A New Fabric Reinforced Geopolymer Mortar (FRGM) with Mechanical and Energy Benefits

Abstract

A large part of the European building Heritage is dated back over centuries. Consequently, its structural and thermal performances are often inadequate. Commonly, different interventions are proposed for solving these issues separately. However, reasonable drawbacks arise when the structural retrofitting requires a direct contact with the target-member while the insulation layer is potentially interposed in between. In this scenario, the present research proposes a novel and unique system able to guarantee both the energetic and the structural retrofitting. Inorganic Matrix Composites (IMCs) are a promising solution in this sense. Among them, the Fabric Reinforced Cementitous Matrix (FRCM) is one of the most used; or rather a composite made of a fabric (open grid or mesh) within an inorganic matrix (lime or cement based). Even if the inorganic matrix has a relevant thickness (if compared with the one of the fabric), its thermal resistance is insufficient. The novelty of this work consists in assessing a new geo-polymeric FRCM-system by combining fly-ash binder (reused material) and expanded glass aggregate (recycled material). Direct tensile tests, for measuring the tensile strength, ultimate strain and elastic modulus, were performed in addition to thermal conductivity tests. The results were compared with those of traditional FRCM (commercially available). The potentiality of the proposal for structural and energy retrofitting is discussed and examples of its possible application are also reported.

1. Introduction

The safety level and the thermal insulation capacity stated in current standard and technical codes often do not meet the actual mechanical and thermal properties of existing buildings, [1,2,3]. For this reason, the interest in retrofitting systems for structural and energetic purposes is steadily increasing [4]. This study focused on the use of Inorganic Matrix Composite (ICM), or more specifically on the Fabric Reinforced Cementitious Mortar (FRCM), which is a composite made up of a fiber mesh (or grid) bounded on a structural member by means of an inorganic matrix, [5,6,7,8,9,10,11]. In the recent past, a combined solution was studied by considering the side-by-side installation of FRCM and thermal insulation layers in order to achieve both the seismic and the thermal improvements of infill [5]. Even if the efficiency of the proposal was found relevant, the methodology requires two separate designs, on-site time-consuming operations and a significant cost. However, the ICM-system is potentially able to improve simultaneously the mechanical and thermal performances of the buildings. In fact, the fabric is a high-strength material while the matrix has a relevant thickness, which may lead to a significant thermal-resistant layer.

As well known in literature, the tensile constitutive law of a FRCM can be approximated to a bi-linear curve where the first and the second slope are dominated respectively by the properties of the matrix and the fabric, [12]. In some cases, an intermediate slope was also found to be corresponding to the cracking phase of the matrix and, at the same time, the slippage of the fabric within it. Generally, this phase is not significant (in term of strain and stiffness) when compared with the other two. Recent studies proved that the matrix plays a key-role in affecting the mechanical performances of the FRCM-system in terms of strength and ductility [12,13,14,15,16,17,18].

In literature, numerous contributions can be found in the field of FRCM characterization referring to the mechanical point of view. The first challenge when dealing with FRCM was computing the tensile characteristic behavior. Different testing methods and specimen preparation were tested in many laboratories in order to reach a standard test procedure. The gripping method is now recognized in European countries. Available technical codes (e.g., in the Italian CNR DT 215, [19]) consider the tensile stress-strain law to be tri-linear depending of the level of damage of the matrix. Hence, the third branch (used for design purpose) assumes a slope almost equal to the elastic modulus of the dry fabric in tension. The scientific literature reports on a significant number of tests on FRCMs exhibiting often a bi-linear approximation.

Three different textiles within the cementitious matrix were investigated in [20]; such as steel, carbon and basalt textile. From tensile tests, it was observed that the steel-FRCM shows a somewhat higher strength before tensile failure and better bonding between the textile and matrix when compared to the carbon-FRCM and basalt-FRCM.

In many cases, FRCM specimens were exposed to a particular environmental condition in order to evaluate the durability and the curing effect [19,20,21,22,23,24]. The specimens were found sensitive to the curing temperature, exposition and relative humidity mainly depending on the properties of the matrix. Furthermore, different systems were tested and compared, by varying both the type of fabric or matrix, [25,26,27,28,29]. The results confirmed the direct influence of the matrix and the fabric regarding respectively the first and the second slope of the tensile law (when bi-linear), respectively. In recent years, some geopolymer-based matrices were tested demonstrating valuable efficiency due to the high level of compressive strength of the matrix [30,31]. Based on authors’ knowledge, the potential thermal application of FRCM (and particularly when geopolymer-based) was never investigated.

