Proceedings of the XXIII Nordic Concrete Research Symposium, Aalborg, Denmark 21-23 August, 2017

The synergetic effect of SCMs, fly ash (F) and calcined clay(C) in ternary mixes at 20% replacement was evaluated. It was found that due to the complementary water demand of F and C, pastes made of ternary blends of FC-OPC showed similar or better rheology than OPC pastes. This was coupled with an increase in heat evolved during early age of hydration and a comparable 28 days compressive strength. The results indicate that ternary mix of FC-OPC can be future green cements, where C can be utilized even in the transition stage with existing SCM, such as F.publishedVersio

Alternative binders based on lime and calcined clay

Alternative binders are in this report defined as binders without Portland cement. In this particular project it will be based on calcined clay or fly ash as a source of reactive silica and alumina in combination with lime and calcium sulphate (e.g. gypsum) and/or calcium carbonate to stabilize special calcium aluminate phases like ettringite and/or calcium monocarboaluminate hydrate, respectively. Gaining strength is all about maximizing the transformation of liquid water to hydrates with hydraulic properties as fast as possible. Hence, admixtures speeding up the reaction kinetics can be part of the formulations as well. Two synergy principles have been described for making improved binders based on slaked lime and pozzolanic SCMs. One showing how neutral salts may accelerate by forming strong alkaline solutions in situ. Another one how calcium carbonate can play a role when alumina containing SCMs are used by leading to an even higher conversion of liquid water into solid hydrates leading to lower porosity and higher strength. An example have been showed for a binder consisting of calcined clay, slaked lime and calcium carbonate on how mortars can achieve a 28 days compressive strength of about 25 MPa and 3 day strength exceeding 10 MPa when cured at 20°C when accelerated by sodium carbonate. The strength may have been improved further by having a slightly higher ratio of calcium hydroxide. Calcined marl may substitute for calcined clay, but then the calcium carbonate content should be reduced in accordance with the remaining calcium carbonate content after calcination. The formulations are not entirely correct according to the outlined 2nd principle since they rely on bulk chemical compositions and the fact that the commercial clay/marl was containing inert quarts or feldspars. Ideally the formulations should be made according to the content of clay minerals and their compositions only after subtracting the inert components. However, such a composition of reactive material may be difficult to arrive at

Microstructure of clinker – fly ash interactions

The first objective was to find out why a white clinker from previous studies has a higher strength potential than the other 3 clinkers tested in the same study, as well as finding out whether or not it contains other aluminate compounds than C3A. Clinker δ gives higher early strength than the other clinkers because it contains anhydrite as flux and therefore has a total higher calcium sulphate content than the other cements when the same amount of gypsum is added to the clinkers. Clinker δ also has a higher C3A content (≈ 6%) than predicted from the Rietveld analysis and part of it probably as a glassy XRD amorphous phase with some fluorine. Clinker δ only has a marginally higher surface than the other clinkers, but substantially higher C3S content that will add to the higher early strength together with excess calcium sulphate not bound early by C3A being able to help accelerate C3S hydration. The other clinkers also contained substantially amounts of C4AF that has a slow hydration with large amounts remaining unhydrated at 28 days. The second objective was to find out why one particular ash out of 4 tested gave much higher strength than the others when replacing cement (clinker with gypsum) in mortars. Fly ash D is not a real fly ash, but a fluidized bed ash that consist of a much more open structure than the closed glassy, spherical particles of the other fly ashes. It also contains a lot more calcium oxide (17.9% CaO) than the other fly ashes (Fly ash B is the second highest with 7.1% CaO). Hence it is assumed to be more reactive than the other ashes. Fly ash D also contains considerable more sulphate (6.6% as SO3) than the other fly ashes (fly ash B is the second highest with 0.5% SO3) which would lead to more ettringite formed on the expense of AFm resulting in more water bound and hence higher strength. The higher sulphate content (and partly less aluminate) for fly ash D compared to the other fly ashes also leads to a less response of this fly ash to the synergy effect with limestone, since AFt is stabilized on the expense of AFm.publishedVersion102000442-