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Service > Harmful effects on the strength of concrete aggregate

Harmful effects on the strength of concrete aggregate

Chemical composition of aggregate under the influence of certain elements of water, and water can chemically react and make concrete changes in the physical and mechanical properties, and reduce the strength of the concrete is destroyed, this aggregate is called hazardous aggregate .
Harmful cement aggregate reaction can be divided into the following categories:
First, the alkali silica reaction
1, the reaction phenomenon
Alkali-aggregate reaction of concrete cement raw materials, alkali (Na2O or K2O) additives, admixtures and water react with the active ingredient in the aggregate, after the concrete pouring molding gradually react to form alkali silicate condensate glue, swelling the reaction product of the concrete internal stress, expansion cracks, leading to the loss of the design performance concrete. Because after reactive aggregate by mixing a substantially uniform distribution. So once the alkali aggregate reaction, concrete expansion stress generated each section, the concrete itself spalling, severe development can only dismantle, can not be remedied, which is called cancer of concrete.
2, reaction mechanism
During the last 40 years, we observed some harmful chemical reaction around the cement paste and aggregate between. The most common reaction is a reactive silicon component and a cement between alkali aggregate. The active form of silica opal (amorphous), chalcedony (hidden crystal fibers), and tridymite (crystals). The active material is present in: opal or chalcedony, chert, siliceous limestone, Wen Shi and andesitic flows and tuffs rhyolite in one thousand.
Reactive silica is characterized by all of the silicon-oxygen tetrahedron be in any network structure, the actual internal surface area is large, alkali ions which will be easier to play the role of silicon-oxygen bond coupling destroy it disintegrated, dissolved into a silicone rubber or according to Under reactor into silicate gel:
Active SiO2 + 2mNaOH (KOH) → mNa2O (K2O) · SiO2 · nH2O
Expansive interpretation can be roughly divided into two theories: one that the alkaline hydroxide alkali aggregate reaction by cement base (Na2O and K2O) formed by the reaction between aggregates siliceous minerals beginning Since the formation of alkali - silica gel matrix interface alteration occurs. This colloid is "infinitely expanded", which have increased in volume after absorbing trend. Because of this colloid by the surrounding cement paste constraints, resulting in internal pressure, leading to expansion of the cement slurry, cracking and damage (sudden burst). In view of this expansion is due to osmotic generated hydraulic pressure caused, but the swelling pressure of alkali silica reaction solid product can also cause swelling. Therefore, we can say the expansion of hard concrete aggregate particles are harmful, some softer gel is due out later flooding and deposited in the fracture due to the expansion of aggregate generated. Siliceous particle size affects the speed of reaction, fine particles (20-30 microns) within a month or two will produce expansion, only relatively large particles to produce expansion only after a few years.
Another wet osmotic theory means surrounding the active aggregate cement paste acts as a semi-permeable membrane, the reaction product of silicate ions difficult to pass, but allows diffusion of water and alkali hydroxides in, so that the osmotic pressure is the main cause swelling.
3. Factors
Generally believed that only a higher total alkali content in cement, while aggregate also contains case of reactive silica, the above adverse reactions will occur.
3.1 alkali content
Since the amount of the base depends only on the amount of cement. They aggregate reaction surface concentration thus indicated to determine the size. The minimum amount of alkali cement expansive cement may occur reaction was 0.6%. Na2O equivalent may be multiplied according to the actual clinker content of 0.658 K2O Na2O content combined with its actual and calculated. However, in exceptional circumstances, even with a low alkali content of the cement can also cause swelling under constraints formulated with reactive aggregate concrete is given, when the cement alkali content is higher, the greater its expansion. In certain cement components, fineness greater, the greater the expansion of concrete.
3.2 reactive aggregate
Particle size and content of the active aggregates have a greater impact on the expansion of the size, it has been found that the addition of finely divided silica in the mixture, allow expansion due to alkali-aggregate reaction caused by reduced or eliminated. This seems contradictory statements can be explained from the following: a low silica content in the range, for a given amount of alkali conditions, the greater the amount of silica, swelling increases, but the silica content than high, the situation is reversed; the larger the surface area of the active aggregate, the less effective the base amount per unit area, which may be generated by the base - the less the amount of silica gel. On the other hand, since calcium hydroxide mobility is very low, only aggregate surface near the calcium hydroxide can participate in the reaction, so that each unit area of calcium hydroxide, independent of the size of the total aggregate amount of surface area. Therefore, increased surface area, so that the ratio of calcium hydroxide with a base in the aggregate at the interface of the solution increases. In this case, they form a harmless (non-swelling) basic calcium silicate product.
