Comportamiento de morteros de cal aérea con aditivos

Resumen

Several additives were added to aerial lime-based mortars in order to check their possible efficiency: two different anionic surfactants -sodium oleate and calcium stearate-, a polypropylene fibre, two different commercial water retaining agents- hydroxypropylmethylcellulose and hydroxypropylguar- and a commercialized potato starch. Different properties of lime-based mortars were evaluated: fresh state behaviour through consistency, density, water retention, air content and setting time; hardened state properties such as density, water absorption through capillarity, water vapour permeability, long-term compressive strengths, pore structure through mercury intrusion porosimetry, and durability assessed by subjecting the samples to different environments -outdoor and indoor exposures, climatic chamber, SO2-chamber and freezing-thawing cycles. A clear improvement in lime-based mortars was achieved when sodium oleate was added: strong capillarity reduction and excellent durability in the face of freezing-thawing processes, without any compressive strength drop. The mechanism for this improvement was related to air void formation due to the air entraining ability of these surfactants. Insolubility of calcium stearate turned out to be responsible for fewer air bubbles and lower effectiveness. An improvement in some properties of aerial lime-based mortars - such as permeability, mechanical strengths, reduction in macroscopic cracks or durability in the face of freezing-thawing cycles - was achieved when polypropylene fibre was added at a low dosage. When a larger amount of additive was used, only the reduction in cracks and the durability of the material were improved. Hydroxypropylguar was observed to be clearly effective: it produced an increase in water retention capacity and setting time, slowing down the hardening process. Furthermore, durability in the face of freezing-thawing cycles improved due to the entrance of air. Hydroxypropylmethylcellulose did not behave as a water retainer in lime-based mortars, owing to its different structure. The behaviour of the starch polymer was found to be strongly dosage-dependent: it acted as a thickener when the incorporated dosage was up to 0.30% of total dry mortar weight (polymer molecules were adsorbed onto lime particles acting as a flocculant); conversely, above that dosage, it behaved as a plasticizer (steric hindrance and electrostatic repulsive forces appeared, leading to a dispersion mechanism). In the last case, the matrix of the hardened mortar presented great coherence, owing to its large density and low porosity, characteristics which led to lower capillarity and permeability absorptions, better mechanical properties and improved durability. For all the studied mortars, the durability was found to depend on the water absorption capacity of the samples, which was related to their pore size distribution and air-content. High dosages of water repellents were necessary to enhance the durability, sodium oleate being the most effective additive to endure freezing processes. Also the lowest tested dosage of fibre, hydroxypropylguar and the starch proved to be useful. SO2 deposition caused the formation of calcium sulphite hemihydrate as the main degradation product. A very small amount of calcium sulphate dihydrate was observed. For the first time, a crystal habit composed of acicular agglomerates of calcium sulphite hemihydrate was detected in SO2 deposition on calcareous materials.