Elsevier

Cement and Concrete Composites

Volume 95, January 2019, Pages 154-168
Cement and Concrete Composites

Pore structure of hardened cement paste containing colloidal polymers with varied glass transition temperature and surface charges

https://doi.org/10.1016/j.cemconcomp.2018.11.001Get rights and content

Abstract

Influences of four polymer latexes with varied glass-transition-temperature (Tg) and colloidal surface properties on pore structure of hardened cement pastes (hcps) were investigated by means of mercury intrusion porosimetry (MIP), nitrogen adsorption measurement (NAM) and SEM. MIP measurement indicates that polymer addition leads to increase of total pore volume and threshold pore size, whereas NAM shows that pore volume in the range of 2–100 nm decreases with addition of polymers, implying a sealing effect. The conflicting results between MIP and NAM originate from destruction and movement of the polymer phase by high pressure mercury intrusion during MIP measurement. SEM reveals that low-Tg polymers form continuous film while high-Tg polymers remain particle form in hcps. The film-forming polymers provide stronger sealing effect than the non-film-forming polymers. Moreover, the non-adsorbing polymers exhibit stronger sealing effect than the adsorbing polymers due to their preferential locations in aqueous phase of fresh cement paste.

Introduction

Hardened cement paste (hcp) is known as a typical porous material with total porosity of 20–40% and pore size ranging from several nanometers to millimeters [1]. Pore structure of hcp is one of the key properties for cement concrete and mortar, which directly determines or indirectly influences many application properties of the cementitious materials, such like mechanical strength [2], permeability and durability [3], shrinkage and cracking [4]. Therefore, understanding of the pore structure of hcp has been a conventional topic and is still one hot topic in the field of cement and concrete research [5,6]. Normally, pores in hcp are divided into three parts according to the size and formation mechanisms, gel pores, capillary pores, and air voids [7]. Csingle bondSsingle bondH gel pore is an inherent property of calcium silicate hydrate gel, including interlayer pore (typical size of 2–4 nm) and inter-gel pore (typical size of 20–30 nm), which mainly influences shrinkage and creep properties of cementitious materials. Capillary pores in a range of 5–5000 nm in size [8], originate from the water that is not consumed during the hydration process of cement, and hence they depend strongly on the water to cement ratio and the hydration degree [[9], [10], [11]]. Usually higher water-cement ratio leads to higher volume fraction of capillary pores and increasing hydration degree of cement results in decrease of the capillary porosity. The capillary pores are believed to have great impacts on the transport process of attacking species in concrete and hence to be the decisive factor for durability of concrete [12]. Air voids, usually introduced naturally or artificially during the mixing of cementitious materials and greater than 50 μm in size, are far larger than the capillary pores in the hcp [13]. In some cases, a certain amount of air voids is introduced by air-entraining agent to improve some properties of concrete, such like freezing-thawing resistance [4].

On the other hand, colloidal polymers in the forms of aqueous polymer dispersion and re-dispersible polymer power are widely used in cementitious materials to acquire superior properties such as higher tensile and flexural strength [14], higher adhesive strength to substrates [15], higher cracking resistance and water proofing property [3], and improved ductility and impact toughness of cement mortar and concrete [16,17]. Upon the incorporation of polymer into cementitious materials, microstructure of the hcps is severely changed from the early age to the final stage of the materials. Several models have been proposed to describe the microstructural development of polymer modified cement pastes for understanding the working mechanism of polymers in cementitious systems. Ohama [18] proposed a basic three steps model to describe the interaction between polymer particles and cement grains. Immediately after mixing, polymer particles stack on the surface of cement grains and then polymer particles flocculate together forming a seal layer on the surface of hydration products. Finally, polymer particles completely melt together to form a continuous polymer network structure. Afterwards, several adjustments and refinements of this model are presented, such as an integrated model in which reciprocal influences between the polymer and the cement are implemented on a time scale presented by A. Beeldens [19] and an early stage evolution model proposed by Z. Su [20]. It is seen from all these models that the synchronous processes of cement hydration and polymer film formation are the decisive factors for the microstructural development of polymer modified cementitious materials.

Influences of latex polymers on cement hydration have been intensively investigated in the last decades [19,21]. General observation is that the inclusion of polymer latexes usually leads to depressed cement hydration and slower strength growth of the cementitious materials [18]. Our previous studies [22,23] have concluded that the retardation effects of the polymer latexes on cement hydration are highly related to the adsorption of the colloidal polymer particles on surface of cement grains, as the covering layer of polymer strongly hinders the nucleation process and reduces the growing surface of the hydration products such as Csingle bondSsingle bondH.

