Optimisation of concrete mix design with high content of mineral additions: effect on microstructure, hydration and shrinkage

linked to the decarbonation of limestone conducted at high temperature and results in a

significant release of CO2. This thesis is part of the project EcoBéton (Green concrete) funded

by the French National Research Agency (ANR), with a purpose to show the feasibility of

high substitution of cement by mineral additions such as blast furnaces slag, fly ash and

limestone fillers. Generally for high percentages of replacements, the early age strength is

lower than Portland cement concrete. To cope with this problem, an optimisation method for

mix design of concrete using Bolomey’s law has been proposed. Following the encouraging

results obtained from mortar, a series of tests on concretes with different substitution

percentages were carried out to validate the optimisation method. To meet the requirements of

the construction industry related to performance of concrete at early age, which determine

their durability, a complete experimental study was carried out. Standard tests for the

characterization of the mechanical properties (compressive strength, tensile strength, and

setting) allowed to validate the choice of mix design on the basis of equivalent performance.

We focused on the hydration process to understand the evolution of the mechanical

properties. Setting time measurement by ultrasound device at different temperatures (10°C,

20°C and 30°C) showed that ground granulated blast furnace slag (GGBFS) and fly ash

delayed the setting process, while use of limestone filler may accelerate this process.

Calorimetric studies over mortars and concretes made possible to calculate the activation

energy of the different mixtures and a decrease in heat of hydration of concretes with mineral

additions was observed which is beneficial for use in mega projects of concrete. Scanning

Electron Microscopy observations and thermal analysis have given enough information about

the hydration process. It was observed that the hydration products are similar for different

concrete mixtures, but the time of their appearance and quantity in the cement matrix varies

for each concrete mix.

Last part of the thesis was dedicated to the study of main types of shrinkage. First of all,

deformations measured were correlated to hydration, capillary depression and porosity

evolution. Results allowed concluding that the use of mineral additions has an actual effect on

the plastic shrinkage behaviour, but its impact is not proportional to the percentage of

additions. Substitution of cement by the additions seems to have a marked influence on the

kinetics of the shrinkage without any effect on its long term amplitude. The study of

restrained shrinkage under drying conditions by means of ring tests showed that concretes

with high percentage of slag addition seem more prone to cracking than the Portland cement

concretes.