This section presents the raw materials and dosages used to produce the cement mortars used in this work, as well as the experimental programme conducted for their physical and mechanical characterisation.
2.1. Materials
The following raw materials were used for the development of this research work: grey cement, natural sand, hemp in different forms and water.
The binder used was grey cement of the CEM II/B-L 32.5-N type. This is a material commonly used in building works, and which is becoming increasingly widespread in façade rehabilitation works for 20th century buildings, due to its excellent technical performance, ease of conservation and accessible cost [
47]. This material, supplied by CEMEX (Madrid, Spain), has a real density of 3060 kg/m
3 determined according to the UNE 80103 standard [
48].
Table 1 shows its elemental composition determined by X-ray fluorescence using a Bruker S2 Puma model (Billerica, MA, USA).
The type of aggregate used was natural river sand, supplied by the company Bricomart (Madrid, Spain). To better understand the properties of these sands used in the production of cement mortars, a physical characterisation was conducted, the results of which are shown in
Table 2.
All the values given in
Table 2 agree with those obtained in previous research [
54]. The fine particle content was close to 2%, defined as the amount of material passing through the 0.063 mm sieve in relation to the total sieved mass, which exceeds that usually obtained in standard sands. It was decided to use this type of commercial natural aggregate as it is the most common in rehabilitation works and would bring the study closer to a real case.
Figure 2 shows the granulometric curve obtained for the natural sands used in the production of the mortars. This granulometric curve was determined using the following series of sieves with standardised mesh sizes: 4.000–2.000–1.000–0.500–0.250–0.125–0.063–bottom (sizes in mm). The method described in the UNE-EN 933-1:2012 standard [
49] for obtaining these curves consists of placing the series of sieves in decreasing order of mesh size, and then shaking the 1000 ± 0.005 g sample mechanically with the aid of a back-and-forth sieve shaker for 1 min, so that the aggregate is retained according to its size in the different sieves.
The aggregates used in this work had a continuous grain size, which was within the limits recommended by the regulations. It should be noted that a continuous particle size favours the workability of the mortar when fresh and improves the compactness and mechanical properties of the mortar when hardened [
57].
Three different hemp morphologies were used as additives in the cement mortars produced: fibres, powder and pellets.
Figure 3 shows the natural state of these three raw materials. These three types of additives have been carefully chosen to produce three diverse types of mortar. Firstly, a traditional cement mortar with a hemp-vegetable-fibre reinforcement, and secondly, two other diverse types: one replacing the aggregate fraction of 0.500 mm or less with hemp powder, and the other replacing the aggregate retained in the 2000–1000 mm sieves with hemp pellets.
The hemp additions were previously washed with water in a way equivalent to what was done in the studies of Ruano et al. and Ferreira et al., to eliminate residues that had adhered to them during the production process [
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39], and then dried in an oven at a temperature of 70 ± 1 °C for a period of 24 h. The hemp fibres were sieved prior to use to obtain two distinct series of fibres, with lengths between 8.0 and 12.0 mm (
Figure 3a). The powder had a particle size of less than 2.0 mm (
Figure 3b). The pellet hemp shown in
Figure 3c had an average diameter of 4.0 mm and a length between 1.0 and 2.0 cm.
Finally, the water used for the mixing of the mortars was tap water from the Canal de Isabel II (Madrid, Spain). This is drinking water fit for human consumption and suitable for use in mortars and concretes [
58], which has been successfully used in previous studies [
59]. Among its main characteristics are its soft hardness (25 mg CaCO
3/L) and neutral pH between 7 and 8 [
60] recommended to avoid setting alterations.
2.3. Experimental Program
The experimental programme developed in this research consisted of a physical and mechanical characterisation of the diverse types of mortar produced, with the aim of analysing the suitability of these cladding and bonding materials for their application in masonry work related to building renovation. In this regard, the tests proposed, and their application regulations, were as follows:
The actual density of hardened mortar was according to standard UNE EN-80103: 2013 [
48]. A pycnometer was used for this test. The process consisted of finely grinding the sample of each type of mortar and depositing it in a Le Chatelier flask, avoiding the formation of pores. Subsequently, the real volume of the sample was obtained by difference of weights, the density being the result of applying the equation:
where
is the mass of the specimen previously dried (70 °C for 24 h) and
is the actual volume obtained according to the method described.
The thermal conductivity coefficient was obtained using the UNE-EN ISO 8990:1997 standard as a reference [
64]. For this test, a Hot-Box was used, as shown in
Figure 4, equipped with thermocouples and a datalogger for the accumulation of temperature data. The test was performed using 24 × 24 × 3 cm
3 mortar plates placed on one side of the Hot-Box, and an internal heat source was applied and after 24 h, when the heat flux (Φ) could be assumed to be stationary, the thermal conductivity was measured and the Fourier equation was applied:
where
is the calculated thermal conductivity coefficient, e is the thickness of the sample, S is the surface of the plate and
is the difference between the inside and outside temperature of the Hot-Box.
