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Superabsorbent polymers (SAPs) have emerged as promising additives to regulate cement hydration via their intrinsic water absorption/desorption characteristics. This paper aims to investigate the influence of SAPs with different functional groups on the development of cement hydration and growth of calcium hydroxide (CH). The current experiment firstly uncovered the active control mechanism of CH in cement paste by SAPs, especially for the nucleation site and crystallization orientation. Specifically, the pores left behind SAPs after water release usually processed higher w/c ratio, Ca2+ concentration and smaller crystallization pressure, which jointly triggered the in-situ nucleation and crystallization of CH along (0 0 1). However, the hydration degree of cement paste was reduced by 0.1∼4.4% with addition of SAPs, especially in the early stage, which was attributed to that less free water can be used for cement hydration due to the strong water absorption property of SAPs. These insights provide a valuable foundation for deploying SAPs in an active control strategy for CH growth position and crystal form in cement-based materials. • The nucleation site and crystallization orientation of CH can be controlled by SAPs. • Higher w/c ratio, Ca2+ concentration triggers the formation of CH around SAPs. • The addition of SAPs reduced the hydration degree of cement paste by 0.1 ∼ 4.4%. • The addition of SAPs primarily affects hydration during the early stage. [ABSTRACT FROM AUTHOR]

By combining the nearest surface distribution function and hydration kinetic equation, analytical equations are proposed to determine the pore phase and solid phase evolution in the interfacial transition zone (ITZ). In addition, the numerical models for concrete with a single flat aggregate are constructed based on the CEMHYD3D hydration model. The results show that the degree of hydration is the main factor affecting the ratio of the average porosity to the matrix porosity in the ITZ. With an increase in the aggregate volume fraction and interface thickness, the volume fraction of the ITZ first increases and then decreases. The degree of overlap increases linearly with the interface thickness and the aggregate volume fraction. As the distance from the aggregate surface increases, high-density C-S-H increases, while low-density C-S-H and calcium hydroxide phases decrease. Finally, the predicted results using the analytical equations and numerical simulation are verified by testing via back-scattered electron microscopy, which indicates that the experimental results are more consistent with the predicted results of the analytical model than with those of the numerical model. [ABSTRACT FROM AUTHOR]

In this work, two PC pastes were used to carry out accelerated carbonation experiments, and phase assemblages of paste samples collected at different sampling depths were analyzed. The source of CaCO 3 originating from the carbonation reaction of hydrates in cement was quantitatively determined by a novel method based on the transformation of hydrates. Experimental results indicated that the phase assemblages in the carbonated zone of two PC pastes showed minor differences. C(A)SH, ettringite, AFm-CO 3 , and unhydrated cement particles in the complete carbonation zone of two cement pastes were carbonated and transformed into other phases. For the surface of two PC pastes (0–3 mm), calculations showed that the CaCO 3 originating from the carbonation of calcium hydroxide and C(A)SH accounted for 75.3% and 72.6% of the total CaCO 3 , respectively. A concentration of 20% CO 2 was not beneficial for the diffusion of CO 2 and the subsequent precipitation of CaCO 3 at the later period of carbonation. • The change in phase assemblages of two carbonated PC pastes collected at different depths was analyzed. • A calculation method was proposed to analyze the source of CaCO 3 originating from the carbonation of hydrates in PC. • For the surface, the CaCO 3 originating from the carbonation of portlandite and C(A)SH accounted for 75.3% of the total CaCO 3. [ABSTRACT FROM AUTHOR]

To solve the problems of low early strength and severe plastic cracking caused by high volume fly ash used in cement-based materials. Triethanolamine (TEA), calcium hydroxide (Ca(OH) 2), and sodium silicate (Na 2 SiO 3) or sodium sulfate (Na 2 SO 4) were selected to conduct a ternary doping test. The compressive strength of samples was measured to determine the best ratio, content, and time effect of the activator, and its action mechanism was studied by various micro test. The quantitative calculation model of main hydration products was established in the fly ash-cement system. Based on the simulation of molecular dynamics, the structure of NASH gel was studied under alkali activation. The results show that the optimal mixing mass ratio of TEA:Ca(OH) 2 :Na 2 SiO 3 is 2:75:25 and the optimal dosage is 1.02% of the cementitious material. There are a large number of needle-like ettringite, petaloid hydrated calcium aluminate and clusters of hydrated calcium silicate gel in the system, whereas the amount of plate-like CH decreased significantly at hydration for 14 days. The Si/Al is three and aluminium coordination is predominantly tetrahedral, and the order of bonds stability and atoms mobility are Si–O > Al–O > Na–O and Na > O > Al> in the NASH gel, respectively. Under the Na+ and alkali environment, the Si(OH) 4 and Al(OH) 4 - formed polycondensation reaction to reform polymers Si–O–Si and Al–O–Si, forming a large amount of NASH gel. • The optimum combination and dosage of activators were determined in fly ash-cement system. • The early activation mechanism of fly ash-cement system was summarized. • The content of hydration products was predicted theoretically. [ABSTRACT FROM AUTHOR]