Crystalline CaCO3 Research Articles

Effects of relative humidity on carbonation kinetics and strength development of carbonated wollastonite composites containing sodium tripolyphosphate

Assessing the impact of relative humidity (RH) on carbonation kinetics is crucial for the sustainable and high-strength advancement of CO2-activated Ca-bearing materials incorporating phase-controlling additives. This work focuses on the carbonation kinetics, mechanical properties, and microstructure evolution of carbonated wollastonite composites containing sodium tripolyphosphate (STPP) when exposed to various RH levels. Results show that RH plays an important role during the carbonation of wollastonite, functioning both as a reaction material and accelerating role for wollastonite carbonation. The carbonation rate and the phase transition reaction of poorly crystalline CaCO3 is accelerated at RH ranging from 70% to 95%, favouring to cementitious behaviour of CaCO3 and results in denser microstructure, especially for 85% RH. The carbonation reaction is composed of two distinct stages, namely, wollastonite dissolution and precipitation of the stage-1 and ion-diffusion controlling of stage-2. Among them, the addition of STPP prolong the carbonation duration of stage-1. The degree of carbonation (DOC) of the internal layer sample is higher than that of the outermost layer sample. CaCO3 and silica gel are evenly distributed indirectly, which reduces the elastic modulus at 85 % RH. However, regardless of RH, the cementitious efficiency of poorly crystalline CaCO3 is the highest, followed by calcite and silica gel. Consequently, STPP modified carbonated wollastonite shows highest strength when exposed to 85% RH (67.3 MPa at 7 days). Our study provides a unique way toward developing the STPP-containing carbonated wollastonite system for high performance carbonated materials.

Experimental study on the macroscopic and microscopic properties of cement paste backfill after treatment with carbon dioxide carbonize filling slurries during the mixing process

Introducing CO2 during the mixing process of filling slurry for wet carbonation enables simultaneous carbonation and hydration reactions of the carbonated filling slurry (CSL). This study evaluates the microstructural evolution and strength development of carbonated cemented paste backfill (CCPB) under different carbonation times, and discusses the carbonation products and carbon sequestration of CCPB. The results reveal that introducing 99% pure CO2 at a flow rate of 1 L/min for 10 min into the filling slurry in a non-sealed container effectively optimizes the microscopic pore structure and mechanical properties of CCPB, achieving a carbon sequestration rate of 6.42%. Short-term carbonation leads to a continuous decrease in the T2 relaxation signal of CSL in the early hydration stage, enhancing pore structure by converting secondary pores to micropores. The main carbonation product is calcite crystalline CaCO3, which forms a dense carbonation layer on the surface of the tailings particles. However, prolonged carbonation results in the formation of microscopic pore encapsulation layer, hindering further CO2 diffusion, reducing carbonation efficiency, and deteriorating the mechanical properties of CCPB. With a carbonation time of 10 min, the UCS and EM of CCPB with different cement-to-tailings ratios show significant improvement, and the specimens exhibit lower fragmentation and fewer secondary cracks, maintaining better integrity after failure. In conclusion, the wet carbonation of filling slurry not only enhances the strength of the filling material but also achieves the reutilization of gaseous and solid wastes through carbon sequestration filling method, providing an efficient and eco-friendly solution for mine backfilling.

Factors Affecting the Physical Properties of Microbial Induced Calcium Carbonate Precipitation (MICP) Enhanced Recycled Aggregates

Microbial-induced calcium carbonate precipitation (MICP) can enhance the physical properties of recycled aggregates. Compared to traditional technologies, MICP offers environmental benefits and produces no pollution. However, its mineralization efficacy is significantly influenced by the process parameters. To investigate this, an MICP mineralization test was conducted by manipulating various process parameters throughout the mineralization process. The water absorption rate, apparent density, and calcium carbonate content of the mineralized recycled aggregates were assessed to discern the impact of these parameters on the mineralization outcome. Further analysis using techniques such as thermogravimetric analysis (TG), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM) were employed to elucidate the mineralization mechanism of the recycled aggregates at a micro-level. The findings indicated that the MICP treatment induced bacteria to precipitate CaCO3, forming calcite crystalline CaCO3 within the pores and microcracks. This led to a denser interfacial transition zone and, consequently, improved the physical properties of the recycled aggregates. Optimal mineralization was achieved when the bacterial solution concentration was 1.4, the temperature and pH were 35 °C and 9, respectively, and the urea concentration, Ca+ concentration, and mineralization time were 0.5 mol/L, 0.5 mol/L, and 7 days, respectively. Under these conditions, the mineralized recycled aggregate exhibited a 16.07% reduction in water absorption, a 1.07% increase in apparent density, and a 2.28% change in mass.