For a given aggregate activity, there is a lead to so-called "most dangerous" the largest amount of expansion content. For opal, "the most dangerous" content can be as low as 3 to 5 percent; and for less active aggregates, "the most dangerous" content may be 10% or 20%, or even up to 100%. That is, in the case of small active particles, with increasing content of alkali silicate gel, the more the number, the larger the expansion. But more than "the most dangerous" content in the future, the situation is just the opposite: the more active particles, a corresponding reduction in the effective base unit area can effect the expansion rate decreases. Thus, adding a sufficient number of active silica powder or ash, fly ash, etc., can effectively inhibit the expansion effect of alkali aggregate reaction.
For the same reason, the active particles are added to the crude fine siliceous material in spite of still and concrete react, but the expansion will decrease the ash admixture indeed reduce the erosion of coarse aggregate particles is effective.
3.3 Moisture and temperature
Alkali-aggregate reaction is usually carried out very slowly, often caused by the destruction was evident after only a few years appear. The presence of water is a necessary condition to obtain alkali aggregate reaction, permeability of concrete have a great impact on the alkali-aggregate reaction. In the case of alternating wet and dry, the reaction is accelerated. Increasing the temperature will accelerate the reaction, at least in the range of 10 ~ 38 ℃ is such, we can see a variety of physical and chemical factors of the alkali aggregate reaction becomes very complicated. Especially due to the absorption of colloidal changed its composition, and a considerable pressure, so in some cases, the occurrence of diffusion of colloidal come from a limited area. Note that in the cement hydration process, a lot of base concentrated in the aqueous phase. So PH value increases, all silica materials have become soluble.
4, aggregate activity test
While we can predict to a certain material will produce concrete aggregate reaction, but generally can not be estimated based on the number of known harmful effects of reactive material. Because the actual aggregate reactivity (activity) affected the particle size and porosity, because these parameters affect the size of the reaction can occur. Although we know that some of the aggregate active trend, but there is no simple way to determine whether a given aggregate will and a base cement and excessive expansion. There method for determining the possibility of potentially reactive aggregate of many kinds, domestic commonly used method is to determine the aggregate physical activity mortar bar test, suspicious aggregate crushing and formulated into a predetermined gradation, for the production of special has a faster expansion rate and higher amount of cement mortar bar expansion, cement alkali equivalents of not less than 0.6%, the test bar 38 ℃ in water conservation, and at this temperature than the above or below this temperature . This reaction is also accelerated because of the high water-cement ratio. Some scholars have proposed a number of better methods, such as test bar expansion at 3 months of 0.05% or more than three months after the expansion of more than 0.1%, then that this aggregate is harmful . But in making judgments on aggregate harmful it will take a long time, on the other hand, chemical test results often can not be made quickly convincing. Similarly, although the petrographic analysis of the mineral composition identification was credible, but can not prove a given mineral will produce abnormal expansion. Therefore, it remains to be developed test methods for aggregate activity of a quick and very confident. The best is at the same time using several existing test methods.
Second, alkali - carbonate reaction
1, the reaction phenomenon
Another type of reaction is the reaction of certain hazardous aggregate dolomitic limestone aggregate and cement base. With a base of concrete expansion occurring in wet conditions - silica reaction similar situation. In general, the formation of the active particles around 2 mm below the reaction zone. Cracks develop in these areas, and produce a crack network and enables adhesion between aggregate and cement paste decline.
2, reaction mechanism
Rock such reactions are limited to fine particulate argillaceous limestone and white clouds, its composition between calcite and dolomite, expanded large rocks often contain 40 to 60% dolomite and 5 to 20% include illite and the like clay and other acid-insoluble material. The reaction mechanism has not yet completely understood. One view is that: if there is presence of a base occurs as follows dedolomitization reaction:
CaCO3 · MgCO3 + 2NaOH = CaCO3 + Mg (OH) 2 + Na2CO3
Generate a swelling Mg (OH) 2, resulting in damage, at the same time, due to the presence of Ca (OH) 2, as the alkali weight resulting reaction will occur:
Na2CO3 + Ca (OH) 2 = CaCO3 + 2NaOH
This makes the above dedolomitization reaction to proceed, and so forth cycle, it may cause serious harm.
