1. INTRODUCTION

This report is a critical literature review of the types, performances and uses of stone consolidants. Processes responsible for the deterioration of stone and criteria for selecting stone consolidants are also reviewed. Stone consolidants are generally considered [1-4] to be deeply penetrating materials which have the ability to re-establish the cohesion between particles of deteriorated stone. A variety of materials have been used in attempting to consolidate deteriorated stone,including inorganic chemicals, organic polymers, alkoxysilanes, resins, and waxes, either by themselves or in combinations. Because of an increasing worldwide interest in preserving historic structures and an apparent acceleration in the rate of stone decay, the use of stone consolidants is growing 5,6]. However, concern is growing over the use of unproven materials on important historic structures. This review has been carried out under the auspices of the National Park Service to assess the current status of stone consolidation technology.

Stone preservatives whose chief functions are to prevent the ingress of moisture into water repellents, have been covered elsewhere [7-9] and are not included in the present report. Treatments with water repellents often do not have long term preservation effects and may even accelerate stone decay through two major processes [10,11]. First, water will often collect behind the treated stone and upon evaporation of the water any salts in solution will be deposited and crystallize in the untreated stone. This may lead to spalling of the treated stone from the untreated stone. In the second process, because of differences in thermal expansive properties of the treated and untreated portions of the stone, shear stresses may be generated that eventually result in interfacial delamination. Sleater [12] recently tested over 50 stone preservatives(1) and found that none of them gave satisfactory performance.

Numerous reports have been published on stone preservation and consolidation and an extensive bibliography covering the literature through 1963 was prepared by Lewin 13]. Excellent sources of information on the mineralogy, weathering, and conservation of building stones are the books by Winkler [14], Schaffer [15], and Warnes [16]. Because of the growing interest in the preservation of historic stone structures, several international symposia have been held recently on the deterioration and conservation of stone [17-20].

In this report the deterioration and consolidation of limestone and sandstone mainly will be addressed. This is because most of the problems encountered in the preservation of historic stone structures located in the United States are associated with these types of stone. Problems are also being encountered with the preservation of marble in countries where it has been extensively used, such as Italy and Greece.