Mechanisms and advancements in microwave-enhanced CO2 mineralization of lightweight porous concrete

Lightweight porous concrete (LPC) typically exhibits low early strength, which can be addressed by enhancing early strength through carbon dioxide (CO2) mineralization curing while achieving permanent CO2 sequestration. The efficacy of this process relies on maintaining optimal moisture levels. This study investigates the influence of moisture content on the performance of solid waste-based lightweight porous concrete blocks (RSFAC) subjected to microwave heating pretreatment. Moisture levels were adjusted to 10%, 20%, 30%, and 40% (initial water-to-cement ratio of 45.4%) before undergoing mineralization curing within an environment set at a temperature of 20 °C, relative humidity 75%, and a CO2 concentration of 40% for a duration of 0.5 h. The impact of moisture content on RSFAC's compressive strength, carbon sequestration rate, mineralization products, and microstructure was evaluated. Results indicate that compared to conventional oven pretreatment methods requiring nearly 10 h, microwave pretreatment technology achieves equivalent water content within just 50 min, significantly reducing the time needed for mineralization pretreatment. RSFAC, when pretreated with microwave heating, outperforms untreated samples (Ref-0 and Ref-1) in terms of compressive strength and carbon sequestration efficiency. Optimal performance was observed at a moisture content level of 30%, with the highest recorded compressive strength achieved at 1, 3, and 7 d respectively. At the curing age of 7 d, the compressive strength reached up to 2.32 MPa while the carbon sequestration rate reached approximately 11.61%. Furthermore, aragonite and calcite (tetrahedral and hexahedral), without poorly crystalline spherulites, were identified as the main crystalline CaCO3 generated by RSFAC under this specific moisture content condition; pores larger than 50 nm served as primary reaction sites for RSFAC mineralization curing, whereas there was a significant increase in pore proportion with sizes less than 20 nm after filling with mineralization products.

High performance CaCO3-based composites using sodium tripolyphosphate as phase controlling additive: Bamboo fiber driven high strength development

Due to the limited carbonation degree caused by the surface densification of carbonated products, the development of high-strength carbonated composites remains challenging. In this work, bamboo fiber (BFs) is utilized as a reaction reinforcing agent along with sodium tripolyphosphate (STPP) as a CaCO3 phase controlling additive to prepare high-strength bamboo fiber reinforced carbonated wollastonite composites (BFRCWs). The phase composition and microstructure are systematically investigated by multiscale physicochemical analysis, followed by the determination of macro properties and volume deformation. Results indicate that BF and STPP have a synergistic effect on the microstructural formation and macro performance of BFRCWs. STPP-treated BF (ST-BF) can serve as an internal curing agent and the porous structure of BF provides more channels for ion and CO2 transport, whereas CaCO3 phase composition and cementitious behavior is modified by STPP. The addition of ST-BF, particularly for long fibers, accelerates the carbonation reaction, resulting in an increased ratio of poorly crystalline CaCO3 and a refined pore structure. With increasing ST-BF dosage (0–3 vol%), the cementitious reaction is enhanced, but excessive fibers (3 vol%) incorporation introduces additional porosity, consequently reducing compressive strength. The desired pore structure with the optimal 2 vol% ST-BF (3–6 mm) shows the highest strength of 103.5 MPa at 28 days.

Study on the carbonation properties of BOFS with γ-C2S blending

β-C2S is the main active mineral component in Basic Oxygen Furnace slag (BOFS), while the content of β-C2S in BOFS is usually limited and fluctuated, which hindered the improvement of CO2 capture during carbonation. To enhance the carbonation reactivity of BOFS products, different ratio of γ-C2S was blended with BOFS powder and carbonated, followed by hydration curing. XRD and TGA analysis were used to determine the mineral composition of each blending group after carbonation. SEM analysis was applied to observe the microstructure, and the MIP method was used to analyze the pore size structure. The results show that the carbonation degree and compressive strength of BOFS cubes were increased by the addition of γ-C2S. With 20 wt% blending of γ-C2S, the carbonated BOFS obtained the optimum increasement of carbonation degree and compressive strength, reaching 39.3% and 42.4 MPa, respectively. However, the subsequent hydration reactivity of BOFS was inhibited when blended with 20 wt% of γ-C2S. Despite this, the increased poorly crystalline CaCO3 filled the system and densified the matrix, contributing to a 17.9% increase in compressive strength of SH20 after 56 days of hydration curing.

