Abstract
The effect of kaolinitic calcined clay and slag on the hydration of limestone-containing ternary blended cements was investigated. The effect of alumina from different sources of SCMs was considered to activate the formation of carboaluminates. Ternary blends with 50% ordinary portland cement clinker, 45% blends of limestone calcined clay (LC2) in 1:2 blend and slag limestone blend (SLS) in 2:1 mix proportion with 5% of gypsum were studied. The hydration behaviour was analysed based on cement mortar compressive strength, heat of hydration using an isothermal calorimeter and bound water measured using thermal gravimetric analysis (TGA). In addition, the degree of hydration of clinker phases and the composition of calcium - alumino - silicate - hydrate (C-A-S-H) gels forming in two different systems were compared on 90 days hydrated samples analysed using X-Ray diffractometry (XRD) and scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDX) respectively. The results show a rapid early strength development in limestone calcined clay cement blend (LC3) but a lower clinker hydration in comparison with slag limestone cement blend (SLSC) at later ages. In both the cement blends the formation of hemicarboaluminate (Hc) and monocarboaluminate (Mc) was confirmed at 90 days, but the conversion of Hc to Mc was higher in SLSC. Results further confirmed a lower degree of hydration and higher alumina incorporation in the C-A-S-H gel in the LC3 comparison to SLSC. The presence of calcium hydroxide was also confirmed in the SLSC blend due to the hydraulic nature of slag that supported the later age conversion of Hc to Mc as not seen in LC3.
Highlights
Despite the relatively lower degree of hydration of limestone (LS) (1-3 g/100 g), it is known to significantly influence phase assemblage and sulphate balance [1,2,3,4,5]
The results show that the LC3 blend achieved a higher early strength at the age of 3 days and 7 days in comparison to the slag limestone cement blend (SLSC), while similar strengths were seen at 1 day
The compressive strength of LC3 was comparable to OPC up to 28 days, whereas the SLSC blend gained a lower strength than OPC and LC3
Summary
Despite the relatively lower degree of hydration of limestone (LS) (1-3 g/100 g), it is known to significantly influence phase assemblage and sulphate balance [1,2,3,4,5]. The formation of carboaluminates is favoured thermodynamically to the formation of monosulphate (AFm) [5], thereby preventing the conversion of ettringite (Aft) and helping in pore refinement due to the lower density of carboaluminate phases [1, 2, 6,7,8]. The conversion of alumina to carboaluminate phases reduces the availability of alumina ions in the solution, preventing the conversion of ettringite (AFt) to monosulphate (AFm) phase. Even when small quantities of LS are added to the cement, the formation of Hc and Mc as hydration products can be observed because only a minor part of LS gets involved in the reaction and most of unreacted LS particles can be found in the microstructure at later ages [9].
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