Influence of trace elements on Ye’elimite-Rich Cements

Abstrakt

The production of CSA cements is more environmentally friendly than that of classic Portland cement as it releases less CO2 and has a good potential for encapsulation of waste from other industries. The CO2 reduction depends on the composition of the CSA and is due to three factors: 1) less emissions from decarbonation in the kilns; 2) lower clinkering temperature, so less fuel is needed; and 3) it is easier to grind, which leads to reduced indirect emissions.

The objective of this research was to shed light on the influence that some trace elements contained in various alternative waste materials have on the properties and characteristics of calcium sulfur-aluminate cement. To conduct this research, the search engine «Google Scholar» was mainly used and a total of thirty studies were collected and selected regarding CSA, BYF, BCSA, CSAB, ACSA clinkers and also of the individual pure clinker phases that make up CSA cement such as ye'elimite, belite, ferrite and ternesite. The mainly observed parameters included the analysis of cement hydration reaction, compressive strength, microstructure, crystal morphology and the immobilization of individual trace elements.

From the results of about thirty studies published in the scientific literature regarding the influence of trace elements on CSA clinker, it was noted that the elements Cu, Li, Fe, Ni, Ba together with Ca and a low quantity of Cr tended to accelerate the hydration reaction of the clinkers. This reaction, on the contrary, was delayed mainly by the elements Zn, Pb, P, F, K together with Na, B, B together with Ba, B together with Na and a high quantity of Cr.

The early and late compressive strength of the cements was observed to be increased by Zn, B, B together with Ba, P and Li in form of LiCl and Li2SO4. The elements K, B together with Na, and Li in form of Li2CO3 were found to tend to decrease the compressive strength in comparison to the cement without pollutant.

Most metals showed a good to excellent degree of immobilization at conditions with not extremely high or low pH (Cd, Ni, Hg, Zn, Pb, Cu, F, As, Mn and Cr3+ at moderate concentrations). In contrast, Cr6+ and Cr3+ at high concentrations showed poorer immobilization. The pH was shown to be important regarding immobilization, in fact at high pH the elements Pb, S and Se and at low pH the element As, emerged their poor ability to be immobilized in the cement.

The comparison of clinkers with and without gypsum addition showed a great similarity of the hydration reaction and compressive strength development between the control and the doped cements, thus showing how the use of gypsum added before hydration can be useful to avoid as much as possible a deviation of the properties and characteristics of the cement to be used.