Early Compressive Strength Research Articles

Mechanism underlying early hydration kinetics of carbonated recycled concrete fines-ordinary portland cement (CRCF-OPC) paste

Aiming to promote applications of carbonated recycled concrete fines (CRCF) as a low-carbon alternative to ordinary Portland cement (OPC), this study explored the effect of CRCF on the early hydration kinetics of OPC paste. The results show that CRCF is not only a highly active pozzolanic material but also an effective accelerator affecting the early hydration kinetics of OPC paste, thereby leading to an increased early compressive strength and a comparable 28-d compressive strength, even at a replacement ratio of 20%. Although the rheological properties and workability are negatively affected due to the high surface area and fine particle size of CRCF, the amorphous silica and silica-alumina gels and calcite present in CRCF facilitate initial ions dissolution and provide additional active nucleation sites for hydrates. As a result, the microstructure is densified due to the formation of additional C–S–H gels with high Si/Ca and Al/Ca ratios and carboaluminate phases, along with the inert filler effect of unreacted calcite particles. The incorporation of 20% CRCF to replace OPC can achieve a CO2 emissions reduction up to 25%. These indicate that the CRCF has a promising application scenario in transforming wasted concrete into high-value industrial products by CO2 activation.

Analysis of the Synergistic Effect on the Strength Characteristics of Modified Red Mud-Based Stabilized Soil.

Based on the existing research results, this research team developed roadbed stabilized soil materials using nano-SiO2 synergistically modified red mud in order to study whether the strength of the stabilized soil materials meets the strength requirements of the roadbed materials, and at the same time, analyze its strength characteristics to make the feasibility of it being used as a roadbed material clear. Through different combination schemes, the effects of different nano-SiO2 and cement contents on the strength of the stabilized materials were explored. The test results show the following: In the synergistic modification of nano-SiO2 and cement, nano-SiO2 can significantly improve the early unconfined compressive strength of red mud-based stabilized soil. In the synergistic modification of nano-SiO2, gypsum, and cement, the 7 d unconfined compressive strength of red mud-based stabilized soil is greater than 2 MPa, which meets the strength requirements of road base materials and shows the superiority of synergism. The nominal stress-strain curves are divided into five stages: compressed and compacted stage, elastic deformation stage, plastic deformation stage, damage deformation stage, and residual deformation stage. The macroscopic compressive damage pattern of the specimens shows that the modified red mud-based stabilized soil mostly exhibits brittle damage. Tests have shown that the strength of modified terracotta-based stabilized soil meets the requirements of roadbed strength.

Experimental Study on Long-Term Mechanical Properties and Durability of Waste Glass Added to OPC Concrete.

This study aims to achieve the sustainable utilization of waste glass resources through an investigation into the influence of three types of admixtures, namely waste glass powder (WGP) (G), waste glass powder-slag (G-S), and waste glass powder-fly ash (G-F), on the mechanical properties and durability performance of waste glass concrete. The experimental results demonstrate that the exclusive use of WGP as an admixture led to the relatively poor early compressive strength of the concrete, which decreased with an increase in dosage. However, at medium to long curing ages, the strength of the waste glass concrete could equal or even surpass that of ordinary concrete. When dual admixtures were employed, the G-S group exhibited higher compressive strength compared to the G-F group. Specifically, within the G-S group, a glass powder dosage of 15% yielded higher compressive strength, and after 180 days, the dual admixture groups exhibited greater strength than ordinary concrete (G0); the compressive strength of the tG1S1 group was 44.57 MPa, and that of the G0 group was 40.07 MPa. The chloride ion diffusion coefficient showed a varying trend with an increase in WGP dosage, initially decreasing and then increasing. The concrete's resistance to erosion was maximized when the glass powder dosage reached 30%. As the WGP dosage increased, the overall frost resistance decreased. For a total dosage of 30%, the optimal glass powder dosage in both G-S and G-F groups was found to be 15%.