From the thermal point of view, the effectiveness of retrofitting essentially depends on two-parameters: the thickness and the conductivity. In FRCMs, the thickness of the matrix is relevant (i.e., 10–30 mm per side in case of single layer application), but the thermal conductivity is not insulating-oriented. Alternatively, an inorganic matrix can be associated with pre-impregnated fabric. In this case, a Composite Reinforced Mortar (CRM) is obtained and a mortar thickness greater than 30 mm is accomplished [32]. Thus, CRMs are even more relevant in the perspective of a novel insulating-oriented matrix; while structural strengthening capacity is well demonstrated in the literature [33,34,35,36].

Usually, insulating plaster (without mechanical strengthening scope) is effectively adopted for energy retrofitting in a sandwich-like assessment (e.g., expanded cork/infill/ventilated wall/finishing), [37]. On the other hand, the application to a substrate of both the insulating plastering and the strengthening system compromises the bond between each other and consequently the mechanical effectiveness of the strengthening. Alternatively, the addition of Light-Weight Aggregate (LWA) into the plastering was examined in the recent past [38]. However, it was found that it may improve the thermal capacity (depending on the volumetric ratio) while dramatically decreasing the mechanical strength making it useless for the scope.

For this reason, a new ICM system, herein called Fabric Reinforced Geopolymer Matrix (FRGM) was assessed by using fly ash, metakaolin and expanded glass aggregate. This type of material is promising, since it exhibited good mechanical properties and lower thermal conductivities with respect to standard structural materials [39,40]; the mortar mix was aimed to obtain comparable mechanical strength, compared to traditionally mortars used for FRCMs, but a significantly lower thermal conductivity. Available results in terms of mechanical and energetic performance were used to assess an optimal mix-design of the proposed geopolymer mortar [41]. In such a way a relevant innovation was provided by this research, in fact, no other materials with comparable double-performances are currently available. Finally, two series of glass-IMC (with traditional and innovative matrix) were tested in direct tension and then compared.

2. Materials and Methods

This section is focused on the description of the used materials and the test methods for the physical and the mechanical characterization.

2.1. Materials and Sample Preparation

The traditional FRCM was made of a Natural Hydraulic Lime (NHL) mortar; which was supplied as a dry powder pack to be mixed with 17% by weight (wt) of tap water. The dry pre-mix consists of hydraulic lime, sand and chemical admixtures according to the commercially available technical sheet. The Geopolymer Matrix (GPM) was made of dry binders (fly ash and metakaolin), alkali activators and LWA (i.e., expanded glass aggregate). The chemical composition and the density of the dry constituents are both reported in

Table 1. Chemical composition of fly ash and metakaolin.

Component	Oxide Concentration %							Particle Density g/cm3
	SiO2	CaO	Na2O	Al2O3	Fe2O3	K2O	TiO2	SO3
Fly ash	37.33	6.13	-	16.14	30.79	5.28	3.65	0.68	2.30
Metakaolin	55.00	0.30	-	40.00	1.40	0.80	1.50	-	2.40
LWA	72.50	9.00	13.00	3.25	-	2.25	-	-	0.57

. The chemical composition was measured by means of XRF for fly ash, while data concerning both the metakaolin and the LWA were declared by the supplier. The preparation of the alkali activator solution was made at least 24 h before mixing all constituents. In detail, it was performed by mixing Sodium Silicate (SS) and Sodium Hydroxide (SH) into distilled water with SS/SH weight ratio set equal to 2.5. Later on, the alkali solution was mixed (low speed) with the dry binders. Finally, the LWA was added into the alkali-activated binder. The GPM was assessed according to [42]. The weight percent mix-proportions of the obtained NHL and GPM is reported in Figure 1a,b, respectively.

