**2.1 Mortars as composite materials**

Composites are materials made by combining two or more other materials. These materials are important in the construction sector as building technology has been favored by the advanced properties that composites can offer. The development of composite materials along with related design and manufacturing technologies constitute one of the most important advances in the history of materials. Composites are multifunctional materials having unprecedented mechanical and physical properties that can be tailored to meet the requirements of a particular application [19]. Thus, new achievements have been constructed as the innovative composites could add new possibilities to the engineers' imagination.

Mortars are a specific type of composite material, which consists mainly of three phases—paste as the matrix, interface transition zone (ITZ), and aggregates. The properties of mortars are influenced by:


At fresh state, mortar should be workable (do not break and do not flow), plastic (to have consistency to hold and not flow on overload loads) and it should show volume stability (do not cause contractions or expansions). At harden state, it should have the required strength and the required porosity. Aggregates constitute the strongest phase, hold a significant percentage in the volume of mortar and are frequently used in sand size (up to 4 mm). Conditions for the use of aggregates in mortars is the health of both the parent rock and the grains (without breaks, cracks, impurities), low porosity—small absorption index, homogeneous granulometric grade, percentage of the fines (<0.075 mm) should not exceed 5% [20]. The presence of fines in lime-based mortars can cause considerable alterations to the properties of the mortars. Their presence significantly reduces the strength and increases the volume shrinkage of mortars [21]. Furthermore, the porosity can be increased, and the same can also happen with capillarity when fine aggregates are participating in excess. Additionally, the type of fines also seems to play a role in their behavior in relation to the basic binder. For example, the strength is decreased in compositions with clay fines while porosity is affected mainly by limestone fines [22]. Capillarity also seems to be affected by the type of fines as the compositions containing fines 10–15% presented low absorption probably because fines block capillary pores [23].

In the case of mortars, as composite materials and keeping in mind that aggregates retain the inherent properties of the rocks from which they are derived, it can explain that the color, the chemical and physical characteristics of the aggregates directly affect the specific weight, the measure of elasticity, the volume stability, the appearance, and the mechanical and physical properties of mortars. The addition of sand to a binding system in mortars technology has proved to confer technical advantages as they contribute to volume stability, durability, and structural performance [16]. The gradation of the aggregates was wide, but the most adequate part was sand of 0–4 mm. Coarse aggregates up to 1 cm were used in thick joints [10] and also combined with sand for structural mortars while sand or finer aggregates (0–2 mm) are usually the constituents of the renders or plasters [24]. Usually, aggregates are obtained after the gentle grinding and sieving (based on EN1015-1) [25]. Even homogeneous distribution of grains is usually obtained as shown in **Figure 2** in a typical bedding mortar.

The ratio of binder to aggregates (B/A) ranges widely but it could be said that for most of the structural mortars, it is 1:2.5 or 1:3 while for the renders and plasters are

**Figure 2.** *Typical gradation of old structural mortar.*

#### *The Role of Sand in Mortar's Properties DOI: http://dx.doi.org/10.5772/intechopen.102489*

richer in binder content and the ratio is mostly 1:1 or 1:1.5 [23]. Apart from the different types of aggregates, as their mineralogy and origin are concerned, the volume content in the mixture, the maximum size, and their gradation influences the structure of a binder—aggregate mixture and the performance of mortars overall [10]. The added aggregates strengthen the composite, and the associated interface weakens it. These two opposite effects offset each other, and the combination of them leads to declined strength. Generally, a strong cohesion between the mortar binder and coarse aggregate confirms the good masonry properties. On the other hand, the increase of aggregate content reduces the workability of a mix and thus, reduces the strength as well [26]. It has been mentioned before that aggregate plays a role in restraining the shrinkage of cement paste, and that the shrinkage of the aggregate itself can be neglected [27]. It has been found in various composite materials that a certain amount and proper size of the aggregate are beneficial to the strength and fracture energy of the composite [28]. For mortar specimens, aggregates have a significant influence on both rheological and mechanical properties. Their specific gravity, particle size distribution, shape, and surface texture influence markedly the properties of mortars in the fresh state. On the other hand, the mineralogical composition, toughness, elastic modulus, and degree of alteration of aggregates are generally found to affect the properties of mortars and in the hardened state [29]. The drying shrinkage strains in investigated mortars are changed significantly by different kinds of fine aggregate materials. The water content of the mortar mix proportion is a major factor in drying shrinkage evolution. Increasing the unit water content can result in an increase in the amount of capillary water, and hence more shrinkage strain would be obtained. The bonding stress of the weak interface zone between the coarse aggregate and paste can be improved when a chemical reaction between the aggregates and the paste [30].

More recently, the role of the recycled sand from waste demolition, when examined in mortars, revealed that it was more beneficial in lime mortars rather than in stronger lime-pozzolan or lime-pozzolan and cement mortars as a decrease in their performance were recorded in the latter cases due to the mortars' structure [31]. It seems that two competitive mechanisms acted in these mortars; high porosity (due to high water content and the nature of the aggregates) which assists toward low strength and durability and the chemical reaction due to the presence of reactive components which creates a strong structure. This chemical reaction is a stronger mechanism in the case of lime mortars and prevails in relation to the competitive mechanisms of the higher porosity [32].

In an effort to test different aggregate-related properties to hydraulic lime mortar, Pavia et al. suggested [33] that an increase in the calcite content of the aggregate lowers the flexural and compressive strength of the mortar. At the same time, they proved that sharp aggregate, as well as aggregate with small average particle size, tends to increase the mechanical strength and bulk density of a mortar simultaneously reducing porosity, water absorption, and capillary suction. Additionally, they concluded that aggregates containing particles of a wide size range can increase the mechanical strength and bulk density of the hardened mortar diminishing porosity, water absorption, and capillary suction.