Resource utilization of drinking water treatment aluminum sludge in green cementing materials: Hydration characteristics and hydration kinetics

As a byproduct of water treatment, drinking water treatment aluminum sludge (DWTAS) has challenges related to imperfect treatment and disposal, which has caused potential harm to human health and the environment. In this paper, heat treatment DWTAS as a supplement cementitious material was used to prepare a green cementing material. The results show that the 800°C is considered as the optimum heat treatment temperature for DWTAS. DWTAS-800°C is fully activated after thermal decomposition to form incompletely crystallized highly active γ-Al2O3 and active SiO2. The addition of DWTAS promoted the formation of ettringite and C-(A)-S-H gel, which could make up for the low early compressive strength of cementing materials to a certain extent. When cured for 90 days, the compressive strength of the mortar with 30% DWTAS-800°C reached 44.86 MPa. The dynamic process was well simulated by Krstulović-Dabić hydration kinetics model. This study provided a methodology for the fabrication of environmentally friendly and cost-effective compound cementitious materials and proposed a “waste-to-resource” strategy for the sustainable management of typical solid wastes.

Mixed artificial intelligence models for compressive strength prediction and analysis of fly ash concrete

The construction industry is facing challenges from the hazardous nature of Ordinary Portland Cement (OPC) production as one of the main contributors to global warming and CO2 emission. Given its increasing demand, the need to replace OPC with sustainable alternatives for green concrete production is essential. The promising results of using fly ash (FA) in concrete mixtures as a supplementary cementitious material (SCM) have attracted great attention. However, conventional laboratory procedures for the analysis of concrete properties like Compressive Strength (CS) are laborious and expensive. The objective of this paper is to predict the CS of fly ash concrete using five integrated models (BP-ANN, BP-GA, BP-PSO, RF, and XGBoost) in order to develop an accurate method for predicting the CS of fly ash concrete and to reduce the error of the prediction model, and to quantify the effect of parameter variations on the CS by using it for parameter impact analysis. In addition, performance statistics for the five algorithmic models mentioned above show that the BP-GA model has the best prediction performance for fly ash concrete CS, with prediction errors distributed within ±5%,The sensitivity investigation concludes that the water-to-binder ratio (w/b) has the greatest influence on the CS of fly ash concrete. Through parametric analysis, it was found that the early CS of fly ash concrete increased with increasing fly ash content when the fly ash replacement rate (FA%) was less than 30%, and decreased with increasing fly ash content when the FA% was greater than 30%.

Evaluation of the early strength of limestone-slag-cement ternary composite mortar improved by aluminum sulfate

Limestone-slag-Portland cement mixtures have a low carbon footprint but have low early compressive strength. This study proposes a method to improve the early strength of ternary mixture mortar using aluminum sulfate, evaluates its effectiveness, and tests the optimal dosage. The fresh paste hydration heat and mortar macroscopic properties, such as mechanical performance, resistivity, and ultrasonic pulse velocity, were measured at different dosages (0, 1, 2, and 3%) of aluminum sulfate. Microscopic testing methods, such as X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy, were employed to analyze the interaction mechanism between aluminum sulfate and the ternary mixture. The research findings demonstrated that aluminum sulfate enhanced the early mechanical properties and significantly improved the 28-day resistivity. Dosages of 1% and 2% had no adverse effects on the 28 d strength, whereas a 3% dosage reduced the 28 d strength. In the presence of aluminum sulfate, the early hydration heat release of the mixture increased significantly, ettringite became the main early hydration product, and portlandite decreased significantly. Considering the changes in the early strength, 28 d strength, and surface electrical resistivity, a 1% aluminum sulfate dosage was determined as the optimal addition level.

Recycling waste dolomite powder in cement paste: Early hydration process, microscale characteristics, and life-cycle assessment

Waste dolomite powder (WDP) is a byproduct obtained from dolomite quarries during the preparation of dolomite products. To study the re-utilisation of WDP, an eco-friendly cement-based material was prepared using WDP as a micro-aggregate. The effects of WDP on the early hydration process, microscale characteristics, and life-cycle assessment of cement paste are discussed in this study. The isothermal calorimetry results showed that the incorporating WDP in cement paste accelerated the early hydration process of cement according to the degree of reaction. In this case, the setting time of the cement pastes with WDP was shortened, and the early compressive strength was significantly improved. The results of X-ray diffraction and scanning electron microscopy analysis at early curing ages (1 and 3 d) showed changes in the peak intensity of ettringite and portlandite and a denser microstructure. Mercury intrusion porosimetry tests showed that the middle and large capillary pores were refined by the nucleation and filling effects of WDP. Based on environmental and economic evaluations, the utilisation of WDP reduced energy consumption, CO2 emissions, and economic costs. Compared to the sample without WDP, the energy consumption, CO2 emissions, and economic cost indices were 42 %, 42.69 %, and 39.4 % lower, respectively. Our results may provide valuable references for the re-utilisation of WDP in low-carbonation cement-based materials.

