Modification of Composition and Microstructure of Portland Cement Pastes as a Result of Natural and Supercritical Carbonation Procedures

Abstract

This study aims to analyze the effects of supercritical carbonation (CO2 at 20 MPa and 318 K) on the physicochemical properties of ordinary Portland cement pastes. The evolution of the main crystalline phases of the cement pastes during carbonation was determined by means of X-ray diffraction and thermogravimetric analysis. The pore structure was analyzed by low-temperature N2 adsorption−desorption and mercury intrusion porosimetry techniques. Finally, the microstructure of the samples was observed by using scanning electron microscopy coupled with energy-dispersive X-ray detection for chemical analysis. For a natural carbonation process, diffusion of CO2 into the pores of the cement paste is considered as the rate-controlling step. Instead, the accelerated reaction kinetics of calcium carbonate precipitation in the supercritical process was chemically controlled by the detachment of calcium ions from solid portlandite or CSH gel. The total pore volume of the studied cement pastes decreased with carbonation, which was associated with the deposition of the formed CaCO3. Samples carbonated under the supercritical conditions developed a higher volume of gel pores than those obtained by natural carbonation.

Supercritical carbonation of calcareous composites: Influence of curing

“…associated with intrinsic physico-chemical changes in the matrix), as the strength is increased independently of the porosity. This is interesting since many literatures [3][4][5][6][7][8]18,19] explain the enhancement in strength of carbonated cementitious matrices purely by reduction of porosity through formation of CĈ crystals and their precipitation in the matrix pores; clearly this is not the case in this instance. Furthermore, the C-S-H phase must be much more susceptible to SCC carbonation than the crystalline phases.…”

“…These reactions form calcium carbonate, hydrated silica (as a result of decalcification of C-S-H gel) and other minor products including sodium and potassium carbonates and hydrated alumina [4][5][6][7][8]. Compared to natural or ordinarily accelerated carbonation however, SCC is greatly accelerated, with complete carbonation of engineering-sized components being achieved in hours rather than years.…”

“…Compared to natural or ordinarily accelerated carbonation however, SCC is greatly accelerated, with complete carbonation of engineering-sized components being achieved in hours rather than years. This is mainly attributable to the significant rise of CO 2 solubility in the pore solution, and the relative ease with which high-density scCO 2 can penetrate and diffuse into the cement paste pore network [7,8].…”

“…The carbonation process also alters the microstructure of calcareous based composites, [1,7,8,[11][12][13][14][15]. The molecular volume of calcium carbonate (normally calcite) is 11.2%…”

“…Thus the calcite precipitated within the hardened cement paste (hcp) matrix during carbonation (both indirectly from the decalcification of C-S-H gel and accelerated hydration/carbonation of residual unhydrated cement particles, and directly from carbonation of the free calcium hydroxide dispersed through the hcp) fills up pores with small tightly packed crystals of calcite. This reduces the total pore volume, pore size and permeability of the composite [4,6,10] and also enhances the compressive and flexural strength [7,8,10,16,17]. Accordingly, much recent work has focussed on documenting the improvements in the microstructural, mechanical and durability properties of cementitious composites afforded by treatment with scCO 2 [6,[10][11][12][13][14][15][18][19][20].…”