Samples of gypsum plasters with initially various water-to-gypsum ratios which contained HEMC admixtures in weight fractions from 0.25 to 1.5% were prepared. Setting, mechanical and DSC tests were made. The results are shown in the following figures. The effect of HEMC admixture on the kinetics of gypsum hydration/crystallization process was discussed.
Setting of material is a transition period during which the physical state of material changes from liquid to solid. This transformation occurs as a result of the development of hydrated products which cause rigid connections between hydrating grains. It is usually characterized by two points in the hydration process, namely initial and final setting time. The development of connected hydration product which reflects the transition of material state was measured by the penetration resistance technique.
Selected results of the Vicat needle test are shown in Figs. 2 and 3. The beginning of the setting period was estimated as the first inflection point on h curve. The time of setting was measured as the time of the intersection of straight line on the curve. The time from the onset of the experiment to the beginning of the setting period is known as the induction period.
Fig. 2Setting of gypsum obtained with various water-to-gypsum ratios (w/g = 0.6–0.74). Weight fraction of HEMC is equal to 0.5%. ts example of setting time measurements, ti induction time of setting
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Fig. 3Setting process of gypsum samples with w/g = 0.66 for various HEMC contents (0.25–1.5%) and for sample without admixtures
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Figure 2 shows setting of the samples with water-to-gypsum ratio x from 0.6 to 0.74. In the case of higher water content (in initial gypsum mixture), the initiation of the setting process (initiation time ti) followed later than in the case of low water content. Gypsum setting time ts also increased with increasing amounts of water. Excess water caused prolongation of the nucleation process and also adsorption/accumulation of ions or other molecular units at the interface due to lowering of the oversaturation degree.
Figure 3 illustrates the setting process of gypsum for samples with w/g = 0.66 and various HEMC contents. In Fig. 3, for comparison, a sample without any admixtures with w/g = 0.66 is also presented. The addition of a small amount of HEMC admixture (lower than 1%) caused an insignificant prolongation of the setting and induction time. Results for the samples with 0.25, 0.5 and 0.75% are very similar. Compared to the samples with a smaller content of admixtures and those without any admixture, the addition of 1% prolongs the setting time. HEMC in the amount of 1.5% causes a significant (almost double) increase in the setting and induction time. The applied polymer prevents diffusion of water molecules and anions to the binder surface due to relatively rigid polymer molecule conformation in the water phase. No electrostatic interactions of polymer active groups with the binder surface of gypsum were predicted because it was found elsewhere that the gypsum zeta potential was close to zero [17].
Dependence of ti and ts on the w/g ratio was presented in our previous paper [2].
The results of mechanical tests are shown in Figs. 4 and 5. Figure 4 presents the dependence of bending stress σ on HEMC content for water-to-gypsum ratio equal to 0.6–0.74. The bending stress increases with increasing HEMC content for chosen w/g. The higher values of σ are received for a smaller water-to-gypsum ratio.
Fig. 4Bending stress σ versus HEMC content for various w/g ratios. Error is equal to 1 MPa
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Fig. 5Bending stress σ versus water-to-gypsum ratio, w/g
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Figure 5 presents bending stress σ versus water-to-gypsum ratio w/g. In both cases (samples without admixture and with 1% HEMC), the bending stress σ decreases with increasing water content. It results from a change in the sample morphological structure and increasing pore content. Greater HEMC content causes the growth of overlapping crystals which leads to significantly increasing bending stress of the sample.
The hydration of calcium sulfate hemihydrate is an exothermic reaction
$${\text{CaSO}}_{4} \cdot 0.5{\text{ H}}_{2} {\text{O}} + 1.5{\text{ H}}_{2} {\text{O }} \to {\text{CaSO}}_{4} \cdot 2{\text{ H}}_{2} {\text{O }} + \, Q,$$
where Q, the amount of heat evolution, depends on a number of factors.
The reaction of gypsum with water [18,19,20] is divided into three main stages which involve a set of coupled chemical processes:
1.
A nucleation period starts immediately after the hemihydrate powder sample is mixed with water solvent and dissolved. The dissolution involves detachment of molecular units from a solid surface in contact with water as well as diffusion and transport of solution components into volume paste. The solution becomes supersaturated with respect to Ca2+ and SO 2−4 ions, which leads to the precipitation of solid grains of calcium dihydrate due to the nucleation process.
2.
An acceleration period in which a complex reaction between ions or solid complexes adsorbed on solid surfaces due to the crystallization/hydration process is observed.
3.
A deceleration period of very slow reactions of adsorption and accumulation of ions or other molecular units at an interface. The late stage of hydration is thus controlled by the diffusion process.
The wide range of properties can be observed as hydration proceeds, including heat of hydration, porosity and setting time as well as phase volume fraction.
The rate of all the reactions can be changed by the presence of a polymer admixture. For example, the induction period can be prolonged with:
a.
Reduced diffusion of water and calcium ions at gypsum surface because the adsorbed polymer (if so) hinders the process,
b.
Formation of a complex between calcium ions and the polymer in the pore solution,
c.
