Calcium silicate pastes products

CH can be observed as areas darker than the unreacted clinker phases but brighter than the other hydration products. As in calcium silicate pastes, these appear to have grown in regions initially occupied by water. Although the areas appear discrete on two-dimensional sections, they are not necessarily so in the three-dimensional material. They can engulf small cement grains. 

The experimental considerations applying to calcium silicate pastes (Sections 5.1 and 5.2) are equally relevant to cement pastes. Of the methods so far used in attempts to determine the degrees of reaction of the individual clinker phases as a function of time, QXDA (C39,D12,T34,P28) has proved much the most satisfactory. Procedures are essentially as for the analysis of a clinker or unreacted cement (Section 4.3.2), but it is necessary to take account of overlaps with peaks from the hydration products, and especially, with the C-S-H band at 0.27-0.31 nm. The water content of the sample must be known, so that the results can be referred to the weight of anhydrous material. If a sample of the unhydrated cement is available, and its quantitative phase composition has been determined, it may be used as the reference standard for the individual clinker phases in the paste. 

As with calcium silicate pastes, the gelatinous nature of the principal reaction product renders any definition of chemically bound water somewhat arbitrary. The three definitions of water content described in Section... 

Silicate anion structures in Portland cement pastes have been studied by the methods described in Section 5.3.2 for calcium silicate pastes. Trimethylsily- i lation (TMS) studies (L20,T12,S69,T36,L31,M43,M44) show that, as with C,S. the proportion of the silicon present as monomer decreases with age and that the hydration products contain dimer, which is later accompanied and eventually partly replaced by polymer (>5Si). Some results have i indicated that fully hydrated pastes of cement differ from those of CjS in that substantial proportions of the silicate occur as monomer (S69,L31), but the results of a study in which pastes of CjS, P-CjS and cement were compared (M44) suggest that the differences between the anion structures of cement and CjS pastes are probably within the considerable experimental errors inherent in the method. The recovery of monomer from unhydrated P-CjS was only 66% and results for cement pastes can only be considered semiquantitative. 

Studies on calcium silicate pastes show that the distribution of silicate anion size is shifted significantly upwards with a rise in curing temperature (Section 5.3.2). XRD gives no definite indication that the C-S-H formed at temperatures up to 100 C is more crystalline than that formed at ordinary temperatures, though the product of the 2-month treatment at 90"C mentioned earlier (T5) contained a little a-CjSH, a crystalline phase which is... 

No attempt is made here to explain the intricate temperature dependence of the hydrations of the three calcium silicates. Work on the kinetics of these hydration processes is not complete, so the authors could give at present only highly tentative and speculative explanations. The conclusion from Figures 2 to 7 for the purposes of this paper is that the surface area development in pastes of all three silicates is determined primarily by the degree of hydration of the silicate. For a simple reaction, in which the reactants have negligible surface areas compared with the reaction products, such a conclusion would be trivial. However, for as complex... 

Stabilisation is a much slower process, which occurs progressively over several months, and involves the reaction of lime with the siliceous and aluminous components of the soil. The lime raises the pH to above 12, which results in the formation of calcium silicates and aluminates. These are believed to form initially as a gel, which coats the soil particles, and which subsequently crystallises as calcium silicate/aluminate hydrates. Those hydrates are cementitious products, similar in composition to those found in cement paste. The rate of crystallisation is temperature dependant and may take many months to reach completion. The resulting gain in strength (measured by the California Bearing Ratio Test [26.11]) is progressive, as illustrated in Fig. 26.2. 

Alite reacts with water to form calcium silicate hydrate and calcium hydroxide, which is also known as portlan-dite. The hardened paste has high strength when the reaction is completed, and because alite is the most abundant compound in cement, it also makes the dominant contribution to the mechanical properties of the final product. The hydration reaction proceeds at an appreciable rate a few hours after the addition of water and lasts up to about 20 days. The reaction of alite with water is accelerated by aluminate and gypsum. 

Concrete is a composite typically composed of aggregate and cement paste. It is the connection between these phases - the interfacial transition zone (ITZ) that is most important.. The ITZ is the weakest link in the composite system with lowest mechanical properties of the three [1-4], The ITZ can affect the overall elastic module and the stress distributions in a concrete material. The ITZ is comparatively more porous than that of bulk cement paste, and often less well bonded to the aggregate [3]. This region can have a low formation of calcium-silicate-hydrate (C-S-H gel), a product of Portland cement hydration responsible for the good mechanical properties and durability [5]. 

Fig. 7a-c show images of the products of hydration in the cement paste. Ettringite, hydrated calcium silicates and Ca(OH)2 were detected due to their typical morphologies. 

The two most important constituents of Portland cement are alite, a form of tricalcium silicate, and belite, a form of dicalcium silicate. In their hydration both calcium silicates yield—in addition to calcium hydroxide—a nearly amorphous calcium silicate hydrate phase (C-S-H phase), and this hydration product is mainly responsible for the strength and other physico-mechanical properties of the hardened cement paste. 

The gel pores form a part of CSH (calcium silicate hydrate), and may be classified as micro pores or meso pores. The principal difference between gel and capillary pores is that the former are too small to be filled by the hydration products and for capillary effects, it means that no menisci are formed. The gel pores occupy between 40% and 55 % of total pore volume, but they are not active in water permeability through cement paste and they do not influence the composite strength. Water in the gel pores is physically bonded. It is believed that gel pores are directly related to shrinkage and creep properties of the cement paste. 

In principle, all the concepts of surface chemistry previously described are also expected to be vahd for cement particles. However, the highly dissolving and reactive nature of such particles introduces a dynamic and complex feature to the particle-water system. This makes it difficult to interpret the data obtained from cement-containing suspensions, which is probably the main reason for the paucity of publications available in the literature that focus on the interactions between additives and high-alumina cement particles. The concepts described herein are therefore based on sffidies performed in aqueous pastes of calcium silicate (Portland) cement, which have been more extensively researched in recent years due to their considerable importance for concrete technology (47-50). Although calcium aluminate and calcium silicate cements are known to display distinct reaction rates and form different hydration products, similar interactions between the cementious phase and the chemical additives (admixtures) are expected to occur in aqueous suspensions. 

The calcium chloride silicate most likely to be formed, in addition to alinite, is Ca, Si04Cl2. A study of its hydration behaviour (K54) showed it to be relatively highly reactive, C-S-H and CH being detectable by XRD in little over 1 h. Ca2Cl2(0H)2-H20 was also formed at w/s = I. though not at w/s = 10, and might thus be expected to be a product in a paste. 

Silicsol is a commercial product introduced in Europe this past decade. It is described as an activated silica liquor with a calcium-based reagent. As opposed to sodium silicate (colloidal silica particles dispersed in soda), silicsol is claimed to be a true solution. Viscosity and penetrability are similar to sodium silicate, but the reaction is different, resulting in a stronger end product more resistant to creep. There is no syneresis associated with silicsol. 

---