Solidification/stabilization of toxic metals in calcium aluminate cement matrices

Abstract

The ability of calcium aluminate cement (CAC) to encapsulate toxic metals (Pb, Zn and Cu) was assessed under two curing conditions. Changes in the consistency and in the setting time were found upon the addition of the nitrates of the target metals. Both Pb and Cu caused a delay in CAC hydration, while Zn accelerated the stiffening of the mortar. Compressive strengths of the metal-doped mortars, when initially cured at 60ºC/100% RH, were comparable with that of the free-metal mortar. Three different pore size distribution patterns were identified and related to the compounds identified by XRD and SEM. Sorbent capacities of CAC for the toxic metals were excellent: a total uptake was achieved for up to 3 wt.% loading of the three metals. In this way, CAC mortars were perfectly able to encapsulate the toxic metals, allowing the use of CAC for waste management as proved by the leaching tests.

Study of the early hydration of calcium aluminates in the presence of different metallic salts

“…Both salts shortened the induction period: ZnCl 2 slightly reduced the induction period in comparison with the control sample resulting in a t max , time of the maximum of the heat release peak, of 7.16 h (7.65 for the control sample), whereas Zn(NO 3 ) 2 gave rise to a more intense reduction, t max = 3.64 h. It can be thus concluded that zinc chloride as well as zinc nitrate acted as accelerators when added to CA samples, shortening the induction period. This finding is in line with results published by Berger et al [17] for calcium sulfoaluminate cement and by our group for calcium aluminate cements [2], in which Zn(NO 3 ) 2 was seen to shorten the setting time of these cementitious materials. Another agreement can be mentioned regarding the moderate level of hydration that was observed for CA and CA 2 : this fact matches the XRD analysis of CAC with Zn(NO 3 ) 2 published in [2], and would explain the moderate decrease in compressive strengths observed for these samples when compared with plain samples.…”

“…This finding is in line with results published by Berger et al [17] for calcium sulfoaluminate cement and by our group for calcium aluminate cements [2], in which Zn(NO 3 ) 2 was seen to shorten the setting time of these cementitious materials. Another agreement can be mentioned regarding the moderate level of hydration that was observed for CA and CA 2 : this fact matches the XRD analysis of CAC with Zn(NO 3 ) 2 published in [2], and would explain the moderate decrease in compressive strengths observed for these samples when compared with plain samples.…”

“…We chose to test the 1 wt. % dosage in coherence with previous works by our research group testing the very same dosage and for comparative purposes [2,15]. This dose lies between lower dosages frequently used when metal salts were employed as setting modifiers and the higher ones reported by many other authors in works dealing with the influence of metals on the hydration of cementitious phases [8,11,16,17].…”

“…In Portland cement, Zn has been reported to show a deleterious effect, delaying dramatically the setting time and causing a severe decrease in mechanical strength [16]. In CAC systems, some controversial data have been reported: while ZnCl 2 has been said to be a strong retarder [11], Zn(NO 3 ) 2 has been identified as an accelerator of the setting [2,9]. In calcium sulfoaluminate cements, Zn 2+ was found to act shortening the setting time, although in this type of cements the effect of the counterion was proven to be essential [17].…”

“…The use of calcium aluminate cements as an efficient binding system to confine hazardous materials has shown a growing interest [1][2][3] 3 ), depending on temperature of the process, curing time, mineralogical composition of the raw cement material and water/cement ratio [4][5]. However, similarly to Portland cement systems, some metals can have a strong influence on the hydration of anhydrous calcium aluminate phases, resulting in a change of the strength development at very early ages [6].…”