The experimental program involved both tests on the matrix and the FRCM. At this scope, two series of specimens were prepared: NHL- and GPM-based. The same fabric was used for FRCM system, i.e., an Alkaline-Resistant (AR) glass fiber arranged as 12 × 12 mm² mesh (40 mm²/m equivalent thickness) with 1400 MPa, 2% and 74 GPa as tensile strength, maximum elongation and Young’s modulus, respectively (according to the manufacturer technical sheet). As suggested by technical Guidelines [43], a total of 9 FRCM samples was tested per series. The coupons, sized 600 × 60 × 10 mm³ in order to include at least four longitudinal yarns of fabric, were cast in a single polystyrene formwork and cured for 28 days at environmental temperature and Relative Humidity (i.e., ~23 ± 2 °C and ~50 ± 5% RH). After curing, steel tabs (80 × 60 × 2 mm³) were epoxy-glued at the specimen ends on both sides.

2.2. Test Methods

The dry mass’ density of the mortar specimens was measured by weighing three cylinders of 35 mm in diameter and 55 mm in height per series. The thermal conductivity was measured against the same oven-dried specimens by the Modified Transient Plane Source (MTPS) technique (TCA Thermal Conductivity Analyzer, C-Therm), according to ASTM standard [44]. The contact surface of the transducer was polished with sand paper, in order to minimize surface roughness, and then cleaned with pressurized air. Furthermore, the contact between the transducer and the sample was ensured by placing a film of thermal paste in addition to a weight (500 g) above the sample. The performance of the device was preliminarily checked by measuring a series of control materials (with standard known property) as suggested by the provider and the results were found reliable.

Two triplets (i.e., NHL and GPM) of 40 × 40 × 160 mm³ prisms were prepared and tested with 3-points bending set (span equal to 100 mm). When the flexural failure occurred, the prisms were divided in two and the resulting parts were tested in compression by considering 40 × 40 × 40 mm³ samples according to UNI EN 1015-11 [45]. Furthermore, the Young’s modulus of the mortars was herein evaluated by computing the secant slope between 5% and 33% of the peak stress in compression. FRCM-specimens were tested in direct tensile stress-state and displacement control (Figure 2a,b for FRCM and FRGM, respectively). The test velocity was set equal to 0.2 mm/min according to [43]. A 100-kN load cell was assigned to the load recording; while deformations were measured with a clip-on extensometer. The latter was equipped with knife edges, positioned at the gage length (=10 cm), in direct contact with the specimens (see Figure 2c). Moreover, a camera was placed in front of the plan view of the specimen in order to video record the crack’s development during the test.

Finally, Scanning Electron Microscopy (SEM) was performed using a Zeiss Scanning Electron Microscope Evo40, [46]. Small fragments, including the fabric, were extracted from the middle cross-section of the FRCM-coupon after the tensile test. The micrographs at the matrix-fabric interface were acquired at 20 kV accelerating voltages.

3. Results

In the following sections, the results about energy and mechanical performance of the tested materials were reported for comparison. The evaluation aimed to evidence the competitive mechanical properties of the FRCM with respect to the FRGM. Moreover, the relevant scatter in terms of thermal capacity between the two systems is underlined.

3.1. Energy Performance

The dry mass density (ρ) and the thermal conductivity (λ) significantly change the potential application field of the material. As well known, a lightweight, porous material generally insulates from both the acoustic and the thermal point of view, even if pores ease the crack opening due to loading. In

Table 2. Dry mass density and thermal conductivity of NHL and GPM.

Label	Dry Mass Density (ρ)			Thermal Conductivity (λ)
	Mean kg/m3	CoV 1 %	ρ GPM/ρ NHL %	Mean W m–1 K–1	CoV 1 %	λGPM/λNHL %
NHL	1540	0.5	66.95	0.830	1.5	26.75
GPM	1031	1.8		0.222	2.7

¹ Coefficient of Variation.

, the results of the mass density and thermal conductivity measurements were collected, as also shown in Figure 3 for the sake of clarity. In particular, the GPM samples revealed a mass density ~33% lower than the NHL ones. The principal reason of this difference was linked to the use of LWA. In fact, the bulk density was 1500 kg/m³ and 450 kg/m³ for the NHL and GPM aggregate, respectively. For this reason, the FRGM is particularly suitable for structural retrofitting because it provides less additional weight with respect to the FRCM. As a consequence, negligible variations of both gravity loads and seismic inertia-like actions affect the strengthened structure.

