Sustainability and cultural heritage conservation: re-use of sand from deconstruction renders

The European project SCORE (Sustainable COnservation and REstoration of built cultural heritage, GA 101007531) deals with a two-way assessment of impacts of the environment on materials and of materials on the environment. Indeed, climate conditions and atmosphere composition determine built cultural heritage (BCH) materials’ behaviour and, at the same time, BCH conservation processes (products and techniques) impact greenhouse gases emissions and then climate change.

The objective of this work is to study the feasibility of recycling old mortars and plasters from an existing building to extract the sand with the aim of reusing it to produce new mortars for the renovation of the built heritage. Old mortars and plasters were mainly made with "natural" sands, generally alluvium and river sands, available near the site. These sources of sand may no longer be available or have become scarce. The use of different sands from the original ones in restoration plasters has consequences on the aesthetic properties of the new plasters applied during the renovation, which often requires the use of additives and dyes. To solve this problem, one possibility is to extract the original sand from deconstruction mortars, taken for example from renovation sites.

We tested the proposed method on a site in Paris. Deconstruction mortars were crushed and the sand was recovered, characterized and used for the formulation of new mortars by adding lime and water. Washing methods with water and acid washing solutions have been tested in order to obtain sands free of debris and lime agglomerates. Citric acid was used because it presents good lime dissolution results and is quite easy to employ in restoration works. Microscopic observations were made to determine the effectiveness of each washing solution. Sands washed only with water show some remains of lime on the grain surfaces whereas washing the grains with citric acid solution (2 %) produces excellent results.

In addition, mortars formulations with different grain size of recycled sands were made. These formulations consist of a mixture of recycled sand, water and an additive lime in equal proportions. Specimens of 4 x 4 x 16 cm were produced and their mechanical properties were measured at 28 days of age. During the maturation time, the samples are kept in an environment with constant humidity and temperature, to optimize the conditions of hydration, carbonation and maturation.

The results obtained show that the mechanical properties of mortars made from recycled sand are acceptable and that these are intrinsically linked to the grain size and quality of the sands. The mechanical properties of mortars formulated with recycled sands of the right grain size are similar to those formulated with commercial natural sand. A large part of the recycled sand, after just washing with water, can be used in the formulation of new mortars with regard to standards and aesthetic properties.