Limestone and slag exert physico-chemical effects on cement hydration and the resultant microstructure. In limestone containing cements, the carbonate species interact with the dissolved aluminates to form carboaluminates, thus stabilizing ettringite. Reaction of slag mainly produces additional C-S-H and a hydrotalcite phase; refining the pore structure. Recent literature suggests an enhancement of the carbonate-aluminate interactions in ternary systems of OPC, limestone and alumina-rich SCMs such as fly ash and slag. Both limestone and SCM exert a filler effect on the cement clinker. However, insufficient information exists on the threshold of limestone-slag combination which maximizes the physico-chemical benefits. Optimizing the limestone-slag content offers the additional benefit of lowering the clinker and/or SCM contents in composite systems. In this paper, the impact of higher limestone contents upon slag hydration and microstructure up to 90 days is presented. Hydration was followed by isothermal conduction calorimetry and chemical shrinkage while microstructure evolution was examined by scanning electron microscopy, x-ray powder diffraction and thermal analysis. The clinker to SCM ratio was kept at 50:50 while the sulfate content in all mixes was 3%. The results show the consumption of limestone in the course of hydration. Limestone, in addition to providing nucleation sites for clinker hydration, also enhances slag hydration. With respect to the hydrates formed, ettringite was also stabilized while hemi-carboaluminate was observed in all mixes after two days. Coexistence of hemi- with mono- carboaluminnate was observed after 28 days in the limestone composite cements. The data shows an increased degree of clinker and slag hydration with increasing limestone content. The possible reasons for the observations, including nucleation, availability of space for hydrates to grow and the effect of the interplay between aluminates, sulphates and carbonates on the microstructure will be discussed. Originality The present study further examines the synergy between carbonates and aluminates in ternary blends. However, previous studies have considered limestone contents of around 5%. Previous studies have also mainly considered fly ash as the alumina source due to alumina being more abundant in this material than in slag. This paper has also considered considerably higher limestone levels, i.e. 10% and 20%. Using a combination of advanced characterization techniques it has been possible to examine the degree of hydration of each of the components within the blends, thus shedding light on the interactions within the ternary blend. The benefit at these levels of limestone is to harness the combined filler effect on clinker hydration as well as the synergy between the extra aluminates dissolved from slag and carbonates from limestone. This work shows that, the interaction between carbonates and alumina significantly affects the porosity and phase assemblages in ternary blends with slag. The observed improvement in slag hydration in the presence of limestone is novel. These results demonstrate that the limestone exercise the filler effect not only on the cement clinker but on slag as well.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)