The role of calcium silicates and quicklime on the reactivity of rehydrated cements

Construction and demolition waste recycling is mandatory in the context of circular economy. Specifically, applications for the cementitious waste fines are less discussed in scientific literature and barely implemented. Apart from the use of cementitious waste fines as raw materials for clinker production, thermal treatment by the decomposition of fines at temperatures lower than 600 °C has been explored to produce an alternative binder implying under this limit in low CO2 emissions during production. However, there are doubts if its reactivity results from the hydration from residual calcium silicates, and quicklime (CaO) eventually formed after the thermal treatment. This paper investigated quantitively the contributions of calcium silicates and quicklime by thermogravimetry (TG) and quantitative X-ray diffraction (QXRD) on the reactivity of the rehydrated cements. After hydration of the high initial strength Portland cement, only 5% (g/g) of calcium silicates remained, mostly larnite. After dehydration at 500 °C, only 6% (g/g) of quicklime was formed due to the decomposition of portlandite. Intermediate silicates increased in function of temperature of dehydration until 500 °C, but the contents were lower than 18%-g/g. Those phases can explain partially the heat released during rehydration. The heat seemed to be also related to the chemically bound water in the reformation of amorphous C-S-H. In fact, for a fixed water/cement ratio, the increase of chemically bound water explained both, the increase of heat released and the compressive strength of the rehydrated cements. This supports our finding that the calcium silicates or quicklime are not the only responsible for the reactivity of this binder.