The water absorption coefficient was obtained by capillarity according to UNE-EN 1015-18: 2003 [
65]. Capillarity is one of the most frequent mechanisms of water penetration in porous materials, and one of the main causes of the deterioration of building materials due to humidity. These lesions arise because of the attraction between water and the surface that forms the pore network of the mortar in accordance with Jurin’s Law. This test was conducted using RILEM half specimens that had been previously flexure evaluated and dried (70 °C for 24 h). The specimens were then immersed in water on the flexural-fracture side, up to a height of one centimetre, for 90 min, calculating the capillary absorption coefficient according to the expression:
where C is the capillary absorption coefficient measured in kg/(m
2min
0.5), M1 is the weight of the sample after 10 min of testing and M2 is the weight at 90 min.
Shore D surface hardness was obtained following the procedure adapted from UNE-EN 13279-2: 2014 [
66]. This is a property of relevance for façade cladding that determines the material’s resistance to scratching on its surface. This was performed using a Shore D durometer and measuring five times on two opposite sides of the standardised RILEM specimens measuring 40 × 40 × 160 mm
3.
Open porosity was determined according to UNE-EN 1936:2007 [
67]. This property is defined as the ratio between the accessible volume of pores and the apparent volume of the material, in relation to the compactness and compressive strength of the mortar. This property is calculated as:
where
is the saturated weight of the sample. To obtain
each sample was completely immersed in water for two hours and then weighed until the difference between two consecutive weighings was less than 0.1%.
is the initial weight of the sample previously dried in an oven for 24 h at a temperature of 70 °C and
is the immersed saturated weight obtained with the aid of a histrostatic balance. This test was conducted on standardised RILEM specimens.
The total water absorption coefficient, according to the recommendations of the UNE-EN 14617-1:2013 standard [
68], was obtained according to the equation:
where
is the saturated weight of the sample and
is the initial weight of the previously dried sample, determined in the same way as in the open porosity. Mortar slabs of 15 × 15 × 2 cm
3 were used for this test.
Determination of the surface permeability was by means of the Karsten pipe test, according to the recommendations of the RILEM method for measuring water absorption under low pressure. The pipe method test (Test nº II. 4) [
69] makes it possible to recreate the conditions of rain and 90 km/h winds and is recommended for determining the effect of pipe additions on the permeability of the enclosures, which is necessary for subsequent cleaning and maintenance work that may affect their durability [
70]. Mortar slabs measuring 24 × 24 × 3 cm
3 were used for this test.
Determination of the dynamic Young’s modulus was carried out by ultrasound (MOE
us). For the determination of this parameter, IBERTEST equipment was used, using 55 kHz receiver–transmitter contact probes. This MOE
us was determined by first calculating the longitudinal transmission velocity of the mortar sample (
), according to the equation:
where L is the normalised sample length RILEM (16 cm) and
is the time for wave propagation measured with the ultrasound equipment. Subsequently, the MOE
us is determined according to the expression:
where
is the density of the mortar evaluated and
the Poisson’s coefficient (which can be estimated at 0.2–0.3).
Determination of the mechanical flexural and compressive strength of the mortars was according to UNE-EN 1015-11:2000/A1:2007 [
71]. Firstly, standardised RILEM prismatic specimens measuring 4 × 4 × 16 cm
3 were evaluated in simple bending, and then the two half-dimensions obtained after the bending test were evaluated in compression. To conduct these tests, six samples of each type of mortar of 28 days of age were used with an AUTETEST 200-10SW hydraulic press. The equations to determine the flexural strength (
) and compressive strength (
) are:
where
is the applied point load, l is the support spacing (approximately 10 cm) and
and
are, respectively, the width and height of the cross-section of the specimen.
Adhesion strength was determined according to UNE-EN 1015-2 [
72]. This is a mechanical property referring to the mortar’s ability to withstand normal stresses on its surface without detaching as a coating. This property depends on multiple parameters in addition to the material used, such as [
73] the mortar–substrate interface, humidity and cleanliness of the substrate, and the method of application and curing. This test was conducted by applying a one-centimetre-thick layer of mortar on a previously moistened ceramic scraper measuring 40 × 50 × 5 cm
3. Once the mortar sample had been hardened and cured in a humid chamber for 28 days, metal discs of 50 mm diameter were glued on its surface, separated at least another 50 mm from each other. Tension was then applied until breakage in the direction normal to the disc, separating it from the application surface, and thus determining the bond strength.
The evolution of shrinkage in the diverse types of mortar produced was measured from the demoulding of the samples up to the age of 150 days. This test was conducted on specimens measuring 2.5 × 2.5 × 28.7 cm
3 and following the recommendations of the UNE 8011289 standard [
74].
Determination of the durability of mortars against repeated freeze–thaw cycles was according to the recommendations of standard UNE-EN 12371:2011 [
75]. This test was conducted on standardised RILEM specimens measuring 4 × 4 × 16 cm
3. A series of three specimens was used and they were subjected to freeze–thaw cycles, while another series of three specimens was used as a reference without cycles. The cycles, 25 in total, consisted of immersing the specimens in water for 18 h at 20 °C and then freezing them for 6 h at a temperature of −12 °C. Subsequently, the loss of flexural strength and mass variation in the aged specimens were evaluated by durability cycling.
Finally,
Figure 4 shows images of the tests and equipment used to obtain the results, as described in the methodology.