivity of water of 0.6 W/(m K) at 294 K is obtained from the existing bibliography [30]. Aggregate porosity (φw) has been calculated according to EN 1097-3 [34]. Table 4 shows the results obtained for the test (𝜆𝜆Reff and ϕw), as well as the estimated particle thermal conductivity for the different models and aggregates evaluated (𝜆𝜆Rs,series, 𝜆𝜆Rs,quadratic , 𝜆𝜆Rs,geometry).

Table 4. Thermal conductivity of the aggregate particles for different prediction models. Aggregate 𝜆𝜆Reff Φw (%) 𝜆𝜆Rs,series 𝜆𝜆Rs,quadratic 𝜆𝜆Rs,geometry

L 1.6 28 2.0 2.1 2.4 S 2.5 32 3.5 3.9 5.0

CDW+F 1.3 45 1.8 2.0 2.3 EAF+F 1.3 32 1.6 1.7 1.9

For all the prediction models proposed, a higher conductivity is observed in the CDW than in the EAF particles, which contrasts with the tendencies observed for each of the resulting mortars. Table 5 shows the results obtained for the proposed volumetric and thermal parameters. For the L, S and EAF-based mortars, accessible porosity has a similar value. As water to cement ratio increases, a decrease of λp is expected, but this phenomenon is compensated by the higher thermal conductivity and volumetric proportion of aggregate present in the mix. However, CDW mixes display a sharp increase in the water to cement ratio with the aggregate proportion and a significantly higher accessible porosity, which causes a sharp decrease in the thermal conductivity of the c-sp-w paste and air, respectively. The higher aggregate

thermal conductivity and its volumetric proportion in the mix are not sufficient to compensate this negative phenomenon, which would explain the decrease in the thermal conductivity with the increase in the aggregate proportion.

Table 5. Volumetric and thermal conductivity values for the evaluated mortars.

Mix s1 s2 w/c 𝜆𝜆𝑔𝑔 Volumetric proportions (%) Va Vs Vl

2 L 0 0.30 1.6 15 35 50 3 L 0 0.37 1.8 9 49 42 4 L 0 0.43 1.8 13 59 28 5 EAF F 0.30 1.4 17 29 46 6 EAF F 0.40 1.5 18 42 40 7 EAF F 0.49 1.5 18 48 34 8 CDW F 0.40 1.3 21 33 46 9 CDW F 0.62 1.2 18 44 38

10 CDW F 0.93 1.0 36 46 18 11 S 0 0.34 1.8 19 34 47 12 S 0 0.50 1.8 17 47 36 13 S 0 0.54 2.1 26 55 19

4. CONCLUSIONS

Limestone, silica, Electric Arc Furnace and Construction and Demolition Waste sands are proposed as main aggregates for mortars used in geothermal closed loop heat exchangers. Limestone filler has been used to correct the grain size distribution of the latter two, with a substitution rate of 25% and 10% by weight, respectively. A flow table test diameter of 260-320 mm has been used for the design of self compactable geothermal grouting mortars. This fact has been verified during the filling of the specimens and the realization of the fresh bulk density test. The objectives proposed in this paper have led to the following conclusions:

• The proposed mixes fulfill the minimum of 15 MPa of compressive strength required for non-structural material in the Spanish concrete standard. However, a maximum proportion of s/c = 2 is proposed for the Construction and Demolition Waste sand-based mortars due to the drastic decrease in the values that the mortars display as aggregate proportion increases.

• The use of the four aggregates proposed improved the thermal conductivity of the pure cement used as reference material and it is independent of its proportion of use for the range evaluated. Silica, Limestone, Electric Arc Furnace slag and Construction and Demolition Waste sand-based mortars have a maximum thermal conductivity of 2.1 W/(m K), 1.8 W/(m K), 1.5 W/(m K) and 1.3 W/(m K), respectively.

• Earth Energy Designer software [35] permits the estimation of the borehole thermal resistance by using the Multipole method [36]. For that purpose, a standard double U configuration is chosen, with a borehole diameter of 140 mm and pipe external and internal diameters of 32 mm and 26 mm, respectively. Effective borehole thermal resistances for Neat cement, L, EAF, CDW and S mortars has been estimated in 0.1139 (m K)/W, 0.0727 (mK)/W, 0.0808 (mK)/W, 0.0901 (m K) and 0.0665 (mK)/ W, respectively. As a consequence, the use of improved mortars permit a reduction of the total borehole length required of 16%, 12%, 9% and 18% for the L, EAF, CDW and S mortars compared to the one required by the neat cement reference material.

• Effective thermal conductivity increases as aggregate proportion increases for Silica sand, Limestone sand and EAF sand, while maintaining similar porosity values. CDW-based mortars decrease as the proportion of use increases, due to the high water content required for the mix. The high porosity observed in the corresponding mixes is attributed to the high water absorption of the aggregate.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the support provided by the GITECO Group of the University of Cantabria and the Department of Construction at the University of Oviedo. The research project ‘Development of systems for rainwater catchment and storage through the use of permeable pavements to

assess its non-potable uses as a resource of low-enthalpy geothermal energy (REV)’ [BIA2009-08272] of the University of Cantabria is possible thanks to the finance of the Research, Development and Innovation State Secretary of the Spanish Ministry for Economy and Competitiveness. Finally, authors wish to acknowledge the financial support provided by the research projects FICYT FC-10-EQUIP10-17 and BIA-2008-00058.

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NOMENCLATURE

Aggregate types L: limestone sand S: silica sand EAF: Electric Arc Furnace slag sand CDW: Construction and Demolition Waste sand F: limestone Filler

Design of the mix proportions w: water kg c: cement kg sp: superplasticizer kg s: aggregate kg w/c: water to cement ratio kg/kg sp/c: superplasticizer to cement ratio kg/kg s/c: total aggregate to cement ratio kg/kg

Properties ρ: bulk density kg/m3 n: accessible porosity m3/m3 msat: water-saturated mass kg mdry: oven dried mass kg λ: thermal conductivity W/(m K) V: volumetric proportion of the hardened mortar m3/m3 φ: porosity of the aggregate m3/m3

Subscripts m-f: mortar in fresh state m-h: mortar in hardened state a: air s: aggregate’s particles w: water p: cement-superplasticizer-water hardened paste sa: aggregate-alone eff: effective series: series composite model quadratic: quadratic composite model geometry: geometry weighted composite model