Change in the growth kinetics and morphology of hydrated phases caused by the dispersive action of the polymer.
Experimental observations have suggested that the formation of a cementitious hydrated product is a rate-controlling process at early stages [19]. Furthermore, this led to the development of hydration kinetics models based on nucleation and growth phenomena such as presented by Avrami et al. [21, 22].
Avrami [21, 22] and also Johnson and Mehl [23], as well as Kolmogorov [24], proposed a simple but widely used equation which is derived using the assumption within the transforming volume in changing liquid on crystal. Avrami power law is as follows:
$$X\left( t \right) = 1{-}\exp \left( { - Kt^{\text{n}} } \right),$$
(1)
where X(t) is the volume fraction of crystalline phase that is transformed at time t, K is the combined rate constant that involves the rates of growth and nucleation, and n is a parameter dependent on the mechanism and dimensionality of growth.
Depending on crystal, growth n can reach a value between 1 and 3. If n ≈ 1, the growth will have one-dimensional (needle) character. In the case of n ≈ 2, two dimensions and n ≈ 3 isotropic growth (sphere) are observed [25, 26].
The volume of the transformed phase will increase with the simple power law (Avrami equation) at early stages of the process before adjacent regions of the growing product impinge. Thus, the overall growth rate in the system decreases with time.
It is commonly assumed that hydration is the diffusion controlled by the rate at which the reactants can diffuse through the nanoporous layer of hydration product around the remaining unhydrated gypsum particles. The point in which the hydration process shifts away from nucleation and growth is not well established, but it is an important aspect of the hydration process.
The increasing use of mineral or polymer admixtures in cementitious materials leads to the question how the admixtures can affect hydration rate, especially at early stages.
The wide range of sample properties can be observed as hydration/crystallization proceeds, including heat of hydrated phase, volume fraction, chemical shrinkage, percolation of capillary porosity and setting time.
Hydration/crystallization of calcium hemihydrates, both pure and with admixtures, was conducted by DSC research. Figures 6 and 7 show curves of the process. Figure 6 presents curves obtained for samples with the water-to-gypsum ratio equal to 0.66. Gypsum hydration occurs in the main three-stage processes, i.e., nucleation (I), acceleration (II) and deceleration (III). The hydration/crystallization process occurs faster in the case of sample 6 (without admixtures). The increasing polymer content causes a delay of the hydration/crystallization process. Not only the induction period is extended, but also the rate of the following hydration reaction is slowed down. This is illustrated by lower values of maximum heat release and broader exothermal peaks in the calorimetric curves of the polymer-modified pastes.
Fig. 6Isothermal DSC curves: heat flow of gypsum hydration/crystallization versus time for various HEMC contents, w/g = 0.66
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Fig. 7Isothermal DSC curves: heat flow of gypsum hydration/crystallization for various w/g (for 0.5% HEMC content)
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Figure 7 shows samples with various water-to-gypsum ratios (0.60–0.74) and HEMC content equal to 0.5%. With an increasing w/g value, the hydration/crystallization process also delays. Both the presence of polymer in water solution and increasing w/g ratio leading to the delay of gypsum hydration/crystallization result from the same prevention of nucleation process of gypsum species.
Figure 8a presents curves transformed in Fig. 6 to the plots of the degree of hydration/crystallization X drawn versus time. Figure 8b shows Log[−ln(1 − X)] versus log t (obtained according to Avrami equation).
Fig. 8a Curves from Fig. 6 transformed into Avrami plots—degree of hydration/crystallization X versus time t. b. Log[−ln(1 − X)] versus log t
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The values of crystal growth rate constant K (dependent on the nucleation rate) are determined from Fig. 8a, b and presented in Fig. 9 (parameter n ≈ 1.4). An increasing polymer admixture content causes a decrease in the K value. The results indicate that HEMC is an efficient agent disturbing the nucleation and crystallization of gypsum (lower K value).
Fig. 9Parameter K versus HEMC content
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The morphology of calcium sulfate dihydrate crystals depends on the formation conditions and the presence of additives [9]. SEM microphotography is helpful for observation of gypsum crystals. Figures 10 and 11 present microphotographs of bending fracture area of the samples with water-to-gypsum ratio equal to 0.6 (with the admixture in Fig. 11—0.5% of HEMC). The crystal habit is affected by the presence of polymer as may be seen in the SEM photographs. Crystals in the absence of admixtures are thin and elongated which is a result of their rapid growth. They are longer than in the case when HEMC is added. The presence of the polymer in the reacting solution enhances the agglomeration of crystals. A decrease in the total pore volume and increase in crystal overlapping lead to a more impact structure and higher mechanical strength.
Fig. 10Microphotograph of bending fracture area of the sample with w/g = 0.6
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Fig. 11Microphotograph of bending fracture area of the sample with w/g = 0.6 containing 0.5% HEMC
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Admixtures in the form of water-soluble polymers change the interaction between Ca2+, SO 2−4 and OH− ions forming hydrated crystals of gypsum. Polymers are not built into gypsum crystals but can form a separate phase (for example thin films in the pores).
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