Synthetic zeolite pellets incorporated to air lime–metakaolin mortars: Mechanical properties
Introduction
The use of natural and artificial pozzolans as supplementary binder materials is widespread throughout the world in order to develop durable and high-performance mortars and concrete. Slag, fly ash, silica fume, rice husk ash, metakaolin, diatomite, zeolite, among others, are being used as blended admixtures.
Natural zeolites have been widely used as supplementary cementitious materials [1]. In the cement industry they are used as natural pozzolan in some regions of the world. In lime mortars, the behaviour of natural zeolites has been studied for a long time. The presence of authigenic zeolites has been detected in ancient Roman mortars cured in aggressive marine environment [47]. The formation of mineral phases on the pozzolan–lime–water system was reported by García et al. [17]. Mertens et al. [27] have studied the parameters that affect the pozzolanic reactions of common natural zeolites with lime. The pozzolanic reaction of lime with natural zeolites was also studied by Snellings et al. [38], Snellings et al. [37] and Moropoulou et al. [29]. The use of natural zeolite in a saturated lime solution at 40 °C was studied in order to determine its pozzolanic activity [46]. Studies related with the incorporation of synthetic zeolites in air lime and air lime–metakaolin mortars are scarce. Andrejkovičová et al. [2] explained the evolution of flexural and compressive strength, and dynamic elasticity modulus of air lime mortars with fine and coarse zeolite pellets up to 180 days of curing time.
Zeolites are hydrated crystalline aluminosilicate materials with three-dimensional skeletal structure that occur in nature, but can be synthetically manufactured for presenting highly porous microstructure promoted by a network of channels and interconnected voids. The frameworks are composed by repeated silicon tetrahedrons [SiO4]4−, that can be substituted by alumina [AlO4]5− in variable proportions. This structure is linked by sharing of oxygen atoms, whereas exchangeable alkaline and alkaline-earth metals compensate for the resulting charge deficit [25].
Molecular sieves are “tailored” synthetic zeolites which due to their precise pore size and shape, molecular polarity and chemical composition have developed selective adsorption properties, based on surface molecular scale phenomena in which gas or liquid molecules are attracted to the internal volume of the porous solid.
The diameter of the open structure typically ranges between 3 to 10 Angstrom [40] and determines the size of the molecules which can be adsorbed, such as NH3, H2O and H2. One of the most common molecular sieves is the “type A” framework or “Linde Type A – LTA” (Fig. 1), where the 3 Å, 4 Å, and 5 Å open-window diameter in α-cage is obtained with potassium (K-LTA), sodium (Na-LTA), and calcium (Ca-LTA) cation inclusion, respectively [24]. Zeolite type A is a synthetic zeolite widespread over the world and its cost is usually lower than other synthetic zeolites.
In many common adsorbent applications coarse spherical agglomerates or pellets of zeolites (Fig. 2) are manufactured, where the pellet contains, in addition to the zeolite material with diameter about 1–4 μm, a binder of an amorphous aluminosilicate (typically, kaolin or bentonite) or alumina. This palletized material has a high adsorbent capacity and presents high potential to be incorporated in mortars enabling adsorbed water to the system, since its structure contains inter-macropores that are inside the pellet and intra-micropores within the zeolitic material (Fig. 2). According to Muller et al. [30], the maximum water adsorption capacity for dry Na-A zeolite powder is 28 wt.% while Hunger et al. [20] present an experimental value around 25.5 wt.%. Whereby, in pellet form it is expected that this property exceeds 30 wt.% value.
The main aim of this study was to evaluate the behaviour of synthetic coarse and fine zeolite pellets incorporated in air lime–metakaolin mortars for repairing ancient masonry (render mortar) with compatible mechanical properties to be used in conservation and restoration of cultural heritage.
A secondary aim of the work was to check the validity of the experimental modified Chapelle test to evaluate the pozzolanic activity of synthetic zeolite pellets.
Section snippets
Materials and methods
Mortars were formulated with powdered commercial air lime (AL) CL 90 (Calcidrata, Portugal) and siliceous river sand. Mortars were prepared with air lime/sand volumetric ratio of 1:3. Air lime binder was replaced by: (a) 10, 20 and 30 wt.% of commercial metakaolin (MK) (C. Condestável, Portugal) and (b) the same mortar compositions in which air lime was substituted by 5 wt.% of coarse and fine synthetic zeolite pellets Phonosorb 551 (Grace Davison, USA).
The coarse zeolite (CZ) pellets used in the
Particle size distribution
The coarse zeolite pellets used in this study commercialized in bead form have a particle size range from 1.2 to 2.5 mm.
Particle size distribution for fine zeolite obtained after milling (grinding) the coarse material, air lime and metakaolin are reported in Fig. 3.
D50 value for fine zeolite, air lime and metakaolin is 16.0 μm, 3.2 μm and 10.0 μm, respectively.
Mineralogical composition
Fig. 4 shows the XRD patterns of the powder random oriented materials used in mortar preparation.
Sand is predominantly composed by quartz as
Conclusions
The main results related with the synthetic zeolite pellets, the incorporation of synthetic fine and coarse zeolite pellets to air lime and air lime–metakaolin mortars are reported:
- (i)
The mineralogical composition of the zeolite pellets does not have a significant influence in the mineralogical composition of the mortar. Leucite and nepheline are unreacted minerals and, except for pozzolanic phases promoted by the zeolite pellets, these sodium materials do not promote any type of salt
Acknowledgments
This research was supported by the Research Project METACAL – Study of Lime-Metakaolin Mortars for Building Conservation (PTDC/ECM/100431/2008) and by the Projecto Estratégico (PEst-OE/CTE/UI4035/2014) financed by the Fundação para a Ciência e a Tecnologia (FCT), to Cerâmica Condestável, Lda for supply the metakaolin and to Calcidrata – Industrias de Cal, SA for supply the air lime used in this work.
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