PSI - Issue 41

Maria-Evangelia Stogia et al. / Procedia Structural Integrity 41 (2022) 744–751 Maria Evangelia Stogia et al. / Structural Integrity Procedia 00 (2019) 000–000

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and the restoration mortars is the main issue to tackle, in order to achieve long-lasting restoration interventions. According to Schueremans et al. (2011), the compatibility will direct the intervention decisions towards the concept of suitable preservation, and will define the boundaries of functional and technical requirements that will limit the impact of intervention. Lime has been used extensively in building conservation actions, before being replaced by cement, earlier in the previous century. However, the use of cement as the main restoration material was strongly criticized in the precious decades, since it resulted in incompatible interventions and acceleration of the deterioration process (e.g. Fang & Chang, (2015)). Therefore, the need to use of lime as a restoration material has been gaining scientific interest again. A. S. Silva et al. (2014), also, suggested that the application of lime-based mortar mixtures is necessary in order to achieve the required compatibility. Many advantages arise from the application of lime-based binders in this field, in contrast to cement mortars. These advantages include low manufacturing temperature of binders, high vapor perme ability and the ability to accommodate movement due to a low modulus of elasticity and through micro-cracking, as well as the ability of self-healing of the mortars. In conservation applications, metakaolin (MK) has emerged as a popular supplementary cementitious material to partially replace ordinary Portland cement (OPC) due to its high pozzolanic reactivity and surface area. Metakaolin by its important specific surface area and its reactivity improves the mechanical behavior at early curing stages. Abdelli et al. (2017) reported that MK is obtained by dehydroxylation of the kaolin by calcination according to the following formula: Al 2 O 3 (SiO 2 )2(H 2 O)2 (kaolinite) → Al 2 O 3 (SiO 2 )2(H 2 O)x + (2-x)H 2 O (metakaolinite) (1) Many researchers exploited the synergistic effect between the metakaolin blended cement and calcium carbonate, so that the performance of the cement mortar can be further improved, including Qian et al. (2019). By partially replacing the OPC with metakaolin, the mechanical properties of the produced concrete can be significantly improved, since calcium silicate hydrates (C-S-H), can be produced through the pozzolanic reaction between the calcium hydr oxide (CH) and the metakaolin. In addition, the large surface area of the metakaolin provides more nucleation sites for the hydration of the OPC. The rate of substitution is the subject of interest of many researchers and generally is fixed after optimization of the mechanical results. Aggelakopoulou et al. (2011) proposed mortars for restoration purposes by mixing metakaolin /hydrated lime (at a ratio of ≤ 1 by mass). As the metakaolin/lime ratio increased, the content of total bound water, the static modulus of elasticity, the compressive and flexural strength increased while the pore size distribution shifted to smaller values. Wianglor et al. (2017) replaced part of the Portland cement with metakaolin at 70, 80, 85, 90, 95 and 100 % by mass of binder. The setting time and drying shrinkage decreased, when metakaolin replacement level increased, while the compressive strength of alkali-activated metakaolin cement increased with the increase of OPC content and increased with temperature. Nevertheless, Silva et al. (2015) noticed that the compressive strength was not improved when natural hydraulic lime or cement contents up to 25 % were added. It was concluded that hydraulic lime or cement can contribute to achieve higher mechanical strength level at early ages and a faster setting time. Metakaolin could greatly enhance the mechanical properties of cement mortars according to Zuo et al. (2020). Additionally, Maravelaki-Kalaitzaki et al. (2005) proposed a repair mortar consisting of natural hydraulic lime as binding material. Due to the absence of soluble salts in the repair mortar after a 3-year application period, together with the development of hydraulic phases like calcium silicate hydrates, consistency between old and repaired structure was noticed. Similar results were noticed by Velosa et al. (2016). A remarkable conclusion that arises is that the use of greater percentages of pozzolan in a mortar doesn’t necessarily imply improved characteristics. The above review indicates the beneficial role of the lime-metakaolin mortar as well as the potential of these binders to be compatible with the target structures. However, in order to support the application of lime-based structures to Cultural Heritage monuments, the compatibility and the performance of the new matrices must be studied extensively, in order to provide safe and reliable solutions and avoid cracks in the interface between the original material and the restoration mortar. In the present article, the mechanical properties of seven building material matrices were examined with different percentages of Lime (L), Metakaolin (MK) and white Cement (CEM). For the accepted water/binder ratio (W/B), the mortars were produced and, then, the mechanical performance of the pastes was studied by means of compression and