Formulación de las Tablas de planificación del Vol. III

The industrial preparation of dry constituents can facilitate the introduction of admixtures, binders and aggregates with different particle size distributions. The addition or substitution of industrial by-products aggregates such as marble dust (MD), metakaolin (MK) and crushed limestone can contribute to performance, sustainability and economy of the mortars since binder is the most expensive component of mortar and concrete. Modification of mortars aggregate can alter the characteristics and microstructure (Faria and Silva 2013).

The addition of calcitic aggregates into lime mortars has many historic precedents and these components are often found in many historic mortars (Hughes et al. 2003, Gibbons et al. 2003). Evidence suggests that inclusions of calcite (i.e. crushed limestone or seashells) are perceived to promote crystal growth and the conversion of Portlandite into calcite (CaCO3) during the setting process thus helping to develop early stage strength (Forsyth 2007, Yoon et al. 2003, Yoon et al. 2004, Liang and Wang 2013, Yang et al. 2010). Finely ground calcite additives in the aggregates have been shown to

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improve the rate of carbonation and prevent strength loss (Taylor 1997, Skoulikidis et al. 1995, Skoulikidis et al. 1996).

Ground limestone (CaCO3) has been used as a replacement of OPC, and its effect on the hydration of OPC has been studied intensively (Hawkins et al. 2003, Li et al. 2009, Pera et al. 1999, Thomas et al. 2010, Guemmadi et al. 2009). These studies have indicated positive effects of the addition of CaCO3 on the hydration of cement and strength development of hardened concrete, especially its accelerating effect on the rate of the hydration.

Fillers can be naturally available materials or synthetic (processed inorganic mineral) materials (Moosberg-Bustnes et al. 2004). The most important factors are the uniform properties and fineness (particle size) (Korjakins et al. 2012). Neville (2002) suggested that fillers must not increase the water content unless the filler was used with water reducing admixture.

The characteristics of mortars comprising several components are influenced by both the particle size distributions and the chemical-mineralogical compositions of the component materials. It is evident that an increase in mass of the mortar is noted within the carbonation process as the diffused carbon dioxide transforms portlandite into calcite (Dheilly et al. 2002). It has been suggested that the presence of calcitic materials aids carbonation as it is typically a relatively porous particulate and therefore facilitates carbon dioxide diffusion through the fresh mortar. In contemporary work, calcitic aggregate additives are used by specifiers and contractors to enhance the initial set in limes in high humidity environments. This is especially common on the West Coast of the UK, well known for high rainfall and low potential evaporation (Hall et al. 2010).

2.16.1 The Influence of Fillers on Strength Development

In their research on lime-based mortars for masonry repair and more specifically their mechanical behaviour, Lanas and Alvarez-Galindo (2006) found the use of calcareous aggregates has a great affect upon the strength when compared to siliceous aggregates (Figure 2.28). The grain size distribution of the aggregates was identified as the most important attribute in relation to aggregate characteristics. It was later concluded that the type and shape of the aggregate influence the mortar strength with angular limestone most improving the strength. The lack of discontinuity between the binder matrix and

the aggregate of the same nature improves the strength, as well as a good packing of the aggregate with angular edges.

Figure 2.28: Porosity and compressive strength versus percentage of mortars specimens with calcitic aggregates tested after 365 days. B/Ag ratios are expressed on top of points (Lanas and Alvarez-Galindo 2006).

2.17 Seeding

Seeding is considered as nanotechnology in civil engineering (Gopalakrishnan et al. 2011). Seeding microstructures has been long recognized as a method for either improving microstructures or to increase the rate of phase formation (Badger et al. 2002). Therefore, seeding is applied to modify the kinetics and microstructure development.

2.17.1 Seeding Materials on Chemical Reactions

It is hypothesized that the establishment of crystal architecture that catalyses the precipitation of Ca2+ ions into growth of calcium carbonate (Gebrehiwet et al. 2012). This concurs with work by various investigators such as Hewlett (2004) and Forster (2004b) that established the influence of temperature and super saturation of Ca2+ ions in the liquid phase upon hydration kinetics. Forster et al. (2013) described the Ca2+ reaction during seeding;

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“The rate of hydration of calcium silicates is restricted by an impermeable hydrate layer of Ca2+ ions that surrounds the grain potentially leading to a temporary period of dormancy. The addition of calcitic material may minimize the effect of the dormancy period leading to an accelerated deterioration of the Ca2+ ion layer as they are readily utilized in the formation of calcite. This is potentially responsible for the reactivation of the hydraulic phase dissolution and precipitation activity”.

The development of early stage products of hydration have been previously associated with an impedance to carbon dioxide diffusion in lime mortars therefore inhibiting carbonation (Radonjic et al. 2001). It was later concluded by Ezziane et al. (2007) that the seeding processes can positively influence the mortar hydration and minimizes disorders caused by the temperature rise.

2.17.2 Contemporary Researches on Seeding

Skoulikidis et al. (1995) showed that presence of additional calcium carbonate as crystallization seed (autocatalysis) in the mortar enhances the rate of carbonation. Seeding with 6% finely ground calcite has been shown to improve the rate of carbonation. This conforms with grain boundary strengthening theory based upon the Hall–Petch equation (Equation 2.2) (Skoulikidis et al. 1996).

Shetty (2013) discusses seeding as unconventional method for making high strength concrete. Sato and Diallo (2010) argued that the accelerating effect of CaCO3 is an advantage and studies should be conducted to maximize its effect. In his study on influence of nucleation seeding on the hydration mechanisms of tricalcium silicate and cement, Thomas et al. (2009) found that the seeding effect of C−S−H also provides a new explanation of the hydration-accelerating effects of various forms of reactive silica because these additives form C−S−H by reacting with aqueous calcium ions released by cement dissolution.

Osman et al. (2004a) highlighted the understanding the rheology of composites is not only essential for optimizing their compounding and processing but also for achieving the targeted end properties. The perceived beneficial nature of seeding is not understood mechanistically. However, several potential reasons have been postulated that could lead their serendipitous addition to have inadvertently enhanced performance. In

contemporary work, calcitic aggregate additives are used by specifiers and contractors to enhance the initial set in limes in high humidity environments. This is especially common on the West Coast of the UK, well known for high rainfall and low potential evaporation (Hall and Hoff 2012).