Microwave-thermal-assisted curing method on geopolymer preparation from Panzhihua high-titanium slag by alkali activation

This paper emphasizes the feasibility of using microwave-thermal-assisted treatment to promote the rapid geopolymerzation and early strength improvement of Panzhihua high-titanium slag (HTS)-based geopolymers. The results reveal that the microwave radiation promotes the geopolymerzation reaction of the HTS during the alkaline activation reaction with a remarkable early compressive strength improvement of 623% only in 20 min. Since the dispersed CaTiO3 particles in the HTS serve as local thermal hot spots and nucleation elements, they will rapidly elevate the temperature in a short time and facilitate the hydration products growing to form a gel network structure. Moreover, due to more water evaporation and accelerating effects of the endothermic reaction in the dissolution-polycondensation stage, the optimum process of microwave-thermal-assisted treatment has further advanced the hydration process, which endowed the highest geopolymerization degree, the lowest residue water, the densest microstructures of geopolymeric network and a desirable mechanical performance (54 MPa) of the HTS-based geopolymer.

Investigation of the relationship among the hydration, microstructure and compressive strength of alkali-activated phosphorus slag

In this study, the relationship among the hydration, microstructure and compressive strength of alkali-activated phosphorus slag (AAPS) was systematically investigated to provide a foundation for its design and application. The alkali dose and silicate modulus had both positive and negative effects on PS hydration due to the change in the pH, the concentrations of Si-containing ions and the hydrate coating on the PS surface, and the overall effect depended on their relative magnitudes. The hydration of waterglass-activated phosphorus slag (WAPS) experienced a transformation of NASH first into CNASH (low Ca) and then into CNASH (high Ca) due to the stronger polarization ability of Ca2+ over Na+, and the gel transformation was accompanied by dealumination. The more early hydrates there were, the higher the early densification and early compressive strength of AAPS. The later densification and later compressive strength were influenced by a combination of the hydrate content, early hydration rate and gel type. CNASH (high Na) was incompatible with CNASH (low Na), which degraded the later densification. When the contents of later hydrates were comparable, the slower early hydration and lower CNASH (high Na) contributed to the greater later densification and compressive strength.

EFFECT OF SEA SAND WITH DIFFERENT CHLORIDE ION CONTENTS ON THE PROPERTIES OF CONCRETE

This article presents the results of a study that evaluated the effects of using sea sand with varying contents of chloride ions on the properties of concrete, including physical and mechanical properties as well as durability. The study was carried out on the sea sand samples with different chloride ion contents, including as-received sea sand samples with a chloride ion content of approximately 0.15%, two washed (desalted) sea sand samples with chloride ion content of 0.024% and 0.05%, and sea sand samples added with chloride salt (0.375%), which were then compared with river sand samples. The sand samples had approximately the same fineness modulus ranging from 2.3-2.5. The research results indicated that compared with washed sea sand and river sand, concrete mixes using sea sand with high chloride ion content increased the mixing water and reduced the slump retainability of the fresh concrete. Furthermore, early compressive strength (7 days) increased but late age strength (91 and 365 days) decreased in both standard curing and wet-dry circle curing cases. Additionally, the impermeability (water and chloride ion) and the ability to protect the reinforcement from corrosion in concrete were reduced when using sea sand with high chloride ion content compared to washed sea sand and river sand.

Utilization of ultra-fine dredged sand from the Yangtze River in alkali-activated slag/fly ash mortars: Mechanical properties, drying shrinkage and microstructure

The ultra-fine dredged sand (DS) from the lower reaches of the Yangtze River is a large amount of solid waste with the potential to be recycled and applied as a replacement for fine aggregate. This article discussed the utilization of dredged sand by producing alkali activated materials (AAMs). The chemical and physical characteristics of the DS were analyzed. The effect of the DS on durability and mechanical characteristics of the alkali activated slag/fly materials were studied with different precursor systems. From the results, it was found that the addition of DS improved the 28 d strength and even 56 d strength. The 25% DS content increased the compressive strength of both slag rich and fly ash (FA) rich AAMs by about 5%, and the microstructure showed a denser matrix at this content. However, excessive DS can cause a decrease in flowability, early compressive strength and a decline in the compactness of the matrix. Adding an appropriate amount of DS can control the drying shrinkage. The 25% DS content has an inhibitory effect on the shrinkage development of slag rich mortar, while the similar effect on FA rich mortar appears at a content of over 50%. The adoption of DS addition to produce AAMs improves the ecological benefits of concrete engineering projects on the Yangtze River coast.