More relevant was the difference in terms of thermal conductivity. In fact, the GPM exhibited a ~73% lower value if compared with NHL. Again, the LWA’s effect was crucial (expanded glass having ~60% porosity). Even the binder (i.e., alkali-activated fly ash and metakaolin) is generally less conductive than traditional binders, i.e., lime and cement based [40].

3.2. Mechanical Performance

The mean values of the mortars’ mechanical properties were reported in

Table 3. Flexural strength, compressive strength and Young’s modulus of NHL and GPM.

Label	Flexural Strength (fb)			Compressive Strength (fc)			Young’s Modulus (E)
	Mean MPa	CoV 1 %	fb,GPM/fb,NHL %	Mean MPa	CoV 1 %	fc,GPM/fc,NHL %	Mean GPa	CoV 1 %	EGPM/ENHL %
NHL	3.10	7.4	66.45	9.13	3.3	64.95	8.58	8.9	37.64
GPM	2.06	6.3		5.93	5.0		3.23	10.2

¹ Coefficient of Variation.

and Figure 4 in terms of flexural/bending (f_b) and compressive strength (f_c) and Young’s modulus (E), with the relative dispersions (i.e., CoV). The GPM reached ~6 MPa compressive strength; which was slightly lower than that obtained for the NHL, but adequate for the ICM-application. Depending on the type of fly ash, the compressive strength of the GPM could be improved as measured in on-going research by the same authors. Generally, NHL matrix exhibited higher mechanical performances, especially in terms of stiffness. Nonetheless, according to the available literature, the proposed GPM has proper characteristics for being involved in FRCM systems, even if a lower initial elastic stiffness of FRCMs is expected.

The stress strain relationships of FRCM and FRGM were reported in Figure 5a,b. In detail, the stress was evaluated dividing the applied load by the cross-section of the fabric, while the strain is related to the gauge length (see Figure 2c). The tensile strength (σ_u) and the ultimate strain (ε_u) are both reported in

Table 4. Tensile strength, maximum strain and elastic moduli of FRCM and FRGM.

Label	Tensile Strength (σu)			Ultimate Strain (εu)
	Mean MPa	CoV 1 %	σu,GPM/σu,NHL %	Mean %	CoV 1 %	εu,GPM/εu,NHL %
NHL	890.75	15.7	97.41	0.87	19.0	100.00
GPM	867.69	10.7		0.87	19.4

¹ Coefficient of Variation.

, while

Table 5. Un-cracked slope and cracked slope of FRCM and FRGM.

Label	Un-Cracked Slope 2 (EI)			Un-Cracked Slope 3 (E*I)			Cracked Slope (EII)
	Mean GPa	CoV 1 %	EI,GPM/EI,NHL %	Mean GPa	CoV 1 %	E*I,GPM/E*I,NHL %	Mean GPa	CoV 1 %	EII,GPM/EII,NHL %
NHL	514.47	8.5	44.59	2.52	8.3	44.59	77.45	5.1	89.26
GPM	229.33	9.8		1.12	9.8		69.13	5.4

¹ Coefficient of Variation; ² Computed according to the fabric cross-section = 2.4 mm²; ³ Computed according to the matrix cross-section = 600 mm².

shows the un-cracked (E_I) and cracked slopes (E_II) of the ICM-systems. As expected, the initial stiffness (i.e., first phase modulus) is directly affected by the stiffness of the matrix used in the FRCM system. Nonetheless, the dispersion area showed that the global behavior of the system is pretty similar in both cases, Figure 6. The stresses are properly transferred from the matrix to the fabric, as proven by the similar slope of the second phase. All the specimens exhibited a quasi bi-linear behavior and a fabric-rupture failure, generally occurring in the middle-height of the coupon (see Figure 7).