Another view is that the expansion of aggregate reaction seems related to the presence of clay, due to the clouds of reacting the dolomite crystals clay wrap exposed, so that the water swelling clay or clay films produced by the osmotic pressure caused by concrete destruction, this view Baiyun reaction merely needs to provide access to the clay water channels, the alkali - carbonate reaction force due to the expansion swelling reaction exposed clouds of dry clay absorbent produced.
Professor Tang Mingshu Nanjing Institute of Chemical to China Alkali - carbonate reaction mechanism was expanded long-term research. First, the use of four dolomitic rocks as coarse aggregate and 1 - cracking damage of three years concrete project conducted a survey which collected three cores of rock and concrete projects and seven for the four projects samples. By means of X- ray (XRD), chemical analysis, petrographic and electron probe microanalysis (EPMA) and other instruments to study the composition and structure of the rock samples, and the results showed that the collected rocks belong to micro dolomitic limestone, dolomitic gray rock, dolomite and shale micro pure microcrystalline dolomite, dolomite rock crystal size of about 5-50μm. Rock column in alkaline solution can produce swelling when soaked part of the rock also cracking damage. In concrete, rock and even cracking expansion, and makes concrete cracking damage. This expansion comes from the rock in which the Baiyun reaction. Concrete core petrographic examination found that most of the concrete crack initiation in the aggregate, and some of the crack by the dolomitic aggregate has been through the mortar, which indicates that the concrete subjected to alkali - carbonate reaction destruction. Ming said that the dolomite with a base to dolomitization reaction brucite, calcite and CO32- ions.
Concrete or cement compacted body, the reaction of CO32- ions generated with cement hydration reaction of hydroxyl calcium stone calcite, and then generate OH- ions, which helps to Baiyun reaction to proceed. Baiyun reaction generated brucite and calcite particles are small, most less than 1μm, and there are more space between the particles. Dolomite powder Baiyun reaction of cement compacts expand and cracking; almost no clay pure microcrystalline dolomite and dolomitic limestone in an alkali solution or reaction Baiyun cement concrete rock also expands and even cracking damage. This indicates Baiyun reaction itself is an expansion of the response. Because between fine particles of calcite and brucite presence of a large number of voids, the volume of the reaction product layer (including brucite, calcite and voids) out of the volume greater than the effect of dolomite. Brucite and calcite limited the growth will cause crystallization pressure, which comes from the Baiyun reaction free energy particles around a reduced pressure is applied to the thrust of the rock or compacted body expands. This mechanism is clearly consistent with the first view.
3. Factors
Differential thermal analysis (DTA) and length measurement results show that the rate of reaction of dolomite and dolomitic rocks are an increasing function of the expansion rate of the solution PH value; the higher the PH value, the faster the reaction, the greater the expansion. When the solution PH value of less than 12, the reaction rate tends to almost zero, and accordingly, the rock hardly swell. Sulphoaluminate low slag cement and plaster pore solution PH value in both dolomite cement concrete does not occur significant reaction to the clouds, and therefore do not produce reactive aggregate expansion. Admixture of a certain degree of inhibition of the expansion. To prevent the alkali - carbonate reaction expansion damage, a lot of mixed material must be mixed with a low alkali Portland cement in the same time. When high admixtures, almost no reaction to the clouds, and thus produce no swelling. The resulting mix of cement hydration CSH gel has a low CaO / SiO2 ratio, high specific surface area and strong adsorption capacity. These gel pore solution in K +, Na +, Ca2 + absorption leads to a large number of OH- ions containing diffusion electric double layer formed so that the uneven distribution of ions in the pore solution and migration deteriorated. Further, since the release of mixed materials base rate is lower than the cement at high admixtures, dilution admixture for cement may also make OH- ion activity greatly reduced. Such activity OH- ions reach the surface of the aggregate and the aggregate reacts greatly reduced, so that the reaction slows down, thereby inhibiting the occurrence of swelling. It is noteworthy that clay is not alkali - carbonate reactions necessary conditions expansion occurs, and humid environment, that there is some moisture present is required.
