Cem ii

CEM II cement is a widely used Portland composite cement. It combines Portland cement clinker with other main constituents such as ground granulated blast‑furnace slag, fly ash, limestone, or pozzolans. For planning, construction, and deconstruction of concrete components, correctly classifying CEM II is important: composition and hydration influence strength, fracture behavior, abrasion, heat of hydration, and durability. These material properties directly affect concrete demolition and special demolition—for example, the selection and parameterization of tools such as concrete pulverizers or rock and concrete splitters from Darda GmbH, as well as downstream processes like building gutting, concrete cutting, and recycling.

Definition: What is meant by CEM II

CEM II is a Portland composite cement whose main portion consists of Portland cement clinker. In addition, it contains defined mass fractions of other main constituents. Typical options are ground granulated blast‑furnace slag (S), fly ash (V/W), limestone (L/LL), pozzolans (P/Q), burnt shale (T), silica fume (D), or mixtures thereof (M). Depending on the proportion of supplementary constituents (e.g., CEM II/A with a lower, CEM II/B with a higher amount), early and final strengths, heat of hydration, and durability characteristics differ. CEM II meets the relevant European cement standards and is produced in the usual strength classes.

Types and composition of CEM II

The composition of CEM II determines concrete properties over its entire life cycle—from the fresh‑concrete phase through structural service to deconstruction. Key main constituents and their typical effect profiles are:

• Ground granulated blast‑furnace slag (S): reduces heat of hydration, increases later‑age strength and sulfate resistance. Often tougher fracture behavior; in demolition, frequently lower spalling tendency but sometimes higher energy needed for crack initiation.
• Fly ash (V/W): slower early strength development, improved workability, dense matrix. In demolition, the finer microstructure can lead to higher fines content.
• Limestone (L/LL): accelerates early strength, eases workability; can moderately influence the E‑modulus. Often favorable fracture pattern for mechanical size reduction.
• Pozzolans (P/Q): improved density and durability, sometimes lower early strength. In deconstruction, often a robust matrix with clear crack guidance under suitable loading.
• Burnt shale (T) and silica fume (D): very fine microstructure, high density. Can influence tool wear and dust generation.

CEM II/A and CEM II/B

CEM II/A contains a lower proportion of supplementary constituents, CEM II/B a higher one. Practically, this often means: CEM II/A shows higher early strength and can be worked faster when the concrete is young; CEM II/B scores with density and later‑age strength, but in demolition it often requires more consistent crack initiation and a well‑matched tool choice.

Material properties and their impact on demolition methods

For choosing between concrete pulverizers, rock and concrete splitters, hydraulic combination shears, or complementary cutting processes, the following parameters are decisive:

Compressive strength and E‑modulus

As compressive strength increases, resistance to crushing and splitting generally increases. A higher E‑modulus favors a more brittle, well‑controllable crack propagation—advantageous for rock and concrete splitters. A tougher, denser matrix often requires higher pressing forces with concrete pulverizers or a sequential approach (pre‑crack, then re‑crush).

Fracture behavior and crack guidance

The supplementary constituents in CEM II influence crack initiation. A limestone‑bearing CEM II tends to form clear fracture edges; slag‑ or pozzolan‑rich systems often need defined attack points, for example through pre‑drilling, to achieve a clean crack line. For linear splitting, a uniform drill‑hole geometry and a controlled increase in splitting force are recommended.

Abrasion and tool wear

Dense, fine‑grained matrices can increase contact stresses at blades and jaws. In practice this means: inspect contact surfaces, rotate/replace wear‑critical parts in time, and set appropriate hydraulic parameters on the Darda hydraulic power units. This keeps cut quality and cycle times consistent.

CEM II in concrete demolition and special demolition

In deconstruction, the cement matrix, aggregates, and reinforcement meet tools with point‑ or line‑type force introduction. Practical decisions can be derived from analyzing CEM II cement:

• Concrete pulverizers: suitable for breaking and size‑reducing members with medium to high strengths. With tough CEM II (e.g., with slag) a two‑stage approach helps: first break edges, then reduce cross‑sections and chase crack fronts.
• Rock and concrete splitters: play to their strengths in low‑vibration methods. With a dense CEM II matrix, precise drill‑hole diameters and spacings are crucial to secure directed crack propagation.
• Hydraulic power packs: constant flow rate and a characteristic‑compliant pressure ramp are essential to achieve reproducible crack patterns and even cycles.
• Combination shears and Multi Cutters: for selective deconstruction to sever profiles, pipes, and embedded items once the concrete matrix has been opened by pulverizers.
• Steel shear: for cleanly cutting exposed reinforcement after the concrete matrix has been reduced with pulverizers.
• Tank cutters: for special operations when steel tanks, lines, or inserts around CEM II concrete must be safely dismantled.