Finally, the SEM micrographs in Figure 8 were taken to make an estimation the short-term effects of the geopolymer binder on AR-glass fabric. In other words, the alkaline environment, generated from the reaction of SS and SH with dry binders (fly ash and metakaolin in this case), represents a potential cause of damaging for both LWA and mesh fiber. A chemical corrosion usually leads to surface cracking of the fabric, [47]. In this study, a first step to explore this issue was made, looking at fabric surface after 28 days. Moreover, the surface of glass in the GPM-matrix was compared with the one in the NHL-matrix. In detail, Figure 8a shows the fabric-matrix interface zone of FRCM, while Figure 8b reports a zoom of the surface of the fabric. Because of the low alkalinity of NHL-matrix, the fabric-surface was smooth and no chemical corrosion was found. The matrix-fabric interface zone (Figure 8c) and fabric surface (Figure 8d) were analyzed also for FRGM sample. Again, it was not noticed any remark of chemical corrosion. In this experimental campaign, only short-term effects were considered. Further studies can estimate the chemical and mechanical characteristics of the material after accelerated aging according to [43].

4. Mechanical and Energetic Retrofitting: Applications

The experimental outcomes demonstrated that the mechanical strength of the FRGM system could be compared to the one traditionally made of lime-based mortar (if same fabric is considered); while a significant reduction of the thermal conductivity was obtained. Moreover, the direct tensile tests highlighted that the performances in terms of tensile stress strain behavior were similar. Thus, the proposed FRGM-system is a candidate for a totally novel application as retrofitting system aimed to both energy and mechanical benefits.

In order to check the effectiveness of the system for possible mechanical and energetic retrofitting, some ideal applications were analyzed in the following section. Two masonry walls typically found in existing building context were considered. The former was a 100 × 100 × 25 cm³ Solid Clay Brick (SCB) masonry wall (Figure 9a,b), the latter a Tuff Brick Masonry (TBM) wall with the same dimensions (Figure 9c,d). For both of them a poor-resistant lime-based mortar was examined for the bed joints.

Results obtained by retrofitting the walls using the FRGM system were reported. The shear strength of the reinforced masonry walls V_eff was evaluated by the analytical model suggested in [13] and reported in Equations (1)–(8), because it is both matrix and fiber dependent (i.e., V_mat and V_f respectively). In fact, the matrix properties (i.e., matrix net area A_n,mas and compressive strength f_mat) were herein the discriminant which affected the mechanical difference between the two reinforcing systems. Whereas, the properties of the fabric (i.e., fabric net area A_n,f and tensile strength f_f) were the same in both the systems. The mechanical properties of the masonry wall (i.e., f_mas and f_t,mas or rather the compressive and tensile strength respectively), which are essential to compute the shear strength, were considered according to [48]. Moreover, to evaluate the energetic performance of the four masonry walls, a calculation of thermal transmittance was performed [49]. The thermal conductivity of the unreinforced masonry walls was estimated according to [50,51]. The mechanical and thermal results were collected in

Table 6. Shear strength and thermal transmittance of solid clay and Tuff brick masonry retrofitted with FRCM and FRGM.

Label	Shear Strength		Thermal Transmittance
	Unreinforced MPa	Reinforced MPa	Unreinforced W m–2 K–1	Reinforced W m–2 K–1
SCB – FRCM	0.85	1.07	1.471	1.330
SCB – FRGM	0.85	1.05	1.471	1.053
TBM – FRCM	0.67	0.90	1.258	1.153
TBM – FRGM	0.67	0.88	1.258	0.939

.

Some interesting remarks can be outlined from the reported results. Despite a matrix with a lower compressive strength, the FRGM system allowed a theoretical shear strength gain very similar to that obtained by using the NHL matrix. In fact, the reinforced/unreinforced shear strength ratios (Figure 10) were equal to 126%, 125%, 133% and 131% for SCB-FRCM, SCB-FRGM, TBM-FRCM and TBM-FRGM, respectively. On the other hand, the thermal transmittance for the considered samples manifested an important gain when involving the GPM-matrix. In the case of SCB, the adoption of the FRGM led to a reduction of heat transfer approximately equal to 29%, while the FRCM reduced it just of 10%. Similarly, if a TBM is considered, the reduction of heat transfers for the case of GPM-matrix and NHL-matrix is about 25% and 9%, respectively.

