ABSTRACT:The increasing use of lime-based mortars for the restoration of historic buildings and structures justifies the research on these materials. The focus of this paper is the effect of technological variables on pore structure and mechanical properties of lime-based mortars. The influence of curing time, binder:aggregate ratio, aggregate attributes and porosity is discussed. Mortars prepared with aerial lime, varying aggregate types and binder:aggregate ratios ranging from (1:1) to (1:5) by volume were tested. Compressive and flexural strength measurements, as well as X-ray diffraction and thermal studies, were performed after 3, 7, 28, 91, 182 and 365 days. A strong increase in strength of mortar mixtures after 365 curing days (as compared to 28 curing days) is found. In spite of the fact that larger amounts of binder increase the total porosity, the strength of these mixtures is also increased. A good interlocked structure is obtained as binder contents increase. Also, higher porosities allow better portlandite carbonation. A relationship between mechanical properties and pore structure was established. However, in case of binder excess, the increase in voids leads to a strength reduction. The use of calcareous aggregates improves strength more as compared to the use of siliceous aggregates. Factors as grain size distribution and grain shape of the aggregates have also been considered.

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Two different anionic surfactants, sodium oleate and calcium stearate, commercialized as water repellents for cement-based mortars, were added to lime-based mortars in order to check whether they were improved by these admixtures. Different properties of limebased mortars were evaluated: fresh state behaviour through 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 means of freezingthawing 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 -as SEM examination revealed -and showed lower effectiveness.3

Specimens of aerial and hydraulic lime-based mortars to be used in restoration works were prepared, hardened and subjected to different environments in order to study their mechanical behavior and durability. Outside exposure, weathering cycles in a climatic chamber, SO 2 -rich environment, freezing-thawing cycles and indoor exposure were selected to expose (as control group) the mortars.Flexural and compression strength tests were performed at 7, 14, 21 and 28 days. Porosity values and SEM-EDAX analysis were used to evaluate the microstructural changes.Flexural strength has been strongly influenced by the RH of the environments. Outside exposure improves, in general, the compressive strength, whereas SO 2 -chamber only provides the strength in hydraulic specimens. Porosity reduction has been related with a strength increment.In climatic chamber, the porosity increment matches a gradual higher degree of alteration. A strength reduction has been determined through a fracture mechanism using the cracks of climatic alteration. In SO 2 -chamber, sulfation appears as a surface phenomenon, giving gypsum in aerial specimens and gypsum and syngenite in hydraulic specimens, as SEM/EDAX confirms. Freezing-thawing cycles showed a high destruction capacity. Hydraulic specimens endured better durability tests than aerial specimens: a discussion on the mortar durability was also introduced.

Lime-based mortars modified with admixtures were prepared and subjected to different environments such as outdoor and indoor exposures, climatic chamber, SO 2 -chamber, and freezing-thawing cycles. The influence that the different admixtures (water repellents, water retainers, polypropylene fibre and a viscosity modifier) had on the pore size distribution of the hardened specimens was assessed and related to the water absorption capacity, and hence to the durability. Ageing resistance and mechanical strengths improved when additives reduced the water intake and increased the aircontent. High dosages of water repellents were necessary to enhance the durability, sodium oleate being the most effective additive to endure freezing processes. Also the low tested dosage of fibre, a water retainer (guar gum derivative), and a starch proved to be useful. SO 2 deposition caused the formation of calcium sulphite hemihydrate as the main degradation product. A very small amount of calcium sulphate dihydrate was observed. A crystal habit composed of acicular agglomerates of calcium sulphite hemihydrate was detected in SO 2 deposition on calcareous materials.3