Low-Grade Kaolin Clay Natural Resource for Use as a Supplementary Cementitious Material in Structural Concrete Elements

At the turn of the century, low-grade kaolin clay (LGK) was presented as a widely available and potential natural resource of supplementary cementitious materials (SCM) for producing high performance concretes. In using LGK as an SCM, clinker production is reduced, aiding environmental concerns with associated CO2 emissions. These types of clays have very heterogeneous mineralogy and microstructural characteristics, causing unique influences on concrete structural and flow performance, although these influences are not well understood. This research studies LGK natural to the state of Florida and assesses their potential use as high performance SCM. Ten kaolin clays were obtained through a detailed search of the Florida Department of Environmental Protection database for clay resources. Prior to assessing performance, a multi-technique approach was used to analyze clay mineralogy using X-ray fluorescence (XRF), X-ray diffraction (XRD), infrared spectroscopy (IR) and thermogravimetric analysis (TGA). The multi-technique approach provided an accurate procedure for assessing LGK mineralogy while considering structural disorder. Accounting for structural disorder during mineralogical characterization is almost always overlooked resulting in semi-quantitative results only. Mineralogical analyses revealed LGK kaolin mineral contents ranging from 70 – 93 wt. %, indicating Florida kaolin clay poses as a potential resource for the production of SCMs. The investigation of various LGK calcination temperatures and accompanying clay mineral transformations are discussed, being that the published literature has focused on that of highly crystalline and pure clay minerals. Furthermore, assessment of calcined clay reactivity with ordinary portland cement (OPC) using mortar compressive strength tests are presented to qualify their strength performance exceeds that noted in ASTM standards. The influence of clay mineralogy, surface area, porosity and kaolin structural disorder on strength gain is discussed. However, the compressive strength of mortars was overwhelmingly dependent on clay kaolin content. All clays tested surpassed ASTM standard specifications and are considered suitable for use in concrete as Class N (natural) pozzolans. Flow investigations of high purity calcined kaolin (metakaolin (MK)) clay have revealed that clay substantially reduces blended cementitious system workability. Yet, there is a lack of literature detailing the rheological influence of calcined LGK with vastly different chemical and microstructural properties compared to that of MK. Variations in the material properties of LGK are expected to significantly influence flow behavior but this has not been addressed. This research provides necessary scientific knowledge on the rheological influence of LGK considering their heterogeneous material properties. Knowledge of LGK material properties is expected to shed insight towards a given clay’s particular influence on rheology, which would yield invaluable information towards system pump-ability, formwork filling, consolidation and self-compaction, all of which are vital properties for LGK to be considered for structural applications. The effects of calcined LGK on the rheological performance of blended cementitious systems was investigated using multiple physical, chemical and electro-chemical characterization techniques. Several material properties were assessed including: clay particle morphology, specific surface area, porosity, particle size distribution (PSD), mineralogy and colloidal properties such as packing density, zeta potential and ionic mobility. The blended system yield stress and viscosity were computed using the Bingham fluid model. The findings indicate that the yield stress and viscosity were significantly affected by the clay kaolin content rather than particle size distribution, surface area or porosity. However, a unanimous relationship between the fluid properties of all calcined clay pastes investigated was not predictable with clay kaolin content alone. The effects of solid particles, present as filler materials, were observed to cause slight shifts in the rheological performance of blended systems from being a perfect function of clay kaolin content. It is shown that the zeta potential and ionic mobility of calcined clay paste linearly relate the to yield stress and viscosity; thus, indicating a direct relationship to the system’s colloidal stability. Both clay kaolin content as well as the filler content control the zeta potential of the blended system. Therefore, it is important to assess the zeta potential of the blends in order to properly understand the rheological performance of LGK blended cementitious systems. Accurately analyzing paste rheology, using concentric cylinders and the Reiner-Riwlin procedure, is commonly overlooked which may lead to biased or erroneous interpretation of the data. Using this procedure, a linear yield stress model was developed for 20 wt. % LGK blended cements with or without fine aggregates present. The model inputs are LGK kaolin content and all solids PSD, which captures any filler effect on the flow of the blended system. The model offers insight into how a given LGK should be processed, whether ground or separated, and how much of a particular aggregate and plasticizer is needed to ensure the desired flow performance of blended cementitious systems containing LGK is attained