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- Jan 19, 2018 -
Polycarboxylate superplasticizers are widely used to improve the performance of concrete. At present, most of polycarboxylate superplasticizer used in key projects or key parts of the concrete, and often these projects have higher durability requirements, and polycarboxylate superplasticizer high water reduction rate, better The plasticity can be applied to the preparation of high strength concrete and large liquidity concrete. However, when polycarboxylate superplasticizer is used in common concrete, it will not produce good compatibility and can not meet the production requirements of normal concrete. In many engineering applications, the problem of incompatibility with cement occurs. Mainly as a superplasticizer performance is poor or increase the mobility of the effect is not good, the coagulation speed is too fast, retardation, slump loss over time. Even reduce the strength of concrete, this incompatibility phenomenon with the superplasticizer variety and mechanism of action, selection of raw materials and synthetic processes, the composition of cementitious materials, fineness differences. Others such as ambient temperature, admixture dosage will also have a certain impact on the compatibility. For the concept of water-reducing agent and cement compatibility, so far there is no clear definition of concrete admixtures in accordance with the application of technical specifications will be tested in line with the relevant standards of a certain admixture, according to the provisions of the admixture can be admixed with the species Of cement (or mortar) formulated with cement, the cement is considered to be compatible with this admixture if it produces its intended effect; on the contrary, there is incompatibility. Aiticin believes that: in the amount of water-reducing admixture is not reached a saturation point, and after 1 h flow time change is small, that the superplasticizer and cement compatibility is good; otherwise, the poor is called compatibility; if between Between the two, then consider its compatibility in general. In a broad sense, the compatibility of cement with water reducer, including cement paste fluidity, mechanical properties, condensation and bleeding phenomenon, and evaluation of the compatibility of cement and water reducer parameters should be fluid, saturated mixed Volume and loss over time, which can objectively and impartially evaluate a certain type of cement and water-reducing agent compatibility. Experts suggest that: cement and polycarboxylate superplasticizer compatibility should be reflected in the initial mobility of a clear point of saturation and less loss of liquid cement and polycarboxylate superplasticizer compatibility issues both depends on The cement production process and cement quality and polycarboxylate superplasticizer varieties and quality.
1, the impact of cement mineral properties on compatibility
The compatibility between cement and polycarboxylate superplasticizer has a great relationship with the mineral composition of cement. The compatibility is not only related to the content of minerals in cement clinker intermediate, but also with the degree of crystallization of clinker intermediate And the intermediate solution of minerals related. The main minerals in cement also affect the compatibility of the most important factor, the impact of mineral composition on the polycarboxylate superplasticizer is mainly achieved by the adsorption of water reducer molecules, the first superplasticizer adsorbed on the surface of cement particles Due to the dissolution of CaSO 4, Na 2 SO 4 and K 2 SO 4 in the initial stage of hydration, the surface of the cement particle is etched and the charge of the different initial charge of hydration of different mineral components in the cement is different. Phase hydration is negatively charged, while aluminate phase hydration is positively charged. The adsorption amount of the same kind of polycarboxylic superplasticizer by different mineral components is: C 3 A> C 4 AF> C 3 S> C 2 S. From the thickness of adsorption layer, the adsorption amount of C 3 S polycarboxylic acid The thickness of the water reducer is smaller than the thickness of the C 3 A polycarboxylate water-reducing agent. Many scholars at home and abroad have in-depth and detailed studies on this effect. According to the study of Liu Bingjing, the larger the ratio of C 3 A / C 4 AF and C 3 S / C 2 S, the more the amount of water- Which will lead to the worse compatibility between cement and water reducer. The adsorption capacity of C 3 A in cement clinker is much higher than that of other minerals. When the content increases by 1%, the standard consistency of cement also increases by 1%, and the concrete water consumption increases by 6 ~ 7kg / m 3. The hydration rates of C 3 A and C 4 AF were faster. After adding water, C 3 A and C 4 AF rapidly started to hydrate, and the hydration products further adsorbed more water-reducing agent molecules and the hydration products lapped , Leading to an increase in the amount of water-reducing agent adsorption, but also led to significant loss of fluidity cement paste increased significantly. Liu Houfen summed up a quantitative analysis: C 3 A content of less than 8%, cement and superplasticizer better compatibility; more than 8.5%, then even if the amount of water reducer dosage, it can not solve the time Loss problem. Sakai et al. Studied the early hydration of C3A in a calcium sulfate system by quantitative X-ray diffraction. It is believed that the use of industrial waste increases the aluminate phase content in the cement and accelerates the slump loss. However, Acid superplasticizer with the bulk price of cement aluminate phase role, control of its hydration, which play a good dispersion. When the content of the aluminate phase or the aluminate phase in the clinker intermediate is constant, the higher the degree of crystallinity, the less favorable the compatibility of the cement and the superplasticizer is. On the contrary, the more favorable Cement and superplasticizer compatibility improvement; and aluminate aluminum content is relatively low, is conducive to the compatibility of cement and superplasticizer to improve.
