Modification of the Physical Properties of Natural and Artificial Adobe by Chemical Consolidation
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MODIFICATION OF THE PHYSICAL PROPERTIES OF NATURAL AND ARTIFICIAL ADOBE BY CHEMICAL CONSOLIDATION Richard L. Coffman, Neville Agnew and Charles Selwitz Getty Conservation Institute, Marina Del Rey, CA 90292 ABSTRACT The use of commercially available chemical consolidants (hexamethylene diisocyanate (HDI) - derived polymers and silane esters) has been shown to increase the compressive strength of adobe and other natural building materials. Laboratory and field testing of chemically-treated adobe and sand-clay mixtures have also revealed an enhanced resistance to disintegration by water. In HDI-treated materials this resistance to disintegration is attained without a significant loss of porosity. Because porosity is not lost and the adobe can still transmit water, the uptake of water by capillary rise is less destructive than for an adobe whose surface is sealed against moisture penetration. BACKGROUND AND INTRODUCTION This study was initiated by our interest in finding a chemical consolidant suitable for the preservation of historic and archaeological earthen (adobe) structures. Many building materials experience accelerated deterioration when subjected to wet-dry, and/or freeze-thaw cycling. This is especially true for adobe which easily loses its structural integrity when wetted. Our preliminary laboratory work examined the interaction of a number of chemical consolidants with adobe [i]. Once it was determined that isocyanates and silane esters could successfully consolidate adobe work focused on the interaction of those consolidants with pure clay components of adobe, because they are most affected by wetting. The results revealed that treatment of clays with isocyanates and silanes markedly increased the clay's water repellency, but did not consolidate the clay (2]. Chemically treated cylindrical plugs of pure clay, whether kaolin, calcium montmorillonite or sodium montmorillonite, did not form mechanically stable plugs with any compressive strength. To determine why consolidation was achieved with adobe but not with pure clay we turned our attention to the reaction of artificial adobes, (i.e. defined silica sand - clay mixtures) with isocyanates and silane esters. MATERIALS AND METHODOLOGY A "typical" artificial adobe mixture was manufactured by combining three parts quartz sand (of uniform particle size) and one part clay. This resulted in a workable, coherent, artificial adobe material. Three different clays were used to make three different sets of artificial adobe. Clays were obtained from the Clay Minerals Society, Source Clay Minerals Repository at the Dept. of Geology, Univ. of Missouri, Columbia, Missouri. They include: A) SWy-I, Na-montmorillonite, Crook County, Wyoming B) SAz-I, Ca-montmorillonite, Apache County, Arizona C) KGa-2, Kaolin, Warren County, Georgia. Mat. Res. Soc. Symp. Proc. Vol. 185. (01991 Materials Research Society
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Test plugs were manufactured by combining the sand-clay mixture with water to form a viscous slurry which was poured into small (22mm x 40mm) cylindrical molds. Plug
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