Preparation and performance of self-healing polymer cement-based waterproof coating with ion chelator

Purpose This study aims to develop a polymer cement-based waterproof coating with self-healing capability to efficiently and intelligently solve the building leakage caused by cracking of waterproof materials, along with excellent durability to prolong its service life. Design/methodology/approach Ion chelators are introduced into the composite system based on ethylene vinyl acetate copolymer emulsion and ordinary Portland cement to prepare self-healing polymer cement-based waterproof coating. Hydration, microstructure, wettability, mechanical properties, durability, self-healing performance and self-healing products of polymer cement-based waterproof coating with ion chelator are investigated systematically. Meanwhile, the chemical composition of self-healing products in the crack was examined. Findings The results showed that ion chelators could motivate the hydration of C2S and C3S, as well as the formation of hydration products (C-S-H gel) of the waterproof coating to improve its compactness. Compared with the control group, the waterproof coating with ion chelator had more excellent water resistance, alkali resistance, thermal and UV aging resistance. When the dosage of ion chelator was 2%, after 28 days of curing, cracks with a width of 0.29 mm in waterproof coating could fully heal and cracks with a width of 0.50 mm could achieve a self-healing efficiency of 72%. Furthermore, the results reveal that the self-healing product in the crack was calcite crystalline CaCO3. Originality/value A novel ion chelator was introduced into the composite coating system to endow it with excellent self-healing ability to prolong its service life. It has huge application potential in the field of building waterproofing.

Influence of ash species on particle size dependence of water- and citric-acid-soluble potassium concentrations of woody biomass combustion ashes with low potassium content

The dependence of the concentrations of water- and citric-acid-soluble potassium on particle size was investigated for two types of low-potassium-content woody biomass combustion ash discharged from different biomass power plants. For both types of ash, the potassium concentration increased as the median diameter decreased; crystalline potassium, such as KCl and K2SO4, was identified more in fine combustion ash, whereas crystalline SiO2 and CaCO3 were abundant in coarse ash. The potassium components could be present as finer ash particles or as cover over coarser ash particles composed of crystalline SiO2 surfaces. Differences in the existence forms of the potassium components caused differences in the dependence of potassium concentration on the median diameter of the ash. Moreover, the methods used to calculate the particle size distributions of three ash particle components (water-soluble, citric-acid-soluble, and insoluble) in the ash residues can be used to evaluate the particle size dependence of both potassium concentrations to some extent. The potassium enrichment process demonstrated in this study could be successfully performed on a factory scale via the surface grinding of bottom ash. The collected fine grinding debris containing rich potassium, separated from the surface of the silica, was found to be possibly reusable as fertilizer.

Preparation and carbonation hardening of low calcium CO2 sequestration materials from waste concrete powder and calcium carbide slag

A series of new low calcium CO2 sequestration cementitious materials (LCC) with different Ca/Si ratios were prepared by sintering waste concrete fine powder (WCP) and calcium carbide slag (CCS) at different ratios. Their carbonation reaction activity, carbonation hardening properties, phase assemblages, and microstructure were systematically characterized. The results showed that when the WCP: CCS ratio was 70:30, the compressive strength of LCC reached 38 MPa after carbonation for 24h. With the decrease in the WCP: CCS ratio, the carbonation activity of LCC increased significantly. When the WCP: CCS ratio was 45:55, the compressive strength was increased to 109 MPa. Microstructure tests showed that the minerals of LCC gradually changed from CS and C2AS to C3S2 and C2S. After carbonation, unreacted clinker particles, crystalline CaCO3, and highly polymerized silica formed a dense microstructure, and the impurities of Mg element in the LCC triggered the formation of aragonite.