Effect of MgO in cement on compatibility
Due to the raw materials, a certain amount of MgO will inevitably be contained in the cement clinker. Because Mg and Ca are the same elements, the physicochemical properties of the two kinds of ions are similar and are easily replaced with each other to form a solid solution. Mg 2+ ions form a solid solution by replacing the calcium ions in the Alite structure. However, an increase in the proportion of Al ions in the Mg ion leads to an increase in the MgO content in the clinker and will result in Alitrides forming the M3 crystal form, MgO being the M3 crystal form stabilizer, while the M3 type C 3 S hydration activity Significantly increased early hydration heat increased significantly. In general, MgO forms free periclase, or enters some solid solution containing phases such as alite and calcium iron aluminate. Periclase in cement is a calcined periclase formed at 1450 ° C. This kind of magnesium oxide crystallizes very slowly at room temperature, causing early hydration of cement to be delayed. After the clinker is fired, the MgO dissolved in the liquid phase will partially crystallize during the cooling of the clinker, and the MgO, which is too late to crystallize, is solidified in the vitreous body of the cement. Therefore, the focus of control over MgO is to strictly control the quality of the raw materials so as to improve the firing quality and performance of the clinker. The inert phase particles are encapsulated by C-S-H gel formed by cement hydration, which act as nucleation and indirectly reduce the thickness of C-S-H gel layer on the surface of cement particles. MgO slows the initial hydration of the cement and increases the setting time of the cement paste. It is because of this characteristic MgO affect the hydration of cement, thereby affecting the cement on the polycarboxylate water reducer molecules adsorption, resulting in poor compatibility. The influence of other factors on the compatibility promotes the post-hydration of cement.
3 mineral admixture compatibility
Water-reducing agents not only have compatibility issues with cement, but also compatibility issues with blended mineral admixtures, affecting the compatibility of cement with water-reducing agents. Mixtures of industrial cement mixed with the main types of limestone powder, fly ash, silica fume, slag and other powder. Due to the different properties of the mixture, which will have different effects on the water reducer, while the physical properties of the mixed material and the amount will lead to water-reducing agent and cement compatibility. In a wide range of mineral admixtures, slag and fly ash are more widely used. Most of the fly ash spherical glass, smooth surface, the main mineral composition of most of the diameter of micron-sized solid beads and hollow beads and a small amount of porous vitreous, vitreous quenching, crystals or unburned carbon particles. Domestic generally divided by fineness of fly ash is divided into three levels, for I-level fly ash, due to the "ball effect" of the spherical glass body, resulting in fly ash on the water reducing agent adsorption capacity than cement, cement can be improved Work performance, improve the compatibility of cement and water reducer. Ordinary fly ash has a high carbon content, resulting in the fly ash particles adsorbing the superplasticizer molecules larger than the cement particles. Since the fly ash itself has a certain water reducing effect, when the superplasticizer is blended , The cement paste with fly ash has better initial fluidity, but the slurry loss over time has increased significantly, which is due to the passage of time, fly ash particles adsorbed more water-reducing agent molecules, Destroyed the original adsorption balance, superplasticizer can not play enough dispersion ability. When the content is too large, since the specific surface area of fly ash is larger than that of cement particles, the adsorption capacity of fly ash of equal mass to superplasticizer is greater than that of cement to superplasticizer, which affects the effect of cement and superplasticizer compatibility.
