Interaction between clinker, calcium sulphate and superplasticisers in cement-based mixes

Stone, Lorna (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Modern cement contains an increasing number of extenders, resulting in a variation in the composition of the cement, while the use of chemical admixtures is also becoming a normal addition to a modern concrete mix. These additions, when used in combination with each other, often result in unpredictable or even undesired concrete behaviour, better known as concrete incompatibilities. Superplasticisers, in particular, are known to be one of the most common admixtures to be associated with these incompatibilities. A complex interaction exists between these additions and the cement, especially the clinker phases thereof, in a concrete mix, and while the mechanism of superplasticisers is relatively well known, it is still not clear as to why these incompatibilities occur. Research suggests that the positively charged C3A component of cement influences the absorption of the superplasticiser in a concrete mix, instead of reacting exclusively with the sulphate found in the concrete mix, as when no superplasticiser is added. While no definite statement can, however, be made as to why these compatibility issues between cement and superplasticisers occur, the reaction between calcium sulphate, C3A and superplasticiser is believed to be the most likely justification and is therefore looked into. This study investigates the effect of added bassanite (CaSO4·0.5H2O) and gypsum (CaSO4·2H2O) respectively when used in combination with different types and dosages of superplasticisers in an attempt to identify incompatibilities that may arise due to the combination of these additions, while also varying the clinker components, especially the C3A percentage. The objective of this study is purely to identify compatibility issues as well as the combination of factors leading up to these incompatibilities, in an attempt to understand the reaction between superplasticisers and cement, which can then be further investigated to eventually be able to eliminate undesired concrete behaviours. The results show that the addition of both the gypsum as well as the bassanite, decreases the flowability of the paste, as the dosage increase. On the other hand, Polycarboxylic Ether (PCE) superplasticisers increase the flowability of the mixture, proportionally to the dosage of the superplasticiser, with the Naphthalene based superplasticiser, being less effective than the PCE superplasticisers. As specified by the manufacturer, CHRYSO Optima 206, a PCE based superplasticiser (SP2), is more compatible with cements manufactured in the Western Cape, which in this study is Clinker 2. CHRYSO Optima 203, a PCE based superplasticiser (SP3), is specified to Various effects on the strength gain, setting times and flowability occurred when considering the different combinations of additions to the reference mixes, with possible compatibility issues being the cause thereof. The use of the clinker containing the medium C3S, C2S and C3A, and high C4AF content was found to result in more compatibility issues than that of the other two clinkers considered, while the high C3S, and a low C2S, C3A and C4AF content clinker was found to be associated with slightly less concrete incompatibilities. The high C2S and C3A, medium C4AF and low C3S content clinker showed hardly any incompatibilities when used in combination with calcium sulphate and superplasticisers. This study only aimed to identify these inconsistent behaviours and incompatibilities as a basis for further research, needed to confirm the chemical reaction responsible for theseVarious effects on the strength gain, setting times and flowability occurred when considering the different combinations of additions to the reference mixes, with possible compatibility issues being the cause thereof. The use of the clinker containing the medium C3S, C2S and C3A, and high C4AF content was found to result in more compatibility issues than that of the other two clinkers considered, while the high C3S, and a low C2S, C3A and C4AF content clinker was found to be associated with slightly less concrete incompatibilities. The high C2S and C3A, medium C4AF and low C3S content clinker showed hardly any incompatibilities when used in combination with calcium sulphate and superplasticisers. This study only aimed to identify these inconsistent behaviours and incompatibilities as a basis for further research, needed to confirm the chemical reaction responsible for these occurrences.

AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107864
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