Thixotropic behaviour and structural breakdown of fresh cement paste: comparison between two types of VMA

A semi-empirical procedure is proposed to obtain a new insight into the structural breakdown of fresh cement pastes by using different nature and the dosages of viscosity-modifying admixture (VMA). For this approach, tests were conducted with two types of VMA at 21°C. The protocol used is based on a down stepwise shear rate sequence after shearing the material at a constant high shear rate and at a variable preshearing time. A destructuration potential K is obtained by the analysis of the breakdown area for each preshearing time. Results suggest different critical dosages of VMA. A maximum value of dosage of VMA is obtained from the results. Beyond this value, the tri-dimensional networks formed by the cement pastes are highly structured and consequently more difficult to deform by shearing.