Abstract:
Clay tolerant superplasticizers can enable the use of low quality, clay containing aggregates and also alleviate the need for washing clay containing sand used in concrete fabrication, thereby promoting sustainable concrete production. Previous studies considered specific aspects of polycarboxylic ether (PCE) superplasticizer molecular design which influence their clay tolerance. In this study, the effect of a combination of various aspects of PCE molecular design on their clay tolerance was investigated. The aim of the study was to determine PCE structural differences that are more tolerant to clay contamination of sands used in concrete making. The objective was to evaluate the effectiveness of six recently developed proprietary PCEs with varying molecular designs in concretes fabricated with clay containing sands sampled from three South African quarries. The PCEs were evaluated by their ability to retain good workability and compressive strength in concretes fabricated using a standard laboratory clay free sand and high clay content sands from the three quarries. Workability was evaluated through slump flow tests and the compressive strength was evaluated through 24 h and 28 d compressive strength tests. Incorporation of an acrylic based PCE with low side chain graft ratio and long side chain lengths in the concretes resulted in a higher initial slump flow of at least 200 mm at 5 min, indicating relatively good workability, which however deteriorated rapidly in concretes made with the clay contaminated sands. Unexpectedly, the 28 d compressive strengths of concretes fabricated with higher clay content sands were found to be comparable to the compressive strengths of concretes made with the clay free standard sand when three of the PCEs were used. High compressive strength in the concretes fabricated with the sand containing the most clay was accomplished by incorporating a PCE with very high molecular weights, very long side chain lengths and a low graft ratio.
