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Supplementary cementitious materials (SCMs) are beneficial for improving concrete's long-term performance, durability, and environmental impact. Impure clay sources are abundant and when calcined, may serve as suitable SCMs. This study presents and evaluates a method for enrichment of low-purity kaolinite clays to improve their reactivity and performance in concrete. A byproduct clay containing 27% kaolinite, 10% muscovite, and 52% quartz was obtained by drying aggregate wash water slurry from a commercial sand and gravel pit. Several methods were evaluated for separating the clay and non-clay minerals. Among others, dispersant-assisted sedimentation, using 0.06% sodium hexametaphosphate, was shown to be effective in enriching the clay content which, upon calcination, produced a purified calcined clay (PCC) containing 64% metakaolinite, 26% muscovite, and 2% quartz. Both the PCC and the non-purified calcined clay (CC) were evaluated for their physical and chemical properties, compliance with ASTM C618, pozzolanic reactivity, and their performance in mortar and concrete mixtures. The results show both SCMs to be valuable while PCC is clearly a higher performance pozzolan.

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This paper presents the results of a study on the evaluation of resistance of pastes from carbonated, low-lime calcium silica cements to external sulfate attack. The extent of chemical interaction between sulfate solutions and paste powders was assessed by quantifying the amount of species that leached out from carbonated pastes using ICP-OES and IC techniques. In addition, the loss of carbonates from the carbonated pastes exposed to sulfate solutions and the corresponding amounts of gypsum formed were also monitored by using the TGA and QXRD techniques. The changes in the structure of silica gels were evaluated using FTIR analysis. The results of this study revealed that the level of resistance of carbonated, low-lime calcium silicates to external sulfate attack was affected by the degree of crystallinity of calcium carbonate, the type of calcium silicate, and the type of cation present in the sulfate solution.

The shortage of conventional supplementary cementitious materials opens up the prospect for broader utilization of nontraditional and natural pozzolans (NNPs). The NNPs used in the study included three calcined clays, three volcanic ashes (also referred to as natural pozzolans), three ground bottom ashes, and two fluidized bed combustion ashes. The study focused on assessment of the performance of these NNPs in portland cement systems by evaluating the following parameters: (a) development of the heat of reaction in cement pastes containing 25, 30, and 35 % of NNPs by weight of cement (bwoc) by means of isothermal calorimetry; (b) measurement of the consumption of calcium hydroxide (CH) in cement pastes containing 0, 25, 30, 35, 40, and 45 % (bwoc) of NNPs after 7, 28, and 56 days of curing using thermogravimetric analysis technique; (c) determination and quantification of the reaction products by quantitative x-ray diffraction in cement pastes containing 0 and 25 % (bwoc) of NNPs after 7, 28, and 56 days of curing; and (d) measurement of the 7- and 28-day compressive strength of mortar cubes. In all cases, the cumulative heat values of pastes containing NNPs were higher than those of reference pastes containing inert fillers, confirming that all NNPs were undergoing pozzolanic reaction. For most of the NNPs, the highest amount of CH consumed was observed in systems with 35 % replacement level. The early-age CH consumption correlated well with the content of alumina in NNPs whereas the later-age data correlated better with the silica content of the NNPs. In terms of compressive strength development, the highest early-age values were observed in mortars containing calcined clays, whereas other NNPs contributed to strength development at latter ages.