Practical guide: Procedure for CEM II concrete

A structured approach increases efficiency and quality in the deconstruction of CEM II concrete:

1. Investigation: review construction documents, derive concrete age and likely cement type. If unclear: sampling and laboratory analysis (e.g., microstructure assessment, binder signatures).
2. Material assessment: evaluate compressive strength, E‑modulus, carbonation depth, and reinforcement ratio. Transfer the result into tool and cycle planning.
3. Method selection:
   • Planar members with good crack guidance: rock and concrete splitters with a defined drilling pattern.
   • Massive members with a tough matrix: concrete pulverizers for a sequence of pre‑cracking and re‑crushing, flanked by steel shear.
   • Selective interventions: combination shears or Multi Cutters for embedded components.
4. Sequencing: weaken edges, reduce cross‑sections, expose reinforcement, cut, lower components. Adjust cycle times to real resistance.
5. Processing: keep concrete and steel fractions separate, adjust grading, condition fines as needed.

Parameters and workflows for splitting and size reduction

The effectiveness of mechanical methods with CEM II strongly depends on preparatory crack control:

• Pre‑drilling: uniform hole diameters and depths stabilize the stress field during splitting and promote a clean crack line.
• Edge strategy: initiating cracks at edges and openings reduces energy demand. With a dense matrix, first generate predetermined breaking points.
• Force dosing: increase pressure stages on the hydraulic power pack gradually to avoid uncontrolled spalling.
• Pacing: with tough CEM II systems with higher later‑age strength, shorter, more frequent cycles are often more efficient than long single loads.

Strip‑out and cutting: Impact of CEM II

In gutting works, CEM II concretes are often found in partition walls, slabs, or columns. The dense matrix influences sawing and milling through higher heat and dust generation. Mechanical alternatives reduce vibrations:

• Concrete pulverizers open components locally to shorten saw paths.
• Rock and concrete splitters create predetermined fracture lines for dimensionally accurate detachment.
• Multi Cutters and combination shears then separate non‑mineral inserts.

Rock demolition, tunnel construction, and natural stone extraction: demarcation and touchpoints

CEM II is a cement for concrete, not a natural stone. Touchpoints arise in tunnel linings, shotcrete, and concrete foundations. There, the relationships described above apply. In actual rock demolition or natural stone extraction, rock split cylinders and rock and concrete splitters act on geogenic crack systems; the cement type only matters when rock and concrete (e.g., injection joints, linings) are processed together.

Carbonation, chlorides, and durability in the deconstruction context

The durability of CEM II concrete shapes the deconstruction strategy. Carbonated zones often show more brittle surfaces and facilitate initial breaking with concrete pulverizers. Dense, low‑carbonation core areas, by contrast, require a clear plan for crack guidance and cross‑section reduction. Chloride contamination or other environmental impacts change the bond between concrete and reinforcement—this affects exposing and subsequent cutting with steel shear.

Recycling and processing of CEM II concretes

The supplementary constituents of CEM II lead to varying fines in the crushed material. For processing, this means:

• Separate fractions: consistently separate steel, crush concrete into defined size ranges.
• Manage fines: depending on CEM II type, higher fine contents are possible. Suitable moistening and process control reduce dust and improve particle shape.
• Reuse: recycled concrete aggregates from CEM II concrete can be used in suitable applications, provided the relevant requirements are met.

Investigation and identification of CEM II on site

Whether a component was built with CEM II can often be clarified by combining indicators:

• Documents: delivery notes, specifications, concrete mix designs.
• Component age and use: in certain construction periods and exposure classes, CEM II was preferred.
• Microstructural clues: dense matrix, finely dispersed supplementary constituents, color, and fracture pattern can provide hints. A laboratory analysis provides clarity.

Safety, emissions, and work quality

Regardless of CEM II type, dust, noise, and vibrations must be minimized. Dense matrices favor the formation of fine dust during size reduction. A coordinated combination of tool choice (e.g., concrete pulverizers or rock and concrete splitters), hydraulic parameters, water mist, and pacing improves work quality and reduces emissions. Measures must always be tailored to the specific project and applicable requirements.