Third, sulfate aggregate erosion
1, the reaction phenomenon
When the aggregate contains sulfate, and hardened cement paste will react when erosion produces concrete and concrete surfaces subject to sulfate attack was characteristic whitish color, damage usually start with the edges begin, followed by further cracking and spalling, resulting in concrete change become brittle and loose state.
2, corrosion mechanism
Aggregate of sodium sulfate, potassium sulfate and other calcium oxide paste can with the role of the hydrogen-containing calcium sulfate, and then the reaction of hydrated calcium aluminate ettringite so solid volume increase a lot, respectively, 124% and 94%, resulting in considerable pressure crystallization, resulting in expansion cracks resulting damage. With sodium sulfate, for example, its role following formula:
Ca (OH) 2 + Na2SO4 · 10H2O → CaSO4 · 2H2O + 2NaOH + 8H2O
4CaO · Al2O3 · 19H2O + 3 (CaSO4 · 2H2O) + 8H2O → 3CaO · Al2O3 · 3CaSO4 · 32H2O + Ca (OH) 2
3. Factors
First, the concentration of sulfate ion, under normal circumstances, with the erosion rate of sulfate ion concentration increase and accelerate, but sulfate ion concentration reaches a certain value (about 0.5% to 1%), corrosion is accelerated with the increase of concentration slowed down.
Second, in addition to the sulfate concentration, speed of concrete erosion rate also depends supplement lost sulphate cement reaction obtained when the concrete under sulfate-containing side in water pressure, the erosion rate of the maximum. On the contrary, when the concrete is completely embedded without groundwater flow channel, erosion will ease much.
Fourth, preventive measures
1, to improve the density of concrete, changing its pore structure
As can be seen from the above discussion, either alkali silica reaction, alkali carbonate reactions, or sulfate attack, in which water plays a crucial role, the more dense concrete, impermeability, the stronger environmental media are harder invasion, therefore, we must ensure that sufficient or appropriate to reduce the amount of cement water-cement ratio, the actual production, the premise of ensuring a minimum amount of cement, we prefer to control water-cement ratio, in order to control the density of concrete.
At the same time, the incorporation of an appropriate amount of causing agent in concrete, permeable pores inside the concrete barrier, making it impermeability and sulfate resistance performance strengthened.
2. Change the mineral composition of cement clinker
First is to reduce the alkali content of the cement clinker, which can effectively suppress alkali - carbonate reactions occur - and alkali silica reaction.
Second, to reduce the C3A, C3S content, a corresponding increase in cement clinker C4AF, C2S content, C3A content is reduced to reduce the number of ettringite, thereby improving its ability of anti-sulfate, although able to produce water of hydration when C4AF sulfur iron, calcium and sulfur calcium aluminate solid solution, but it is distributed more evenly than ettringite expansion of small and hydrated calcium ferrite but also in the surrounding free of hydrated calcium aluminate to form a protective film .
When C3S hydration to precipitate more Ca (OH) 2, and the presence of Ca (OH) 2, but also the cause of a large main reason sulfate attack, which should be appropriate to reduce the clinker content of C3S corresponding increase C2S content.
3, adding a mixed material
With "fire with fire" approach, with an alkali reactive aggregate powder acts as a cement admixture to replace part of the base can be effectively suppressed - silica reaction expansion, and its inhibiting effect increases with the amount of and improve, increase and decrease with the alkali content. When Na2O≥3.0% more than about 50% of the powder to the mortar expansion at 0.1% or less inhibition; when 2.0 <Na2O <at 3.0 percent, about 35 to 40% of the powder can also achieve the same effect; and when Na2O≤2.0% less soot can be achieved when the same requirements. Powder on the expansion of the inhibition even in the long-term conservation process is quite stable.
After mixing the concrete mixture in granulated blast furnace slag or silica fume, clinker hydration precipitated Ca (OH) 2 can be combined with slag active silica or alumina hydrates generate low alkalinity so sulphoaluminate crystalline hydrated CaO concentration in the lower liquid phase expansion more relaxed. Thus effectively improve their resistance to sulfate attack capability.

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