#### **2.2 Different sands in mortars**

The role of aggregates on the structure and behavior of lime-based mortars is examined by studying the influence of the aggregate content, type, and grain size on the strength, porosity, and volume stability of the mortars. Trying to understand how

**Figure 3.** *(a) Black sand, (b) yellow sand, and (c) blonde sand.*

the properties of the sands influence important macroscopic properties of pure lime mortar, threes sands that were available in the market were selected and analyzed in the laboratory. All of them were river origins of siliceous content (**Figure 3**).

X-ray diffraction analysis (XRD) using a D2 Phaser 2nd generation, Bruker instruments, indicated that blonde sand was containing quartz, feldspar, magnetite, calcite, hornblende. Yellow sand contained quartz, feldspar, magnetite and black contained quartz, feldspars, biotite, and hornblende. Physical properties, such as water absorption, specific gravity, and sand equivalent (S.E.), are shown in **Table 1** while the chemical analysis revealed the silicic nature of the sands (**Table 2**).

Lime mortars were prepared using lime CL90 (based on EN459) [34] and the compositions were produced, as shown in **Table 3**. The workability was measured with a flow table as described in EN1015-3 [35].

The samples were cured based on EN456 and at 28 days, the compressive strength and the open porosity were recorded (**Table 4**).

The results show that there are different properties recorded in the produced mortars even when siliceous sands are used. The different properties, such as S.E. and the


#### **Table 1.**

*Physical properties of sands.*


**Table 2.** *Chemical analysis of sands.*

*The Role of Sand in Mortar's Properties DOI: http://dx.doi.org/10.5772/intechopen.102489*


#### **Table 3.**

*Composition of trial mortar mixtures.*


#### **Table 4.**

*Properties of the produced mortars at 28 days.*

water absorption capacity of the sand grains, influence both the fresh (workability) and the hardened properties (porosity, strength) of the produced mortars.

The natural sands can be of similar origin with the crushed but weathering not only rounded the particles but also changed the proportions and removed most of the light minerals, such as the flaky micas. Due to these differences, mixtures with crushed sand often display higher water demand and lower workability than the corresponding composite with glaciofluvial sand. Additionally, crushed sand has a positive impact on long-term strength. It seems that, when rough-grained sand is used, strong cohesion with the binder can be achieved, as shown in **Figure 4**, where mortars with rounded and crushed sand were examined under scanning electron microscopy (SEM) [17].

The mechanical features, particle shape, grading, and physical properties, such as moisture absorption, sand equivalent value, are what can be labeled as properties of interest in the aggregates when used in mortars. Some of these most important properties are shown in **Table 5**.

#### **Figure 4.**

*SEM examination of rounded sand grain (left) where there is a gap in the contact zone and angular grain with strong cohesion (right).*


#### **Table 5.**

*Important properties of aggregates to be used in mortar production.*

The bond behavior in the interface between the binder and the aggregates has a strong effect on the mortar properties since the effectiveness of the reinforcement provided by the addition of particles depends on the interfacial bond (**Figure 5**). This is since the size, shape, and content of the particles predominantly control the morphological features of the internal structure of the composite.

The test results showed that with increasing volume fraction of aggregate, the compressive strength of the composite decreases, which is different from the prediction of conventional composite theories. The possible explanation of this result is

#### **Figure 5.**

*Macroscopic examination of contact zones of natural aggregates and binders in old mortars. Despite the presence of cracks in the binder in the left image, the cohesion is strong. On the right, there are pores on the interface probably due to the high content in aggregates in relation to the binder.*

**Figure 6.**

*Examination under SEM of natural aggregate and lime binder with weak ITZ (left) and crushed brick as aggregate and lime-pozzolan binder with strong ITZ (right).*

based on the interface transition zone (ITZ) around the aggregate, which is the weak zone in composites (**Figure 6**) [15]. With more aggregate added into the mixtures, more interfaces are formed in the hardened material. The compatibility between the aggregate of the paste affects the development of strong cohesion at the aggregatematrix interface in many cases and that usually indicates the good performance of the mortar. As aggregates are, by weight or by volume, the major component of mortars, they can be a source of silica, which can react in certain conditions with the binder, leading to the formation of reaction rims at the edge of the grains and recrystallization along with the pre-existing cracks (**Figure 7**).

Apart from the different types of aggregates as their mineralogy is concerned, the volume content in the mixture, the maximum size, and their gradation influences the structure of a binder—aggregate mixture [3, 5]. The analysis of mortars reveals that higher strength values are attained for lime mortars of low binder/aggregate (B/A) ratio (1:1.5, 1:2.5, and 1:3) which contained sand (0–4 mm). Coarse aggregates have

#### **Figure 7.**

*Old mortars under the polarized microscope (x10). Reaction rim in the interfacial zone of the binders and the aggregates used.*

#### **Figure 8.**

*Pores and cracks in the structure of lime mortar with coarse aggregates (polarized microscope, x15).*

**Figure 9.** *Cracks inside the binder where they meet the aggregate volume as the obstacle.*

contributed positively to the volume stability of lime mortars. The microstructure has recorded the restriction of volume changes in cases where coarse aggregates have been used in the structure of lime mortars (**Figure 8**).

However, it is well recognized that coarse aggregate particles can act as crack arresters, as they restrict the shrinkage of the binder so that under an increasing load, extra energy is absorbed for the formation of a new crack (**Figure 9**) [36].