Effect of strontium slag on early hydration and mechanical properties of belite-C4A3$ cement

Belite-calcium sulfoaluminate (Belite-C4A3$) cement is one type of low-calcium clinker with good long-term compressive strength and durability. Yet, its early mechanical property is lower than P·І 42.5. In this paper, the effect of strontium slag on the preparation and performance of belite-C4A3$ cement was investigated. Moreover, the effect of SrO on crystal form and hydration characteristic of silicate minerals was studied. It was found that SrO could improve the early hydration activity of silicate minerals by stabilizing the β-type C2S and M-type C3S. The experiment results showed that the addition of strontium slag improved the burnability of cement, stabilized the high temperature active crystalline form of silicate minerals and promoted its formation, which were benefited to the rapid generation of hydration products and increase of early compressive strength of belite-C4A3$ cement. It was found that the compressive strength of belite-C4A3$ cement at different ages was increased to various degrees under the content of 1∼8% strontium slag. The optimal content of strontium slag was 5%, and its compressive strength of 1, 3, 7 and 28 days was 8.43, 35.83, 71.25 and 103.73 MPa, respectively. Compared with the blank group without strontium slag, the compressive strength was increased by 270%, 210%, 320% and 13%, respectively. This indicated that strontium slag had a significant effect on improving the early mechanical properties of belite-C4A3$ cement.

Study of fly ash-slag geopolymer mortar as a rapid strengthening agent for concrete structures

A fly ash-slag geopolymer mortar (GPM) was developed as an efficient bond material for rapidly strengthening concrete structures. To determine the most significant factors affecting fluidity, setting time, and early compressive strength, we studied the following six factors using an orthogonal test: the amount of slag in the binder (SL/B), the mass ratio of sodium silicate to sodium hydroxide (SS/SH), the molarity of NaOH (MN), the mass ratio of alkali solution to binder (A/B), the superplasticizer addition to binder (SP/B), and the mass ratio of fine aggregates to binder (F/B). Different admixtures have been studied to improve the fluidity of the GPM. Bonding tests were conducted to investigate the differences in the bond behavior of the GPM. The results indicate that SL/B and A/B significantly affected the properties of the GPM. Increasing SL/B was beneficial for enhancing the early compressive strength of the GPM and decreasing its fluidity and setting time, whereas A/B exhibited the opposite trend. It is challenging to meet the requirements of high early strength and fluidity of the GPM by only changing the chemical composition, and admixtures should be added. The 1-day GPM without the interfacial agent exhibited a bond strength similar to that of the PCM at 7 days and the existing interfacial agent was not suitable for the early-strength GPM.

The Effect of RHA and GBFS on the Mechanical and Physical Properties of Cementitious Composites with High Early Age Compressive and Flexural Strength: Bayesian Algorithm for the Design

The Effect of RHA and GBFS on the Mechanical and Physical Properties of Cementitious Composites with High Early Age Compressive and Flexural Strength: Bayesian Algorithm for the Design

Influence of main components of accelerators on mechanical property and hydration of Portland cement in a dry-hot geothermal environment

