Concrete admixture has become in addition to cement, aggregate (sand, stone) beyond the essential composition of concrete materials. At present, the most widely used admixture in civil engineering construction is water reducing agent. From the engineering construction practice, water reducing agent and cement have good adaptability to reduce the project cost and improve the working performance of concrete. But in practice, it is found that some cement with water reducing agent after the increase of concrete slump is not much, the need for a larger amount to make the slump significantly increased; Sometimes it is the concrete slump by the loss of the slump, and even the slump quickly becomes zero. The above reasons are mainly due to the poor adaptability of cement and water reducing agent. At present, the adaptability of cement and water reducer has been paid more and more attention by engineering construction personnel. This paper analyzes and discusses the adaptability of cement composition to water reducer in combination with engineering practice.

1. Engineering Examples

1. 1 Problem Discovery

The climate of the location of a reservoir project is characterized by abundant light and heat resources, large temperature difference, less rainfall and intense evaporation, belonging to the continental desert climate of warm temperate zone. The average annual precipitation is 53.3 mm ~ 62.7 mm, the average annual evaporation is 2273mm ~ 2788mm, the average relative humidity is 45% ~ 47%, the average annual temperature is 11.5 ℃, the summer is hot, and the extreme maximum temperature is 43.6 ℃. It is mainly sunny throughout the year, with long sunshine duration, much solar radiation energy, large temperature difference between day and night, and windy spring.

The dam structure of this project is sand and gravel homogeneous dam, and the slope protection adopts cast-in-place concrete slab, and the design index of cast-in-place concrete slab is C30F300. The mix ratio submitted by the laboratory through the test is shown in Table 1

In order to check the mixture ratio and mixing system of concrete and ensure the normal construction of concrete, the trial pouring of dam slope concrete was carried out before the formal construction of slope protection concrete. In the field test casting of dam slope concrete, it was found that the slump of concrete mixture was less than 1cm, much lower than the slump of 7cm in the test, and the setting time was too short to collect the surface at the site. After many times of trial adjustment the results are not ideal, can not guarantee the normal construction.

1.2 Experimental comparison

In order to analyze and find out the reasons for the failure of the test casting, the test personnel inspected the raw materials used in the construction site. The inspection results show that the quality of the raw materials are in line with the national standard requirements. After the research and analysis, the final focus on the cement and the adaptability of high efficiency water reducer, and arrange the site cement and different dosage of high efficiency water reducer slurry fluidity test. Through the test found:

Site of concrete (cement factory, on April 14, referred to as "site) and the adaptability of high efficiency water reducing agent is extremely bad, on February 3, much less used in the design of concrete test to the same manufacturer of varieties of cement, the cement is in late January factory of 32. 5 r Portland cement, referred to as" the test of cement ") with high efficiency water reducing agent to adapt to the matching result, see table 2.

It is obvious from Table 2 that the fluidity of the test cement net slurry mixed with 0. 5% high efficiency superplasticizer reaches 220mm, and the fluidity of the site cement net slurry mixed with 2% high efficiency superplasticizer is less than the former, and the difference of the content of the latter is four times that of the former. It shows that the main reason of failure of concrete test pouring is the poor adaptability of cement and high efficiency water reducing agent.

Because there are still samples of the high-efficiency superplasticizer used in the test and no samples of cement were retained, in order to exclude the factors of the high-efficiency superplasticizer, the superplasticizer used in the test and the cement on the site were used for the adaptability test, as shown in Table 3.

Compared in table 3 with high efficiency water reducing agent dosage of flowability of net, found that test using water reducing agent and water reducing agent on site on site of net cement slurry to adapt to the matching effect is basically the same, are poor, it's ruled out the use of concrete test of water reducing agent and water reducing agent on a project site performance differences on the impact of net cement paste fluidity. This shows that the factors affecting the poor adaptability of site cement and high efficiency water reducer have nothing to do with water reducer, but mainly lie in the change of cement.

According to the research at home and abroad, there are three main reasons that cement affects its adaptability to water reducer. One is that the coagulant gypsum mixed with cement is not two water gypsum (CaSO42H2O), but half water gypsum; Secondly, the content of C3A in cement clinker is too high. Third, the alkali content in cement (R2O =Na2O+Q 653K2O) is on the high side. One of the first reasons for the use of retarded superplasticizer wood calcium can be identified, because the cement containing anhydrite met sugar, wood sulfonic acid superplasticizer, will cause the cement rapid coagulation; The latter two can be explained by the chemical analysis of the cement clinker used to determine the level of C3A and R20. To this end, we use the site cement in the addition of high efficiency water reducer, but also add the retarding water reducer wood calcium 0.2%, to observe whether it is retarding. The test results are shown in Table 4.

The results show that the wood calcium can promote the coagulant of the site cement, and the net slurry has no fluidity after 25min, which indicates that there is hemihydrate gypsum in the coagulant of the site cement.

1. 3 Cause analysis

Through the understanding of the production technology of cement on the construction site, we think that the different contrast between the cement of the same variety in late January and April 14 in the test and the adaptability of water reducer is closely related to the production season of cement. The reasons are mainly as follows:

(1) Influence of clinker grinding temperature. The temperature in January is low, and the cement clinker temperature into the mill is also low. Because the temperature is negative, the temperature of the mill is also low. Temperature rise in April, the increase of demand for cement, into the mill clinker temperature is higher, when the mill operation temperature is higher than in January, when the temperature is higher than 45 ℃, incorporating the dihydrate gypsum in clinker, dehydration, occurs within 45 ~ 170 ℃ dihydrate gypsum dehydration type into beta half water gypsum (beta CaSO4 1/2 h2o), dihydrate gypsum on cement hydration retarded the coagulation, beta type half water gypsum coagulating effect. In the process of grinding cement, the more the amount of dihydrate gypsum becomes semi-hydrate gypsum due to the high temperature, the weaker the retarder effect and the stronger the procoagulant effect.

(2) Influence of clinker calcination process. According to the manufacturer, the clinker used in the production of common cement in this factory has two kinds: rotary kiln and vertical kiln, and vertical kiln is more than rotary kiln. The contents of C3A and alkali in rotary kiln clinker were low, while those in vertical kiln clinker were high. January cement demand less, cement clinker in rotary kiln production is given priority to, so the number of C3 A and alkali content in cement is low, the weather gets warmer, the construction season, cement demand increase, to ensure the supply of cement used in Portland cement clinker, increase the proportion of the shaft kiln clinker, is relative increase in the number of C3A and alkali content in cement.

The superposition of the above several reasons, caused the reservoir project trial mix use of this cement, the cement and water reducer adaptive deterioration of the phenomenon. Here it needs to be explained again that the use of this cement, only by adding water and reducing agents will have the problem of "adaptability". And with the reduction of the water-binder ratio of concrete its reaction is stronger. Because the water glue of the concrete of the reservoir project is relatively low, the reaction is obvious.

2 the conclusion

To sum up, the adaptability of cement and water reducer directly affects the workability of concrete mixture. Therefore, in the construction of the choice of cement should consider the following aspects: (1) try to choose the cement with two water gypsum as a coagulant. Avoid the use of cement containing anhydrite, and strictly control the temperature of the mill during cement grinding to avoid or reduce dehydration of gypsum. (2) choose cement with low alkali content. Alkali content not only makes the adaptability of water reducer and cement worse, but also causes the alkali aggregate reaction of concrete. Therefore, the alkali content in cement should be strictly controlled. (3) Choose cement with low C3A content; (4) Preferential choice of cement produced by rotary kiln