$$V_{eff}=\alpha f_{mas}{}^{0.5}·A_{n,mas}+V_f+V_{mat}$$

(1)

$$V_f=\frac{2}{3}{A}_{n,f}·0.5f_f$$

(2)

$$V_{mat}=0.1·A_{n,mat}·f_{mat}{}^{0.5}$$

(3)

$$\alpha =\frac{0.363}{1+e^{-\nu }}$$

(4)

$$\nu =-0.52+1.36\beta +0.05\gamma$$

(5)

$$\beta =\frac{f_f·A_{n,f}}{f_{t,mas}·A_{n,mas}}$$

(6)

$$f_{t,mas}=0.7\sqrt{f_{mas}}$$

(7)

$$\gamma =\frac{f_{mat}}{f_{mas}}$$

(8)

Another interesting application that was explored is the retrofitting of horizontal structural elements, as masonry vaults. Generally, the issues of this application are linked to the weight density of the proposed solution and its thermal properties, besides the mechanical performances. The FRGM herein proposed represents a competitive system, which can inhibit the failure mechanism of a vault or an arch (typically linked to a cinematic configuration, rather than to a material failure) without adding a relevant weight and preserving the thermal insulation capacity, or adding more insulating power. An example of thermal simulation was performed by a 2D model of a TBM vault, involving a multi-physics computation tool [52]. The material above the vault is typically a masonry filler (Figure 11a), but it was replaced with an ICM 10 cm layer in Figure 11b to evaluate the difference in thermal conductivity of the horizontal structure. The section of the vault and the filler were modelled using a material with the same thermal properties of the above-defined TBM. The surfaces were inserted in a conductive medium set along the borders with a fixed temperature, 10 °C. The ground has a low thermal conductivity, i.e., 0.001 W m^–1 K^–1, so it behaves as an adiabatic plane. In both figures all the elements were initially set to 10 °C, except the heater, whose surface is fixed at 50 °C. From the initial set until the equilibrium, the heat transfer occurred because of conduction and convection (i.e., solar radiation was neglected in this simulation).

The results referred to the stage corresponding to the thermal equilibrium, i.e., when thermometers did not register any further time-dependent change. The recorded surface’s temperatures highlighted that the layer of GPM is more efficient to insulate than the infill usually involved in vaults, despite a much lower thickness. Furthermore, it is well-known that the configuration in Figure 11b is more performant from the mechanical point of view, because of the ICM-strengthening [53].

5. Conclusions

Building heritage is nowadays inadequate with respect to the prescriptions of current technical Codes, particularly referring to the thermal and the mechanical (static or seismic) performances. Generally, practitioners have experience in one of these two fields. Thus, independent retrofitting solutions are commonly accomplished in order to mitigate the energy dispersion or the structural vulnerability; often generating conflicts. The present investigation was aimed to study a unique ICM-based system focused on the thermal and mechanical capacity improvement of buildings. In fact, the fabric was chosen based on the high tensile strength; while the matrix was designed in order to have low thermal conductivity. Fly ash, metakaolin, alkali activators and expanded glass aggregates were combined in a proper proportion (geopolymer) for the scope.

Experimental results are promising and it is the authors’ opinion that the proposed material is worthy of further research due to its total innovative nature. In fact, the mechanical properties of FRGM are comparable to traditional FRCM. In particular, the scatter between the two ICMs in terms of ultimate strength and strain is negligible (i.e., <3%), while a lower initial stiffness was recorded for FRGM. These results are consistent with the characterization of FRGM-mortar, which is less resistant and stiff. On the other hand, the FRGM-system exhibited a higher thermal insulation power, which decreased the thermal conductivity of about 25%. Another interesting aspect is related to the mass density of GPM, which is 33% lower than the NHL one. Thus, the magnification of gravity and inertial loads is limited when compared to current market-available solutions. An open issue, related to the long-term behavior of the system is target of further investigations, despite the 28 days SEM analysis was found encouraging.

Finally, some design examples were proposed in the present work, supported by theoretical calculation. The mechanical results highlighted that FRGM is a valid alternative for retrofitting with respect to the FRCM, since the scatter in terms of shear strength gain is lower than 5% (when the same type and amount of fabric are considered). Contrarily, the reduction in thermal transmittance is significant, as the application of FRGM leads to a ~25% reduction instead of ~10% for the FRCM.