The behavior of accelerators in the shotcrete of tunnel structure is significantly affected by the high geothermal tunnel environment. The objective of this study is to investigate the influence of main components of alkali-free accelerators (aluminum sulfate, AS) and alkaline accelerators (sodium aluminate, NA) on mechanical property and hydration of Portland cement in a dry-hot geothermal environment of 80 ℃ (DH). Setting time test, compressive strength test, XRD, TGA, MIP and SEM were performed on cement systems with AS and NA. Results show that relative to standard curing environment, the cement hydration was promoted and the pore structure at early stage was optimized in DH, thereby significantly accelerating the setting time of cement pastes and improving the early compressive strength development of mortars. In contrast, the later cement hydration was retarded and pore structure was detrimental in DH, resulting in abnormally slow development of later strength and even reduction. Additionally, the stability of hydration products such as ettringite (AFt) and monosulfoaluminate hydrate (AFm) would decline in DH. DH could accelerate the dehydration and decomposition of AFt, and promote the transformation process from AFt to AFm at early stage. After 28 days, AFm would convert into hemicarboaluminate due to carbonation. The addition of AS elevated the hydration degree and optimized the pore structure, especially at later stage, thus exhibiting more stability in long-term strength. Nevertheless, the incorporation of NA would bring about serious deterioration phenomenon in DH, and the compressive strength almost stagnated after 1 day. This work provides a guidance for the real application of accelerators in the shotcrete of tunnel structure in a dry-hot geothermal environment.

Effect of steel slag on pores and strength of fly-ash-based adsorbent geopolymers

A study was undertaken into the preparation and properties of fly-ash-based geopolymers that can be used to make multistage-pore materials able to adsorb gases such as carbon dioxide. The influence of steel slag on the mechanical properties and microstructure of the final precursor products was investigated and characterised by mechanical strength tests, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry–differential thermal analysis, scanning electron microscopy–energy-dispersive X-ray spectroscopy and mercury intrusion porosimetry. The results showed that a low dose of steel slag significantly promoted the early compressive strength of a multistage-pore geopolymer. However, when the slag dose reached 30%, the strength decreased dramatically. The pore classification effect was more obvious when the capillary pores in the geopolymer disappeared and the pores mainly existed in the form of gel pores and voids.

Hydration of Portland slag cement in the presence of nano silica

The cement industry accounts for nearly 7% of carbon dioxide (CO2) emissions globally and is facing tremendous pressure to reduce the emission intensity. Supplementary cementitious materials (SCM) are a quick and effective lever to lower CO2 emissions. The cement hydration process is a sensitive mechanism and is greatly influenced by many factors such as hydration environment, type of SCM, and the amount used in the cement, which affect the cement's physical and mechanical performance. The SCM adversely impacts cement performance, especially prolonged setting and slow initial compressive strength development, compared to ordinary Portland cement (OPC). The importance of nanomaterials has been known for decades; their usage in cementitious materials is now accelerated as they can play an essential role by offsetting the adverse effects of the high usage of SCMs in cement. They help in reducing the intensity of CO2 and offer value-added features. A lab experiment is carried out to understand the role of nano SiO2 at different dosages on the Portland slag cement (PSC) hydration by studying heat evolution profiles, compressive strengths, TG-DTA, XRD, and SEM images. Results have revealed that 1% nano SiO2 accelerates early settings time and compressive strength compared to OPC. It will promote slag-based blended cement (PSC) and replace ordinary Portland cement in the construction industry to reduce the CO2 footprint of the cement and construction industry. A model of NS's role during PSC hydration is also given.

Investigation on the influential mechanism of FA and GGBS on the properties of CO2-cured cement paste

In the present work, the synergetic effect of fly ash (FA) and ground granulated blast furnace slag (GGBS) on the early compressive strength and microstructure development of CO2-cured mortars was investigated. A rim of several micrometers was found around carbonated cement particles, which contained not only silica-rich gel but also crystal calcium carbonate. The calcium carbonate formed around the cement particles were surrounded by an amorphous layer of around 3 nm, while no layers were observed around the calcite formed on FA or GGBS particles. The calcite formed on FA particles were hexagonal plate shaped, while the one on GGBS particles were rhombohedral shaped. The use of FA resulted in the increase of crystal size and crystallinity of calcite, while GGBS decreased the crystal size of calcite. The incorporation of FA and GGBS increased the calcite content and polymerization of silica-rich gel. However, this didn't result in a higher compressive strength. This was due to the looser microstructure and nanopores within the carbonation products compared to the pure ordinary Portland cement sample. The compressive strength of the ternary binder system showed linear relationship with capillary pores and the crystal size of calcite. Moreover, the effect of GGBS on the compressive strength reduction was more obvious than that of FA.