Blast furnace slag, which is used extensively in the cement industry, is called slag for a long time and its particle shape is poor. It belongs to the polygonal shape, which reduces the contact area with the cement particles or slag particles. The slag particles also have water repellency. Adsorption of the agent decreases, the relative adsorption is less than C 3 S, will not have a greater impact on the compatibility of cement and water reducer, so the use of slag admixture can improve the fluidity of the slurry. However, when the amount of slag increased to a certain value, the specific surface area increased, the adsorption was significantly enhanced, resulting in reduced fluidity of the slurry.
4 soluble sulfate on the compatibility
Polycarboxylate superplasticizer is a kind of relatively free chemical structure of the copolymer, compatibility with the cement and its molecular structure, while being affected by the concentration of soluble ions in the slurry. Yamada et al. Found that the increase of the sulfate ion concentration in the liquid phase leads to the contraction of the space volume of the polycarboxymethyl water reducer molecules, the steric hindrance effect of PEO side chains in the water reducer molecule, and the sulfate ion And carboxylic acid ions produce adsorption competition, thus affecting the dispersion of polycarboxylate superplasticizer. Nava's research also found that: when Sulfate is added, the amount of C 3 A and C 4 AF adsorption superplasticizer is reduced, but the amount of adsorbed silicate is increased. Silicate phase dispersion increases, reducing the viscosity of cement paste to increase the fluidity of cement paste. In the process of cement production, gypsum is the main source of sulfate ion in cement. Therefore, the content and types of gypsum in cement will have certain influence on the compatibility of polycarboxylate superplasticizer and cement. And gypsum dehydration will affect its retarding effect, from accelerating the hydration of cement, thereby affecting the compatibility of water-reducing agent and cement. When the cement contains a high proportion of anhydrite, then the liquid water reducer content, loss of cement dispersion, accelerated slump loss. In addition, anhydrite can accelerate the hydration of both silicate minerals and aluminate minerals, and increase the hydration products of cement to adsorb a large amount of water-reducing agent. Superplasticizer has changed the cement hydration, reducing the solubility of gypsum, gypsum in the liquid phase of the lower concentration. Therefore, the fluidity of a system incorporating a superplasticizer is strongly dependent on the type and amount of gypsum used in the cement.
5 cement specific surface area on the compatibility
Polycarboxylate superplasticizer cement-water suspension system can be divided into three parts. The first part is interspersed with cement hydration products or is involved in the formation of hydration products, thereby changing the hydration products morphology; the second part is adsorbed on the surface of cement particles or hydration products and the formation of a certain thickness of adsorption layer, This part of the superplasticizer disperses the cement particles. The last part of the superplasticizer remains in the solution. This part of the superplasticizer maintains a dynamic equilibrium with the adsorbent layer and replenishes the superplasticizer consumption due to factors such as cement hydration Agents, the main slump retention performance plays an important role. According to the adsorption principle between polycarboxylate superplasticizer and cement, it can be seen that when the specific surface area of cement increases, the contact area between cement particles and molecules of water and water reducer increases, resulting in an increase of surface energy and hydration Speed up, enhanced adsorption. As a result, the loss of cement slurry over time will increase, bleeding will occur, resulting in poor compatibility.
The promotion and application of polycarboxylate superplasticizer in concrete engineering has promoted the rapid development of modern concrete technology, improved the quality of concrete production (increased durability), and achieved the effects of energy saving, material saving and land saving. Saving resources is the fundamental measure to protect the ecological environment, thus reducing the intensity of energy consumption, increasing the efficiency and effectiveness of the use of the increasingly prominent importance. Polycarboxylate superplasticizer with its series of superior performance, the use of engineering in the construction of a growing share of a large share of superplasticizer has entered the polycarboxylate era, which polycarboxylate superplasticizer Application is also more critical. In polycarboxylate superplasticizer engineering practical applications there will be more adverse phenomena, can not meet the actual project requirements, often caused by this reason is that the superplasticizer and cement compatibility issues. Due to the complex mineral composition of cement, cement production quality fluctuations, which superplasticizer and cement compatibility posed a great challenge. By studying and analyzing the compatibility of polycarboxylate superplasticizer and cement from the perspective of cement, it can objectively and accurately guide various problems that should be noticed when cement is actually produced, and is of high quality and good compatibility Cement provides theoretical support.