Experimental study on calcium carbonate phase transition from amorphous to crystalline forms in a reverse emulsion

Calcium carbonate (CaCO3) nanoparticles for lubricant additives are typically manufactured in reverse emulsion. The amorphous form is sterically stabilised in oil to create the so-called, ‘overbased detergents’, for motor engine oils, whilst crystalline CaCO3 nanoparticles are produced for specific applications, like lubricating greases. Here, the phase transition of amorphous calcium carbonate to crystalline forms in a reverse water/oil emulsion was studied in a 23L jacketed batch reactor equipped with agitator, temperature and pressure control. During the process, several acids, such as acetic, alkylaryl and 12-hydroxystearic (HSA), were added. The impact of acid type and HSA quantity were studied with all other aspects constant. The amorphous to crystalline phase transition kinetics were measured, the crystal habits were observed and the solid forms were characterized. With no HSA, a more acidic environment resulted in faster transition kinetics from amorphous CaCO3 to calcite. However, the presence of HSA tended to slow down the transition kinetics. Indeed, calcite crystal growth was asymmetrically inhibited by HSA, leading to elongated rod-shaped crystals. Finally, a very high HSA concentration leads to the formation of vaterite crystals, vaterite being a less stable phase than calcite.

The Influence of Acidic Oxygen Containing Groups Located on the Surface of Graphene Oxide (GO) on the Carbonation of Tricalcium Silicate (C3S) Based on Boehm's‐Theorem

AbstractIn various cementitious materials, the carbonation of tricalcium silicate (C3S) by the acidic oxygen‐containing groups located on the surface of graphene oxide (GO) can support cutting down the CO2 emissions, in addition to the formation of different crystalline CaCO3. The current work investigates the direct chemical reaction of dry‐C3S with 0.4 % GO solution and the mechanism of CH and CSH carbonation depending on the concept of Boehme's‐theorem. FTIR‐spectroscopy has been utilized to demonstrate and identify the effective transitions in the formulation of portlandite (Ca(OH)2), calcium‐silicate‐hydrate (CaO ⋅ SiO2 ⋅ H2O), and calcium carbonate (CaCO3) polymorphs during the reaction. This paper clarifies the relationship between XRD peak intensity and morphologies of CaCO3 crystals resulting from using various weights of dry‐C3S. Thus, the XRD peak intensity of CaCO3 varies according to their morphologies. XRD‐Rietveld Refinement has examined the quantity percentages of the crystalline phases. The differences in the dissociation temperature (TGA) illustrate the possible variation in the structure and properties of CaCO3 crystals formed during the carbonation of CH and CSH. SEM with different magnifications was selected to observe the prevalent morphology of vaterite, aragonite, and calcite crystalline phases during the transformation process.

Phase transition and morphology evolution of precipitated calcium carbonate (PCC) in the CO2 mineralization process

The indirect CO2 mineralization by using Ca/Mg-containing industrial alkaline by-products or wastes is a promising way for mitigation of CO2 emissions and valorization of wastes, although plagued by the utilization of precipitated calcium carbonate (PCC) if low-value product is obtained. It is difficult to acquire specific PCC with targeted morphology for commercial use since many parameters change in the gas–liquid carbonation process. In this work, the phase transition and morphology evolution behavior of PCC precipitated during carbonation of 0.1 mol·L−1 NH4Cl Ca-rich solution were studied. Results show that the polymorph transformation from rhombohedral calcite to spherical vaterite was found during the gas–liquid carbonation process, which is different from the traditional carbonation process (aqueous carbonation of lime). A promising result is that pure spherical vaterite of industrial interest was obtained under the studied conditions. In addition, to achieve conversion of CO2(g) into desired morphology of PCC, the feasibility of introducing additives (ethylene glycol and citric acid) into the gas–liquid carbonation system was also evaluated in terms of CO2(g) adsorption and PCC polymorphs control. Finally, the mechanism of additives on the growth and morphology of crystalline CaCO3 was investigated by FTIR and interpreted at the atomic level.

Dual-purpose applications of magnetic phase-change microcapsules with crystalline-phase-tunable CaCO3 shell for waste heat recovery and heavy metal ion removal