Durability performance of waste glass powder and calcium oxide added sodium carbonate-activated slag mortar

Cement manufacturing consumes large quantities of natural resources, emits considerable amounts of greenhouse gases, and brings harm to the environment. Alkali-activated cementation materials are promising candidates to replace ordinary Portland cement. However, Na2CO3-activated slag mortars suffer from long setting times, low early strengths, and poor durability. In this study, CaO and recycled waste glass were added to Na2CO3-activated slag mortars to overcome these issues. Accordingly, in this experiment, the effects of the CaO and glass powder (GP) contents on the setting time, fluidity, compressive strength, drying shrinkage, chloride ion electric flux, sulfate erosion, and fire resistance of Na2CO3-activated slag mortar were investigated. X-ray diffraction and nuclear magnetic resonance were used to characterize the reaction products and pore structure. It was found that the initial setting time and final setting time of slag mortar activated by sodium carbonate are more than 3 days. The setting time and fluidity of mortar decrease with the increase of calcium oxide. When 4% calcium oxide was added, the initial and final setting time was reduced to 95 min and 160 min, and the fluidity was reduced to 170. However, after GP replaced part of the slag, the setting time and mobility increased. The effect of CaO content on compressive strength and drying shrinkage of sodium carbonate activated slag mortar is similar (increasing first and then decreasing). The main reason may be that when CaO content is less than 6%, the calcite generated in the reaction fills the pores, which makes the number of pores in smaller pores (less than 50 nm) increase and the structure becomes more dense. GP is good for reducing drying shrinkage, but not good for early compressive strength. CaO is beneficial to improve the chloride ion erosion resistance of sodium carbonate activated slag mortar, which is related to the porosity and the adsorption of hydrotalc. The addition of GP has no effect on the improvement of chloride ion erosion resistance of CaO and sodium carbonate composite activated slag mortar, but the chloride ion electric flux is lower than 1200 C at 28 d. CaO and GP are beneficial to improve the resistance of alkali-excited slag mortar to sulfate attack. The effect of CaO on the resistance to sulfate attack is mainly related to the porosity and the content of hydrotalcite. The effect of GP on the resistance to sulfate attack is mainly related to the silicon and aluminum separated by GP. CaO and GP are beneficial to improve the fire resistance of alkali-excited slag mortar. The increase of fire resistance is related to decomposition temperature and content of reaction products. The combination of CaO and Na2CO3 to activate the slag and the addition of GP were significantly better than the use of only Na2CO3 to activate the slag.

Effect of curing temperatures on geopolymerization and heavy metal solidification in alkali-activated zeolite/MSWI fly ash specimens

Admixtures of pozzolanic materials or solid wastes with municipal solid waste incineration fly ash (MSWIFA) can improve heavy metal stabilization/solidification (S/S) properties in alkali-activated MSWIFA specimens. In this work, the compressive strength of MSWIFA admixes with zeolite activated by Na2O·1.7SiO2·nH2O (FZNS) specimens with different curing times (3, 7, 14, and 28 d) and temperatures (20, 50, and 80 ℃) is investigated. The results reveal that a 40 wt% zeolite replacement of MSWIFA in FZNS specimens (FZNS40) not only increases the early compressive strength by 129% but also reduces strength loss during continuous high-temperature (50 or 80 ℃) curing. Moreover, zeolite addition promotes the creation of geopolymers at 28.5°–30.5°in X-ray diffraction (XRD) curves. During 50 or 80 ℃ curing, the NaCl peak disappears and a marginal Friedel's salt peak can be detected at 11.4°. The gel contents are more concentrated on C-S-H in FZNS0 and C-(A)-S-H/N-A-S-H in FZNS40, as demonstrated by differential thermogravimetry (DTG) and magic angle spinning nuclear magnetic resonance (MAS NMR) analysis. With zeolite addition, long-time and high-temperature curing can improve the heavy metal solidification ratio in FZNS specimens. After high-temperature curing, the honeycomb or cluster morphologies of the gel products, visible by scanning electron microscopy (SEM) in the FZNS specimens, reveal high heavy metal stabilization properties. It is easier to form the fibrous C-(A)-S-H/N-A-S-H gel near zeolite owing to the high content of SiO2 and Al2O3. Zeolite modifies the compressive strength, high-temperature stability, and heavy-metal S/S efficiency in FZNS specimens, which promotes the utilization of MSWIFA as an ecologicalresource in the future.