Film formation of polymer in hydrating cement pastes remains a hot topic in recent years. Knapen et al. [24] found that polymer bridges form between the layered Ca(OH)2 crystals by means of SEM investigation. Plank et al. [25] found that film formation is significantly retarded when the latex dispersion is present in cement pore solution. Mattea et al. [26] tracked the film formation process of PVOH-solutions using a low-field, single-sided nuclear magnetic resonance (NMR) scanner and found that the depth-dependence of the relaxation time T-2 could be as a signature of dynamic heterogeneities in the drying process under which the film is gradually forming. Nevertheless, a combined approach where both, the film formation as well as the hydration degree of the cement are highly accurate determined, is not published in the literature. The film formation process of polymers in cementitious materials needs to be further studied by means of new technologies.

Disclosing the impacts of colloidal polymers on pore structure of hcps is one of the essential aspects for understanding the working mechanism of the polymer in varying the shrinkage and the transportation properties of hardened cementitious materials. Only in recent years, influences of polymer films on the pore structure of cementitious materials became a research subject in the field of polymer modified mortar and concrete [14,27]. Ramli [3] found that the presence of polymer latexes (styrene-butadiene rubber SBR, polymer esters PE and vinyl acetate ester VAE) in cement mortars greatly changes the pore structure of the cement paste, resulting in increased waterproofness and impermeability. D.A. Silva's study revealed that vinyl acetate/ethylene copolymer (EVA) increases the total porosity and the threshold pore diameter of hcps [28]. Mateusz [29] studied the effect of different types of cellulose ethers on pore structure of tile adhesive mortars and found that a higher dosage of CE results in a slight pore volume increase and a clear coarsening of the air voids due to higher extent of air bubble agglomeration at a higher CE dosage.

To sum up, it is seen that in the previous studies, major focuses are the film forming process of polymer phase during the hydration process of cement and the influence of film-forming polymers on cement hydration [19,22,23]. However, the film formation property of polymer latexes is closely related to the glass transition temperature (Tg) of the polymers. Polymers with Tg higher than the curing temperature may not form homogenous film during the hardening process of cement pastes. On the other hand, cement hydration in the presence of polymer latexes is highly dependent on the adsorption behavior of polymer particles on cement grains, which is determined by the surface charges of the colloidal particles [30]. From the polymer side, the adsorption behaviors of polymer particles on cement grains and the film formation property of polymer latexes could be easily tuned by adjusting the surface charges of the colloidal particles and the glass transition temperature of the polymer in latexes.

The aim of this paper is to advance the understanding of the mechanisms of how colloidal polymers affect the pore structure of hardening cement paste with consideration of the adsorption property of colloidal particles on cement grains and the film formation property of polymer particles during cement hydration. Self-synthesized styrene-butyl acrylate co-polymer (SA) latexes, with varied surface charges of colloidal particles and Tg of the polymers were used in this study. The colloidal polymers are mixed into cement paste at dosages of 5% and 15% by weight of cement (bwoc). Pore structure of the hcps was characterized using mercury intrusion porosimeter (MIP) and nitrogen adsorption measurement (NAM). Moreover, the hydration degree of cement and the micrographs of the hcps containing the SA latexes were determined by the measurement of the chemically bound water (CBW) and SEM observation for a thorough understanding of the pore structure.

Section snippets

Materials

Ordinary Portland cement 42.5 R complying GB8076-2008 was provided by China United Cement Corporation. The specific surface area of the cement measured by Blaine method is 346 m2/kg. The chemical and mineralogical compositions of the cement are indicated in Table 1.

Four SA latexes, named T1, T2, C1 and C2, with varied Tg and surface functional groups were synthesized through the emulsion polymerization and provided by BASF Advanced Chemicals Co. Ltd (Shanghai, CHINA). Full characterization of

Adsorption of polymer on cement grains in fresh cement paste (fcp)

The adsorption of polymer particles is one of the key parameters determining the cement hydration in the presence of latex polymers [36]. Kong et al. concluded that more adsorption of SA polymer leads to more retarded cement hydration [30]. On the other hand, the adsorption behavior of polymer particles is also the key factor determining the location of the polymers after hardening of the cement paste. That is to say in the hcps, the adsorbing polymers are rather located in the surrounding area

Conclusions

Pore structure of hardened cement pastes is a decisive factor for transportation of attacking species and hence durability of concrete, and one of the critical factors for mechanical strength, shrinkage and cracking of hardened cementitious materials. In this paper, the influences of four SA latexes with varied Tg and surface charges on pore structure of hcps are studied by means of MIP, NAM and SEM etc. According to the abovementioned experimental results, the following conclusions are

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2017YFB0310002) and the National Natural Science Foundation of China (51778333).

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