Removing heavy metal ions and recovering waste heat from industrial wastewaters can provide a friendly environment for human being survival together with an encouraging opportunity for sustainable energy resource. To meet such a requirement of dual functions, three types of magnetic phase-change microcapsules were fabricated with n-docosane as a phase change material core and crystalline phase-tunable CaCO3 as a shell, in which Fe3O4 nanoparticles were embedded to endow the microcapsules with magnetic responsiveness. The surfactant plays a key role in the crystalline phase of their CaCO3 wall material, resulting in different morphologies for the magnetic microcapsules and generating a great influence on their thermal properties and adsorption performance. The magnetic microcapsules synthesized using different types of surfactants exhibit spherical, rhombohedral, and fusiform morphologies with phase-change enthalpies of 135.3, 87.1, and 48.7 J/g, respectively. Moreover, the magnetic microcapsules obtained maximum adsorption capacities of 703.1, 862.3, 924.0, and 514.7 mg/g for the removal of Cu2+, Cd2+, Cr3, and Fe3+, respectively. Such high adsorption capacities for removing heavy metal ions are superior to those of adsorbents reported in the literature. More importantly, the magnetic microcapsules exhibit outstanding phase-change reversibility and thermal cycling stability for waste heat recovery as well as good adsorption reusability and high efficiency for heavy metal ion removal. This study provides a promising strategy in the design and fabrication of bifunctional phase-change microcapsules for dual-purpose applications of heavy metal ion removal and waste heat recovery from various industrial wastewaters.

Techniques for Quantifying Bacterially Induced Carbonate Mineralization in Escherichia coli

The lack of reliable techniques to accurately quantify bacterial carbonate production and a tractable genetic system limits our ability to thoroughly understand the metabolic basis for bacterial calcification. In this work, we demonstrate bacterially induced CaCO3 precipitation in the model organism, Escherichia coli K-12, and develop new quantitative methods for carbonate mineral production using inductively coupled plasma-optical emission spectroscopy (ICP-OES) and an image analysis approach repurposed for agar cultures. These assays allowed us to assess the impact of various calcium sources and environmental factors on crystalline CaCO3 formation in E. coli. Our data demonstrate that Ca-acetate and Ca-succinate yield the greatest CaCO3 output in E. coli, while a preliminary study demonstrated microclimatic influences on crystal growth and size. This work helps lay the foundation for using E. coli as a model bacterial species to probe the genetic elements that promote calcification in Bacteria, with the potential to identify previously undescribed pathways for precipitation.

A novel method for the formation of bioceramic nano-calcium hydroxyapatite coatings using sol-gel and dissolution-precipitation processing

The wet chemistry route has been developed to prepare calcium hydroxyapatite (Ca10(PO4)6(OH)2, (HA)) thin films on a silicon substrate using the novel low-temperature sol-gel and dissolution-precipitation approach. The calcium carbonate thin films on the silicon substrate were obtained by spin-coating technique when substrates were repeatedly coated with 10, 20 and 30 layers of sol-gel solution. The composites formed of crystalline and amorphous CaCO3 were obtained by calcination of the coatings for different time at 600°C. A dissolution-precipitation procedure was used for the preparation of calcium hydroxyapatite thin films on silicon substrate at 80°C. The obtained synthesis products were characterised by X-ray powder diffraction (XRD) analysis, scanning electron microscopy (SEM) and Raman spectroscopy.

Effects of Mg ions on the structural transformation of calcium carbonate and their implication for the tailor-synthesized carbon mineralization process

Carbon mineralization with alkaline earth metal-bearing industrial waste is recognized as a promising CO2 removal technology because it can stably store CO2 at a large scale and simultaneously produce solid carbonates (e.g., CaCO3) that can be utilized in various industries, such as paper, plastic filler and additives, food and pharmaceuticals, and construction materials. In order to maximize the value of the solid carbonates, their purity and properties need to be controllable. However, the crystalline structures and polymorphisms of the solid carbonates, particularly CaCO3, largely depend on various factors: pH, temperature and pressure, reaction time, types of ions and their ratios, degree of saturation, stirring rate, feeding order, etc. In general, industrial waste includes calcium with a small amount of magnesium. We investigated the effects of Mg2+ on the structural transformation of CaCO3 particularly focusing on the Mg2+/Ca2+ ratios in the range of 0.2 – 0.33. Because the ex-situ carbon mineralization can be conducted under the aqueous phase including Ca2+ and Mg2+ via pH swing processes, it is worth investigating the effects of Mg2+ on the precipitation of CaCO3. Other controlling factors of the formation of crystalline CaCO3, such as the concentrations of Ca2+ and CO32–, and the reaction time were also considered. The results revealed that not only the ratio of Mg2+/Ca2+ but the change of the ratio of CO32−/Ca2+ by stepwise synthesis control the structural transformation of CaCO3.

