Structure of Hydrated Cement
The below figure shows the structure of hydrated cement and the reaction of cement between aggregate creating a transition zone.
- Transition zone represents the inter-facial region between the particles of coarse aggregates and hardened cement paste.
- Transition zone generally a plane of weakness. because the quality of paste in the transition zone is of poorer quality.
- firstly due to internal bleeding, water accumulate below elongated, flaky and large pieces of aggregates. this reduces the bond between paste and aggregate.
- and also the size and concentration of crystalline compounds such as calcium hydroxide an ettrigate are also larger in the transition zone hence causes lower strength of transition zone.
- Capillary pores represent that part of the gross volume which has not been filled by the products of hydration. capillary pores can be seen at any stage of hydration.
- the capillary porosity of the paste depends on the W/C ratio of the mix and degree of hydration.
- the rate of hydration of cement does not matters but the type of cement influence the degree of hydration matters.
- at W/C ratio>0.38 volume of gel is not sufficient to fill all the space available to it. so there will be some volume of capillary pores decrease with the progress of hydration where as the total volume of gel pores increases along with the volume of gel as the hydration proceeds.
- size of the capillary pores is of the order of 10-50 nm. although it may be larger (3-5 mm) for higher W/C ratio.
- larger voids affect strength and permeability. whereas smaller voids impact shrinkage.
- Gel pores are the interconnected interstitial spaces between the gel particles.
- the gel pores occupy about 28% of the total volume of gel.
- gel pores are much smaller in size than capillary pores with size normally between 0.5 and 2.5 nm.
- gel pores have little impact on strength and permeability of concrete and can influence the shrinkage and creep of the concrete.
- the total volume of gel pores increases along with the volume of gel as the hydration proceeds. however the volume of capillary pores decreases with the progress of hydration.