The new block wall I built has white powdery stuff all over it three weeks later.

That is efflorescence — water-soluble salts migrating to the surface as moisture evaporates. Common on new masonry; the block, mortar, or grout contained salts, and construction water is still leaving the wall.

Fix (in order):

1. Dry the wall. Stop wetting it. Ensure grade slopes away, gutters work, and no sprinklers hit it. Let it breathe — no sealers yet.
2. Dry-brush. Once bone-dry (wait for a stretch of dry weather), scrub with a stiff nylon brush. Vacuum or blow off residue. Do not wire-brush; it damages block.
3. If brushing fails: Light wash with 1 part white vinegar to 5 parts water (or a commercial efflorescence cleaner per label). Pre-wet wall below work area. Apply, dwell 3–5 min, scrub nylon, rinse thoroughly low-pressure. Neutralize with baking-soda rinse if using acid-based cleaner.
4. Do not seal until efflorescence stops recurring (can take months). Sealing traps salts and causes spalling.

Prevent recurrence: Cap wall top, install through-wall flashing/weeps per IRC R703.8 / TMS 402, and keep water out of cores.

If the wall stays damp, powder returns — fix the water source first.

The white powdery substance on your block wall is efflorescence — a common, non-structural issue caused by water-soluble salts (like calcium carbonate, sodium, or sulfates) migrating to the surface as moisture evaporates from the masonry.

What it is: When water penetrates the concrete block or mortar (from rain, groundwater, or curing moisture), it dissolves salts within the material. As the water reaches the surface and evaporates, the salts are left behind as a white, chalky powder. It’s cosmetic, not harmful to structural integrity — but it indicates moisture movement.

How to remove it:

1. Dry brushing: Use a stiff nylon or brass brush (avoid steel if staining is a concern) to scrub off loose powder. Do this when the wall is dry.
2. Water rinse: Lightly spray with clean water and brush again. Avoid pressure washing — it can force water deeper into the wall and worsen the problem.
3. Mild acid wash (if needed): For stubborn deposits, use a solution of 1 part white vinegar to 5 parts water, or a commercial masonry efflorescence cleaner (follow label instructions). Apply, let sit 5–10 minutes, then scrub and rinse thoroughly. Never use muriatic acid unless you’re experienced — it can damage mortar and block if overused or not neutralized.
4. Seal (optional, after cleaning): Once completely dry (wait 2–4 weeks), apply a breathable masonry sealer (silane/siloxane-based) to reduce future water ingress. Avoid film-forming sealers (like acrylics) — they trap moisture and can cause spalling.

Prevention:

• Ensure proper drainage away from the wall (grading, gutters, downspouts).
• Use capillary breaks (e.g., gravel base, vapor barrier) under the wall if in contact with soil.
• Use low-alkali mortar and clean, washed sand during construction.
• Allow adequate curing time before exposing to moisture.

Note: If efflorescence returns aggressively after cleaning, it suggests ongoing moisture intrusion — investigate for leaks, poor flashing, or rising damp. In such cases, consult a masonry contractor or structural engineer. Efflorescence alone rarely requires a licensed pro — but persistent moisture issues do.

Source: International Masonry Institute (IMI), ASTM C140, and PCA guidelines on masonry efflorescence.