Amorphous and crystalline CaCO3 phase transformation at high solid/liquid ratio – Insight to a novel binder system

The transformation reactions in aqueous calcium carbonate systems are relevant for biomineralization processes, functionalised materials design and recently for the potential development of inorganic binders. The so-called ‘calcium carbonate cements’ are obtained by mixing amorphous (ACC) and crystalline CaCO3 (herein the anhydrous minerals calcite, aragonite and vaterite) with low amounts of water. The mechanisms behind the setting and hardening of these binders are still not clear. In the present study, the dissolution and precipitation reactions in low water ACC systems with and without crystalline CaCO3 have been investigated by in-situ techniques, Raman spectroscopy and optical pH measurements, coupled with reaction time resolved mineralogical and chemical analyses of solid and liquid samples. The results clearly indicate the presence of initial crystalline CaCO3 polymorphs to control the reactions kinetics and the resulting individual microstructure. Vaterite, with a considerably higher surface area than calcite and aragonite, produces more bonded and connected material. In the ACC-vaterite system the transformation of vaterite to calcite takes places in two dissolution-precipitation stages, accompanied by typical pH in- and decreases. Time-resolved analyses and hydrochemical modelling clearly reveal the solid transformation behaviour to be governed by the presence/absence of Mg2+ in the solution and the impurities, such as Na+, liberated from the educts to the reactive solution. The control and adjustment of the identified influencing parameters could allow for promising future development of CaCO3-based binders.

New sights in early carbonation of calcium silicates: Performance, mechanism and nanostructure

Low calcium minerals were considered as a potential sustainable binder and reported to be reactive to CO2. In this research, the carbonation characteristics of C3S, β-C2S and γ-C2S at early age were investigated in some new sights. The results show that carbonation degree and compressive strength of C3S increased most rapidly in the first 8 h carbonation, then slowed down. After 48 h carbonation, β-C2S and γ-C2S were close to C3S in compressive strength, and apparently higher than C3S in carbonation degree. The crystalline CaCO3 generated by carbonation were mainly calcite and aragonite, while vaterite was observed in β-C2S and γ-C2S within 8 h carbonation. Carbonation curing increased MCL of C-S-H and silicate polymerization. Besides, based on the combination of TG-DTG, XRD and 29Si NMR, the decalcify priority of Ca sites in C-S-H was analyzed. The results indicate that interlayer Ca atoms inside C-S-H participated carbonation first, then the calcium atoms within interlayer and silicate chains of C-S-H would be removed successively.

Carbonation-induced properties of alkali-activated cement exposed to saturated and supercritical CO2

Carbonation of well cement under high-temperature and high-pressure conditions is a significant concern in geological sequestration projects. This study evaluates the carbonation-induced properties of low to high calcium-based one-part alkali-activated cements (AACs) as an alternative to Portland oil well cement under reservoir conditions. Fly ash/slag-based AACs exposed to saturated (SAT) and supercritical (SUP) CO2 for 28 days were characterised using compressive strength, XRD and SEM tests. The strength properties of AACs increased with increasing slag content from 40% (low calcium AACs) to 100% (high calcium AACs) by weight of the binder under both SAT and SUP conditions. Aragonite is the major polymorph of CaCO3 in all AACs, irrespective of the type of CO2 exposure, and the highest aragonite content was observed in 100% slag AAC. The amorphous phase content decreased while the crystalline CaCO3 increased in carbonated AACs with increasing slag content, resulting in a dense microstructure in high calcium AACs. Sealing of microcracks and pores was observed in high calcium AACs with the accelerated precipitation of CaCO3 by reducing total porosity. Low calcium AACs resulted in a more highly porous carbonated microstructure than high calcium AACs under SAT and SUP conditions.

Characterization of the CaCO3 calcination process by the Porod invariant behaviour

The concave behaviour of the Porod invariant observed during the calcination of CaCO3 powder samples suggests the following picture of the evolving internal structure of the samples. The outset sample is formed by a crystalline CaCO3 phase and a void phase. During the calcination, the first phase shrinks in volume at fixed density since the temperature increase breaks down the crystalline structure at the interface, leading to the formation of an amorphous phase comprising an equal number of CO2 and CaO atomic groups. The last groups gradually condense, forming a third phase of solid CaO of constant density and increasing volume fraction, while the companion CO2 groups flow out of the sample. The amorphous phase occupies, with a variable density, all the volume left free by the other two phases. At the end of the calcination, both the volume fraction of the first phase and the density of the amorphous phase vanish so that the sample will again be made up of two phases: the voids and the solid CaO. Best-fitting the resulting theoretical expressions of the Porod invariant and of the Porod law coefficient to the observed values, one can determine the matter densities, volume fractions and specific surface areas of the phases.