Cement paste hydration development

As a result of exposure to humidity and even a slightly elevated temperature, a conversion reaction may start in hardened cement paste. It develops in the calcium aluminate hydrates where hexagonal crystals are transformed into cubic ones, which have smaller volume. This causes an increase of porosity and considerable decrease of strength. It has been proved that even a temperature over 20°C can initiate the reaction for which the remaining amount of mixing water may be sufficient. The conversion reaction may... 

Chemical admixtures in small amounts are added to concrete to enhance the physical, mechanical, chemical, and durability characteristics of concrete. Superplasticizers are admixtures that have the ability to increase the workability of concrete (for easy placement), but also produce high strength concretes. The relative rates of hydration of cement containing superplasticizers at different temperatures are conveniently followed by conduction calorimetry. In Fig. 11 both the rates of hydration and the cumulative amounts of heat developed in cement pastes hydrated at temperatures of20,40, and 55" C are plotted as a function of time. In the figure, SMF refers to the superplasticizer based on sulfonated melamine formaldehyde. The addition of the superplasticizer retards the hydration of cement. The retardation increases with the dosage of the superplasticizer. Also, the retardation effect becomes less significant as the temperature of hydration is increased. 

Figure 3.4 Thermodynamic modelling of phase development in a portland cement paste hydrated at 20°C. Cement composition and modelling are detailed in (Lothenbach et al. 2008). (Courtesy of Barbara Lothenbach.)...

Supercritical C02 treatment affects the microstructure of the cement paste. In the first stage of the sc C02 treatment, free water in the cement pores is extracted. As a consequence of this dehydration process, channels of about 50-pm diameter develop. Dissolved calcium in the free water reacts with the C02 and crystallizes with the C02 as calcite along the channel walls. In the second stage, the structural water of the hydrated cement phases is extracted. The carbonation of the portlandite to form more calcite takes place. Water, bound to the CSH surrounding the partially hydrated cement clinker particles, is partially replaced by a carbonate formation. The short fibers of the CSH-cement framework, which are responsible for the physical properties of the cement, are not affected (Hartmann et al., 1999). 

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40"C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

Pozzolanic and blast furnace cements (or, alternatively, addition of fly ash or GGBS to Portland cement at the mixing plant) may be the most suitable choice for many stmctures that are critical from the durability point of view. In fact, they reduce the rate of development of heat of hydration, they lead to a lower content of alkalis and Ume in the cement paste, and they can produce a denser cement paste. They should be preferred, for instance, for massive structures (to reduce the rate of development of heat of hydration), or in sulfate-contaminated environments (Section 3.3), when there is risk of ASR (Section 3.4), or in chloride-contaminated environments (Section 12.5.1). 

The aim of the work is to develop new cement admixtures by using bentonite clay, PVA and their mixture. The study of the effect of PVA and clay on the hydration rate of cement paste will be done, as well as, characterization of the final modified cement by means of FTIR, TGA and SEM. From our knowledge, there is not previous publication utilizing these admixture combinations in cement. 

The heat of hardening can be determined not only by a simplified ealculation, but by direct calorimetric measurements, or indirectly applying the Hess rule. According to this rule, the heat of reaction depends only on the initial and final state and this is the basis of the dissolution method. In this dissolution procedure, the heat of hardening is determined as a difference between the heat of neat cement dissolution and heat of hydration products (cement paste) dissolution. The mixture of nitric and fluoric acids is used for this purpose. Only the dissolution method is applied in the case of the longer hydration time. The most accurate, differential calorimetric method has been developed by Zielenkiewicz [164]. The completely hydrated mortar is placed in one container of calorimeter and in the second one— the fiesh mortar. 

Wood fibres are produced in the form of chips, which is usually a waste material in the wood industry. Wood chips mixed with cement paste have been used since the 1920s for the production of sheets applied for thermal insulation in housing. The chips are subjected to chemical pre-treatment to avoid any disturbance of cement hydration by organic acids. The application of wood-origin fibres as a reinforcement for minor structural elements has been developing at a local level. 

Some indication of the respectively contributions of the clinker phases to the strength development of cement is given in Fig. 9. However, these results obtained for individual phases cannot be directly applied to the conditions actually occurring in cement paste, as is apparent also from the heat of hydration values given in Table 10. Fig. 17 schematically shows the sequence of formation of the hydrate phases and the structure development in the setting and hardening of Portland cement. 

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