## 6 Microstructural Analysis of Cement Paste with Water Reducer
### Rheological Microstructural Model of Cement Paste with Water Reducer after Carbonation  
(1) Construction of the rheological microstructure model  
The rheological microstructural model of cement paste with water reducer after carbonation is shown in Figure 13. Adding water reducer to fresh cement paste not only prolongs the chemical reaction of the cement effectively but also changes the volume and space distributions of the products, thus decelerating coagulation after hydration of the cement. Moreover, water-reducer molecules adhering to particle surfaces may induce changes in electrical properties. Influenced by \"steric hindrance\", flocculates disperse and flocculation water is released, thus increasing free water [29; 30]. According to the mechanism of the water reducer and combined with the above rheological analysis, a rheological microstructural model of paste with water reducer can be derived.  
Since water reducer can delay the hydration of cement, it not only decelerates the coagulation of cement to some extent but also changes the volumetric and spatial distribution of hydration products and hinders the flocculation of cement particles. As a result, the liquidity of cement can be maintained to some extent in the early stage. According to the mechanism of the water reducer and with considerations of the above rheological analysis, the rheological microstructural model of paste with water reducer can be gained [31].  
(2) Rheological model analysis of cement paste with water reducer after carbonation  
Figure 13a shows that flocculants in cement paste without CO\({}_{2}\) disperse uniformly under ultrasonic vibration. After water reducer is added, flocculation water in flocculants is released and flocculants are dispersed into several small cement particles. Cement pastes supplied with 0.44%, 0.88%, 1.32%, 1.76% and 2.20% CO\({}_{2}\) are shown in Figure 14b-f. Clearly, CO\({}_{2}\) and cement hydrates react to produce CaCO\({}_{3}\) crystals, which adhere to surrounding flocculants. With increases in CO\({}_{2}\) AA in fresh cement paste, CaCO\({}_{3}\) crystals adhering to surrounding flocculants also increase continuously, thus restricting the effective release of free water from the fresh cement paste. Moreover, free-water release decreases with increases in CO\({}_{2}\) AA, thus making the liquidity of the cement paste decline gradually.  
Moreover, flocculants in fresh cement pastes are dispersed effectively under ultrasonic vibration after water reducer is added, thus enabling the release of water wrapped in the flocculants. Hence, the free water in fresh cement paste increases and the liquidity is improved. It can be seen from Figure 13b-f that with increases in CO2 AA, the number of flocculating structures that are separated effectively decreases, thus resulting in a relatively small amount of released free water. The liquidity of cement paste declines continuously.  
(3) Summary of the rheological model of carbonated cement paste with water reducer  
The liquidity of cement paste with water reducer deteriorates after CO2 absorption. This is mainly because with increases in CO2 AA, the CaCO3 crystals on the surfaces of flocculants and cement particles increase, including AFt crystals produced by the hydration of cement [32]. After water reducer is added, water-reducer molecules enter organic mineral phases. Since excessive CaCO3 crystals and AFt are wrapped on the surfaces of flocculants and cement particles, water-reducer molecules cannot develop the role well. During vibration, only some flocculants are dispersed, thus preventing flocculant water from being released well. Accordingly, free-water growth declines. As a result, the liquidity of cement paste worsens after CO2 absorption. In a word, the liquidity of cement paste is negatively related to its CO2 AA.