Improving lime-based rendering mortars with admixtures

The present work presents focuses on the use of different admixtures for the development of rendering lime-based mortars with improved adhesion and durability, as well as reduction of cracking. To this aim, combinations of an adhesion improver (ethylene-vinyl acetate copolymer, EVA), a water repellent agent (sodium oleate), a viscosity enhancer (a starch derivative) and a mineral admixture (pozzolanic addition of nanosilica or metakaolin) were tested. The renders were applied on four different substrates (sandstone, limestone, granite and brick) to assess their performance. The influence of the admixtures’ combination on fluidity, stiffening time, adhesion, cracking, compressive strength, pore structure, frost resistance and durability against magnesium sulfate attack was evaluated. The EVA admixture was seen to enhance the adhesion when used in combination with oleate, metakaolin and starch. This combination also led to a minimized cracking. Opposite trends between adhesion and cracking were observed as a function of the porosity of the substrates and of the presence of small-sized capillary pores. The interferences with the carbonation accounted for the drops observed in compressive strength for the nanosilica-free tested renders; nanosilica-containing renders showed good compressive performance, due to the filling effect of the admixture and to the C-S-H formation. The use of most of the admixtures’ combinations was seen to clearly enhance the durability of the renders, in the face of freezing-thawing cycles as well as sulfate attack, proving the applicability of these lime-based renders for repair works of the Cultural Heritage and for new Civil Engineering applications